Grounded middle phase on secondary Wye CCVT

[etronics]

Can anyone explain why you would ground the middle phase on the secondary of a CCVT metering circuit. I have come across this in substations where generators are located and also old substations where there is no generator. Thanks for any response in advance. The CCVT feeds metering and protetive relays of the microprocessor type, though there used to be electro mechs. in their place.

 

[DiscoP]

In the past (and perhaps today in some regions) Lower cost VTs were used for metering purposes that only had two windings and they were earthed on the mid point which was also the midddle phase.

Many places continued to use this earthing method when 3 winding VTs were used either because there was still some older VTs in the sub or simply to keep with a convention.

 

[scottf]

etronics-

Can you be a little more specific on the secondary wiring.

If your in the IEEE/ANSI world, you probably have 2 or 3 secondary windings on your CCVT like X, Y, and Z and each one is probably tapped, i.e. X1, X2, X3, etc...

Which terminal is grounded? If, for example, the X windings of all phases are connected in a grounded-wye configuration, then the X3 of each phase would be tied together and then grounded, either in the junction box, or in the control house.

You may have the X3 lead from A and C phase run to the B phase terminal box and then grounded? Would essentially be the same thing.

 

[busbar]

 
If the relaying needs zero-sequence voltage for operation, only 3-wires from the VT set will not do the job. Positive, negative and zero sequence voltages need 4-wire wye secondary connections.

An exception would be tertiaries on the VTs connected in broken-delta fashion to provide independent zero-sequence voltage from primary-side neutral-shift conditions.

 

[Deansharafi]

In australia, all the middle phases are grounded and it is industry common practice, the reason being as per Discop's explanation.

 

[scottf]

Deansharfi-

When you say a grounded middle phase, exactly what connections are grounded?

 

[buzzp]

Must be an open delta connection (two windings).
Busbar, your comments have sparked my interest again. You mention that only zero sequence voltages will be seen with an open delta connection by the relaying equipment. Just curious as to any protection that would look at negative and positive sequence voltages? I know that negative and postive sequence currents are an issue with heating and such. Does pos or neg seq voltages only produce a pos or neg seq current (causing heating problems) or is there other ill affects of neg or pos seq voltages?
Just wondering what we may be missing by using an open delta VT connection for our protective relays. Thanks and sorry for pulling this thread off track.

 

[scottf]

buzzp....that's what I was thinking, but they specifically mention metering, which wouldn't be a broken delta connection. Since it's with CCVT, that implies HV application, which is generally 3 element metering in a grounded wye configuration.

 

[DiscoP]

Scottf - on the B Phase of the VT is connected to ground. The star point of the VT is not grounded. Your Vb input on the relay will be at ground potential

There is a fuse/MBC on the A, C and Neutral legs, with a solid link on the B phase.

Relays and other equipment work perfectly, the only real issue is that VT fuse supervision on relays doesn't work if the neutral fuse blows. But there are ways around this (eg use a MCB with an auxiliary contact instead)

You also need to ensure that the voltage inputs on the equipment connected to the VT aren't earthed inside the box.

 

[scottf]

Sorry to be a pain, but I'm still missing what you're trying to say.

From the secondary of each VT there are 2 terminals (or 3) for each winding. I think in the AS standard they're called A1-An for instance (X1-X3 in the IEEE world). You would have these terminals for all 3 phases. For wye connections, typically, the An (or X3) terminals would be tied together and grounded.

If I read your post correctly, are you saying that the A1 terminal for the B phase (center phase) is grounded and the An of each phase tied together and ungrounded? if this is correct, why in the world would you wire like that?

 

[davidbeach]

I believe that grounding one phase of a wye, particularly a wye derived from a CCVT, is done to minimize ferroresonance.

 

[DiscoP]

Scottf - you have interpretted that correctly.

The reason is simply an historical convention as stated in an earlier post.

There is no real benefit in doing it this way, but there is nothing wrong with it either - it is one of those "if it aint broke - don't fix it" situations.

Although in substations with 2 winding and 3 winding VTs it makes good sense to keep them all the same. On a 2 winding VT there is only one option available.

I've heard this convention is used by quite a few utilities in the UK and Australia - there's probably some others too

 

[busbar]

I overlooked etronics’ comment that this case is a metering and relaying application. For both, there are several variations of ANSI {an no doubt other regions} VT-secondary grounding methods.

In North American parlance, open-delta and broken-delta VT-secondary configurations have different applications. Broken delta is useful for zero-sequence voltage for neutral-shift sensing in protective relaying, and is likely useless for metering.

 

[busbar]

 
Assorted VT-application urls..

http://www.geindustrial.com/pm/notes/artsci/art08.pdf

http://www.areva-td.com/static/largefiles/06-current_and_voltage.pdf