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Placement of VTs/PTs
3

Placement of VTs/PTs

Placement of VTs/PTs

(OP)
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?

RE: Placement of VTs/PTs

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

RE: Placement of VTs/PTs

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

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

(OP)
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.

RE: Placement of VTs/PTs

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

RE: Placement of VTs/PTs

(OP)
This is main and transfer buss.

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

(OP)
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.

RE: Placement of VTs/PTs

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]

RE: Placement of VTs/PTs

(OP)
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?

RE: Placement of VTs/PTs

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]

RE: Placement of VTs/PTs

(OP)
Any particular reason for placing them at the entry?

RE: Placement of VTs/PTs

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?

RE: Placement of VTs/PTs

(OP)
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?

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

(OP)
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.

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

(OP)
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?

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

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

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

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

RE: Placement of VTs/PTs

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).

RE: Placement of VTs/PTs

(OP)
Thanks! :)

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

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

(OP)
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.

RE: Placement of VTs/PTs

(OP)
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.

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

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

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

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

RE: Placement of VTs/PTs

(OP)
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?

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

(OP)
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?

RE: Placement of VTs/PTs

Way back on 1 May, crshears wrote:

Quote:

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:

Quote:

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]

RE: Placement of VTs/PTs

(OP)

Quote (crshears)

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.




Quote:

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)...

RE: Placement of VTs/PTs

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.

RE: Placement of VTs/PTs

(OP)
Hard to believe [surprise]. Modern relays can and do everything except the mundane. Thats how it is I guess. Then again it could be worse LOL. Either way thanks again, your knowledge on protective relaying is worth its weight in gold. :)


....................

A bit of a side note (since switching CTs was brought up in relaying by position), you might have to log in, but for anyone with a 3 CT only deferential relay who parallels CTs briefly while switching bays on M&T:


https://cdn.selinc.com//assets/Literature/Publicat...

RE: Placement of VTs/PTs

(OP)
Ok, so I've been heavily researching this and out of all the options I have seen thus far I think I will go with DavidBeach's recommendation. Option one of having multiple or "one size fits all" relays dedicated to the transfer buss coupler I have rejected due to complexity & high probability of error and the original idea having single CT input relays with external CT switching has been put on hold. My current research yields that when CTs are paralleled (see PDF above) a risk of error is possible with line differential protection.


My current plan is to use an SEL-487E, SEL421, SEL487V, realys while having the transfer buss coupler breaker CTs & single phase VT string rack to rack to each relay. Under normal conditions relays will ignore "breaker 2" CT and VT inputs. During switching procures breaker 2 summing will be enabled via dedicated operator push button on the relay. Once done the normal bay and transfer breaker will sum both inputs. The bay to transfer buss disconnect can than be closed. After this step current readings will be human verified from both breaker CTs (normal and transfer) to verify current is about equal. This is done to indicate that the disconnect has closed correctly and parallel switching can take place if the normal breaker can not be opened for any reason. After that the normal breaker CT links are opened and work can begin. To bring back the normal breaker the reverse is done. Normal breaker CT links closed, normal breaker closed, and transfer is removed. Final step is to place the relay back into ignore "breaker 2".

Its a work in progress, but thus far I believe I have things sorted out.


However, I have one hiccup. When the transfer breaker is not being used, Id like to operate it as normally closed while having 50/51 protection on it for the transfer buss. I am stumped as to best to go about doing it.


RE: Placement of VTs/PTs

We have an overcurrent relay on the bus tie breaker. We don't keep the aux (transfer) bus energized, decision made long, long ago, so the overcurrents are on for the first few minutes after closing the breaker (SOTF time set to max) and then they drop out. If the breaker is normally closed you may need to indicate to the bus tie relays that they are protecting the aux bus. Depending on what you're using for bus protection, and how you indicate to other relays that they need to include the bus tie CTs, you could have a separate bus zone for the aux bus using only the CTs on the bus tie breaker. Any faults on the aux bus while not switching will trip just the bus tie. Once you start switching, you'll trip both the bus tie and the position breaker for any aux bus faults.

