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!

 

[DTR2011]

Just put a 587Z scheme into service yesterday. Steady state readings with ~120MW @ 138kV had readings less than 50 mV at the relay.

A few things to look into:

Check individual CT currents at the summation point.

Perform a Burden Test (or lead resistance) on CT secondary circuits. A poor or loose connection could be present. CT Lead resistance is an important factor in the settings calculation. Using a DMM is not accurate enough. A micro ohm meter is suggested.

Have the CT's been tested for ratio, polarity and Excitation / Saturation, or review the commissioning data. I would not rely solely on specifications of the CT's, but actual test data. It is not uncommon to find incorrect CT's installed, or an installation error while commissioning CT's and breakers. Verify that only a single ground is present.

Are there any Surge Arrestors in the protection zone?

How was the 68V Trip setting derived? What is the available fault current? What is CT ANSI Class? What is the CT Lead resistance? Is 68V perhaps a bit too sensitive? This does not change the fact that 8V steady state is not (in my opinion) an acceptable value for a properly commissioned scheme.

Where are the CT's summed? Is this an outdoor AIS or indoor Metal Clad arrangement? Is the CT summing point more or less allowing for equal secondary lead resistance?

 

[eletest]

In such a case, my first doubt would be the polarities of CT connections for the particular feeder. Normally this would be identified if a 'stability test' had been carried out for all the feeders at the time of commissioning of the switchboard/ differential protection . If it is possible to take a short shutdown of the bus, carrying out a stability test by primary injection would reveal the problem.

Also check whether the stabilising resistance values are calculated and set correctly.However since the rest of the feeders are stable, chances of this being the reason are low.

 

[cranky108]

SEL has papers on both switchgear applications, and arresters in the differential zone, for this relay.

68V does sound light, and may be worth a recalculation. But 8V standing on any phase in my applications cause an alarm for a CT problem.

 

[luffy2011]

DTR,
All standard tests were performed for the CTs, wiring and relays - results are satisfactory. Basically, we have two major concerns here:
1. Tripping during load switching
In this case, the CTs and arresters are the primary suspects as they have transient characteristics. However, a delay was already set to cover arrester operations as with previous high impedance bus protection installations. The CTs though may need to be retested thoroughly and will be compared to other feeder CTs.

2. The 8 V reading during steady-state.
Here, CTs and loose wiring could be the culprits - more on CTs and less on wiring. Nonetheless, we've already considered to checking these.

For your queries:
1. Yes, there are arresters.
2. CTs are C800
3. CT currents are sum up in a terminal block near the relay. The wire lengths are not really equal due to different distance from switchgear.
4. Switchgear is indoor GIS.
5. The setting was calculated using SEL guideline. Actually, our previous installations have setting values on the same range.

I'll get back to you for other details you've asked.

Thanks much for your reply.

http://www.powerqualityworld.com

 

[luffy2011]

Eletest,

Thanks much for the reply.

Yes, we've conducted PCIT prior to energization and everything was normal. Also, I forgot to mention that aside from the 8 V in 87A & 87C, the steady-state diff currents are about 5-7 A only, thus, lowering the polarity problem as the cause.

Thanks..

 

[HamburgerHelper]

One Valero's plants tripped out due to setting the relay as the manual described. The white paper came from investigating the operation. I believe that Valero won't use a SEL-587Z anymore due to not liking SEL's explanation.

 

[HamburgerHelper]

file:///C:/Users/00224811/Downloads/AN2008-01_20080522.pdf

 

[HamburgerHelper]

https://www.selinc.com/WorkArea/DownloadAsset.aspx?id=1124

 

[luffy2011]

Cranky,

Yes, we're also looking at the CTs as the prime suspects but will also consider other possible reasons that forumers here may have encountered from similar experiences.

Also, the 8 V is bothering us, since our other 587Z installations would only show 1 V the most..

Thanks!

