high impedance faults

Not a lightweight concept!

Searching "hirop" or hirop and electrical protection systems may lead to a definition. It would probably be worthwhile having a good grip on 3 phase HV protection systems before you dig into HIROP.

recyclecomputer@nospam##hotmail.com for direct response

A high impedance fault is simply resistive, capacitive, or inductive leakage of a high impedance, in contrast to a low impedance dead short.

Harry C.

Every fault will have some resistance to ground (or whatever the return path might be). As with any other path for current flow, some are better than others. A high resistance fault will be one which has a very high resistance path for the fault current to get back to source.

One example would be:

Snow at very low temperatures. Snow, at low temperatures, is a relatively poor conductor. A few years ago, locally an overhead 25kV line had one phase broken by equipment passing below. The broken phase conductor fell to earth, landed in the snow, and the path was so poor that the recloser on the system operated 7 times before the current path got to be good enough for two consecutive trips to be close enough together for the relay tro know that it was "seeing" an actual fault, and it should trip.

(There was sufficent resistance in the return path for current flow that it actually limited the amound of current which flowe3d, so the clearing device did not recognioe this as an actual phase to ground fault, in spite of the fact that the phase conductor was resting on the ground.)

Does this help?

HR.

As the name implies, they are simply faults with more than zero impedance. The problem can be when protective devices/relaying doesn't sense enough fault current to operate. For example, a broken phase hanging in a tree or across a structure. Perhaps the impedance is high enough that the fault currents don't immediately trip the protection devices.

But you might have nearly full rating of the line being dumped into a fault (something that can't necessarily 'use' that kind of power safely). Burning, fire, bad things. If the fire progresses of course, the fault gets 'worse' and eventually trips.

This is one reason why a variety of protective relaying is used, not just short-circuit protection. Phase imbalance, differential relaying and ground-fault are some ways to detect this type of fault and deenergize the equipment before too much damage.

daestrom