Voltage Variation

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jghrist said:
Because transmission lines and transformers have more reactance than resistance.

VD = I_real?R + I_imaginary?X
Click to expand...

Wrong formula,

VD = I*(R*cosx + X*Sinx)

Now, I am not agree with you in this because.
1-In design we all consider reactance and resistance.
2-low voltage feeder has high resistance than reactance
3-My question is about power flow.
 
Hameedulla-Ekhlas said:
Wrong formula,

VD = I*(R*cosx + X*Sinx)

Now, I am not agree with you in this because.
1-In design we all consider reactance and resistance.
2-low voltage feeder has high resistance than reactance
3-My question is about power flow.
Click to expand...

I_real = I?cosx
I_imaginary = I?sinx

Real power = I?V?cosx
Reactive power = I?V?sinx

Are you sure that "system voltage variation are more sensitive to reactive power flows than to active power flows" in low voltage feeders?
 
jghrist said:
I_real = I?cosx
I_imaginary = I?sinx

Real power = I?V?cosx
Reactive power = I?V?sinx

Are you sure that "system voltage variation are more sensitive to reactive power flows than to active power flows" in low voltage feeders?
Click to expand...

I am only sure that system variation are more sensitive for reactive power for both low voltage and high voltage. since we are talking about power flow, it doesnt make any difference of low or high. But wanted to know the reason
 
It seems like there are two different subjects here, one is voltage drop on a feeder (for simplicity) and the other reactive and real power flow in a grid connected system. Remember one way a power provider controls reactive power flow is by changing field excitation which would otherwise change output voltage (in a system without grid connection). This book has a reasonably easy to read description of power flow... check around page 202.

http://books.google.com/books?id=bW...&resnum=1&ved=0CBMQ6AEwAA#v=onepage&q&f=false
 
Over exciting a generator will increase voltage and the generator will push Vars out onto the system. Lower voltage, by lowering excitation will take Vars from the system. It's a balancing act for system operators. The generators ability to do this has a lot to do with the impedance that was selected for the step up transformer.

Take a look at IEEE C57.116 which discusses transformer impedance and how it relates to generator reactive capability. That may answer some of your questions.

Otherwise, I'll need a little more help on fully understanding the basis of the question.
 
Hameedulla-Ekhlas said:
I am only sure that system variation are more sensitive for reactive power for both low voltage and high voltage. since we are talking about power flow, it doesnt make any difference of low or high. But wanted to know the reason
Click to expand...
But as you said before, low voltage feeders have higher resistance that reactance. Because of this, low or high does make a difference.

Power flow is related to current flow.
 
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