EET 3092 SWITCHGEAR AND PROTECTION - …foe.mmu.edu.my/lab/lab...

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EEL3086 SWITCHGEAR AND PROTECTION 1 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).

Transcript of EET 3092 SWITCHGEAR AND PROTECTION - …foe.mmu.edu.my/lab/lab...

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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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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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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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 10 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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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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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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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 lbf-

in), 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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11 . 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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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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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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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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circuit and no current flows in the coil of the protective relay (IR = 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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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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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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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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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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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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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 I2 = ___ A

E3 = ___ V I3 = ___ A

Is the differential protection system perfectly balanced? Briefly explain?

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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, andR3 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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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?

Is the 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

Yes

No

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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 phase-

to-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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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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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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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)