RE: Placement of VTs/PTs

(OP)
In this case the buss tie has no relay, the plan is that any normal relay can use the transfer breaker. It sounds like I need to basic SEL-501 dedicated to the transfer breaker, however I am unsure of how to go about wiring/programing the logic. I looked into a new buss zone, however the primary high impedance (SEL) buss differential relays only allows one buss zone. A GE B90 can be configured for multiple zones, so I can research that further if need be.

RE: Placement of VTs/PTs

In that application we use the 487B rather than the 587Z. We also have a 351A on the bus tie so that we have a single relay doing things like trip coil monitor and CT fail monitor functions.

RE: Placement of VTs/PTs

(OP)
Yup, spot on SEL-587Z. However, correct me if I am wrong, the 487B can be configured to provide 50/51 only to the transfer buss breaker under a dedicated zone 2. However, how would I also include the transfer breaker into zone 1 differential? And remove the 50/51 under transfer? In all honesty (I've never done it before) I am hesitant about using the buss protection scheme to also provide over current protection to one breaker. In my eyes (Im sure SEL has worked around this) added complexity can increase the likely hood of inadvertent misoperation. I myself have always had the philosophy of having the buss protection dedicated and separate from any other function, but Im sure thats about to change :)

RE: Placement of VTs/PTs

There's torque control equations for nearly everything and for what there aren't torque control equations there's always PSVs to supervise the result. If you can define the conditions you can get the relay to do what you want it to do. Without a pile of drawings to peruse, it's hard to say exactly how I'd go about it.

On the other hand, what I'd do on our system is immaterial if you and yours aren't comfortable implementing it in your system. For us, now, that would be a step change; evolution not revolution. Wasn't always so, may not yet be for others. Better to do something you thoroughly understand today and in five years say "we could have done better" than to do something on the cutting edge now and in five years still be saying "why the heck did we do that?" It hasta work for you; all else is secondary.

RE: Placement of VTs/PTs

(OP)
Understandable. Using something that requires reliability as a learning tool is not best imo. That and the fact this will become a none standard substation in terms of relaying which increases errors taking place once the special setup is long forgotten. Not that I will not right this down for future reference.

Anyway, Im thinking of an SEL-501 and programming an external input (switch) that disables 50/51 when the TBB is being used. Sound doable?

RE: Placement of VTs/PTs

Sure, that's doable. Personally I'd move up the product line to something with SER capability. The 551 has a sort-of SER, for a real SER the minimum would be 351A or a 751A.

RE: Placement of VTs/PTs

(OP)
SER? Sorry LOL


RE: Placement of VTs/PTs

Sequence of Events Report. What inputs/outputs/internal elements picked up or dropped out at what time. The event reports (oscillography) which even the 501 does provide brief snapshots in response to some triggering event. The SER is a running journal of what the relay saw and did; as long as you have the right elements in the SER list and as long as there haven't been too many other things since then as each has a finite length.

RE: Placement of VTs/PTs

(OP)
I wouldn't worry about a Sequence of Events for a buss fault (I never even bother setting them in buss differential relays)unless you believe its worth it for repair.

RE: Placement of VTs/PTs

Perhaps not of much use for the actual bus fault. But what about that trip when there wasn't actually a bus fault?

RE: Placement of VTs/PTs

(OP)
If set correctly the breaker should not trip. I can't think of any scenario where a properly set 50/51 breaker would trip on anything besides hundreds if not thousands of amperes flowing. Perhaps on a transmission line where sympathetic tripping is possible, misapplied settings due to complexity and of course needing to know what and where a fault has occurred out the line make SER a valuable tool, but not for simple over current on a buss that can be inspected by foot IMO its not of any advantage.

RE: Placement of VTs/PTs

Murphy can, and does, find the durndest things to have fun with.

RE: Placement of VTs/PTs

Agreed, davidbeach; we generally find the value of using a Sequence of Events Recorder is proven to its greatest extent not when things operate the way they should, but rather when they don't.

CR

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

RE: Placement of VTs/PTs

(OP)
Are there any real life cases justifying such? It sounds like SER is indeed needed for buss bar protection the way its being described.

