Bus Bar protection relay operated

[rajendrarohra1978]

Hi all,

We have 34.5 KV switchgear with Bus Bar protection relay 7UT6 ( Siemens Siprotec)
The system has incomer & around 4 outgoing feeders. The system is working fine till recently when the new feeder for different plant was connected. The feeder was energized & minimum load was available. owever when the major load was started the Bus bar Protection for Bus-A section operated. The previous commissioning report during installation of 34.5 KV system showse testing report for all CT's & their polarities. We suspect the polarity of CT in new feeder may not be OK. Is there no Harmonic inrush blocking feature same as Transformer differential not available in Bus bar differential.

Thanks.

 

[davidbeach]

Harmonic blocking isn’t necessary for bus protection, the harmonic currents come in and go out, balancing along the way. Your issue is almost certainly a miswired CT or three.

 

[Mbrooke]

Look and verify everything including if the correct ratios are entered into the settings and the polarity. Make sure the CTs neutrals are grounded at one end only.

 

[JG2828]

Also check that the CT shorting terminal blocks are not shorted. The ones feeding that relay of course.

 

[submonkey]

Hi,

The oscillographic fault records in the 7UT6 will almost
certainly confirm:

(a) Whether the switchgear is likely to be healthy.
(b) The probable cause of the trip.

Download the records with DIGSI and get an appropriate
person to have a look at them.

Thanks,
Alan

 

[krisys]

From the past experience I can say that the bus bar differential operating with the increased load current is typical of

a) CT mismatching
b) CT polarity reversal
c) Forgotten to remove the CT shorting at the test terminals
d) Check the stabilizing resistor tap (if any), for the heating of the resistor etc.

The best thing to do is to periodically measure the spill current across the relay, while the load is increasing. Observe the reading and then you can establish what is the cause.