Corel oop vs El-cid test 1

[Sargardani]

Is it necessary to carry out cote loop test along with an El-cid test for an old stator core before and after rewind? My question is: is El-cid good enough to give a good idea of the condition of the core?

Sarg

 

[electricpete]

My impression is El Cid is better for localized problems, but core loop test is better for overall condition of the core.

It is very common for the core to be loop-tested before and after stripping in order to identify whether the stripping has significantly degraded the core. 5% increase in core losses is deemed unacceptable by many specificaitons.

I don't know if any comparable quantitative measure can be given by El Cid.

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[kh2]

When rewinding a machine, the core loop test is performed after the removal of the old coils and before the new coils (or bars) being installtion. Any hot spots 5 degrees C or more from the ambient will be idendified. The El-Cid test (Electromagnetic Core Imperfection Detection)is performed after the completion of the installation of the new windings and wedge system to identify any damage to the core which resulted from the installation.

The core loop test excites the core to its full rated flux. The fault currents will be similar to those produced if the machine is in operation and the hot spots and temperature rises aproximate to the normal operation condition but without the cooling system. It is an expensive test that might cost 4 to 5 times the El-Cid test and is very much time consuming depending on the size of the machine to be tested.

The El-Cid test applies a low flux about 4% of rated flux. The fault current is measured by an electromagnet detector and the signal is recorded in milliamperes. The El-Cid test if it detected about 100 milliamperes reading or more, the spot will be invistigated.

Few years ago an article in Hydro Review was written about one of Irrigation Districts in Northern California that performed both tests on the same core and found little differences in the results. But still most utilities prefer to perform the core loop test. One of the reasons I think of is that the Core Loop Test is recommended on the IEEE Stnadard 56-1977 and not the El-Cid test.

 

[QCE]

I have never seen any companies that do both tests. EL-CID test is becoming more popular lately in my experiences. For large units some people do it before stripping, after stripping and after rewind. I think it is only necessary after stripping and before you rewind the unit. Maybe do it after the winding if you think some damage has been done. All rewind projects that I have worked on I have never seen it done after the winding installation. Also if problem areas are found and repairs are done redo the EL-CID test to confirm the repair.

 

[kh2]

I may say that it is a fact, both tests are specified in Utilities specifications (the ones I have). They are specified for the rewind and for a new stator core. When there is a rewind, the core loop test was performed after the old windings were removed and the El-Cid performed afer the rewind is completed. In case of a new stator core installation, the core loop test was performed after the core is torqued to design levels but before the windings are installed and the El-Cid test is performed after the new winding and wedging were completed.

The core loop test is more representative of the machine operation since it requires to apply the full flux (or close to) and it is recommended and described on IEEE Std 56-1977 and Std 432-1992. It is an expensive test and takes days in preparation and performing the test itself. Therefore, some Utilites decided to perform the loop test first and later after finishing the job they follow with the El-Cid test which is simpler and less expensive. Is this the reason?

Appreciate the comments.

 

[QCE]

Sorry, I meant I have never seen a company do both tests at the same stage of the rewind. Both tests give similar results.

Note 56-1977 does not mention the EL-CID test because it was not being used in 1977 and 432-1992 mentions the EL-CID test but only briefly because it was just starting to be used in the early 90's.

kh2 sounds like your guy, it appears that he has more experience with this type of work with a large utility.

kh2 or other members - Do you think it is necessary to EL-CID test a hydro generator core after a rewind if the hydrogenerator core was tested before the rewind started? If yes, what type of damage would you be looking?

 

[kh2]

QCE,

The purpopse of performing the El-Cid after the rewinding is to identify any damage to the core that might be caused as a result of the winding installation, packing, and wedging.

The El-Cid test is one of other tests to be performed in the Field to assure the high quality of work before energizing the machine. Failure of any test could be grounds for the rejection. The other tests are: Winding Hi-pot test per ANSI C50.10, IEEE 95 & 115; Armature (stator) resistance per IEEE Std 115; Insulation Resistance and Polirization Index; and Blackout test after all other field tests have been successfully passed. Also, will add testing the RTDs for continuety and insulation resistance and to insure proper temperature indication. The idea behind all the tests is to cover all the possibilites of failure before the start-up, and loading the machine.

Hope this will help. Regrads.

 

[QCE]

I understand the testing procedures, we do all the tests that you listed except the blackout and final EL-CID during a rewind. I can't think of any reason why major damage would be done to the core during the winding process. By major damage I mean - enough damage to fail an EL-CID test. If major damage was caused during the winding I would hope it would be reported to someone then I could see doing an EL-CID but to just do it as routine seems a bit much.

 

[kh2]

I agree with you.

 

[edison123]

Minus points of Elcid

1. Requires skilled and experienced operator to interpret the results.
2. If the signal processor is not calibrated properly (which itself requires skill and knowledge), you land up with false readings. You have to recheck with full flux test.
3. It does not work properly with multi-section stator assemblies (hydro generators) where the axial core splits will give false readings.

As a rewinder, I always trust & perform full flux test (however difficult it may be) on bare core for it simulates real life scenario. One can always measure the hot spot temps directly instead of depending on some mA vs core health chart given by Elcid kit manufacturer. Btw, did you know that 100 mA thumb rule often quoted users of Elcid test is only for 5 v/m induced voltage along core length? If your v/m is less than 5 v, then the upper limit must be reduced proportionately i.e 80 mA for 4 v/m etc.

 

[QCE]

Thanks agian edison,

You straightened me out in two threads today. This is a very good point that I had forgot about. You get funny readings at the core splits and also I have come across other units where conducting the EL-CID before coil remove gives a lot of funny readings. Seems to have a lot of interference.