Megger Value of 38MVA Alternator

[gsjhand]

My question is regarding IR value of an alternator which is at it’s commissioning stage. It’s a 38 MVA, 11KV alternator and was stand still for many years at some other site. The megger values which I am getting is 1.3 Mega Ohm Phase to ground for all three phases and 2.7 Mega ohm for Phase to Phase. We tried meggers with different voltages 1KV, 2KV, 5KV but results were always same. Before taking any megger value we passed around 1200 amps current and kept it at 70-80 degrees C temperature for about 24hours. Any suggestion what could be causing such a low megger value and what can I do to raise it?

 

[gsjhand]

Just to help one more thing in case it helps.
Year of manufacture of alternator is 1966.

 

[Zogzog]

The NETA/ANSI spec for windings made before 1970 is kV+1, so you should have 12 Megohms. That value is based on 40 degrees C so if your winding was 80C when you tested that would be 8.2M phase to ground and 17.1M phase to phase after temperature correction. That makes your readings borderline acceptable, about what I would expect from a motor sitting idle for many years.

Your values may rise after it runs for awhile, but that is a little risky (Maybe someone with more experience than me on this can provide more insight on that). You can bake the motor, any motor shop should be able to help with this, if that dosent work you will need to rewind it.

 

[rbulsara]

1966? Have a good motor winding shop check it out. The insulation may have deteriorated beyond use even it had remained in use for such vintage machine, let alone staying in disuse for many years.

Perhaps 24 hr drying was not enough, try a little longer and let it cool down before meggaring.

Rafiq Bulsara

http://www.srengineersct.com

 

[edison123]

With 1966 vintage, it is a probably class B, bituminous insulation, which is hygroscopic. You may need many hours of drying out at about 90 deg C to achieve a good megger value and polarization index.

Of course, a good megger value is not an indication of winding dielectric strength, which was what matters most. This 43-year old generator winding is at the end of its life. If you want a reliable performance and a long life, you need to rewind it with new class F insulation.

Muthu

http://www.edison.co.in

 

[rasevskii]

Certainly 1.3 Mohm phase to ground is not acceptable. Did you disconnect everything else and are only megging the stator itself? Do not have any external cables, switchgear, PTs etc. connected. Make sure everthing is clean and condensation free.

If there are surge suppression capacitors connected at the stator terminals, these have to be disconnected as they have an internal discharge resistor inside. Had a lot of trouble with that once...

Each phase should be megged separately with the other two grounded. That means the neutral point has to be opened.

I assume this is a GT which was moved from elsewhere. Did the last owner/operator provide any maintainance records..?

On a 10 minute test /you need an electronic or motor driven megger for this/there should be a PI value of around 2 to 1 at the end of the test.If not you are megging dirt or moisture. Or a bad winding.

Did you really mean 1.3 Mohm or 13...?

regards, rasevskii

 

[waross]

I had a small power plant with several small generators (600 kW and 350 kW) contaminated with salt water after hurricane Mitch.
The plan was to see what could be done with one generator and then decide what to do with the others.
The first machine meggered zero. It read low ohms with a multi meter and the reading changed when the meter leads were reversed. Then I tried the Volts scale and read a small voltage coming off the windings. The windings were in such bad shape that in the presences of salt water the windings and the stator were acting as a battery.
We spent days washing and drying and eventually got the megger reading up into the megohm range. We actualy got the reading about twice as high as the normal resistance reading. (We had a bench mark megger reading recorded from a better time.)
We were desperate for power and put the machine into service.
I recommended sending the other generators up to the US to be cleaned, dipped and baked in a well equipped shop.
The first set did service until it could be replaced with the first returned rebuilt generator.
I left warnings that once the first set was taken offline and allowed to cool down, it was not to be reused before it was sent out for repairs.
But, hey, what's a gringo know? This set was working fine and it would be convenient to use it today. They started it up from cold. That was the last day in the life of that generator.
The point is:
I wish you had taken a bench mark reading before you heated the machine up. I have no comment on the condition of a machine that was heated before any readings were taken.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wolf39]

gsjhand:

Your generator is at it's commissioning stage. That means that the complete stator and rotor was properly inspected and cleaned (especially the end winding portion of the stator). The unit is ready for a balancing and bearing run. You could therefore run the unit in an excited mode with the terminals solidly short-circuited for rated current in order to dry out the stator winding internally and externally. I assume at this point that an external excitation system is available. The air to water heat exchangers are on (propably with reduced cooling water flow) in order to get 90 to 100 degrees centigrade at the resistance slot thermometers. Any moisture escaped from inside the generator is condensed at the heat exchangers, which are the coldest parts in the cooling air circuit. Then measure the megger values over a 10 minute period and calculate the polarization index (divide the 10 minute figure by the 1 minute figure). A figure of 2 should be obtained as a very minimum.

Traditionally the megger values double with every 10 degrees centigrade reduction. A figure of 12 megohms at 40 degrees centigrade sounds reasonable.

Whatever figures and values you get: Your problems start when you have all data on hand. Do you risk to apply an a.c. test voltage of 1.5 times rated voltage? This should be the minimum figure, applied to every phase separately, with the other two phases grounded. Having gone that far you propably have no choice. Otherwise you have to recommend to your boss a complete rewind at this point.

I agree with the others that a rewind is to be recommended for a reliable long-term service and higher generator efficiency (with a new winding the slot space factor goes up because of less groundwall insulation). But until the new winding arrives at site a temporary service may be possible.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM