AC Generator Brushless Excitation not working

[electrageek]

I have a 45MW 13.8Kv 60HZ Generator with a Brushless Excitation system on it. The Generator is a 3600 rpm two pole machine. There is a PMG (Permanent Magnet Generator) mounted on the end of the Brushless Exciter that puts out 292Vac at 240HZ to a Brush Prismic A30 Automatic Voltage Regulator (AVR). In the manual mode of the AVR we can slowly increase the DC current to the Pilot Exciter field from 0 to 5Adc. That DC current is used to excite four (4) stationary field coils around a shaft mounted 3 phase AC Generator. The AC Generator should produce 3 phsae AC voltage which is then full wave rectified by 6 diodes located on a diode wheel which in turn produces 0 to 350Vdc which is passed through to the Main Generator Field. The nameplate on the Generator indicates that approximately 2.5Adc should be enough to get to full voltage no load.

As the Generator is accelerated to 3600rpm we can see a small amount of residual voltage buildup on the Generator stator. (approximately 50Vac per phase) by watching the Generator Potential Transformer (PTs Open Delta)

As we increase the DC current from the AVR from 0 to 2Adc we see no buildup of voltage on the Generator Stator.

We have checked the diodes and fuses and verified the main Generator field (0.485 ohms) is correct. We have disassembled and reassembled the entire diode wheel and fuses. Is it possible that if the field coils were not connected in the correct polarity it could prevent the shaft mounted AC Gnerator from producing any voltage?

Does anyone have any ideas of any tests I can run to prove the various parts are working?

 

[waross]

Overall polarity is a factor with self excited machines. Reversed polarity tends to reduce the field buildup rather than increase it.
With a PMG that is not an issue.
However, check the polarity of the four field coils. If they are not connected with the correct individual polarity that could be the problem.
You haven't said if this is a working machine that has failed or a machine being started up new or after repairs.
I would put a small current through the four field coils and check the polarity with a compass.
I would expect North-South-North-South.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rasevskii]

As waross has said: was the machine ever disassembled and reassembled? Or was it working properly and suddenly failed? If you have possibly shorted diodes on the rotating rectifier, then the AVR will attempt to raise the stator voltage by overexciting the exciter field, resulting in burnout unless there is a timelagged overcurrent protection on the exciter field circuit. It can be that the machine was subjected to an out of sync closure onto the system, this may have destroyed the overvoltage diode (shorted it out) on the rotating rectifier.

Some more background info please.

rasevskii

 

[electrageek]

thanks gentlemen. the unit was running in Pakistan a year ago. It was moved to Venezuela and changed from 50hz to 60hz operation. The Generator is rated for both. However the change from 50Hz to 60Hz involves removing the gearbox between the Turbine and Generator so that the Generator now spins at 3600 instead of 3000. When that occured it also caused the Generator to rotate the opposite direction and the cooling fan on the exciter end had to be reversed. That is when the two top poles of the Brushless Exciter AC Generator were removed and then replaced after the repair. In my opinion I believe we may have the coils now connected for S S N N instead of N S N S. What symptom with this cause? The compass is a good idea I will try to get one for testing.

 

[waross]

"In my opinion I believe we may have the coils now connected for S S N N instead of N S N S."
That is most likely the problem.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electrageek]

Well today we tried running again with the field poles changed. I was able to get a compass and we now do have N S N S around the outside and when we swap the DC wires it does change to S N S N. However when we run we still get no build up of voltage. The other thing that bothers me is that we are only getting the residual voltage on the Stator to build up to 50Vac. I usually get around 300 to 500Vac at full speed no-load without excitation from the residual field in the Generator.

I am beginning to wonder if there is something open circuiting when we are at 3600rpm. Something in the Main Field. Tomorrow we are going to try providing the field current from a simple Variac and diode bridge instead of the AVR. If that doesn't help we are going to take the exciter apart and check all the windings, diodes, and fuses again. We are going to ran an AC impedance test of the main field, and check the resistance of the main field and the AC Generator armature.

 

[rmw]

One possibility - You may need to remagnetize your PMG's. During the period of inactivity (especially if you had the rotor out for any length of time, and failed to wrap a chain around them) they have become de-magnetized. Brush has a procedure for doing that. It is not difficult. Contact them.

If by the "fan rotor on the exciter end" you mean the small fan on the diode wheel, you would be right. If you meant the rotor cooling fan on the exciter end, then you still have two to do, the small fan on the diode wheel and the other rotor fan on the driven end. Both rotor fans have to be reverse when direction of rotation is changed. Plus, the fan on the diode wheel can't be change because it is aluminum shrunk onto aluminum and you can't heat it fast enough for it to expand away from the diode wheel.

If you haven't reversed both fans on the rotor, it may be a good thing that you can't get it to work.

Be double sure that you are right about the ability of the generator to operate 50 or 60 Hz. If it is only 50 Hz design, the rotor may not be designed with enough metal in it for 60 Hz and also may be near or at a critical.

rmw

 

[electrageek]

Thanks rmw,

Yes the Generator is rated for 60Hz and has all the manufacturers nameplates for the 3600rpm ratings. We have all the factory tests for both 50 and 60Hz ratings. It is not a Brush generator it is a Meidan Unit from Meidansha Electric out of Japan. We are using a Brush A30 AVR. The exciter we have does not have any fans located on the diode wheel that I can see. The PMG is eight poles and is putting out 295Vac at 240Hz and doesn't seem to vary. The fans they reversed are inside the main generator housing and circulate air through the Generator stator windings.

 

[rmw]

My bad. I saw the reference to the Brush A-30 AVR and made the wrong leap. Sorry about your getting saddled with the clone generator.

Maybe it is just that the AVR has to much pride to run a Meiden generator - HA!

Hope you get it sorted. Where in Venezuela are you?

rmw

 

[waross]

You may try exciting the main field directly with AC. Turn the rotor one or two turns. You should see the voltage build up and drop down in each stator winding in turn from transformer action. The leads must be long enough to wrap around the shaft once or twice.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[redlinej]

Did you check the exciter field (stator) resistance and you can also do a drop test on the main rotor.

 

[electrageek]

I believe we have found the problem. I removed the fuses on the diode wheel which basically removed the load from the Exciter AC Generator. I attached a 120Vac Light Bulb across the Exciter AC Generator phase A to Phase B and made it easliy viewable from the end of the machine. We ran the unit up to 2000 rpms and I provided 3Adc to the field coils of the Exciter AC Generator. 2.5Adc should have been enough to provide full voltage at 3600rpm. We observed the light bulb did not glow at all. Under test the Light bulb was easy to see glowing at 20Vac. We now believe we have shorted turns in the Exciter AC Generator and will proceed to get it repaired.

I have one question about the failure. During the reversing of the cooling fan the field coils were accidently swapped so that the oreintaton was NNSS instead of NSNS. What damage could this have caused to the Exciter AC Generator Stator windings?

 

[waross]

Probably none with DC.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electrageek]

Additional data recently received from the manufacturer told us the Exciter AC Generator winding resistance, normalized to 95 deg C, should be 0.0057 ohms per phase. We checked and our impedance was 0.0018 ohms per phase, normalized to 95 deg C.