 Comparison of D.C. and A.C.

Transmission
The electric power can be transmitted either by means of d.c. or a.c. Each system has
its own merits and demerits.
1. D.C. transmission.
For some years past, the transmission of electric power by d.c. has been receiving the
active consideration of engineers due to its numerous advantages.

Advantages. The high voltage d.c. transmission has the following advantages
over high voltage a.c. transmission :
1. It requires only two conductors as compared to three for a.c. transmission.
2. There is no inductance, capacitance, phase displacement and surge problems in
d.c. transmission.
3. Due to the absence of inductance, the voltage drop in a d.c. transmission line is
less than the a.c. line for the same load and sending end voltage. For this
reason, a d.c. transmission line has better voltage regulation.
4. There is no skin effect in a d.c. system. Therefore, entire cross-section of the line
conductor is utilized.
5. For the same working voltage, the potential stress on the insulation is less in
case of d.c. system than that in a.c. system. Therefore, a d.c. line requires less
insulation.
6. A d.c. line has less corona loss and reduced interference with communication
circuits.
7. The high voltage d.c. transmission is free from the dielectric losses, particularly in
the case of cables.
8. In d.c. transmission, there are no stability problems and synchronizing difficulties.
9. There is low insulation required in DC system (about 70%).
10. The price of DC cables is low (Due to Low insulation)
11. In DC System, The Value of charging current is quite low, there fore, the length
DC Transmission lines is greater than AC lines.
Disadvantages
1. Electric power cannot be generated at high d.c. voltage due to commutation
problems.
2. The d.c. voltage cannot be stepped up for transmission of power at high
voltages.
3. The d.c. switches and circuit breakers have their own limitations.

2. A.C. transmission.
Now-a-days, electrical energy is almost exclusively generated, transmitted and
distributed in the form of Ac.

Advantages
1. The power can be generated at high voltages.
2. The maintenance of a.c. sub-stations is easy and cheaper.
3. The a.c. voltage can be stepped up or stepped down by transformers with ease
and efficiency. This permits to transmit power at high voltages and distribute it at
safe potentials.
Disadvantages
 1. An a.c. line requires more copper than a d.c. line.
2. The construction of a.c. transmission line is more complicated than a d.c.
transmission line.
3. Due to skin effect in the a.c. system, the effective resistance of the line is
increased, the losses in AC system are more.
4. An a.c. line has capacitance. Therefore, there is a continuous loss of power due
to charging current even when the line is open.
5. In AC line, the size of conductor is greater than DC Line.
6. The Cost of AC Transmission lines are greater than DC Transmission lines.
7. Due to Skin effect,
8. Other line losses are due to inductance.
9. More insulation required in AC System
10. Also corona Losses occur In AC System,
11. There is telecommunication interference in AC System.
12. There are stability and synchronizing problems in AC System.
13. DC System is more efficient than AC System.

 Basic components of power system:

Power system is divided into three basic component as

1. Generation 2. Transmission 3. Distribution

1. Generation:
The process of generating electricity by different sources of energy such as Hydel,
Fuel, thermal, nuclear, wind and solar is Called Generation of Electricity. TheGenerator
is important part of Generation used to generate electricity by mean of different sources
of energy. A generator is a device that converts mechanical energy to electrical energy
It works on the principal of electromagnetic induction stated by Faradays.The
generating voltage are in the range of 11KV to 13.2KV and these voltages step up by
Step-UpTransformers in 132KV, 220KV and 500KV.
2. Transmission:
Second phase of power system is transmission of electricity at higher voltages
(132/220/500KV).Electricity is transmitted through transmission lines to longer distance
at higher voltage level to reduce losses and increase capacity of power to be
transmitted. These lines are called primary transmission lines. The main component of
transmission line are conductors, insulators, tower, damper and spacer.These voltages
are step down at primary substation to 132/66KV with the help of step down transformer
and then transmitted to Secondary substation. Secondary substation step down
voltages to 11KV.For controlling and protection circuit breakers and Relays used.
3. Distribution:
Third phase of power system is called distribution of electricity to consumer. The
main function of an electrical power distribution system is to provide power to individual
consumer premises. Distribution of electric power to different consumers is done with at
low voltage level 11KV/400V by mean of distribution transformer.

 ProtectiveRelay:

A protective relay is a device which operates when the electrical quantity to

which it responds changes in prescribed manner. It is designed to trip a circuit
breaker when a fault or abnormal operating conditions is detected such as over-current,
over-voltage, reverse power flow, over- and under- frequency.Relays are used for three
purpose

1. Protection 2. Control 3. Regulation

 Function of Protective Relay

1. To detect the presence of a fault
 2. To identify the faulted components
3. To initiate the appropriate circuit breaker
4. To remove the defective component from system
 Circuit Breaker
A circuit breaker is an automatically operated electrical switch designed to
protect an electrical circuit from damage caused by overload or short circuit. Its basic
function is to detect a fault condition and interrupt current flow.It is ON Load Device.It is
used for making and interrupting a circuit under normal and abnormal condition.
Types of Circuit Breaker
According to their arc quenching media the circuit breaker can be divided as-
1. Oil circuit breaker.
2. Air circuit breaker.
3. SF6 circuit breaker.
4. Vacuum circuit breaker.
According to their services the circuit breaker can be divided as-
1. Outdoor circuit breaker
2. Indoor breaker.
According to the operating mechanism of circuit breaker they can be divided as-
1. Spring operated circuit breaker.
2. Pneumatic circuit breaker.
3. Hydraulic circuit breaker.
According to the voltage level of installation types of circuit breaker are referred
as-
1. High voltage circuit breaker.
2. Medium voltage circuit breaker.
3. Low voltage circuit breaker.

 Protection scheme
 Primary protection:
Transformers, line, reactors, buses and generators are protect by at least one
sensitive relay which will trip quickly (about 20ms) when a fault occurs.These
relays are first line of defense against fault of the system
 Back up protection

All power circuits are protected by a second or backup relay which is more or
less independent of the other set (Primary Protection).The back operates when primary
fails with an intentional time delay.It is used to increase the reliability or safety of the
power system.
 Duplicate Protection:
`Now a days, relay schemes have backup relay.The new standard to protect the
power system consisted of two indepented relay schemes where neither of them has
intentional time delay.In addition Breaker Failure Protection is provided on all high
voltages and some low voltages breakers.
 Overcurrent Relay:

A relay that operates or picks up when it’s current exceeds a predetermined

value (setting value) is called Over Current Relay.
Over current protection protects electrical power systems against excessive currents
which are caused by short circuits, ground faults, etc.
Application
1. Transformer
2. Generator
3. Transmission Lines
 4. Feeder protection Scheme
5. Motors
 Current Transformer:
A current transformer (CT) is used for measurement of alternating electric
currents. Current transformers, together with voltage (or potential) transformers (VT or
PT), are known as instrument transformers. When current in a circuit is too high to
apply directly to measuring instruments, a current transformer produces a reduced
current accurately proportional to the current in the circuit, which can be conveniently
connected to measuring and recording instruments. A current transformer isolates the
measuring instruments from what may be very high voltage in the monitored circuit.
Current transformers are commonly used in metering and protective relays

 CT Ratio:
Current transformers where the secondary current under normal operation is
practically proportional to the primary current. e.g 3000/1A would be 3000A primary
(input) with 1A secondary (output).
 Burden Of CT:
Burden is the load imposed on the secondary of the CT at rated current and is
measured in VA (product of volts and amps).The burden of CT are based on protection
& metering .Protection CT have more VA burden then metering.

 Measuring CT:

The CT used for metering will have to carry only full load current thats why it is
designed near to knee point.as per accuracy concern the metering Ct should be more
accurate under normal operating condition. The standard accuracy classes according
IEC are class 0.2, 0.5, 1, 3 en 5. For classes 3 and 5, no angle error is specified. The
classes 0.2S and 0.5S have their accuracy shifted toward the lower currents

 Protection CT:

The CT using for protection will have to carry the fault currents which are 10
times the normal full load current thats why it is designed at much below the saturation
point
(knee) in order to avoid saturation.Protection CT have more VA Burden than Measuring
CT. Under normal operating condition the protection CT accuracy not important.

 Their accuracy is not very high but most important is that the accuracy in fault
conditions is high enough. This can only be the case when the core is not saturated
in case of a fault current. They will be connected to one or more protection
relays.Accuracy classes are 5p/10p.

 Distance Protection:

There is one type of relay which functions depending upon the distance of fault in the
line. More specifically, the relay operates depending upon the impedance between the
point of fault and the point where relay is installed. These relays are known as distance
relay or impedance relay.
Working Principle of Distance or Impedance Relay

The working principle of distance relay or impedance relay is very simple. There is
onevoltage element from potential transformer and an current element fed from current
transformer of the system. The deflecting torque is produced by
secondary current of CT and restoring torque is produced by voltage of potential
transformer. In normal operating condition, restoring torque is more than deflecting
torque. Hence relay will not operate. But in faulty condition, the current becomes quite
large whereas voltage becomes less. Consequently, deflecting torque becomes more
than restoring torque and dynamic parts of the relay starts moving which ultimately
close the No contact of relay. Hence clearly operation or working principle of distance
relay, depends upon the ratio of system voltage and current. As the ratio
ofvoltage to current is nothing but impedance a distance relay is also known as
impedance relay.

The operation of such relay depends upon the predetermined value

of voltage to current ratio. This ratio is nothing but impedance. The relay will only
operate when this voltage to currentratio becomes less than its predetermined value.
Hence, it can be said that the relay will only operate when the impedance of the line
becomes less than predetermined impedance (voltage / current). As the impedance of a
transmission line is directly proportional to its length, it can easily be concluded that a
distance relay can only operate if fault is occurred within a predetermined distance or
length of line.

Instead of single step impedance protection we use a three step Distance Protection
consisting of Zone 1, Zone 2, Zone 3 and a non-directional zone if required.To avoid
over reaching first 85% of line is protected in zone 1 rest 15% in zone 2 and zone 3 is
used as back seeing of adjacent line.

 Dehydration of Transformer Oil and its importance

The process of removing the moisture (water content) from the body. Dehydration is
one of the methods to improve the efficiency of Transformer. In Dehydration process
Transformers is dried up so that the moisture and sludge could be removed out from
winding. Moisture in transformer oil can cause arcing, corona discharges, and
overheating, thereby reducing the electrical efficiency and life time of the transformer.
Water contamination at levels as low as 30ppm can adversely affect the insulating
strength of the oil. Removing water is called dehydration or dewatering. After results of
DGA and DES of oil dehydration is necessary if water contents (moisture) increased
and dielectric strength of oil is below range.

Procedure of Dehydration of Transformer Oil

Dehydration is done by mobile dehydration plant that is high temperature vacuum

process unit.When the oil is heated and enters the tank in a high degree of vacuum, the
water contained in the oil tends to vaporize and is carried away by the vacuum pump.
When the oil flows into the filter, the impurities in oil cannot pass through it due to
extremely fine mesh of the filter core. In this way, the treated oil is improved with regard
to moisture content, gas content.

Water content should be less than 10ppm after dehydration.

 Skin Effect and its losses in power system:

 Skin effect is the tendency of an alternating electric current (AC) to become
distributed within a conductor such that the current density is largest near the surface of
the conductor, and decreases with greater depths in the conductor. The electric current
flows mainly at the "skin" of the conductor, between the outer surface and a level called
the skin depth. The skin effect causes the effective resistance of the conductor to
increase at higher frequencies where the skin depth is smaller, thus reducing the
effective cross-section of the conductor. The skin effect is due to opposing eddy
currents induced by the changing magnetic field resulting from the alternating current
 According to faradays law of electromagnetic induction, a conductor placed in a
changing magnetic field induces an emf. The effect of back emf is maximum at the
centre because of maximum lines of field there. Hence the maximum opposition of
current at inner side of conductor and minimum opposition at the surface. Hence the
current tries to follow at the surface. It is due to this reason that we take hollow tube
conductors in bus duct.
 Taking into account the inductance effect, its simple consider the DC current.
Since its constant & not varying hence no back emf but if we gradually start increasing
the frequency then the flux cutting the conductor goes on increasing, hence greater the
frequency greater the alternating flux cutting the conductor & hence greater the back
emf & therefore greater the skin effect.
In AC systems the copper losses are higher due to skin effect. Due to skin effect, the
flux density at the center of the conductor is greater and current flow towards the
surface of the conductor is greater. Therefore the skin effect increases the resistance
and thus the power loss. The increase in resistance is proportional to the frequency of
the AC signal.

 What is meant by regulation in a transformer?

Voltage regulation in transformers is the difference between the no load voltage
and the full load voltage. This is usually expressed in terms of percentage.

Voltage regulation of transmission line is measure of change of receiving

end voltage from no-load to full load condition.

 How Corona Discharge Effect Occur in Transmission Line?

 In a power system transmission lines are used to carry the power. These
transmission lines are separated by certain spacing which is large in comparison to
their diameters.
 In Extra High Voltage system (EHV system ) when potential difference is applied
across the power conductors in transmission lines then air medium present between
the phases of the power conductors acts as insulator medium however the air
surrounding the conductor subjects to electro static stresses. When the potential
increases still further then the atoms present around the conductor starts ionize.
Then the ions produced in this process repel with each other and attracts towards the
conductor at high velocity which intern produces other ions by collision.
 The ionized air surrounding the conductor acts as a virtual conductor and
increases the effective diameter of the power conductor. Further increase in the
potential difference in the transmission lines then a faint luminous glow of violet color
appears together along with hissing noise. This phenomenon is called virtual corona
 and followed by production of ozone gas which can be detected by the odor. Still
further increase in the potential between the power conductors makes the insulating
medium present between the power conductors to start conducting and reaches a
voltage (Critical Breakdown Voltage) where the insulating air medium acts as
conducting medium results in breakdown of the insulating medium and flash over is
observed. All this above said phenomenon constitutes CORONA DISCHARGE
EFFECT in electrical Transmission lines.

