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Plant Performance Q&A
This issue's question and answer are reprinted from the April 1999 issue of General Physics' "Utilities in General".
Question
Joe,
Yesterday, I was confronted with an interesting question that I couldn't answer. I figured if anyone could answer the question, you could. The question is: How do VARS impact heat rate? When we perform heat rate calculations, we base the generation upon the real MW generation. We don't make any corrections for power factor (VARS). On some days, the unit VARS can be quite sizeable, and we know that our generators have VAR limitations. If you were to hold the generator MW output constant, and increase the VARS on the unit, the generator winding temperatures would increase. This would lead one to believe that the shaft horsepower to the generator must also increase and that some form of VAR correction would be required when the heat rate calculations are performed. I'd appreciate your thoughts on the matter. Thanks!
Dave
Answer
Dave,
Good to hear from you - you have the concept correct and that's a great beginning. Assuming that shaft horsepower is held constant (i.e., turbine power), the gross generation from the generator will decrease by the additional amount of heat generated in the windings (rotor and stator i2r losses) as a result of "picking up" the reactive load (first law effect - energy flowed out of generator as heat instead of electric power). Generally, this is minimal and is pretty much ignored except for actual acceptance testing when you're looking for every BTU you can get. On a typical 500 MWe generator, when going from 500 MW at unity power factor (i.e., no reactive load = 500 MVA output) to carrying 200 MVAR with the same shaft power (i.e., MVA = 538 output), the generator losses will increase from 6.4 MWe to 6.6 MWe (i.e., 200 KWe). You should be able to calculate this for the specific machine in question using the Generator Losses curve provided by the manufacturer (usually in the generator book). Since 200 KWe is approximately 0.04% of the generator's output, the unit's heat rate will therefore increase by 4 Btu/Kwh (assuming a 10,000 Btu/Kwh unit heat rate).
Joe
Joseph R. Nasal, PE: Mr. Nasal is Vice President of Energy Services at GP. Over his 33 year career, Joe has helped thousands of power plant personnel assess and improve plant performance with his practical, hands-on approach.
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