Partial discharge generator testing

[TurbineGen]

Can anyone comment on partial discharge testing for mica insulated rotating machines? We have a 13.5kV generator which is tested for PD each year.

1. Should once a year be sufficient to determine when the machine is going to fail?

2. Is there and difference between online and offline test data in these machines?

3. Is a DC hipot test still neccessary, or does the PD test eliminate the need for it?

Thanks in advance.

 

[electricpete]

1. Should once a year be sufficient to determine when the machine is going to fail?
Iris recommends 6 month interval, and a lot of people check at 3 month interval.

2. Is there and difference between online and offline test data in these machines?
Yes. For one thing, the neutral portion of the winding is at low voltage during the on-line test.

3. Is a DC hipot test still neccessary, or does the PD test eliminate the need for it?
That's a judgement call. Many users do not do routine dc hi-pot tests, or only very infrequent tests.

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[olicg]

1 & 2. The frequency depends on current state of the winding, machine age, PD activity levels, etc. If you have carried out tests recently, you can decide based on the results. For newer machines upto 5-8 years in service life without severe signs of PD (during your initial testing and physical inspection), initially one year interval for testing is fine. However, online monitoring with capacitive couplers installed is a much better option to me. Any increasing trend in PD levels would require investigation with inspections and offline test.

If you have budget issues, you can get couplers installed on machine (and a transimitting device with alarms only). For detailed analysis, you can get the services anytime for online testing.

As far as difference in data is concerned, i think it is important to see the PD levels with change in load on generator as it changes thermal stressess on windings. It is only possible with online testing.

3. Hipot should be avoided unless you have carried some sort of repairs and want to make it as an acceptance test. After all generator is an expensive and long lead time item. Even premature failure during hipotting would cause lots of revenue to your company. PD is definately a better test for keeping track of generator health.

 

[slavag]

Hi.
Are PD online test have some influence on protection function of generator?
Thanks.
Slava

 

[electricpete]

no - pd is non-intrusive condition monitoring. It has nothing to do machine protective relaying and does not interfere with protective relaying

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[slavag]

Dear Electricpete.
Thank for your reply.
Maybe I ask other.
Are PD on-line test influence on stator/rotor 100% with injection principle?
Thanks
Slava

 

[azril83]

dear slavag,
PD measurement does not injecting any current or voltage to the stator winding. it was just a measurement to check the existing of corona sign at the stator end winding(usually). so, with that we can check the insulation strength at the stator winding. Hope this will clarify everything, if anybody have any idea, can give a feedback

 

[slavag]

Hi Azril83.
On this year I finished commissioning of two generators
with REG protection and with REX 100% stator E/F injection principle protection.
One generator have secondary principle injecion and next one primary principle.
In both of generators customer add on-line PD sytem ( Iris).
In generator with primary injection I had big problems with calibration of REX, after PD capacitor disconnection
system started work propertly.
Up today I don't have any excplanation from ABB and Iris about this issue.
Regards.
Slava

 

[azril83]

Dear slava,
what is your generator design (spec)? so far as i know, i already done for 4 units of PD online measurement, i did online measurement at pf=0.85 and pf=0.98. the measurement never influence stator/rotor 100% E/F. unless there is bad corona sign at your stator winding. maybe you have to check with ABB or IRIS about your stator winding insulation condition. i just afraid if your stator winding have a bad corona sign. look forward for your reply or opinion.