Capicitors on Induction Generators 1

[TerrySmith]

I am working on the design of a Distributed Generation system, using 2 400kw natural gas driven induction generators. I am using Basler protective relaying. In the application data, and other material I have read, it is recommended that capicitors be used wherepower factor may be below .9.

I am looking practical application advice, to ensure the selection is made correctly, and in fact that they are actually required.

Anyone experienced with induction generator use. In this case I am using induction in lieu of synchronous, because the Utility in my area wants the gererator excited from the
utility source, so that unit can not run if the utility is down.

 

[busbar]

Induction generation is also real popular where wind or water is the energy source. There are two major considerations in your query.

1. Capacitors can provide reactive (magnetizing) current for induction machines—be they motors or generators. Aside from an operational aspect, a careful study of the tariff/contract is needed to specifically determine what reactive power will cost you if the utility supplies it. They may want a lot of money for their providing you reactive power in the form of kVAR-demand or kVAR-hour support. That directly affects your operating income and should be given careful study…like using a spreadsheet to estimate your income based on differing modes of operation. [You may be well aware that synchronous machines can “make their own” reactive power, but interconnection protective requirements get more complex.]

For smaller non-utility generators, it’s normal to connect to the utility at a distribution voltage—like in the 7-25kV range. The utility may provide the transformer bank served by the distribution circuit in the case the induction machines are low voltage; id est, 480V. An alternative may be that you provide your own transformer set, but the payback will be defined in the tariff/contract.

The metering point may be at low or medium voltage, and that will affect the hardware cost of the metering node.

2. The utility is concerned about an islanded condition where a feeder breaker or fuse isolates the area around your generators, but other customers’ loads are still connected on your side of the open breaker or fuse. A common method of protection is negative-sequence overvoltage on the transformer high side, that would trip your local {medium- or low-voltage} breaker for a line fault.

One compromise that makes good use of protective and metering components is to meter at the high side of the transformer, and use the voltages provided for metering to also connect to a protective relay to isolate the generators from other local loads in the case of a line fault.

There are some IEEE standards available in the US that may be applicable to your situation. IEEE Std C37.102-1995 Guide for AC Generator Protection and IEEE Std C37.95-1989 - Guide for Protective Relaying of Utility-Consumer Interconnections. Remember that the utility has the “upper hand” in deciding on the protection scheme, for they have safety concerns that they view take precedence over the sale of electric power and operational protection of your equipment.

That’s the short explanation. Myself and other readers may be able to explain related aspects to get you pointed in the right direction—don’t hesitate to ask. You will need to retain an electrical PE that has non-utility generation experience to deal with the details.

 

[jbartos]

Suggestion: There are also state guidelines, e.g.

http://www.nrri.ohio-state.edu/programs/electric/DistributedGeneration/data/state/wisconsin/wip.pdf

and industry standards