Voltage Regulation / Field Resistance 1

[altoon44]

We have 3 hydor units rewound, with new solid state exciters, 2-125vdc machines and 1-250vdc. Through trending software in SCADA, field current for a 125vdc and 250vdc units are stable with very little corrections made. The other 125vdc unit has a high frequency ocillation at an amplitude of 15 amps. Megger readings of this rotor have dropped to 500k ohms and are not stable, and we have not experienced a field ground. Can this fluctuating field resistance be a factor in the regulator's inability to hold a stable field current? Thank you for any help.

 

[ScottyUK]

What size is this unit? What form is the AVR on this unit? What does 'high' frequency mean - 500Hz or 500MHz? Numbers convey so much more than words!

Most AVRs are designed to continue running with one field pole earthed until an outage can be planned in so I doubt a 500M[Ω] Megger result will be significant. The instability sounds more like an AVR tuning problem. Most AVRs have enough phase lags in the control loop to be able to produce instability at some frequency where the loop gain is high enough to sustain it. Does this unit have a power system stabiliser fitted? They are known to encourage a badly tuned AVR to oscillate.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[altoon44]

This unit is 10.5 mw, the AVR is Eaton dualbridge (main & redundant) scr firing. My fault in using "high" frequency, it is 30-40 cps and the megger reading was K ohms, I wish it was Mohms. We recently had Eatons field rep in to check tuning, the two 125vdc machines are identical and this problem is only on one unit. Yes we do have PSS but it does not change the condition. Thanks.

 

[ScottyUK]

Sorry, I read k[Ω] and wrote M[Ω]. 500k[Ω] is low but I don't think it is the cause of the problems you are seeing. You could have one pole with a dead short to earth and the AVR should continue running, although it would be tempting fate to leave it in that condition for any length of time. Does the problem only manifest itself when operating in parallel with the utility or is it present when unsynchronised? I guess you might not know for sure because you probably can't raise excitation current very far when unsynchronised.

I don't know your AVR at all so some of the below comments may seem silly or inapplicable. If so I apologise.

Does your AVR have a manual mode where you can directly set the field current? Does the oscillation calm down or go away when the AVR is in manual? As this is a redundant system, does the problem persist when running on the main and the reserve AVR? If so are there any components which are shared? Have you checked for simple problems such as a missing or reverse polarity firing pulse, blown fuse on the rectifier or at the excitation transformer, or a dead thyristor? Reverse connected QCC current transformer (likely to have adverse effect only when under load, probably worsening with load)?

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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[altoon44]

We performed a pole drop test and then seperated the pole connections, everything was uniform. We cleaned and heated the rotor, the resistance improved for a short period of time then dropped again. The condition is slightly better when this unit is on line alone, then gets worse as the other two units sync to the line.
I have not checked the field current unsynchronised, you bring up a good point I will look at it.
Manual mode is slightly worse, 30-40 cps at 17-20 amps swing. The firing command is a feed back loop from redundant to main and main to redundant, the shared component is the briged firing control circuit, we have replaced this with no improvment.
We have taken this exciter off line, performed a complete diagnostic and used calibrated field simulators and every thing looks good. The ocillation is not present, it looks as good as the other two units. The reason I keep looking at the rotor is the swing in Rmin and Rmax readings given by the field ground detector circuit of the exciter, I don't see this on the other two units. Thanks.

 

[ScottyUK]

Sorry, should have asked: is this a directly excited machine (slip rings carrying main field current) or a solid state excitation system driving a rotating diode bridge with a shaft-mounted exciter? Failed rotating diodes alter the overall loop gain.


----------------------------------

Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[altoon44]

They are 55 year old slip ring machines, each had the pmg, pilot and main exciter. Now it is exciter to main field slip rings. Thanks.