We have a captive power plant which includes 4 GTs. i want to know whether i should run the plant on high frequency, say 50.10Hz or on low frequency say 49.95 Hz? which will be more economical? which will have give better GT efficiency?

 

Manoj... the technically correct answer is, “It depends on the load-characteristic!”

a) If frequency increases and the load is mostly inductive, i.e., blowers, fans, compressor, pumps, and the like, then power will increase due to the "Affinity Law!"

b) The Law states that power for the loads mentioned above, power increases as the cube of speed! Thus for a frequency increase of 0.1 Hz the speed increase is 0.2%! The resultant power increase for machinery alone will be about 6.1%. In addition, cable losses are proportional to the square of current!

c) If the load consists of mostly electrical heaters, then there is no difference in power, as long as voltage is unchanged!

d) If the load is a combination, then an equivalent load-impedance must be determined! Consequently the effect of a frequency change on the reactive component will impact on net kVA, thus impacting on generator output!

Regards,
Phil Corso

 

Thanks to the person who noted my error, thus expressed Thumbs-Down!

The correct increase should have been 1.002^3!

Phil Corso

 

Thanks Phil,

My question was if i only want present my GT report. in that case when will my heat rate show lower in a system of low frequency or high frequency considering my load is mainly pumps and fans/blowers?

again i am not talking about complete system; only about GT heat rate/efficiency.

thanks again for the reply

 

Manoj... sorry, but my contribution demonstrated how very small frequency changes affect the kW component of the connected load!

To determine GT efficiency, measured in heat-rate, i.e., kW per unit of fuel used, requires testing!

The test method I'm thinking of is similar to that for finding the minimum excess-air value associated with combustion!

I'm sure CSA, or others, can shed much more detail then I can!

Regards,
Phil

 

If you look at the performance curves provided with the gas turbine you will have your answer! I know for GE design heavy duty gas turbines, maximum efficiency (minimum heat rate) is at the design 100% speed. Go above or below 100% speed and the efficiency drops. I would expect this is true for all the other manufacturers.