Generator differential protection 87 2

[Smasem]

Hellow experts
I will appreciate any suggestion regarding this issue.
a generator 11kv 5.5 kw rated current 361 A.
the generator tripped 87 Diff protection. while sharing the load with the other units.
check the CT's all good and IR test was good. cleaning the lugs and start the generator but it tripped on 87 Diff again after 2 weeks.
found the DE Vibration probe wire burn out also the RTD module, seems to be result for high leakage current.
IR test 10 kv 1 mins.:
phaseA 27Gohm ,phaseB 21Gohm and phaseC 4Gohm.
High pot test:
phase A 29 kv, leakage current was 30 micro ampere
phase B 29 kv, leakage current was 20 micro ampere
phase C 22 kv, leakage current was 250 micro ampere (phase C break down after 22KV)

high pot test for phase C not good, CT ratio 500/5 Pick up current 0.2 in the protection relay,
so the required cuurent to energize 87 protecion = 0.2 *361/CT ratio = 0.722 A. the leakage current still in micro ampere and IR test still good why the diff. energized ?
based on the previous tests,is the generator can be repaired on site or not? what is repairing required?
regards

 

[edison123]

Given the microamps, is the hipot test DC?

DC hipot and AC hipot are two different tests where the leakage current will be much higher in milliamps in AC.

IR value is not an indication of dielectric strength of the winding insulation.

If your generator differential acted, pretty sure winding has suffered ground fault.

Do a 1.5 times rated voltage ac hipot to check the insulation. Most likely the winding fail this test.

Such small machines would be cheaper to rewind in a shop than at site.

Muthu

http://www.edison.co.in

 

[beyond86]

Can you attach .cfg file from IED protection?

 

[Smasem]

thanks edison for this valuable information,
yes,microamp is the high pot test
happy if you advise me about rewinding precautions and tests after rewinding and its accepted values.

 

[electricpete]

I'm assuming (there's that word) that the tests were dc, test voltage would be way to high for ac... far above even factory hi-pot 2x11+1=23kv ac.

I agree, DC tests do not necessarily create similar voltage distribution on the surface of the endwindings and to a lesser extent in the body of the insulation.

That works both ways... the fact that you are testing with dc instead of ac might make you view the situation more severely or less severely depending on the context. For example, the grading coating is designed to minimize an abrupt change in surface voltage of the coil near the slot exit... it's designed to control the ac stress distribution in that area but it doesn't work well for dc. Your result could potentially reflect a minor deviation in surface conditions that does not dictate rewind. Off line tests/inspections might provide better diagnosis (for example visual inspection and lights out test with ac). It may be that you have something like localized surface contamination on one coil. It disrupts the dc measurement and might explain the intermittent trips. The fact that the machine trips and then apparently operates fine afterward (twice?) may give some reason to suspect the condition as something other than a traditional failure of ground or turn insulation.

By the way I think 29kv is on the high side for dc maintenance test of 11kv machine. There is one standard (I think IEEE95) that suggests ac maintenance test be done at 125-150% of line to line voltage and dc maintenance tests be done at 1.7 of ac maintenance tests. So that would be around 23-28kv.
V_LL 11.00
AC Maint Hi 150% VLL 16.50
AC Main Lo 125% VLL 13.75
DC Maint Hi *1.7 28.05
DC Maint Lo *1.7 23.38

With that said, I would expect the winding to pass at 22kv dc and beyond... the results definitely indicate some problem but whether or not that's an automatic rewind I wouldn't decide yet... it may be that after shop inspection/testing a cleaning is all that is needed (along with more testing after cleaning). Of course that decision also varies with difficulty of removing the machine for maintenance... if that's a real pain and disruption, then you may want to proactively rewind the winding when you've gone to all the trouble to remove it even if there are other options that would resolve the reading.

=====================================
(2B)+(2B)' ?

 

[Smasem]

Sorry beyond,i am not not on site.i have only the attached photos now.
we reviewed all setting and configuration information all was good.

 

[edison123]

Smasm

First do the AC hipot at 1.5 times (16.5 KV AC) for one minute after proper cleaning of the winding and checking the PI value (10 min IR/1 min IR) which should be more than 2. If PI is less than 2, you need to dry out the winding with external heaters, DC current heating etc. If the winding passes this AC hipot, it is good to go.

After a rewind, the new stator winding should pass 2U+1 = 23 KV AC for one minute test and surge comparison test at 23 KV between phases.

Muthu

http://www.edison.co.in