POWER POINT PRESENTATION

ON

EHVAC Transmission
2018 - 2019
IV B. Tech II semester (JNTUH-R15)

Mr. B NAVOTHNA, Assistant Professor

ELECTRICAL AND ELECTRONICS ENGINEERING

INSTITUTE OF AERONAUTICAL ENGINEERING

(Autonomous)
DUNDIGAL, HYDERABAD - 500 043
General Overview of HVDC
Transmission System
HISTORY
 The first 735 kV system was commissioned in Canada in 1965.
 Since then, voltage levels up to 765 kV have been introduced in Russia
with neighboring countries, U.S.A, South Africa, Brazil, Venezuela and
South Korea.
 The general trend of 800 kV investments is indicated in the diagram,
which shows the total capacity of power transformers and generator step-
up transformers for 800 kV delivered by ABB.
 Since the 90’s, the investments in 800 kV systems have been much lower
compared to the 70’s and 80’s.
 However, plans are under way for future introduction of 800 kV in India
and China.
UNIT-I
INTRODUCTION
WHY EHVAC?

Reduced line losses

High transmission efficiency

Improved voltage regulation

Flexibility for future system growth

Increase in transmission capacity of the line

INTRODUCTION

•The increased demand of electricity needs more

generation of electrical power.
•As the generation takes place at remote places, an
efficient distribution system is necessary.
TYPES OF HVAC TRANSMISSION

HVAC Transmission system can be divided into two parts

(a) Transmission System

(b) Distribution System

Each part is again subdivided into two parts:

1. Primary Transmission

2. Secondary Transmission
HVAC TRANSMISSION SYSTEM
ADVANTAGES
 With increase in the transmission voltage size of the conductors is
reduced (Cross section of the conductors reduce as current required to
carry reduces).

 As the reduction in current carrying requirement losses reduces results

in better efficiency

 Due to low current voltage drop will be less so voltage regulation

improves
COMPARISON WITH HVDC

POINTS EHVAC HVDC

Voltage levels 220 KV,400 KV,765 KV ± 500

Amount of power There is limit due to No limit

delivered power angle and
inductance
Equivalent essential Step-up transformer and Step-up transformer,
step-down transformer rectifier and inverter, step-
down transformer

Economical viability EHVAC is economical for HVDC is economical to

bulk power is to be transmit bulk amount of
transmitted over a long power and above. Over a
distance. 500 KM and long distance(800 KM and
above above)
 POWER TRANSFER CAPABILITY

V(KV) 220 400 765 1200

P(KW) 193.6 640 2341 5760

•One765kv transmission line can carry nearly 4 times the

power carried by 400kv line.

•One 1200kv transmission line can carry more than twice

that of 765kv line.
POWER EQUATION
 The electric power transmitted by overhead AC system is approximately
given by

P=V2/Z

Where , V= Operating Voltage

Z= Surge Impedance

 The power transfer capability increases as it proportional to square of

voltage
POWER HANDLING CAPACITY AND LINE
LOSS
 The power handling capacity of a single circuit is

P=E2 sinδ/Lx

where P = power in MW, 3-phase, Es, Er = voltages at

the sending-end and receiving end, respectively, in kV line-line, δ =
phase difference between Es and Er, x = positive-sequence reactance per
phase, ohm/km, and L = line length, km.
 MECHANICAL CONSIDERATIONS IN LINE PERFORMANCE

 Conductor tension,
 Span length,

 Conductor size,

 Type of conductor

 Terrain of line,

 Direction of prevailing winds,

 Type of supporting clamp of conductor-insulator assemblies from the tower,
 Tower type,
 Height of tower,
 Type of spacers and dampers, and

 The vegetation in the vicinity of line.

