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.

Insulation Resistance to Earth (Low Voltage Generators 100 – 690VAC)

Insulation Resistance to Earth

Low Voltage Generators 100 – 690VAC.

Note
When conducting high voltage test to earth, it is advisable to
either disconnect or short out any electronic devices, such as the
Automatic Voltage Regulator, (AVR), and Main rotor diodes.
Short circuiting the terminals can be achieved with a piece of fuse
wire, which must be removed immediately after the tests are
completed.
Caution: Running the Generator before removing the short circuit
connection could seriously damage the Generator.
When Megger testing a machine, failure to protect the voltage
control unit and diodes could result in permanent damage to one
or more of the electronic components.
The resistance of the insulation between the copper conductors
and the frame of the machine, (earth or ground), is measured by
means of a high voltage tester, or "Megger", which applies a D.C.
potential of 500 or 1000 volts across the winding insulation.

The high voltage causes a current to ‘leak’ through the insulation
system. This current produces an output reading on the Insulation
tester (‘Megger’), which is measured in Megohms (resistance to
earth or ground). A normal value for a low voltage Generator
winding should be higher than 1 Megohm to earth.
Generators with an output voltage of between 100V to 600V
should be tested as above. If the output winding (stator) is lower
than 1 Megohm to earth, the windings should be cleaned, dried,
or removed to a workshop for complete refurbish.