Page 329 - Practical Power System and Protective Relays Commissioning

Protection Relays Settings Chapter | 19 323

19.8.2.5 Overload Protection
Thermal replica relays are used to protect the motor from overloads using a
separate relay in each phase of the motor. Thermal relays are also used with
thermal replica relays (resistance temperature detectors embedded in
machine winding) to give complete protection for the motor.

19.8.2.6 Low-Voltage Protection
Undervoltage time delay relay is used as follows:
V set 5 0.5 V nominal
Action: Trip after a few seconds.

19.8.2.7 Phase Rotation Protection
A negative-phase sequence voltage relay is used.
v 2 set 5 10% V 2

19.8.2.8 Phase Unbalance Protection
Detect I 2 and set as follows:
I set 5 0.15 I N
To protect the motor against single-phasing condition:
2
I 2 t 5 40 in general

19.8.2.9 Out-of-Step Protection for Synchronous Motors

A relay is used to detect the condition in which the motor draws a heavy
current in a low-power factor. Therefore, this relay will be a power factor
relay and will issue a signal to disconnect the field winding and let the motor
run as an induction motor.

19.8.2.10 Loss of Excitation Protection for Synchronous Motors
Undercurrent relay is inserted in a field circuit.