Ze of Generator

[Cookie]

How do you find the the Ze of a diesel generator when power a building, for example a 400kw unit?

In so far I have FLC of 1,110 amps. 7.5 (L-E) x 1,110 = 8327. 120/8327= 0.015 ohms

Somehow this seems just to easy.

 

[Lucien Nunes]

The machine's own resistance and reactance would normally dominate for a bolted fault at the main panel. The stator reaction depends on the magnitude and phase of the current w.r.t the EMF, not the terminal voltage. The difference between the two is no longer nominal nor hidden behind the AVR.

 

[Cookie]

It will, but whether it makes a significant difference is less clear.

Your example 400 kcmill supply cable is a little bigger than our 185mm (for which I have a table handy) where the cable Z is about 20% higher than R due to inductance, where as the 4/0 AWG cables are a little bigger than our 95mm size where the difference is around 5%.

With large cables, transformers, etc, it is always best to run the calculations using the full information of X & R values and then see what comes out, even if in reality something like a 5% difference is likely lost in the uncertainty of fault magnitude, etc.

Alright and agree.

However, what R & X to I assume for the generator windings when doing the equation to get the total Zs? And what voltage? I know this is a dynamic 4-D scenario, but I want to keep it simple (if practicable) to a 2-D set of equations.

 

[Cookie]

The machine's own resistance and reactance would normally dominate for a bolted fault at the main panel. The stator reaction depends on the magnitude and phase of the current w.r.t the EMF, not the terminal voltage. The difference between the two is no longer nominal nor hidden behind the AVR.

As I am reading up on it, and I could be wrong, the kw on the prime mover actually drops due to the R going down and the X going up. The current is reactive, and as the voltage goes down the field is "forced" to its max excitation to keep voltage up.

 

[Cookie]

As I am reading up on it, and I could be wrong, the kw on the prime mover actually drops due to the R going down and the X going up. The current is reactive, and as the voltage goes down the field is "forced" to its max excitation to keep voltage up.

Anyone know? R goes up as shirt circuits move away from the generator.