Voltage Balance Relay / Switching VT inputs

[Mbrooke]

Does any have a general wiring diagram of a voltage balance relay application used to switch CVT circuits feeding protection relays? Or is there another recommended way to do it?

Also, does anyone know how a 311C or 421 behaves (step distance and directional over current wise) immediately after 3 phase voltage has been briefly removed and then re-applied?


In simple terms I want to be able to switch AC relay voltage between bus CVTs.

 

[DTR2011]

Either SEL relay should have a “LOP” available-there are several flavors, or
You can use the LOP element with SEL logic.

Our disco uses a simple AC ice cube relay (Potter Brumfield type) for this exact application. Loss of normal source transfers to alternate source. I suppose you could use something like a voltmeter selector switch to accomplish the same if the intent is for switching out a bus.

A voltage balance relay (Dev 60) is generally used to block operation of 21/40/67, etc.

 

[Mbrooke]

Your ice cube has a delay I'm assuming? During a fault I can see the bus voltage sag causing the relay to drop out. My plan is to use the voltage balance relay as a delay device, and its output contacts connected to DC which will de-energize a DC coil aux relay switching the VT circuit after a time delay say 75 cycles.

 

[RRaghunath]

Mbrooke, it is not advisable to have aux contactors in the VT secondary circuit. The VT secondary should be directly wired to the relay (through fuse or MCB of course) without any switching.
Distance relays especially are sensitive and need to be blocked on VT fail and the distance relays are designed to sense and block in less than a cycle.
The VT switching is done generally in metering circuits.

Rompicherla Raghunath

 

[Mbrooke]

I agree that distance relays need to block for loss of voltage, however in my humble view automatic transfer of a VT circuit prevents a sustained loss of voltage provided the cause is not after the VT switching point.


However, my real reasoning behind this scheme is so that I can take out a bus section for service without losing any protection.

 

[DTR2011]

Understanding your intention, why not use something like an Electroswitch manual handle switch. Place into substation operating instructions to transfer from Bus X to Bus Y prior to power circuit breaker switching. Not all that different than manually transferring station AC supplies in a distribution sub (assuming station power is derived from MV Bus Xfmr's). LOP in relays will allow for a small blip.

You are correct, BTW, generally an Agastat timer supervises the transfer.

 

[Mbrooke]

Honestly, I think I will go that route considering the sheer simplicity.

Only down side will be in inability to obtain correct VT inputs from remote SCADA switching... but I shall see. If more experienced folks advise against it, then there is certainly good reason for following that advice.

 

[DTR2011]

We have a few subs where a contact from the Transfer SW (Bus Volts), also drives an input to SCADA to get bus volts from alternate source. Ex - normally Bus 1 SEL is SCADA volts, when transfer (or loss of comm from B1 SEL), alternate comes from B2 SEL.

****
This scheme is not universal and used in old "special" stations in the boonies.

 

[Mbrooke]

If anyone is curious here is a single line which may clear up confusion. 21 and 67 voltages come off the bus, while only a single CVT is used on the line terminal for sync check for auto re-closing. Either bus must have the ability to be taken out of service without comprising protection.

 

[Mbrooke]

I think I found a good solution. I have limited knowledge of relays outside SEL and GE, however ABB and Micom relays appear to have the ability to take two bus voltage inputs in addition to the single phase sync check VT. Logic selection based on isolater inputs.

The other option I am considering is two external aux relays controlled by isolaters' aux position indicating switches. I see no issue in briefly having both VT set paralleled unless anyone can some up with a scenario where that could come back to bite me.

From ABB's REL manual:

https://files.engineering.com/getfi...2d2e32d944&file=ABB_line_distance_relay_1.jpg

 

[Mbrooke]

[highlight #FFFFFF].....[/highlight]

https://files.engineering.com/getfi...bbaa17357d&file=ABB_line_distance_relay_2.jpg

 

[Mbrooke]

Does anyone know of a good Email address for SEL regarding general questions in relay application to specific scenarios? Nothing over the top, just something general that will get taken with genuine interest.

 

[davidbeach]

From what I've experienced and heard from others, the local application engineer is always a good starting point.