Diode and Thyristor Failure in Generator Exciter 1

[eskim]

Dear All,

I've problem above for the last two days. The six-pole, full wave rectifier have one diode and thyristor broken at the same time. We have over current and over voltage relay picked up at excitation control (V~4000V and I~40A).

My question:
1. Why the current and voltage at excitation control so high ?
2. What is possible cause of diode and thyristor failure ?

Sorry for my english
Thank you for assistance.

Regards,
SMD

 

[powerjunx]

eskim,
Talking about Exciter, is it Static or Rotary Exciter Assembly? I hate to keep on guessing.

 

[ScottyUK]

4000V on an exciter? Wow! What size machine is this? 4000V and 40A doesn't just sound right for an excitation system: the current is way too low for the voltage - even big machines with direct static excitation usually run at <2kV and <6kA.

Please post some ratings of the excitation system, especially type (static & slip rings, static & rotating rectifier, static & pilot exciter, etc) and generator and nominal running conditions for both exciter and generator.

Are the relays you mention the main relays protection for the generator, or part of the excitation control & protection scheme? Also, what condition provoked the operation of the relays?

Your English is fine, you just haven't included enough detail.


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I don't suffer from insanity. I enjoy it...

 

[eskim]

The exciter is staked from PMG/pilot exciter --> AVR --> exciter stator --> exciter rotor --> finally to main field winding. The broken diode and thyristor is at the exciter rotor put in generator shaft, unfortunately I can't attach the drawing.

In the normal running with 27MW (our Turb Gen is 34MW) excitation current 6.8A and voltage 81.6V. The prot relay is part of excitation control. This relay is wired between AVR and exciter stator.

We have trip turbine by MFT, after fixing we tried to roll turbine run up to 90% of speed then automatically the exciter field breaker closed. Suddenly our turbine have trip by excitation prot. In the investigation we found high I and V, diode and thyristor failure.

Is the high V and I at exct control caused by shorted circuit diode ?

Thank you again.

 

[ScottyUK]

So you are saying that there are thyristors on the rotating shaft? What are the gate firing arrangements?

Am I correct in assuming that the 40A referred to initially was excitation current, and the 4000V was generator output voltage? What is nominal output voltage for the set? If the 4000V was recorded at the excitation control, what did you measure it with? 4kV seems excessive for a circuit which nominally operates at 80V.

One possibility is that the failure of part of the shaft-mounted rectifier has effectively reduced the gain of AVR because the failure will make the three-phase rectifier behave as a single-phase rectifier. The AVR must therefore deliver greater exciter field current to maintain generator field.


Aside: The pilot exciter and PMG are different items of equipment - the pilot exciter is a smaller auxiliary machine mounted on the shaft which is used to control the main exciter, which in turn controls the generator field. The AVR controls the pilot exciter. They aren't so common now power electronics has reached ratings which allow the pilot exciter to be dispensed with.

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I don't suffer from insanity. I enjoy it...

 

[waross]

Hi ScottyUK;
I am seeing a lot of systems with Permanent Magnet Generators on smaller machines.
Rather than a pilot exciter controlled by the AVR, A PMG is mounted on the shaft. The ones I have been seeing produce about 220 volts three phase with enough energy to excite the main exciter. The AC output of the PMG is rectified and controlled by the AVR which then supplies and controls the field of the main exciter.
Good point about the 4000 volts being generatopr output. That makes more sense than 4000 volts at the AVR.
I suspect there may be problems with either the exciter windings or the field windings.
The first check is the resistance of the exciter field followed by checks of the exciter rotor and the field windings.
I wonder if the silicon rectifiers have been mistaken for silicon Controlled rectifiers (Thyristors).
respectfully

 

[itsmoked]

Scotty, those pilot exciters the black thing at the top?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[powerjunx]

eskim,

"The broken diode and thyristor is at the exciter rotor put in generator shaft, unfortunately I can't attach the drawing."

A thyristor mounted on a rotary shaft together with diode assembly, i wonder how the triggering circuit it was installed. Maybe, you'll post a picture.

I figure out , i think, you mean about Thyristor and Diode combination is usually found on AVR circuitry where the Thyristor (SCR) controls the Field excitation voltage output (F+/F-), in which its power supply for triggering circuit is derived from Diode assembly (rectifier unit).

A diode assembly found in rotary part, is different where it is use to rectify the 3 phase output of rotary winding to produce DC voltage to supply directly the Main rotor field winding.

Probably, shorted diode would be the cause - a faulty diode. How many times does it occurs?
If this failure repeats, dont hesitate to replace a good quality and genuine diode. I experienced this before.

 

[ScottyUK]

Keith,

Those are hydro machines which aren't my strongest area of knowledge, but I think the little black devices are probably the PMG's. The pilot exciter is the smaller yellow item just below it, with the main exciter being the larger yellow lump immediately above the stator housing.

waross,

The main field current of a big machine might be 4000A or more. The pilot exciter is an intermediate gain stage similar to, but smaller than, the main exciter. The AVR is fed from the PMG as you describe, and the AVR controls the pilot exciter, which in turn controls the main exciter. The pilot exciter adds lag into the excitation system which slows the unit's response under transient conditions. Nowadays they're pretty much consigned to history because the advent of high power electronics doesn't need the additional gain stage provided by the pilot exciter. In the old days (before my time!) the pilot exciter might have had a field of a few amps, controlling a main field current of several thousand. It was just easier to make that gain in power in two steps. Modern power electronics allows direct control of currents in the kA region. Directly excited machines with a very high power static rectifier and slip rings are perhaps the fastest to respond, which is good from the system operator's point of view. The excitation system you describe is common on utility-sized machines, although the field is maybe 100 - 200A and a couple of hundred volts. Sometimes a variation is to use a flashing battery to establish terminal voltage and then allow the AVR to draw on the generator terminal voltage, rather than using the PMG.


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I don't suffer from insanity. I enjoy it...

 

[itsmoked]

Thanks Scotty

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rmw]

"Six-pole full wave rectifier...." Could this be parallel diodes?

rmw

 

[waross]

Thank you for the explanation ScottyUK.
My experience is with small diesel sets. Only up to about 1.6 kw.
rmw, The supply to the diodes is usually 3 phase, hence six diodes for full wave rectification.
respectfully

 

[itsmoked]

1.6kW huh... really small.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi Keith;
have you ever noticed how close the "k" is to the "m" on the keyboard?
Sorry, I'm on the road and using borrowed computers and slow connections. I'm short of time and this machine will only let me review and spell check with a lot of waste time. Hence even more typos than usual. Sorry.
should have been 1.6 mw.

 

[itsmoked]

So 1.6 milliwatt generators?




Sorry waross couldn't resist! Just pulling your chain.

Hope you have a good road trip.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[motorspert]

Itsmoked
Those small black thiongs look too small for a pmg on a slow speed machine like a hydrogenerator. i have seen mechanical overspeed devices located up there and sometimes the governer device. I suspec the overspeed switch.

Eskim
is the thyristor part of the rectiofier circuit or does it form a protection path in anti-parallel with the diodes?
Possible causes of failure are ususally surges, ie switching or lightning. Have you replaced the devices?

Have we established where the 4000v applies? 4kV at 35MW is an unsusual rating.

R

 

[powerjunx]

motorspert,
you may suspect as speed sensor as you think.