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EET3086 SWITCHGEAR AND PROTECTION
EET 3086 SWITCHGEAR AND PROTECTION
LAB ASSESSMENT
(Report – 10 Marks)
Total 10 Marks = 5% Course work 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 threephase 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
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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).
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: _______________________________________
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7. Transmission grid – A: _____________________________________________
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.
4. Connect the LOW POWER INPUT of the Prime Mover / Dynamometer module to the 24
V - AC output of the Power Supply.
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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 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 switches of current transformers CT4, CT5, and CT6 on the current transformers
module to the O (open) position.
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EET3086 SWITCHGEAR AND PROTECTION
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 FourPole 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.
1 1 . Repeat the previous step a few times.
Is the overcurrent protection system stable when the induction motor is starting?
Ye
N
s
o
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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.
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13. 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.
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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?
N

Yes
o
On the Universal Fault Module, place the INITIATE FAULT button in the released
position.
14. 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.
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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)
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COMMENTS
Write briefly your comments about this experiment.
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EET 3086 POWER TRANSMISSION AND DISTRIBUTION
FACULTY OF ENGINEERING, MULTIMEDIA UNIVERSITY, CYBERJAYA
LAB REPORT (10 Marks)
ID
NAME:
EXPERIMENT DATE:
EXPERIMENT TITLE: ______________________________________________________________________________
______________________________________________________________________________
OBJECTIVE:
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
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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)
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EET3086 SWITCHGEAR AND PROTECTION
EXPERIMENTAL PRECAUTIONS: (Precautions related to experiment alone)
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_____________________________________________________________________________
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Experimental/Design Calculations: (show detailed calculations)
EXPERIMENTAL RESULTS: (Refer labsheet)
EXPERIMENTAL RESULTS ANALYSIS: (plot graph and analyze results)
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CONCLUSIONS: (Discuss whether experimental results met the objectives or not)
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