15kV Bus Diff Operation Question
15kV Bus Diff Operation Question
(OP)
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
Any help is appreciated.
Thanks
jt






RE: 15kV Bus Diff Operation Question
When differential protection incorrectly operates on energisation, the problem is usually due to incorrect CT polarity.
RE: 15kV Bus Diff Operation Question
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
RE: 15kV Bus Diff Operation Question
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.
RE: 15kV Bus Diff Operation Question
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.
RE: 15kV Bus Diff Operation Question
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