Delta connected CT¦s for overcurrent protection 2

[aherbas]

I need to understand when and why should i use delta connected CT´s for ocrecurrent protection. I saw this kind of connection to protect grounding transformers, and one to protect a bus bar with tap relay adjusted to pickup 1.2 times 3 transformer rating capacity

thanks

 

[davidbeach]

When using modern numeric relays, I doubt there is ever a reason to use delta connected CTs. So much useful information is lost when CTs (or VTs) are connected in delta that one should almost always (I know of no exceptions other than a broken delta set of VTs for 3V0) connect in wye. The relays can do the necessary corrections in differential schemes where one would have used delta CTs with electromechanical or solid state relays.

 

[RRaghunath]

aherbas,

From your description, it appears the delta connection of CT secondaries is done to exclude zero sequence currents from coming to the overcurrent relay.

This is a sure way of making the overcurrent protection exclusive to phase faults (as earth faults produce zero sequence currents).

I am not sure if there is any specific advantage in doing so except, may be, making the relay coordination more straight forward.

 

[DiscoP]

A ground fault on the system will result in current flowing through each of the phases of a grounding transformer.

Connecting the CTs in delta means that this current circulates and no current is measured by the relay.

The only time the relay measures current is for a fault on the grounding transformer, hence a faster operation is possible.