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Sizing of Gas Turbine Generators

Sizing of Gas Turbine Generators

(OP)
I am trying to figure out the sizing criteria of large gas turbine generators (GTG) when non-linear loads are present. Information is easily available on the web for sizing the small diesel generators, but not much information is available for large gas turbine generators.

For sizing the small diesel generator: If the largest motor is VFD-operated, then we take 150% of the load of the largest VFD-operated motor, and 100% of all other loads, and sum all these loads. And if 50% of loads are non-liner then we de-rate the generator by 20%.

However larger GTG generators have higher inertia so they are different than smaller diesel generators.

What should be the criteria for sizing two large gas turbine generators (GTG) running in parallel, each of 30MW 13.8kV 0.8PF; when voltage harmonics is reduced to 5%, the largest motor is VFD-operated, and about 50% of the loads are non-liner loads? Thanks

RE: Sizing of Gas Turbine Generators

A single-line diagram it would help to interpret the problem

RE: Sizing of Gas Turbine Generators

Inertia:
Generally larger machines have more inertia than smaller similar type machines.
But inertia related to KVA may be the more important metric.
That said per unit inertia may have more to do with accepting and recovering from block loading and block load shedding than with non linear loads.
Your AVR may be a more important concern.
The AVR samples or monitors the generator output voltage and adjusts the field current to keep the generator voltage constant.
The harmonics of some non linear loads may badly confuse some AVRs.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Sizing of Gas Turbine Generators

In the US IEEE C50.13 is the generator standard for that type of machine (round rotor, above 10 MVA). I don’t remember off the top of my head what the harmonic limits or derated are, if I get the chance I’ll peruse it again.

RE: Sizing of Gas Turbine Generators

To size two large gas turbine generators (GTG) running in parallel, each 30 MW, 13.8 kV, 0.8 PF, with voltage harmonics reduced to 5%, and 50% non-linear loads, follow these steps:
Largest VFD-operated Motor Load (L3): 5 MW
VFD Adjustment: 1.5 × L3 = 7.5 MW

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