Source Impedance

[jpcqub]

Hi, a few general questions on source impedance:
1. Why is mains a low source impedance?
2. Why are generators a high source of impedance?
3. and how does this high source impedance result in higher voltage waveform distortion where harmonics are concerned?

Thanks for your help in advance...

 

[Skogsgurra]

I wouldn't say that mains is always a low impedance source, compared to a generator. But, in general, that is the case.

The main's impedance is determined by the distribution transformer and the associated cables. And since the transformer usually is in the 300 - 1500 kVA range, it has to have a rather low impedance to give good voltage regulation (keeping voltage constant when load changes).

A generator in a power station also has a rather low impedance, but there are at least three different kinds of impedance, the x (which is delta-U/delta-I) and the transient impedance (or reactance, x') which is the impedance you measure before the AVR has done its work. Then, there is the sub-transient reactance, x''. It is determined by stray inductance in the generator. It is the x which is low.

A MG set generator or an emergency generator usually has a much lower power capåacity than the mains. That usually also means that you get a larger voltage drop for a certain load than you get if you load the mains with the same current. And load transients are usually not handled very well by small generators.

Gunnar Englund

http://www.gke.org

 

[jghrist]

A high source impedance results in higher voltage harmonic distortion because harmonic currents are produced by non-linear loads. The harmonic currents flow through the source impedance which causes the harmonic voltage distortion. The voltage at each harmonic is a product of the current and the impedance, so the higher the impedance, the higher the voltage.

 

[jpcqub]

Thanks guys...

 

[satnam123]

Jghrist,
"A high source impedance results in higher voltage harmonic distortion because harmonic currents are produced by non-linear loads"

If this is correct, why increasing the source impedance by using reactors is considered one of the solutions for reducing harmonics. Does that measn we are reducing current distortion and increasing voltage distortion.

I was once going through one of the threads and David Beach & Warooss commented that increasing impednace as one of the solutions.

 

[davidbeach]

Increasing the impedance on the branch of the system that feeds the harmonic source reduces the effects of the harmonics on the rest of the system by concentrating the voltage distortion in the impedance rather than over the entire system.