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?

 

[davidbeach]

Let's say you have line relay A and relay B in series from the bus tie and that position B is on the bus tie. If you then do something, testing say, at position A that in anyway affects the CT circuit you can get undesired results at B. Say that B is a transformer diff and you short two phases at A then that current disappears at B, possibly resulting in an undesired trip of the differential.

 

[Mbrooke]

But won't proper input CT selection/switching take care of this?

 

[davidbeach]

The whole point is no CT switching; just run them from relay to relay. There's test switches at every relay, but stuff can still happen.

 

[Mbrooke]

No CT switching even if relaying per position? Just double checking everything thoroughly.

 

[davidbeach]

Right, the relays use the current(s) they need and ignore the other. I did a paper on the scheme at WPRC in 2014 (I think).

 

[Mbrooke]

Thanks!

WPRC? If the doc is public I would love to read it.

 

[davidbeach]

Western Protective Relay Conference. I don't know what their policy is about copies of past papers and presentations. But I do know that if you register to attend (October in Spokane WA) you can buy a compilation CD with many years worth.

 

[Mbrooke]

Seeing your posts here, I think you should write a book or at least make a sticky on ET. By far the most informative help. I have to admit that certain aspects of protective relaying aren't taught or covered as well as they should be. Perhaps preferences between utilities play a role, but none the less information many are left wondering about regardless.

 

[Mbrooke]

One last question, does relaying by breaker vs relaying by position change buss differential protection in any manner? Currently a GE B90 is being applied as primary protection and an SEL-587Z as auxiliary backup. Single zone of protection only in this case.

 

[davidbeach]

Note, for the bus protection it only matters that the breaker is connected to the bus. For us that would be each breaker wired to the SEL-487B relays.

 

[Mbrooke]

And just let the SEL relay exclude the CT input from the sum on the breaker being put out of service?

 

[davidbeach]

The breaker out of service is expected to contribute zero amps to the diff. If we're going do some testing that might result in current we'd open the CT test switches for that position.

 

[Mbrooke]

Testing is my concern, but it looks like the same topology as prior can be applied. Awesome, thanks!

 

[Mbrooke]

Sorry to drudge this old thread up, but I just want run by a quick question instead of opening a new thread. Im hearing that in main and transfer applications some step distance and sync check relays want 3 VTs for the line itself instead of one like mentioned. Does this hold any truth?

 

[davidbeach]

Yes and no. In the relays we use, the relay internally calls the 3-phase voltage "line" and calls the single phase voltages "bus". Nothing stops us from connecting the 3-phase voltage to 3-phase VTs on the bus and the single phase voltage to the line VT. We just have to keep track of what's what. When we want to allow closing hot bus to dead line we need to use the internal hot line dead bus logic.

 

[Mbrooke]

Sounds almost like the relay manufacturer has it backwards. In nearly all cases we close a breaker its a live bus and de-energized line. Do your relays do sync check through that single phase VT when wired this way?

 

[Mbrooke]

Update: it looks like the 1 phase VT can indeed do a sync check, but I am puzzled why the buss is 3VTs and the line only one. Perhaps to save the customer costs on VTs I guess.

https://cdn.selinc.com//assets/Literature/Product Literature/Flyers/311L_Flyer.pdf?v=20150812-085254

 

[crshears]

Way back on 1 May, crshears wrote:

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.

To which MBrooke queried:

Any particular reason for placing them at the entry?

Answer: I'm not privy to the reasons why those a lot further up the food chain do what they do, but almost all of our stations are breaker-and-a half schemes with disconnects on either side of each breaker. For line protection VTs/PTs it's about a 50/50 split as to whether these are connected to either the line itself or the T-shaped line terminal bus on the station side of the line disconnect switch.

Main point being that because pot sources are not located inside breaker disconnects, line protections can remain fully in service without modifications required if one of the two breakers is out of service for maintenance. Touching on something from another thread, if the line is two-ended and the entire terminal [ meaning "outside breaker disconnects by line disconnect" ] is out of service, the Zone 2 relaying at the remote end is typically made instantaneous to preclude timed clearance of line faults.

Edit/addition: I have NEVER seen CT wiring equipped with blocking switches; in my utility, across the board, links are used, with the rigid practice used of ALWAYS applying shorting links first before opening the CT circuit splitting links. Presumably there is an expectation of less human errors being made using this approach, although I couldn't say for sure that's why...

CR

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

 

[Mbrooke]

Answer: I'm not privy to the reasons why those a lot further up the food chain do what they do, but almost all of our stations are breaker-and-a half schemes with disconnects on either side of each breaker. For line protection VTs/PTs it's about a 50/50 split as to whether these are connected to either the line itself or the T-shaped line terminal bus on the station side of the line disconnect switch.

Main point being that because pot sources are not located inside breaker disconnects, line protections can remain fully in service without modifications required if one of the two breakers is out of service for maintenance. Touching on something from another thread, if the line is two-ended and the entire terminal meaning "outside breaker disconnects by line disconnect" ] is out of service, the Zone 2 relaying at the remote end is typically made instantaneous to preclude timed clearance of line faults.

You would be correct and that does make sense. Unless you had a VT on each breaker and the relays to handle them, you need to have the VTs between both breakers in breaker and a half. In double breaker double bus I guess it might be possible if each breaker had its own relay but the DB DB deigns Ive dealt with are not like that.

Edit/addition: I have NEVER seen CT wiring equipped with blocking switches; in my utility, across the board, links are used, with the rigid practice used of ALWAYS applying shorting links first before opening the CT circuit splitting links. Presumably there is an expectation of less human errors being made using this approach, although I couldn't say for sure that's why...

CR

Same here, but it might have something to do with M&T vs BR & 1/2. For POCOs that have main and transfer, what method or switches are used to transfer between the normal bay and transfer bay CTs? Id imagine you would need to parallel the CTs while switching the disconnects and then finally open the normal bay CTs after the normal breaker is tripped (no current on the CTs)...

 

[davidbeach]

Three phase voltage is the protection voltage. On most two breaker applications - ring, breaker and a half, double bus double breaker, the three phase voltage is on the line in front of the relays, so calling the three phase voltage "line" makes sense. That's the application they had in mind with the relay was designed. For simple bus and M&T there is just one set of 3-phase VTs and they're located on the bus. Relay still wants to call it line.

The prior generation of relays, with only one set of current inputs, treated the 3-phase voltage as bus and the single phase as line. When we used to apply those on two breaker applications we also had to keep track that the references were backward.

Ideally there would be a setting in the relay that tells it whether 3-phase was line or bus, but that setting doesn't exist.