Neutral Transformer Protection

Er, perhaps there is no neutral on the secondary?

The primary transformer is Delta, and one of the secondary is also Delta.

That means I have to provide a ZCT either on primary or secondary for the 50N and 51N protection right?

Why not ask the Client?

I am afraid that your terminology is incorrect. In the Primary Delta as well as in the secondary delta, there cannot be any neutral and hence there cannot alos be neutral protection. I assume that you want protection against earth faults in the delta side. If so, this can be easily acheived by connecting a sensitive earth fault realy in a residual connection of the phase CTs or a CBCT or go for a relay which can provide earth fault protection with zero sequence sensing.

@electricalexpert65:

You are right, that's what I meant for the protection. Then agan, what is the difference, for example, regarding the sensitivity etc, if I put the CT on the primary or on the secondary? Or I can just put it on the primary side, compare to providing 2 sets of CT on the secondary?

Does earth fault current exist when both sides are Delta?

We may need two simultaneous earth faults to get fault currents !!!

OP actually says 3 winding transformer. May be it is actually D/Yn/d with secondary Y.

In this case the star side can have neutral - 51N relays.

If both primary , secondary are D, no neutral current exist as there is no path for the same.

@raghunath7

Actually its a 3 winding transformer to feed 12 pulse drive. So the vector group on the secondary are D and Y.

Assuming your source end on primary side is grounded, you can measure the neutral current on the source side [Delta - Primary] for earth fault location from the source end CT's to Primary Delta winding location.

Again you can measure secondary Yn side neutral current using CT's for earth faults on Yn side for faults from the secondary Yn CT locations to the load supplied.

However, you cannot measure the earth fault current on the other secondary Delta side, unless you intentionally ground this second delta winding on secondary side.