Motor Protection – Over Current and Earth Fault:
Motor Protection – If the motor is heavily loaded beyond its capacity, then it is said to be in an overloaded condition, in which it draws heavy current from the supply and there will be a simultaneous rise in temperature of winding and deterioration of the insulation resulting in damage of winding. Hence, the motor must be protected against this mechanical overload. Thermal overload relays, over current relays or miniature circuit breakers with built-in trip coils, may be used.
Due to Excessive temperature rise, the insulation may get damaged which may lead to stator earth fault or stator phase to-phase fault. For low rating motors fuse, MCB and MCCB provide sufficient protection against fault while for large motors, overcurrent, earth fault protection and differential protection are used. This blog discusses the Overcurrent and Earth Fault Protection of motors.
Over Current Protection:
51- IDMT Over Current Protection
Pickup Current (I >):
Time Multiplier Settings (T >):
TMS is selected above the starting characteristics of all the three voltages (100%, 80%, 110% voltage) of the motor to avoid mal-operation during starting and below the motor hot withstand curve.
Manual Calculation:
Rated Voltage : 3.3 kV
CT Ratio : 350/1A
Protection Relays : ABB REF615
Motor Full Load Current : 262.7A
Non-directional over current protection 51 (I>) REF615 relay:
Required Current Setting = FLA x 110% / CT Primary Amps
Pickup = 0.83 (290.5A)
Curve = IEC Extremely Inverse
TMS = 1.5 (From ETAP Graph)
Instantaneous Phase protection 50 (I>>) REF615 relay:
Required Current Setting = LRA x 130% x 1.6 / CT Primary Amps
Pickup = 10.15 (3552A)
DT = 0.02
Non-Directional Earth fault protection (51N) REF615 relay:
Required Current Setting = 10% of Load Current / CT primary amps
Pickup = 0.08 (28A)
Curve = IEC Normal Inverse
TMS = 0.1
Instantaneous Earth fault protection (50N) REF615 relay:
Required Current Setting = 100% of Load Current / CT primary amps
Pickup = 1 (350A)
DT = 0.02