Auxiliary generator with isolated neutral

[RRaghunath]

I am going through the drawings of a 55MW power plant. The plant is a very old one and is due for relocation in an Indian site from its original location in France. The plant has strange (un-familiar, ofcourse interesting to me for the same reason)) configuration.

The generating unit has a 50MW main generator and a 5MW, 3kV auxiliary generator on the same shaft.

The auxiliary generator is like the unit auxiliay transformer connected generator terminals that engineers of my generation are familiar with and feeds to a 3kV auxiliary power supplies switchboard.

Further, the generator neutral is isolated from ground and there is no earth fault protection in the 3kV system (there may be open delta connected residual voltage sensing relay in the VT secondary cicuit, I am yet to check-up).

I would like to hear from any one who has worked in such a power plant and learn from their experiences. Presently the prospect of uncleared earth faults in the 3kV isolated system and arcing grounds / flashovers is scaring me.

 

[busbar]

ANSI/IEEE C37.101 …Generator Ground Protection is one reference for generator stator-coil grounding and protective schemes. IEEE C62.92.3 …Application of Neutral Grounding in Electrical Utility Systems, Part III—Generator Auxiliary Systems is another.

In .101, eight grounding connections and 22 protection schemes are discussed. “A disadvantage {in ungrounded systems} is that excessive transient overvoltages may result from switching operations or intermittent faults.” Boy Howdy!

 

[mc5w]

Some of our 480 volt ungrounded systems here in the U.S. are supplied from a 277Y480 volt transformer secondary with the secondary neutral terminal taped up and not connected to anything.

You might want to convert that 3 KV auxiliary generator to some type of impedance grounding with a potential transformer that would allow you to hook up some 230 volt or 400 volt resistance heaters or high frequency signal generators to it. This would help you track down faults. I high frequency tracing system only removes about 90% of the guesswork that is involved without it.

Another interesting way to trace faults is to amplitude shift key part ( say 50% ) of your grounding impedance and then look for which ammeters have a corresponding wiggle. Amplitude shift keying can operate with a signal that is smaller than the noise around it which is how television remote controls send a signal in spite of infrared noise from light bulbs and people.

Mike Cole