Corona loss is the other major type of power loss in transmission lines. Essentially,
corona loss is caused by the ionization of air molecules near the transmission line
conductors. These coronas do not spark across lines, but rather carry current (hence
the loss) in the air along the wire. Corona discharge in transmission lines can lead to
hissing/cackling noises, a glow, and the smell of ozone (generated from the breakdown
and recombination of O2 molecules). The color and distribution of this glow depends on
the phrase of the AC signal at any given moment in time. Positive coronas are smooth
and blue in color, while negative coronas are red and spotty. Corona loss only occurs
when the line to line voltage exceeds the corona threshold. Unlike resistive loss which
where amount of power lost was a fixed percentage of input, the percentage of power
lost due to corona is a function of the signal's voltage. Corona discharge power losses
are also highly dependent on the weather and temperature.

 Factors Affecting Corona Discharge Effect:

Corona Discharge Effect occurs because of ionization if the atmospheric air
surrounding the voltage conductors, so Corona Discharge Effect is affected by the
physical state of the atmosphere as well as by the condition of the lines.
(1) Conductor: Corona Discharge Effect is considerably affected by the shape, size
and surface conditions of the conductor .Corona Discharge Effect decreases with
increases in the size (diameter) of the conductor, this effect is less for the conductors
having round conductors compared to flat conductors and Corona Discharge Effect is
concentrated on that places more where the conductor surface is not smooth.
(2) Line Voltage: Corona Discharge effect is not present when the applied line
voltages are less. When the Voltage of the system increases (In EHV system) corona
Effect will be more.
(3) Atmosphere: Breakdown voltage directly proportional to the density of the
atmosphere present in between the power conductors. In a stormy weather the ions
present around the conductor is higher than normal weather condition So Corona
Breakdown voltage occurs at low voltages in the stormy weather condition compared
to normal conditions
(4)Spacing between the Conductors: Electro static stresses are reduced with
increase in the spacing between the conductors. Corona Discharge Effect takes
place at much higher voltage when the distance between the power conductors
increases.

 Effects of Corona
1) Line Loss – Loss of energy because some energy is used up to cause
vibration of the air particles.
2) Long term exposure to these radiations may not be good to health (yet to be
proven).
3) Audible Noise
4) Electromagnetic Interference to telecommunication systems
5) Ozone Gas production
6) Damage to insulation of conductor.

 How to Minimizing Corona Effects

 Critical Breakdown voltage can be increased by following factors
 By increasing the spacing between the conductors:
  Corona Discharge Effect can be reduced by increasing the clearance spacing
between the phases of the transmission lines. However increase in the phases
results in heavier metal supports. Cost and Space requirement increases.
 By increasing the diameter of the conductor:
 Diameter of the conductor can be increased to reduce the corona discharge
effect. By using hollow conductors corona discharge effect can be improved.
 By using Bundled Conductors:
 By using Bundled Conductors also corona effect can be reduced this is because
bundled conductors will have much higher effective diameter compared to the
normal conductors.
 By Using Corona Rings or Grading Rings:
This is of having no greater significance but i presented here to understand the
Corona Ring in the Power system. Corona Rings or Grading Rings are present on the
surge arresters to equally distribute the potential along the Surge Arresters or Lightning
Arresters which are present near the Substation and in the Transmission lines.
Advantages
 Due to corona formation, the air surrounding the conductor becomes conducting
and hence virtual diameter of the conductor is increased. The increased diameter
reduces the electro-static stresses between the conductors.
 Corona reduces the effects of transients produced by surges.
Disadvantages
 Corona is accompanied by a loss of energy. This affects the transmission
efficiency of the line.
 Ozone is produced by corona and may cause corrosion of the conductor due to
chemical action.
 The current drawn by the line due to corona is non-sinusoidal and hence non-
sinusoidal Voltage drop occurs in the line. This may cause inductive interference
with neighboring Communication lines.

 Why We use of Stones/Gravel in electrical Switch Yard

1. Reducing Step and Touch potentials during Short Circuit Faults
2. Eliminates the growth of weeds and small plants in the yard
3. Improves yard working condition
4. Protects from fire which cause due to oil spillage from transformer and
also protects from wild habitat.
5. Maintain moisture of earth for better grounding.
 What is Boucholz relay and the significance of it in to the transformer?
Boucholz relay is a device which is used for the protection of transformer from its
internal faults,
it is a gas based relay. whenever any internal fault occurs in a transformer, the
boucholz relay at once gives a horn for some time, if the transformer is isolated from
the circuit then it stop its sound itself otherwise it trips the circuit by its own tripping
mechanism.
 What is BIL and how does it apply to transformers?
 BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests
that consist of the application of a high frequency steep wave front voltage between
windings, and between windings and ground. The Basic Impulse Level of a
transformer is a method of expressing the voltage surge (lightning, switching surges,
etc.) that a transformer will tolerate without breakdown.
 All transformers manufactured in this catalog, 600 volts and below, will withstand
the NEMA standard BIL rating, which is 10 KV.
 This assures the user that he will not experience breakdowns when his system is
properly protected with lightning arrestors or similar surge protection devices.

 The difference between Ground and Neutral?

NEUTRAL is the origin of all current flow. In a poly-phase system, as its phase
relationship with all the three phases is the same, (i.e.) as it is not biased towards
any one phase, thus remaining neutral, that’s why it is called neutral.
Whereas, GROUND is the EARTH on which we stand. It was perceived to utilize this
vast, omnipresent conductor of electricity, in case of fault, so that the fault current
returns to the source neutral through this conductor given by nature which is
available free of cost. If earth is not used for this purpose, then one has to lay a long.
long metallic conductor for the purpose, thus increasing the cost.
Ground should never be used as neutral.

 What is % impedance of a transformer?

 The percentage impedance of a transformer is the volt drop on full load due to
the winding resistance and leakage reactance expressed as a percentage of the
rated voltage."

"It is also the percentage of the normal terminal voltage at on side required to
circulate full-load current under short circuit conditions on other side."

The impedance of a transformer has a major effect on system fault levels. It

determines the maximum value of current that will flow under fault conditions. More
the %Z of transformer, more Copper used for winding, increasing cost of the unit. But
short circuit levels will reduce, mechanical damages to windings during short circuit
shall also reduce. However, cost increases significantly with increase in %Z.
 Lower %Z means economical designs. But short circuit fault levels shall increase
tremendously, damaging the winding & core.
 The high value of %Z helps to reduce short circuit current but it causes more
voltage dip for motor starting and more voltage regulation (% change of voltage
variation) from no load to full load.
Z% = Impedance Voltage x 100
Rated Voltage
 What is the reason of grounding or earthing of equipment?
with a ground path, in case of short circuit the short circuit current goes to the
body of the equipment & then to the ground through the ground wire. Hence if at the
moment of fault if a person touches the equipment body he will not get a shock cause
his body resistance (in thousands of ohms) will offer a high resistance path in
comparison to the ground wire. Hence the fault current will flow thru the ground wire &
not thru human body.
Providing a ground path helps in clearing the fault. A CT in the ground connection
detects the high value fault current hence the relay connected to the CT gives breaker a
trip command.
Grounding helps in avoiding arcing faults. IF there would have been no ground then a
fault with the outer body can cause a arcing to the ground by breaking the air. This is
dangerous both for the equipment & the human beings.

 Why the secondary of CT never open when burden is connected on the CT.?
CT is always connected in series with the load, when CT is shorted or connected to
certain load,the impedeance become negligible compared to the circuits it
connected, so voltage drop across CT will be negligible.When CT get opened then
the impedance

 What is Ferrari Effect?

 Ferranti Effect is due to the rise in voltage at the receiving end than that of the
sending end. This occurs when the load on the system reduces suddenly.
 Transmission line usually consists of line inductance, line to earth capacitance
and resistance. Resistance can be neglected with respect to the line inductance
.When the load on the system falls the energy stored in the capacitance gets
discharged. The charging current causes inductive reactance voltage drop. This gets
added vector ally to the sending end voltage and hence causes the voltage at the
receiving end to raise
 A Long transmission line draws significant amount of charging current. If such
line is open circuited or very lightly loaded at the receiving end, the voltage at the
receiving end may become greater than sending end voltage. This effect is known
Ferranti effect and is due to the voltage drop across the line inductance (due to
charging current) being in phase with the sending end voltages. Therefore both
capacitance and inductance is responsible to produce this phenomenon.
 The capacitance (charging current) is negligible in short lines, but significant in
medium and long transmission line. Hence, this phenomenon is applicable for
medium and long transmission line.
The main impact of this phenomenon is on over voltage protection system, surge
protection system, insulation level etc.
 Why transformer rating is in KVA or KW?
 Because power factor of the load is not defined in case of transformer that’s why
it is not possible to rate transformer in KW.
 The losses (cu loss and iron loss) of the transformer depends on current and
voltage purely, not on load i.e, phase angle between the current and voltage i.e. why
transformer rated in kVA
 Transformer is not a load and having no effect on P.F (that’s why no change in its
power factor) and it only transfer the constant power from one voltage level to
another voltage level without changing frequency. since both the losses viz copper
loss(depends on current) and iron loss(depends on voltage) are independent of
power factor, that is why a Transformers rating is not on kW, but on KVA

 Parallel Operations Of Transformer:

Parallel operation of transformers involves two or more transformer connected to
carry common load when a given transformer is insufficient in capacity to deliver a
particular load it may either be taken out of circuit and replaced with a larger unit or
an additional unit may be added to the circuit by connecting its primary side to the
same source of supply and its secondary side to the same load circuit. The second
unit is then operating in parallel with the first unit.
 Conditions for Parallel operation
 The same polarity
 The same phase sequence
 The same inherent phase angle difference between primary and secondary
terminals.
 The same percentage impedance
 KVA rating should not exceed three to one(3:1)
 The same voltage ratio with almost same characteristic of tap changer
 Difference Between Circuit Breaker and Isolator
Isolator is an off-load device while, circuit breaker is an on-load device. Isolator
is a switch operated manually, which separate the circuit from the power main and
discharges the trapped charges in the circuit.The isolator have
no special arrangement or mechanism to extinguish the arc
 Circuit breakers operate automatically, triggered by electromechanical
mechanism inside and are a safety feature for abnormal loads and voltages in the
circuit. Make or break both normal and abnormal currents. Its basically off load device.
 B/C RATIO:

A benefit-cost ratio (BCR) is an indicator, used in the formal discipline of cost-benefit

analysis, that attempts to summarize the overall value for money of a project or
proposal. A BCR is the ratio of the benefits of a project or proposal, expressed in
monetary terms, relative to its costs, also expressed in monetary terms. All benefits and
costs should be expressed in discounted present values.
Benefit cost ratio (BCR) takes into account the amount of monetary gain realized by
performing a project versus the amount it costs to execute the project. The higher the
BCR the better the investment. General rule of thumb is that if the benefit is higher than
the cost the project is a good investment.

 Current Energy Crises in Pkaistan & its Rectification:

Pakistan is facing a big and serious energy crisis which is increasing with the passage
of time and the government of Pakistan has remained fail to control this and fix this
issue. In spite of the growing ratio of economic growth in the entire world, Pakistan is
stuck with the same energy crisis specifically from the last 5 years. Government has
conducted many National Energy Conferences on provisional and national level, but
they all are in vain. In this conference, the government authorities just use to talk about
the methods to control the energy crisis in country and the masses have dejected over
extended outage of electricity energy. Although government has conducted many
conferences before this to control the shortage of energy and all of them remained
useless and this time, government authorities have also made a use less conclusion of
this conference.
The country is going down fall and has plunged into the worst crisis of energy since
2007; it is because of the rising demand for electricity in the country. A study has told
that the demand for electricity has entered into double digit figure following increasing
sale of electrical and electronic appliances. The Pakistan Economic Survey for the year
of 2003 and 2004 has told us that the consumption of electricity has increased by 8.6
per cent in the entire. People are now demanding and adopting more luxurious life style
which has increased this demand. This survey has unveiled that the household sector is
the biggest consumer of electricity which use to use almost 44.2 per cent of total the
electricity consumption in the country. Where as industrial sector is using 31.1 per cent
of electricity, agriculture sector is consuming 14.3 per cent, all other government sector
are consuming 7.4 per cent, the commercial sector is liable to consume 5.5 per cent
and the rest of 0.7 per cent is being consumed on the street lights.

The major management-related causes of the crisis are:

Management Information System (MIS) not fully utilized.
Failure to forecast and plan for the future.
Failure to set up new generating stations in time.
No new Transmission/Distribution networks & grid stations setup.
Unexpectedly rapid growth of load.

