REF SETUP ON TWO LV SWITCH PANNELS WITH ONE MCAG

[warmulli]

I have come across a REF setup causing some confusion, any help. Comments are welcome!

Setup: One 11KVA transformer with two LV feeds supply two C&H LV Switch panels with one switch panel containing a interconnector supply another LV panel.
The MCAG setup is as follows: MCAG relay located on the main LV Switch panel with CT's installed on both LV switch panels protecting the supply cable with no CT's installed on the Sub panel as this is fed via the interconnector.

This is where the confusion starts, On the LV switch panel with the MCAG relay fitted there was 4 CT's installed 800/5 ratio main incomer ACB rated at 800Amps, on the other switch panel there was 3 CT's installed ratio 1600/5, Main incomer ACB rated at 1600amp, missing the Neutral CT causing the relay to trip when the load become unbalanced due to the missing Neutral CT not be able to observe the Neutral current. This was rectified by installing the missing Neutral CT. The arguments is some people are saying that the ratio of the CT's at both panels have to be the same ratio and that the 800 CT's will need to be changed for 1600 CT's for the correct operation of the MCAG relay. My understanding is as long as the set of 4 CT's are matched then there is no need to swap the 800 for 1600. By changing out for 1600 ratio CT's your reducing the required fault current needed to trip the relay.

Upon viewing the as built drawings the above setup is as per drawing including the missing Neutral CT, there is also a N-Earth link missing on one of the LV panels. The panels are around 12 years old and as being cutler and Hammer there is no way of getting any information out of them.

 

[Marmite]

A picture speaks a thousand words and it would be useful if you could post an AC schematic showing the location of the CT's and all connections.
However, as a general point, the CT's have to be closely matched in terms of ratio and magnetisation characteristics for high impedance protection to ensure correct operation for in zone faults and stability for through faults. The MCAG brochure and manual is available electronically at

http://www.alstom.com/Global/Grid/R...ty circulating current relay Brochure GB.pdf.

This states clearly that the CT's have to match.

Regards
Marmite

 

[warmulli]

Hi Marmite,

I have attached the drawings showing the two sets of CT's, you can see the neutral CT is missing.

Regards

 

[waross]

I understand that the CTs shown as 400:5 are in fact 800:5. No problem.
The CTs in each set must be matched. There is no need to match one set with another set. If this has been in service for 12 years then I assume that the original loads were all line to line and more recently line to neutral loads have been added.
The 1600:5 CT that was added should match the existing 1600:5 CTs. That is one set. No need to match the 800:5 CTs.
BUT
One drawing is labeled Mains incoming and one is labeled Gen incoming. Do you have the correct drawings?

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

 

[warmulli]

Hi Bill,

Please see attached drawing i have added, it shows the transformer with the two LV supply’s coming off it feeding the two LV panels. Both LV panels contain a set of CT's and one panel contains the relay MCAG16 looking at both sets of CT's protecting the supply cable/transformer. Relay set at 0.5A with a stabilizer resister to filter out any untoward. One set of CT's are 1600/5 ratio and had a missing neutral CT that has been replaced and the other set are 400/5 ratio with all 4 CT's present. The problem is that im get confecting arguments say that both Sets have to be the same ratio and matched. So the 400/5 would have to be changed for 1600/5 to match the other set. However i can get my head around why this would have to be done. The other thing is the neutral earth link located after the CT's on the LV panel one panel has it the other don’t, am I wrong in saying that high neutral faults could develop and without the link you can’t keep the potential at a safe level?

Thanks
Regards
Warren