In general, the most severe vibration conditions are created by winds without turbulences
that hills, buildings, and trees help in reducing the severity.
TYPES OF VIBRATION

(1)Aeolian Vibration,

(2) Galloping, and

(3) Wake-Induced
Oscillations.
Fig 1:Stockbridge
Damper
Fig 2: Suspension Clamp
Fig 3: Spacer for two-
conductor bundle

DAMPERS AND SPACERS

COMPARISON OF AC AND DC TRANSMISSION
 RESISTANCE OF CONDUCTORS
•Conductors used for e.h.v.
transmission lines are always
stranded. Most common
conductors
•Use a steel core for reinforcement
of the strength of aluminium, but
recently high tensile strength
aluminium is being increasingly
used, replacing the steel.
•The former is known as ACSR
(Aluminium Conductor Steel
Reinforced) and the latter ACAR
(Aluminium Conductor Alloy
Reinforced).
 SKIN EFFECT RESISTANCE IN
ROUND CONDUCTORS
•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.
WHAT IS THE NEED OF BUNDLED
CONDUCTORS IN EHV TRANSMISSION
LINE?
 Primarily used to reduce the potential gradient on the surface of the
conductors so that occurrence of Corona can be reduced.
 As an advantage we get less Inductance more capacitance less radio
interference, reduced value of surge impedance and hence more power
carrying capacity of the line.
ADVANTAGES OF BUNDLED CONDUCTORS

 Bundled conductors are primarily employed to reduce the corona loss and radio
interference. However they have several advantages:
 Bundled conductors per phase reduces the voltage gradient in the vicinity of
the line. Thus reduces the possibility of the corona discharge. (Corona effect will be
observed when the air medium present between the phases charged up and start to
ionize and acts as a conducting medium. This is avoided by employing bundled
conductors)
 Improvement in the transmission efficiency as loss due to corona effect is countered.
 Bundled conductor lines will have higher capacitance to neutral in comparison with
single lines.
 Thus they will have higher charging currents which helps in improving the power
factor.
RESISTANCE AND INDUCTANCE OF GROUND
RETURN
 Situations that occur in practice when ground currents have important
effect on system performance.
Some of these are:
 Flow of current during short circuits involving ground.
 Switching operations and lightning phenomena.
 Propagation of waves on conductors.
 Radio Noise studies.
UNIT II
VOLTAGE GRADIENTS OF
CONDUCTORS
ELECTROSTATICS

 Conductors used for e.h.v. transmission lines are thin long cylinders
which are known as 'line charges'.
 Their charge is described in coulombs/unit length which was used for
evaluating the capacitance matrix of a multi-conductor line
 Types of important electrode configurations useful for extra high voltage
practice in the field and in laboratories.
 Examples of this type are sphere-plane gaps, sphere-to-sphere gaps,
point-to-plane gaps, rod-to-plane gaps, rod-rod gaps, conductor-to-tower
gaps, conductor to-conductor gap above a ground plane, etc.
•This is one of the oldest technique
adopted for the measurement of all the
types (dc =, ac ~ and impulse ) high
voltages of either polarity.
• It remained the most widely used
method for decades. The field between
two identical spheres is a classical
example of "weakly non
uniform" field.
• The breakdown characteristic of such a
gap is linear for the gap distances not
greater than the radius of the spheres.
•Measurement voltage is made as a
function of minimum distance at which
it can flash over or spark over.

FIELD OF SPHERE GAP

SURFACE VOLTAGE GRADIENT ON
CONDUCTORS

 The surface voltage gradient on conductors in a bundle governs

generation of corona on the line which have serious consequences
causing audible noise and radio interference.
 They also affect carrier communication and signaling on the line and
cause interference to television reception.
 Since corona generation depends on the voltage gradient on conductor
surfaces, this will be taken up now for e.h.v. conductors with number of
sub-conductors in a bundle ranging from 1 to N.
 The maximum value of N is 8 at present but a general derivation is not
difficult.
CALCULATION FOR CONDUCTOR SURFACE GRADIENT

 The conductor surface gradient is calculated from the following equation:

E=(V/√3)*(b/(r*ln ((a/Re)*2h/ √(4h2+a2))))

where E : conductor surface voltage gradient (kV/cm)
V : line voltage (kV)
b : factor for multiple conductors
r : radius of conductor (cm)
R : outside radius of bundle (cm)
Re : equivalent radius of bundle conductor (cm)
S : distance between component conductor centers (cm)
a : phase spacing (cm)
h : height of conductor above ground (cm)
n : number of component conductors in bundle
WHAT ARE BUNDLED CONDUCTORS?

