Voltage Regulation / Field Resistance
Voltage Regulation / Field Resistance
(OP)
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.






RE: Voltage Regulation / Field Resistance
Most AVRs are designed to continue running with one field pole earthed until an outage can be planned in so I doubt a 500MΩ 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!
RE: Voltage Regulation / Field Resistance
RE: Voltage Regulation / Field Resistance
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!
RE: Voltage Regulation / Field Resistance
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.
RE: Voltage Regulation / Field Resistance
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Sometimes I wake up Grumpy.
Other times I just let her sleep!
RE: Voltage Regulation / Field Resistance