Over Frquency happened at a windy winter night

khaled hamza · #1 (**permalink**) 12-15-2010

Over Frquency happened at a windy winter night

at 3:00 am , A sudden frequency fluctuation happened , that cause the MW output fluctuate with the grid frequency , Unit 7 was at Coordinated Mode ( Boiler Master – Auto , Turbine Set – Auto ) , The load fluctuated accordingly to the frequency from 105 MW to 90 MW, and increased to 105 MW.
After a little time, frequency reached 50.1 HZ – therefore the MW output reached 20 MW,
Also, Boiler Master responds by reducing Air Flow and Fuel Gas Flow for the boiler. The load still decreased until it reached 2 MW, and to (-2 MW), without coming the signal of Reverse Power.

At this instant we trip the turbine manually.
We wanted to conserve the boiler to be in service, but HP-TBV opens widely within 30 sec., and sucking all drum level to +267 mm, resulting MFT Protection to the boiler.

The first out indication of MFT at BMS was (Drum level HH). “

There were a lot of discussion and arguments about what is the cause of this accidents , and what should operators do ?

I attached A chart Table shows the different parameters changed at each same time !! , If any one has an opinion about what happened , plz tell me !!

khaled hamza · #2 (**permalink**) 12-17-2010

After I discussed what happened the outage three days ago - the situation I posted previously - I found an explanation for that: -

In winter days , Electrical loads are decreasing here in Egypt especially at night , therefore the frequency increases from 50 HZ .
the Dispatch should decrease the loads of the generating units to limit the decrease of frequency , and the generating unit's Operator should take care of such these situations.

At that night , Turbine's operator should realize the increase of frequency and the decrease the actual MW , he should switch the turbine control to manual , to limit the decrease of MW , Then he should switch the HPTV to manual and start open it gradually to release the over pressurized steam at HPTV .

The Boiler's operator should maintain the main steam pressure to the actual MW by the percentage opening of HPTV , therefore the boiler master in that situation will control main steam pressure tracking to the opening of HPTV , then he should try to limit the load as possible away from Fluctuations .

After frequency is settled we can increase the MW and close the HPTV , then switch again the Turbine set and Boiler Master to Automatic Mode.

That is a brief conclusion for what should happened in such that situation.

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