EET 3092 SWITCHGEAR AND PROTECTION

EEL3086 SWITCHGEAR AND PROTECTION
EEL 3086 SWITCHGEAR AND PROTECTION
Experiment 1 (5 marks)
OVERCURRENT PROTECTION OF A THREE-PHASE INDUCTION MOTOR
Objective
• To analyse the overcurrent protection of three-phase induction motors.
• To describe the operation and setting of overcurrent relay.
• To explain the motor supply voltage and current waveforms during the relay
operation.
• To evaluate the overcurrent protection performance
Introduction
Overcurrent protection is often applied to protect three-phase induction motors against
phase faults at the motor terminals, such as terminal short-circuits, terminal flashovers,
etc. The current related to these faults is usually greater than any normal operating
current of the motor. For this reason, instantaneous overcurrent. Relays with a high
current setting are normally used to obtain fast, reliable, inexpensive protection. Figure 1
is a simplified diagram showing overcurrent protection applied to a three-phase induction
motor.
Note that the secondary windings of the line current transformers are connected together at one
end to form a neutral point. This neutral point is connected to the neutral point of the three
overcurrent relays. This reduces the number of connections between the line current
transformers and overcurrent relays (four instead of six). Furthermore, this allows the same line
current transformers to be used for both overcurrent protection and earth fault protection, by
connecting the neutral point of the transformers to that of the overcurrent relays through an earth
fault relay (another overcurrent relay).
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EEL3086 SWITCHGEAR AND PROTECTION
Care must be taken when adjusting the current setting of the overcurrent relays. It must be high
enough to prevent undesired relay tripping on the initial peak of the motor starting current, which
can be many times the normal operating current of the motor. On the other hand, it must be low
enough to provide effective protection against phase faults occurring at the motor terminals. In
the case where the initial peak of the motor starting current could exceed the overcurrent relay
setting, a short time delay can be added. This, however, slightly delays fault clearance, and
may not be acceptable in certain situations.
To obtain additional information on overcurrent protection applied to three-phase induction
motors, refer to section 20.14.4, entitled ‘Terminal faults’ in the third edition of the Protective
Relays Application Guide published by GEC Alsthom Measurements Limited.
Procedure Summary
In the first part of the exercise, set up the equipment in the EMS Workstation and the
Protective Relaying Control Station.
In the second part of the exercise, connect the equipment as shown in Figures 2 and 3. In this
circuit, a three-phase induction motor is protected by an overcurrent protection system. When a
fault occurs at the terminals of the induction motor, a high fault current flows in the line current
transformers and the three-phase overcurrent relay trips. This initiates a trip current in control relay
CR1. Contact CR1-C closes to memorize the fault and light up the corresponding reset button.
Contact CR1-B opens to open contactor CR1, thereby disconnecting the induction motor from
the power source.
;

Turn on the power source and set the mechanical load so that the torque produced by the
induction motor is equal to the nominal full-toad torque. You will turn the power source on
and off a few times and observe whether or not the overcurrent system is stable when the
motor is starting.

Produce a fault at the induction motor terminals and observe the operation of the
overcurrent protection system.
EQUIPMENT REQUIRED
Protective Relaying Control Station: (record the equipment rating)
1. Three-phase overcurrent relay: ____________________________________
EMS Workstation: (record the equipment rating)
2. Power supply: ___________________________________________________
3. Interconnection module: ___________________________________________
4. Universal fault module: ____________________________________________
5. Four-pole squirrel cage induction motor: _______________________________
6. Prime mover / Dynamometer: _______________________________________
7. Transmission
grid
–
A:
_____________________________________________
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EEL3086 SWITCHGEAR AND PROTECTION
8. Current transformers: ______________________________________________
9. AC ammeter: ______________________________________________________
10. AC voltmeter: ___________________________________________________
PROCEDURE
CAUTION!
High voltages are present in this laboratory exercise! Do not make or modify any
banana jack connections with the power on unless otherwise specified!
Setting up the Equipment
1. Ensure that the Protective Relaying Control Station is connected to a three-phase power
source.
Make sure the DC Power Supply of the Protective Relaying Control Station is turned off.
Make sure that all fault switches on the Three-Phase Overcurrent Relay are set to the O (off)
position then install it in the Protective Relaying Control Station.
