Breaker and a half protection scheme

[APHK]

I have a question regarding a breaker and a half protection scheme. Please see the PDF sketch I have attached for reference.

This is the current set up we are looking at: A breaker and a half configuration with the Tie Breaker as a normal open point. The protection (drawn in blue) is in a summing set up and if a fault is detected a trip command is sent to both breakers in the protected zone. Eg a fault on Load 1 under normal operating conditions a trip command will be sent to both Breaker 1 and the Tie Breaker.

Now the problem I am running into is if Breaker 1 is out of service for maintenance or whatever, Breaker 1 will be open and the Tie Breaker will be closed supplying Load 1 and Load 2 from Bus 2 if there is a Fault on Load 1 Relay 1 will still have to be in service as Relay 2 will sum the fault to 0 and not trip.

I would like to be able to take both the Breaker and the relay out of service during the maintenance outage and have Relay 2 do some smart tripping. The constraints I am working with are the protection Relay has to be a SEL-351A relay so we only have one set of current inputs. I was trying to come up with some logic or alternate CT configuration that makes sense.

Has anyone else run into a similar situation what are your suggestions?

 

[slavag]

Hi.
APHK, waht is a voltage of your system ?
one and half scheme is a not simple solution.
Usually used for more tnah 220kV.
It's a not some 50/51 functionality. Such schemes are include 87B, 87L, stub protection, etc....
It is also depend on CT disposition, before , after disconnectors, CT on the line or w/o.

Best Regards.
Slava

 

[oldfieldguy]

I've seen quite a few "breaker and a half schemes employed in medium voltage switchgear.

What kind of relays are you using?

In a normal breaker and a half scheme, both sets of protective relays remain in service, and in your scenario, with Breaker 1 open, Relay 1 would still function to trip the tie breaker on a Load 1 fault. If you're taking the relay out of service in your scenario, though, you kill that function.

If you used some of the modern digital relays, such as the GE UR, you could do a bit of fancy programming and set up the relay to cover both conditions, but basically you end up with something that is not a classic breaker and a half arrangement.

Instead, you have two overcurrent elements to coordinate for each relay, one for the main and the other for the tie, with both overcurrent elements residing in a single relay. You cannot do your summation of the currents outside the relay as is the case with the classic breaker and a half scheme.

The GE UR platform allows you multiple sets of CT inputs and lets you sum them or use them individually and this is what it seems you'd need. If you're using relays that do not give you this functionality, then it looks like you may need a separate set of protection for the tie breaker.

old field guy

 

[bacon4life]

If you short tie break cts at the breaker and open the relay side, seems like relay 2 could protect both of them. Not sure if you could do that while energized.

 

[jghrist]

Perhaps you could have a scheme that takes the tie breaker CTs out of service by shorting and disconnecting them when Breaker 1 and Relay 1 are out of service. Then all of the current in Breaker 2 CTs will flow into Relay 2 whether the fault is on Load 1 or on Load 2.

 

[Electic]

You should talk to the SEL representative.

Generally SEL351 is over current protection and would operate on CT's installed on the load. It might be possible to create zone differential protecton scheme by having two SEL 351 relays talk to each other but that seems like a complicated solution. SEL 487 or some other bus differntial relay will be configured to what you need without a complicated communication program. Relays are relatively economical and it would be better to have the proper unit installed.

 

[slavag]

Hi.
For my pinion, isn't possible protect such system with two overcurrent relay.
If..if..only start think about used overccurent relay, for my pinion, need add aditional two CT set's and additional 3 overcurrent relays.
1. One for summation CT ( after/before CB-A)
2. One for summation load 1 ( stub protection)
3. One for summation CT ( after-before bus-tie CB-C)
4. One for summation CT on load 2 ( stub protection)
5. One for summation CT ( after/before CB-B).
Possible use same CT with few cores, not additional CT .
In such option, overcurrent relays will operated only in case of fault in protective zone and are possible used relays with two current set inputs: summation current used as diferential protection, and one input as simple overcurrent protection, as back-up protection.

In such option, is possible take out CB, relays, w/o any problem. Possble add reverse blocking, etc...

Best Regards.
Slava

 

[cranky108]

Breaker and a half wasen't intended to do what you are wanting to do.

If you make it an odd scheme then the next guy has a trap, because it won't work like a breaker and a half was intended.

 

[rterickson]

The whole point of breaker-and-a-half is to achieve the reliability of the double-bus scheme at the economy of only 1.5 breakers per line. If the shared breaker is left closed, your scheme should work just fine as drawn. Operating with the shared breaker open (as a tie breaker) completely defeats the purpose of having breaker-and-a-half in the first place.

Breaker maintenance does require an outage of one load though.

I you absolutely cannot take a load out for maintenance, you would be better off with double-bus, double-breaker.

 

[stevenal]

Are you summing the currents by paralleling the CTs? If so, you are betting that when B1 is open and a fault occurs on L1, the two CTs feeding R2 saturate equally. Not too sure this is a safe bet.

 

[odlanor]

This is a classical busbar arrangement for EHV systema. They always require double protection completely independent in order to possibility a sequencial maintenance.
Tie-breaker has the same responsibility then other breakers. The designation is central breaker.