Differential Trip 2

[wr180561]

Can anyone tell what is the reason for a current differential trip in a genset knowing that the genset is protected by a differential trip relay connected by CTs on both the neutral & bus sides of the genset.

Thanks for your help. WR

 

[busbar]

 
“Burn and learn” or blind-reclose procedures can be very expensive from a facility-loss standpoint, or career-changing blame.

There are two aspects of evaluating the problem that should have close to equal consideration. One is to perform an ‘audit’ of CT electrical characteristics, secondary wiring, relay-installation instructions and relay-setting calculations. The other is to thoroughly test the protective relay that asserted the trip with injected AC-current quantities to prove correct [and maybe as importantly—anticipated/personally expected] response of the relay.

[There are quite a few hits reported with the search phrase “Differential Trip” at this site.]

 

[rbulsara]

Are you saynig that the generator is tripping on 'current' differential?

Or do you think that current differential trip is something different than the differential relay with CTs as you describe? It is not.

 

[dpc]

Your question raises a lot of other questions:

Is this a new installation undergoing checkout, or an existing unit where the 87 relay has operated without problem for many years?

Did the relay trip as soon as the generator was enerigzed or some time after?

Is this a one-time event, or has it happened repeatedly.

In theory, for a properly installed and tested 87 relay, its operation indicates a winding fault or other type of fault within the differntial zone.

 

[swgrfpso]

By comparing current in the generator and current in the switchgear, the generator is to be protected.
By the way, I do not know how the differential protection is set(%) and it has to be set depending on how big comsumer is loaded.
Anyone tell us how differential protection is set, let say 20% or 40%.

 

[rbulsara]

I beleive its usually 5%-10%.

20% or 40% sounds very high.

 

[busbar]

Depending on project status and outage undesirability, it may be time and money will invested to retain a skilled consultant for a site visit or design review.

 

[wr180561]

Thanks all for your replies, however here are some facts:

1- The installation is not new, it is ongoing for almost 8 months now.
2- The genset was inoperative for some time due to other mechanical problem and we started to notice this problem when it was put back to service 2 weeks ago.
3- The genset occasionally trips on (differential trip) after a while from start up when it operates as a single unit.
4- When this genset operates in parallel with other genset in the same power house, this problem seems to disappear, i.e. it does not trip due the above problem.
5- The other three gnesets on this installation does not suffer the same problem even when they run as single unit.
6- Settings of the relevant 87 relay on all gensets are as under :
- minimum pickup 0.07 x CT
- Slope 1 10 %
- slope 2 20%
- delay 60 cycles.


7- CT ratio is 250/5
8- Genset FLA 172 A
9- Genset rating 3.3 MW
10- Voltage 13.8 kV
11- Frequency 60 Hz.

Details of the last two trips are as under:

1- Current on phase A 129A
Current on phase B 123A
Current on phase C 123A

Phase A differential 17 A (genset tripped)


2- Current on phase A 111A
Current on phase B 108A
Current on phase C 109A

Phase A differential 28 A (genset tripped)

Hope the above information will help you to help me identifying the problem.

Thanks you. W.R.

 

[rbulsara]

Do you mean the installtion is not more than 8 months old? That is new in my book.

What type of commissioning or startup testing was done?

It appears that the gen may be developing slow internal fault.

Have the mfr. or a good rewindig shop thoroughly inspect/test the alternator, it is not worth taking risk to restart a gen after a differential trip, even if it turns out to be a nuisance trip.

Your diff. pick up sounds very low, it should be somewhere between 0.3 and 0.5

If alternator checks out OK, revise the setting of the relay, of course check all relay wiring, and see what happens.

 

[corrosivo]

wr180561

I think you do not have a real fault in the windings, if that were the case you will have other symptoms on your machine. An internal fault in a generator rapidly becomes catastrophic.
Looks that the relay tripping is dependent on the loading of the generator. Did the generator tripped on an external fault recently? Maybe your generator CTs are magnetized and are not sending a faithful representation of the primary currents to the 87 rly. I guess you already checked the relays. The fact that this machine have operated for 8 months indicates that your CTs connections are correct. Maybe a loose or a momentary short circuit in the connections on the CTs secondary circuit is causing the misrelaying as rbulsara suggest. Regards.

 

[wr180561]

I'm going to the site tomorrow to do further checks on the relay and relevant CTs if all seems to be fine then the last thing will do is to change the settings. Will start with increasing the time delay from 60 cycle to 120 cycles (2 sec). Will keep you posted.

Best Regards
W.R.

 

[dpc]

It's extremely unusual to have even a 60 cycle delay in a differential relay, much less 120 cycles. One of the main benefits of differential protection is the ability to rapidly clear faults within the differential zone. 120 cycles is an eternity for a generator fault.

What type of relay is this and does the differential zone include a step-up transformer?

As busbar indicated, automatically assuming the problem is in the relay is not a wise approach. If you can't find a CT wiring problem then I would want to break the generator leads and high-pot the machine, just to be sure.

For a straight generator differential, if it is successfully synchronized and able to carry any load at all, the CT wiring just about has to be correct (or non-existent). Because the generator differential relay does not have to deal with transformer inrush, it normally operates with low restraint.

 

[wr180561]

Dear all

Thanks for your valuable responses.

We have changed the pickup set point temporary to 18 % and the delay to 100 cycles and the genset is under close monitoring now. However can anyone tell me why the genset in question never tripped on differential current when it ran in parallel with another genset ? ( even before changing the settings. Thanks. WR.