Ahsanullah University of Science and Technology

Department of Electrical and Electronic Engineering

LABORATORY MANUAL
FOR
ELECTRICAL AND ELECTRONIC SESSIONAL COURSES

Student Name:
Student ID:

Course No: EEE 4252

Course Title: Power System Protection Lab

For the students of

Department of Electrical and Electronic Engineering
Year: 4 Semester: 2

1
Experiment Page
No. Name of the Experiment No.

1 Familiarization with different kinds of insulators, fuses, and miniature circuit 03-12
breakers & Determination of the Time Current Characteristics (TCC) curve of
a rewire able fuse & MCB.

2 Study of the performance of an electro-mechanical over current relay and 13-17

thermal overload relay.

3 Study of different components and their functions of an Air Circuit Breaker 18-20
(ACB).

4 Performance study of different types of substation and trip circuit for a protected 21-29
line.

5 Performance study of directional relay. 30-35

6 Performance study of an O/C relay, O/C relay co-ordination, advantage of 36-41

parallel feeder.

7 Performance study of a differential relay for the protection of transformer. 42-50

8 Study of different types of motor protection system. 51-60

9 Generator protection by using microprocessor based VAMP relay (PART-1). 61-71

10 Generator protection by using microprocessor based VAMP relay (PART-2). 72-82

2
 AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB
Experiment: 1

Experiment name: Familiarization with different kinds of insulators fuses and miniature circuit
breakers & Determination of Time Current Characteristics (TCC) curve of a rewire able fuse &
MCB.

Objective:
1. To be familiar with electrical devices like insulators, fuses, lightning arrester & MCBs.
2. To take at least 5 set of reading of current and their corresponding fuse blow out time.
3. To take at least 5 set reading of currents and their corresponding tripping time of MCB.
4. To draw the TCC curve of fuse and MCB from the data.

Insulators:
Line conductors are bare. Insulators are used between line conductors and supports.
Function of Insulator:
• The main function of insulators is to provide perfect insulation between line conductors and
supports and prevent any leakage current from the line conductors to earth through support.
• Mechanical strength should be adequate so that it can carry the conductor’s weight.

Types of insulator:

Pin Insulators: Pin type insulator is secured to the cross-arm on the pole. There is a groove on the
upper end of the insulator for housing the conductor. The conductor passes through this groove and
is bound by the aluminum wire of the same material as the conductor.
Pin type insulators are used for transmission and distribution of electric power at voltages up to 33
kV. Beyond operating voltage of 33 kV, the pin type insulators become too bulky and hence
uneconomical.

Fig1.1(a): Showing the 11 KV and 33 KV pin insulators.

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Connection:

Fig1.1(b): Connection of pin type insulator.

Spool / Shackle Insulators: These insulators are used in the 0.4 KV overhead service lines.

Fig 1.2: showing the spool/shackle insulators and its connection.

Disc Insulators: These Insulators are used normally in HV overhead transmission lines. Each unit
or disc is designed for low voltage; say 11.5 kV. Total Insulation of the string can be increased by
increasing the number of disc unit in the string to use in EHV lines.

Fig 1.3 (a): Showing the Disc insulators

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 Typical number of disk insulator units for
standard line voltages
Line voltage
Disks
(kV)
34.5 3
46 4
69 5
92 7
115 8
138 9
161 11
196 13
230 15
287 19
Fig 1.3 (b): Connection of Suspension type insulators 345 22
360 23

Strain Insulators:
When there is a dead end of the line or there is corner or sharp curve, the line is subjected to
greater tension. In order to relieve the line of excessive tension, strain insulators are used. For low
voltage lines (< 11 kV), shackle insulators are used as strain insulators. However, for high
voltage transmission lines, strain insulator consists of an assembly of suspension insulators as
shown in Figure. The discs of strain insulators are used in the vertical plane. When the tension in
lines is exceedingly high, at long river spans, two or more strings are used in parallel.

Fig 1.4: Showing the Strain insulators

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Post Insulators:
Also called support insulator. It provides support to electrical equipments and separates them from
ground.

Fig 1.5: Showing the Post insulators

Fuse:

Fuse is essentially a small piece of metal connected in between two terminals mounted on
insulated base which forms a series part of the circuit. The duty of a rewire able fuse wire is to
carry the normal working current safely without heating the wire but when the normal operating
current is exceeded it should rapidly heat up to the melting point and eventually circuit is opened.
It can provide two types of protection.

1. Short circuit protection

2. Over load protection

The melting point follows inverse characteristics between the melting time and the melting current.
At normal rated current the fuse element will never be heated to its melting point. At overloaded
current the melting will occur after certain time. As the amount of overloading is increased the
melting time will be shorter.

Semi-enclosed or Re-wire able Fuse:

These types of fuses are used for the protection of appliances at 0.4 KV voltage level and usually
called cut-out. The Fuse carrier can be pulled out and the blown out fuse element can be replaced.

Fig 1.6: Showing the Re-wire able fuse

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Totally Enclosed or cartridge Fuse:
The Fuse Element (the conductor which melts) is enclosed in a totally enclosed
container and is provided with metal contact on both side.

Fig 1.7: Showing the totally enclosed Fuse.

Drop out Fuse:

A fuse link in which the fuse carrier drop out after melting the fuse wire thereby providing
isolation between the terminals. This type of fuse is normally used in 11 KV side of a 11/0.4 KV
distribution transformer.

Fig 1.8: showing the drop out fuse

HRC (High Rupturing capacity) cartridge Fuse:

A cartridge fuse link having breaking capacity higher then
certain specified value (e.g. above the 16 KA for medium
voltage)

Fig 1.9: Showing the HRC fuse

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Lightning Arrestor:
The main function of a Lightning arrestor is to divert any surge over voltage caused by lightning to
the ground, so that equipment or devices behind the arrestor are saved from insulation failure and
eventually short circuit fault.

Fig 1.10: showing the Lightning arrestor

Low Voltage Circuit Breaker

Miniature Circuit Breaker (MCB):
MCBs are used extensively in LV domestic, commercial and industrial applications. They replace
conventional fuses and combine the features of a good HRC fuse and a good switch. For normal
operation it is used as switch. During overloads or faults, it automatically trips off. The tripping
mechanism is actuated by magnetic and thermal sensing devices provided within the MCB. Over
current is sensed by over current release which helps to open the contact of the MCB. On the other
hand short circuit is sensed by magnetic release which provides the means of opening the contact
of MCB.
Tripping mechanism and the terminal contacts are assembled in a moulded case, moulded out of
thermo setting powders. They ensure high mechanical strength, high dielectric strength and
virtually no ageing. The current carrying parts are made of electrolytic copper or silver alloy
depending upon the rating of the breaker. All other metal parts are of non ferrous, non rusting type.
Sufficient cross section for the current carrying parts is provided to ensure low temperature rise
even under high ambient temperature environment. The arc chute has a special construction which
increases the length of the arc by the magnetic field created by the arc itself and arc chute is so
placed in the breaker that the hot gases may not come in contact with any of the important parts of
the breaker.

Fig 1.11(a): Showing the MCBs

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 Fig 1.11(b): Construction and working principal of MCB

Types of MCB: MCBs are classified into three major types according to their instantaneous
tripping currents. They are
i. Type B MCB
ii. Type C MCB
iii. Type D MCB

Ir= Rated current

Molded Case Circuit Breaker (MCCB):
The current rating of the MCCB is higher than 63 Amp.Its also provides short circuit and overload
protection. Additional fact is, operating current setting can be controlled in a possible range.

Fig 1.12: Showing the MCCBs

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Residual Current Circuit Breaker (RCB):
Residual Current Circuit Breaker basically is installed to prevent human from shocks or death
caused by shocks. It prevents accidents by disconnecting the main circuit within fraction of
seconds.

Fig 1.13: Showing the RCBs

The basic operating principle lies in the toroidal Transformer shown in the following diagram
containing three coils. There are two coils say Primary (containing line current) and Secondary
(containing neutral current) which produces equal and opposite fluxes if both currents are equal.
Whenever in the case there is a fault and both the currents changes, it creates out of balance flux,
which in-turn produces the differential current which flows through the third coil (sensing coil
shown in the figure) which is connected to relay. The toroidal transformer, sensing coil and relay
together is known as RCD - Residual Current Device.

Residual Current Circuit Breaker With Over Current Protection (RCBO):

A combination of RCB+MCB (miniature circuit breaker) is known as a RCBO.

Fig 1.14: Showing the RCBOs

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Instrument and Components:
1. Current Injector.
2. Clamp on meter.
3. Rewire able Fuse Wire (5 A).
4. Wooden Board fitted with Fuse Holder.
5. MCB
6. Connecting Wire.

Procedure:
Connect the current injector set to a 230 V supply line. There are two output current terminals, one
is of 0-20A and other is 0-200A. Use 0-20A output terminal. Set the output current at a desired
value by changing the current varying knob. This can only be achieved by shorting the output
terminals by a thick wire. Keeping the knob position at the desired current value, switch off the
current injector and connect the fuse holder fitted with fuse wire across the output terminals. Then
switch on the injector. The desired current flows through the fuse wire. Measure the blow out time
of the fuse wire. As the increased current flows through the fuse wire, the fuse blow out time
reduces. Measure and record the currents and the corresponding fuse and blow out time in the table
1.1 and for MCB, connect directly it to the 0-200A output terminals after disconnecting the
shorting wire. Then switch on the current injector set. The desired current flows through the MCB.
Measure and record the tripping time of the MCB. As the increased current flows through the
MCB, the tripping time of the MCB is reduced. Measure and Record the currents and their
corresponding tripping time of the MCB in Table 1.1

Table 1.1

Fuse blow out

Sl. No Current (A) Current (A) Tripping time of the MCB (sec)
time (Sec).
1

4
5

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Circuit Diagram:

Testing of a Fuse

Testing of a MCB

Warning:
There are two variable voltage output terminals, one is 110 V dc and other is 230 V ac. Do
not touch the terminals of those voltage output.