Solutions to cover-up Energy Crisis:

Line losses control:
This is the easiest way which can provide the relief on short term level. Pakistan is
currently bearing losses up to 24% of its total power generated. These 24% losses
include losses incurred during transmission and distribution as well as due to theft and
the late night shifts of offices and markets. If government takes a step and reduces
these to 10 % then we can save up to 300 MW of electricity which can be useful and
ultimately the rates of electricity will be reduced.
Wind Energy:
Many countries have found wind energy a best substitute of electricity. America, China
and Canada have done a lot of research and made developments to get energy from
wind. After these countries, many other countries are also thinking to get electricity from
wind. Pakistan should also develop such strategies to get energy from the wind, it will
reduce the shortfall of electricity and the rates will also decrease automatically.
Solar Energy:
Solar energy is the ultimate and free source of energy. Pakistan is going down and the
demand of electricity is increasing where as the supply is decreasing continuously. A
very large part of the rural population does not have the facility of electricity because
they are either too remote or it is found too expensive to connect their villages to the
national grid station. Pakistan can easily utilize the solar energy although in some areas
of Pakistan this kind of energy is becoming common, but still there are a lot of things to
be done in the real world, in the major cities of Pakistan.
Coal Potential in Pakistan:
Pakistan is very rich in its natural resources and it comes at the 5thnumber in the entire
world for having largest coal resources. The resources of coal in Pakistan are more than
185 Billion tones. With this huge and massive reserves of coal can help Pakistan in
generating energy from coal. The annual report of WAPDA told that Pakistan is
generating energy from coal not more than 0.79% where as the other developed
countries are generating a huge amount from coal, as USA generates 56%, UK 58%
and China is generating 81% of coal energy.
Hydro-electric power potential:
Pakistan has a massive possible opportunity to produce electric power from
hydroelectric power plants. Also focus on small hydropower project along with major
project like Dasu and Diamir Dam.
 COMPONENTS OF A Power Transformer

The following are the basic components of a transformer.

1. Core
2. Windings
3. Transformer oil
4. Tap changer
5. Conservator
6. Breather
7. Cooling tubes/Radiators
8. Buchholz Relay
Power Transformer Accessories
1. Pressure Relief valves
2. Fire sensor
3. Temperature sensor
 4. Oil level indicators
5. Fans
 Vacuum Circuit Breaker:
Salient Features:
1. Restrike free CB
2. Arc quench in ½ cycle
3. Closing its contact when separated from mechanism
4. Vacuum interrupter design upto 36KV
5. Interrupter life 20 years or upto 30000 triping
6. Pressure 110-6mm mercury is considered high vacuum
7. Vapor condensing shield used to prevent condensing on insulating enclosure.
Contact Material:
1. High electrical conductivity
2. Low contact resistance
3. High terminal conductivity
4. High melting point
5. Low tendency to weld
6. High arc with stand ability
Advantages:
1. Self contained and does not need filling of gas
2. Oil free
3. No emission of gas
4. Modest maintenance
5. Non explosive
6. Silent operation
7. Long life
8. Suitable for repeated operation
9. Constant Dielectric strength
10. Constant contact resistance
Disadvantages:

1. Expensive
2. Rated voltage per interrupter 36kv
3. Interrupter cannot repair in the event of vacuum loss
 Types Of Insulators:
There are mainly three types of insulator used as overhead transmission lines

1. Pin Insulator
2. Suspension Insulator
3. Strain Insulator
4. Shackle Insulator
5. Stay Insulator
6. Pin Insulator
1. Pin Insulator:
It look like a shaped of pin.The pin type insulator is secured to the cross arm on the
pole.There is groove on the upper end of insulator to grove the conductor by mean of
annealed wire of same material like conductor.Pin insulator secured with a steel or lead
bolt onto the transmission pole.
It is used on 11kv and 33kv lines. For 11kv single pin insulator is required but for higher
voltage it may be two are three.For 33kv lines it is not feasible.

2.Suspension Insulator:
In suspension insulator numbers of insulators are connected in series to form a string
and the line conductor is carried by the bottom most insulator. Each insulator of a
suspension string is called disc insulator because of their disc like shape.
Each suspension disc is designed for normal voltage rating 11KV(Higher voltage rating
15KV), so by using different numbers of discs, a suspension string can be made
suitable for anyvoltage level.

2. If any one of the disc insulators in a suspension string is damaged, it can be replaced
much easily.

3. Mechanical stresses on the suspension insulator is less since the line hanged on a
flexible suspension string.

4. As the current carrying conductors are suspended from supporting structure by

suspension string, the height of the conductor position is always less than the total
height of the supporting structure. Therefore, the conductors may be safe from
lightening.

3.String Insulator:

When suspension string is used to sustain extraordinary tensile load of conductor it is

referred as string insulator. When there is a dead end or there is a sharp corner in
transmission line, the line has to sustain a great tensile load of conductor or strain.
A strain insulator must have considerable mechanical strength as well as the
necessary electrical insulating properties.

5. Shackle Insulator:

The shackle insulator or spool insulator is usually used in low voltage distribution
network. It can be used both in horizontal and vertical position. The use of such
insulator has decreased recently after increasing the using of underground cable for
distribution purpose. The tapered hole of the spool insulator distributes the load
more evenly and minimizes the possibility of breakage when heavily loaded. The
conductor in the groove of shackle insulator is fixed with the help of soft binding
wire.

6. Stay Insulator:

For low voltage lines, the stays are to be insulated from ground at a height. The
insulator used in the stay wire is called as the stay insulator and is usually of
porcelain and is so designed that in case of breakage of the insulator the guy-wire
will not fall to the ground.

 Method improving string efficiency or Equalizing Potential of insulators

There are three method to equalize the potential across the insulators as

A) Reducing Ground capacitance 2. Grading 3. Guard Ring

1. Reducing Ground capacitance:

Reducing the ground capacitance relative to the capacitance of the insulators units
(reduce m where m=Ce/Cs).This can be done by increasing the length of the cross arm
and hence taller supporting tower which is uneconomical.

2. Grading Of Insulators Unit:

It can be seen that unequal distribution of voltages is due to the leakage current
from the insulators pin to tower structure.The solution is to use insulators of different
capciatnces.this requires that unit nearest the cross arm should have minimum
capacitance(maximum Xc) and the capacitance should increase as we go towards the
power line.

3. Static Shielding or Guard Ring:

This method uses a large metal ring surrounding the bottom insulators unit and
connected to the line. This ring is called Guard ring which gives a capacitance which will
cancel the charging current of the ground capacitance. Guard ring equalizing the
voltage drop across each insulator unit. Protect the insulator against flash over.

 Frequency Relay:

Relay which functions at a predetermined value of frequency; may be an over-

frequency relay, an under-frequency relay, or a combination of both. It isolate that
portion of the system under abnormal condition meet according to predefined value of
frequency.

It play vital role in performance and efficiency of the power system.If frequency of
the any part of the system either power station or substation changed then it will affect
the whole power system network. So, it is necessary to use frequency relay on Power
station generators and in substation.

Over Frequency Relay:

The relay measures line frequency and if it exceeds from the set threshold is
called over frequency.

In over system 50Hz frequency used for power system. The value set in over
frequency relay is 51.2Hz approximately.

Under Frequency Relay:

The relay measures line frequency and if it decrease from the set threshold in
case of over frequency.
 The value set for under frequency is 48.8HZ approximately.

 TYPES OF BATTERIES used at Substation:

Two types of batteries i.e (i) lead acid batteries and (ii) alkaline batteries are used in grid
stations. But lead acid batteries are most commonly used in grid stations.

LEAD ACID BATTERIES

Lead–acid batteries, invented in 1859 by French physicist Gaston Planté, are

the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio
and a low energy-to-volume ratio, their ability to supply high surge currents means that
the cells maintain a relatively large power-to-weight ratio.

MAIN PARTS OF LEAD ACID BATTERY

The main parts of lead acid battery are electrodes, separators, electrolyte and
container.

Electrodes:

Two dissimilar metals used for electrodes in lead Acid Batteries.

Anode(+ve): Porous Lead oxide PbO2

Cathode(-ve): Spongy lead on lead grid (Pb).

Electrolyte:

It consist of 35% sulphuric acid and 65% water. This solution causes a chemical
reaction that produce electrons.

Separator:

Separators between the positive and negative plates prevent short-circuit through
physical contact. Separators obstruct the flow of ions between the plates and increase
the internal resistance of the cell. Wood, rubber, glass fiber mat, cellulose, and PVC or
polyethylene plastic have been used to make separators. Wood was the original choice,
but deteriorated in the acid electrolyte. Rubber separators were stable in the battery
acid.

TYPES OF LEAD ACID BATTERY:-

There are three types of Lead acid battery, (i) Lead antimony batteries (ii) Lead planti
battery and (iii) Lead calcium battery. But mostly Lead antimony batteries are used in
grid stations because these batteries are suitable for cycling i.e for frequent charging /
discharging and also these batteries are less expensive.

SPECIFIC GRAVITY (S.G) OF LEAD ACID BATTERIES:

S.G = density of a substance/density of water = weight of a substance / weight of equal
volume of water.

S.G of water = 1, S.G of concentrated H2SO4 =1.834 and S.G of all Lead acid batteries
used in grid stations is from 1.200 to 1.220 at 77ºF or 25ºC electrolyte temperature. S.G
of a battery is commonly written as 1200 to 1220 points at77ºF or 25ºC electrolyte
temperature. Specific gravity is inversely proportional to temperature.

SG X 1/ TEMPERATURE
Generally the S.G of a new battery slightly increases from its initial value for first few
years (2 years) and then gradually decreases till its end of life.

For every 3°F above 77°F, add I point (0.001 specific gravity) to the hydrometer
reading. For every 3°F below 77°F, subtract 1 point (0.001 specific gravity) from the
hydrometer reading. When using a centigrade (degrees Celsius) thermometer, the
equivalent correction is 1 Point (0.001) for every 1.5°C.

Hence specific gravity reading must be corrected to 77ºF electrolyte temperature.

 SAFETY POLICY:-

“ No 0perating conditions or urgency of work can ever justify endangering the life
of someone”.

SAFETY CONSIDERATIONS:
 Worker Safety
 Public Safety
 Equipment Safety

WAPDA SAFETY POICY OBJECTIVES:

 To Completely integrate safety with production, construction, maintenance and
operation.
 To provide safe working conditions, proper and adequate tools, equipment and
protective devices
 To train employees in practice for safe conduct of their work
 To enforce safety measures.

SAFETY PRINCIPLES
FIVE SAFETY PRINCIPLES:

Before performing any job or work, the following five safety principles must be
considered.
1. “Know and identify hazard.”

This means that what kind of hazard or danger may exist in the work to be done.
Generally two hazards exist in any kind of work to be performed in a grid station.
i. Electric hazard
ii. Falling hazard

Sometimes mechanical hazard i.e. hazard due to accidental closing / opening of

breaker during work on breaker, also exists. Electrostatic and electromagnetic
inductions are also examples of hazards.
2. “Eliminate or remove hard, if possible.”

Electric hazard must always be removed by isolating the circuit, grounding, bonding,
use of caution notice and line teasing etc.
3. “Control or reduce hazard, if it cannot be removed”.

4. “Minimize injuries by use of approved protective gears when hazard is out of

control.”

5. “Minimize severity of injury, if an injury occurs.”

This is accomplished by use of “first aid” and artificial respiration.

 POWER SUBSTATION:
An Electrical Power Substation receives electric power from generating station via
transmission lines and delivers power via the outgoing transmission lines. Substations
are integral parts of a power system and form important links between the generating
stations, transmission systems, distribution systems and the load points. Various power
substations located in generating stations, transmission and distribution systems have
similar layout and similar electrical components. Electrical power substation basically
consists of number of incoming circuit connections and number of outgoing circuit
connections connected to the busbars. Busbars are conducting bars to which number of
circuit connections is connected. Each circuit has certain number of electrical
components such as circuit breakers, Isolators, earth switches, current transformers,
voltage transformers, etc.
In a Power Substation there are various indoor and outdoor switchgear and equipment.
Transformers are necessary in a substation for stepping up and stepping down of a.c
voltage. Besides the transformers, the several other equipment include busbars, circuit
breakers, isolators, surge arresters, Substation Earthing System, Shunt reactors, Shunt
Capacitors etc . Each equipment has certain functional requirement. The equipment are
either indoor or outdoor depending upon the voltage rating and local conditions.
In a large power System large number of Generating stations, Electrical Power
Substations and load centers are interconnected. This large internetwork is controlled
from load dispatch center. Digital and voice signals are transmitted over the
transmission lines via the Power substations. The substations are interlinked with the
load control centers via Power Line Carrier Systems (PLCC). Modern Power System is
controlled with the help of several automatic, semi - automatic equipment. Digital
Computers and microprocessors are installed in the control rooms of large substations,
generating stations and load control centers for data collection, data monitoring,
automatic protection and control.
FUNCTIONS OF ELECTRICAL POWER SUBSTATIONS ARE:

limits

TYPES OF ELECTRICAL POWER SUBSTATIONS:

1. BASED ON SUBSTATION DESIGN:
i. Outdoor Electrical Power Substations:
In Outdoor Power Substations, the various electrical equipment are installed in the
switchyard below the sky. Electrical equipment are mounted on support structures to
obtain sufficient ground clearance. Such as conventional or Air insulated Grid Station.
Air insulated Grid Station: An Air Insulated Switchgear substation (AIS substation)
uses atmospheric air as the phase to ground insulation for the switchgear of an
electrical substation.
The main advantage of the AIS substation is the scope of the substation for future
extension,simple handling, easy access.
The main disadvantage to the AIS substation is its overall size, regular maintenance
required, insulation deterioration with ambient conditions and susceptibility to pollutants.

ii. Indoor Electrical Power Substation:

In Indoor Power Substations the apparatus is installed within the substation building.
Such substations are usually for the rating of 66kV. Indoor Substations are preferred in
heavily polluted areas and Power Substations situated near the seas (saline
atmosphere causes Insulator Failures results in Flashovers).such as GIS.
Gas Insulated Grid Station:
A Gas Insulated Switchgear substation (GIS substation) uses Sulfur hexafluoride gas
(SF6 Gas) whose dielectric strength is higher than air, to provide the phase to ground
insulation for the switchgear of an electrical substation.
The main advantage of the GIS substation is that lesser space required, higher
insulation strength as compared to air.
2.SUBSTATION BASED ON FEEDING:
 i. Primary Substation:
The substation feeding the primary as well as secondary transmission lines
but not basically meant for feeding the distribution system are called primary
substation.
For example 500KV Nokhar,220KV Ghakar.
ii. Secondary Substation:
These are grid station being fed from secondary transmission line or even
primary transmission lines but are basically meant for feeding 33KV or 11KV
Distribution system so, even if a grid station is fed directly by a power house,
but is only meant for catering power to distribution system, it will be called a
Secondary Substation.
 ADVANTAGES OF GIS OVER OUTDOOR CONVENTIONAL GRID STATION
i. Low area requirement. 550 kV models, for example, take only fraction
of the space required by conventional – air types.
ii. Environmental adaptability. GIS is suitable for installation almost
anywhere: in or out of doors, even underground; near the sea, in
mountainous areas, in regions with heavy snowfall, etc.
iii. High margin safety. The high voltage conductors are securely
enclosed in grounded metal.
iv. High reliability. The chemically inert SF6 enveloping the conductors
and insulators preserves them for years of trouble free operation.
v. Long maintenance intervals. SF6 gas’s arc-quenching properties
reduce contact wear. Technological advancements over the years have
seen GIS continues to grow smaller and lighter.
vi. Low Maintenance Cost: GIS are highly reliable and maintenance free.
No inspection is required before ten years.
vii. Long Life: The operating life of GIS is 40 to 50 years compared to 25
to 30 years of conventional outdoor grid station.
viii. Personnel Safety: GIS causes no risk of injury to operating personnel.
ix. Short Circuits by Wildlife: Fully encapsulated enclosures reduces risk
of outages caused by lizards and vandalism.
x. Unbeatable Performance: Factory assembled and tested units offers
unbeatable performance in terms of reliability and continuity of power
supply.
xi. Unaffected by Environmental Conditions: GIS is unaffected by
environmental factors. It is most suitable for harsh environmental
conditions i.e. where humid, saline, polluted atmosphere laden with
industrial exhausts prevails.
xii. Economical: SF6 plants are more economical than conventional
equipment despite the higher cost of switchgear.
xii. Phase to Phase spacing:its phase to phase Spacing decreases significantly.