2. Make the following settings on the Universal Fault Module:
TD1 time delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~1 s
SST1 time interval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~3 s
SST2 time interval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 1 0 s
Note: The control knobs for adjusting the time delay and time intervals are located
on time delay relay TD1 and solid-state timers SST1 and SST2 in the Universal Fault
Module.
3. Install the Interconnection Module, Power Supply, Universal Fault Module, Four-Pole
Squirrel-Cage Induction Motor, Prime Mover / Dynamometer, Transmission Grid "A",
Current Transformers, AC Ammeter, and AC Voltmeter in the EMS Workstation.
Mechanically couple the Four-Pole Squirrel-Cage Induction Motor to the Prime Mover /
Dynamometer using the timing belt.
Make sure the Power Supply is turned off and its voltage control knob is set to the ‘0’
position. Connect the Power Supply to one of the three-phase power outlets on the back
panel of the Protective Relaying Control Station.
On the Current Transformers module, make sure that all switches are set to the I (close)
position to short-circuit the secondaries of the current transformers.
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EEL3086 SWITCHGEAR AND PROTECTION
4. Connect the LOW POWER INPUT of the Prime Mover / Dynamometer module to the 24 V
- AC output of the Power Supply.
o On the power supply, turn on the 24 V AC power source.
Overcurrent Protection of a Three-Phase Induction Motor
5. Connect the interconnection Module installed in the EMS workstation to the
interconnection panel of the protective relaying control station using the supplied
cables.
Figure 2 and 3: Connection diagram of the equipment in the EMS workstation
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EEL3086 SWITCHGEAR AND PROTECTION
Figure 4: Connection diagram of the equipment in the protective relaying control station
Connect the equipment as shown in figures 2 and 3.
Note: There are three current transformers in Figure 2. However, they are labeled CT4, CT5,
and CT6 as on the front panel of the current transformers module.
6. Make the following settings:
On the Prime Mover/ Dynamometer
MODE switch………………………………………………………………. DYNamometer
LOAD CONTROL MODE switch………………………………………………….MANual
MANUAL LOAD CONTROL knob……………………………………………...MINimum
DISPLAY switch…………………………………………………………………..TORQUE
On Transmission Grid “A”
Switch S1 …………………………………………………………………………...O (open)
On the universal Fault Module
INITIATE FAULT button………………………………………………….released position
FAULT DURATION switch……………………………………………………….0.05 – 5 s
Make sure that the current transformers are connected as shown in Figure 3 then set the
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EEL3086 SWITCHGEAR AND PROTECTION
switches of current transformers CT4, CT5, and CT6 on the current transformers module to
the O (open) position.
7. Set the current set point of the Three-Phase overcurrent relay to approximately 125% of
the nominal full-load current of the three-phase induction motor, taking into account the
transformation ration of the current transformers.
Full-load current of the induction motor: ______ Current Setting of the relay: ______
NOTE: The rating of the induction motor (nominal voltage, frequency, full-load
current, power, speed, etc.) is indicated in RATING on the front panel of the Four-Pole
Squirrel-Cage Induction Motor. Induction Motor Rating:
Nominal voltage: ________________
Frequency: _____________________
Full-load current: ________________
Power: ________________________
Speed: ________________________
Set the time delay of the Three-Phase Overcurrent Relay to 1 s.
8. Turn on the DC Power Supply of the Protective Relaying Control Station.
On Transmission Grid "A", set switch S2 to the O (open) position to open contactor CR2.
This will prevent operation of the overcurrent protection system and allow the operation of
the Three-Phase Overcurrent Relay to be observed.
9. Turn on the Power Supply while observing the motor currents indicated by the AC
Ammeter. The induction motor should start rotating.
On the Prime Mover/Dynamometer, set the MANUAL LOAD CONTROL knob so that
the mechanical load torque (indicated on the module display) is equal to 1.0 N-m (9.0 lbfin), which is the nominal full-load torque of the motor.
Turn off the Power Supply.
10. Turn on the Power Supply while observing the motor currents and the tripping indicator
(red LED) on the Three-Phase Overcurrent Relay. The induction motor should start
rotating.
Turn off the Power Supply.
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EEL3086 SWITCHGEAR AND PROTECTION
1 1 . Repeat the previous step a few times.
Is the overcurrent protection system stable when the induction motor is starting?
