SUBJECT : FAILURE OF 11 KV CABLE TERMINATION AT MOTOR TERMINAL.

Normally all BHEL supplied 500 MW units are equipped with one Motor Driven Boiler Feed Pump (MDBFP) and two nos. of Turbine Driven BFPs. While initial commissioning, MDBFP will be used for Boiler light up, Alkali boil out safety valve floating and Turbine rolling & synchronization.

In one of the recently commissioned 500 MW units, after synchronization and while increasing the load of the unit, the MDBFP tripped on Core balance relay earth fault protection.

On investigation, it was observed that the motor terminal in “U” phase, was found loose and symptoms of arcing has taken place. This was due to discrepancy in cable termination (customer scope) at the motor terminal. The lugs, supplied along with the cable jointing kit were not used for cable end termination at motor terminals. In that place. Some other lugs having lesser thickness (approx. 5 mm less in thickness) than the original ones have been used. This has caused improper tightness on electrical connection made at the motor end. The motor terminal bush has stud type connectors (as shown in Figure) with studs which were not having thread to the full depth, suitable only for original lugs supplied along with the bush and cable end termination kit.

When load on the motor was increased during loading the machine to 100 MW, this particular terminal connection has pulled out from the lug, leading to tripping of motor on Core balance relay protection. The has caused Boiler trip (due to drum level low) and ultimately unit trip. After identifying the problem, all the lugs in MDBFP at the motor terminal end were cut and the lugs supplied along with the cable jointing kit were made use of.