Powering of Generator AVRs

[Rodmcm]

Does anyone have a source of typical drawings showing the methods that an generator AVR can be powered.I am aware of the usual, pmgs,separate winding in the exciter etc, but I am more interested in other forms such as VTs and CT in series etc. We have a problem with an old french ( we think) generator.

 

[waross]

There was an old regulation system that I still see every 10 or 12 years. It was bullet proof.
A satureable voltage transformer and rectifiers were used to energise the field. A series resistor would give fine tuning of the no-load voltage.
A current transformer would develop a voltage across a variable resistor. This voltage would be rectified and summed with the PT excitation voltage. This would give the required increase in excitation under load. This system was capable of voltage regulation in the range of 5%. I beliee that it could be set to over compensate and actually increase the voltage under load to compensate for line losses when it was a long distance from the generator to the load. Not sure about that though. I never had that situation.
It was possible to add an Automatic voltage Regulator to improve regulation to around 1% or 2%.
respectfully

 

[ScottyUK]

The CT in series sounds like a compounding CT which delivers additional power to the excitation system when a fault occurs resulting in high current from the machine terminals. This scheme is frequently found with self-excited machines which derive their main excitation supply from the generator terminals via a transformer.

This series of threads might be of help:
thread238-150867
thread238-150959
thread238-151204
thread238-151797
thread238-158342

Or maybe they won't! Good luck.


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Sometimes I only open my mouth to swap feet...

 

[Rodmcm]

Thanks for that, I didn't search compounding CTS. Another question though, for a self excited machine, how does the manufacturer guarrantee that there will always be enough residual magnetism to get the AVR away?

 

[ScottyUK]

On bigger machines the field is 'flashed' by a battery source which pushes enough current through the field of either the exciter (for a brushless machine) or the main field winding (for a machine with sliprings and a collector). This current is large enough to raise the terminal voltage to a point where the AVR can draw power from the machine terminals, although terminal voltage will usually be well below rated working voltage at this point. The AVR can then continue buildup until the machine has rated voltage.

The battery is isolated against backfeed from the field supply by a large high voltage diode. Voltage build-up begins when a contactor in the DC circuit closes to allow current to flow from the battery; usually there is a large wattage resistor or a few ohms in series with the battery to limit the flashing current.


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Sometimes I only open my mouth to swap feet...

 

[Rodmcm]

Yes, thank you, I know and have used these techniques. However, this question is around the VT/CT type of self excited systems on the smaller machines, ie 3 to 5MW self excited.

 

[ScottyUK]

Sorry, don't know the answer to that one. This is right in Waross' field of expertise - he should be waking up about now.


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Sometimes I only open my mouth to swap feet...