TRIPPING FOR NO FAULT & NO TRIPPING FOR FAULT

TRIPPING FOR NO FAULT

Transformer Earth fault protection was operated when load increasing.
Then investigation started and team checked all power cables transformer IR value. Then primary injection test was done and found "B" phase CT secondary wire loose connection. " B" phase current not reached to relay and residual current developed. Residual current flow  through neutral coil and earth fault operated during load condition only.

 NO TRIPPING FOR FAULT

Transformer Earth fault protection was operated during fault condition. After investigation found the secondary wire going to neutral protection coil was ground and residual current taken essay path to return. So neutral trip coil not getting the fault residual current to trip the breaker.

TRIPPING CIRCUIT PROBLEM

380KV HYUNDAI 380KV CB TRIPPING CIRCUIT PROBLEM

During trip the breaker from trip relay (LOR) Pole Discrepancy Stage-1 was operated.
After investigation the following point were observed.

1.The overall trip circuit length more than 300 mts.
2.The resistance of the circuit loop 14 ohms. ( Resistance measurement refer drawing)
3. Resistance of trip coil unit ( Coil + Stabilizing resistance + Capacitor ) 40 ohms.
4. Trip circuit cable size 2.5 Sqmm.
5. Trip circuit voltage used 127 dc.

As per stranded minimum operating voltage of trip coil is 70 v dc.

Due to more loop resistance and coil resistance voltage drop was more in the trip circuit. So during tripping operation ( by operating last lockout of the circuit) trip coil not getting enough voltage to trip the breaker and PD operated immediately ( Pole Discrepancy Stage-1 lock out relay very near to the trip coil).

We are decided to reduce the loop resistance of the trip circuit by increasing the cable size upto 12 Sq.mm. ( ( 2 x 6Sq.mm).

The loop resistance was reduced upto 8 to 9 ohms from 14 ohms and circuit was worked properly.

But this is the temporary  solution and permanent solution is reduce the trip coil unit resistance from 40 ohms to below 20 ohms.

 

Resistor & Capacitor unit