Bus Differential and Lock Out

[DTR2011]

I came across something that did not 'feel right' when reviewing a new installation. 138kV Utility switching station with several large generation sources tied into a ring bus.

There is an upgrade to the bus for new 87B (Bus Differential) relays. New panels, Lock Out Relay, Test Switches, etc. This scheme uses a Primary (Low Impedance) and Back Up (High Impedance Scheme). The protection is via SEL 487B (Primary) and 587Z (Back Up) relays.

The back up scheme (587Z) does everything I would expect. Has independent CT's, wired to relay (86B), 86B shorts CT's, etc. 86B trips all zone breakers (Trip Coil #2) and BLOCKs close, via 86B 'b contact' on associated zone breakers.

The Primary Relay (487B) has more contacts available and trips Trip Coil #1 & #2 directly. What troubles me is that the 487B relay does not trip the 86B to block close on the associated zone breakers. Some of the associated zone breakers are lines connected to generation and use a reclosing scheme that is not inherent to the digital relays internal logic (SEL 321 Series). There is nothing in the schematic to suggest that the 487B sends a block close to the line relaying reclosing scheme or SCADA.

There is a single 86B relay, which also serves as the 86BF in this scheme.

My concern is that the primary relaying (487B) does not at least block close on the associated zone breakers. I don't have the settings for the 487B relay yet, but for sure, there are no other contacts in the close scheme in the associated zone breakers that would replicate the 'b contact' of a normal 86 relay.

I suppose a latched trip on the 487B relay could leave a 'standing trip' on the associated zone breakers (and reset via a push botton on the 487B), but then why have the back up relay trip an 86B?

I can think of all kinds of reasons how the back up protection (SEL 587-High Impedance...Shorted Ct's??) , which trips and blocks close via the 86B relay could be out of service and the primary 487B relay could trip, but block close / reclosing.

There are no SEL Mirrored Bits or 61850 schemes involved. This is all traditional hard wired logic schemes.

At this point, it is easy enough to add a few wires and at least have the Primary Relays trip the 86B for the sole purpose of the block close function.

A request for information has been sent. I am just trying to imagine a reason to not include a 'block close' for a low impedance diff scheme.

 

[crshears]

Within our utility there is a more-or-less standing policy that any switchyard circuit breakers that could re-synchronize generation following a contingency have their reclosing features normally blocked; the thinking is that the consequences of an out-of-synch reclosure for any reason are almost certain to be more severe than the corresponding loss of generation.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[crshears]

...the reasoning seeming to be 'why take a chance that a reclosure blocking scheme could malfunction? The supposed increase in system security and reliability afforded by such a scheme is not worth the risk of the consequences should it fail to operate as designed.'

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[cranky108]

Would you not expect that a fault that trips the 87P also trips the 87S?

Could this also be a young engineer design, where they don't understand?

 

[mgtrp]

If you are right in thinking that the SEL-487B is working as a lockout relay until its target reset button is pressed, then this same scheme could not be applied to the backup relay as the SEL-587Z is a more primitive relay whereby the target reset button is not available as a relay word bit and therefore not available for use in the relay logic to unlatch the trip signal.

That being said, I am not a fan of the protection relay being used as a lockout relay, and suggest that if you want relay operation to lock out the breakers that you provide a lockout relay (preferably separate from the secondary relay lockout).

 

[bacon4life]

In a ring bus, what do the bus differential relays protect? Perhaps this is a breaker and a half scheme?

If I understand correctly,an external reclosing relay could automatically reclose the circuit breakers after the 487B trips them. In this case, it seems like an oversight to not provide blocking contacts. If instead the 321 only initiates reclosing for line faults the 321 detects, providing redundant blocking may be less important.

Does the 487B use any zone selection logic such that only some of breakers would be tripped?

If this is a high lightning area, do the utility allow bus reclosing after a lightning strike?

Does adding the additional trip from the 487B to the 86B make testing significantly more complicated? When two relays trip the same 86 coil, testing both of them can be challenging. When only one trip path exists, actual fault operations can be used for PRC documentation.

 

[DTR2011]

Thanks for all of the comments. I've only spent a few hours (a few days ago), reviewing the site and new 87B panels. This is an existing (large) station that has been modified (bus configuration) over the years. When viewing the single line, I would call it a radial/ring and a half. Let's just say that the 87B scheme is zone protection with sources and feeders.

The 321 is both the Line Protection and an SELogic enabled recloser (there is an app note on this from 1997 or so). There is back up distance and sync check via a 200 series SEL distance relay.

The issue that I believe exists is the lack of some kind of 'block close' contact from the 87B-P system. I believe this is some kind of oversight as the back up 87B does as expected.

As far as testing, the 487B has sufficient spare outputs and FT Test Switches to route a contact to the respective 86B relay in the same new panel (yet to be installed). The 86B has sufficient spare FT Switches for isolation of the input from the 487B.

There is nothing in the schematic to suggest something like zone selection, the 487B does not have any breaker or switch position contacts wired to it.

I havent tested the relays together to see who would win the race, but there is always the possibility that the 87B- backup could trip first under certain conditions and then the 86B does it's job, trip and block close. Since the two relays have different operating principals, it would take some meditation to test for equivalent conditions and I don't think I would prove much.

The 487B is directly tripping each breaker in the zone and there is a possibility (I suppose) that it may win a race between it's direct trip and the 587Z>>86B>>breakers. I haven't verified if the 487B contacts used for tripping are the high speed variety.

Under this condition, the bus zone is dead and the control center does not have a proof positive of an 86B operation, which is monitored several different ways into the RTU.

I have researched several threads on this forum regarding the application of an 86 device and agree that it's initiation is due to a serious fault, requiring a site visit to hand reset.

The breaker failure relays trip the same 86B relay, in lieu of a dedicated 86BF relay, so I can't see that adding an addition trip to the device would change much at this point.