Operation of Synchronous Generator as Induction Generator.

[RAgrawal]

There is a small about 10 MW, 3000 RPM Generator coupled to a back pressure Turbine in our plant. One day, Excitation of the generator failed along with station DC power failure (supplied by 220 DC battery bank). Due to failure of DC power, no relay could the detect the loss of field / trip the generator. Even operation people could not trip the generator, as there was no DC power for that particular machine.

Since system was strong enough to supply the increased reactive power demand, there was no other disturbance in system e.g. voltage dip etc. After 8 minutes, machine copuld be isolated by tripping upstream breaker. Tripping of machine caused the over speeding of Turbine but this whole incidence caused the tremendous damage to the Generator. Its rotor was flied off towards Turbine by breaking the coupling and shaft by about a meter. Fire also taken place in the plant. Machine was operating at full load even after the loss of excitation.

Can any one comment on this incident and have any idea about the capacity of machine to operate as Induction Generator? Trends of system parameters are available, if any one of you is interested.

 

[electrageek]

dpc

I am surprised that a new turbine would come without a mechanical overspeed bolt. Its a device that operates on centrifugal force, the faster you go the more force to throw the bolt outward from the shaft. The bolt hits a lever that dumps the oil from the stop valve. Its amazing how accurate and reliable they are. I thought all turbines had them. We normally test them after every overhaul and at least once a year. They can get sticky and miss-operate. The overspeed bolt is equal in importance to the batteries. Without a mechanical overspeed bolt I would want a completely separate system for redundancy.

RAgrawal

I have been at sites where there has been no protection due to failed lockout relay coils, blown fuses, failed protective relays, and failed batteries.
The Generator's were still running and no one ever noticed. The problem with protective systems is that you don't need them very often and we tend to forget about them until its too late. Don't put off testing your protective systems.

The Generator capability curves usually give you the range of control for the Generator field. On the lagging Power Factor side you have the maximum field current that produces Vars to the grid. The limitation is when the rotor field current begin to overheat the rotor windings. On the opposite end (leading Power
Factor) is the minimum Generator field current, the Generator absorbs or uses Vars from the grid. As the field is reduced the Generator stator current increases because it needs Vars from the grid to make up for the Vars not being produce by the Generator field. The limitation is end turn heating of the Stator windings. This is where your loss of excitation (40) relay should kick in. They normally do not look at the actual field current they look at the Generator PT's and CT's to calculate when the Generator has ventured into a dangerous excitation area. With most loss of excitation relays the point at which the relay operates and the timing of the relay are dependent on the Generator manufacturers design data. They won't tell you where to set the relay, due to liability, but they do give you the heating factors needed to calculate the trip points. Many protective relay manufacturers (Beckwith, GE, Basler) have excellent instruction books, available on the web, that explain this a lot better then I can.

 

[peterb]

electrageek -
I fully agree with everything that you are saying, except to note that the problem arises when the stop valve does not fully close (as described by dpc). This would mean that the mechanical overspeed could very well have dumped the oil, but the valve just did not close.
If you have a full load rejection under these conditions, there will be a continuing overspeed condition. This is why it is critical to monitor power output to interlock the trip, to ensure that there is not enough steam passing into the turbine to cause an overspeed when the breaker is opened.

 

[electricpete]

I'm not much on turbine or generator controls, so having a hard time following the discussion.

electrageek - you say you're surprised they don't have overspeed bolt. But as I read the problem they did trip due to mechanical overspeed trip set at 115%. Would bolt have provided a lower setpoint? why would it have been significant?

It still sounds to me like the damage was in fact done by overspeed condition (correct me if that doesn't make sense). That leads me to several questions which might explore why this would occur.

1 - Had the overspeed trip been tested? Do we know for a fact that it tripped at 115%? Is 115% the proper setpoint provided by the manufacturer?
2 - Was there any abnormal vibration problem associated with this generator during normal operation prior to the event which might have limited its ability to withstand overspeed? High imbalance? abnormal degree of misalignment?
3 - Perhaps the abnormal operation prior to the overspeed trip caused some abnormal heating of rotor which might cause it to bow and therefore it could no longer withstand the same degree of overspeed as a healthy rotor? To confirm that theory - Is there continuous monitoring of vibration which might indicated whether running speed vibration increased during the latter stages of induction motor generator operation but still prior to the trip?

And just to confirm one more time my basic understanding of the events (RAG): Is it correct that the generator self-destructed at about the same time that the overspeed trip tripped which was almost immediately after the system breakers were opened?

 

[electrageek]

electricpete

Actually I was responding to dpc about his turbine overspeed example on the stuck stop valve. He is correct that if the valve is really stuck open the speed will continue right on up to destruction. The mechanical overspeed bolt will not be any better at shutting the valve. The same problem exists with non-return valves that stick open. They are the check valves that are suppose to prevent downstream extraction steam from being push back into the turbine and again causing an uncontrolled overspeed.

In reference to the original problem with RAG's turbine. He lost all excitation and normally would have tripped on a 40 relay function. I would think that someone could have grabbed a broom stick and reach in to trip the Generator Breaker manually. There is usually a manual trip button located on the Breaker. I wouldn't stand right in front of the Breaker.

 

[electricpete]

electrageek - I'm starting to understand better now the scenario that has been discussed of leaking or stuck-open stop valve, which renders the overspeed trip useless even if it does actuate.

If that were the case, wouldn't the turbine overspeed also even after the generator destructed? How was steam to the turbine ever stopped if the stop valve was stuck open? (perhaps the governor valve?)

 

[peterb]

Electricpete -
What stopped the steam to the turbine? If there wasn't an accessible manual valve, then the answer is - when the boiler was tripped and the steam supply was exhausted. I saw this happen many years ago, where a turbine-driven compressor self destructed and exhausted all of the steam into the 13.8 kV switchgear located on the floor above. That took many days and nights of cleanup to get the plant back up.
This is obviously serious stuff and points to the need for coordinated design effort between electrical & mechanical disciplines and also for rigorous operating practice in testing and maintenance of stop valves and overspeed trips.

 

[electrageek]

electricpete

The interesting thing about a steam turbine is that it take very little steam to keep the turbine running once it is moving. The journal bearings, condensor vacuuum, and inertia of the system provides a very efficent low loss machine. Sometimes it is necessary to break vacuum just to get the machine to stop in a resonable time. Because of this Steam Turbines have a very low reverse power setting. A 20MW Turbine may only pull 200KW of reverse power. It takes a very low range relay to reach this level. The Stop Valve is the primary protection for overspeed, the Govenor valves can also help but generally are not required to completely seal. Even a small leak can gradually cause the speed to go up and up and up. The thing is the amount of steam is fairly small so once something gives up is doesn't take much load to stop the machine.