Interposing Potential Transformer

https://www.quora.com/What-is-an-interposing-current-transformer-and-why-is-it-used

And, to think that you have thought that I do not know anything about ICT that you are giving me a Quora link to know what is ICT....

Please read and comprehend the question fully before answering hastily. I have asked about IPT and NOT ICT.

Anonymous may just be complaining about your use of a TLA...

Yes, auxiliary PTs have been used to get a phasing configuration different from what the main PTs provide, of which what you describe is a very common example. Modern digital protective relays can usually work with Wye-connected PT inputs and compensate in their calculations for the configuration really needed, but older relays aren't as flexible. And I think the term you really want is "open corner delta" or "broken corner delta." Just plain "open delta" is a little different, where only 2 PTs are used to get 3 phases of secondary.

Of course! The customer has a older version relay. And, I meant broken delta.

The high voltage wye is grounded on those systems, and it appears the primary wye of the auxiliary PTs must also be grounded. This is as mentioned by C. Russell Mason, The Art And Science Of Protective Relaying, see Page 320. He also mentions that you need a significant burden on the secondary to avoid an unstable neutral condition when the line section is indeed operated ungrounded, to avoid false trips. Normally your 59G relay comes with that resistor. The VA capacity of your auxiliary PTs you may want to match to your primary PTs. He references an AIEE Transaction from 1941 that perhaps gives information to size that resistor, and whether it is better installed in line with the primary wye ground connection.

I think I’ve seen dual secondary PTs used for both broken delta ground relay polarization and metering, BTW.

Interposing CTs are not uncommon. So no reason interposing PT should be problem.

As for CT, you do not want interposing transfo to give extra error. Relays 59 are not high burden.

I would look at an interposing PT with twice the saturation voltage of existing & see how it works out on availability, burden etc.

Is it normal to use metering core for protection? Even if you use a IPT, input for that IPT will be from metering core of existing PT?

If yes, we can replace existing meter with protection relay which has 59N protection as well which will derive from three phase VT connection.

If you don't want to disturb the meter, I think there are some stand alone 59N relays which can still calculate V0 from three phase voltage. This can be connected in parallel to meter. See one type in below link.

https://www.prokdvs.com/items/20

Thanks,

Vijay