Fault currents differences

[wonderchi]

I have done a fault analysis on a power station, and obtained results from a software. The report shows initial symmetrical currents, asymmetrical currents and initial symmetrical netural currents.

The asymmetrical currents are a lot bigger than the initial symmetrical currents. Is this normal?

Why are the asymmetrical currents bigger than the initial asymmetrical currents?

When I am sizing circuit breakers, should I looked at the initial symmetrical currents, or should I looked at the asymmetrical currents.

Thanks for helping

 

[dpc]

I suggest you obtain a basic reference book on electrical power systems. Your questions will be answered there.

Asymmetrical fault current values will always be equal to or greater than symmetrical values. The difference between the two is determined by the X/R ratio of the system impedance at the fault point. In real life, it is also a function of the voltage phase angle at the time of the fault. Asym values can be significantly higher than sym values, especially near large transformers and generators.

Modern circuit breakers are generally rated in symmetrical amps, but the breaker specifications will tell you that. The symmetrical rating is based on a maximum X/R ratio. If the actual X/R ratio exceeds the specified maximum, the interrupting rating must be de-rated accordingly.