Overall Cable and Transformer Differential Protection

[Zahi1]

We are using Siemens 7SD522 relays (V4.6) for the overall differential of a transformer cable feeder. The protected zone consists of a 66kV power cable (~580m length) and a 50/60 MVA, 66/11KV, YnD1 power transformer. The protection topology consists of one 7SD522 relay at each end of the protected zone communicating through a direct fiber optic link. The first relay is connected to CTs (ratio 1200/1A) at the 66kV GIS Line feeder and the other relay is connected to CTs (ratio 4000/1) at the 11KV side of the power transformer.

I was wondering if anyone has used such a scheme before, and would appreciate any feedback on the subject. The difference in CT ratios can be compensated with a correction factor setting in the relay. However, I'm concerned about the effect of the phase shift introduced by the transformer vector group since it will cause the relay to always see a differential current of a magnitude that will vary with the load and may cause the relay to trip on heavy external faults if this is not considered in the settings (typically the setting is 0.2-0.3xIn).

Thanks in advance

 

[dpc]

The phase shift in the transformer must be accounted for, either by the CT connections, or by the relay.

With digital relays, it is typical for the relay to compensate for the phase shift - this must be configured into the relay settings.

With electromechanical relays, the delta winding used wye-connected CTs and the wye wnding used delta-connected CTs.

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[slavag]

Hi.
You have check your Siprotec 7SD522 relay order number, if it is include power transformer protection function.

http://siemens.siprotec.de/download_neu/devices/7SD5xx/Catalog/7SD5xxx_Catalog_SIP-2008_A2_en.pdf

It's standard function in the newer numerical line differntial relays (87L)and this relay have it.

Are you provide stability test for the block: line-trafo?
Best Regards.
Slava

 

[dpc]

I'm not familiar with this relay, but as Slava points out, you need a differential relay designed for transformer protection. In addition to dealing with the delta-wye phase shift, it should also have harmonic restraint.

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[Zahi1]

Thanks for the feedback, I have checked the relay ordering number against the catalogue sheets given in the link. The relay has the provision to be used in the same application with an optional function of having a power transformer in the protected zone and that has to be specified during ordering. Our protection designer and panel manufacturer has failed to take this into consideration and ordered the relays without this function.

I guess we will need to upgrade the relays firmware to enable this function.

Thanks again, your feedback has been most valuable.

 

[slavag]

Hi Zahi1.
It's also not problem.
Siemens will send to you some small upgrade file.
Few minutes upload to each relay.
You are not first and not last, we was in same situation
with other functionality in this relay .
Good Luck.
BTW, is really good relay and Im not work for Siemens.
Slava