reactive power and fuel consumption

Ramona0304,

This is correct as per theory. The power factor of generator is proportional to ratio MW/MVAr. So any increase in numerator with denominator remaining constant will result in improved power factor. similarly any increase in denominator alone (i.e. MVAr) will result in dropping of power factor.

In a grid, the MW is directly proportional to speed reference. Any change in it will not affect MVAr. Similarly MVAr is directly proportional to magnitude of terminal voltage which is directly proportional to excitation.

So by increasing field excitation, you are increasing terminal voltage of generator. This added terminal voltage will result in generator supplying more MVAr & thus resulting in dropped power factor
 
Keep in mind that whatever heat is generated during a fault is real power and real MW's. I have seen an off line fault of a bad lightning arrestor that caused the turbine speed control system to call for more steam to maintain speed. I agree that most of the current flow is reactive, but not all.
 
Prasad: remember, current does not "contain" MW.

Watts are produced when current flows thru a resistance and all generators, bus work, transformers etc have some unless they are superconducting. So when current flows thru a resistance, watts are produced in the form of heat in an amount equal to I-squared-R. Those watts dissipate energy which must have come from somewhere. In this case, "somewhere" is your generator which converted mechanical energy into electrical energy. Your generator received the mechanical energy from the conversion of thermal energy in your turbine, therefore the turbine must have burned more fuel. So increased reactive power generation increases fuel consumption.

The law of conservation of energy always applies.
 
S
> how can the reactive power affect the fuel consumption

> i'm confused about this plz help me

Reactive power can't affect fuel consumption. Because if reactive power(say lagging) increases then the frequency of excitation voltage decreases, but that doesn't mean that the angular velocity of rotor changes.

So to get back the actual frequency we use power factor correction and after that automatically the reactive power decreases and the frequency gets back to its actual value.
 
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Engr. Md. Zakiul Hoque

I agree with you partially but not fully with your comment "this should increase the fuel consumption, but it is so small that it will not reflect in the fuel flow much." Actually this amount depends on the Alternator Efficiency Change. The Alternator efficiency changes @ of change of power factor. Suppose for p.f 0.95 the efficiency = 97% but for pf 0.80 it is 96% (can check from the alternator test results). So, this change in efficiency needs more power from the prime mover (Engine or turbine). Hence it requires more fuel consumption accordingly. So, you cannot say this is not so much. it is eventually a lot for large scale generation.

Thanks.
Zakiul.
 
Simply speaking

Reactive power is the power which is given to the system in the positive half of an AC cycle and received back in the negative half. overall average power through one complete cycle is zero. thus theoretically speaking there is no increase in fuel when you decrease your PF of simple increase MVAr.

But then why do we try to maintain high PF? It is because any system is not ideal. All system will have resistive losses which are in turn directly proportional to square of current through the system. even though as mentioned reactive power over a complete cycle is zero, reactive current will still cause a resistive loss in cables, transformers, etc. thus decreasing efficiency. but as mentioned in earlier posts, the effect is seen over a period of time. when you calculate you will have a substantial saving overall in an annual period.
 
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