Report:

1. Explain why pin insulators are not used above 33 KV voltages.

2. Explain how a fuse can provide time delayed protection for normal overload and high
speed protection for short circuit.
3. What are the differences between a MCB and a MCCB?
4. Draw the TCC curve on a graph paper from the data of table 1.1. Use Current in the X-axis
and time in Y-axis.
5. Discuss the special feature for selecting the fuse rating for the protection of motor.
6. Discussion the special feature for selecting rating of MCB for the protection of the motor.

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 AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB
Experiment: 2
Experiment name: Study of the performance of an electro-mechanical over
current relay and thermal overload relay.

Objectives:
1. To observe the performance of IDMT O/C relay and thermal overload relay.
2. To draw TCC curve from the data (over load currents and their corresponding
relay tripping times) for different over load currents.
Theory:
Protective relay senses the abnormal conditions in any part of a power system and
gives an alarm or isolates the faulty part from the healthy system. The relays are compact,
self contained devices which respond to abnormal conditions.
The relays distinguish between normal and abnormal condition. Whenever an abnormal
condition develops, the relays close its contacts. Thereby the trip circuit of the CB is
closed. Then the contacts of the CB are opened and the faulty part is disconnected from
the supply.

The functions of a protective relaying include the following-

1. To sound an alarm or close the trip circuit of the CB so as to disconnect a
component during an abnormal condition from the system. The abnormal
condition include- overload, under voltage, temperature rise, balanced load,
reverse power under frequency, short circuit etc.
2. To disconnect the abnormal operating part so as to prevent the subsequent fault.
3. To disconnect the faulty part quickly so as to minimize the damage to the faulty
part.
4. To localize the effect of fault by disconnecting the faulty part from the healthy
part, causing least disturbances to the healthy system.
5. To disconnect the faulty part quickly so as to improve the system stability, service
continuity and system performance.

Trip Circuit:
CB CT

Protected line PT
Solenoid coil

Relay
Auxiliary
contact

Battery

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Types of Relay:

Relays are basically of three types:

1) Electromechanical relay
2) Electronic/ Static relay
3) Microprocessor based relay
Electromechanical relays can be further classified into two types:
• Electromagnetic attraction
• Electromagnetic induction

Electromagnetic attraction type:

This is the simplest type of relays. This relay has an electromagnet energized by coil. The
coil is energized by the operating quantity which may be proportional to the circuit
current or voltage. Working principle is “instantaneous” in nature.
N/O To Trip
Contact Ckt
of Relay

Spring
Electromagnet
Armature
Current coil

Current settings of this kind of relay can be changed in two ways:

i. Electrically- by tapping

ii. Mechanically- by increasing the distance between electromagnet and armature.

Also by increasing the strength of spring connected to armature.

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Electromagnetic induction type:
It is the most widely used for protective relaying purpose involving ac- quantity. They are
not usable with dc quantity. Basically it is like a split phase induction motor. Actuating
force is developed in a movable element (either disc or other form of rotor of non
magnetic current conducting material) by the interaction of electromagnetic fluxes with
eddy currents that are induced in the rotor by these fluxes.

Current
coil Disk

By varying Time Setting dial (disc position), time setting of this kind of relay can be
controlled.

Thermal over load relay:

Thermal overload relays are 3 poles. The motor current flows through their bimetals (1
per phase) which are indirectly heated. Under the effect of the heating, the bimetals bend;
cause the relay to trip and the position of the auxiliary contacts to change. The relay
setting range is graduated in amps. In compliance with international and national
standards, the setting current is the motor nominal current and not the tripping current (no
tripping at 1.05 x setting current, tripping at 1.2 x setting current. The relays are built to
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be self protecting in the event of an overload until the short circuit protection device is
activated. To make a fine adjustment, change the distance between the heater and the
heat-sensitive element. An increase in this distance increases t he tripping current. You
can make another form of adjustment by changing t h e distance the bimetal strip has to
move before the overload relay contact is opened.
B I M E T A L . In the bimetal relay, the heat-sensitive element is a strip or coil of two
different metals fused together along one side. When heated, the strip or coil deflects
because one metal expands more than the other. The deflection causes the overload relay
contact to open.

Thermal over load relay

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Settings of IDMT O/C relay:
The standard IDMT relay has two controls- Plug setting (PS) and Time Setting Multiplier
(TSM). The PS is a device used to provide a range of current settings at which the relay
starts to operate. The setting ranges from 50% to 200% in the steps of 25% of the relay
rated current. The TSM is a means adjusting the moveable backstop which controls the
travel of the disc and thereby varies the time at which the relay closes its contact for a
given value of fault current.

TEST Connection

Procedure:
The PS of the relay is set at 5 amps, and the TSM at 0.9 sec (say). The current coil of the
relay is connected to (0-20 A) output terminals of the current injector set. Adjust the
current output at little over 5 A. Then observe the operation time of the relay. As the
current in the operating coil is increased, the relay operation time is reduced. So the
operating current and time are recorded in the following table.

Current in the relay coil (A) Operating time of the Relay (Sec)
5.5
6.0
6..5
7.0
7.5

Current in Thermal O/L Relay (A) Operating time of the Relay (Sec)

Reports:
1. What is a relay?
2. Draw the TCC of the IDMT relay using the data of the table with current in X
axis and time in Y axis.
3. Give Examples where times delayed O/C relay are applied?
4. What is the function of the O/C relay?

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 AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB
Experiment: 3

Experiment name: Study of different components and their functions of an Air

Circuit Breaker (ACB)

Objectives:
• To observe the contact closing operation manually
• To observe the contact closing operation automatically
• To observe the tripping mechanism
• To observe the under voltage shunt tripping mechanism
Theory:
In the ACB under discussion the over current relays and their corresponding CTs in three
lines are built inside the ACB. The under voltage relay coils are also built inside the
ACB.There is a spring charging motor in the ACB. When the motor is supplied from a
single phase 230 V, the motor is started and the contact of the ACB are closed keeping
the spring fully charged and latched. When there is any over current in all the phases or in
any of the phases, the built in over current relay closes its trip circuit and the trip coil
unlatch the fully charged spring, then the contacts are opened by the mechanical energy
charged in the spring. The arc is extinguished in the medium of normal atmospheric air.
If the voltage in any of the three phases or in all three phases, supplied to the load
through the ACB, are reduced below a preset level, the under voltage relay will be picked
up and closes the trip circuit; consequently the trip coil will unlatch the fully charged
spring and the contacts will be opened.
The closed contacts of the ACB can be manually opened by pushing the OFF button,
which closes the trip circuit, then trip coil is energized and unlatch the spring to open the
contacts.
Circuit diagram for motor operating mechanism, opening closing and under voltage
release are shown in the figure.

1
18
Spring Operation:

Circuit diagram:

Line

u1 V
C1 C11 D1
SW1 A
M Charging Motor
YU R
[Interlocking] I YC Closing Coil
A
M YC YO C YU YO Tripping Coil

YU Under Voltage Coil

Push for
Push for YU
Motor u2 OFF D2
ON
Limit C2 C12
SW

Neutral

2
19
Procedure:
For energizing the spring charging motor, close the SW1 switch. Now this charging
motor will charge up the closing spring. One incandescent lamp is connected across one
of the three phases of the contacts and the neutral. When the ‘ON’ switch is pushed the
contacts will be closed and the lamp will glow and when the “OFF” switch is pushed the
contacts will be opened and the lamp will stop glowing. It is important to keep in mind
that the voltage across the under voltage coil (i.e D1 and D2) should be maintained at
about 220 V (ac) otherwise contacts cannot be closed by pushing the ‘ON’ switch. If the
voltage across the under voltage coil is reduced below 200 V (approax.) the breaker will
automatically be tripped. Now to observe the interlocking mechanism, reduce the voltage
to 160 V approximately. The circuit breaker is tripped in this condition. Now if you
energize the closing coil by pressing ON switch, the circuit breaker will not close even
though the closing spring is charged.

Experimental Setup

Motor switch Motor Coil U/V Coil

U1 U2 D1 D2

Bulb

220V

220V

150V

Report:
1. What are the difference between a MCB and the ACB used in this experiment?
2. What are the means of arc extinction in ACB?
3. Explain which condition is necessary to close the circuit breaker in HV system.
4. Explain why interlocking is necessary during the closing of CB.

3
20
 AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB
Experiment: 4
Experiment name: Performance study of different types of substation and trip circuit for a protected
line.
Introduction:
A substation is a part of an electrical generation, transmission, and distribution system. Substations
transform voltage from high to low, or the reverse, or perform any of several other important
functions. Between the generating station and consumer, electric power may flow through several
substations at different voltage levels. A substation may include transformers to change voltage levels
between high transmission voltages and lower distribution voltages, or at the interconnection of two
different transmission voltages.

Transmission substation

A transmission substation connects two or more transmission lines. The simplest case is where all
transmission lines have the same voltage. In such cases, substation contains high-voltage switches
that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station
may have transformers to convert between two transmission voltages, voltage control/power factor
correction devices such as capacitors, reactors or static VAR compensators and equipment such as
phase shifting transformers to control power flow between two adjacent power systems.

Distribution substation

A distribution substation transfers power from the transmission system to the distribution system of
an area. It is uneconomical to directly connect electricity consumers to the main transmission
network, unless they use large amounts of power, so the distribution station reduces voltage to a level
suitable for local distribution. The input for a distribution substation is typically at least two
transmission or sub-transmission lines. Input voltage may be, for example, 132 kV, or whatever is
common in the area. The output is a number of feeders. Distribution voltages are typically medium
voltage, between 2.4 kV and 33 kV, depending on the size of the area served and the practices of the
local utility. In addition to transforming voltage, distribution substations also isolate faults in either

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the transmission or distribution systems. Distribution substations are typically the points of voltage
regulation, although on long distribution circuits (of several miles/kilometers), voltage regulation
equipment may also be installed along the line.