12. Turn on the Power Supply.
On the Universal Fault Module, depress the INITIATE FAULT button to produce a fault
at the terminals of the induction motor. While doing this, observe the circuit currents and
the tripping indicator on the Three-Phase Overcurrent Relay.
Describe what has happened.
On the Universal Fault Module, place the INITIATE FAULT button in the released
position.
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
On Transmission Grid "A", set switch S2 to the I (close) position to close contactor CR2.
This will allow operation of the overcurrent protection system.
On the Universal Fault Module, depress the INITIATE FAULT button to produce a fault at
the terminals of the induction motor. While doing this, observe the circuit currents and the
tripping indicator on the Three-Phase Overcurrent Relay.
Describe what has happened.
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
Has the fault been cleared by the overcurrent protection system?
Yes
No
Does the overcurrent protection system provide fast, effective protection against faults at
the induction motor terminals?
On the Universal Fault Module, place the INITIATE FAULT button in the released
position.
13. Turn off the Power Supply then turn off the 24-V AC power source.
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EEL3086 SWITCHGEAR AND PROTECTION
Turn off the DC Power Supply of the Protective Relaying Control Station. Remove all leads
and cables.
CONCLUSION
In this exercise, you learned that overcurrent protection is often provided to protect against phase
faults at the terminals of induction motors. You saw that instantaneous overcurrent relays with a
high current setting can be used in most cases, because the fault current caused by a terminal fault
is usually higher than any normal operating current of the motor.
_____________________________________________________________________________
_____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
___________________________________________________________________________
REVIEW QUESTIONS
1. Overcurrent protection is normally applied to induction motors to protect against
a.
Earth faults.
b.
Terminal faults.
c.
Thermal overload.
d.
Both b and c.
2. When overcurrent protection is applied to a three-phase induction motor,
a. Thermal overload protection is not injured.
b. Instantaneous overcurrent relays, with a current setting a little higher than the
motor nominal full-load current, are used.
c. Instantaneous overcurrent relays, with a current setting of approximately three to
six times the motor nominal full-load current, are used.
d. Both (a) & (b).
3. In an overcurrent protection system, connecting the secondary windings of the line current
transformers together at one end allows
a. Decreasing the number of connections between the line current transformers and
the overcurrent relays.
b. Protection of induction motors with stator windings connected in delta.
c. The same line current transformers to be used for both overcurrent protection and
earth fault protection.
d. Both (a) & (c)
COMMENTS
Write briefly your comments about this experiment.
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
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EEL3086 SWITCHGEAR AND PROTECTION
EEL 3086 POWER TRANSMISSION AND DISTRIBUTION
FACULTY OF ENGINEERING, MMU, CYBERJAYA
LAB REPORT
ID
NAME:
EXPERIMENT DATE:
EXPERIMENT TITLE:
______________________________________________________________________________
________________________________________________________________________
OBJECTIVE:
_____________________________________________________________________________
_____________________________________________________________________________
Instruments/Software required: (refer labsheet and lab equipment)
NAME
RATING/RANGE/DETAILS
NUMBER
Circuit/Schematic Diagram: (draw a neat sketch of diagram and indicate ratings)
EXPERIMENTAL PRECAUTIONS: (Precautions related to experiment alone)
Experimental/Design Calculations: (show detailed calculations)
EXPERIMENTAL RESULTS: (Refer labsheet)
EXPERIMENTAL RESULTS ANALYSIS: (plot graph and analyze results)
CONCLUSIONS: (Discuss whether experimental results met the objectives or not)
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EEL3086 SWITCHGEAR AND PROTECTION
EEL 3086 SWITCHGEAR AND PROTECTION
EXPERIMENT 2 (5 Marks)
DIFFERENTIAL PROTECTION OF A THREE-PHASE TRANSFORMER
Objective
•
To analyse the differential protection scheme as applied to a three-phase power
transformer
•
To describe the operation and setting of differential protection.
•
To explain the transformer supply voltage and current waveforms during the
differential relay operation.
•
To evaluate the differential protection performance.
Introduction
The differential protection scheme can be used to protect both the primary and secondary
windings of a three-phase transformer against earth faults and phase-to-phase faults. This is
possible because the efficiency of the power transformers is high and the magnetizing
current is negligibly small. In a differential protection scheme a circuit compare the current
entering the protective equipment to the current leaving the equipment, in each phase.
