Diode failure by bad sync

[Yuma]

Hello to all;

Talking about a turbogenerator with rotating diodes exciter, is it normal to have a diode failure condition because of bad paralleling to the network?
If so, can you tell me why?

Thanks

 

[davidbeach]

Normal? I don't know that I'd say normal, but certainly possible. If the changes in stator currents and voltages associated with paralleling with the utility as sufficient to cause the field current to attempt to flow in the opposite direction, or have an AC component, the reverse current could create a reverse voltage across the diodes sufficient to cause the diodes to fail. A good sync check relay is very inexpensive insurance.

 

[Yuma]

Thanks a lot, David! and now, a slightly different question. Again, in a turbo generator with rotating diodes exciter. Assuming that everything is OK, and diodes are healthy. Suppose that you close the excitation breaker, the field flashing works, the generator voltage goes up to rated voltage, everything is normal so far. And after that, you open the field breaker, let the generator voltage fall down to, say, 70% of rated voltage, and close the field breaker again.
Would you expect any damage in the rotating diodes?

 

[Skogsgurra]

Have a look at thread238-149416

We had a varistor failure and it happened because we lost excitation and lost synch so the pole wheel was slipping against grid.

The diodes were OK in this case.

In your second scenario, I would say that you would not see any damage to the diodes (assuming islanding). If you do the same thing when tied to a grid, the breaker will probably open long before anything happens to the diodes.

Gunnar Englund

http://www.gke.org

 

[Yuma]

Thanks, I’ve read that thread and found it really interesting. I’ve definitely learnt something from it.
But I think it is not related to my problem. The thing is, what you call ‘my second scenario’ actually happened, and some of the rotating diodes were damaged. And sorry, I forgot to tell you that also one of the fuses that protect the thyristor bridge was blown.

Yes, the generator was running at FSNL, (islanding). No bad paralleling, no pole slipping. Just closing the field breaker, opening it, and closing it again. And voilà, you get one fuse blown and some diodes damaged.

Beats me. So far I haven’t been able to find an explanation, and all the people I’ve asked have the same opinion as you: nothing should happen to the diodes.

I hope some of you who are more experienced can offer some explanation...

Thanks in advance.

 

[rmw]

Any chance you slipped a pole in the process? If that occurred, then I can see the PMG/exciter going through a transient that might have caused a problem that trashed the diodes.

rmw

 

[Skogsgurra]

Yuma,

There is another failure mechanism that may have been at play.

When the field breaker opened, the voltage went down. The AVR then tried to compensate and probably went to its positive limit. When the breaker closed again, the controller was already demanding maximum field current and that may have resulted in an unusually high field current.

This can happen when the AVR is slow and when there is ample reserve in the power circuits. But I have never seen it happen personally.

Gunnar Englund

http://www.gke.org

 

[Yuma]

Maybe I didn't put it clear. The generator was not paralleled to grid at all, it was running at full-speed-no-load. So, as I understand it, there is no possible pole slipping.

 

[Yuma]

Sorry Gunnar, in my last reply I was responding to to rmw. I think our posts crossed.

Now about your post: in this excitation system, when you manually give the order of opening the field breaker, the Automatic Voltage Regulator goes for a moment into inverter mode (this is, it starts to feed the inductive energy that is stored it the field, back into the excitation transformer). So it doesn’t go into full rectifier mode, but the opposite.

That would have been the case if the field breaker would have open for no reason, I mean, if the AVR had not received the ‘opening command’. It is true that then the AVR would react to the falling voltage by trying to feed more DC current into the exciter armature winding. But in this excitation, the order of opening the field breaker goes both to the AVR and to the breaker itself at the same time.