 At voltages above 300 kV, corona causes a significant power loss

and radio interference if a single conductor per phase is used.
 Instead of using a single conductor, it is preferable to use two or
more conductors per phase, in close proximity, which is
called bundled conductors.
 Thus, a bundled conductor is a conductor made up of two or more
sub-conductors and is used as one phase conductor.
 The high voltage gradient is reduced considerably by the use of
bundled conductors.
NUMBER OF
SUB-
CONDUCTORS
•The number of sub-
conductors used per phase in
an Indian transmission system
are two (for 400 kV), and four
(for 765 kV). Fig.1: Bundled conductors with twin, triple and quadruple sub-
conductors.
•The bundled conductors for
the proposed 1200 kV Ultra
High Voltage (UHV)
transmission system in India
will have 8 sub-conductors.
• Figure 1 shows Bundled
conductors with twin, triple
and quadruple conductor.
•Figure 2 shows a 765 kV
Fig.2: 765 kV transmission line in India with 4 sub-
Transmission line in India conductors per phase (Quadruplex bundle).
with quadruplex sub-
conductors.
BUNDLE SPACING:

 The spacing between adjacent sub-conductors is called bundle spacing and is

almost 30 cm or more. In figure 1, bundle spacing is denoted by 'B'.
 In almost all cases, the sub-conductors are uniformly distributed on a circle.
 The radius of the pitch circle on which the sub-conductors are located is called
bundled radius.
 The various advantages of using bundled conductors are reduced reactance,
voltage gradient, corona loss, radio interference, and surge impedance.
UNIT III
CORONA EFFECTS
WHAT IS CORONA?
 Electric transmission lines can generate a small amount of sound energy as a result of corona.
 Corona is a phenomenon associated with all transmission lines. Under certain conditions, the
localized electric field near energized components and conductors can produce a tiny electric
discharge or corona that causes the surrounding air molecules to ionize, or undergo a slight
localized change of electric charge.
 Utility companies try to reduce the amount of corona because in addition to the low levels of
noise that result, corona is a power loss, and in extreme cases, it can damage system
components over time.
 Corona occurs on all types of transmission lines, but it becomes more noticeable at higher
voltages (345 kV and higher). Under fair weather conditions, the audible noise from corona is
minor and rarely noticed.
 During wet and humid conditions, water drops collect on the conductors and increase corona
activity. Under these conditions, a crackling or humming sound may be heard in the
immediate vicinity of the line.
 Corona results in a power loss. Power losses like corona result in operating inefficiencies and
increase the cost of service for all ratepayers; a major concern in transmission line design is
the reduction of losses.
CORONA PHENOMENON AT HIGH VOLTAGE

 Corona phenomenon is the ionization of air surrounding the power

conductors.
 Free electrons are normally present in the atmosphere.
 The free electrons will move with certain velocity depending upon the
field strength.
 These electrons on their movement collide with the molecules of air and
liberate more electrons.
 The process of ionization is cumulative and ultimately forms and
electron avalanche.
ELECTRICAL BREAKDOWN
 The electrical breakdown of surrounding air around the conductor is
accompanied by-

 a faint glow around the conductor,

 a hissing sound,
 vibration in conductors,
 formation of ozone and oxides of nitrogen,
 loss of power, and
 radio interference.
FACTORS AFFECTING CORONA LOSS IN
AN OVERHEAD TRANSMISSION LINE
 The important factors that affect the corona loss in an overhead transmission
line are -

 frequency and waveform of supply,

 spacing between conductors,
 condition of conductor surface,
 atmospheric conditions,
 conductor diameter and
 number of conductors per phase.
 CORONA CALCULATIONS
 The following corona calculations are from Dielectric Phenomena in High Voltage
Engineering

1. For Concentric Cylinders in Air:

 Corona will not form when RO / RI < 2.718. (Arcing will occur instead when the voltage is
too high.)

2. For Parallel Wires in Air:

 Corona will not form when X / r < 5.85. (Arcing will occur instead when the voltage is
too high.)