Switching station

A switching station is a substation without transformers and operating only at a single voltage level.
Switching stations are sometimes used as collector and distribution stations. A switching station may
also be known as a switchyard, and these are commonly located directly adjacent to or nearby a
power station. An important function performed by a substation is switching, which is the connecting
and disconnecting of transmission lines or other components to and from the system. Switching
events may be planned or unplanned. A transmission line or other component may need to be de-
energized for maintenance or for new construction, for example, adding or removing a transmission
line or a transformer. To maintain reliability of supply, companies aim at keeping the system up and
running while performing maintenance. All work to be performed, from routine testing to adding
entirely new substations, should be done while keeping the whole system running.

Power System Protection – Main Functions

1. To safeguard the entire system to maintain continuity of supply.
2. To minimize damage and repair costs.
3. To ensure safety of personnel.
Power System Protection – Basic Requirements
1. Selectivity: To detect and isolate the faulty item only.
2. Stability: To leave all healthy circuits intact to ensure continuity of supply.
3. Speed: To operate as fast as possible when called upon, to minimize damage, production
downtime and ensure safety to personnel.
4. Sensitivity: To detect even the smallest fault, current or system abnormalities and operate correctly
at its setting.

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Power System Protection – Basic Components
1. Voltage transformers and current transformers: To monitor and give accurate feedback about the
healthiness of a system.
2. Relays: To convert the signals from the monitoring devices, and give instructions to open a circuit
under faulty conditions or to give alarms when the equipment being protected, is approaching towards
possible destruction.
3. Fuses: Self-destructing to save the downstream equipment being protected.
4. Circuit breakers: These are used to make circuits carrying enormous currents, and also to break the
circuit carrying the fault currents for a few cycles based on feedback from the relays.
5. DC batteries: These give uninterrupted power source to the relays and breakers that is independent
of the main power source being protected.
The sequence of operation during abnormal condition:
1. Fault occurs.
2. Relay sense the fault and close the trip circuit.
3. Energize the trip coil unlatch the spring.
4. Contacts start to apart and arc is drawn between the contacts of CB.
5. Arc is extinguished at the instant when fault ac current becomes zero.
6. Fault interruption is completed i.e. fault is cleared.
Objective:
Observe and draw the one line diagram of 132kV/33kV step down substation and switching
substation model, connection diagram, and power flow and trip circuit operation during faulty
condition.
Apparatus:

1. Circuit breaker----------33kV BUS Side

2. Protected line----------- 33kV BUS
3. Battery------------------- 110V DC
4. CT------------------------ 600/1
5. VAMP 50--------------- O/C IDMT Relay

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Circuit Diagram:

CB CT

Protected line PT
Solenoid coil

Relay
Auxiliary
contact

Battery

Figure 4.1: Trip Circuit

Observation: Observe different types of relays in substation I and substation II, their
configuration & how they operate during fault condition.
Procedure:
1. Switch ON Substation I and II.
2. Switch ON every breaker of the substations by turning the breaker control switch.
3. Observe the current flow on the over current relay display by switching 100 Amp Load
only.
4. Draw the one line diagram of the substations.
5. Observe the BUS trip circuit carefully and draw the diagram of this trip circuit.
6. Connect VAMP50-I relay with the communication cable to the computer.
7. Open the relay software, and assign output relay T1 to trip the breaker for earth fault on
the 33kV incoming bus, and assign fault LED to show E/F trip from matrix menu.
8. On the scaling menu give CT ratio 600:1.
9. Activate Earth fault Protection. Assign definite time (DT) 3sec and current setting 0.1In
of the relay.
10. Observe the connection diagram of the relay.
11. Now create fault on the 33kv incoming BUS and observe tripping operation of the
breaker.
12. Find magnitude of the fault current on the earth fault recording menu.

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 O/C -I
132KV/33KV FAULT
CB1 150:1 600:1
33 KV Bus

CB2 600:1
Feeder 1 Feeder 2
DIFF
CB3 CB4
132 KV 3 Phase Transmission line

O/C-III
O/C-II
300:1 300:1

Substation1

TX TX

Transmission line

RX RX

CB5 CB6

DIR-II
DIR-I

300:1 300:1

Substation 2

33 KV/110 V 33 KV/110 V

33 KV Bus

LBS LBS

Load 1 Load 2 Load3

100A 200A 400A

Figure 4.2: Substation one line diagram.

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 NOTE: TO RESET ALL LATCHE OF O/C RELAY PRESS HOME KEY AND THEN
PRESS OK BUTTON.
Power Supply:
1. 3 phase 400 V/132 V Transformer.
2. 110 V DC for relay biasing.
Differential relay comprising of:
1. Primary CT 150:1 [132 KV side]
2. Secondary CT 600:1 [33 KV side ]
3. BT : Bucholz Trip
4. PRD: Pressure release device
5. WT: Winding Temperature.
6. BA : Bucholz Alarm
7. TA : Temperature Alarm
8. TT : Temperature Trip
9. Neutral CT secondary 600:1 for restricted earth fault protection.
10. 132 KV/ 33 KV 3 Phase DY connected transformer which is to be protected.
IDMT O/C relay-I for over load protection of transformer & backup protection of Feeder
comprising of:
1. CT 600:1 [33 KV side]
IDMT O/C relay-II for protection of feeder I comprising of
1. CT 300:1 [33 KV side]
IDMT O/C relay-III for protection of feeder II comprising of
1. CT 300:1 [33 KV side]
Directional O/C relay-I for protection of feeder I of substation -2comprising of
1. CT 300:1 [33 KV side]
2. PT 33 KV/110 V
Directional O/C relay-II for protection of feeder II of substation -2comprising of
1. CT 300:1 [33 KV side]
2. PT 33 KV/110 V

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Figure 4.3: O/C relay.

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Figure 4.4: Block Diagram of O/C relay.

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 Figure 4.5: Connection example
Report:
1. Why do you need a trip circuit for the power system?
2. Explain the objective of different sub-station.
3. What is the function of auxiliary contact in the trip circuit?
4. Mention the name and usage of the equipment that is used in the substation.

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 AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB
Experiment: 5
Experiment name: Performance study of directional relay.
Introduction:
Directional protection responds to the flow of power in a definite direction with
reference to the location of CT’s and PT’s. Directional relay senses direction of power flow
by means of phase angle between voltage (V) and current (I). When this angle exceeds
certain predetermined value, the directional relay operates. A directional relay is a double
actuating quantity relay with one input as current I from CT and the other input as voltage, V
from PT. With the electromagnetic directional O/C relays, discrimination is affected when
the voltage drops down to a low value due to faults close to the location of PT. With static or
digital directional O/C relay can function well up to 1% of the system voltage.
Working principle:
Active power flowing through a part of an electric circuit is P = VI Cosθ, where θ is the
angle between voltage and current.

θ = 90o
Region of +ve power
Region of -ve power

Vr
θ = 180o θ

Ii
θ = 270o

Fig 1: Vector diagram of power.

From the above vector diagram,
For θ< ±90o, Cosθ is positive, hence the real power is positive.
For θ> ±90o, Cosθ is negative, hence the real power is negative.
For θ=90o & θ=270o, Cosθ is zero, hence the real power is zero.

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The direction of power flow can be sensed by sensing the magnitude and sign of power. Here
we used microprocessor based directional relay.
Directional phase fault Protection: (67)

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31
Directional Earth fault Protection (67N)

Description of the set up

The following figure shows the single line diagram of a power system which feeds 33 KV
consumers of bus 3. The bus 3 is fed from bus 2 at 33 KV through two circuits in parallel.
There are two breakers and associated relays at the two ends of each circuit. These relays are
A & B and C & D as shown in the figure. Of these, A&C are non directional O/C relays
whereas B and D are directional O/C relays.

When the fault current flows through the relays B and D in the direction of arrow as shown in
the figure, the relays operate and trip associated breakers.

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32
Case I: For a fault at ‘P’ on one of the circuits, the direction current flow is as shown.

Under this fault condition, the non directional relay A and directional relay B operate and trip
the associated breakers to isolate the fault.
Case II: For a fault at ‘Q’ in the other circuit, the direction of current flow is as shown.

Under the fault condition, the non directional relay C and the directional relay D operate and
trip the associated breakers to isolate the faulty circuit.
Case III: For a fault at ‘W’ in the other circuit, the direction of current flow is as shown.

Under the fault condition, the non directional relay C and A operate and trip the associated
breakers to isolate the faulty circuit. In this condition the directional relay should not work.
Apparatus:
1. Circuit breaker.
2. Protected line.
3. CT 300:1 – 6 no’s
4. PT 33KV/110V – 6no’s
5. Microprocessor based directional O/C Relay.

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33
Circuit Diagram:

Procedure:
1. Switch ON Substation I & II.
2. Switch ON every breaker of the substations.
3. Connect 100amp load of the substation -II.
4. Draw the circuit diagram of this directional protection.
5. Observe the feeder trip circuit carefully.
6. Connect the directional O/C relay I and II with the communication cable to
the computer.
7. Open the relay software. Activate directional protection Iφ>. Set pick up setting
0.1In and angle offset -100 degree for each relay to set correct coordination.
8. Assign output relay (T1 & A1) and fault LED to trip the breaker for directional
line and directional earth fault from Matrix menu.
9. On the scaling menu give CT ratio 300:1 and PT ratio 33kV/110V.
10. Activate directional phase and earth fault Protection.
11. Assign time 0.2Sec DT of the directional relay Iφ>.
12. Assign time 0.2Sec DT of the directional earth fault relay Ioφ>

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34
 13. Now create fault on feeder I, feeder II and BUS II and observe tripping
operation of the associated breaker.
14. Find magnitude of the fault current on the directional fault recording menu.
NOTE:
• TO RESET ALL LATCHES OF VAMP 52 RELAY PRESS TWICE THE
FUNCTION KEY F1.

