Steam Turbine with Woodward 505e Speed Drop

[electrocuted1]

Hello,
We've been facing a problem with our 15 MW steam turbine recently. It has Woodward 505e governor controller installed and configured in 4% droop mode, although normally it is running alone. At other times (during startup), it is synchronized to other DGs.

The problem we faced recently was that when our turbine went up to about 11 MW (with all other DGs shut down), it kept normal for about 10-20 minutes. Then all of a sudden, without an increase in the load, its speed suddenly dropped from 3000 to 2950 rpm. Operators tried to increase Woodward speed setpoint manually, but it did not respond. Operators then switched off some 1.5 MW load manually and the turbine speed normalized to 3000 rpm. All this happened within 30-40 seconds. During this time there was no change in load, no change in steam pressure or flow (extraction was off at the time), no physical change in HP or LP valve.

The two questions which are looking to be answered are:
1. Why would the turbine suddenly slow down?
2. Even if somehow the turbine slowed down, why did 505e not respond by increasing its output to maintain the rpm?

Any suggestions would be most welcome!

 

[waross]

Possible a change in steam super-heat and resulting lower steam energy.
The symptoms may indicate an overload condition. Either too much load or too little steam energy
The inability to increase the speed manually and the recovery when the load is reduced indicate an overload.
Another possibility is a failing AVR.
If the excitation is increased on an islanded set, the voltage will rise. A rise in voltage may increase the MW enough to overload the set.
This is a factor worth checking and eliminating as a possible cause.
Could you have had a rise in back pressure? A flow restriction in the cooling tower circuit?

Bill
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"Why not the best?"
Jimmy Carter