15kV Bus Diff Operation Question

[james001]

I just installed the BE1-87B relay with all CT characteristics etc. are the same. The 87B has one main breaker and 5 feeders. The 87B relay operated fine until I energized the last feeder and close in the breaker, it hit the lockout relay and tripped the entire section. Afterward, I noticed that the last feeder load was unbalanced, 136A on A-phase, 319A on B-phase and 601A on C-phase. Does the unbalance current cause the 87B to operate?

Any help is appreciated.

Thanks

jt

 

[DiscoP]

Im not familiar with that relay type, but it shouldn't operate the busbar protection.
When differential protection incorrectly operates on energisation, the problem is usually due to incorrect CT polarity.

 

[dpc]

My money is with DiscoP's. This is almost always due to incorrect CT polarity or phasing, or missing CT signal.

Go back to the breaker that caused the trip and trace out the CT wiring. It's easy to get a polarity backwards, and that's all it takes.

(Assuming you don't actually have a fault in this breaker

 

[DanDel]

How did you notice that the last feeder load was unbalanced if closing it in tripped out the system? Did you have one trip and then it closed in ok? This is not clear from your post.
If it is tripping all the time, I'd go with DiscoP and dpc about the CT polarity. You could also have a defective CT. Check these things, and if you can't find anything, then megger check that last feeder.
Unbalance will not cause the differential relay to trip, but depending on the settings, you may have had a trip on inrush current, even if the CTs are matched. I believe the Basler BE1-87B has either adjustable or a fixed variable slope which may be too tight for certain applications.

 

[peterb]

In addition to the CT check suggested, do a magnetization curve check on the suspect circuit CTs (in fact, do this test on all CTs during commissioning). Also check that there is only one ground point on the CT secondary wiring. Multiple grounds can lead to sneak current circulating outside of the secondary wiring.
Don't know if it's possible with your system arrangement, but a foolproof way of checking out a circuit being added to a differential scheme is to do a primary injection test of the CTs against an adjacent circuit that is already proved correct for the scheme.
Lastly, verify your setting calculations against max inrush current and for the actual CT kneepoint voltage.

 

[jlazucena]

Bus differential (87B) is to detect faults inside the zone defined by the CTs. But 87B may misoperate when one CT gets saturated during a fault. As per above comments you have already checked polarity, Breaker insulation, wiring, CT magnetizing curve, etc.

If the CTs are multiratio, Have you checked that all CTs have connected to the same CT ratio?
Have you checked for phasing?(All phase A polarity to the point No 5, etc)
In my opinion there are three posibilities: polarity or ratio or phasing