Generator Terminal Voltage

[JSLeith]

I am in the process of selecting the terminal voltage for a 140MVA 50Hz 3000rpm generator driven by a gas turbine. I would appreciate any comments regarding the pros and cons of selecting 15kV over 11kV. Our preliminary analysis has clearly shown there is an advantage when considering losses over the projected life of the plant. Are there any reasons that would make selecting a 15kV generator less desirable?

 

[rbulsara]

I am no expert in large power plant, but can certainly speak for 15kV vs 11kV issue. I am using USA's standard voltages, there may be some deviation where you are.

15kV is a voltage class that is the insulation rating of the equipment. 11kV would be the operatong voltage. Equipment insulation class below 15kV is 5kV (in the USA). One above may be 25kV or 35 kV. You certainly do not want your generator actual terminal voltage to be 15kV when its insulation class is 15kV. 11kV makes most sense.

For 11kV you still have to used switchgear rated for 15kV. In fact maximum nominal system voltage used on 15kV switchgear equipment is 14.4 kV. in the USA 13.8kV is most common other are 12.47 and 13.2 kV.

As for advantage or disadvantge goes there is nothing to evaluate in your case. It is a matter of the standard voltages used in your region and 11 kV or any other value below 15kV would be acceptable.

You can certainly have a 15kV generator but have to use equipment rated higher than that.

 

[ScottyUK]

From the voltages and frequencies quoted, I'm guessing this might be a UK application. Voltage classifications are a little different over here: 11kV is a standard distribution voltage, and switchgear and cable rated for 11kV duty is commonplace. What is definitely not standard is 11kV gear rated for the output of a 140MVA generator, so there's no real advantage gained by using 11kV over 15kV in this respect. The manufacturers might arguably save a few pounds by using standard 11kV ancillaries like VTs, but not so you would notice.

It would help if the OP would disclose whether the generator is intended to be embedded in the 11kV network - I haven't heard of an embedded set this big in the UK, but what do I know? - or whether it will have a close-coupled GSU transformer and switchgear only on the HV side. If this is the case then it is probably down to economics. The IPB will probably be larger and more expensive for an 11kV system. A GSU transformer of this size will be a custom or semi-custom design so it is unlikely to be vastly different in cost.

One thing to consider is that some 15.5kV machines of British origin are, allegedly, an adapted 11kV design. We have a number of machines which our generator consultant believes fall in this category. He's forgotten more about generators than I know, so I take his opinion seriously. Many of the clearances are reduced to the bare minimum leading to problems with insulation deterioration due to discharge activity. If you are buying a 15kV machine, be sure that is what it really is, and not an 11kV design with go-faster stripes.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[rcw retired EE]

Use 15 kV - less current and better short circuit ratings on the switchgear. Most suppliers of Gas Turbines in this size will supply a 15 kV, 50 Hz generator. The manufacturer's standard generator voltage for 60 Hz is 18 kV. Keeping the V/Hz constant, so the generator design doesn't have to change means the standard generator for 50 Hz is 15 kV.

Buying an 11 kV generator will be more expensive, even if you could convince a supplier to buld you one.

 

[ScottyUK]

rcwilson,

That's interesting: I often wondered how Westinghouse arrived at 16kV for our steam turbines. It's been noted a few times that it is a big machine for its rating - it's probably because it is a rebadged 18kV 60Hz machine since it is of US origin. Sometimes the simplest answers are hardest to see.





----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!