High Impedance Bus Protection Relay Tripping

[luffy2011]

Hi Guys,

I’d like to know if any of you have a similar experience and kindly give your analysis on this situation: High impedance relay for bus protection (SEL 587Z) trips during switching of load on one particular feeder only.

Details:

> In steady-state, the relay impedance element reading is 87A=8, 87B=0, 87C=8. Other substations with similar setup measures 87A=87B=87C=0.
> As mentioned, tripping occurs only during feeder load switching with values of 87A & 87C = around 80 V, while 87B = 0. Trip setting is 68 V.
> CTs used have the same specifications. Also, another substation has the same setup with no recorded problem for the past two years.
> Power transformer was already energized prior to loading of that particular feeder.
> No problem switching adjacent feeders of the same substation.
> The same feeder line was transferred/bypassed to adjacent feeder – no tripping.
> Relay event records show that relatively high differential currents are only transient in nature (about a second). A small differential current values are measured at steady-state but not enough to cause a trip.

Please don’t hesitate to ask for additional details...

Basically, we already have an initial assessment and action plans. However, I would appreciate if any can give their analysis or share a similar experience.

Thanks!

 

[stevenal]

Why do you expect swapped phases to provide more summation current? What are your loadings?

Not a whole lot of value to insisting things tested out correctly. Either your testing method was incorrect or it fell short of testing what is wrong.

(86V/2000 ohm relay burden)*(1500/5) = 12.9 A primary summation current at pickup. You are running at over 50% of pickup with no fault on the system. This is guaranteed to trip on the first through fault even if you set the delay to ride out the switching transient.

You offered to provide more info if asked and I'm asking. Number of circuits summed? Steady state loadings on each?

 

[luffy2011]

stevenal,

There were only two feeders up, the one with the problem (70 A) and the adjacent line (130 A)...

Anyway, I'd appreciate to see your calculations for the differential current in case of a phase swap...

Thanks...

http://www.powerqualityworld.com

 

[cranky108]

One problem with testing CT polarity is when the tech.'s encounter a reverse CT they assume it is there testing, and will reverse there CT test leads. I've seen this happen, and the test is invalid.

The best method to detect polarty is with a pulse of DC current on the primary, and watch the secondary with a sensitive center balance meter (not a digital meter). All the CT's should deflect the same direction, provided the primary DC is in the same direction. This test is followed by demagnitizing the CT (which could also be a problem in itself).

Also if this is switchgear, SEL recommends pickup values above 150 volts, which is higher than what you have set.

Question: Why do you have the settings so low? Are you concerned about insulator leakage?

 

[stevenal]

I'm not sure how 70 and 130 are expected to sum to zero. Perhaps there is a third circuit supplying 200 amps so Kirchoff is satisfied?

Also, you state that only two feeders are up. Do you have more CTs wired in from open feeders?

DTR already said it, but ensure your CT secondary neutral is grounded at one location only for the entire system, typically at the relay. Multiple grounds will cause circulating currents that will affect your results.

 

[luffy2011]

Cranky,

The settings were calculated based on SEL guidelines. The 68 V trip setting have a time delay of 1.25 cyc to compensate arrester operation and/or CT unbalance during high current (inrush or external fault). Since you can set two 87 stages, the SEL guideline also recommends to set the other stage to 200 V but without a delay.

I'm not sure about the polarity problem since the differential current is only 5 A. As I understand, a polarity problem shall add up the current of that particular phase, if I'm not mistaken.

Thanks...

 

[luffy2011]

stevenal,

I apologize for the confusion...

The circuits summed involve two feeder and the main line. Actually, there are four feeders in total but at that time we only loaded two feeders.

Main Line - 200 A
Problem Feeder - 70 A
Adjacent Feeder - 130 A
Other 2 Feeders - No Load (Breakers Open)

Yes, we'll include checking for possible multiple grounding on the secondary circuits of the problem feeder...

Thanks...

 

[luffy2011]

stevenal,

I apologize for the confusion...

The circuits summed involve two feeder and the main line. Actually, there are four feeders in total but at that time we only loaded two feeders.

Main Line - 200 A
Problem Feeder - 70 A
Adjacent Feeder - 130 A
Other 2 Feeders - No Load (Breakers Open)

Yes, we'll include checking for possible multiple grounding on the secondary circuits of the problem feeder...

Thanks...

 

[luffy2011]

Guys,

Aside from possible wiring mistakes, we're also looking at CT remanence as the possible cause - since it affects CT performance during high current such as load switching and reduces CT accuracy (e.g. ratio) even at steady-state.

Kindly share your experiences...

Thanks...

 

[stevenal]

With just the three circuits, I agree a phase swap would cause a much higher summation current.

Since all the CT secondaries are tied together, you need to look everywhere for grounds, not just on the one feeder. A likely location may be one of the unused feeders.

 

[luffy2011]

stevenal,

Well noted... Thanks much for giving time to share your thoughts...


Thanks...