Reports:

1. In what conditions is it necessary to use directional protection? Explain how close up

faults affects the relay operation.
2. How does directional protection determine the direction of current?
3. What is relay characteristic angle, and what are usual values for this angle?

Page 6 of 6
35
 AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB
Experiment: 6
Experiment name: Performance study of an O/C relay, O/C relay co-ordination, advantage of
parallel feeder.
Introduction:
Though it may be possible to grade the relay settings based on the fault currents,
it is noted that the fault currents in a series network differs marginally when the sections are
connected by cables without any major equipment like transformers in between the two ends. In
such types, if networks grading the settings based on current values do not serve the purpose. It
is required to go for time grading between successive relays in most of the networks.
To achieve selectivity and coordination by time grading two philosophies are available,
namely:
1. Definite time lag (DTL), or
2. Inverse definite minimum time (IDMT).
For the first option, the relays are graded using a definite time interval of approximately
0.5 sec. The relay R3 at the extremity of the network is set to operate in the fastest possible time,
whilst its upstream relay R2 is set 0.5 s higher. Relay operating times increase sequentially at 0.5
sec intervals on each section moving back towards the source as shown in Figure 1.

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36
 The problem with this philosophy is,
the closer the fault to the source the higher the
fault current, the slower the clearing time –
exactly the opposite of what we should be
trying to achieve. On the other hand, inverse
curves as shown in Figure 2 operate faster at
higher fault currents and slower at the lower
fault currents, thereby offering us the features
that we desire.

Protection of parallel distribution lines:

The usage of multiple lines improves the availability of power, i.e. using multiple lines in
parallel allows more power to be conveyed to a given location. Two parallel connected lines are
the simplest and most frequently encountered example of a closed ring. The protection system
must be designed in such a way that when a fault is on one line, the other distribution line should
continue with its normal operation. For better explanation why the directional protection is
necessary in this case, the protection system will be firstly explained just with regular over
current relays and it will be explained why it is not good solution and why directional protection
must be used if we want to have good selectivity of relay protection. As it is already mentioned,
the simple system with two parallel lines will be observed. Such a system is shown on this
figure.

Page 2 of 6
37
One of the possible protections of this simple system is to use 2 over current relays and a circuit
breaker at the source end of each line as it is shown on the following figure. This solution
protects the line properly, but the selectivity is not achieved at all. In other words, wherever the
fault occurs, both lines would be out of function.

When a fault occurs on feeder 1, the fault is supplied with current form both lines, which means
that both over current relays will react after 1.5 Sec (because of the time-delay), thereby opening
circuit breakers B and C. This opens both power lines and the result is: power in no longer
available at substation bus 2.
To solve this problem, i.e. to archive discriminative protection of the two power lines from above
figure, a directional over current relays are required. The protection system with directional units
is shown on the following figure:

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38
The analysis will be done when the fault is on the feeder 1. In this case, fault current IF1flows
through feeder 1 and fault current IF2 flows also through feeder 2, but in the opposite direction.
The protection equipment time delays are shown in the figure. The current IF2 flows though
relay B, D and E. Relay B detects the fault but because of time delay it won’t react immediately,
but after 1.5 Sec. The relay D doesn’t detect the fault at all because the direction of current is
opposite to the direction set in the relay. Finally, relay E would react because the direction of the
current flow is the same as the direction set in the relay and there is time-delay of just 0.2s (1.5s
lower than for relay B). With the reaction of the relay E, the over current relay B resets before its
time delay has elapsed. The situation with current IF1 is much simpler for this fault because this
current flows only through relay C. At the moment when the circuit breaker at location E opens,
the circuit breaker at location C is still closed although this relay has reacted. The reason for this
is that this relay has time -delay of 1.5s. After this time, the circuit breaker on location C opens
and the fault is isolated while the healthy line is still in operation, providing electrical energy to
the bus bar 2. A time setting of 1.5s is used on the over current relay at location B to allow the
directional over current relay at location E to trip when a fault occurs on line. This ensures that
feeder 2 is not disconnected when a fault occurs on feeder 1, thereby achieving proper fault
discrimination. For the same reason the relay on location C has the same time delay, just this
configuration has sense when the fault is on the feeder 2.

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39
Circuit breaker failure protection:
The circuit breaker failure protection is based on supervision of phase and earth current after
tripping events. The test criteria is whether all phase current have dropped to less than 5% of in
within tCBFP. If one or more of the phase currents have not dropped to specific current within
this time, CB failure is detected and the assigned output relay is activated to trip the upstream
breaker.
Apparatus:
1. Circuit breaker.
2. Protected line.
3. Bus CT 600:1 – 3 no’s
4. Feeder CT 300:1 – 3+3 no’s
5. Feeder PT 33V/110V 3 no’s
6. Microprocessor based IDMT O/C Relay: 3 no’s
7. Microprocessor based directional O/C Relay: 2 no’s
Procedure:
1. Switch ON the Substation.
2. Switch ON every breaker of the substations.
3. Connect 100 Amp load of the substation -II.
4. Connect the O/C relay –II and III with the communication cable to the computer.
5. Activate over current Protection I> & Breaker failure protection for feeder-2
of O/C relay-III, Earth fault protection I0> for feeder-1 of O/C relay-II.
6. Open the relay software and assign output relay T1 & A1 and assign Fault LED to
trip the breaker for over current on the bus, feeder I and feeder II from matrix
menu.
7. On the scaling menu give CT ratio 300:1 on each feeder.
8. Assign delay type DT 1.5 Sec for Feeder O/C RELAY II and III and set current
setting 0.8In of I> for O/C relay-III and 0.08In of Io> for O/C relay-II.
9. On over current relay-II of feeder -1 activate Earth fault protection and on over
current relay-III of feeder -3 activate over current and breaker failure protection.
10. Activate breaker failure protection on Over current relay-II and assign output relay
T3 and assign fault LED to trip the upstream breaker of over current relay –I.

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40
 11. Now create fault on Bus-I, Feeder-1, Feeder-II and Bus-II and each case observe
which breaker are tripping for each faults.
12. Now open the trip coil of breaker CB4 by pressing breaker failure switch and create
fault on feeder -2 and observe the tripping operation.

Report:
1. What are the advantages of Parallel feeder?
2. How correct co ordination is obtained by ring grid system?
3. Discuss the significance of the back up protection.
4. Under what condition back up protection is economically justified?

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41
 AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB
Experiment: 7
Experiment name: Performance study of a differential relay for the protection of transformer.

Objectives:

The objectives of this experiment are to observe the performance of the following schemes of a
power transformer.

1. Buchholz alarm
2. Buchholz trip
3. Temperature alarm
4. Temperature trip
5. Differential relay trip due to phase to phase and ground fault
6. Restricted E/F relay trip
7. PRD trip

Introduction:

The choice of the protection for any power transformer depends upon a number of factors such as its
size, importance and cost. Power transformers are used in transmission network of higher voltages
for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally
rated above 200MVA.

Differential Relay:

Differential relay provides unit protection. This relay is one that operates when there is a difference
between two or more similar electrical quantities exceeds a predetermined value. In differential relay
scheme circuit, there are two currents come from two parts of an electrical power circuit. These two
currents meet at a junction point where a relay coil is connected. According to Kirchhoff Current
Law, the resultant current flowing through the relay coil is nothing but summation of two currents,
coming from two different parts of the electrical power circuit.

The polarity and amplitude of both the currents are so adjusted that the phasor sum of these two
currents is zero at normal operating condition. Thereby there will be no current flowing through the
relay coil at normal operating conditions. But due to any abnormality in the power circuit, if this
balance is broken, that means the phasor sum of these two currents no longer remains zero and there
will be non-zero current flowing through the relay coil thereby relay being operated. In current
differential scheme, there are two sets of current transformer each connected to either side of the
equipment protected by differential relay. The ratio of the current transformers are so chosen, the
secondary currents of both current transformers matches each other in magnitude.

Page 1 of 9
42
 O/C Protected element
Relay 2
CT1 CT2 1

G UNIT

I1 Pilot Operating I2
wire D/R
coli

Figure: Differential protection for radial feeder

At healthy condition: I1-I2=0. For a unidirectional feeder, if fault occurs at location 1 (outside of
unit), then current through the operating coil will be I1-I2=0. But for location 2, I1-I2≠0.In case of
bidirectional feeder, if fault occurs within unit, direction of I2 current will be changed and current
through the operating coil will be I1+I2 which is definitely not equals to zero.

Fault outside the unit: Only over-current relay will operate

Fault inside the unit: Both over-current realy and differential will operate

Construction of Transformer:

A transformer is a static piece of equipment used either for raising or lowering the voltage of an a.c.
supply with a corresponding decrease or increase in current. It essentially consists of two windings,
the primary and secondary, wound on a common laminated magnetic core. The winding connected to
the a.c. source is called primary winding (or primary) and the one connected to load is called
secondary winding (or secondary).

1. Transformer Tank
2. High Voltage Bushing
3. Low Voltage Bushing
4. Cooling Fins/Radiator
5. Cooling Fans
6. Conservator Tank
7. System Ground Terminal
8. Drain Valve
9. Dehydrating Breather
10. Oil Temperature/Pressure
gauges
11. Bushing Current
Transformers
12. Control Panel

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43
Working Principle:

I2
I1
Primary Secondary
V1 E2 V2 LOAD
E1 N1 N2

When an alternating voltage V1 is applied to the primary, an alternating flux φ is set up in the core.
This alternating flux links both the windings and induces e.m.f.s E1 and E2 in them according to
Faraday’s laws of electromagnetic induction. Magnitudes of E2 and E1 depend upon the number of
turns on the secondary and primary respectively.

