Secondary connection in VT for Neutral Voltage Displacement 3

[Rwelagedara]

Hi Guys,
Please guide me for the following.
We are to provide neutral voltage displacement protection in a solar power plant. For this, a 3 phase protection VT is utilized.Primary winding of the VT is connected to 33kV lines in the power system.Primary winding is star connected and neutral is grounded. The secondary winding is connected as open delta and this open delta terminals will be connected to the protection relay. Attached sketch shows the connections.

However, as per our undestanding , we should connect one terminal of open delta to ground(ground connection shown in red in the attachment) when connecting to relay terminals, in order to provide a reference to the relay. Is it correct to connect one open delta terminal to ground, since it seems like a line to ground short circuit when looking at the VT.Can a relay operate without connecting one terminal to ground?Will there be an issue to 3 phase VT with this connection?
Please advice.

 

[waross]

IEC open delta = US broken delta.
If the relay does not have one terminal grounded internally, it is good practice to ground one leg of an instrument transformer input.
This avoids a possible common mode voltage rise on the PT secondary circuit due to capacitive coupling.
There should be one and only one ground connection in an instrument transformer secondary circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Kiribanda]

As Waross mentioned you are using the BROKEN DELTA VT connection and not OPEN DELTA.
In fact, OPEN DELTA is the VEE connection used in two VT arrangement inside MV metalclad switchgear to save space.
One side of the broken delta connection has to be earthed and the stabilizing resister has to be
properly sized based on the VA rating of the VTs used to avoid any ferroresonance issues.

 

[prc]

Some additional points-
1) The PTs shall be single phase units or if 3 phase unit with 5 limbed core.
2) An over voltage factor of 1.9 for PTs
3) Resistor value may be such that about 50 % loading of secondary VA