Differential Trip (Device 87) 4160vac 6 Gen Parallel System

[EmergencyPowered]

Six Generator Detroit Diesel(1750Kva units) 4160vac ASCO system carrying building load varying around 6000Kva. One unit trips 86 lockout from 87 device. Flag on 87 fires so it is not control circuit issue. Will stay on bus at lighter loads but right around this level it trips. Swapped Relay, relay case and reactor unit and it still trips. Put same parts in other generator and they work fine. Testing firm tested all CT's, meggered the cable and local disconnect switch, all good. 3ph Current looks correct on ASCO EDGEWISE meters. Also swapped SR voltage regulator, no change. Rotating rectifier is good. I have HIOKI and a Dranetz Power Analyzer (old monster). Any ideas! I have 3 years of BSEE but didn't finish 4th year. Vectors stuff is not my strong suit. Now I will say "HELP". Uses cross-current CT's on B phase aloso.....

 

[rasevskii]

Quite possibly the CT connections for the 87 are wrong, i.e. phases rotated, reversed or other mistake in connections. The relay won't trip in this case (falsely) until a certain load current is reached. Has this been a problem from the beginning on one unit, or did it develop later? Is everything new or old or has something been modified recently?

Just testing the CTs individually won't find a mistake in connections. Same with the relay.

regards, rasevskii

 

[davidbeach]

What do the currents look like in the relay event reports?

 

[EmergencyPowered]

System is unchanged for twenty years with no similiar problems, but customer believes he may have more load than he has ever had in the past. HIOKI will be used to prove CT connections by viewing current waveforms this week. I have always had problems at much lower loads when CT's were wrong, but I have had very little 87 problems except on initial system startup so I am no expert here.

These BBC relays are old analog type, there are no reports.

 

[EmergencyPowered]

Oh yeah, I have set the pickup to MAX but it still occurs. I can not control the testing loads as it is real building load whichs fluctuates around 6000Kva. Customer is curtailing load for CON ED on very hot days. I have no load banks on site and wish to avoid the high expense of load banks and cables if possible. There are five identical units and I hope to compare all my traces with a known good one. Also, nothing bad happened when I risked disabling the relay. Does this prove that no REAL problem exists?

 

[DTR2011]

I would rent a high current test set and run primary current through the CT's.

 

[rasevskii]

If you can get an outage on the unit that has problems, try to inject primary current from a single phase test set (your testing Co. can do this)and see that the secondary current appears at the relay for each phase on the correct phase. This will verify the phasing but not the polarity. To check the polarity a battery can be touched to the primary across each CT (all other voltages off, and the CT primary open) (main breaker off), then using a center zero analog meter check the direction of the DC voltage at the relay input This is known as a flick test. Again your testing outfit should do this. Compare the result with another unit known to be correct. Observe polarity.

Ideally a 3 phase SC test should be done using the generator as a source to verify all the CT circuits. Probably beyond the capability of the available personnel. A separate excitation of the exciter field from an external DC source needed. Should have been done at the original commissioning but who knows what was done 20 years ago...

IF you disable the relay and actually have a fault, it will get very ugly, with possible severe damage. High Short Circuit power. Some severe Legal and possible Insurance issues.

Get a professional in to survey the situation and advise your organization.

regards, rasevskii

 

[waross]

Are the governors running in droop mode, or can you go to droop mode? If so, you can change the load on an individual machine by adjusting the speed setting for the machine in question.
And regardless of the control scheme of the other generators, If you can run the governor in manual mode you can control the loading. With the governor in manual mode the load on the machine under test will be determined by the throttle setting. If the building load varies, the other machines will automatically adjust their throttles to compensate leaving the load on the machine under test unchanged.
As long as the building load does not drop below the loading of the machine under test there should be no problems. An easy and safe way to load the machine is to put the governor in droop mode and then adjust the frequency slightly. Then, should the building load drop below the setting of the machine under test, the frequency will rise slightly.
With a building load of 6000KVA available you should not need a load bank to load test a 1750 KVA machine.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rasevskii]

Oh yes, also do a visual check to see that no short circuit link or screw has been left in place that might be shorting out a CT secondary. Also that the grounds are connected correctly and that no double ground exists on a CT secondary
that would be effectively a shorted secondary.

If these are the old BBC mechanical relays, please be kind to them, they are irreplaceable and out of manufacture for years.
Such good things will never be made again. Maybe in India?...

rasevskii

 

[davidbeach]

Again, what do the relay event reports show? That's where the answer lies.

 

[DTR2011]

These sound like ITE/BBC/ABB Circuit Shield Relays (Allentown, PA). Static, single phase. A type 87G I would presume. No event reports, but good relays for the technology.

 

[edison123]

I too would check and rule out the CT's first. May be the secondary leads are switched.

Muthu

http://www.edison.co.in

 

[Zogzog]

"If these are the old BBC mechanical relays, please be kind to them, they are irreplaceable and out of manufacture for years."

I stock hundreds of these, so they are still replaceable, same goes with the powershields.

A simple test using something like a MS-1A will verify operation.

 

[ScottyUK]

At 1750kVA I wonder is this a true differential scheme with a CT on each winding end, or a restricted earth scheme with a CT on the neutral after the common point? Reversed neutral CTs can lie dormant without affecting anything for years until somthing causes a neutral current to flow.

I agree with rasevskii, use the machine as its own primary injection set. 1750kVA won't need even particularly heavy shorting links.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[EmergencyPowered]

Thank you all. I learned more from your help here than I lesrned from the "Paid Consultants". Almost ashamed to admit that it was simply a poorly crimped 2 cent ring terminal done twenty years ago. Hot weather, generator room around 120 degrees and large load step caused a intermittant lug to become barely connected. I just wonder if CT has been damaged by being unburdened.

 

[xnuke]

Good troubleshooting, and thank you for providing feedback on the cause! We always like to see the resolution to a problem.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[DTR2011]

I would recommend an excitation test on the CT in order to verify if the CT has been damaged.

 

[EmergencyPowered]

Btw, Who is John Galt?

 

[waross]

John seems to have dropped out of sight during these difficult times.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

I run across this type problem on a fairly regular basis unfortunately, just recently at a waste water plant with 4 gensets. In my experience likely a CT or CT wiring if you neutral side CT's are mounted in the generators. If they worked good for 20 years, the relay moves and the problem stays with the unit, take a good look at the CT's and the wiring. The CT's sit in the end of a high speed engines generator, high vibration, temperature, and not much TLC unless something goes wrong. The last time I found this the unit would run for 3-4 hours then trip. Older system with single purpose relays, no SER or event report. We have an SEL300G we use as a training aid, setup it up temp and caught the problem. Customer liked the idea of getting an event report after a failure and we got a retrofit out of it.

Hope that helps,

Mike L.