RE: Placement of VTs/PTs

Hi Mbrooke, don't misunderstand; a Sequence of Events Recorder / Reporter is merely an information gathering tool, and as such is not "needed" for any type of protection scheme...however the data it collects can be of almost inestimable valuable when analyzing the performance of protection schemes and equipment operation during real-time operating scenarios; we particularly employ its records to determine whether equipment functioned as designed. Mind you, sometimes the design ends up being less than stellar, and the record of how the equipment behaved under actual fault conditions frequently pinpoints with painful accuracy the exact nature of the holes / gaps / flaws / deficiencies in the protection scheme "as designed" - but such findings can also confirm in some instances that there was no actual malfunction of any components, meaning no troubleshooting or repair is warranted.

CR

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

RE: Placement of VTs/PTs

(OP)
I understand, and welcome your concern :) However, IMHO, while event data recording is for all practical purposes essential to operating a system correctly regarding transmission lines, such rich data does not justify the added cost for what is essentially a buss stub. In my limited knowledge, a bus bar either functions or faults showing some visible sign thereafter. The exception would be 15, 25 and 35kv class distribution bus bars which are susceptible to transient wildlife faults to which one reclose attempt is performed (lock out relay eliminated for differential).

RE: Placement of VTs/PTs

(OP)

Quote (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.


Quote (Crshears)

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...

Adding this for future readers who may stumble upon this thread having the same question. I am at the drawing board and shorting links are indeed needed for every relay panel and must be kept closed (shorted) for any relay not using the TB CTs. The reason is two fold:

1. Each relay adds impedance to the CT circuit, so any relay that does not require the current input must be taken out of the circuit.

2. If any relay is taken out of service the loop must be complete and the shoring links will do this.

RE: Placement of VTs/PTs

In the electromechanical days the relays accounted for much of the CT secondary burden, but now the bulk of the burden is the cable between the breaker and the control house.

Opening the current blades of the test switches before removing a relay takes care of keeping the loop intact.

RE: Placement of VTs/PTs

(OP)
How much burden does a modern microprocessor relay add? Id imagine looping through 12 relays adds substantial burden?

RE: Placement of VTs/PTs

At 15A secondary current, an SEL-487E has 2.51VA of burden. At 8 times tap, the HU that the SEL replaced has a burden in the 50-60 VA range for each restraint; plus operate circuit has a burden of about 1.5 times that much. Three KD line relays would have had a burden of about 6 VA at 5 A secondary vs less than .5 VA for the SEL-411L.

RE: Placement of VTs/PTs

(OP)
Sounds like I could get away without shorting any of the CT inputs when not in use an just tell the relays to ignore breaker 2 current when desired via user interface.


Thank you so much with all your time and help. :) I just want to say your idea about using an SEL-411L was a life saver and I truly appreciate it. I certainly would have missed it. Looking at the manual it even lets you configure the actual bus bar being used (on screen) so it truly does work for any application, even those outside being advertised in the brochure.


RE: Placement of VTs/PTs

(OP)
For future readers who may stumble upon this same issue or thread I finally found the papers you were referring to btw.



https://conferences.wsu.edu/forms/wprc/2014/Papers...

https://conferences.wsu.edu/forms/wprc/2014/Presen...

I just want to say that these are worth an IEEE (or equivalent) publication IMO. International recognition is much needed, which I will explain. I can say for a fact there are many engineers/utilities all over the world who encounter this very same problem regarding M&T where this simple solution often gets missed. For example, I've been told in India main and transfer buss is used on nearly all 66 and 132kv with half of 220kv substations holding it as well. The rest of 220kv and half of 400Kv is single breaker-double main with transfer which also holds the same philosophy. An example of such:


http://www.bsptcl.in/FINAL_SLD/Gaurichak.pdf


Single 3 phase CT Electromechanical relays are being replaced with modern microprocessors and new substations receiving the same. Single 3 phase CT relays are chosen simply because engineers are unaware that dual breaker relays can take care of the complexity despite being available from IEC manufactures. To add insult to injury main and transfer substations dominate new construction in developing countries. As load grows necessitating higher reliability, the transfer disconnect is removed and a compact disconnecting circuit breaker takes it place (or a modified GIS solution) similar to what is mention here:

https://www.google.com/url?sa=t&rct=j&q=&a...

