RELAY COORDINATION PART -III

Dead End Equipment (Motor Protection)- Motor Negative Phase Sequence Settings

These settings are based on following Negative Phase Sequence (NPS) withstand values

• Continuous NPS rating I2 p.u.
• Short time NPS I2
• I2 2 t rating (K).

Motor continuous I2 withstand pick up should be set based on maximum unbalance voltage that the motor can withstand as this determines the continuous I2  flow in the motor.

For motors above 2500 kW the NPS thermal damage limit as I2 2 t constant K is taken as 15 sec when data has to be assumed.

For motors below 2500 kW it is 10 sec when data has to be assumed.

Actual I2 2 t constant K value should be obtained from vendor.

Assuming 2% unbalance in the supply voltage and LRC to be 6 times of FLC, the continuous withstand negative sequence of the motor, I2 would be 6×2=12%.

• Typical negative sequence relay setting shall be
• The I2 current pick up of 15%, with I2 2 t curve set below

                I2 2 t  = K (Typically K = 15) withstand curve of motor.

• For faster definite time trip, the current pick up of 57% of full load current with time delay of 1 sec, this delay time is required for the coordination with downstream relay.

Dead End Equipment (Motor Protection)- Motor Negative Phase Sequence Settings