Large Generator Exciter Air gap 2

[Drivesrock]

Hi all.

Has anyone had any (bad) experiences with an irregular exciter air gap of a synchronous generator?

Generator in question: 31.2MVA/25MW, 13.8kV, 60Hz; Brushless Exciter 76V, 9.8Amps.

The exciter air gap was measured by the original manufacturer and was 4.3mm, 2.7mm, 0.95mm, 3.0mm at the 90-degree positions (0-270 degrees). The nominal gap was not informed but manufacturer says the air gap should not be less than 70% of the nominal air gap and that the gap should be corrected with shims but did say the generator could be used.

I have searched and found plenty on the effects of the main rotor to stator air gap and what the differences can cause. Can I relate them to the exciter air gap and what could happen to the AVR?

 

[waross]

The AVR is very forgiving.
There may be a concern for the armature windings and the diodes.
It depends on the type of exciter and how the exciter windings are connected internally.
Depending on the physical spacing relative to the poles, there may be uneven division of current between the windings.
That may lead to overheating of some windings.
Depending on the physical spacing relative to the poles, one phaae may be running at a higher voltage than the others.
One phase running at a higher voltage may cause overheating of that phase windings and may stress the diodes for that phase.
Is this exciter brushed or brushless. AC or DC, shaft mounted on the main generator or independently drive?
It depends.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Just like in the main generator, the voltage waveforms will be distorted, plenty of harmonics and electrical vibration in the exciter rotor (though it won't affect the bigger animal). Not sure if AVR can handle such a scenario.

The air-gaps measured don't show a round rotor. If 4.3 mm was at the top and if the exciter stator has shims, you can lower the stator to even out the air-gaps. If no shims, you are screwed in the sense that you need to machine the exciter stator base or the foundation frame to bring it down. If 4.3 mm was at the bottom, you just add shims.

At 0.95 mm, you also run the risk of rotor rub due to thermal expansion, vibration, bearing float etc. I am surprised OEM said it's ok to run.

My thumb rule of air-gap variation is +/- 5% of average.



Muthu

http://www.edison.co.in

 

[panter]

It seems that someone brave simply need to pull out the 2.2 mm shimps from the lower pole and insert it under the upper one. But that is not case because sometimes it is important disposition of shimps respecting they are made of magnetic or non-magnetic material.
Good luck

 

[Drivesrock]

Hi guys, thanks for the replies.

With regards to the AVR: The generator can run in parallel and the loads are very constant. It behaves ok with the VAr loadsharing voltage raise/lower pulses for many hours. BUT, when a large load is started - like a 4.5MW compressor motor - the AVR fails. I caught an internal flashover. It's happened a few times unfortunately!

Any thoughts?

 

[wcaseyharman]

What does “internal flashover” mean? Arcing in the AVR rectifier? What equipment is arcing? And you can restart your machine?

Anytime we had failed equipment in the AVR cabinet (granted, most of them are static exciters with a large bridge) something fried, like a SCR “puck” and the exciter wouldn’t work any more.

 

[electricpete]

The exciter air gap was measured by the original manufacturer and was 4.3mm, 2.7mm, 0.95mm, 3.0mm at the 90-degree positions (0-270 degrees).

You can further characterize the situation by rotating the rotor 90 degrees and repeating the measurements.

If the gap readings remain the same for each clock position (relative to the stator), that tells you the rotor axis of rotaiton is offset with respect to the stator bore axis.

If the gap readings rotate 90 degrees with the rotor, that tells you the rotor od is not concentric to it's own axis of rotation (could be a bow for example).

the correction for these two conditions is different. and you may have some of both.

 

[Drivesrock]

Thanks everyone for the replies so far.

The AVR is a 20Amp nominal unit, with thyristor rectifier for a brushless generator. The operating power is from a PMG.

The unit that was seen to have the internal flashover was opened and had a thyristor snubber resistor (R-C circuit) with a small burn mark. Another had cracked enamel.

A previously failed unit was opened and other components were seen to have overheated/burnt - the gate driver outputs circuits for gating the SCRs. This could mean the PMG frequency is higher than the AVR is rated for. PMG is 170V, 180Hz but AVR is rated for much higher than this.

Next steps are a thorough inspection of the exciter and PMG which, in this case, is mounted behind the exciter. Perhaps that has rubbed due to the low air gap and sustained damage.

 

[edison123]

"This could mean the PMG frequency is higher than the AVR is rated for. PMG is 170V, 180Hz but AVR is rated for much higher than this."

Contradictory.

You need to set right the air-gaps both in ac exciter and the PMG stat.


Muthu

http://www.edison.co.in