The single CT relays are then called upon to run 2 breakers when a dual breaker relay could have been selected in the first place. In fact the added cost of a dual bay relay is easily off set by those dedicated relays which can be eliminated for the bus tie. In fact its things like this sold by manufactures which reinforces the myth dedicated relays or CT switching is needed for M&T:

https://cdn.selinc.com/assets/Literature/Product%2...



For reasons I have yet to understand I have not heard a word from manufactures regarding dual breaker relays being applied to M&T applications. While M&T and other transfer variant substations are historically less common and fading away in North America, they make up for what I would say 70% of all substations around the world.

For this reason I believe manufactures should be offering literature on this solution along with options to configure software and relay labeling to reflect it. (like changing line VT to buss VT and buss VT to line VT).

Have you presented any of this to say SEL?

RE: Placement of VTs/PTs

They are well aware of my feelings about their failure to consider that bus configuration.

RE: Placement of VTs/PTs

(OP)
And oh, the protection issues mentioned with paralleling CTs (such as a station latter going to double breaker double buss) are documented and legit.

RE: Placement of VTs/PTs

(OP)

Quote (DavidBeach)

They are well aware of my feelings about their failure to consider that bus configuration.

Sounds like they are refusing to listen. Most of the guys which I tell them about this are hearing it for the first time.

RE: Placement of VTs/PTs

(OP)
Update... This should perhaps be a new thread but it ties into this. SEL's latest versions of the 421 have 6 voltage inputs (VAY, VBY, VCY & VAZ, VBZ, VCZ) instead of the older VA, VB, VC, VN & VS, NS)Page 22:


https://selinc.cachefly.net/assets/Literature/Prod...



I am just digging into the manual, however would this be any indication that I can swap bus and line voltage inputs on the relay to get the intended displays? Or is there another reason for the change and if so would anyone know?

RE: Placement of VTs/PTs

Nope; you can tell the relay to base the protection on VY or on VZ. But which ever one you choose, the 3-phase voltage used for protection is by definition (in the relay) the line voltage.

RE: Placement of VTs/PTs

(OP)
Ok, thanks :) But why do they offer a full 3 phase VZ? And am I correct to assume that on the 311C, you have the option to tell the relay to base VA,VB,VC on the line instead of the bus bar? I ask because the 311C's manual appears to offer such an option (see attached pic).

RE: Placement of VTs/PTs

If you don't want synch voltages, you can set the relay up to use either of two 3-phase voltages. Say for instance you're doing line protection in a double bus-double breaker station and the only 3-phase voltages on on the buses rather than on the lines. You could have a preferred voltage, say the north bus if the north breaker is closed, but switch to the south bus voltage if the north breaker is open and the south breaker closed. I wouldn't, but maybe somebody else would...

RE: Placement of VTs/PTs

(OP)
Another bizarre relay function that has no bearing to the real world LOL! In the very least, can LOP (loss of potential) be set to switch between those 2 sets of 3 phase inputs or am I being to optimistic?

RE: Placement of VTs/PTs

If you can define the criteria, you can program the relay based on that criteria. The problem with alternate protection voltages is that you give up on synch check capability. Personally, I'd rather wire the A and B relays to different voltages (at least different secondaries with different fuses), provide LOP alarms, and keep the synch check/voltage supervision capability that have multiple protection voltage sources. We also program overcurrent elements that turned on an LOP condition is declared; obviously these are non-directional overcurrents and may not coordinate well, but some allowance is given for last ditch efforts to keep the line protected. PRC-023, for instance, excludes overcurrents active only during LOP conditions.

RE: Placement of VTs/PTs

(OP)
I will make note of losing sync check on 2, 3 phase inputs (one would think the relay manufacture would not do that). As for the rest it makes sense and pretty much the same here. Straight buss for me always has 2, 3 phase VTs on the buss with each set dedicated to either A or B relays. Less critical stations have one 3 phase set on the buss, but each individual winding is dedicated to either A or B relay group.