 TRANSMISSION LINE: -
Power transmitted from power houses to consumer by means of transmission lines at
large distances.Transmission line is comprised of three power conductors. These three
power conductors are also referred to as phases and for purposes of identification are
referred to as Red, Yellow and Blue. These conductors are attached to steel arms on
steel towers by suspension type insulators.
Components of Transmission Line:
1. Conductors 2. Tower 3. Insulators 4. Spacer 5. Dampers

 Types Of Transmission Lines On Basis of Feeding Source:

There are two basic types of transmission line on the basis of feeding source
2. Primary Transmission Line:
It normally includes voltages levels of 500KV,220KV & 132KV.A primary
Transmission line transmits power from power house to primary Grid station or from one
primary grid station to another primary grid station.e.g 500KV Tarbela-Gatti,220KV
Mangla-Nokahr , 220KV Nokahr-Ghakar .
3. Secondary Transmission Line:
 It normally includes voltages levels of 132KV & 66KV secondary Transmission
line transmits power from primary Grid station to another primary grid station or from
one secondary to another secondary grid station.e.g 132kv Nokhar-Gujranwala.

 Types Of Transmission Lines On Basis of Lentgh:

1. Short Transmission Line less than 80Km
2. Medium Transmission Linemore than 80 km but less than 250 km,
3. Long Transmission Line above 250km

 Types Of Transmission Lines On Basis of Voltages:

1. Low voltage transmission line upto 1KV
2. Medium voltage transmission line 1KV upto 66KV
3. High voltage transmission line 132KV upto 220KV
4. Extra Highvoltage transmission line 230KV to 800KV
5. Ultra High voltage transmission line Above 800KV

 ASCR Conductors:
Conductor Name Current
Capacity
RAIL 625A
DRAKE 565A
LYNEX 330A
RABIT 140A
GOPHER 85A
DOG 225A

 AAC Conductors
Conductor Name Current
Capacity
ANT 181A
WASP 271A
GNAT 124A
HOWTHRON 720A
COREPSIS 1300A

 Load Factor:
load factor is defined as the average load divided by the peak load in a specified
time period.
Load Factor=Average Load/Maximum load
 Diversity Factor:
The ratio of sum of the individual maximum demands of various subdivisions of
the system to the maximum demand of the complete system. The diversity factor
is always less than or equal to 1.
Diversity Factor= sum of the individual maximum demands/maximum
demand of the system
 Power Factor:
the power factor of an AC electrical power system is defined as the ratio of
the real power flowing to the load, to the apparent power in the circuit.
P.F=P/S (P is real power in watt & S is apparent power in VA)
P.F=CosΦ (angle between voltage & current)
P.F=Z/R (impedance to resistance ratio)
For Resistive load PF is unity, for inductive load PF is lagging and for capacitive load
PF is leading.
 Load Curves:
The number of KWH consumed varies by over the period being observed. The
trace of demand against time is a load curve. The period may be day, week,
month or year reach plot will yield different information.
 Single Line Diagram:
 One-line diagram or single-line diagram (SLD) is a simplified notation for
representing a three-phase power system.[1] The one-line diagram has its largest
application in power flow studies. Electrical elements such as circuit breakers,
transformers, capacitors, bus bars, and conductors are shown by standardized
schematic symbols.[1] Instead of representing each of three phases with a separate line
or terminal, only one conductor is represented.

Single Line pupose

i. Physical layout
ii. Nature of equipment,Tansformer vector group, breaker type
iii. Capacity of the equipment
iv. Operational information
v. Protection equipment
Color Scheme in Single Line Diagram
500KV Red, 220KV green, 132kv Gray, 11KV blue
 capacitor bank
A capacitor bank is a grouping of several identical capacitors interconnected in
parallel or in series with one another. These groups of capacitors are typically used to
correct or counteract undesirable characteristics, such as power factor lag or phase
shifts inherent in alternating current (AC) electrical power supplies. Capacitor banks
may also be used in direct current (DC) power supplies to increase stored energy and
improve the ripple current capacity of the power supply. Capacitors are electrical or
electronic components which store electrical energy. Capacitors consist of two
conductors that are separated by an insulating material or dielectric. When an electrical
current is passed through the conductor pair, a static electric field develops in the
dielectric which represents the stored energy. Unlike batteries, this stored energy is not
maintained indefinitely, as the dielectric allows for a certain amount of current leakage
which results in the gradual dissipation of the stored energy. The energy storing
characteristic of capacitors is known as capacitance and is expressed or measured by
the unit farads. This is usually a known, fixed value for each individual capacitor which
allows for considerable flexibility in a wide range of uses such as restricting DC current
while allowing AC current to pass, output smoothing in DC power supplies, and in the
construction of resonant circuits used in radio tuning. These characteristics also allow
capacitors to be used in a group or capacitor bank to absorb and correct AC power
supply faults.
The use of a capacitor bank to correct AC power supply anomalies is typically
found in heavy industrial environments that feature working loads made up of electric
motors and transformers. This type of working load is problematic from a power supply
perspective as electric motors and transformers represent inductive loads, which cause
a phenomenon known as phase shift or power factor lag in the power supply. The
presence of this undesirable phenomenon can cause serious losses in terms of overall
system efficiency with an associated increase in the cost of supplying the power. The
use of a capacitor bank in the power supply system effectively cancels out or
counteracts these phase shift issues, making the power supply far more efficient and
cost effective. The installation of a capacitor bank is also one of the cheapest methods
of correcting power lag problems and maintaining a power factor capacitor bank is
simple and cost effective. One thing that should always be kept in mind when working
with any capacitor or capacitor bank is the fact that the stored energy, if incorrectly
discharged, can cause serious burns or electric shocks. The incorrect handling or
disposal of capacitors may also lead to explosions, so care should always be exercised
when dealing with capacitors of any sort.

 Causes of Fatal/Non-Fatal accidents to line Staff:

1. Not follow safety precaution
2. Use of Improper PPE(Personal Protective Equipment)
3. Improper Grounding
4. Work Without Grounding
5. Assign technical job to untrained person
6. Lack of communication regarding instruction to work
7. Without PTW
8. Without complete isolation of faulty part of the equipment or line
 9. Over confidence
10. Poor planning
11. Work in not suitable weather condition
12. Improper T&P
13. Taking Unsafe Position or Posture
14. Insufficient Light
 Methods of Theft of Electricity:

1.Direct hooking
2.Meter tampering
3.Use earth for a return path instead of Neutral coming out from meter
4.Bypassing the energy meter(the input terminal and output terminal of the
energy meter is short-circuited)
5. inverting the meter (turning the meter upside down)
Technical Remedies:
1. Electronic Tamper detection meter
2. Smart meter by using GSM system for information transfer
3. Pre-payment meters such as prepaid cards
4. Anti theft cable
5. Plastic meter encasement
6. Using PLC

Step taken by the department to minimize the theft of electricity:

1. Proper checking of meter by reader & related SDO randomly
2. Take strict action if someone found guilty
3. Recovered amount of energy theft or fine to accused consumer
4. Check and verify the actual & connected load of the consumer
5. Temporarily disconnected his connection

 Causes of Technical losses in Transmission & Distribution System:

1. Poor quality conductor material used
2. Loosening of Joint cause Leakage current
3. Transformer losses (4% due to cupper & core losses )
4. Corona Loss
5. Skin Effect
6. Power dissipation due Resistive loss
7. Less capacity conductor or equipment used
8. Poor preventive maintenance of lines & equipment
9. Excessive or Non-linear load distribution
10. Different accuracy class of metering equipment used
11. Low voltages at consumerterminals causing higher drawl of currents by
the inductive loads.
Remedial Measures:
Some of the losses can be rectified by proper management, preventive
maintenance, planning and designing of the power system. Some losses are
inherent to Transmission & distribution system cannot be eliminated but can be
reduced.
1. Bundled conductors
2. By using good quality conductor
3. Proper load distribution
4. High accuracy class protection & metering equipment used
5. Proper preventive maintenance
6. Conductor or equipment proper sizing used

 Role Of National Grid stations & Transmission in our country :

An electric grid station is an interconnection point between two transmission
ring circuits, often between two geographic regions. They have a transformer,
depending upon the possibly different voltages, so voltages level can be adjusted as
needed.
Grid station control and regulate the power between interconnected transmission lines
to increase the reliability of the power system. It receives power from power station at
very high voltages and convert them to low voltage levels and supplied to other
substation to different or same voltages level depending upon the requirement.
National grid system of Pakistan contains an interconnected group of transmission
lines in a ring system. It covers most of the power station of the country in this single
ring and supplied electric power to different regions in the country.Main function of
substation is switching between the connected line stations and the load
centres.NTDC operates and maintains twelve 500 KV and twenty nine 220 KV Grid
Stations, 5077 km of 500 KV transmission line and 7359 km of 220 KV transmission line
in Pakistan.
In National Grid system of Pakistan several power station are interconnected in
ring main system to supply power to different areas of country under the supervision of
WAPDA.All station are transmitting their power to transmission lines and regional gris
supplied to their own areas.by connecting several power station into ring main system
stability & reliability of the system increased.
The generated voltages of the power station or either 11KV or 13.2KV and
stepped up at higher voltages level such as 132kv,220kv and 500kv by means of step
up transformer.The power is transmitted at higher voltages to minimize losses and to
increase capacity of power to be transmitted by means of high voltages transmission
lines to primary grid station.At primary grid station these voltages step down to
different voltages and transmit to secondary substation by means of secondary
transmission line
 Substation Grounding/Earthing :
The sole purpose of substation grounding/earthing is to protect the equipment
from surges and lightning strikes and to protect the operating persons in the substation.
The substation earthing system is necessary for connecting neutral points of
transformers and generators to ground and also for connecting the non current carrying
metal parts such as structures, overhead shielding wires, tanks, frames, tower etc to
earth. Earthing of surge arresters is through the earthing system. The function of
substation earthing system is to provide a grounding mat below the earth surface in and
around the substation which will have uniformly zero potential with respect to ground
and lower earth resistance to ensure that
-gaps, surge
arresters, and shielding wires etc. .

the substation

discharge the trapped charge (Due to charging currents even the line is dead still
charge remains which causes dangerous shocks) to earth prior to maintenance and
repairs.
Components of Grounding System:
Grounding systems typically include the following:
1. equipment grounding conductors - the conductors used to connect the metal
frames or enclosures of electrical equipment to the grounding electrode conductor;
2. grounding electrode conductors - the conductors connecting the grounding
electrode to the equipment grounding conductor; and
3. grounding electrodes - usually driven rods connected to each other by suitable
means, buried metal, or other effective methods located at the source, to provide a low
resistance earth connection.
Types Of GROUNDING
Solid grounding or effective grounding:
The neutral is directly connected to the earth without any impedance between neutral
and ground.
Resistance grounding:
Resistance is connected between the neutral and the ground.
Reactance grounding:
Reactance is connected between the neutral and ground.
Resonant Grounding:
An adjustable reactor of correctly selected value to compensate the capacitive
earth current is connected between the neutral and the earth. The coil is called Arc
Suppression Coil or Earth Fault Neutralizer.

 Causes and Effects of Failure of Earthing and Remedial Measures

 Following are the causes and effects of failure of earthing system,
1. Corrosion
Earthing conductor, rod, electrode plates and their joints are buried under the
ground where water logging. Under the ground, salinity badly affects the termination
and produce oxide compound at the joint and electrode which can cause damage to the
equipment.

