diference on current ripple on excitation

[uhpo]

i have a problem with an excitation system, i have a short diode alarm, i know the ripple must be higher than normal, how much % of ripple i must have? and now we suppose we have a 6 pulses bridge on the avr, and if one of the SCR have the driver disconected, so it´s not working, how would change the current ripple? cause the voltage ripple have change, and how much the voltage ripple must change? could somebody help me with some literature....

thanks a lot

 

[electricpete]

Just thinking out loud...


I would think that if you have an open diode on a rectifier, then ripple would go up.

But if you have a shorted diode, could be high currents circulating about to damage something else until some other component or fuse interrupts the circuit.... wouldn't it?

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(2B)+(2B)' ?

 

[rasevskii]

If this is a brushless exciter then:

The diode fail protection should sense the failed/shorted diode and trip the unit within about 10 seconds. This protection works by sensing the exciter field current, if it is abnormally high (over ceiling current) for more than the set delay time, a trip will occur. During normal regulation, and on start-up ceiling current may be reached for some seconds or fraction of a second.

If it is a full static exciter then:

Other means are used: Blown fuse detection (tripping flag on the bad fuse has operated), or current comparison circuit between sections of the bridge(s)has operated. In that case likely the unit is designed to operate with one failed diode at reduced rating.

rasevskii

 

[catserveng]

Go to the Basler website and download the DECS200 manual, in section 3 (about page 3-8/9) is an excellent explanation of an exciter diode monitor and how to set it up properly.

I have used this information to help develop settings on other manufacturers AVR's with similar functions with pretty good success.

http://www.basler.com

Hope that helps, Mike L.

 

[uhpo]

the question is how to discriminate the ripple caused by a problem on the thiristors of avr (open tiristor) or a problem with a short diode on the indirect excitation, i mean on the rotating diodes.

 

[rasevskii]

uhpo:

Why are you concerned about ripple? Excitation systems are designed with reliability in mind, and all the necessary protection functions are included with any system from any reputable supplier.

It sounds like a theroretical exercise that you are in the midst of. The field supply of any generator or brushless exciter is not affected by ripple due the high inductance, all diode and thyristor rectifiers produce some degree of ripple but it doesn`t matter.

See above posts. Do not tinker with excitation equipment. Call in someone from the OEM or an expert. Do not change or disable the protection settings from the original commissioning, which was likely done by the OEM or his rep.

rasevskii

 

[uhpo]

je,je i explain it, it´s not theorical, i explain, i have an AVR, with an hydro genset, 17 MW... with indirect excitation,some days ago, we have an alarm from AVR and a trip, diode short circuit, so we change all diodes, then try to start unit, but again the same problem, we check diferents questions on generator, cause AVR shows up the problem must be on generator, after several days with unit stopped i noticed that one little cable from one of the thiristor on AVR was disconnected, so we connect it again, and right after it..we started unit and the unit syncronize and worked properly...
After we reclaim manufacturer, he said they could feel on their control the diference between a ripple change on the voltage cause of the SCR open, and i wonder how much would be on % the change on the ripple of voltage, and % on change on the ripple of current.....
And how could i comparate it with the ripple on the current with a short diode or open diode.....
As you see is just to know a little more about it just to discuss with the manufacturer.
Thanks a lot for your ideas, maybe i must explain it better on first time.

 

[ScottyUK]

I'm not sure how you would get a short circuit alarm from and open-circuit gate, I'd expect quite the opposite because that thyristor would never conduct. If you take one diode / thyristor out of a six-pulse rectfier then you behaviour more akin to a single-phase rectifier.

The ripple voltage will be significantly higher. It is a fairly trivial task to sketch out the waveforms - the generator field is a textbook 'highly inductive load' so you can find plenty of examples to start you off, but obviously you'll need to assume that one switch is open circuit.


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If we learn from our mistakes I'm getting a great education!

 

[uhpo]

you´re right the behaivor of the six pulses rectifier, without an SCR is more close to a single rectifier, so the ripple must be higher, how much? and how could i relate with the % on the ripple of the current...i just want to compare it, cause i think the avr must have some way to discriminate between ripple caused by short diode on rotating rectifier and ripple caused by an open sCR on AVR rectifier.
thanks