Double Earthing of CT Circuits 1

[Vegemite]

We have a three phase overcurrent plus earth fault relay installed on a bus tie breaker in which the earth fault element is fed from the residual connection of the three phase circuits. The CTs are IEC P class 1200/1 A. Recently, when the circuit was passing 700 A, the relay tripped on earth fault. The pick up current of 60 A was exceeded and after 90 seconds the trip occurred.

To the best of our knowledge there was no earth fault current in the primary system. Thus our attention is focused on the secondary side. We believe that the CTs are sufficiently accurate and could not have caused this trip.

My question is, "If the CT secondary wiring has been earthed both at the CTs in the metal clad switchgear as well as at the relay panel which is in a control room 80 metres away, what form of EMC or other mechanism could cause secondary circuit zero phase sequence current to flow?"

 

[waross]

As for the double earthing, that is considered bad practice.
Most of us here have much more experience doing it right than doing it wrong.
But, could it cause a trip? Well with 1200/1 A CTs it would only take a little more than 0.5 Amps on the ground to cause a trip.
Two things to try:
Measure the voltage between both ground points. Any potential difference between the two ground points may cause tripping.
Measure the current to ground at one of the CTs. There should be zero current. Any current to ground may become worse and cause a trip, depending on the cause.
Also, a residual connection sums the phase currents and assumes that any non-zero sum is a ground fault.
Unbalanced currents between phases may cause a ground trip on some schemes.

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

 

[Vegemite]

Waross,

Thank you. We shall try to tong test the CT neutral - earth wire to see if anything is flowing in it. That should be possible for our testing people.

 

[waross]

60 Amps at 1200/1, 60/1200 = 0.05 Amps.
That may not show up with a tong tester.
Also it may have been a temporary condition causing the current.
Is it possible that the phase currents were unbalanced?

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

 

[marks1080]

Ground CT circuits at a single point only. Most likely the cause of your false trip.

 

[stevenal]

When rebuilding a substation, we wired the CTs with the normal grounds at the panel. The bushing CTs on the open breakers were still shorted and grounded from the factory when we saw double digit currents on the panel meters. Currents went to zero once the factory shorts and grounds were removed. No earth fault is needed to make double grounds a bad idea. Single point grounding is necessary for accurate relaying and metering.

The suggested testing above seems more complicated than necessary and small secondary currents may go undetected. With the bus tie open, first look for current. Then lift the intended ground and use a continuity tester to test for remaining grounds.

 

[scottf]

echoing the above comments...I recommend you remove one of the secondary ground connections. Not have 2 points of ground on an instrument transformer secondary circuits is one of the "basics".

 

[HamburgerHelper]

I don't know what anyone would gain by having two grounds other than it is twice as unlikely for the circuit to become ungrounded.

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