EET3086 SWITCHGEAR AND PROTECTION EET 3086 SWITCHGEAR AND PROTECTION LAB ASSESSMENT (10 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. 1/16
Transcript
EET3086 SWITCHGEAR AND PROTECTION
EET 3086 SWITCHGEAR AND PROTECTION
LAB ASSESSMENT (10 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
IntroductionOvercurrent 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
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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: _______________________________________7. Transmission grid – A: _____________________________________________8. Current transformers: ______________________________________________9. AC ammeter: ______________________________________________________
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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:
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.
o On the power supply, turn on the 24 V AC power source.
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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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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:
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?
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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.
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.______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
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.
14. Turn off the Power Supply then turn off the 24-V AC power source.
Turn off the DC Power Supply of the Protective Relaying Control Station. Remove all
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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 againsta. 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 allowsa. 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
