Breaker and One Half Tripping Schemes

[DTR2011]

Hi All-

I was reviewing some potential work and came across what I would call a convolution of security vs. dependability. Curious how others handle tripping. I am concerned about local tripping onlt.

Thanks

 

[odlanor]

Do you have a question?

 

[rcw retired EE]

Many issues enter into the trip circuit design.

Some things that created issues for us in the past:

Assigning lockout relays to the breakers instead of the protection zones - That made it difficult to automatically determine which side of the breaker the fault was on. We were trying to autotransfer downstream equipment using lockout relay contacts. The same lockout rolled for faults on either side. (This is not a problem with modern programmable relays and communications). This can be an issue on ring bus setups also.

When one breaker and/or its disconnects are open or out of service, do you inhibit tripping through auxiliary contacts? Contact failure can compromise trip schemes.

Use redundant control power sources, redundant relays and dual trip coils- It can be difficult to keep the two tripping systems isolated and still have all protections or controls functional on both power sources.

These issues are also present in other configurations and not just 1-1/2 breakers.

What was it that raised your concerns?

 

[rasevskii]

I agree with rcwilson...On breaker and a half schemes and ring bus schemes, the tripping system gets very complicated, and relies heavily on correct operation of aux contacts on breakers and disconnect switches. Circuits have to be switched by multi-contact auxiliary relays, often latching type.

Depending on the exact scheme, even CT secondaries may have to be switched around to the protection relay inputs, carrying the risk that some secondary gets opened or shorted out if the switching system malfunctions, or the DC supply fails or has an earth fault somewhere.

Can be a nightmare to commission or maintain such a system.

rasevskii

 

[davidbeach]

No need for aux relays or CT circuit switching. Each position takes care of its own needs, and occasionally sees a breaker opened by a different position.

 

[rasevskii]

But: If two busbar differential protections are involved with overlapping zones, it can be necessary to switch over CT secondaries on each side of the center breaker from the protection on busbar A to that on busbar B. I am referring to a breaker and a half scheme here, where each busbar has its own busbar differential protection.

In this case the protection for busbar A has a CT on the "other" side of the center breaker (overlapping), if the main circuit through this breaker is in fact connected at a certain moment to busbar B, then that same CT secondary has to be disconnected from A busbar protection and onto B busbar protection. The same would happen for the bus diff on busbar B (mirror image).

If the two protections do not overlap the center breaker, this could be simplified with no CT switching, but then this breaker and associated disconnectors, etc, would be "outside" the bus diff protection, perhaps not an acceptable situation.

A bit hard to explain without a diagram.

I was once involved in such a scheme in the field.

rasevskii

 

[davidbeach]

Bus protection for a breaker-and-a-half scheme does not include the center breaker. Bus protection extends only to the line/transformer side of the breakers connected to the bus. The center breaker is part of two adjacent zones, lines or transformers, but wouldn't be part of the bus zone. You can't get to the middle breaker without including another zone of protection; including a line in a bus zone is a good way to get misoperations.

 

[DTR2011]

Thanks for the responses so far. I am just getting into the details of the project right now.

This is to be a "smart grid" substation, with the intention of doing away with traditional lock out relays, trip/close switches, etc. Everything is to be operated via HMI through SEL protection logic processors or Novatech Orion Communications processor. That part I get.

The substation is 161kV, 4 lines, 2 transformers and 9 SF6 breakers. The line relaying is SEL 421 (Primary)& 311C(Secondary), with PLC based POTT scheme, 2 shot reclosing with HBDL supervision. There are two sets of station batteries for secondary relaying, 2 trip coils on the breakers. The SEL relays are equipped with the high-speed/ high-interrupting contacts. This is where things get convoluted.