If N2 > N1, then E2 > E1 (or V2 > V1) and we get a step-up transformer.
If N2 < N1, then E2 < E1 (or V2 < V1) and we get a step-down transformer.

Types of faults:

1. Phase to phase fault &

Phase to neutral fault
2. High voltage surge due to lightening
3. Fault inside the tank below oil level
4. Magnetic inrush current
5. Tank E/F protection

Protection Scheme:

1. Phase to phase and phase to neutral fault protection: DR+ O/C relay

In protection of a transformer, the CT connections and CT ratios are such that current fed into the
pilot wires from both the ends are equal during normal and for through fault conditions. During any
kind of internal fault, like phase to phase faults or phase to ground faults, the balance is disturbed.
The out of balance current (I1-I2) flows through the relay operating coils. To avoid unwanted relay
operation on through faults, restraining coils are provided in series with the pilot wires. The average
current through the restraining coil is (I1+I2)/2. As a result the restraining current increases with the
increase of (I1-I2) in the operating coil for a through fault condition. An additional over current relay
is also used to provide over load protection.

2. High voltage surge due to lightening:

Lightening Arrester: For large transformers

Horn gap: in small distribution transformer

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44
 Fig.: Horn gap or Arcing horn

Arcing Horns bypasses the high voltage across the Insulator using air as a conductive medium
between the Horns. The small gap between the horns ensures that the air between them breaks down
resulting in a flashover and conducts the voltage surge rather than cause damage to the insulator.

Arcing Horns basically form a Spark Gap across the Insulator with a lower breakdown voltage than
the air path along the insulator surface, so an overvoltage will cause the air to break down and the arc
to form between the arcing horns, diverting it away from the surface of the insulator. An arc between
the Horns is more tolerable for the equipment because it provides more time for the fault to be
detected and the arc to be safely cleared by remote Circuit Breakers. Air gap length should be
provided is such a way that at normal system voltage, there should not be any current flow through
this horn gap path.

3. Fault inside the tank below oil level:

Overheating Protection:

The rating of the transformer is based on the temperature rise above an assumed maximum ambient
temperature; under this condition no sustained overload is usually permissible. At lower ambient
temperature some degree of overload can be safely applied. Short time overloading are also
permissible to an extent dependent on the previous loading conditions. No precise ruling applicable
to all conditions can be given concerning the magnitude and direction of safe overload.

Thermocouples or resistor temperature detectors are kept near each winding. These are connected to
a bridge circuit. When temperature increases above safe value, an alarm is given. If measures are not
taken, the circuit breaker is tripped after a certain temperature. Some typical settings for oil
temperature are as follows-

At 60 0C, Switch on cooling fans

At 95 0C, give an alarm

At 1200C, give a trip signal to trip the CB.

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45
A temperature of about 950C is considered to be the normal maximum working value. Any further
rise of 80-10 0C beyond this 950C will make the life of the transformer half if this rise is sustained.

Buchholz relay (Gas relay / Gas actuated relay):

All faults below oil in a transformer result in the localized heating and break down of the oil. Some
degree of arcing will always take place in a winding fault and resulting decomposition of the oil will
release gas such as hydrogen, carbon monoxide and light hydro carbons. When the fault is of a very
minor type, such as a hot joint, gas is released slowly, but a major fault involving severe arcing
cause’s rapid release of large volumes of gas as well as oil vapor. The action is so violent that the gas
and oil vapor do not have time to escape but instead build up pressure and bodily displace the oil.
When such faults occur in transformers having oil conservators, the faults causes a blast of oil to pass
up the relief pipe to the conservator.

(a)Buchholz Alarm:

The incipient faults (gradually developing faults in the winding below oil level) produce the gas and
it gets collected in the upper portion of the relay, thereby the oil level in the relay drops down. The
float, floating in the oil in the relay tilts down with lowering the oil level. While doing so the
mercury switch attached to the float is closed on to the alarm circuit.

(b)Buchholz trip :

The short circuit fault causes a blast of oil rushes towards the conservator through Buchholz relay.
The baffles (plate) in the Buchholz relay get pressed by the rushing oil. Thereby it closes another
switch which in turn closes the trip circuit of the circuit breaker.

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46
Pressure Relief Device:

The working principle of transformer pressure

relief device is very simple. If pressure arises
inside a transformer and exceeds a pre-set
pressure limit, the pressure safety valve opens its
valve clap, which is held by a spring and releases
the internal pressure until it declines. After
decrease of the pressure, the pressure valve clap
moves back to its origin position and closes
completely. Normally, the pressure relief device
will be mounted on top of the transformer. Due
to internal faults, it is suggested to have such
pressure relief valves to protect the transformer
and release arising pressure quite suddenly.

4. Magnetic Inrush Current Protection:

The transformer inrush current is the maximum instantaneous current drawn by the primary of the
transformer when their secondary is open circuit. The inrush current does not create any permanent
fault, but it causes an unwanted switching in the circuit breaker of the transformer. During the inrush
current, the maximum value attained by the flux is over twice the normal flux.

In the FFT analysis of inrush and fault current, it’s been observed that the second harmonic content
of the inrush current is lying in the high range almost above 60%. The second harmonic content for
fault in all the cases is lying below 25%. So, on the basis of FFT analysis the inrush and fault
transients can easily be differentiated.

Unnecessary tripping can be occurred due to inrush current. In order to avoid this phenomenon, a
second harmonic filter is used whose NC contact is connected in series with the operating coil of
differential relay.

5. Restricted Earth Fault Protection of Y-winding

When E/F occurs very near to the neutral point of Y-winding of the transformer, the voltage
available for driving earth fault current is small. Hence the fault current is low. If the normal biased
different relay is to sense such faults, it has to be too sensitive and would therefore operate for
spurious signal like, external faults and switching surges; under this condition restricted earth fault
protection scheme has evolved. Here the practice is to set the relay such that it operates for earth
fault current of the order of 15% of rated winding current, such setting protects restricted portion of
the winding.

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47
Procedure:

A 3φ, 38 MVA, 33/11KV, Δ- connected power transformer feeds power to an 11 KV bus from a
33 KV bus as shown in the figure below-

Relay
Stabilizing Resistor
A B C

CB CB

c b a

11 KV bus
33 KV bus

Procedure:

Temperature Alarm:

For pushing the temperature alarm switch, which represents the closing of a contact due to rise in
winding temperature, an alarm signal will be displayed on the relay display board.

Temperature trip:

If the winding temperature goes to a very high level, the transformer should be isolated from the
system. By pushing the temperature trip switch, the temperature relay essentially closes the trip
circuit and fault is cleared by two breakers on the two sides of the transformer.

Buchholz Alarm:

Pushing this button means closing the contact of Buchholz relay as an indication of incipient fault in
the winding inside the oil, so an alarm indication is displaced on the relay.

Buchholz trip:

Pushing this button means closing the contact of Buchholz rely as an indication of internal short
circuit fault. So the breakers on both sides of the transformer are tripped.

Page 7 of 9
48
Internal Fault:

A short circuit fault in the winding is created by shorting the two phase of any side. This fault is
detected by the differential relay and the breakers on both sides of the transformer are tripped to
isolate the fault.

Restricted E/F Protection:

An earth fault close to the neutral end of the Y- winding of the transformer is created by shorting the
phase terminal and neutral terminal. This fault is detected by the concerned relay and the breakers on
both sides of transformer are tripped to isolate the fault.

Apparatus:
1. Circuit breaker: 2 no’s
2. 3 Phase 38.1 MVA Δ-Y 33KV/11KV transformer.
3. CT 600:1 – 3 no’s
4. CT 1800:1 – 3 no’s
5. Microprocessor based differential Relay: P632
6. Microprocessor based O/C Relay: P122

Circuit Diagram:

BT/TT/WT/PRD VAMP
50

132kV/33kV
600:1 1800:1

1800:1

VAM 265

Observation:
• How VAMP 265 relay operate during fault condition.
• VAMP 265 relay configurations.
• Transformer voltage.
• Power
• REF

Page 8 of 9
49
 • CT/PT ratio
• Differential protection
• Vector group.
• BT, TT, WT, PRD trip
• LED configures.

Report:

1. What do you mean by incipient faults in the transformer winding? What are the possible causes if
this fault?

2. Is the earth fault close to neutral end of a wye connected winding very common? Why?

3. Explain why percentage differential relay is not suitable for detecting the E/F near neutral end of
an wye connected winding whose neutral is grounded through high resistance.

Page 9 of 9
50
 AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB
Experiment: 8

Experiment name: Study of different types of motor protection system

Objectives:
• To observe the connection diagram of DOL protection system and its operation.
• To observe the connection diagram of MMS protection system and its operation.
• To observe the connection diagram of Inverter protection system and its operation &
control.
Theory:
The following two basic protections are provided for every motor:
1. Thermal over load protection
2. Short circuit protection.
The switchgear used for the motor protection can also be classified into the following two groups
depending on the size of the motor:
1. For small motor (up to 150 hp), fuse and thermal over current protection are used
2. For large motors, circuit breakers and associated relays are used.
For small motors:
Short circuit protection:
Fuse will provide the short circuit protection of stator winding. The operating time current
characteristics of the fuse should be such that the fuse should not blow during the motor starting
which could be 5 to 7 times the motor full load current. The fuse should blow at current more
than those which can be interrupted by the contactors. Here we used magnetic contactor for short
circuit protection.
Over load protection:
Thermal relay should provide the overload protection. Thermal relay should not operate during
starting period of the motor. Starting period is generally considered to be 5 to 10 seconds.