Any difference of current of sufficient magnitude operates a relay, which in turn indicates fault
clearance. Figure 1 shows a simplified diagram of a single-phase differential protection
scheme.
The currents entering and leaving the protected equipment (Ipin and lpout) are sensed through
two identical current transformers. When there is no fault in the protected equipment,
currents Ipin and Ipout are equal and the currents at the transformer secondaries are also equal
(Isin = Isout) because the current transformers are identical. When the current transformers
are connected with the polarities indicated in Figure 1, the secondary currents flow round the
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EEL3086 SWITCHGEAR AND PROTECTION
circuit and no current flows in the coil of the protective relay (I R = 0), which can be an
overcurrent relay. However, when a fault occurs in the protected equipment, currents lpin
and lpout are no longer equal. Consequently, currents Isin and Isout are also no longer equal.
The current resulting from the difference between these two currents (Isin – Isout) flows in
the protective relay coil. This trips the protective relay, there by, initiating fault clearance.
Similar differential protection scheme can be employed for the protection of
transformers. In this case, when the turns ratio of the protected transformer is not unity,
the primary and secondary currents are different, and thereby, current transformers with
different turns ratio are required for the CT secondary currents to be equal and the residual
current IR to be zero under no fault condition. When protecting three-phase power
transformers, some additional considerations must be taken into account:
• There is a 30° phase shift between the primary and secondary currents of a three-phase
power transformer that is connected delta-wye or wye-delta and supplies a balanced load.
• When a three-phase power transformer is connected delta-wye or wye-delta, the zero sequence
current on the wye side of the power transformer has no replica on the delta side.
The 30° phase shift must be compensated and the zero sequence current on the wye side of the
power transformer must be eliminated, for the CT secondary currents to be equal under no fault
condition. This is achieved by proper connections of the current transformer secondary
windings. A general rule for connecting the current transformers states that the CT
secondary windings should be connected in delta when the power transformer windings are
connected in wye, and vice versa. Figure 2 shows typical connections of the current
transformers for three-phase power transformers connected delta-wye and delta-delta. Note that it
is assumed that the ratios of the current transformers have been selected so that the secondary
currents supplied by the two groups of current transformers are equal, thereby ensuring balance
of the currents in the differential protection system.
In practice, it is very difficult to maintain perfect balance of the currents in a differential
protection system protecting a three-phase power transformer. This is mainly due to the
following factors:
• Change in the power transformer turns ratio (on transformers with a tap-changing facility).
• Current transformer mismatch (difficulty in having current transformers with ratios that
perfectly balances the differential protection system).
• Transformer magnetizing current.
All these factors unbalance the differential protection system and produce a residual current IR
in the differential relay coil. This residual current increases as the line currents flowing through
the three-phase power transformer increase. Therefore, the current setting of the differential
relay must be increased to prevent undesired relay tripping, thereby reducing the system
sensitivity. Differential relays with bias coils are often used in transformer differential
protection systems to reduce the negative effect of current unbalance on the system
sensitivity. Figure 3 shows the bias characteristic of a differential relay. This characteristic
shows that the current required to trip the differential relay (differential operating current)
increases as the current flowing through the transformer increases. Note that, in general, the
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EEL3086 SWITCHGEAR AND PROTECTION
sensitivity of transformer differential protection systems is less than that achieved in differential
protection systems protecting the stator windings of a synchronous generator.
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EEL3086 SWITCHGEAR AND PROTECTION
Figure 3: Typical bias characteristics of a differential relay
Transformer magnetizing inrush, discussed in the first exercise of this unit, is another source
of unbalance in transformer differential protection systems. This is because the magnetizing
inrush current in the energized winding of a transformer is not replicated in the other windings
of the transformer. This appears as a current unbalance to the differential protection system,
which superficially, cannot be distinguished from a current unbalance caused by a fault in the
transformer. When magnetizing inrush is severe, the current unbalance may easily exceed
the current required to trip the differential relay and cause undesired disconnection of
the power transformer. Fortunately, transformer magnetizing inrush is a transient
phenomenon occurring on transformer energization. Undesired transformer disconnection
can therefore be avoided by adding a short time delay to prevent the differential protection
system from tripping on transformer magnetizing inrush.
Note that after the magnetizing inrush, the magnetizing current stabilizes to a very low value.
This current, however, causes a slight current unbalance that is stable under normal operating
conditions. To preserve the system stability, this slight current unbalance must be taken into
account when setting the differential relay operating current.