 

[luffy2011]

HH,

Thanks, have already read it. That's why we've added a time delay of about 1-2 cyc. Other installations were stable but I think we still need to review it.

Also, I forgot to mention that high diff readings persist for about 30 cyc as per event records.

Thanks.

 

[stevenal]

87A=8, 87B=0, 87C=8 and it trips when switching one particular feeder? I'd say you've got A and C CT secondaries rolled on that feeder. Disable the bus differential and swap the A and C polarity leads (after shorting of course) from that feeder at the summation point and recheck voltage at the relay.

 

[dpc]

I have to agree with stevenal - the steady-state voltages and the fact that it is tripping for switching on one feeder really points to a phase swap - somewhere - despite the previous testing.

 

[DTR2011]

Secondary testing on the CT wiring can be easily be performed with a 3 phase relay test set. Apply 3 Ph voltages of dissimilar magnitudes (Ia=20V, Ib=30V, Ic=40V) at the relay. Check all the way back to the CT's with a DMM.

IF primary injection testing was performed properly, I can't imagine how a phase swap would not be picked up. I'm assuming a stability or through fault test was performed and voltages checked at the relay.

I ran into a situation in GIS years back where the termination from the Oil Cable to the GIS was not properly isolated, creating a current split between the GIS and every other grounded item in the substation and the primary circuit. This was found during primary injection of the GIS. It turned out to be the bonding jumpers between the termination and the GIS was touching the GIS.

 

[luffy2011]

stevenal and dpc,

As what I've replied to Eletest, it would seem at first that it is simply a polarity problem (A & C interchanged). However, I'd like to add that aside from the high impedance element voltages, the steady-state differential currents are relatively low: IA = 5-7, IB = 1, IC = 5-7. If it was simply a swapping of phases A & C, then, differential currents would have been much higher given that the angle difference would be around 120 deg.

Additional details:

Feeder Load = around 70 A @ 13.8 kV
CTR = 1500:5

This would mean differential currents of more than 70 A at steady-state for both IA and IC, if it was a case of phase swap...

Sorry for missing out these info...


Thanks!

 

[luffy2011]

DTR,

Yes, PCIT and secondary loop tests were performed to check the phasing and metering accuracy of the relay...

When you say bonding jumpers, are you referring to the cable shield wires?

Thanks...

 

[stevenal]

You will not see 120 degree phase displacement, since you will be summing the wrong currents from just the one feeder with the correct currents from all the others. The relay only sees the summation. You will also not see the 70A magnitude.

Swapped phases is not the same as a polarity problem, where the CT phases are correct, but the leads are reversed. Might be multiple problems present of course, but it's better to take one step at a time.

I disagree that your steady state differential currents are relatively low. A and C phases are significantly higher.

How many feeders/circuits are present? What are the currents, and are they sources or loads?

 

[stevenal]

DTR,

Your unequal voltage application sounds pretty good. You suggest measuring at the CT, though. In my experience,

http://www.eng-tips.com/the

CT is rather inaccessible; and I've never even dealt with GIS. The closest one can reasonably get is at the CT shorting blocks. Verifying proper phasing at the shorting blocks does not ensure the correct shorting block was wired to the correct CT. The very best test is to use live load for this purpose, since it checks all the way through.

 

[stevenal]

Don't know how that dead link got there when I hit the submit. Please substitute "the" above with no link.

 

[luffy2011]

stevenal,

The trip occurred when we first load that feeder... When we tried loading the adjacent feeder, it was successful. So what we did was lengthen the delay to around 5 seconds, and load back the feeder that caused the trip. There was no trip but the relay recorded that the high impedance element was activated and would have trip had we not adjusted the delay. Subsequently, at steady-state the values I mentioned were noted...

Anyway, this is not a polarity problem. For the possibility of swapped phases, we also thought so at first, but the differential currents should be higher than 5-7 A. Also, PCIT, secondary loop tests and wire-wire as standard tests were conducted prior to energization.

Thanks...