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?

 

[Mbrooke]

Hard to believe . 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.


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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/Lite...ingCTs_DC_20150217_Web4.pdf?v=20151112-100936

 

[Mbrooke]

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.

 

[davidbeach]

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.

 

[Mbrooke]

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.

 

[davidbeach]

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.

 

[Mbrooke]

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

 

[davidbeach]

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.

 

[Mbrooke]

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?

 

[davidbeach]

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.

 

[Mbrooke]

SER? Sorry LOL

 

[davidbeach]

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.

 

[Mbrooke]

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.

 

[davidbeach]

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

 

[Mbrooke]

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.

 

[davidbeach]

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

 

[crshears]

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]

 

[Mbrooke]

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

 

[crshears]

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]

 

[Mbrooke]

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).

 

[Mbrooke]

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.

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.