2. Improper Material;
If the material is not used as per specification, the earth resistance increases and
will resist the flow of fault current. This badly affects the performance of the equipment.
3. Arcing
Arcing in earthing system may be due to the following reasons,
• The grid station earthing resistance is approximately 2 ohm. If the resistance of
the earthing is increased, then during the flow of fault current, arcing may result.
• If the two different earthing conductors have different ohmic value, then during
fault condition, very heavy current will flow towards the lower resistance conductor,
which can produce arcing in the system.
4. Loose Connections:
Loose connections and joints is also the cause of earthing failure.
5. Digging/Excavation
During digging of the ground, the mesh conductor can get injured or broken can
become failure of earthing.
6. Not proper jointing:
Due to not proper jointing earthing resistance increased.
7. Bimetallic Action:
It is one of the cause of failure.Bimettalic joints of two conductor e.g ACSR and
copper conductor damaged due to chemical action

Remedial Measures:
Following remedial measures are taken to achieve proper ohmic resistance
according to standard specification.
• Soil conditioning agents are introduced into ground to reduce the soil resistivity
and also to reduce earth resistance.
• In order to avoid corrosion effect, the earthing material should be galvanized and
standard copper conductor as per standard specification may be used.
• All joints should be cad welded used, then minimum corrosion occurs.
• Loose connections should be avoided.
• When digging in grid station yard with respect to proper layout of earth mesh,
proper route for fresh excavation should be selected.
• All the metallic parts of grid station should be solidly earthed at least two points
through standard copper conductor to mesh. The termination joints of the metallic
body should be inspected regularly and tightened every time.
• The neutral point of the transformer should be earthed separately with insulated
copper conductor from the bushing to avoid contact with transformer body.
• Earth testing should be carried out regularly and compared with last result.
• Lighting arrestors should be earthed with insulated conductor.

 Effect of Failure of Earthing on Grid Station Equipment

1. Effect on Power Transformer:

In the neutral of power transformer, if the failure of earthing occurs, then voltage of
transformer winding will rise which causes failure of installation. During the fault, heavy
current flows and this heavy current will not pass through the neutral of the earthing
system but keeps on circulating in the transformer winding producing excessive
thermodynamic stress. This will result in the deformation of the core and damages the
winding. Therefore, the body of the transformer is earthed at least two diagonally
different conducting points. If the earth of transformer body is open having high value of
resistance greater than 2ohm, then the transformer bushing provides the protection.
2. Effect on Lightning Arrestor:
Lightning arrestors are installed at every grid station for safety against lightning
strokes but it is always earthed. The lightning stokes are diverted through a non-linear
resistance connected with ground. However, if earth is disconnected due to any fault,
 then the lighting stroke will not be grounded. This will cause heavy heating in the non-
linear resistance that can damage the arrestor.
3. Effect on Instrument Transformers
Potential transformer is a step down transformer. One terminal of potential
transformer is connected with line voltage and other terminal is solidly grounded. If the
potential transformer is opened due to any fault, then the line voltage will appear on the
neutral terminal and insulation is damaged. This will damage the potential transformer.
A current is a step up transformer. The secondary side of current transformer is earthed.
If the earthing connection gets open, then high voltage will appear on the secondary
winding. This can damage the insulation of secondary winding and destroy the current
transformer.
 C & DF TESTING:
Electrical properties of insulating systems change due to age and continuous
electrical stress. Tan-Delta testing system also called Loss Angle or Dissipation Factor
testing, is a diagnostic method of testing electrical equipment to determine the integrity
of the insulation. By measuring electrical properties such as Capacitance and Tan-Delta
regularly it is possible to ensure the operational reliability of HV insulating systems and
to avoid costly insulation breakdown. This is particularly important for High-Voltage
Bushings, Power Transformers, Generators, Power Capacitors, H.T. cables etc.
Tan-Delta is a very important diagnosis parameter that is statistically used to
determine the degradation of insulation. Electrical properties of insulating systems
change due to age and Continuous electrical stress. By measuring values
of Capacitance and Tan-Delta regularly it is possible to ensure the operational reliability
of H.V. insulating systems and to avoid costly breakdowns. C&DF test gives us an
alarm to be ready for things to be happening and to perform maintenance works etc. for
example de- hydration of T/F.
CAPACITANCE:
The ability of a system to store an electric charge.
 the ratio of an electric charge in a system to electric potential.
C=Q/V (Q for charge And V voltage)
also C=KA/d
(K Dielectric constant K= εr ε0 where εr is the relative permittivity of the material, and ε0 =
8.8541878176.. × 10−12F/m is the air permittivity.)
A area of plates and d distance between paltes.
Dissipation factor:
A pure insulator when is connected across line and earth, it behaves as a capacitor. In
an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure,
the electric current passing through the insulator, only have capacitive component.
There is no resistive component of the current, flowing from line to earth through
insulator as in ideal insulating material, there is zero percent impurity.

In pure capacitor, the capacitive electric current leads the applied voltage by 90°.

In practice, the insulator cannot be made 100% pure. Also due to ageing of insulator the
impurities like, dirt and moisture enter into it. These impurities provide conductive path
to the current. Consequently, leakage electric current flowing from line earth through
insulator has also resistive component.Hence, it is needless to say that, for good
insulator, this resistive component of leakage electric current is quite low. In other way
the healthiness of an electrical insulator can be determined by ratio of resistive
component to capacitive component. For good insulator this ratio would be quite low.
This ratio is commonly known as tanδ or tan
delta. Sometimes it is also referred as
dissipation factor.
INTERPRETATION:
1. If both C & DF values increase from their previous values then insulation is
contaminated definitely with moisture. In this case T/F must be de-hydrated.
2. If C value is unchanged but DF Value increases from its previous value then
insulation is deteriorated chemically (may be due to aging) or insulation may be
contaminated with varnish etc. (but not with moisture).
3. If DF of inner winding (i.e. LV) to ground increases and C value decreases from their
previous values then core ground circuit is open.
4. If DF is unchanged but C value changes from its previous values, then suspect some
mechanical damage for example shifting of winding etc.
EFFECT OF HEAT ON C & DF: With heat DF increases and vice versa. So DF
readings are always corrected to 20°c oil temperature. For this tables are attached.
Temperature has no affect on C values of Transformer insulations.

EFFECT MOISTURE ON C & DF:-

With moisture C & DF both increases.
Modes Of C & DF Test:

1. Ungrounded Specimen Test(UGST):

This test is made when both specimen terminal can be insulated from ground.the
test is often used to reduce the effect of stray capcitance losses to the ground
and reduce the effect of interference pickup from nearby energized equipment.i.e
CHL
2. Grounded Specimen Test(GST):
This is most frequently used test specimen and involves all insulation between
high voltages connection to ground. i.e CHG &CLG
3. Grounded Specimen Test with Guard connection(GST-G):
This test is used to separate the total values of GST into separate part for better
analysis.often this test is used with the GST to confirm the reading made using
UGST.i.e CHL+CHG

 FAULTS IN POWER TRANSFORMER:

There are two type of faults in power transformer
1. Internal Faults:
 Short Circuits/Insulation failure
i. Between winding
ii. Between Turns
 Ground Faults
 Over temerature/heating
 Over pressure
 Miss of oil/detereriotion of oil

2. External or Through Faults:

 External short circuit
 Overloads
 Over volatges
 Core faults/damaging of core

 CAUSES OF DISTRIBUTION TRANSFORMER BURNOUT

 Over Voltages
  Not proper preventive maintenance
 Imbalanced loading of phases
 Loose connections/jumper
 Improper rating fuses used
 Over loaded
 Decomposition of oil/low oil level
 Unavoidable weather condition(wind,storm,rain)
 old age transformer/weak insulation
 Mishandling & Improper Installation.
 Clogging of cooling pipes/ fins.
 Switching transit surges
 Energization of Transformer and LT with out patrolling & rectification of fault.
 Damage of Transformer due to fault in LT Network
Remedies to avoid damaging Of Distribution Transformer
 Proper preventive maintenance in time
 Proper installation
 Check the DES of Oil and its level
 Avoid overloading
 Balanced load on each phase
 Control excessive voltages
 Avoid unwanted switching
 Correct ratings fuse used
 Tight all jumpers & connection
 Dehydration of oil

Transformer Ratings HV Current LV current

10KVA 0.52KA 13.91KA
15KVA 0.78KA 20.86KA
25KVA 1.31KA 34.78KA
50KVA 2.62KA 69.56KA
100KVA 5.24KA 139.12KA
200KVA 10.49KA 278.24KA

 DC Power in Substation:
AC power is required for substation building small power, lighting, heating and
ventilation, some communications equipment, switchgear operating mechanisms, anti-
condensation heaters and motors. DC power is used to feed essential services such as
circuit breaker trip coils and associated relays, supervisory control and data acquisition
(SCADA) and communications equipment. For an effective or safe operation of the
substation direct current auxiliary systems used for substations.
The most critical component of protection, control, metering and monitoring
system is the auxiliary DC power control system. Failure of DC control power can
rendered fault detection devices unable to detect faults, breaker unable to trip on fault.
Theauxiliary DC control system consist of Rectifier, Batteries, distribution system,
switching & protective devices and any metering & monitoring equipment. Due to
advancement in technology all the protection relays, control system, transducer,
communication equipment works on DC system. DC system is more reliable than AC in
substation because of its storage capability by use of batteries. Rectifier convert AC
power to DC by means of Thyristor controlled bridge. In cause of failure of AC auxiliary
supply battery will supply uninterruptable DC power as a back source.
 Radial Feeders:
The feeder that originated at its source of supply, ending at the consumer.
Disadvantage is their low reliability i.e zero. When it is tripped by fault, the supply is cut.
 Loop Feeders:
Loop feeders originating from one common source as two feeders but with
additional connection in between. With one feeder tripped then other feeder remains
fed, the factor of reliability equal to one.

 Mesh Feeders:
 Mesh feeders are a system of conductors spread over an area to form a meshes.
The knots of meshes are the points where distribution transformers are connected.
Reliability of feeder is at least 3.

 Methods of Protection against Lightning

These are mainly three main methods generally used for protection against
lightning. They are

1. Earthing screen.
2. Overhead earth wire.
3. Lighning arrester

Earthing Screen

Earthing screen is generally used over electrical sub-station. In this arrangement a net
of GI wire is mounted over the sub-station. The GI wires, used for earthing screen are
properly grounded through different sub-station structures. This network of grounded GI
wire over electrical sub-station, provides very low resistance path to the ground for
lightning strokes.

This method of high voltage protection is very simple and economic but the main
drawback is, it can not protect the system from travelling wave which may reach to the
sub-station via different feeders.

Overhead Earth Wire

This method of over voltage protection is similar as earthing screen. The only difference
is, an earthing screen is placed over an electrical sub-station, whereas, overhead
earthwire is placed over electrical transmission network. One or two stranded GI wires
of suitable cross-section are placed over the transmission conductors. These GI wires
are properly grounded at each transmission tower. These overhead ground wires or
earthwire divert all the lightning strokes to the ground instead of allowing them to strike
directly on the transmission conductors.

 Lighting arrestors:- It is a type of protective device used to save the costly power
system equipment from the damaging effects of over voltages due to lightening
strokes and surges.
The lightning arrester is a devices which provides very low impedance path to the
ground for high voltage travelling waves.
The concept of a lightning arrester is very simple. This device behaves like a
nonlinear electrical resistance. The resistance decreases as voltage increases and vice-
versa, after a certain level of voltage. Once Lightening arrester operate at lightening
stroke then it have to replace.

The functions of a lightning arrester

1. Under normal voltage level, these devices withstand easily the

system voltage as electrical insulator and provide no conducting path to the
system current.
2. On occurrence of voltage surge in the system, these devices provide very low
impedance path for the excess charge of the surge to the ground.
3. After conducting the charges of surge, to the ground, the voltage becomes to its
normal level. Then lightning arrester regains its insulation properly and prevents
 regains its insulation property and prevents further conduction of current, to the
ground.

 Types of Lightening Arrestors: -

i. Spark gap type lightening arrestor ii Zinc-oxide type lightening arrestor.

 Reactors & their role in National Transmission & Grid

StationReactors and Series Reactors are used widely in AC networks to limit the
overvoltage or to limit the shortcut current. With more high-voltage overhead lines for
long transmission distance and increasing network capacity, Reactors andSeries
Reactors play an important role in the modern network system.

Reactors
Reactors
For extra-high-voltage (EHV) transmission lines, due to the long distance, the
space between the overhead line and the ground naturally forms a capacitor parallel to
the transmission line, which causes an increase of voltage along the distance.
Depending on the distance, the profile of the line and the power being transmitted,
a Reactor is necessary either at the line terminals or in the middle. The advanced
design and production technology will ensure the product has low loss and low noise
level.It is used as a shunt to circuit also called shunt reactor.

Shunt reactors carry out different types of tasks:

 They compensate the capacitive reactive power of the transmission lin, in

particular in networks with only light loads or no load.
 They reduce system-frequency over voltages when a sudden load drop occurs or
there is no load.
 They improve the stability and efficiency of the energy transmission.

Series Reactor:
When the network becomes larger, sometimes the short-circuit current on a
transmission line will exceed the short-circuit current rating of the equipment. Upgrading
of system voltage, upgrading of equipment rating or employing high-impedance
transformers are far more expensive than installing oil-immersed Series Reactors in
the line.

 Fuel Price Adjustment:

"Fuel cost adjustment system" is a system designed to automatically adjust
monthly electricity fees based on fluctuations in (actual recorded) fuel prices for crude
oil, liquefied natural gas (LNG) and coal.