51
For large motor:
Overload and short circuit protection:
Over current relay and earth fault relay (either instantaneous or inverse time or both depending
on the importance of the motor) are used to protect against phase fault and earth fault on stator
winding. If the motor is very large and expensive, it is essential to provide differential protection
for the winding. The short circuit protection characteristic is set just above the maximum starting
current of the motor.

Types of motor Short Circuit O/L protection Phase unbalance

protection protection
Small motor (up to Fuse/ CB Thermal O/L relay ---
150 hp )
Large motor Differential relay + O/C relay + CB NPS relay + CB
CB

NPS Relay:
A relay which protects the electrical system from negative sequence component is called
a negative sequence relay or unbalance phase relay.

When a three phase rotating electric machine, including an alternator is connected to the
perfectly balanced three phase power system, no negative sequence current is developed in the
rotor winding. If, however, the power system is unbalanced, as usually in the case, a negative
sequence current double the system frequency is induced in the rotor winding. This naturally
causes motor rotor overheating that in the absence of this current. Flow of large amount of
negative phase sequence current in the rotor winding for long period can cause damage to the
rotor winding. Under this situation, a necessary measure must be taken to save the machine. So,
the negative phase sequence current can be used as a parameter in the design of negative
sequence protection scheme of large and expensive rotating electric machines including
generator.

52
Protection Types:
• DOL (Direct on line)
• MMS (Manual motor starter)
• Y-∆ starter
• Resistance starter
• Auto transformer method
• Inverter method
• Soft starter method
DOL protection system:
In electrical engineering, a direct on line (DOL) or across the line starter starts electric motors
by applying the full line voltage to the motor terminals. This is the simplest type of motor starter.
A DOL motor starter also contain protection devices, and in some cases, condition monitoring.
Smaller sizes of direct on-line starters are manually operated; larger sizes use an
electromechanical contactor (relay) to switch the motor circuit. Solid-state direct on line starters
also exist.
A direct on line starter can be used if the high inrush current of the motor does not cause
excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a direct
on line starter may be limited by the supply utility for this reason. For example, a utility may
require rural customers to use reduced-voltage starters for motors larger than 10 kW.
DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor
belts. In the case of an asynchronous motor, such as the 3-phase squirrel-cage motor, the motor
will draw a high starting current until it has run up to full speed. This starting current is
commonly around six times the full load current, but may be as high as 6 to 7 times the full load
current. To reduce the inrush current, larger motors will have reduced-voltage starters or variable
speed drives in order to minimize voltage dips to the power supply.

53
 VR VY VB N

Fuse

Thermal N/C
O/L relay Push
OFF

Magnetic
contactor A/C
Push
ON

M/C
COIL

DOL protection system.

54
Magnetic Contactor:

MMS protection system:

MMS is the integrated form of magnetic
contactor, overload relay and a switch. You
can use an extra magnetic contactor for
remote control. The characteristics of manual
motor starter are:

• Overload protection
• Phase failure sensitiveness
• Disconnect function for safety isolation of
the installation and the supply
• Temperature compensation from -25 …
+60 °C
• Adjustable current setting for overload
protection
• Suitable for three- and single-phase
application

55
Y-∆ starter Protection Scheme:
A, B, C: Magnetic Contactor
To make Y: A+B
To make ∆: A+C
LINE

Push A
ON M/C

N/C
TOR

Push
OFF

N/C N/O
Timer Timer

M/C -C M/C -B
M/C Timer N/C N/C
COIL-A 5 sec

M/C M/C
COIL-B COIL-C

Neutral

Resistance starter:
In this method, external resistances are connected in series with
each phase of stator winding during starting. This causes voltage
drop across the resistances so that voltage available across motor
terminals is reduced and hence the starting current. The starting
resistances are gradually cut out in steps (two or more steps) from
the stator circuit as the motor picks up speed. When the motor
attains rated speed, the resistances are completely cut out and full
line voltage is applied to the rotor.
This method suffers from two drawbacks. First, the reduced
voltage applied to the motor during the starting period lowers the
starting torque and hence increases the accelerating time.
Secondly, a lot of power is wasted in the starting resistances.

56
Auto transformer starter:
This method also aims at connecting the induction motor to a
reduced supply at starting and then connecting it to the full
voltage as the motor picks up sufficient speed. The tapping on
the autotransformer is so set that when it is in the circuit, 65% to
80% of line voltage is applied to the motor.
At the instant of starting, the change-over switch is thrown to
“start” position. This puts the autotransformer in the circuit and
thus reduced voltage is applied to the circuit. Consequently,
starting current is limited to safe value. When the motor attains
about 80% of normal speed, the changeover switch is thrown to
“run” position. This takes out the autotransformer from the
circuit and puts the motor to full line voltage. Autotransformer
starting has several advantages viz low power loss, low starting
current and less radiated heat. For large machines (over 25 H.P.),
this method of starting is often used. This method can be used for
both star and delta connected motors.

Inverter protection system:

You can control and protect the motor using inverter. A frequency
inverter controls AC motor speed picture. The frequency inverter
converts the fixed supply frequency (50 Hz) to a variable-frequency,
variable-voltage output to enable precise motor speed control.

A variable frequency drive (VFD) is a motor control device that

protects and controls the speed of an AC induction motor. A VFD
can control the speed of the motor during the start and stop cycle, as
well as throughout the run cycle. VFDs are also referred to as
adjustable frequency drives (AFDs). VFDs are used in applications
where complete speed control is required, energy savings is a goal
and custom control is needed.

VFDs convert input power to adjustable frequency and voltage

source for controlling speed of AC induction motors. The frequency
of the power applied to an AC motor determines the motor speed.
The VFD's input power comes from the facility power network
(typically 400V, 50 Hz AC). It has a rectifier that converts network
AC power to DC power. A filter and DC bus work together to
smooth the rectified DC power and to provide clean, low ripple DC
power to the inverter, which uses DC power from the DC bus and
filter to invert an output that resembles sine wave AC power using a
pulse width modulation (PWM) technique.

57
Soft Starter method:

A soft starter continuously controls the three-phase motor’s

voltage supply during the start-up phase. This way, the motor
is adjusted to the machine’s load behavior. Mechanical
operating equipment is accelerated smoothly.

Soft starters are used in applications where speed and torque

control are required only during startup (and stop if equipped
with soft stop) or where there is a need to reduce large startup
inrush currents associated with a large motor is required.
Electrical soft starters temporarily reduce voltage or current
input by reducing torque. Some soft starters may use solid-
state devices to help control the flow of the current. They can
control one to three phases, with three-phase control usually
producing better results. Most soft starters use a series of
thyristors or silicon controlled rectifiers (SCRs) to reduce the
voltage. In the normal OFF state, the SCRs restrict current, but
in the normal ON state, the SCRs allow current. The SCRs are
engaged during ramp up, and bypass contactors are pulled in
after maximum speed is achieved. This helps to significantly
reduce motor heating.

58
Report:
1. What kind of protections is given in small and large motor?
2. How can you control the speed of a motor?
3. Draw the control circuit of resistance and auto transformer starter.
4. Why thermal over load relay is used in motor protection system?

59
 Experimental Setup

10

60
 AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB

Experiment 9: Generator protection by using microprocessor based VAMP relay. (PART-1)

Generator Fault:
Generator O/C Protection
Reverse Power protection
Generator earth fault protection by calculating zero sequence
Generator earth fault protection by calculating zero sequence voltage.

Theory: A modern generating unit is a complex system comprising the generator stator winding
and associated transformer and unit transformer, the rotor with its field winding and exciters, and the
turbine and its associated condenser and boiler complete with auxiliary fans and pumps. Faults of many
kinds can occur within this system for which diverse protective means are needed. The amount of
protection applied will be governed by economic considerations, taking into account the value of the
machine and its importance to the power system as a whole. In this experiment we will show how
different faults occur in a generator and how the protections are given by using microprocessor based
VAMP RELAY.

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Page 2 of 11

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Over current protection I> (50/51)

Over current protection is used against short circuit faults and heavy overloads. The over current
function measures the fundamental frequency component of the phase currents. The protection is
sensitive for the highest of the three phase currents. Whenever this value exceeds the user's pick-up
setting of a particular stage, this stage picks up and a start signal is issued. If the fault situation remains
on longer than the user's operation delay setting, a trip signal is issued.

Reverse power and under-power protection P< (32)

Reverse power function can be used for generators against motoring to protect the prime mover against
over-speeding or to disconnect a motor in case the supply voltage is lost and thus prevent any power
generation by the motor. Under-power function can be used to detect loss of the mechanical load of a
motor. Reverse and under power function is sensitive to active power. Whenever the active power goes
under the pick-up value, the stage picks up and issues a start signal. If the fault situation stays on longer
than the delay setting, a trip signal is issued. The pick-up setting is proportional to the nominal power of
the prime mover parameter Pm , which is part of the basic configuration.

If two or more generators are run in parallel then if one of them drags behind due to any reason,
instead of delivering electric power to the bus, it takes power from the bus acting like a motor and trying
to drive the turbine. Then generator can enter a state called "motoring", where the prime mover is no
longer providing the power for the generator to the electric grid. Under this condition the generator is
using power instead of providing power. Hence power flows in reverse direction. This condition is known
as reverse power.

Reasons of Reverse power

There are mainly two reasons for which reverse power can occur.

I. Motoring
The failure can be caused to a starvation of fuel in the prime mover, a problem with the
speed controller or other breakdown. When the prime mover of a generator running in a

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 synchronized condition fails. There is a condition known as motoring, where the generator
draws power from the bus bar, runs as a motor and drives the prime mover. This happens as
in a synchronized condition all the generators will have the same frequency. Any drop in
frequency in one generator will cause the other power sources to pump power into the
generator. The flow of power in the reverse direction is known as the reverse power relay.

