Current Tranformers secondary earthing 2

[giuseppe]

Hi to all.

I have an issue on the earthing (or grounding) at the secondary of Current transformers.

Generally speaking, the Primary of a Current Transformer is marked P1 and P2, with P1 normally being positioned where the Current enter the Transformer and P2 at the exit. Accordingly, S1 (or X1) is the mark for the secondary terminal carrying the power and S2 (or X2) is the secondary terminal normally earthed (grounded) for safety and references.

A Board manufacturer is now arguing that the earthing of the secondary is irrilevant for the correct functioning of Protection Relays installed on the circuit.

Is that right? I grew up knowing that this connection is important for Protection Sytstems as to avoid false alarms or, worse, reverse current circulation in the system. That's why the Polarity check on the Current Transformer is important...or is it not?

Could somebody please give me advise?

Thanks to all. Keep on with the good job so far.

Giuseppe

 

[slavag]

Hi Giuseppe.
We had a lot of threads on the topic.
Please search on the Forum ( CT grounding).
In all cases:
CT grounding/earthing NOT influence on the protection.
CT grounding is safety issue and as saied on the Forum:

"The point of providing a ground reference to the secondary windings of CTs is to provide a voltage reference to ground of the windings in order that the winding does not "float up" towards the voltage of the primary winding via capacitive coupling."

That means,A Board manufacturer must grounded the CT's.
Good Luck.
Slava

 

[giuseppe]

Thanks Slava,

I cannot find the other posts as they are shortened as CT... how dummy of me!

I think it is quite fine what you say, so i will take it for good. Much appreciated indeed.

Kind regards.

Giuseppe

 

[davidbeach]

Draw out your CT circuit without marking polarity. Connect as necessary for proper operation of the protection and ground appropriately. Now mark the polarity as installed; it won't matter if the secondary polarity is grounded.

Conventional installation practices are that the primary polarity is toward the breaker in switchgear and away from the device for free standing equipment such as transformers and free standing breakers.

 

[stevenal]

Earthing does affect protection. Without a ground reference, secondary voltage can rise up toward primary levels stressing and possibly failing the insulation of your relay. Multiple grounds will cause incorrect readings and mis-operation. If proper single point grounding is practiced is does not matter if polarity or non-polarity is grounded so long as the drawings are correct and the techs expect it.