Alternate Supply Instead of PMG

Yes provided it can support the proper voltage and current sourcing capacity that is needed.

It depends. I'm assuming that you're referring to your failed PMG from an earlier thread. If so then you need to know the details of exactly what type of excitation system you have, along with details of the manual control and AVR. How does the main field get its power, through a Pilot Exciter (brushless) or directly (brushes)?

Using the PMG specs that you provided, it looks like you have to be aware of how the internals of the AVR utilize the output of the PMG. Why? Because it has 8 magnets/poles and its output is 200Hz, meaning if the AVR has any frequency sensitive circuits (over/underfrequency, Volts/Hz to name a few) you may find that it doesn't respond the way you expect if it's fed from a 50Hz supply. Also, if the output of the AVR drives some type of thyristor phase angle firing control to boost/buck the input to the field, it may not work at all.

You have an interesting set of compromises to make. If the field is fed through brushes you could bypass the AVR altogether and just power the field off of a 2kW variable DC power supply, but then you have to power that supply from a source other than the station power. Why? Because under any type of severe voltage dip the variable supply may not be able to provide enough regulation to boost the field current enough to keep the generator up at a time when it needs it the most, especially during any type of fault near the generator, its transformer, and switchyard.

You could also try a VFD instead of the PMG, but they have no real inertia so will suffer the same as the above.

Also do not use anything other than DC rated switches/interrupting devices on the DC side, fields are really big inductors with long time constants and will quickly destroy anything not rated for the proper interrupting duty.

If you want to be safe call the OEM, maybe they can point you to a utility that has a spare PMG for sale if you don't want to pay for a factory unit.

Please let us know what you decide to do.

Hi

Thanks for a very useful info. Further to my query, i would like to add that we have 4 turbines running in parallel out of which three of them are on automatic regulation and one in manual i.e. through MVR, which has the damaged PMG.

Below i have jotted down the single line.

The MVR of this turbine is feed through a trafo with 100, 180 and 200 Volts options on the primary side and 98 volts on the secondary which is fed in to MVR from where it is feed to the excitor.

So if i rephrase my question

What if i connect a 3KVA (200V) instead of PMG, will it work?

Regards

Unfortunately your diagram didn't come through, but if the OEM says you can do this and the manual control works then you have proven that you can feed AC into the MVR and it will provide a variable DC voltage/current to the field of the machine.

The static (steady state) case is easy, now it's time to do some "what-if" thinking to your setup. In particular "what-if" the supply to your transformer is interrupted, a fault occurs that causes the supply to sag significantly, there are times that the supply voltage rises unexpectedly, a system transient causes a wild fluctuation in load; i.e., the dynamic response to a sudden transient of the field current.

There are many who would argue that what you have set up is no different running the machine on MVR so you are exposed to the same risks. The thing that I have observed that is different is the exposure that you have if anything affects the output of the transformer This is exactly why the PMG system was developed, to eliminate that risk and isolate/insulate the overall excitation system from those fluctuations.

You need to ensure that the supply does not have any large equipment that cycles and/or causes significant/unexpected swings in the input voltage to the MVR.

AVR - Definition by AcronymFinder

MVR - Definition by AcronymFinder

PMG - Definition by AcronymFinder

UPS - Definition by AcronymFinder