If the 311C (or 421) can legitimately be programmed recognize 3 phase sensing on the line in straight buss applications, it has been spoken placing 3 VTs on the line for step distance protection and using the buss only for single phase sync check. That way if an LOP occurred, it would effect the relays only for one bay (circuit) and not every single A or B relay in the station.

When all of A or B are effected, two general options exist: 1. disable all protection for the group under LOP. Works well provided all the remaining groups are in service and none will fail. 2. Have the LOP group revert to over-current only. The risk here is that a fault elsewhere on the system can trip multiple lines due to lack of coordination on one group even though the other group is fully functional. Of course setting LOP overcurrent times delays slower than zone 1 reduces some risk of miscordination, but not all.


Now, when 3 phase current sensing is on the line, an LOP will only cause one bay to become a trade off, not all of them. Is my thinking correct here?


RE: Placement of VTs/PTs

2
Davidbeach-

Your comments reminded me of a situation coming up with more regularity in the IEEE market. We have a lot of customers now requesting MV VTs with 2 secondary windings, where the traditional requirement has been 1 secondary winding for these voltage levels. The feedback we get is that the protection group wants 2 secondary windings for redundancy or the meter group and the protection group want to each have their own winding to separate the voltage source.

Given that VT outputs typically go through a junction/marshalling box where fusing is provided, it seems to me to be much simpler to just provided multiple fused circuits for each phase to provide isolation/redundancy. Reason being is that a VT with 2 secondary windings does not provide true isolation between the 2 windings. If there is a short/fault on one winding, it impacts the voltage on the other secondary windings, since they are both wound about the same core.

I now see a lot of utilities buying larger and more expensive VTs, which in reality, do not provide any significant improvement on redundancy over just adding an additional fused circuit from a single seocndary winding.

RE: Placement of VTs/PTs

For better or worse, NERC recognizes two VT secondaries as being redundant even if they share the same primary.

RE: Placement of VTs/PTs

Interesting, thanks. Is there a specific code/clause you could refer me to?

RE: Placement of VTs/PTs

(OP)

Quote (scottf)

Given that VT outputs typically go through a junction/marshalling box where fusing is provided, it seems to me to be much simpler to just provided multiple fused circuits for each phase to provide isolation/redundancy. Reason being is that a VT with 2 secondary windings does not provide true isolation between the 2 windings. If there is a short/fault on one winding, it impacts the voltage on the other secondary windings, since they are both wound about the same core.


This is exactly why I use individual, dedicated VTs for step distance protection.



Quote (David Beach)

For better or worse, NERC recognizes two VT secondaries as being redundant even if they share the same primary.

Perhaps this may not be the place to say this, but NERC is not on my favorites list. Countries that do not deal with such standards are way ahead imo.

RE: Placement of VTs/PTs

I'm not sure there's anything enforceable at the moment, but a few years make there was a data request survey of the degree of redundancy of components of protection system components, they stated that a VT with independent secondaries would be considered redundant, at least for that purpose. Formal rules requiring redundancy haven't arrived yet. We lose VT fuses much more often than we lose VTs, so separate secondaries are far more redundant than one secondary daisy chained between relays.

Then there's the question of how do you lose a VT primary without tripping the bus or line it is connected to? I guess you might get an open, but anything nasty happening to one VT primary would render any other VT on the bus or line temporarily unavailable. It seems to me that a second, fully redundant, VT adds a lot of expense, space, and complexity for a fairly small gain in redundancy.

RE: Placement of VTs/PTs

(OP)

Quote (David Beach)

Then there's the question of how do you lose a VT primary without tripping the bus or line it is connected to?

One of these ;) :P

http://www.sandc.com/products/fusing-outdoor-trans...

I've seen it done at a few 66/69kv stations before. But outside of that I do agree with you, a VT fault is going to clear what ever its attached to.



Quote:

I guess you might get an open, but anything nasty happening to one VT primary would render any other VT on the bus or line temporarily unavailable. It seems to me that a second, fully redundant, VT adds a lot of expense, space, and complexity for a fairly small gain in redundancy.

Two arguments Ive heard:

In double bus, single breaker applications with dynamic zones a VT tripping out any buss would not result in the remaining circuits loosing step distance voltages.

Second, any buss VT can be taken out of service as needed.