 Facts Controllers:
A power electronic based system & other static equipment that provide control of
one or more AC transmission parameters are called Facts Controller or Flexible AC
transmission System.
AC transmission systems incorporating the power electronic-based to enhance
controllability and increase power transfer capability.
Controllable parameters
Control of the line impedance
current and active power control
Control of angle
current and active power control
Series voltage injection
Current, active, and reactive power control
Parallel voltage injection
Current, active, and reactive power control
Types of Fact Controller:
 Static Var Compensator(SVC)
 Static Synchronous Compensator (STATCOM)
 Series Active Power filter (Series APF)
 Static Synchronous Series Compensator (SSSC)

 Static Var Compensator(SVC)

A static VAR compensator (var is defined as volt ampere reactive) is a set of electrical
devices for providing fast-actingreactive power on high-voltage electricity
transmission networks. SVCs are part of the Flexible AC transmission systemdevice
family, regulating voltage, power factor, harmonics and stabilizing the system. Unlike
a synchronous condenserwhich is a rotating electrical machine, a static VAR
compensator has no significant moving parts (other than internal switchgear). Prior to
the invention of the SVC, power factor compensation was the preserve of large rotating
machines such as synchronous condensers or switched capacitor banks.
The SVC is an automated impedance matching device, designed to bring the system
closer to unity power factor. SVCs are used in two main situations:

 Connected to the power system, to regulate the transmission voltage

("Transmission SVC")
 Connected near large industrial loads, to improve power quality ("Industrial
SVC")
In transmission applications, the SVC is used to regulate the grid voltage. If the power
system's reactive load is capacitive(leading), the SVC will use thyristor controlled
reactors to consume VARs from the system, lowering the system voltage.
Under inductive (lagging) conditions, the capacitor banks are automatically switched in,
thus providing a higher system voltage. By connecting the thyristor-controlled reactor,
which is continuously variable, along with a capacitor bank step, the net result is
continuously-variable leading or lagging power.
Typically, an SVC comprises one or more banks of fixed or switched shunt
capacitors or reactors, of which at least one bank is switched by thyristors. Elements
which may be used to make an SVC typically include:

 Thyristor controlled reactor (TCR), where the reactor may be air- or iron-cored
 Thyristor switched capacitor (TSC)
 Harmonic filter(s)
 Mechanically switched capacitors or reactors (switched by a circuit breaker

 Switching Impulse Test Of Transformer

The switching impulse test is applied to confirm the withstand of the transformer’s
insulation against excessive voltages occurring during switching. During switching
impulse voltage test, the insulation between windings and between winding and earth
and withstand between different terminals is checked.
The switching impulse voltage is generated in conventional impulse voltage
generators at the laboratories. Due to over-saturation of the core during switching
impulse test, a few low amplitude, reverse polarity (e.g. positive) impulses are applied
after each test impulse in order to reset the transformer core to it’s starting condition
(demagnetised). By this way,the next impulse voltage waveform is applied. The tap
position of the transformer during test is determined according to test conditions.
The on-off impulse voltages are applied to each high voltage terminal sequentially.
 Differential Relay:
Differential protection as its name implies, compare currents entering and leaving
the protected zone and operates when the differential current between these currents
exceeds a predetermined level.
The differential relay actually compares between primary current and
secondary current of power transformer, if any unbalance found in between primary and
secondary currents the relay will actuate and inter trip both the primary and
secondary circuit breaker of the transformer.
The type of differential scheme used normally applied to a transformer is called
current balanced or circulating current scheme
Differential protection using current balance scheme for external conditions.The
CTs are connected in series and secondary current circulate between them.The relay is
connected across the mid-point where the voltages is theoretically/ideally nil, therefore
no current passes through the the relay, hence no operation for faults outside the
protected zone.If an equipment within the protection Zone is functioning correctly, then
the sum of the currents entering the zone must equal the sum of the currents leaving i.e
their difference must be zero.
Under internal fault conditions (the fault between CT) the relay operates, since
both the CTs secondary currents add up and pass through the relay.
Most of differential relays have slope setting of 20% to 40%.
 ENERGY METER:
An electric meter is an electrical instrument which measure and records the amount of
electrical energy, consumed by an load.The internal mechanism of meter is such that
when the meter is connected to a power supply it registers and display the running total
units consumed.

Single Phase Static Energy Meters

Single phase static energy meter used for measurement of active(kWh) energy for
Domestic & commercial consumers.
Some of the main features include:
Capable of recording on the basis of current with assumed voltage and power factor in
the absence of voltage on its terminals.
Display of energy (kWh) during power outages.
High performance Lithium battery with long storage life for display of data during power
outages.
Window based application software for configuration of parameters.
Anti-tamper indication on meter LCD
Electromechanical Energy Meter:
These are old type of energy meter based on analog technology. They work on the
principal electromagnetic induction. The deflecting torque is produced on pivot disc by
mean of the interaction of two fluxes and the currents that they induce in the disc.Thus
pivot disc rotates like a high precision motor.
 Types of Energy Meter:
Different types of energy meter are being used depending upon nature of load
and requirement of tarrif.
 Direct Kwh meter
 Kvarh meter
 CT operated Kwh meter
 CT & PT operated Kwh meter
 TOD meter/Kwh
 MDI kwh meter
 Method of reduction of interference of power line with communication line
Electromagnetic interference effects of transmission lines upon nearby
communication lines are real problems.Power line attenuate the communication
signal.Noise and distortion added by Power lines.There are following techniques
used to reduce these effects
 Proper distance maintain from Power lines
 Proper Shielding/insulation used in Communication line
 Where possible used underground power cables
 Proper Filters used
 Use separate pole communication lines
 Follow precaution at crossing
 Separation in the soil between telecommunication cables and earthing
system of power facilities
 Proper earthing of communication equipment
 Parallel run between power line & communication line avoided
 Transposition of power lines
 Use Isolation transformer
 increase use of optical fibre
 STEP POTENTIAL:
The potential experienced by a person between his feet when taking step at the
ground towards energized grounded equipment is called “Step Potential.”
If the person move towards an energized equipment which is at higher potential,
then the foot near the equipment during a step forward will be at higher potential as
compared to the back foot, a potential difference is created between the foots of the
person. The person may feel shock depending upon the voltage.It can be dangerous for
the person working at the site.To reduce the step potential at substation gravel is used.
 TOUCH POTENTIAL:
In case of ground fault or leakage current due to induction the potential
experienced by a person standing on earth or ungrounded mat while touching the tower
is called “Touch Potential”.
Equipment should be earthed properly at least at two points.Check the earthing
resistance properly.

 Power-line communication :
Power-line communication (PLC) carries data on a conductor that is also used
simultaneously for AC electric power transmission or electric power distribution to
consumers. It is also known as power-line carrier communication.

Power-line carrier communication (PLCC) is mainly used for telecommunication, tele-

protection and tele-monitoring between electrical substations through power
lines at high voltages, such as 132kV, 220 kV, 500 kV. The major benefit is monitoring
of electric equipment and advanced energy management techniques. The speech/data
signal having frequency 300 to 400Hz and this audio frequency is mixed with the carrier
frequency. The carrier frequency is again filtered, amplified and transmitted.

The principle of PLCC consists in superimposing a high frequency signal (24khz

to 500khz) at low energy levels over the 50 Hz electrical signal.
To sectionalize the transmission network and protect against failures, a "wave trap" is
connected in series with the power (transmission) line. They consist of one or more
sections of resonant circuits, which block the high frequency carrier waves (24 kHz to
500 kHz) and let power frequency current (50 Hz – 60 Hz) pass through. Wave traps
are used in switchyard of most power stations to prevent carrier from entering the
station equipment. Each wave trap has a lightning arrester to protect it from surge
voltages.
A coupling capacitor is used to connect the transmitters and receivers to the high
voltage line. This provides low impedance path for carrier energy to HV line but blocks
the power frequency circuit by being a high impedance path. The coupling capacitor
may be part of a capacitor voltage transformer used for voltage measurement.
Optical fiber channel also used a back up.
 WAVE TRAP USED IN SUBSTATION:
The Wave Traps extract the high frequency information from the power lines and
route it to the telecomm panels. They also block any surges from passing through.

Wave Traps are simply resonant circuits(combination of capacitor & inductor) that
produce a high impedance against PLCC carrier frequencies (24kHz - 500kHz) while
allowing power frequency (50Hz - 60Hz).
 Capacitor Voltage Transformer (CVT):
A capacitor voltage transformer (CVT), or capacitance-coupled voltage
transformer (CCVT), is a transformer used inpower systems to step down extra high
voltage signals and provide a low voltage signal, for metering or operating aprotective
relay.
the device consists of three parts: two capacitors across which the transmission line
signal is split, and inductive element to tune the device to the line frequency, and
a voltage transformer to isolate and further step down the voltage for the metering
devices or protective relay.
The tuning of the divider to the line frequency makes the overall division ratio less
sensitive to changes in the burden of the connected metering or protection devices The
device has at least four terminals: a terminal for connection to the high voltage signal, a
ground terminal, and two secondary terminals which connect to the instrumentation or
protective relay.
In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected
in series. This provides a large voltage drop across C1 and a relatively small voltage
 drop across C2. As the majority of the voltage drop is on C1, this reduces the isolation
level of the voltage transformer. This makes CVTs more economical than the wound
voltage transformers under high voltage (over 100kV), as the latter one requires more
winding and materials.

Main applications of CVTs in HV Networks

 Voltage Measuring: They accurately transform transmission voltages down to
useable levels for revenue metering, protection and control purposes
 Insulation: They guarantee the insulation between HV network and LV circuits
ensuring safety condition to control room operators
 HF Transmissions: They can be used for Power Line Carrier (PLC) coupling
 Transient Recovery Voltage: When installed in close proximity to HV/EHV Circuit
Breakers, CVT’s own High Capacitance enhance C/B short line fault / TRV performance

 Smart Grid:
A smart grid is a modernized electrical grid that uses analogueor
digitalinformation and communications technology to gather and act on information,
such as information about the behaviours of suppliers and consumers, in an automated
fashion to improve the efficiency, reliability, economics, andsustainability of the
production and distribution of electricity.
 Smart Meter:
A smart meter is usually an electronic device that records consumption
of electric energy in intervals of an hour or less and communicates that information at
least daily back to the utility for monitoring and billing purposes.Smart meters enable
two-way communication between the meter and the central system. Unlike home
energy monitors, smart meters can gather data for remote reporting.

 SCADA (supervisory control and data acquisition)

It is a system operating with coded signals over communication channels so as to
provide control of remote equipment (using typically one communication channel
per remote station)

A SCADA system performs four functions:

1. Data acquisition

2. Networked data communication

 3. Data presentation

4. Control

These functions are performed by four kinds of SCADA components:

1. Sensors (either digital or analogue) and control relays that directly interface with
the managed system.
2. Remote telemetry units (RTUs). These are small computerized units deployed in
the field at specific sites and locations. RTUs serve as local collection points for
gathering reports from sensors and delivering commands to control relays.
3. SCADA master units. These are larger computer consoles that serve as the
central processor for the SCADA system. Master units provide a human interface
to the system and automatically regulate the managed system in response to
sensor inputs.
4. The communications network that connects the SCADA master unit to the RTUs
in the field.

 SF6 Circuit Breakers:

High-voltage circuit breakers have greatly changed since they were first introduced in
the mid-1950s, and several interrupting principles have been developed that have
contributed successively to a large reduction of the operating energy. These breakers
are available for indoor or outdoor applications, the latter being in the form of breaker
poles housed in ceramic insulators mounted on a structure.
Current interruption in a high-voltage circuit-breaker is obtained by separating two
contacts in a medium, such as sulfur hexafluoride (SF6), having excellent dielectric and
arc-quenching properties. After contact separation, current is carried through an arc and
is interrupted when this arc is cooled by a gas blast of sufficient intensity.
Characteristics of SF6 circuit breakers:
 Low noise level
 Simplicity of interrupting chamber
 Highest performance
 Reliability & availability
 Integrated closing resistors or synchronized operations to reduce switching over-
voltages
 Short break time of 2 to 2.5 cycles
 Less maintenance
 Safe operation
Properties Of SF6 GAS:
 Color less
 Odour less
 Non toxic
 Non inflameable
 Density 5 times greater than Air
 Dielectric Strength at atmospheric pressure is 2.35 times more than Air and 30%
less than air
 Vector Group:-
A vector group is the International Electro technical Commission (IEC) method of
categorizing the high voltage (HV) windings and low voltage (LV) winding configurations
of three-phase transformers. The vector group designation indicates the windings
configurations and the difference in phase angle between them. For example, a wye HV
winding and delta LV winding with a 30-degree lead is denoted as Yd11.
Different combinations of winding connections will result in different phase angles
between the voltages on the windings. Transformers connected in parallel must have
the same vector group; mismatching phase angles will result in circulating current and
other system disturbances.

In the IEC vector group code, each letter stands for one set of windings. The high-
voltage (HV) winding is designated with an uppercase letter, followed by medium or low-
voltage (LV) windings designated with a lowercase letter. The digits following the letter
codes indicate the difference in phase angle between the windings, with HV winding is
taken as a reference. The number is in units of 30 degrees. For example, a transformer
with a vector group of Dy1 has a delta-connected HV winding and a wye-connected LV
winding. The phase angle of the LV winding lags the HV by 30 degrees.
Note that the high-voltage (HV) side always comes before the low-voltage (LV) side,
regardless of which is the primary winding. This means that the vector group symbol will
always start with a capital letter.
The letters indicate the winding configuration as follows:

 D or d: Delta winding, also called a mesh winding.

 Y or y: Wye winding, (also called a star).
 Z or z: Zigzag winding having both star & delta connection
 N (uppercase): indicates that a system neutral is connected to the high-voltage
side.
 n (lowercase): indicates that a system neutral is connected to the low-voltage
side.
 String Efficiency:-

The ratio of voltage across the whole string to the product of number of discs
and the voltage across the disc nearest to the conductor is known as string
efficiency i.e.

String Efficiency=voltage across string/(n x voltage across the disc nearest the
conductor)

where n = number of discs in the string.

String efficiency is an important consideration since it decides the potential
 distribution along the string. The greater the string efficiency, the more uniform is
the voltage distribution. Thus 100% string efficiency is an ideal case for which
the volatge across each disc will be exactly the same. Although it is impossible
to achieve 100% string efficiency, yet efforts should be made to improve it
as close to this value as possible.

