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?
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RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
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
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
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
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
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
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 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
RE: Placement of VTs/PTs
"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
RE: Placement of VTs/PTs
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
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
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
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
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RE: Placement of VTs/PTs
WPRC? If the doc is public I would love to read it.
RE: Placement of VTs/PTs
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RE: Placement of VTs/PTs
https://cdn.selinc.com//assets/Literature/Product%...
RE: Placement of VTs/PTs
To which MBrooke queried:
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
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.
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
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
....................
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
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
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
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
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?
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RE: Placement of VTs/PTs
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
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
Opening the current blades of the test switches before removing a relay takes care of keeping the loop intact.
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
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
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
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
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
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?
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RE: Placement of VTs/PTs
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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
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
RE: Placement of VTs/PTs
RE: Placement of VTs/PTs
This is exactly why I use individual, dedicated VTs for step distance protection.
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
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
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.
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
Thanks for your comments.
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
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
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
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.
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