Insulation Resistance to Earth (Medium to High voltage Generators)

Insulation Resistance to Earth
Medium to High voltage Generators, 1k Volt or higher
 
Insulation testing of Medium and High voltage
generators .
The effectiveness of a particular on-site test will depend to a large
extent on the machine application. In many situations,
measurements of insulation resistance and polarisation index
only will be appropriate. More detailed testing involving loss
tangent, dielectric loss analysis, partial discharge measurement,
is undertaken at intervals in order to establish the extent of
deterioration of insulation condition. Other tests such as high
voltage withstand tests are particularly effective for investigative
work in order to identify the onset of fault conditions.

Polarisation Index Test (P.I.)

The P.I. test is used as a guide to the dryness, cleanliness and
safety of the winding insulation system.
A special motorised insulation tester is required, which can
maintain a test voltage of 1 - 2.5kV, (medium voltages), or 5kV,
(high voltage), for a period of 10 minutes.
Readings are taken (in Megohms) following a 1 minute and 10
minute time interval: -
 
The resultant ratio is called the P.I. index, and should be a
minimum of 2 at 20°C.

A P.I. index below 1.5 suggests the windings are wet, dirty or
faulty, and should be cleaned, dried, and refurbished as
necessary.


Caution! Do not test any winding other than the main
stator with this method. 
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