II. Synchronization
Another cause of reverse power can occur during synchronization. If the frequency of the
machine to be synchronized is slightly lesser than the bus bar frequency and the breaker is
closed, power will flow from the bus bar to the machine. Hence, during synchronization
(forward), frequency of the incoming machine is kept slight higher than that of the bus
bar i.e. the synchro-scope is made to rotate in the "Too fast" direction. This ensures that the
machine takes on load as soon as the breaker is closed.

III. Load variation problem

Effects:
I. Steam unit: Overheating of turbine and turbines blades.
II. Hydro unit: Cavitation of the blades.

Over current:

Over current protection is used against short circuit faults and heavy overloads.

Reverse Power:

Reverse power relays are used on generators to trip them if this condition occurs. A reverse power relay
is a directional power relay that is used to monitor the power from a generator running in parallel with
another generator or the utility. The function of the reverse power relay is to prevent a reverse power
condition in which power flows from the bus bar into the generator.

The reverse power relay operates by measuring the active component of the load current, I cos φ. When
the generator is supplying power, the I x cos φ is positive, in a reverse power situation it turns negative.
If the negative value exceeds the set point of the relay, the relay trips the generator breaker after the
preset time delay.

Procedure:

• Connect the Generator with the CT’s and PT’s like above figure.[Already given]
• CT’s and PT’s output should be connected with the relay like above figure. [Already given]
• Output relay should be connected with the breaker like the diagram [Already given]
• Now configure the relay by using VAMP setting software.
• Connect the relay with the VAMP setting software by the given USB cable.
• After open the software set the required port.

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• And then click connect.
• Default password is 2.
• Now configure CT setting, generator rating from the scaling menu.

• Configure over current protection stage.

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• Configure reverse power protection stage.

• Configure output relays and output LED on the output matrix menu.

• Now synchronize the generator with the line. [Note: To synchronize the generator with the line
you should match the voltage frequency, phase sequence and phase angle. Everything should be
approximately same.]
• Match the phase sequence of the line and generator. If the phase sequence of the generator is
different from the source, then the phase sequence meter given an un-even movement. To
match the sequence you should interchange any two phases. This is done by phase sequence
switch. Then you can show the even movement of the phases from the sequence meter.
• Now vary the excitation by tuning excitation knob to match the generated voltage with the
system voltage.
• Now vary the frequency by tuning frequency knob to match the generated frequency with the
system frequency. Here we change the frequency by varying the coupling voltage of the VS drive
motor.
• You can see the system and generated voltage and frequency in the two digital multifunction
meters.
• Synchroscope is used to detect the phase displacement between system and generator.

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 • When the synchronizing conditions are satisfied then green LED of the synchroscope will be
glowing, then ON the generator breaker.
• Now after synchronizing condition if you increase the frequency by tuning frequency knob you
can see the over current scenario happens and if it is cross the threshold value of the relay
setting it will instantly pick up and trip the breaker and alarming a signal.
• Synchronize the generator again.
• Now after synchronizing condition if you decrease the frequency by tuning frequency knob you
can see the reverse power scenario happens and if it is cross the threshold value of the relay
setting the relay will instantly pick up and trip the breaker and alarming a signal.
• You can see the actual current, harmonics, phasor diagram from the respective menu.

Communication:

• Communicate with the relay to the given software and USB cable.
• Port selection must be OK to communicate with the relay.

Observation:

• Device info
• Measurement
• Phasor diagram
• Event buffer
• Scaling
• Output matrix
• Over current stage
• Reverse power stage
• Harmonics
• Scaling
• Clock sync.
• You can create stator fault line to line and line to ground by pushing of push button on the top
of the Generator diagram.

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Generator earth fault protection by calculating zero sequence current and zero sequence voltage by
using microprocessor based VAMP relay.

Theory:

Causes of stator ground fault:

• Transient over voltage due to lightning.
• Temporary over voltage.
• Degraded insulation due to high temperature or ageing.
• Mechanical impact.

Impact:
• Damages on the stator iron.
• Increase voltage on healthy phases.
• Small fault currents.

In case of short-circuits between phases in the stator winding or between the generator terminals, the
machine must quickly be disconnected from the network and brought to a complete shutdown in order
to limit the damage. Phase short-circuits on the generator bus, in the unit transformer or in the high
voltage winding of the unit transformer, must also be quickly disconnected from the network. The
generator must be brought to a complete shutdown in case of a transformer fault if there is no circuit-
breaker between the machine and the transformer.

Earth fault protection I0 > (50N/51N)

Un-directional earth fault protection is used for generator's stator earth faults in low impedance earthed
networks. In high impedance earthed networks, compensated networks and isolated networks un-
directional earth fault can be used as back-up protection. The un-directional earth fault function is
sensitive to the fundamental frequency component of the residual current 3I0 . The attenuation of the
third harmonic is more than 60 dB. Whenever this fundamental value exceeds the user's pick-up setting
of a particular stage, this stage picks up and a start signal is issued. If the fault situation remains on
longer than the user's operation time delay setting, a trip signal is issued.

Zero sequence voltage protection U0 > (59N)

The zero sequence voltage protection is used as unselective backup for earth faults and also for selective
earth fault protections for generators having a unit transformer between the generator and the bus bar.
This function is sensitive to the fundamental frequency component of the zero sequence voltage. The
attenuation of the third harmonic is more than 60 dB. This is essential, because 3n harmonics exist
between the neutral point and earth also when there is no earth fault. Whenever the measured value
exceeds the user's pick-up setting of a particular stage, this stage picks up and a start signal is issued. If
the fault situation remains on longer than the user's operation time delay setting, a trip signal is issued.

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Procedure:

• Connect the Generator with the CT’s and PT’s like above figure.[Already given]
• CT’s and PT’s output should be connected with the relay like above figure. [Already given]
• Output relay should be connected with the breaker like the diagram [Already given]
• Now configure the relay by using VAMP setting software.
• Connect the relay with the VAMP setting software by the given USB cable.
• After open the software set the required port.
• And then click connect.
• Default password is 2.
• Now you should configure CT setting, transformer setting on the scaling menu.
• Configure Earth fault protection stage for I0.

• Configure Earth fault protection stage for U0.

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 • Configure output relays and output LED on the output matrix menu.

• Now start the generator.

• Now observe the third harmonics components of current and voltage.
• Now push the stator earth fault switch by selecting knob at Io.
• Observe the third harmonics of the current wave shapes.
• Observe how this protection system works.
• Now start the generator.
• Now observe the third harmonics components of current and voltage.
• Now push the stator earth fault switch by changing knob position to Uo.
• Observe the third harmonics of the voltage wave shapes.
• Observe how this protection system works.
• You can see the actual current, harmonics, phasor diagram from the respective menu.

Equipments:

Generator 2KW: 1
VS Drive motor 2KW: 1
Unit transformer 0.6KVA 3
R.P.M adjustment mechanism Load control device. 1
Voltage adjustment mechanism/Excitation control device. 1
Synchronizing mechanism:
1
Analog synchroscope.
Multifunction digital meter 2
Miniature CB 20A 1
Magnetic contactor 20A 4
CT 10/5 A
6
5VA
PT 400/110
2
5VA
PT 220/110 V
2
5VA
Fault simulators 10
Control switches 6A 10
Neutral grounding resistance
1
200 ohm
Generator protection relay: Vamp210 1

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Circuit diagram:

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71
 AHSANULLAH UNIVERSITY OF SCIENCE AND TECHNOLGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
SWITCHGEAR AND PROTECTION LAB

Experiment 10: Generator protection by using microprocessor based VAMP relay. (PART-2)

Generator Fault
Generator rotor fault
U/F detection and protection by
Generator unbalance loading
Under excitation
Generator over voltage
Over frequency protection by using microprocessor based VAMP relay.

Theory:

Rotor earth fault: I02

While the winding in the rotor is insulated from the ground during normal operation, the Rotor is
subjected to stresses due to vibration, heat, etc. These stresses can cause the winding to give way in a
particular place and the winding can get earthed.

Reasons of Rotor Earth Fault:

Failures in the rotor, caused by low exciting voltage do not occur that often and single earth faults are
not that dangerous. But it was obvious, that in case of a second breakdown of isolation, the turn-to-
turn-fault exerts a force on the axle. The detection of turn-to-turn faults is difficult. With a low exciting
voltage they occur in case of operation of the machine only. Centrifugal forces and heating utilize the
winding mechanically and thermally.

This explains why a turn-to-turn fault occurs during special load conditions and not if the generator was
out of service. Measurements are difficult due to a low resistance of the rotor winding. Only a careful
assembly of the winding was a sufficient protection because the isolation was aged by the de-excitation.

Effects of Rotor Earth Fault:

The currents produced during a rotor earth fault can cause excessive vibration and disturb the magnetic
balance inside the alternator. These forces can cause the rotor shaft to become eccentric and in extreme
cases cause bearing failure.

Protection Method:
There are several methods of power system grounding. These include low-resistance grounded (LRG),
effectively grounded, reactance grounded, high-resistance grounded (HRG), and ungrounded. Source
grounding may be accomplished by the grounding of the generator(s). The rotor earth fault protection
can be realized with the non-directional earth fault stage using the energizing current input I02 in
combination with a simple current injecting device. Here we used resistive grounding system.

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Over Excitation (V/F)
A synchronous generator is driven by a prime mover to run at a constant speed.
Based on the dc excitation current flowing through the rotating poles, the stator armature coils generate
more and more current--- all at a constant voltage. Voltage is dependent on the speed, which is
constant, because it is being run at synchronous speed. But the process has a limit. After a certain stage,
sending higher and higher dc current through the poles (Over-excitation) will make the generator
unstable! It will start hunting, and there is even risk of the generator falling out of synchronism.

Reasons of Over Excitation Fault: Excessive dc excitation causes over excitation.

