Doble Tests on Generators

Kelly McGrath

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May 27, 1996, 3:00:00 AM5/27/96

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Can someone tell me the purpose of a doble test, as in comparison with
either meggering a generator or taking a polarization index?

Thanks,
Kelly

MBurton130

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May 29, 1996, 3:00:00 AM5/29/96

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In article <4od5i2$g...@dub-news-svc-1.compuserve.com>,
76735...@compuserve.com (Kelly McGrath) writes:

>Can someone tell me the purpose of a doble test, as in comparison with
>either meggering a generator or taking a polarization index?

A Doble test is a Power Factor tip up test as described in IEEE Std 286.
The applied test voltage is at ac line frequency and is typically applied
in steps up to 10KV. The power factor is measured at the various voltage
steps. An increase in insulation power factor for a given voltage as
compared to historic data for a given machine or type of machine indicates
voids or pockets in the insulation. Doble Engineering Co. leases the
equipment to perform the tests and offers training, client
conferences,etc. They can also provide field personnel with the equipment
and training to perform the basic tip-up test as well as a series of other
generator tests.
By contrast,a megger test is typically a low voltage
(500,2500,5000VDC) test of winding insulation resistance. The minimum
insulation resistance for continued service is typically 1Megohm per
thousand volts of machine voltage rating plus one megohm.The polarization
index is the ratio of the 1 minute to the 10 minute insulation resistance.
A polarization index of greater than 2 indicates a clean dry winding.
Though a reliable basic test capable of detecting a serious problem, the
low DC voltage delivered by the megger test instrument does not search
out insulation degradation as reliably as AC tests or high voltage DC
tests.
There has been much debate over the most useful tests for evaluation
of generator insulation. IMHO, the most economical yet widely accepted
test for field evaluation of generator insulation resistance is the
polarization index applied via 2500v or 5000v megger, followed by a DC
overpotential (hipot) test applied in steps up to the maintenance voltage
recommended by IEEE Std or the equipment manufacturer. Competent plant
electricians are easily trained in these methods while lease expenses,
field service personnel charges and capital investment in equipment is
minimized.

mburt...@aol.com Mike Burton, P.E.
"You can observe a lot just by watching"-Yogi Berra

anthony r zamberlan

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May 29, 1996, 3:00:00 AM5/29/96

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How about really opening the can of worms and ask everyone what hipot
values they use on their generators? Strictly what is recommended by
IEEE or is that value fudged a little based on age of the winding or
whether it is a water-cooled stator or what?
My feelings on this subject are that it really depends on the machine and
the equipment you have available as to whether you do a DC hipot, a 60 Hz
hipot, or a 0.1 Hz hipot. Of course, it you perform the DC hipot as a
current leakage test and controlled overvoltage test, it provides the
most information of them all.

I hope more people respond to this one.

THANKS.
Tony Zamberlan
azamb...@ue.com

anthony r zamberlan

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May 29, 1996, 3:00:00 AM5/29/96

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James D. Hake

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May 30, 1996, 3:00:00 AM5/30/96

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Mike Burton wrote

>>>
An increase in insulation power factor for a given voltage as
compared to historic data for a given machine or type of machine indicates
voids or pockets in the insulation.
<<<

Mike - can you comment on what might happen if Doble testing
is performed on a vacuum contact which is sealed in a housing
(epoxy, porcelain or fiberglass) where most of the 'void'
area between the housing is filled with a solid dielectric
foam

Given that vacuum contacts may have varying capacitances
as a result of the most recent interruption, do you think
Doble can have any validity for vacuum contact testing.

We normally recommend only a high potential test and resistance
check. We find in our experience Doble gives unreliable and/or
unrepeatable results.