Placement of VTs/PTs 3

[Mbrooke]

Is any one aware of, or sees an advantage in placing PTs within breaker bays (between the circuit breaker and disconnect) instead of on the outgoing line positions?

 

[Hoxton]

Synchronising. Then you need a breaker either side of the breaker.

 

[Mbrooke]

Im confused lol, what do you mean by this? (My apology)

 

[Hoxton]

Sorry. If one of your incoming supplies is an ac generator then you have to match speed, phase angle, phase rotation, voltage before closing the circuit breaker between the generator and the switchgear bus. In this case, you have to have a VT either side of the breaker in order to measure these quantities, and then adjust the speed (frequency) and voltage to match the generator with the bus.

This assumes that the bus is live. If the generator is the first supply to be live, then you use the bus VT to detect a "dead bus" in order that the generator breaker may be permitted to close.

Google "generator synchroniser" for more details.

 

[davidbeach]

I think the question had more to do with which side of the breaker disconnects the VT is placed rather than whether or not to have the VT.

The location of the VT relative to the disconnects determines how much you have to take a clearance on to do any work on the VTs. Local preference ("that's the way we've always done it here") will also play a large part in making that determination.

 

[Mbrooke]

My apologies about being vague, but none the less thank you, I will certainly use this info latter on.

As for the current application I have a multi terminal transmission substation. The PTs are normally connected directly to the line arrestors leads, however an assessment is being done if these PTs can be moved into the breaker bays.

 

[stevenal]

What's your breaker arrangement? Many arrangements have more breaker bays than lines, so your number of VT sets will increase.

 

[Mbrooke]

This is main and transfer buss.

 

[jghrist]

If the VTs are on the line side of the breaker disconnect, they will measure the line voltage whether or not the line is connected to the main or transfer bus.

If the VTs are on the breaker side, they will measure no voltage when the breaker is out of service and the line is connected to the transfer bus. This is probably what you want because the VT on the breaker protecting the line will be the one measuring the line voltage.

 

[Mbrooke]

Sounds more reasonable the way to describe it. When the line is on the transfer buss the bus coupler breaker will have its own set of VTs for the line bay that was taken out of service.


The reason for moving the VTs is two fold. 1. VTs can be serviced and tested at the same time while the breaker is being tested/maintained. 2. The line does not have to be taken out of service for VT testing or replacement. What leads me to question this however is nearly all exiting stations have the VTs immediately connected to LAs which tap right into the incoming/outgoing lines. There is no disconnect for these VTs so in order to do any work on them it requires removing the line for service.

 

[crshears]

Where I work the vast majority of the stations use either ring busses or the breaker-and-a-half scheme with multiple "diameters," with the line pots interchangeably found either on the line itself or on the line terminal bus between a pair of breakers, and with each breaker having a pair of disconnects on each side of it.

Where the line pots are on the line itself, obviously the entire line has to be out of service to work on the pots, meaning that if there are tapped transformer stations on the line, isolation will have to be provided at those taps as well as at the far terminal. The flip side is that with the line disconnect open, all three breakers in the "diameter" can remain in service, thus maintaining dual connectivity to the other element in the diameter.

Where the pots are on the terminal bus, however, they can be serviced with just one end of the line off load, with any taps along the line remaining on load from the line's other end.

And whichever way it's done above, any single breaker can be removed from service without unloading any elements.

Some of the utilities we supply use transfer busses, but I can't think of anywhere at all that we do, and I know little about them...but I'm surmising that where more than one supply of potential is available to feed a line's protections there would also have to some sort of pot transfer scheme, either manual or automatic, which in my limited experience would pose a needless additional complication and expense...but admittedly this is not my area of expertise, and perhaps the extra trouble is worth it in some way...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

The transfer buss bay feeding the transfer buss will have its own PT between the breaker disconnect and breaker. In your case, do you know of any station with a PT in between a breaker disconnect and circuit breaker? Do your PTs have disconnect by chance or tap directly off line positions?

 

[crshears]

Hi Mbrooke, no, I can't recall ever seeing my utility having any type of PT/VT between a breaker and its disconnects.