Procedure:

• Connect the Generator with the CT’s and PT’s like above figure.[Already given]
• CT’s and PT’s output should be connected with the relay like above figure. [Already given]
• Output relay should be connected with the breaker like the diagram [Already given]
• Now configure the relay by using VAMP setting software.
• Connect the relay with the VAMP setting software by the given USB cable.
• After open the software set the required port.
• And then click connect.
• Default password is 2.
• Now you should configure CT setting, transformer setting on the scaling menu.
• Configure Rotor earth fault protection.

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 • Configure Earth fault protection stage for U0.

• Configure output relays and output LED on the output matrix menu.

• Now start the generator.

• Now push the rotor earth fault switch.
• Observe how this protection system works.
• Now start the generator.
• Now fix the voltage at approximately 234 Volt and frequency approximately at 48 Hz. Now
observe how this protection works.
• You can see the actual current, harmonics, phasor diagram from the respective menu.

Observation:

• Device info
• Measurement
• Phasor diagram
• Event buffer

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 • Scaling
• Output matrix
• Rotor fault
• U/F

Generator unbalance loading and under excitation protection by using microprocessor based VAMP
relay.

Causes of under excitation:

• Open field circuit
• Field short circuit
• Accidental tripping of field CB
• AVR
• Loss of field at the main exciter
Consequence:
• Machine speed higher than synchronous speed
• Asynchronous running of synchronous machine without excitation
• Stator end core heating
• Induced rotor current

Reasons of Negative-sequence

When the generator is connected to a balanced load, the phase currents are equal in magnitude and
displaced electrically by 120°. The ampere-turns wave produced by the stator currents rotate
synchronously with the rotor and no eddy currents are induced in the rotor parts.

Unbalanced loading gives rise to a negative sequence component in the stator current. The negative-
sequence current produces an additional ampere-turn wave which rotates backwards, hence it moves

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relatively to the rotor at twice the synchronous speed. The double frequency eddy currents induced in
the rotor may cause excessive heating, primarily in the surface of cylindrical rotors and in the damper
winding of rotors with salient poles.

Effects of Negative-sequence fault

The approximate heating effect on the rotor of a synchronous machine for various unbalanced fault or
severe load unbalance conditions is determined by the product I22 t = K, where I2 is the negative
sequence current expressed in per unit (p.u.) stator current, to the duration in seconds and K a constant
depending on the heating characteristic of the machine, i.e., the type of machine and the method of
cooling adopted. The capability of the machine to withstand continuously unbalanced currents is
expressed as negative sequence current in percent of rated stator current.

Procedure:

• Connect the Generator with the CT’s and PT’s like above figure.[Already given]
• CT’s and PT’s output should be connected with the relay like above figure. [Already given]
• Output relay should be connected with the breaker like the diagram [Already given]
• Now configure the relay by using VAMP setting software.
• Connect the relay with the VAMP setting software by the given USB cable.
• After open the software set the required port.
• And then click connect.
• Default password is 2.
• Configure under excitation protection.

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• Configure phase unbalance protection stage.

• Configure output relays and output LED on the output matrix menu.

• Now synchronize the generator with the line. [Note: To synchronize the generator with the line
you should match the voltage frequency, phase sequence and phase angle. Everything should be
approximately same.]
• Match the phase sequence of the line and generator. If the phase sequence of the generator is
different from the source, then the phase sequence meter given an un-even movement. To
match the sequence you should interchange any two phases. This is done by phase sequence
switch. Then you can show the even movement of the phases from the sequence meter.
• Now vary the excitation by tuning excitation knob to match the generated voltage with the
system voltage.
• Now vary the frequency by tuning frequency knob to match the generated frequency with the
system frequency. Here we change the frequency by varying the coupling voltage of the VS drive
motor.
• You can see the system and generated voltage and frequency in the two digital multifunction
meters.
• Synchroscope is used to detect the phase displacement between system and generator.
• When the synchronizing conditions are satisfied then green LED of the synchroscope will be
glowing, then ON the generator breaker.

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 • Now after synchronizing condition if you reduce the excitation by tuning excitation knob you can
see the under excitation scenario happens and if it is cross the threshold value of the relay
setting it will instantly pick up and trip the breaker and alarming a signal.
• For creating unbalance condition you have to open one phase or just rotate the unbalance
switch position and observe how this protection system works.

Observation:

• Device info
• Measurement
• Phasor diagram
• Phase unbalance protection
• Under excitation protection
• Output matrix
• Over current stage
• Reverse power stage
• Harmonics
• Scaling
• Clock sync.
• You can create stator fault line to line and line to ground by pushing of push button on the top
of the Generator diagram.

Generator over voltage and over frequency protection by using microprocessor based VAMP relay.
Over frequency:
Over frequency results from the excess generation and it can easily be corrected by reduction in the
power outputs with the help of the governor or manual control.

Under frequency operation:

Under frequency occurs due to overload, generation capability of the generator increases and reduction
in frequency occurs.
Protection:
The power system survives only if we drop the load so that the generator output becomes equal or
greater than the connected load. If the load increases the generation, then frequency will drop and load
need to shut down to create the balance between the generator and the connected load. The rate at
which frequency drops depend on the time, amount of overload and also on the load and generator
variations as the frequency changes. Frequency decay occurs within the seconds so we cannot correct it
manually.

Therefore automatic load shedding facility needs to be applied.

These schemes drops load in steps as the frequency decays. Generally load shedding drops 20 to 50% of
load in four to six frequency steps. Load shedding scheme works by tripping the substation feeders to

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decrease the system load. Generally automatic load shedding schemes are designed to maintain the
balance between the load connected and the generator. The present practice is to use the under
frequency relays at various load points so as to drop the load in steps until the declined frequency
return to normal. Non-essential load is removed first when decline in frequency occurs. The setting of
the under frequency relays based on the most probable condition occurs and also depend upon the
worst-case possibilities. During the overload conditions, load shedding must occur before the operation
of the under frequency relays. In other words load must be shed before the generators are tripped.

The under-frequency relay is basically a protection for various apparatuses in a network, which in case
of a disturbance, may be separated from the rest of the system and supplied from one generator.
Operation at low frequency must be limited, also, in order to avoid dam- age on generators and
turbines.

In practice, prolonged generator operation at low frequency can only occur when a machine with its
local load is separated from the rest of the network.

The necessity of under-frequency protection has to be evaluated from knowledge of the network and
the characteristics of the turbine regulator.

Over voltage protection:

Over voltage

During the starting up of a generator, prior to synchronization, the correct terminal voltage is obtained
by the proper operation of the automatic voltage regulator (AVR). After synchronization, the terminal
voltage of the machine will be dictated by its own AVR and also by the voltage level of the system and
the AVRs of nearby machines.

Effects of over voltage fault

Generally, the rating of one machine is small in comparison with an interconnected system. It is,
therefore, not possible for one machine to cause any appreciable rise in the terminal voltage as long as
it is connected to the system. Increasing the field excitation, for example owing to a fault in the AVR,
merely increases the reactive Mvar output, which may, ultimately, lead to tripping of the machine by
the impedance relay or the V/Hz relay. In some cases, e.g. with peak-load generators and synchronous
condensers, which are often called upon to work at their maximum capability, a maximum excitation
limiter is often installed. This prevents the rotor field current and the reactive output power from
exceeding the design limits.

Protection

If the generator circuit breaker is tripped while the machine is running at full load and rated power
factor, the subsequent increase in terminal volt- age will normally be limited by a quick acting AVR.
However, if the AVR is faulty, or, at this particular time, switched for manual control of a volt- age level,
severe over voltages will occur. This voltage rise will be further increased if simultaneous over speeding
should occur, owing to a slow acting turbine governor. In case of a hydroelectric generator, a voltage

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rise of 50 - 100 % is possible during the most unfavorable conditions.

Modern unit transformers with high magnetic qualities have a relatively sharp and well-defined
saturation level, with a knee-point voltage between 1.2 and 1.25 times the rated voltage Un. A suitable
setting of the overvoltage relay is, therefore, between 1.15 and 1, 2 times Un and with a definite delay
of 1-3 s. An instantaneous high set voltage relay can be included to trip the genera- tor quickly in case of
excessive over-voltages following a sudden loss of load and generator over-speeding.

For high impedance earthed generators, the over-voltage relay is connected to the voltage between
phases to prevent faulty operation in case of earth-faults in the stator circuits.

Procedure:

• Connect the Generator with the CT’s and PT’s like above figure.[Already given]
• CT’s and PT’s output should be connected with the relay like above figure. [Already given]
• Output relay should be connected with the breaker like the diagram [Already given]
• Now configure the relay by using VAMP setting software.
• Connect the relay with the VAMP setting software by the given USB cable.
• After open the software set the required port.
• And then click connect.
• Default password is 2.
• Now you should configure CT setting, transformer setting on the scaling menu.
• Configure over voltage protection.

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 • Configure over frequency protection stage.

• Configure output relays and output LED on the output matrix menu.

• Now start the generator.

• Increase the voltage by increasing excitation.
• Observe the protection system.
• Observe how this protection system works.
• Now start the generator.
• Now increase the frequency by varying frequency knob.
• Observe over frequency protection stage.

Observation:

• Device info
• Measurement
• Phasor diagram
• Event buffer
• Scaling
• Output matrix
• Over voltage stage .
• Over frequency stage.

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Equipments:

Generator 2KW: 1
VS Drive motor 2KW: 1
Unit transformer 0.6KVA 3
R.P.M adjustment mechanism Load control device. 1
Voltage adjustment mechanism/Excitation control device. 1
Synchronizing mechanism:
1
Analog synchroscope.
Multifunction digital meter 2
Miniature CB 20A 1
Magnetic contactor 20A 4
CT 10/5 A
6
5VA
PT 400/110
2
5VA
PT 220/110 V
2
5VA
Fault simulators 10
Control switches 6A 10
Neutral grounding resistance
1
200 ohm
Generator protection relay: Vamp210 1

Circuit diagram:

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