 How do the insulator fail?

An insulator performs dual functions: mechanically, it holds the conductor or

busbar at a certain distance from ground; and electrically it also provides the necessary
insulation. Therefore, one way to define insulator failure is when either or both of these
functions are no longer being fulfilled.

Causes of Insulator Failure

There are different causes due to which failure of insulation in electrical power system
may occur. Let's have a look on them one by one-

Cracking of Insulator
The porcelain insulator mainly consists of three different materials. The main porcelain
body, steel fitting arrangement and cement to fix the steel part with porcelain. Due to
changing climate conditions, these different materials in the insulator expand and
contract in different rate. These unequal expansion and contraction of porcelain, steel
and cement are the chief cause of cracking of insulator.

Defective Insulation Material

If the insulation material used for insulator is defective anywhere, the insulator may
have a high chance of being puncher from that place.

Porosity in The Insulation Materials

If the porcelain insulator is manufactured at low temperatures, it will make it porous, and
due to this reason it will absorb moisture from air thus its insulation will decrease and
leakage current will start to flow through the insulator which will lead to insulator
failure.

Improper Glazing on Insulator Surface

If the surface of porcelain insulator is not properly glazed, moisture can stick over it.
This moisture along with deposited dust on the insulator surface, produces a conducting
path. As a result the flash over distance of the insulator is reduced. As the flash over
distance is reduced, the chance of failure of insulator due to flash over becomes more.

Flash Over Across Insulator

If flash over occurs, the insulator may be over heated which may ultimately results into
shuttering of it.

Mechanical Stresses on Insulator

If an insulator has any weak portion due to manufacturing defect, it may break from that
weak portion when mechanical stress is applied on it by its conductor. These are the
main causes of insulator failure. Now we will discuss the different insulator test
procedures to ensure minimum chance of failure of insulation.
 What is open delta connection?
Open delta transformers are not the commonly used. Typically they would be
used for small loads where cost is important. Alternatively, they could be used as an
emergency measure, should one winding only of a transformer fail.Open delta is also
called as v-connection transformer.
Some time the power delivered by an open delta transformer is compared to that
of an equivalent three winding transformer. Typically figures like having 57.7% of the
capacity of an equivalent three winding transformer or 87% of two winding transformer.
 Disadvantages of low Power Factor:
In case of Low Power Factor, Current will be increased, and this high current
will cause to the following disadvantages.
1.) Large Line Losses (Copper Losses):
We know that Line Losses is directly proportional to the squire of Current “I2”
Power Loss = I2xR i.e., the larger the current, the greater the line losses i.e. I>>Line
Losses
In other words,
Power Loss = I2xR = 1/CosФ2 ….. Refer to Equation “I ∝ 1/CosФ”….… (1)
Thus, if Power factor = 0.8, then losses on this power factor =1/CosФ2 = 1/ 0.82 = 1.56
times will be greater than losses on Unity power factor.
2.) Large kVA rating and Size of Electrical Equipments:
As we know that almost all Electrical Machinery (Transformer, Alternator, Switchgears
etc) rated in kVA. But, it is clear from the following formula that Power factor is inversely
proportional to the kVA i.e.
CosФ = kW / kVA
Therefore, The Lower the Power factor, the larger the kVA rating of Machines also, the
larger the kVA rating of Machines, The larger the Size of Machines and The Larger the
size of Machines, The Larger the Cost of machines.
3.) Greater Conductor Size and Cost:
In case of low power factor, current will be increased, thus, to transmit this high current,
we need the larger size of conductor. Also, the cost of large size of conductor will be
increased.
4.) Poor Voltage Regulation and Large Voltage Drop:
Voltage Drop = V = IZ.
Now in case of Low Power factor, Current will be increased. So the Larger the current,
the Larger the Voltage Drop.
Also Voltage Regulation = V.R = (VNo Load – VFull Load)/ VFull Load
In case of Low Power Factor (lagging Power factor) there would be large voltage drop
which cause low voltage regulation. Therefore, keeping Voltage drop in the particular
limit, we need to install Extra regulation equipments i.e. Voltage regulators.

5.) Low Efficiency:

In case of low Power Factor, there would be large voltage drop and large line losses
and this will cause the system or equipments efficiency too low. For instant, due to low
power factor, there would be large line losses; therefore, alternator needs high
excitation, thus, generation efficiency would be low.
6.) Penalty from Electric Power Supply Company on Low Power factor
Electrical Power supply Company imposes a penalty of power factor below 0.95 lagging
in Electric power bill. So you must improve Pf above 0.95.

7.) Loss of generating capacity:

 The lagging power factor reduces the handling capacity of all the elements of the
system.
 It is because the reactive component of current prevents the full utilization of
installed capacity.
8.) Poor power factor means more line loss and low transmission efficiency
 For a given cross-sectional area of the line conductors, line losses are
proportional to 1/cos2Φ.
 Poor power factor means more line losses and low transmission efficiency.

 Methods of Power Factor Improvement

• Capacitors:Improving power factor means reducing the phase difference between
voltage and current. Since majority of loads are of inductive nature, they require some
amount of reactive power for them to function. This reactive power is provided by the
capacitor or bank of capacitors installed parallel to the load. They act as a source of
local reactive power and thus less reactive power flows through the line. Basically they
reduces the phase difference between the voltage and current.

• Synchronous condenser: They are 3 phase synchronous motor with no load

attached to its shaft. The synchronous motor has the characteristics of operating under
any power factor leading, lagging or unity depending upon the excitation. For inductive
loads, synchronous condenser is connected towards load side and is overexcited. This
makes it behave like a capacitor. It draws the lagging current from the supply or
supplies the reactive power.
• Phase advancer: This is an ac exciter mainly used to improve pf of induction motor.
They are mounted on shaft of the motor and is connected in the rotor circuit of the
motor. It improves the power factor by providing the exciting ampere turns to produce
required flux at slip frequency. Further if ampere turns are increased, it can be made to
operate at leading power factor.

 What is earthing mat? What is its use?

A grounding system formed by a grid of horizontally buried conductors - Serves to
dissipate the earth fault current to earth and also as an equipotential bonding conductor
system.

 What is difference between system earthing & equipment earthing?

System earthing (Connection between part of plant in an operating system like LV

neutral of a power transformer winding) and earth.This earthing for flow of current in
case of fault.

Equipment earthing (safety grounding) connecting bodies of equipment (like electric

motor body, transformer tank, switchgear box, operating rods of air break switches, LV
breaker body, HV breaker body, feeder breaker bodies etc) to earth.

Necessity of Equipment Earthing

Protection (a) Safety of personnel (b) Safety of equipment Prevent or at least minimize
damage to equipment as a result of flow of heavy currents. (c) Improvement of the
reliability of the power system.

 What is the reason for providing delta tertiary winding in star-star

connected auto-transformer?

In delta-delta, delta-star and star-delta transformers all voltages are balanced and
there is no floating of neutral. The floating of neutral is developed in the case star-
star connection only. The transformers are sometimes constructed with three
windings. The main windings are connected to form star-star connection and the third
winding known as tertiary winding is used to make a closed delta connection to
stabilize the neutrals of both primary and secondary circuits. The tertiary winding
carries the third-harmonic currents. Third harmonic magnetizing currents flow in
closed delta, making induced voltages and core flux almost sinusoidal.

The tertiary winding connected in delta reduce the impedance offered to the zero
sequence currents so as a larger earth fault current flows for proper operation of
protective equipment.it permits the flow of third harmonic current to reduce the third
harmonic voltage.

The winding is generally delta connected. Thus any fault or short circuit occurs on the
primary or secondary sides, there will be large unbalance of phase voltage which is
compressed by the large tertiary winding circulating current. The reactance of tertiary
winding must be such as to limit the circulating current to that which can be carried
by copper in order to avoid over heating of tertiary winding under fault condition.

HARMONICS are caused because of use of high flux densities in the core. If the
core gets saturated during part of the sinusoidal wave, then secondary wave will be
non-sinusoidal. This may be due to inadequate core area or characteristics of core
material.
Harmonic currents result in higher copper loss, core loss, magnetic interference and
interference with communication systems. Harmonic voltages result in increased
dielectric loss, interference with communication systems and resonance
between inductance of the winding and capacitance of transmission line.
 What is meant by protective angle? Give its value for reliable operation?
Protective angle is the vertical angle through the ground wire axis and the line
passing from the ground wire axis to the outermost phase conductor. Its value for
reliable protection is taken equal to 20-30 degrees.
 Voltage drop and Voltage Regulation:
The allowable voltage drop is considered as critical factors in determining the
conductor size for 11kV and Low Tension (LT) distribution line with thermal loading
(ampere loading) about 80 percent of the normal thermal rating based on the maximum
operating temperature. Large conductor size (cross section) employed in distribution
lines reduces the resistance of the line and hence the I 2R losses and voltage drop in the
line; and hence voltage regulation of the line improves. But using large cross section
conductor size will increase the cost as the material required is more. Hence an
optimum value must be chosen in between the cost and improving voltage regulation
while designing the conductor size for distribution power system.

 What is meant by IDMT relay?

It is an inverse definite minimum time relay. In IDMT relay its operating is
inversely proportional to fault current and also a characteristic of minumum time after
which this relay definitely operates.

 Basic Requirements of Power System protection:-

1. Reliability
2. Stability
3. Sensitivity
4. Selectivity
5. Discrimination
6. Speed
Functional Requirements of Protection Relay

Reliability
The most important requisite of protective relay is reliability. They remain inoperative for
a long time before a fault occurs; but if a fault occurs, the relays must respond instantly
and correctly.

Selectivity
Only the effected parts of the power system shall be disconnected.
• Is achieved by two main methods – Time-grading/Current Grading
• Relays are set to operate depending on the time and current characteristics –
• Current is measured at several points and compared.

Sensitivity
The relaying equipment must be sufficiently sensitive so that it can be operated reliably
when level of fault condition just crosses the predefined limit.
Speed
Faults must be isolated as fast as possible. • Speed is necessary for two main
reasons – Maintain stability of the overall power system – Reduction of damage
to equipment & property

Stability:-

The protection system shall not react to non-fault situations • The protection system
must not react to faults in neighboring zones or high load currents.

 Power System component:-

The symmetrical component is a mathematical tool that simplifies the
analysis of power system during unbalanced system conditions. The theory of
symmetrical components and the synthesis of sequence networks for three-phase
power systems are instrumental for solving most unbalanced problems such as
asymmetrical faults, as well as for understanding the unbalanced operating conditions
of a normally balanced system and the behavior and influence of harmonic voltages and
currents.

The three elements that make up the symmetrical components are

1. Positive sequence

2. Negative sequence

3. Zero sequence

The following information about fault;

1. Balanced system operation or balanced 3-phase faults have positive

sequence elements only.

2. Any fault involving ground must have zero sequence elements.

3. Negative sequence elements show up in unbalanced systems only.

Positive Negative Zero

Type of Fault Sequence Sequence Sequence

Balanced 3-phase X
 Line to
Ground X X X

Line to Line X X

Double Line
to Ground X X X

One Open
Conductor
(due to single
pole CB or
fuse
operation) X X X

Two Open
Unbalanced Conductor X X X

 Fault which do not have zero sequence current flowing?

Phase to phase or 3 phase fault don’t have zero sequence current .

 Breaker and Half scheme:-

The Breaker and Half scheme has two main buses. Both the buses are normally

energised. Three breakers are connected between the buses. The circuits are

terminated between the breakers as shown. In this bus configuration for two circuits
three numbers of breakers are required. Hence it is called one and half scheme. It is

something like, for controlling one circuit we require one full and a half breakers. The

middle breaker is shared by both the circuits. Like the ring bus scheme here also each

circuit is fed from both the buses.

Any of the breakers can be opened and removed for maintenance purposes without

interrupting supply to any of the circuits. Also one of the two buses can be removed for

maintenance without interruption of the service to any of the circuits. If fault happens on

a bus it is isolated without interruption of supply to any of the circuits. If the middle

circuit breaker fails then the breakers adjacent to the buses are tripped so interrupting

both the circuits. But if a breaker adjacent to the bus fails then the tripping of middle

breaker does not interrupt power supply to circuit associated with healthy breaker. Only
the circuit associated with failed breaker is interrupted.
This configuration is very flexible and highly reliable. The relaying of the scheme is
complicated as the middle breaker is associated with both the circuits. This scheme is
economical in comparison to Double Bus Double Breaker scheme. This scheme also
require more space in comparison to other schemes to accommodate more equipment.

In one substation you can find two or more schemes implemented as per the
requirement. In most of the modern substations it is usual to add one transfer bus in
most of the schemes above. Which enhances the availability and maintainability of the
system and operational flexibility
 Short circuit study and its purposes:
A short-circuit study is an analysis of an electrical system that determines the

magnitude of the currents that flow during an electrical fault. Comparing these

calculated values against the equipment ratings is the first step to ensuring that the

power system is safely protected. Once the expected short-circuit currents are known, a

protection coordination study is performed to determine the optimum characteristics,

ratings and settings of the power system protective devices.

 Reduces the risk a facility could face and help avoid catastrophic losses.

 Increases the safety and reliability of the power system and related

equipment.

 Evaluates the application of protective devices and equipment.

 Identifies problem areas in the system.

 Identifies recommended solutions to existing problems. -

 Load Flow or Power flow study:-

In power engineering, the power-flow study, or load-flow study, is a numerical

analysis of the flow of electric power in an interconnected system. A power-flow study
usually uses simplified notation such as a one-line diagram and per-unit system, and
focuses on various aspects of AC power parameters, such as voltages, voltage angles,
real power and reactive power. It analyzes the power systems in normal steady-state
operation.
Power-flow or load-flow studies are important for planning future expansion of power
systems as well as in determining the best operation of existing systems. The principal
information obtained from the power-flow study is the magnitude and phase angle of the
voltage at each bus, and the real and reactive power flowing in each line.
 Infinite Bus bar:-
A bus is called infinite bus if its voltages remain constant and does not alter by
any change in generator excitation.