3. For Equal Spheres in Air:

 Corona will not form when X / R < 2.04. (Arcing will occur instead when the voltage is too
high.)
 Arcing difficult to avoid when X / R < 8
EFFECTS OF CORONA

(1) Audible Noise

(2)Radios and Television Interference

(3) Gaseous Effluents

(4) Induced Currents

(5) Fuel Ignition

(6) Cardiac Pacemakers

(7) Computer Interference

AUDIBLE NOISE
 During corona activity, transmission lines (primarily those rated at 345 kV and
above) can generate a small amount of sound energy. This audible noise can
increase during foul weather conditions.
 Water drops may collect on the surface of the conductors and increase corona
activity so that a crackling or humming sound may be heard near a transmission
line.
 Transmission line audible noise is measured in decibels using a special
weighting scale, the “A” scale that responds to different sound characteristics
similar to the response of the human ear.
 Audible noise levels on typical 230 kV lines are very low and are usually not
noticeable.
 For example, the calculated rainy weather audible noise for a 230 kV
transmission line at the right-of-way edge is about 25 dBA, which is less than
ambient levels in a library and much less than background noise for wind and
rain.
CORONA LOSS
 When an alternating potential difference is applied across two conductors
whose spacing is large in compares ion with their diameter then the atmosphere
air surrounding the conductor realize electrostatic stress.
 At low voltage there is no change in the condition of atmospheric air around the
conductors but when the potential difference is gradually increased, a stage
arrive when fan is luminous glow of violet color appears with the hissing sound
.
 This phenomenon is known as visual corona. If the potential difference raised
still further, the glow and the noise will increase eventually a spark over of air
insulation will take place.
 This phenomenon is lies in transmission line of 100KV and above.
REDUCING POWER LOSS DUE TO
CORONA
 The use of bundle conductors reduce corona loss
 Spacing between conductors is selected so that corona is tolerable
 Since the shape of conductors affect corona loss, cylindrical shape conductors
have uniform field that reduces corona loss than any other shape
 The voltage stress and electric field gradient should be minimized which can be
accomplished by using good high voltage design practices. Using conductors
with large radii reduce corona loss
 Void free solid conductors and insulators should be used
 Corona formation can be suppressed, if the terminals on high voltage
equipment are designed with smooth round diameter rounded shapes like balls
and the addition of corona rings to insulators of high voltage transmission lines
 CHARGE-VOLTAGE DIAGRAM WITH
CORONA
•When corona is absent the
capacitance of a conductor
is based on the physical
radius of the metallic
conductor. The charge-
voltage relation is a straight
line OA as shown in Figure
3 and C = q0/V0, where
V0 = the corona-inception
voltage and q0 the
corresponding charge.
Fig. 3. Charge-Voltage diagram of corona.
RADIO INTERFERENCE
 Pulse type of corona gives interference to radio broadcast in the range of 0.5 MHz to 1.6
MHz.
 In addition to corona generated on line conductors, there are spark discharges from
broken insulators and loose wires which interfere with TV reception in the 80 – 20 MHz
range.
 Corona on conductors also causes interference to Carrier Communication and Signaling
in the frequency range 30-500 KHz. As in most gas discharge phenomenon under high
impressed electric fields, free electrons and ions are created in space which contain very
few initial electrons.
 We can therefore expect a buildup of resulting current in the conductor from a zero value
to maximum or peak caused by the avalanche mechanism and their motion towards the
proper electrode. Once the peak value is reached there is fall in current because of
lowering of electric field due to relatively heavy immobile space charge cloud which
lowers the velocity of ions.
 We can therefore expect pulses to be generated with short crest times and relatively
longer fall times.
UNIT IV
ELECTRO STATIC FIELD
ELECTROSTATIC EFFECT ON
ELECTRIC-FIELD OF EHV LINE
 Electric fields are due to voltage so they are present in electrical appliances and
cords whenever the electric cord to an appliance is plugged into an outlet (even
if the appliance is turned off).
 Electric fields (E) exist whenever a (+) or (-) electrical charge is present. They
exert forces on other charges within the field.
 Any electrical wire that is charged will produce an electric field (i.e. Electric
field produces charging of bodies, discharge currents, biological effects and
sparks). This field exists even when there is no current flowing.
 The higher the voltage, the stronger is electric field at any given distance from
the wire.
 The strength of the electric field is typically measured in volts per meter (V/m)
or in kilovolts per meter (kV/m). Electric fields are weakened by objects like
trees, buildings, and vehicles. Burying power lines can eliminate human
exposure to electric fields from this source.
ELECTROSTATIC EFFECT ON HUMAN
BEINGS

