Neutral CT sizing 1

[Golfex]

Hi all,

In a substation in Europe we have a YD transformer with the neutral solidly earthed as per the picture below.

The transformer is a 155/66 kV 160 MVA.

How can I calculate the primary rating of the Neutral CT?

Thanks in advance for your help.

 

[waross]

why do you want a neutral CT. This is a serious question. The purpose of the CT will influence the sizing.
You are aware that any primary voltage or phase angle inequality will be reflected in neutral current? This may affect your CT sizing.
Faults on either the primary or the secondary will be reflected in large neutral currents.
I spent about 15 years in a land where grounded wye to delta, transformer banks were common. I don't want to go back.
There are current backfeeds and neutral currents that are unique to that transformer connection.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

At 155kV, neutral CT primary rating will be same as that for phase CT. There is no reason to be different.
A neutral CT based earth fault protection is likely to operate for an earth fault in the upstream 155kV transmission lines etc. This happens because the Yd connected transformer acts like a zero sequence current source for earth faults elsewhere in the 155kV system due to Delta on the secondary.
Hence, while a Restricted Earth fault protection for winding faults is OK on primary, standby earth fault protection could prove to be a nuisance. Thus, if you must provide, delay the protection sufficiently to prevent nuisance operation.