Is it ok to tolerate small amounts of GROUND CURRENT in medium voltage feeders?

[bdn2004]

I'm looking at one of these Littlefuse Startco ground relays... SE-325. This relates to my other post but I'm little confused...is it ok to just let ground current stay on the ground system ? This relay has three settings 1, 2 or 4 amps all definite time.

Is it ok to pick 4 amps and definite time 2 seconds? That's the max settings.

What happens when 3 amps appears on the ground circuit ? That's ok to just let be ?

 

[Parchie]

@bdn2004,
A rule of thumb, set your earth fault protection to about 10-20% of the available ground fault of your system.
Or, to say it differently, the ground fault setting should be just a bit higher than the amount of charging current in your system. Set it lower and you will trip.

 

[bdn2004]

Does it work the same way on a high resistance ground? Per the literature for this relay...the setting should be 1A per every 1000kVA. This is a 7500kVA,4160V transformer with a 400A resistor . The relay only goes up to 4A. It looks undersized. We didn't design this....

I was reading that stray ground currents are always present at the substation and up to 80 Amps and the earth ground relay is usually set higher than this and greater than 1 sec. and is a last resort.

 

[waross]

Uhh, What ratio of CT are you using?
Multiply the relay setting by the CT ratio to get the tripping current.
That relay is not intended for direct connection.

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

 

[bdn2004]

It’s a 400/5 CT on a 400A high resistance ground. I’m only going by the calculator on their website. There are 3 settings: 1A, 2A, 4A. And that matches exactly with the EasyPower software.

 

[waross]

The 4 Amp setting will trip at a ground current of 320 Amps with that CT.

What happens when 3 amps appears on the ground circuit ?

3 Amps on the ground circuit will develop 37.5 milli-Amps at the relay.
3 Amps at the relay indicates 240 Amps on the ground circuit.

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

 

[stevenal]

320 is below the 400 rating of the resistor. There should be a time limit on the resistor as well, usually 10s. If relay trips at a current and duration less than the resistor, the resistor is protected. I don't see how transformer kVA plays into it.

 

[bdn2004]

Perhaps I'm wrong that's why I'm asking the question. Attached is the manual for this relay. It's set by the primary current, not the secondary.

 

[bdn2004]

And see article 2.1.5 that refers you to a Littlefuse calculator that tells you setting. That's what's attached.

 

[bdn2004]

And EasyPower has this relay in it's library. It's not going to model this wrong. Attached is the ground curve I get with this.

 

[waross]

It looks as if you have this well handled.
Congratulations.

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

 

[bdn2004]

I’m not trying to prove anything. But my question still hasn’t really been answered.

I used to work at an aluminum smelter plant that was right next to a huge power plant. The maintenance guys would claim there was 200A on the ground grid all the time in the switchyard. It came it up once they were taking this metal siding off the switchgear building and witnesses said they saw an arc 3’ long.

I guess there’s just stray ground currents that you can’t do much about.

 

[ppedUK]

Looks like you have the wrong relay. This relay is designed for NGR ratings of 5A, 10A, and 20A, hence the 3 available settings. You have a 400A NGR!

 

[bdn2004]

ppedUK,

I know - that's what I've been saying - it's the wrong relay - and that's per the Vendor's website also. But the capacitance charging current appears not to be a number that you can exactly calculate, and then go measure the value in the field to see if it's right when the system is energized. It's like a rule of thumb based on capacity.

The designers of the system have been told and they apparently don't think it's an issue.

The other function of this relay is to trip if the NGR connection is lost. And when asked - that's the reason they put it in. That's a good idea. However it is also in the trip logic for overcurrent. I set it at the maximum current setting (4A) and the maximum time delay (2 seconds).