 The human body is a composed of some biological materials like blood, bone,
brain, lungs, muscle, skin etc. The permeability of human body is equals to
permeability of air but within a human body has different electromagnetic
values at a certain frequency for different material.
 The human body contains free electric charges (largely in ion-rich fluids such as
blood and lymph) that move in response to forces exerted by charges on and
currents flowing in nearby power lines. The processes that produce these body
currents are called electric and magnetic induction.
 In electric induction, charges on a power line attract or repel free charges within
the body. Since body fluids are good conductors of electricity, charges in the
body move to its surface under the influence of this electric force.
 For example, a positively charged overhead transmission line induces negative
charges to flow to the surfaces on the upper part of the body. Since the charge
on power lines alternates from positive to negative many times each second, the
charges induced on the body surface alternate also.
ELECTROSTATIC EFFECT ON HUMAN
BEINGS-1

 Negative charges induced on the upper part of the body one instant flow into the
lower part of the body the next instant. Thus, power-frequency electric fields
induce currents in the body (Eddy Current) as well as charges on its
surface.
 The currents induced in the body by magnetic fields are greatest near the
periphery of the body and smallest at the center of the body.
 It is believed that, the magnetic field might induce a voltage in the tissue of
human body which causes a current to flow through it due to its conductivity of
around them.
 The magnetic field has influence on tissues in the human body. These influences
may be beneficial or harmful depending upon its nature.
 The magnitude of surface charge and internal body currents that are induced by
any given source of power-frequency fields depends on many factors.
ELECTROSTATIC EFFECT ON HUMAN
BEINGS-2

 These include the magnitude of the charges and currents in the source, the
distance of the body from the source, the presence of other objects that might
shield or concentrate the field, and body posture, shape, and orientation.
 For this reason the surface charges and currents which a given field
induces are very different for different Human and animals.
 When a person who is isolated from ground by some insulating material comes
in close proximity to an overhead transmission line, an electrostatic field is set
in the body of human being, having a resistance of about 2000 ohms.
 When the same person touches a grounded object, it will discharge through his
body causing a large amount of discharge current to flow through the body.
Discharge currents from 50-60 Hz electromagnetic fields are weaker than
natural currents in the body, such as those from the electrical activity of the
brain and heart.
ELECTROSTATIC EFFECT ON HUMAN
BEINGS-3

 Short term Health Problem

 Headaches.
 Fatigue
 Anxiety
 Insomnia
 Prickling and/or burning skin
 Rashes
 Muscle pain
 Long term Health Problem

 Risk of damaging DNA.

 Risk of Cancer
 Risk of Leukemia
 Risk of Neurodegenerative disease:
 Risk of Miscarriage
ELECTROSTATIC EFFECT ON ANIMALS

 Many researchers are studying the effect of Electrostatic field on animals. In

order to do so they keeps the cages of animals under high Electrostatic field of
about 30 kV/m.
 The results of these Experiments are shocking as animals (are kept below high
Electrostatic field their body acquires a charge & when they try to drink water, a
spark usually jumps from their nose to the grounded Pipe) like hens are unable
to pick up grain because of chattering of their beaks which also affects their
growth.
 ELECTROSTATIC EFFECT ON PLANT
LIFE

 Most of the areas in agricultural and forest lands where high power transmission
lines pass. The voltage level of high power transmission Lines are 400KV,
230KV, 110KV, 66KV etc. The electromagnetic field from high power
transmission lines affects the growth of plants.
 Gradually increases or decreases and reaches to maximum current or minimum
current and thereafter it starts to fall down to lowest current or raises to
maximum current or a constant current. Again the current, it evinces with little
fluctuations till the next day morning.
 Current in Power transmission lines varies according to Load (it depending
upon the amount of electricity consumed by the consumers). Hence the effect of
EMF (due to current flowing in the power lines) upon the growth of plants
under the high power transmission lines remains unaltered throughout the year.
 From various practically study it was found that the response of the crop to
EMF from 110 KV and 230 KV Power lines showed variations among
themselves. Based on the results the growth characteristics like shoot length,
root length, leaf area, leaf fresh weight, specific leaf weight, shoot/root ratio,
total biomass content and total water content of the four crop plants were
reduced significantly over the control plants.
ELECTROSTATIC EFFECT ON PIPE
LINE/FENCE/CABLES