The SEL relays are not utilizing the optional high-speed / high-interrupting contacts, but rather the standard rating contacts and one trip signal is sent from SEL to ABB combiflex Aux Tripping relay. From what I've researched about the ABB relays, they have an ~4ms operate time and are rated no higher in interrupting capacity that what the SEL relays have. In addition, there are literally 4 FT test switches from SEL to ABB to breaker trip coil. The scheme is similar for the secondary protection. I could understand the need for an Aux relay if there were concern about the SEL relay contact ratings or the need for contact multiplication. I've never heard of any problems with the high-speed/ high-interrupting ones. I fail to see why the relay, which has probably 10 unused contacts would not be utilized and why would anyone want to add so much additional wiring, devices and a quarter cycle delay?

I haven't fully investigated the 87B scheme, however it does incorporate SEL 487 and GE B90 relays. I see the ABB combiflex relays here, but would expect them to be acting as a LOR.

I have duly noted the discussions on the 87B as listed above.

Thanks

 

[davidbeach]

Since the relay trip contacts never actually have to interrupt the current through the trip coil, the standard outputs are perfectly adequate. The HS/HI contacts are fine too, if you get the polarity right. Never heard anything good about anything that has the word "combiflex" in it. I'd also never do a 421/311C combination. With the CTs paralleled outside the relay, the 311C is at a significant disadvantage compared to the 421. All you're reclosing and breaker failure protection is only in the 421.

 

[odlanor]

In Breaker Failure event the relay trip contacts have to interrupt the current through the trip coil, and the standard outputs will be damaged.
A very fast relay has only 4 to 6 contacts or less.This is sufficient for single-pole trip.
We use standard ABB(4ms) to trip coil and robust power auxiliary relay(20ms,like CS1-C) in paralell to reset trip coil;

 

[DTR2011]

The CT's for the line relays are paralleled externally. All CT's same ratio, C800. I could see a problem if on the SEL421 the summation was made inside the relay and on SEL311C it was external.
I'm not sure I understand if it is a problem if CTs from both breakers are both paralleled externally and taken into the relay as a single, 4 wire current circuit, assuming approximately equal burden.

Breaker Failure and Reclose / Sync are handled by external SEL 451. As I can see for now, reclose only initiated by Primary 21 Relay.

I guess my original question is why use an Aux relay, which is just more wiring, complexity and another device to fail (NERC PRC-005 test, etc), when the relays themselves have a sufficient number of contacts which are appropriately rated for tripping & interrupting (30A DC) duty?

As I am digging through the 11"x17", poorly pdf'ed files, it appears that the Aux relays are referred to as a trip bus - 2 breakers. Some of this reminds me of 30 year old circuit designs. Again, this is a "Smart Grid" substation.

Maybe the bigger question is for new construction as IED's are defacto in the substation, are AC/DC P&C design standards evolving in harmony?

 

[davidbeach]

Why would you have a relay that can accept inputs from two sets of CTs and then parallel the CTs outside the relay?

 

[DTR2011]

I am the commissioning engineer, not the designer....This is why I ask. If there is an obvious reason (Labor / Training), I could understand.

I was reviewing the Main Transformer Diff Scheme AC Ckts. The 87T protection systems, SEL 487E & GE T-60 which have 4 sets of 3 phase inputs - freely assignable to various protection functions. 3 Phase Auto Transformer with Delta Tertiary. Station service is fed by tertiary to ~150kVA Pole Top Transformers(Aux) in an open delta/3P 4W wye configuration. The Aux transformer has its own primary fuses and LV panelboard.

The (2) CT's for the Delta Tertiary are wired in what I would call it a partial differential to 51G. A single phase CT circuit feeds the 87T IEDS. With the available CT inputs on the relays and the IEDS able to process this, why not?

This looks like an old GE or Westinghouse design from 50 years ago applied to a 21St Century Substation. It just makes me think that this will stump somebody 25 years down the road. If the Main transformer fails and is replaced and the new unit recommissioned, this may look obscure compared to someone. I can't think of too many reasons, other than a lack of updated design standards by the utility. Or the SCADA guys got all the budget money.

 

[slavag]

Protection scheme of one and half breaker is depend on the CT disposition.
We are use 87B for diameter ( three breakers line) too.
Lot of things depend on the type of Line and transformer differential protection, in few cases possible add stub protection.

as usually, all depend