Generator rotor and stator tests

[Steve200]

Is there information that I can refer to regarding tests on generator rotors. I am specifically looking for lamination tests.

Also, I am wondering what is an acceptable Partial Discharge level to warrant stator and rotor rewinds.

 

[DougMSOE]

There is a pile of information on lamination testing and the coatings that are applied to magnetically insulate the laminations. Strongly suggest that you contact the OEMs for the various types of steel that they carry and the types of coatings that are available.
About the partial discharge levels, You will find 2 schools of thought on the 2 common units that are used
1. pico-coulombs is used by some
2. milli-volts is used by most other mfgs
I have used both and have found the mV to be far superior.
The pC threshold value tends to be 10 to 100 for alarm. 1k to 10k to remove the machine from service and repair.
The mV is more sensitive than the pC. Typically 10mV is aprox = to 0.1pC
The best indication as to when to rewind a stator is to track the power factor of the insulation. This is done by a standard test.

If you have further questions feel free.

 

[neps3]

Steve 200

Making a decision for rewind of generators is not by one test. It is a composition of multiple tests and moreover a visual inspection is quite necessary.

1. Power Factor & tip-up test
2. Offline pd test
3. Online pd monitoring
4. DCCO test
5. Visual
6. Historical record of operating pattern
7. Operating environment
8. Base load or two shifting because the number of starts derates the remaning life
9. ELCID test on the core
10.If necessary a core flux test.
11.IR/PI the standard test.
12.Slot wedges test because the wedges influence on pds.
13.RSO test on the rotor
14.Voltage drop test
15.Heat rejection test.

Merit rate the factors and conclude on the assessment.

kantor

 

[neps3]

Moreover a steam generator rotor forging is tested by NDT and is called TOFD method. This is an ultrasonic testing which detects any inclusions or cracks inside rotor forging. This test is done once atleast in 20 years or for growth detection depending on the crack size.

Talk to a metallurgist about the NDT and a generator engineer about the life assessment.