 A fence, irrigation pipe, pipeline, electrical distribution line forms a conducting

loops when it is grounded at both ends. The earth forms the other portion of the
loop. The magnetic field from a transmission line can induce a current to flow
in such a loop if it is oriented parallel to the line.
 If only one end of the fence is grounded, then an induced voltage appears
across the open end of the loop. The possibility for a shock exists if a person
closes the loop at the open end by contacting both the ground and the conductor.
 For fences, buried cables, and pipe lines proper care has been taken to prevent
them from charging due to Electrostatic field.
 When using pipelines which are more than 3 km in length & 15 cm in Diameter
they must be buried at least 30 laterally from the line center.
UNIT V
VOLTAGE CONTROL
SYNCHRONOUS CONDENSER
 Synchronous Condenser is also known as Synchronous
Compensator or Synchronous Phase Modifier.
 A synchronous condenser or a synchronous compensator is a synchronous motor
running without a mechanical load.
 It can generate or absorb reactive volt-ampere (VAr) by varying the excitation of its
field winding.
 It can be made to take a leading current with over-excitation of its field winding.
 When the motor power factor is unity, the DC excitation is said to be normal. Over-
excitation causes the motor to operate at a leading power factor. Under excitation
causes it to operate at a lagging power factor. When the motor is operated at no load
with over-excitation, it takes a current that leads the voltage by nearly 90 degrees.
 Thus, it behaves like a capacitor and under such operating conditions, the
synchronous motor is called a synchronous capacitor.
 Since a synchronous condenser behaves like a variable inductor or a variable
capacitor, it is used in power transmission systems to regulate line voltage.
VOLTAGE CONTROL BY
SYNCHRONOUS CONDENSERS:
 Synchronous condensers are over excited synchronous motors installed in the
power system to deliver the reactive power.

 These synchronous phase modifiers are located near the load improves the
voltage profile of the power system.

 The main advantage of synchronous phase modifiers are the ability to deliver
the reactive power can be adjusted unlike static shunt capacitors.
SERIES COMPENSATION
 Voltage Control by Series Capacitors

 In Extra High Voltage (EHV) or Ultra High Voltage System (UHV) systems
series capacitors are connected in series with the transmission line to reduce the
effect of inductive reactance XL between the sending end and receiving end of
the line.

 One of the major drawbacks of series capacitors is that high over voltages are
produced across the capacitor terminals under short circuit condition.

 Series capacitors are usually employed for increasing the power transfer
capability of the transmission line and not for voltage regulation
SHUNT COMPENSATION
 Voltage Control by Shunt reactors

 Voltage Control by Shunt Capacitors

 Voltage Control by Static Shunt Compensation

VOLTAGE CONTROL BY SHUNT REACTORS
 Shunt reactors are provided at sending end and receiving end of the long EHV
and UHV transmission lines.
 They are switched in when the line is to be charged or during line is on low load
 When the line is on no load or low load, shunt capacitance predominates and
receiving end voltage is higher than the sending end voltage. This phenomenon
is called Ferranti effect.
 The receiving end voltage of 400kV, 1000 km long line may be as high as
800kV. The shunt capacitance of such lines is neutralized by switching in the
shunt reactor.
 During high loads, the series inductive reactance of the line produces IXL drop
and the receive end voltage drops, the shunt reactors are switched off Shunt
treactors may be connected to the low voltage tertiary winding of a transformer
via a suitable circuit breaker, EHV shunt reactors may be connected to the
transmission line without any circuit breaker.
VOLTAGE CONTROL BY SHUNT
CAPACITORS

 Shunt capacitors are usually switched in during high loads.

