Eet 3092 Switchgear and Protection

Eet 3092 Switchgear and Protection

EEL3086 SWITCHGEAR AND PROTECTION

EEL 3086 SWITCHGEAR AND PROTECTION

LAB ASSESSMENT

OVERCURRENT PROTECTION OF A THREE-PHASE INDUCTION MOTOR

Objective

• To analysethe 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.

1/11

EEL3086 SWITCHGEAR AND PROTECTION

1/11

EEL3086 SWITCHGEAR AND PROTECTION

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).

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 inFigures 2and 3. In this circuit, a three-phase induction motor is protected byan overcurrent protection system. When a fault occurs at the terminals of theinduction motor, a high fault current flows in the line current transformers and thethree-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 resetbutton. Contact CR1-B opens to open contactor CR1, thereby disconnecting theinduction 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)

  1. Power supply: ______
  2. Interconnection module: ______
  3. Universal fault module: ______
  4. Four-pole squirrel cage induction motor: ______
  5. Prime mover / Dynamometer: ______
  6. Transmission grid – A: ______
  7. Current transformers: ______
  8. AC ammeter: ______
  9. 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.

  1. 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.

  1. 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.

  1. Connect the LOW POWER INPUT of the Prime Mover / Dynamometer module to the 24 V - AC output of the Power Supply.
  • On the power supply, turn on the 24 V AC power source.
Overcurrent Protection of a Three-Phase Induction Motor
  1. 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

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.

  1. 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.

  1. 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.

  1. 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.

  1. 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.

  1. 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.

11.Repeat the previous step a few times.

Is the overcurrent protection system stable when the induction motor is starting?

1/11

EEL3086 SWITCHGEAR AND PROTECTION



1/11

EEL3086 SWITCHGEAR AND PROTECTION

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.

______

  1. 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?

1/11

EEL3086 SWITCHGEAR AND PROTECTION



1/11

EEL3086 SWITCHGEAR AND PROTECTION

On the Universal Fault Module, place the INITIATE FAULT button in the released position.

  1. 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 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

  1. Earth faults.
  2. Terminal faults.
  3. Thermal overload.
  4. Both b and c.
  1. When overcurrent protection is applied to a three-phase induction motor,
  2. Thermal overload protection is not injured.
  3. Instantaneous overcurrent relays, with a current setting a little higher than the motor nominal full-load current, are used.
  4. Instantaneous overcurrent relays, with a current setting of approximately three to six times the motor nominal full-load current, are used.
  5. Both (a) & (b).
  6. In an overcurrent protection system, connecting the secondary windings of the line current transformers together at one end allows
  7. Decreasing the number of connections between the line current transformers and the overcurrent relays.
  8. Protection of induction motors with stator windings connected in delta.
  9. The same line current transformers to be used for both overcurrent protection and earth fault protection.
  10. Both (a) & (c)

COMMENTS

Write briefly your comments about this experiment.

______

EEL 3086 POWER TRANSMISSION AND DISTRIBUTION

FACULTY OF ENGINEERING, MULTIMEDIAUNIVERSITY, 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)

1/11

Top View