Current Transformer orientation 2

[BritEng]

Hi All,

Please could someone answer my query.

On my current project we have installed an 11kV switchboard and a 400v switchboard/MCC. My question is regarding the orientation of the CT's.

On the 400v switchboard the P1 and P2 primary markings are in the direction of the power flow. On the 11kV board the CT's are all in the same direction, i.e. all P1's are closest to the CB on both incomers and feeders. This means that on the incomers the power flows from P2 to P1. Both boards are from the same manufacturer albeit in different countries.

Could some please advise on the correct convention. I have always seen P1 & P2 in the direction of power flow, however, we are still debating the point.

The CT's on the 11kV board are feeding a mulfifunction protection relay incorporating directional protection.

Many thanks

 

[Skogsgurra]

P1 towards incoming and P2 towards load is what I am used to.

Gunnar Englund

http://www.gke.org

 

[waross]

If you ever have the adventure of working in a part of the world where conventions are not followed, you will follow the normal conventions for the sake of future sevicing without massive confusion.
The transformer will function either way if the secondary connections are reversed when the primary connectioons are reversed, but you are well advised to follow skogsgurra's advice.
respectfully

 

[davidbeach]

Ideally CTs would always be installed such that the non-polarity of the CTs would be toward the protected portion of the circuit and connected in wye with the non-polarity of the CTs forming the mid point of the wye. The polarity side of the CTs are then connected to the relay.

But the world is not ideal. The same results - the relay won't know any difference - can be achieved by having the polarity of the CTs toward the protected portion of the circuit and connected as the mid point of the wye as long as the non-polarity of the CTs are connected to the polarity terminal of the relay. The currents all still go through the relay in the proper direction.

Why would the CTs be installed "backward"? In switchgear you will often find that CTs are installed with the polarity toward the circuit breaker. That is done to so that the label on the CT will be toward the breaker and available for inspection without having to remove the CT.

If the only measurement being used is phase current, the individual CT orientation does not matter. If zero sequence or negative sequence current it being measured, all of the CTs need to be in the same direction, but the direction does not matter. If directional overcurrent is being measured, the orientation of the CT connection to the relay is critical.

Regardless of the CT orientation, it is imperative that the project as-built documentation correctly indicated how the CTs are oriented and connected.

 

[hogdawg]

Ammeters don't care. All others do! Polarity always toward the source!

 

[ravishankarazad]

Hi BritEng,
CT will perform its action irrespective of its conection. However, the importance of this conncetion is critical for the following application:
1) Differential protection
2) Di-rectional O/C or E/F protection

Under this circumstances, it is required to test scheme through primary injection test. If required, interchange the secondary core connection S1/S2 and keep the as-commissioned drawing for future reference.


Regards,
Ravishankar Azad

 

[PhilGavin]

Different countries have different conventions. Follow your geographical regions convention to avoid confusion.

On HV CT's the casing is live and is bonded to either P1 or P2 and sometimes selectable.
In this instance, the orientation of a CT with respect to protected plant is important for real reasons.

A flashover to earth from the live CT casing, will now either bypass the CT winding (show as an internal to the CT)or appear as a fault on the other side of CT.

To try and describe this practically:
The same CT casing flashover could be judged by protection as either a busbar fault or a line fault dependant on whether the casing was bonded towards the busbar or the line.

Obviously, one would want to create electrical impression that such a fault was a line fault and only trip the line breaker as opposed to all the breakers on that buszone, therefore the CT bonding should face out towards the line.

As a rule of thumb, and with good reason, the CT bonding should always face plant with the least consequences caused by a trip.