"Our" PT's are generally connected to the line/station entry bus-work by means of individual "pigtails" that can be readily removed/installed with a live line tool. This can be handy if one goes bad, since the line can often be returned to service with just two out of three PT/VT's functional, provided Protection and Control personnel make any necessary adjustments.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

Any particular reason for placing them at the entry?

 

[davidbeach]

Ring and breaker-and-a-half need to have the VT on the line, but perhaps between the line disconnect and the the breaker disconnects. If the VT were between a breaker and the associated disconnect, any time you have that breaker out of service you'd force out the protection of the line. With the VT on the line, either breaker could be out of service. With Main and transfer it gets more complicated; the protection VT is on the bus, it is the single phase line VT that could be either with the breaker or with the line. We have examples of both, each has its pluses and minuses. Overall, I'd rather those single phase VTs be the breaker than with the line, so we can do maintenance on one set while we are doing maintenance on the associated breaker, but the overall schematic is a bit more complicated that way. Simple bus has the 3-phase on the bus and it really makes no difference if the single phase VT is on the breaker or line side of the disconnect.

So, in general, 3-phase VTs not between the breaker disconnect and the breaker, but stand-alone single phase VTs between the breaker and the breaker disconnect. Clear as mud, right?

 

[Mbrooke]

Getting clearer as I understand it more. In the few breaker-and-a half schemes Ive seen 3 phase VTs were on the buss as well, but not sure why. I assumed it was part of the protection scheme for the line.

As for main and transfer, its not a concern if the breaker is taken out of service because the transfer buss coupler will have its own set of VTs and those will take the protective relaying over when a single line is placed on the transfer buss for that line.

In larger stations with 2 main buses, or a transfer buss having the ability serve as a back up main buss (breakers can pick from either buss), each buss will have their own 3 phase VTs for line protection.


As for the overall schematic, how does it become more complicated? Are you referring to the control/relay wiring?

 

[davidbeach]

For Main and Transformer it depends in part on whether you relays for each position or relays for each breaker. If it's relays by position, you have to get the right currents and voltages to the relay while the position is on the transfer bus; if the VT is on the line it doesn't matter which breaker is being used but if it is VT by breaker you have to switch the voltage too. On the other hand, if you have relays by breaker (settings have to change for each position put on the bus tie), then having VT by breaker is much easier than VT by position; the one VT needs to be wired to the bus tie breaker relays and no need to worry about any of the rest.

The basic arrangements are fairly consistent between utilities, but the actual implementation there of, well that's a whole different ball game. We've always been a relay by position outfit, but our largest neighbor is a relay by breaker outfit. The couple of times we tried relay by breaker we have to stop and think about what we're doing much more than when we stay with what we know.

 

[Mbrooke]

If the transfer buss (breaker) also has the ability to do transformer protection, does relaying by position vs by breaker make a difference?

In so far I have relayed mostly through position, but I guess that is about to change. In my eye (I could be wrong) relaying by breaker seems easier in the long run. It has that "mathematically elegant" feel.

FWIW I had a similar debate regarding CTs a while ago. With live tank breakers the option exists of having the CTs on the outgoing line or between the breaker and disconnect. It was 50/50, but when dead tank breakers became more common (for me) I started to design every station with relaying taking into account CTs on the breaker's bushing.

 

[davidbeach]

Using the SEL-4xx family of relays, we relay by position and have the normal breaker wired as breaker 1 and the bus tie breaker wired as breaker 2. The relays need to know which position is using the bus tie; if it is the relay looks at the breaker 2 CTs and trips breaker 2, otherwise use breaker 1. Add in a bit of logic to handle the switching between the two and we have a decent solution. There's a certain risk associated with that bus tie CT circuit that strings from rack to rack, but that can be managed. This way we use line relays for lines on the bus tie breaker and transformer relays for transformers on the bus tie. When there's more than one type of position that can go on the bus tie, the relaying per breaker does get a bit more complex.

 

[Mbrooke]

This application calls for both transformer and line protection via transfer buss coupler when required. What is the risk which you are aware of stringing rack to rack?