Time delay setting on a zero sequence relay due to capacitance charging/discharging in the feeder? 2

[bdn2004]

We've a 12.47kV feeder breaker that feeds a 7500kVA 12.47kV/4160V transformer, delta-wye. 2500'.

There's a 50/5 zero sequence CT around this feeder. I was told there is a capacitance charging and discharging in the feeder and to delay the trip because of it. I realize there shouldn't be any zero sequence current in this circuit - unless faulted. Is this really a concern?

If so, what kind of delay is typical?

 

[Parchie]

@bdn2004,
As far as I can remember, you are allowed to set a maximum delay of 1 second for circuits 3000A or above. Please see 230.95

 

[bdn2004]

there is a residual ground fault element....51N and this 50N zero sequence relay. Both are set to trip on like 20A (50N) and - 60A (51N). They protect feeders to a delta-wye transformers. A ground fault detection on these relays would be a fault on the feeder. My question is just how sensitive do you make these? Can capacitance discharge to ground be enough to nuisance trip these relays ?

 

[davidbeach]

The charging current should show up on all three phases approximately equally and therefore not be seen by a residual element. What can be of concern is inrush that can show up as containing a lot of 3I0 immediately on close.

 

[ppedUK]

For de-sensitising the GF relay against leakage capacitance (to ground), time delay won't be of any use. On very long cable feeders you would normally set the pick-up higher than the leakage current of the longest feeder on that switchboard. Directional GF (67N) looking into the feeder allows lower GF settings, however I wouldn't expect a pick-up of 20A to be a problem on feeder of the length you describe.

 

[bdn2004]

What is the calculation for the leakage current ?

And on the inrush issue... what do you do?

 

[protoslash]

For inrush, you set logic to disable 50N and 50phase upon breaker close. Since inrush currents has high 2nd order harmonic, modern SEL relays also have 2nd harmonic restraint feature to supervise your 50N and 50P element.

 

[bdn2004]

I thought the even numbered harmonics cancelled each other... ?

 

[protoslash]

just because all even harmonics appear to cancel in the overall waveform doesn't mean 2nd harmonic is not present. The relay filters for this and helps increase security.

 

[waross]

You may wish to coordinate the time delay with any reclosers on the circuit.
If the ground fault relay trips too soon, it will prevent the proper operation of the reclosers.
Set the tripping current above the steady state charging current.
Any switching transients due to charging will be short in relation to a recloser cycle.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bdn2004]

I appreciate the advice on setting the harmonic restraint. I'll find out if the relay was programmed to disable these elements on breaker close. It's either one or the other right?

I was watching a video and this guy makes the same point you do. His slide is attached. They've had issues at this plant energizing these transformers, as it trips the breaker and they try again. That's as much as I know. The existing relays are 20+ years old. Is this a feature that's been around a long time?

Thanks for the advice on reclosing - but this is not a Utility or overhead lines, which is the only place I've seen reclosers?

 

[protoslash]

you don't have to choose one, it's ok to have both logic active. At the end it depends on your use case and various scenarios that result in in-rush condition. For our utility practice for example, we have a third logic in there that blocks 50N and 50P upon loss of source voltage, this way when source comes back, in-rush does not mis-trip protections.

The harmonic restraint existed for a while, modern relays from GE or SEL all have this feature.

 

[bdn2004]

I've found out the GE relays used do indeed have this harmonic restraint function. That's the good news. But you have to order it with the relay and it's not field upgradable, and these were not specified with this feature.

It's been suggested to write in Flexlogic a delay on startup to ride out the inrush. I'm not sure what all that entails.

 

[ppedUK]

The O.P. here was regarding capacitive leakage current in the feeder, how has it developed into a discussion around transformer inrush and 2nd Harmonic restraint which has nothing to do with cable capacitive current. Am I missing something?