Help requested setting a 51N element on a 12.47kV switchgear 1

[bdn2004]

Attached is a 4page pdf of the single line, the vendor prints and a TCC of a 12.47kV switchgear ground curves.

We worked out the phase settings on the Main breaker to coordinate with the Utility, but I'm confused about the 51N settings. The Utility said to set them the same as the phase settings. That doesn't seem right even though the curves look coordinated. I didn't think the 51N worked on line currents but on differential current and its supposed to be a definite time setting. Any thoughts ?

 

[newelecteng]

Yes are right
There is time discrimination.
What is the relay type?

 

[bdn2004]

It's a GE 850 relay. This is set up just like the phase relay, and I agree it's coordinated but how is this working?

This is a 3-phase, 3-wire, wye connected, solidly grounded 12.470 kV system.

With only three CTs how does it know it's got a ground fault ? Is it looking an current imbalance at a certain threshold? And if it's looking at current imbalance why would it be set up like I've got it shown? Seems like you would just want to immediately trip.

On another note I've looked at other relays in the system at the medium voltage level like this one, and they all seem to be set up similarly.

 

[stevenal]

It looks at the summation of the three phase currents, 3I0, which will only be present during ground faults and unbalanced loading. The curve defines the threshold. Instantaneous tripping would not coordinate for feeder ground faults.

 

[mvcjr]

In your case, the relay internally calculates the sum of the three phase currents to determine the ground fault current. In some relays, there is a fourth current input (In), aside from Ia, Ib, and Ic, which is physically wired to take the sum of the three phase currents and thus it is measuring the ground current.