Distance relay

[haisa]

Hello
My distance relay at a 132/33kV terminnal sub-station is tripping due to faults on 33KV feeders. My suspicion is that it is looking at the wrong side. How do I confirm and correct this please?

 

[dpc]

You'll need to confirm phasing and polarity of all the CTs and PTs and compare with the relay manufacturer's recommended wiring diagram.

You may be able to look at the actual phasors in the relay and see if they look as they should.

There are a lot of ways to hook up the CTs and voltages - but only one way is correct.

Sometimes the hardest part is figuring out how it is supposed to be connected. You should also review the relay settings and make sure it is set to look in desired direction.

 

[DTR2011]

If it is a digital relay, you can follow DPC's advice and use the metering function of the relay and plot out the phasors. If it is an older electro mechanical device, you will need a phase angle meter, etc.

Where are the CT/VT supposed to be installed? What type of scheme do you have? Do you have any commissioning records?

Some relays have default settings that need to be changed or disabled, depending on the application.

As always here, the more detailed information will yield a better answer.

 

[DiscoP]

Assuming this is a numerical relay, I would recommend examining the fault records first. It could be that the direction is wrong, but it might be something else too. These records will help you pin point the problem if available.

 

[slavag]

Two options:
1. Wrong direction of forward zones.
2. Wrong setting of back zone.

What is type of protective relays?

 

[haisa]

The relay is micom p 422.
Is it possible to change its direction by changing the star point of the CT or the phase rotation of VT?

 

[mgtrp]

To change the direction that the relay sees as forward, you need to swap the polarity of either the CT's or the VT's.

Please bear in mind that there is more than one way to misconnect the instrument transformers to the relay. You have a numerical protection relay, which makes confirming CT connections a breeze. If the system is already in service, then go and check the phasors online and compare with what you would expect (perhaps compare to metering or another relay on the same line?). For new relays, we often use line charging current after first energisation to gather phasor information.

From this, you should be able to work out exactly how your relay is misconnected (if at all), rather than a "try it and see" approach.

 

[DTR2011]

I am not 100% familiar with the Micom relay, however "star point of the CT", if that would be a setting may change the polarity of the CT.