High impedance wires

[wwhitney]

I've continued to gnaw at the idea of adding inductance to drop the fault current.

Wanted to check my understanding:

The only way to drop the fault current is to add impedance. One reason to add reactance rather than resistance is that if the load is power factor 1, then the voltage change at the load due to the extra reactance will not (to first order) occur as a drop in the magnitude of the voltage, but as a phase shift, so the load performance is not affected. Correspondingly, if the load power factor were say 1/sqrt(2) (45 degree power factor angle), then you'd want to add resistance and reactance (of opposite sign(*) to the load) in equal parts to achieve the same effect.

Is that right?

Cheers, Wayne

(*) I'm not sure if the convention is that inductance or capacitance causes positive reactance (I think it's inductance?) and if non-unit power factor loads are typically inductive or capacitive (I think inductive, since you hear of capacitors being used for PFC?)

 

[winnie]

In a bolted fault, the load impedance is considered to be zero, so the load power factor doesn't matter.

You are correct in thinking that adding impedance that is 'perpendicular' to the load impedance would minimize voltage drop during normal operation.

You are correct in thinking that to maximize the effect of added impedance, you'd want 'in phase' with whatever impedance remains in the system during a fault, to maximize the reduction of fault current.

-Jon

 

[wwhitney]

In a bolted fault, the load impedance is considered to be zero, so the load power factor doesn't matter.

Right, I understand that. Thanks for answering both the question I asked and the questions I meant to ask. : - )

So far the discussion has been about passive elements which I understand behave linearly as a fixed impedance through the full range of currents from 0 to operating levels to fault levels.

Nobody has devised a passive non-linear circuit element that can withstand fault currents and behave as a near zero impedance at operating current levels and a higher impedance at fault current levels (which would correspondingly be lower)?

Cheers, Wayne

 

[winnie]

I remember seeing something about supercondor based systems which have very low impedance in their operating state, but if current gets too high they cease to superconduct. Don't think it is an available product.