 Static shunt capacitors are installed near the load terminals, in industries,
substations, … Most of the industrial loads (induction motors, transformers,
welding sets, furnaces) draws inductive current of poor power factor (0.3 to 0.6
lag).
 The shunt capacitors provide leading VARs there by the total KVA loading of
substation transformer and the current is reduced.
 Thereby IXL drop in the line is reduced and voltage regulation is improved.
 Shunt capacitors are switched in when KVA demand on the distribution line
goes up and voltage on the bus comes down.
 Switching in shunt capacitor should improve the bus voltage if the
compensation is effective
VOLTAGE CONTROL BY STATIC SHUNT
COMPENSATION

 A step-less variable compensation is possible by thyristorized control of shunt

capacitor and reactors.
 During heavy loads, the thyristors of the capacitor control are made to conduct
for longer duration in each cycle.
 During low loads, the thyristors in reactor circuit are made to conduct for longer
duration in each cycle.
 Thus a step-less variation of shunt compensation is achieved by means of static
shut compensation
SUB-SYNCHRONOUS RESONANCE
PROBLEM

 When the electrical system operates in such a manner that the rotating fields in
the generator due to sub-synchronous currents produce torques of the same
frequency as one of the mechanical torsional frequencies of the shaft and of the
correct phase, torques up to 10 times the break away or ultimate strength of the
shaft can be reached resulting in shaft damage.
 This phenomenon of electromechanical interaction between electrical resonant
circuits of the transmission system and the torsional natural frequencies of the
T-G rotor is known as "Sub-Synchronous Resonance", and designated SSR.
 Three distinct problems have been identified in SSR problem which are called
 Induction Generator Effect,
 Torsional Interaction, and
 Transient Torque Problem.
INDUCTION GENERATOR EFFECT
 An oscillatian of generator rotor due to disturbances produces two
components of currents or voltages.
 To the super synchronous component the generator is an induction
motor. To the subsynchronous component the generator is an induction
generator.
 At resonant electrical frequency f, below the synchronous frequency, the
apparent, generator rotor resistance is negative. The equivalent
resistance contains a term R/s where R is the effective rotor resistance
and 's' is the slip. If this apparent negative resistance exceeds the
network resistance i.e., when the system resistance is negative, the
subsynchronous component will be amplified with time.
 Such a condition will result in self excitation of oscillntory currents at
the resonant frequency. The phenomenon is known as induction
generator effect.
TORSIONAL INTERACTION

 When the rotor field flux which rotates at the speed of the rotor, overtakes the
more slowly rotating positive sequence subsynchronous flux of the-rotor, a
subsynchronous oscillatory component of electrical torque is produced whose
frequency is the difference between fr and fs, If this torque frequency coincides
with or in close to a rotational modal frequency fn, of the shaft system a
regenerative feed back saturation may arise.
 If the enhanced subsynchronous electrical torque exceeds the inherent
mechanical damping of the rotating system, the system will become self
excited.
 This interply between electrical and the mechanical system is called the
torsional interaction
METHODS OF ANALYSIS
 Methods of analysis of SSR The various methods available in the frequency
domain to analyse the phenomenon of SSR are:

 Routh- hurwitz stability criteria

 Frequency scanning techniques and

 Eigen value analysis.

COUNTER MEASURES
 Remedy for Countering Induction Generator Effect:
 Addition of pole-face
 Addition of series reactance in stator circuit
 System switching and unit tripping
 Armature-current relay protection
 Remedy for Countering Torsional Interaction
 Dynamic Stabilizer
 Reduction in Series-Capacitor Compensation or Complete Removal
 Filters
COUNTER MEASURES-2
 Remedy for Countering Transient Torque Problem.
 Filtering and Damping: These utilize
(a) Static Blocking Filter,
(b) Line Filter,
(c) Bypass Damping Filter,
(d) Dynamic Filter,
(e) Dynamic Stabilizer,
(f) Excitation System Damper.
 Relaying and Detecting: These are
(a) Torsional Motion Relay,
(b) Armature Current Relay,
(c) Torsional Monitor.
 System Switching and Unit Tripping.
 Modification to Generator and System: These include
(a) T-G modifications for new units altering stiffness, inertia and damping of rotors,
(b) Generator Series Reactance,
(c) Pole-Face Amortiseur Winding.
 Removal or Short-Circuiting the Series Capacitor
THANK YOU