To obtain additional information on transformer differential protection, refer to section 16.7,
entitled "Differential protection", in the third edition of the Protective Relays Application
Guide published by GEC Alsthom Measurements Limited.
Procedure Summary
In the first part of the exercise, set up the equipment in the EMS Workstation and the
Protective Relaying Control Station.
In the second part of the exercise, connect the equipment as shown in Figures 4 and 5. In
this circuit, power transformers connected delta-wye are protected by a differential
protection system which mainly consists of a current sensitive relay and line current
transformers. When a fault occurs in the power transformers, the overcurrent) relay trips.
This energizes time delay relay TD1. Once the time delay is elapsed, contact TD1-A closes
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EEL3086 SWITCHGEAR AND PROTECTION
to initiate a trip current in the coil of control relay CR1. Contact CR1-C closes to memorize
the fault and light up the corresponding reset button. Contact CR1-B opens to open
contactor CR2, thereby disconnecting the power transformers from the three-phase power
source.
Open contactor CR3 to prevent operation of the differential protection system. Check whether or
not the differential protection system is perfectly balanced when no load is applied to the power
transformers. Adjust the current setpoint of the overcurrent relay.
With the power transformers supplying power to a balanced three-phase load, initiate earth
faults at the primary and secondary windings of the power transformers and observe what
happens in the differential protection system.
You will close contactor CR3 to allow operation of the differential protection system. You will
verify whether or not the differential protection system is stable on transformer magnetizing
inrush. You will initiate earth faults and a phase-to-phase fault in the power transformers, and
observe the operation of the differential protection system.
Equipment Required Protective Relaying Control
Station: (record the equipment rating)
1. AC/DC current sensitive relay: __________
2. EMS: ____________________________
Workstation (record the equipment rating)
14. Power supply: _________________________
15. Interconnection module: _________________
16. Universal fault module: __________________
17. Faultable transformers: __________________
18. Transmission grid – A: _________________
19. Current transformers: ___________________
20. Resistive loads: ________________________
21. AC ammeter: __________________________
22. AC voltmeter: _________________________
PROCEDURE
CAUTION!
High voltages are present In this laboratory exercise! Do not make or modify
any banana jack connections with the power on unless otherwise specified!
Setting Up the Equipment
5. Ensure that the Protective Relaying Control Station is connected to a three-phase power
source.
Make sure the DC Power Supply of the Protective Relaying Control Station is turned off.
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EEL3086 SWITCHGEAR AND PROTECTION
Install AC/DC Current sensitive relay in the Protective Relaying Control Station.

Make the following settings on the Universal Fault Module:
TD1 time delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -1 s
SST1 time interval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~5 s
SS12 time interval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ 1 0 s
Note: The control knobs for adjusting the time delay and time intervals are
located on time delay relay TD1 and solid-state timers SST1 and SST2 in the
Universal Fault Module.
3.Install the Interconnection Module, Power, Supply, Universal Fault Module, Faultable
Transformers, Transmission Grid "A", Current Transformers, Resistive Load, AC
Ammeter, and AC Voltmeter in the EMS Workstation.
Make sure the Power Supply is turned off and its voltage control knob is set to the O
position. Connect the Power Supply to one of the three-phase power outlets on the back
panel of the Protective Relaying Control Station.
On the Current Transformers module, make sure that all switches are set to the I (close)
position to short-circuit the secondary’s of the current transformers.
Differential Protection of a Three-Phase Power Transformer
6. Connect the Interconnection Module installed in the EMS Workstation to the
Interconnection Panel of the Protective Relaying Control Station using the supplied cables.
Connect the equipment as shown in Figures 4 and 5
Note: Since a single AC/DC Current Sensitive Belay is available, terminals A2
and A3 are connected to terminal A4 to avoid disturbing the operation of the
differential protection system.
