non load break switch

[protoslash]

We have a simple non load break switch on a 44kV line (circuit switcher, in-line disconnect w/e you call it).

The switch is energized but not carrying any load downstream. My understanding is that the switch can be opened manually as it is technically not breaking load current. However, when we did this, the switch drew arc and faulted.

One of my theory is charging current due to mutual capacitance of 3 phase line that is causing current to flow in the switch. If there is very little current (<1A), what would happen if the switch is opened? Should it still be able to handle it?

 

[Bill West]

Thanks for the follow up, Proto. Couldn't find any magnetizing curves for a PT but we can get around that. A 200va PT connected L-N on 44kv only draws 8 milliamps load. I would think that the magnetizing current would be less than that. Also I think MV voltage PTs are rated for about 125% continuous overvoltage so the magnetizing knee point is fairly high.

The low current however suggests that the induction would be very high, does that lead to a strong "kick" reaction, especially with no damping load around? Your 600ma line charging current leaves some capacitance to echo with too. All we need is one crest at 20-30% over normal voltage and the weak bushing could give in. Sorry, stretching my figuring here.

Bill

 

[protoslash]

The metering configuration is 2 element on a 3 wire system. There are only 2 PT, connected line to line with the common phase as reference.

Recreating the transient event mathematically is a tough ask. I think i will leave it at the qualitative level. Thanks for all the inputs.