RE: Placement of VTs/PTs

Davidbeach-

Thanks for your comments.

Quote (davidbeach)

We lose VT fuses much more often than we lose VTs, so separate secondaries are far more redundant than one secondary daisy chained between relays.

I'm not following you here. Normally, each circuit would be fused separately. So, assuming you fuse the secondary circuits of VTs already, I don't see how creating a second fused circuit off of the same winding is any less redundant that 1 fused circuit from each secondary.

The only type of fault in which 2 separate secondary windings provides redundancy is an open-circuit in one of the secondary windings. In the 25 yrs I've been working for instrument transformer manufacturers, I've never seen that happen in the field. Primary turn-to-turn shorts, primary insulation break-down, and secondary turn-to-turns short are the types of failures I've seen and having 2 secondary windings isn't going to help in any of those situations.

Also, since we're discussing medium voltage VTs here, primary fusing is common and is present to clear a faulted VT from the line.

RE: Placement of VTs/PTs

I thought we were discussing transmission level VTs; yes, we have primary fuses on our distribution VTs.

Typical, long standing transmission design, is to fuse the VT secondary circuits at the VT location and bring the fused secondary circuit into the control house.

As with many other aspects of substation design, there are myriad ways of getting to the same result; for better or worse an awful lot of it depends on decisions made decades ago. In our case it used to be one secondary in wye, the other in broken delta. We no longer have need for the broken delta to polarize the electromechanical directional ground overcurrent relays that we no longer have, so we have made that secondary available as a redundant voltage source.

Sure, I still see a significant common-mode failure risk of taking out both voltages; that's why we turn on overcurrent based protection during LOP conditions. Given that ability to provide alternate protection, I'd much rather push hard on dual batteries and dual trip coils than for a second VT. I know you'd like us to buy twice as many, but that do well enough that other non-redundancies are much more pressing.

RE: Placement of VTs/PTs

Yes...for HV VTs, 2 secondary windings is standard.

The issue here is that we're seeing the requirement a lot now for MV VTs, where 1 secondary winding is the standard. In many cases, to go to 2 secondary windings requires a larger, more expensive VT. Hence the point about just having separate fused circuits being much more economical and providing about the same level of redundancy.

Yes..I would like to sell twice as many, but not what I'm pushing for here :) I'm actually pushing for folks to use the standard smaller, more cost effective MV VTs and spend an extra $10 on an additional fuse set-up.



RE: Placement of VTs/PTs

(OP)

Quote (David Beach)

Sure, I still see a significant common-mode failure risk of taking out both voltages; that's why we turn on overcurrent based protection during LOP conditions. Given that ability to provide alternate protection, I'd much rather push hard on dual batteries and dual trip coils than for a second VT. I know you'd like us to buy twice as many, but that do well enough that other non-redundancies are much more pressing.

Question. Have you ever considered 3 phase VTs on the line for Relay A and 3 phase VTs on the bus for relay B? I know it sounds odd but as for late this is seriously being discussed for existing straight bus applications being overhauled.

RE: Placement of VTs/PTs

No, we have not.

RE: Placement of VTs/PTs

(OP)
Ok, just wondering. Thanks :)

RE: Placement of VTs/PTs

What market are you in that 3-phase VTs are available? They are not typically used/available in North America.

RE: Placement of VTs/PTs

Pretty sure that's just short hand for a group of three singles used for protection purposes, as opposed to one single phase VT used for synch check purposes. We regularly refer to a group of three as a three phase source. Just like referring to a bank of 3 single phase power transformers as a single three phase transformer.

RE: Placement of VTs/PTs

(OP)
My mistake, I obfuscated my point. 3eyes I wrote without thinking. By 3 phase I meant 3 single phase VTs used for step distance protection, and single phase VT meaning only one VT is present for sync check. I do not know of any actual 3 phase (as a unit) VTs.

RE: Placement of VTs/PTs

Got it. There actually are/were 3-phase VTs, which is what I thought of when I read the post :)

RE: Placement of VTs/PTs

(OP)
If anyone has a pic by chance, wouldn't mind seeing it. I myself have never seen a 3 phase unit VT. Learn something new everyday.

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