I have a client that has a large unit connected generating station.

The main auxiliary bus has transformer differential protection for the main auxiliary transformer.

Their question is if they are racking in the breaker on the secondary of the main auxiliary transformer with the generator running and have an arcing event (on the line side of the breaker), what should the generator contribution be after the excitation has been removed?

The other sources (motors and utility) will be removed after the lockout trip and the excitation will be off, but there will still be some energy left in the generator.

How long will the generator still provide energy to the arcing fault? I talked to SKM, and they felt that the additional energy would be minimal, which seems like it might be not conservative enough, while a flat two seconds seems too conservative.

Has anyone come across this before?

It’s inductor but we usually measure impedance. Convert back to inductance and calculate joules of energy. Maximum power transfer to the arc occurs if the arc impedance equals the system impedance (we’re following the Lee derivation). Finally as with Lee calculate the energy over a 2 Square cm area of a sphere with radius equal to working distance. This gives incident energy.

We can really cast any electrical system into a case where we can calculate the energy in the system then divide by two (maximum power transfer argument) followed by the working distance calculation as a generalized Lee calculation. It’s use is the simplicity and that it produces conservative results useful as a first pass to determine if there might be a hazard.

Similar calculations are used for capacitors and DC arcs as given on Annex D.8 of 70E. After doing this exercise for a lot of similar situations and never getting to 1.2 cal/cm2 this is probably just due diligence. As with Lee the calculated value will be off by a huge multiple (measured DC arcs were 2-5 times lower) but since it’s a due diligence exercise anyways why beat yourself over accuracy.

Thanks Paul.