Protecciones Electricas

Exercise Phase Reversal Protection of a Three-Phase Induction Motor
EXERCISE OBJECTIVE

3-3

When you have completed this exercise, you will be familiar with phase reversal protection of three-phase induction motors. DISCUSSION The direction of rotation of three-phase induction motors is determined by the phase sequence of the voltages applied to the stator windings. Most motor applicationssuch as fans, centrifugal pumps, hoists, cranes, conveyors, etc. are designed to be driven in only one direction. Reversal of shaft direction of rotation can be very dangerous and expensive in terms of safety and equipment damage. Under normal circumstances, the power system can be expected to maintain a constant reliable phase sequence. However, there are occasions when the phase sequence couldbe changed inadvertently. For example, after a substation power failure due to a defective transformer, the new transformer could be accidentally rewired with a phase reversal. Another case might be after a maintenance shutdownto replace old equipment: an accidental phase reversal could take place during rewiring of the motor or its controls. Phase reversal protection is applied to an inductionmotor when injuries to maintenance personnel or damages to the mechanicalload may occur. It is in no way intendedfor protectingthe induction motor. Figure 3-10 is a simplified diagram showing phase reversal protection of a three-phase induction motor. In this circuit, a phase sequence relay monitors the supply voltages through potential transformers. When correct phase sequenceis detected, the phasesequence relay allows an ac circuit breaker (a three-phase contactor) to be energized, thus connecting the induction motor to the power source. On the other hand, when reverse phase sequence is detected, the phase sequence relay prevents energizationof the circuit breaker,and no power can be supplied to the motor. Note that the primary windings of the potential transformers must be connecteddirectly to the three-phasepower sourceto allow the supply voltages to be monitored when the circuit breaker is not energized.

3-29

Phase Reversal Protection of a Three- Phase Induction Motor
INDUCTION MOTOR STATOR WINDINGS A

B

C
I I I I POTENTIAL TRANSFORMERS

O
I I I I I I

CIRCUIT (DEVICE

BREAKER TYPE 52)

J

PROTECTIVE RELAY (DEVICE TYPE 47)

Figure 3-10. Simplifieddiagram of phase reversal protection applied to a three-phase induction motor.

Procedure Summary In the first part of the exercise, you will set up the equipment in the EMS Workstation and the Protective Relaying ControlStation. In the second part of the exercise, you willconnect the equipment as shown in Figures 3-11 and 3-12. In this circuit, a three-phase induction motor is protected againstphase reversal using a phase balance/sequence relay. This relay detects reverse phase sequence as well as unbalance of the supply voltages. With the correct phase sequence on power up, the output relay of the phase balance/sequence relay is energized (the relay normally-open contact closes). When switch S2 (motor starting switch) is set to the I (close) position, contactor CR2 closes and energizescontactor CR1, thereby applying the supply voltages to the motor. However, when a reverse phase sequence is detected on power up, the output relay of the phase balance/sequence relay remains "deenergized" (the relay normally-opencontact remains open). As a result, contactor CR1 remainsopenwhencontactorCR2is energizedusingswitch S2, thereby preventing the supply voltages from being applied to themotor. Furthermore,once the time delay of time delay relay TD1 has elapsed,current flows in control relay CR1 through the normally-closecontact of the phase balance/sequence relay.This lightsupthe CR1 RESETbuttonto indicatethat the phasesequenceis ¡ncorrect.

3-30

Phase Reversal Protection of a Three- Phase Induction Motor'
You will observethe operation of the phase reversal protection...