Bus Differential Protection using relay

[budhiman]

Hello

I am looking for guidance on how to set bus differetial protection for four circuit single breaker bus configuration. Is there a good reference on how to calculate Settings. For this application schwietzer relay is used 787. This relay is typically used with transformer differential.

 

[DTR2011]

SEL has plenty of application guides available. Simply register on the site and search away. You may also check SEL 587 for a similar application. Also consider using a SEL-387 or better yet a SEL-487B for your application. The SEL 387 has 4 sets of CT inputs. The 487 can have up to 6, I believe.

If you are stuck using the 787, Select the SEL-787-4X for current-based, four-winding differential protection. The 787 has configurable I/O cards, so you may be able to add additional CT inputs. This is a must if your CT's are not the same ratio / class. There have been applications in the past where the CT's are paralleled together, but for the low cost of the I/O cards vs. the cost of a misoperation there should not be a question to upgrade.

Better yet, contact SEL directly. Search this site for the large number of users here that speak very highly of SEL customer service.

 

[Mbrooke]

FWIW the 487B can take up to 21 CTs if 3 identical 487Bs are run in parallel.

 

[davidbeach]

Minor quibble - each 487B can take 21 CTs. That can be 7 three-phase circuits, or you can use a relay per phase for up to 21 positions on a bus.

Back in the beginning, when the earlier 487B only had 18 CT inputs, they were saying that you could use one relay for up to six positions, two relays for up to nine positions and three relays for up to 18 positions. I fell for that on a seven or eight position bus and did the two relay thing; one relay for phases A and B and the other for phase C. Never again, three relays would have been much simpler. I also haven't seen them touting the two relay solution for quite some time.

After an installation where we had six relays per bus on a 9 bay 1.5 breaker installation, twelve bus relays total for the station, we're trying a slightly different approach for the next 1.5 breaker station, four bays initially and a maximum of 7 when the station runs out of room. In that one we're going to use a total of six relays for the whole station; CT inputs 1-4 are one bus and 17-21 are the other bus with two zones defined in the relay. Three relays for one set for the whole station and three more for full redundancy for the whole station.

It may seem like overkill to use the 487B on a four position bus, but it's not. It may be a bit of a set into the abyss to go from never having used a 487B to programming the first one, but they can be a relatively simple relay to work with once you get the hang of them and you realize that many of the "features" can be safely left turned off. I don't think we've ever bothered with the check zone logic, but we've definitely used the dynamic zone definitions.