Generator became Synchronous Motor

It was beginning from 125V DC power supply loss. Central Control Room (CCR) got blank indication about all status equipment. The sequence of event (SOE) shown that Unit got MFT (master fuel trip) by all ID and FD trip, but actually they were still in service.

The SOE only shown on the boiler area so that was way the operator concentrate or first checking on the boiler area and they found that fans group (ID Fan & FD Fan) and Pulverizers still in service (PA Fan and coal feeders already trip from MFT signal) and we initiated to stop all pulverizers via 10kV motor CB.

The 2nd checking was on the turbine area and found that generator seal oil pump was in service. That was good news cause we didn't necessary to hydrogen blow off.

The turbine and generator as usually still in rotating caused by inertia force remaining and will slowly decrease to 4 RPM (tuning gear operation). We didn't realize yet that we had on special case.

The 3rd checking was How effect to the Unit if we had loss of 125V DC power? So we must found out where the 125 DC use for and normalize the 125 DC Power.

During the tracing check we found that the speed of turbine still steady at 3000RPM. Further tracing check found that Generator CB (GMCB) still in close position, it was meaning the generator became synchronous motor. We have tried to open GMCB but didn't work due to no power.

What is the different about a synchronous and asynchronous motor? A synchronous motor runs at a speed which is synchronised to the frequency of the AC voltage that supplies it. For example if the motor is supplied at a frequency of 60Hz then it will run at either 60, 30, 20 etc revs/second, depending on the number of magnetic poles (2, 4, 6 respectively) that it has (by design).

An asynchronous motor generally runs slower than the above with a speed that usually varies somewhat with the amount of mechanincal load that it is driving.

Read more: http://www.physicsforums.com

Only one way remaining to de-energize the generator and transformer either via 500kV substation but the unit became blackout status. It was done. After generator de-energize, field CB also opened safely from local by spring mechanical.

During the unit in blackout status, the generator thyristor cooling fan was stop either. That was consequence that our taken for thyristor fuses condition was 75% found broken.

Any other defect correlation with generator became synchronous motor are not found.