5.Make the following settings:
On the Faultable Transformers
Transformer T1 Fault Switches (FS1 to FS3).............. O
Transformer T3 Fault Switches (FS1 to FS3) . . . . . . . . . . . . . . O
On Transmission Grid "A"
Switch S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I (close)
Switch S2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O (open)
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EEL3086 SWITCHGEAR AND PROTECTION
Figure 4: Connection diagram of the equipment in the EMS workstation
Figure 5: Connection diagram of the equipment in the protective relaying control station
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EEL3086 SWITCHGEAR AND PROTECTION
On the AC/DC Current Sensitive Relay
INPUT switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . … … … AC
MODE switch . . . . . . . . . . . . . . . . . . . . . . . . OVER CURRENT
Current setpoint . . . . . . . . . . . . . . . . . . . . minimum (fully CCW)
Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . … … . . . ~7.5%
On the Universal Fault Module
INITIATE FAULT button . . . . . . . . . . . . . . . . . released position
FAULT DURATION switch . . . . . . . . . . . . . . . . . . … . 0.05 - 5 s
Make sure that the current transformers are connected as shown in Figures 4 and 5
then set the switches of current transformers CT1 to CT6 on the Current Transformers
module to the O (open) position.
6.On Control Relays 2 of the Protective Relaying Control Station, set the time delay of control relay
TD1 to approximately 2 s.
Note: -Access to the time delay adjustment knob of control relay TD1 is
through a panel on top of the Protective Relaying Control Station.
Turn on the DC Power Supply of the Protective Relaying Control Station.
On Transmission Grid "A", set switch S3 to the O (open) position to open contactor
CR3. This will prevent operation of the differential protection system and allow the
operation of the AC/DC Current Sensitive Relay to be observed.
7.On the Resistive Load module, set all toggle switches to the O (open) position to
temporarily disconnect the load (resistors R1, R2, and R3) from the secondary windings
of the power transformers.
Turn on the Power Supply and set the voltage control knob so that the line-to-neutral
voltage at the secondary windings of the power transformers
Record the circuit voltages and currents in the following blank spaces.
E1 = ___ V
I1 = ___ A
E2 =___V
I 2 = ___ A
E3 = ___ V
I3 = ___ A
Is the differential protection system perfectly balanced? Briefly explain?
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EEL3086 SWITCHGEAR AND PROTECTION
11. Adjust the current setpoint of the AC/DC Current Sensitive Relay to approximately
110% of the residual current (I3) measured in the previous step. To do so, slowly turn the
current setpoint adjustment knob clockwise until the tripping indicator (red LED) of the
AC/DC Current Sensitive Relay turns OFF.
On the Resistive Load module, set the resistance of resistors R1, R2, andR 3 to the
value indicated in Figure 2
4. On the Faultable Transformers, set fault switch FS1 of transformer T1 to the I position to
insert an earth fault near the middle of the primary winding of transformer T1. While
doing this, observe the circuit currents and the tripping indicator on the AC/DC
Current Sensitive Relay.
Record the circuit voltages and currents in the following blank spaces.
E1 = ______V
I1 = ______A
E2 = ______V
I2 = ______A
E3 = ______V
I3 = ______A
Describe what happens when an earth fault occurs near the middle of one of the power
transformer primary windings.
On the Faultable Transformers, set fault switch FS1 of transformer T1 to the O position
to remove the fault.
10.On the Faultable Transformers, set fault switch FS3 of transformer T1 to the I position to
insert an earth fault near the neutral end of the secondary winding of transformer T1.
While doing this, observe the circuit currents and the tripping indicator on the AC/DC
Current Sensitive Relay.
Record the circuit voltages and currents in the following blank spaces
E1 = ______V
I1 = ______A
E2 = ______V
I2 = ______A
E3 = ______V
I3 = ______A
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EEL3086 SWITCHGEAR AND PROTECTION
Describe what happens when an earth fault occurs near the neutral end of one of the
power transformer secondary windings.
On the Faultable Transformers, set fault switch FS3 of transformer T1 to the O position to
remove the fault.
11.On Transmission Grid "A", set switch S3 to the I (close) position to close contactor CR3. This
will allow operation of the differential protection system.
4. On Transmission Grid "A", set switch S1 to the O (open) position to open contactor CR1
and remove power from the power transformers.
Energize the power transformers by setting switch S1 on Transmission Grid "A" to the I
(close) position. While doing this, observe the circuit currents and the tripping indicator on
the AC/DC Current Sensitive Relay.
13. Repeat the previous step at least ten times.
Does the residual current (I3) sometimes exceed the current setpoint of the AC/DC Current
Sensitive Relay on transformer energization?
No
Ye
Is thes differential protection system stable on transformer energization?
