VTs / Potential Transformer for Protection Relay

We have someone trying to get details off the VT. What you have suggested makes a lot of sense and explains why L1 and L3 are fused and L2 is a link.

Peter,

you've just answered something that's been in the back of my mind for years.

The first gear I worked on was Whip & Bourne and they were Yy for the VT's.

It just carried it on as "the norm" in my mind.

I'm ashamed to say how many makes of 11KV gear I've worked on without question.

I'm an old dog, you've taught me a new trick.

Thank you.

The VT is probably a 3phase British make to standards BS 81, BS2046 or BS3941.

The most modern BS for this VT is BS7729 & a schmematic is included in all the BS's referred to above.

If you are now contemplating installing a more modern relay this 3PhUVT will not be usable & the 3PhUVT will have to be replaced with 3-1PhEarthed VT's to IEC 60044-2, BS7625 or a 3Ph5Limb to BS7729.

This may open up a "can of worms" as the original 3PhUVT(3Ph3W) was inversion free, the replacements(3Ph4W) may not be so due to neutral distortion.

The grouding is only for safety, you can change provided you earth the secondary at one point only.

y-phase grounding of VT secondary in generator feeders is quite an established practice. This is especially relevant for generators with high resistance grounded neutral.

IEEE242 has good discussion on the benefits of this arrangement.

install a 5 limb 3 ph VT. 3 limb VT is for 3P 3W system not for3P4w. in 3P3W system Y ph always earthed as it is a reference for 2 element metering. generally in 11 KV system PT is used for metering only a there is no directional element in O/C relays

Take all the three phases out and the neutral grounded to the protection Relay.That would be sufficient but disconnect the y phase earthing, instead of y phase we use to ground neutral and that neutral closes its path through the common point of relay star point.

Many thanks for your useful comments. We have managed to get to the VT and have found A,B,C,N and A1,B1, C1 and N1 coming out on terminal posts. Likely the VT is 30 years old.