Fault impedance - resistive or reactive? 1

[brainsalad]

Hello,

I am inserting fault impedance into 3-phase and line-to-line faults in order to find a limiting condition for overcurrent and undervoltage relay coordination.

This is for industrial 4.16kV and 480V systems, which have a fairly high X/R ratio.

Since the system is mostly reactive (high X/R ratio), inserting a purely reactive fault impedance produces the lowest post-fault bus voltage and lowest short circuit current, which is the limiting case for overcurrent vs. undervoltage coordination. For the same magnitude of fault impedance, the purely resistive fault impedance case results in a higher post-fault bus voltage and short circuit current, due to the large X/R ratio of the system.

What is the reality of fault impedance? When someone is talking about a sustained high impedance fault, as far as modeling goes, is the fault impedance itself resistive or reactive? Does anyone have any reference or basis for the nature of fault impedance, being resistive or inductive?

Thanks for your help.

 

[davidbeach]

Fault impedance resistance is almost always highly resistive. There's not really anything that can make a fault inductive. Look at the circuit elements that can be the fault connection. All you'll find are failed insulations and those will always appear to be resistive.

 

[ChristopherWalken]

It would be nice if faults were purely reactive. Nothing would be burning or blowing up by the fault.

 

[pwrtran]

Fault impedance is most reactive around 60 to 80 degrees depending on your system. Arc impedance is most resistive though.

 

[ChristopherWalken]

Why would the fault impedance be mostly reactive? Are you sure you aren't thinking about the PF of the fault current?

 

[davidbeach]

The fault current of a bolted fault will be at the line angle, generally highly reactive. But the fault impedance itself; that portion of the fault circuit that is between components not generally allowed to make contact, will be resistive.

Unless, perhaps, you have an unconnected reactor just sitting there for something to flash into. ;-)