Yes
No
Note: The AC/DC Current Sensitive Relay is fairly insensitive to the residual
current resulting from the transformer magnetizing inrush. A time delay relay is,
however, included in the differential protection system to prevent transformer
disconnection in case a high magnetizing inrush would trip the AC/DC Current
Sensitive Relay.
3. On Transmission Grid "A", set switch S1 to the I position to close contactor CR1 and
energize the power transformers.
On the Faultable Transformers, set fault switch FS3 of transformer T1 to the I position to
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EEL3086 SWITCHGEAR AND PROTECTION
insert an earth fault near the neutral end of the secondary winding of transformer T1. While
doing this, observe the circuit currents and the tripping indicator on the AC/DC Current
Sensitive Relay.
Describe what has happened.
Has the fault been cleared by the differential protection system?
On the Faultable Transformers, set fault switch FS3 of transformer T1 to the O position to
remove the fault.
e. On the Faultable Transformers, set fault switch FS1 of transformer T1 to the I position to
insert an earth fault near the middle of the primary winding of transformer T1. While doing
this, observe the circuit currents and the tripping indicator on the AC/DC Current Sensitive
Relay.
Describe what has happened.
Has the fault been cleared by the differential protection system?
On the Faultable Transformers, set fault switch FS1 of transformer T1 to the O position to
remove the fault.
16.On Control Relays 1 of the Protective Relaying Control Station, press the RESET button of
control relay CR1 to reset the differential protection system.
On the Universal Fault Module, depress the INITIATE FAULT button to produce a phaseto-phase fault at the secondary windings of the power transformers. While doing this,
observe the circuit currents and the tripping indicator on the AC/DC Current Sensitive
Relay.
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EEL3086 SWITCHGEAR AND PROTECTION
Describe what has happened.
Has the fault been cleared by the differential protection system?
Does the differential protection system protect the power transformers against earth
faults as well as phase-to-phase faults? Briefly explain.
Turn off the Power Supply.
17. Turn off the DC Power Supply of the Protective Relaying Control Station. Remove
all leads and cables.
CONCLUSION
In this exercise, you learned that differential protection can be used to protect the primary and
secondary windings of a three-phase power transformer against earth faults and phase-to-phase faults.
You also learned that the sensitivity of transformer differential protection is limited by several factors
explained in the discussion of this exercise. You saw that transformer magnetizing inrush unbalances
the circulating current circuit of a differential protection system, and may cause undesired
transformer disconnection. You saw that transformer disconnection on a magnetizing inrush can be
prevented by adding a time delay relay in the differential protection system.
REVIEW QUESTIONS
1. A differential protection system protects




•
Power transformers against magnetizing inrush.
The primary and secondary windings of a power transformer against earth faults
Power transformers against phase-to-phase faults
Both b and c
In general, when using differential protection to protect a three-phase power
transformer, the secondary windings of the line current transformers should be connected
in
• Wye when the power transformer windings are connected in delta, and vice-versa.
11. Wye on-both sides of the power transformer
12. Delta on both sides of the power transformer
13. None of the above.
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EEL3086 SWITCHGEAR AND PROTECTION
3. In transformer differential protection systems, a time delay relay can be used to
o Increase the sensitivity to earth faults occurring on the secondary windings of power
transformers
o Compensate the 30° phase shift of the line currents in power transformers connected
delta-wye or wye-delta
o Prevent undesired transformer disconnection on transformer magnetizing inrush
d. None of the above.
CONCLUSIONS
Write briefly your own conclusions about this experiment and the theory you have understood
and concept learned
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EEL3086 SWITCHGEAR AND PROTECTION
EET 3086 SWITCHGEAR AND PROTECTION
LAB REPORT
ID
NAME:
EXPERIMENT DATE:
EXPERIMENT TITLE:
_____________________________________________________________
_____________________________________________________________
OBJECTIVE:
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
Instruments/Software required: (refer labsheet and lab equipment)
NAME
RATING/RANGE/DETAILS
NUMBER
Circuit/Schematic Diagram: (draw a neat sketch of diagram and indicate ratings)
EXPERIMENTAL PRECAUTIONS: (Precautions related to experiment alone)
Experimental/Design Calculations: (show detailed calculations)
EXPERIMENTAL RESULTS: (Refer labsheet)
EXPERIMENTAL RESULTS ANALYSIS: (plot graph and analyze results)
CONCLUSIONS: (Discuss whether experimental results met the objectives or not)
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