87G when fault on transmission line 1

[electmags]

When ever is a problem or surge on the transmission line our generator unit # 3 87G relay activate on A phase, why?
All Ct's are ok, but when the surge capacitor for A phase is disconnect we trip on 59N.Do any have any idea why?

 

[rbulsara]

Is the surge arrester within the zone of protection for 87G? If so what is happening is normal.

59N is a voltage bases relay, that is it senses ground faults based on voltage unbalance created by a fault. It is typically employed in impedance grounded systems. Operation of surge arrester or loss of a phase could be seen as ground fault.

Firstly, look up theory of operation of the relays you are mentioning. Then have someone experienced look at the relay settings and ratings of surge arresters.

Other reason for 87G could be harmonics during surges, see it your relay has harmonic restraints. While not transmission related, see thread238-164352 for some insight.

 

[ScottyUK]

Depending on how big the surges and 'problems' on the system are, unmatched CT saturation characteristics could be a cause of the 87G operating. A plain ratio test won't reveal this unless you have the means to generate fault-level current (which is highly unlikely) so the next best thing to do is plot the mag curve. Check the results of the test against the design of the scheme to ensure that the scheme will remain stable under through-fault conditions. Also look for anything untoward such as stabilising resistors being incorrectly set.

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Sometimes I only open my mouth to swap feet...

 

[Alex68]

Real experience:
We had the same problem: the 87G always tripped in case of fault on a trasmission line. The explenation was extremely difficult to find but it was demonstrated by several tests. The generator CTs at the terminals side were close to the enclosure of the generator busducts and very close each other. It was demonstrated that the flux of the currents on one phase involved also the CT of another phase.

You can say: current of phase A doesn't flow through the CT of phase B so you are wrong.

But you don't consider that the CTs are very big and the CT of phase B has one side close to phase A and one far fron it. So the flux in the far side is quite less than that on the close side. We have demonstrated that this difference was able to produce a current flowing in the CT which tripped the 87G.

 

[Borti]

I would check the slope of the differential relay. That may take care of the CT saturation problems if you have any. The currents during the external fault should be in the restraining region.

When it comes to 59N, neutral displacement relay, your time delay may be too short and you trip on the capacitor switching transient. These are all "mays". There could be something completely different.