 Swing Bus bar or slack bus:-

A bus is called swing bus for when the magnitude and phase for bus voltages are
specified for it.The swing bus is the reference bus for load flow solution and it is
required for accounting line losses. Usually one of the generator bus selected as
swing bus.
Surge Impedance Loading (SIL) of Transmission Line
Loading of any transmission line depends on,

 Thermal limitation (I2R limitation)

 Voltage regulation
 Stability Limitation

This is defined as the load (of unity power factor) that can be delivered by the line of
negligible resistance.

Where VLL is the receiving end voltage in kV and Zo is the surge impedance in ohms,
and SIL is the surge impedance loading or natural loading of the line
The above expression gives a limit of the maximum power that can be delivered by a
line and is useful in designing the transmission line. This can be used for the
comparison of loads that can be carried on the transmission lines at different voltages
From the above expression power transmitted through a long transmission lines can be
either increased by increasing the value of the receiving end line voltage (V LL) or by
reducing the surge impedance (Zo). Voltage transmission capability is increased day by
day, this is the most commonly adopted method for increasing the power limit of the
heavily loaded transmission line. But there is a limit beyond which is neither economical
nor practical to increase the receiving end line voltage
By applying some methods such as introducing series capacitors (capacitors in series
with the transmission line) or shunt capacitors (capacitors in parallel with transmission
lines) can be used to reduce the value of surge impedance (Zo).

Surge Impedance Loading (SIL) can be increased by reducing the Surge impedance of
the line. From the above expression Zo can be decreased by either increasing the
capacitance (C) of the line or by reducing the inductance (L) of the line. Inductance (L)
of the transmission line cannot be reduced easily
By use of the series capacitors surge impedance (Zo) and the phase shift get reduced
due to decrease in the line inductance (L). This improves the system stability limit.
These capacitors also helps in reducing the line drops and so voltage variations. But
this method causes difficulty under short circuit conditions of system as capacitors will
get damage.
By use of shunt capacitors though the surge impedance (Z o) is reduced but the phase
shift of the system increases this affects the poor stability in the system specially when
synchronous machines are under the load. This method is not employed in long
transmission lines specially when stability limits are present.

 Differential Relay:-

Generally Differential protection is provided in the electrical power transformer rated

more than 5MVA. The Differential Protection of Transformer has many advantages
over other schemes of protection.

1) The faults occur in the transformer inside the insulating oil can be detected by
Buchholz relay. But if any fault occurs in the transformer but not in oil then it can not be
detected by Buchholz relay. Any flash over at the bushings are not adequately covered
by Buchholz relay. Differential relays can detect such type of faults. Moreover
Buchholz relay is provided in transformer for detecting any internal fault in the
transformer but Differential Protection scheme detects the same in more faster way.

2) The differential relays normally response to those faults which occur in side the
differential protection zone of transformer.

Differential Protection Scheme in a Power Transformer

Principle of Differential Protection

Principle of Differential Protection scheme is one simple conceptual technique. The
differential relay actually compares between primary current and secondary current of
power transformer, if any unbalance found in between primary and secondary currents
the relay will actuate and inter trip both the primary and secondary circuit breaker of the
transformer.

Suppose you have one transformer which has primary rated current Ip and secondary
current Is. If you install CT of ratio Ip/1A at primary side and similarly, CT of ratio Is/1A at
secondary side of the transformer. The secondaries of these both CTs are connected
together in such a manner that secondary currents of both CTs will oppose each other.
In other words, the secondaries of both CTs should be connected to same current coil
of differential relay in such a opposite manner that there will be no resultant current in
that coil in normal working condition of the transformer. But if any major fault occurs
inside the transformer due to which the normal ratio of the transformer disturbed then
the secondary current of both transformer will not remain the same and one resultant
current will flow through the current coil of the differential relay, which will actuate the
relay and inter trip both the primary and secondary circuit breakers. To correct phase
shift of current because of star - delta connection of transformer winding in case of three
phase transformer, the current transformer secondaries should be connected in delta
and star as shown here.

At maximum through fault current, the spill output produced by the small percentage
unbalance may be substantial. Therefore, differential protection of transformer
should be provided with a proportional bias of an amount which exceeds in effect the
maximum ratio deviation.

III. CAUSE OF FALSE DIFFERENTIAL CURRENT Certain phenomena can cause a

substantial differential current to flow, when there is no fault, and these differential
currents are generally sufficient to cause a percentage differential relay to trip. However,
in these situations, the differential protection should not disconnect the system because
it is not a transformer internal fault. Such phenomena can be due to the non-linearities
in the transformer core. Some of these situations are considered below.

1 Inrush currents Magnetizing inrush current in transformers results from any abrupt
change of the magnetizing voltage. Although usually considered as a result of
energizing a transformer, the magnetizing inrush may be also caused by [2]: •
Occurrence of an external fault, • Voltage recovery after clearing an external fault, •
Change of the character of a fault (for example when a phase-to-ground fault evolves
into a phase-to- phase-to-ground fault). • Out-of-phase synchronizing of a connected
generator. Since the magnetizing branch representing the core appears as a shunt
element in the transformer equivalent circuit, the magnetizing current upsets the
balance between the currents at the transformer terminals, and is therefore experienced
by the differential relay as a “false” differential current. The relay, however, must remain
stable during inrush conditions. In addition, from the standpoint of the transformer life-
time, tripping-out during inrush conditions is a very undesirable situation (breaking a
current of a pure inductive nature generates high overvoltage that may jeopardize the
insulation of a transformer and be an indirect cause of an internal fault) [3]. The
following summarizes the main characteristics of inrush currents [4] [5]: • Generally
contain dc offset, odd harmonics, and even harmonics. • Typically composed of unipolar
or bipolar pulses, separated by intervals of very low current values. • Peak values of
unipolar inrush current pulses decrease very slowly. • Time constant is typically much
greater than that of the exponentially decaying dc offset of fault currents. • Second-
harmonic content starts with a low value and increases as the inrush current decreases.

2. Over excitation conditions Overexcitation of a transformer could cause

unnecessary operation of transformer differential relays. This situation may occur in
generating plants when a unit-connected generator is separated while exporting VARs
[3]. The resulting sudden voltage rise impressed on the unit transformer windings from
the loss of VAR load can cause a higher than nominal volts per hertz condition and,
therefore, an overexcitation event. This could also occur in transmission systems where
large reactive load is tripped from a transformer with the primary winding remaining
energized. When the primary winding of a transformer is overexcited and driven into
saturation, more power appears to be flowing into the primary of the transformer than is
flowing out of the secondary [6]. A differential relay, with its inputs supplied by properly
selected CTs to accommodate ratio and phase shift, will perceive this as a current
differential between the primary and secondary windings and, therefore, will operate.
This would be an undesirable operation, as no internal fault would exist, with the current
imbalance being created from the overexcitation condition. Since overexcitation
manifests itself with the production of odd harmonics, and since the third harmonic (and
other triples) may be effectively cancelled in Δ transformer windings, then, the fifth
harmonic can be used as a restraining or a blocking quantity in the differential relay in
order to discriminate between the over-excitation and the faulty state. [4].

3. Current transformer saturation [4] The effect of CT saturation on transformer

differential protection is double-edged. Although, the percentage restraint reduce the
effect of the unbalanced differential current, in the case of an external faults, the
resulting differential current which may be of very high magnitude can lead to a relay
male-operation. For internal faults, the harmonics resulting from CT saturation could
delay the operation of differential relays having harmonic restraint.

HARMONICS RESTRAIN Harmonics restrain is based on the fact that the inrush
current has a large second-harmonic component of the differential current which is
much larger in the case of inrush than for a fault (see fig 2). The over-excitation current
has also a larger fifth-harmonic component. Therefore, these harmonics may be used to
restrain the relay from tripping during those two conditions. In contrast to the odd
harmonics ac that CT saturation generates, even harmonics are a clear indicator of
magnetizing inrush. As even harmonics resulting from dc CT saturation which are
transient in nature are a clear indicator of magnetizing inrush, then it is important to use
them (and not only the second harmonic) to obtain.
 Causes of Voltages drop:-
Voltage drop is caused by resistance in the conductor or connections leading to the
electrical load. There are many causes of resistance in the conductor path. There are
four fundamental causes of voltage drop:
1. Material - Copper is a better conductor than aluminum and will have less voltage drop
than aluminum for a given length and wire size.
2. Wire Size - Larger wire sizes (diameter) will have less voltage drop than smaller wire
sizes (diameters) of the same length.
3. Wire Length - Shorter wires will have less voltage drop than longer wires for the same
wire size (diameter).
4. Current Being Carried - Voltage drop increases on a wire with an increase in the
current flowing through the wire.
Transmission lines Losses:-

Copper Losses

One type of copper loss is I2R LOSS. In rf lines the resistance of the conductors is
never equal to zero. Whenever current flows through one of these conductors, some
energy is dissipated in the form of heat. This heat loss is a POWER LOSS. With copper
braid, which has a resistance higher than solid tubing, this power loss is higher. Another
type of copper loss is due to SKIN EFFECT.

Dielectric Losses

DIELECTRIC LOSSES result from the heating effect on the dielectric material between
the conductors. Power from the source is used in heating the dielectric. The heat
produced is dissipated into the surrounding medium. When there is no potential
difference between two conductors, the atoms in the dielectric material between them
are normal and the orbits of the electrons are circular. When there is a potential
difference between two conductors, the orbits of the electrons change. The excessive
negative charge on one conductor repels electrons on the dielectric toward the positive
conductor and thus distorts the orbits of the electrons. A change in the path of electrons
requires more energy, introducing a power loss.

Radiation and Induction Losses

RADIATION and INDUCTION LOSSES are similar in that both are caused by the fields
surrounding the conductors. Induction losses occur when the electromagnetic field
about a conductor cuts through any nearby metallic object and a current is induced in
that object. As a result, power is dissipated in the object and is lost.

Radiation losses occur because some magnetic lines of force about a conductor do not
return to the conductor when the cycle alternates. These lines of force are projected into
space as radiation and this results in power losses. That is, power is supplied by the
source, but is not available to the load.

 open-circuit test:-
The open-circuit test, or "no-load test", is one of the methods used in electrical
engineering to determine the no-load impedance in the excitation branch of
a transformer.
The secondary of the transformer is left open-circuited. A wattmeter is connected to the
primary. An ammeter is connected in series with the primary winding. A voltmeter is
optional since the applied voltage is the same as the voltmeter reading. Rated voltage is
applied at primary.
If the applied voltage is normal voltage then normal flux will be set up. Since iron loss is
a function of applied voltage, normal iron loss will occur. Hence the iron loss is
maximum at rated voltage. This maximum iron loss is measured using the wattmeter.
Since the impedance of the series winding of the transformer is very small compared to
that of the excitation branch, all of the input voltage is dropped across the excitation
branch. Thus the wattmeter measures only the iron loss. This test only measures the
combined iron losses consisting of the hysteresis loss and the eddy current loss.
Although the hysteresis loss is less than the eddy current loss, it is not negligible. The
two losses can be separated by driving the transformer from a variable frequency
source since the hysteresis loss varies linearly with supply frequency and the eddy
current loss varies with the square.
Since the secondary of the transformer is open, the primary draws only no-load current,
which will have some copper loss. This no-load current is very small and because the
copper loss in the primary is proportional to the square of this current, it is negligible.
There is no copper loss in the secondary because there is no secondary current.
Current, voltage and power are measured at the primary winding to ascertain
the admittance and power-factor angle.

 Why shunt capacitor preferred over series capacitor for power factor
improvement:-
Capacitors aid in minimizing losses and helps in reducing operating expenses.
Capacitors should be placed where the reactive power demand is more as capacitors
generate reactive power and helps to maintain the voltage. Basically reactive power
demand will be more at load side of the power system, hence capacitors should be
placed as close as possible to load. In specific, capacitor banks should be placed where
the low voltage problem occurs.Shunt capacitors are series capacitors are employed in
power system for different purposes

 Shunt capacitors and series capacitors in the power system generate the
reactive power to improve the power factor and voltage, thereby enhancing the
power system capacity and reducing the losses. In series capacitors reactive
 power generation is proportional to the square of the load current (I2Xc), whereas
in shunt capacitors reactive power generation is proportional to the square of the
voltage (V2/Xc).
 The cost of installation of series capacitors is higher than that of the shunt
capacitors. This is because the protective equipment for the series capacitors is
often complicated. In addition to that, series capacitors are generally designed for
higher power to cope up with the future increase in the load
 For the same voltage improvement, the rating of the shunt capacitors will be
higher than that of series capacitors. Series capacitors compensation may create
certain disturbances: ferro-resonance in transformers, sub-synchronous
resonance during motor starting, shunting of motors during normal operation and
difficulty in protection of capacitors from system faults.
 Series capacitors are more effective on distribution circuits with higher X/R ratio
and for load variations involving a higher reactive content.
 Series capacitors are generally employed to improve the stability of the system
and shunt capacitors are generally employed to improve the power factor of the
system

Some of the factors which influence the choice between the shunt and series capacitors
are tabulated below:

Series Shunt
Objective
Capacitors Capacitors

Improving Power Factor Secondary Primary

Improving Voltage level in

overhead line system with a Primary Secondary
normal and low Power Factor

Improving Voltage level in

overhead line system with a high Not Used Primary
Power Factor

Reduce Line Losses Secondary Primary

Reduce Voltage Fluctuations Primary Not used