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Capacitor Voltage Differential relay measures twice expected value

Capacitor Voltage Differential relay measures twice expected value

Capacitor Voltage Differential relay measures twice expected value

(OP)
When a fuse blows on one of our capacitor banks, the differential voltage reported is about twice as high as predicted for a blown fuse.  The differential voltage recorded by the SEL 287V appears to be very slowly decaying over a couple of seconds. Since the relay has DC filtering, any decay would have to be an actual 60Hz decay.  Does anyone have any ideas on what might temporarily cause extra differential voltage.

The  115 kV externally fused capacitor bank made up of 5 series sections each with ten 13.8 kv cans.  The voltage differential measures between the 115 kV bus, and a PT connected to the lowest voltage capacitor (13.8) kV.  The relay has an alarm set point for one fuse failure and a higher set point for multiple failures.  In all cases after the fuse has been replaced, the voltage differential is approximate zero.

This substation has two identical banks and the problem has happened on both of them on different phases.  Each bank has it's own PT and SEL relay.
 

RE: Capacitor Voltage Differential relay measures twice expected value

If I understand correctly, your "Tap" PT is not at the center point of the string, but rather at the bottom.  Do you have a CVT on the Bus? Perhaps a bit of ringing is occurring that eventually damps down.  Are there any other devices in the CVT/PT secondary circuits that may have undue burden?

I am assuming that the alarm condition causes concern after a fuse replacement.  Try installing a scope and see what happens after re energizing.  Maybe a small delay on the alarm is all you need.

RE: Capacitor Voltage Differential relay measures twice expected value

(OP)
The tap PT is at the 20% point instead of the 50% point. Both the tap PT and the bus PT are inductive type.

The relay is set to alarm for a single blown fuse and trip for two blown fuses.  Unfortunately, after a single fuse blows, the differential voltage is above set point for tripping.  

The relay already has a 30 cycle pickup delay. Does that sound long enough?  

RE: Capacitor Voltage Differential relay measures twice expected value

Are the relays set up for your PT tapped @ 20% vs. 50%?  A quick scan of the 287 manual suggests the 2nd alarm delay @ 60 cycles (pg 5-21).  If you could list you settings this would be helpful.

RE: Capacitor Voltage Differential relay measures twice expected value

(OP)
Yes the voltages were calculated for the 20% PT.

I don't know why they relays is set to 30 instead of the SEL recommended 60 cycles. However in the attached PDF, it is clear that the voltage on the phase with the blown fuse continues to to change even at 60 cycles.

We did attempt to use a scope to capture the transient after intentionally installing a very small fuse, unfortunately the scope did not trigger properly.  The relay captured an event report at 30 cycles when the Loss of Potential timer expired, then  another one at about 60 cycles when the 87 trip timer expired.

At 30 cycles, the differential was 3.67 volts, at 60 cycles it was 1.7 volts and the calculated value was 1.4 volts.

The text file has the two event reports and the relay settings. The original 187V could not handle different settings for positive vs negative differentials, so the alarm/trip settings are only coordinated for the upper 4 sections.  All of the blown fuses have been in the upper sections.   

http://files.engineering.com/getfile.aspx?folder=5f32bb26-b5eb-424d-b8fc-9c73e7eae4b6&;file=cap_bank_trip_graphs.pdf
  
 http://files.engineering.com/getfile.aspx?folder=bf1c095a-d35f-4fec-b4a8-e26ee7fc5500&file=Cap_bank_trip_1573324_2_14.TXT


 

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