Page 246 - Industrial Power Engineering and Applications Handbook

Winding insulation and its maintenance 9/225
9.5.3 Making-up of the insulation level in large resistance readings. It will not be less than 1.5 for class
machines (1000 kW and above) A, 1.75 for class E and 2 for class B insulations for the
windings to be regarded as suitable for a high-voltage
IEEE 43 places special emphasis on determining the test or actual operation. This index is based on the
insulation condition of such machines before energizing phenomenon that the insulation resistance of a winding
and even before conducting a high-voltage test. This can rises with the duration of application of the test voltage.
be determined by the insulation test as noted below. The rise is rapid when the voltage is first applied and
then it almost stabilizes with time, as noted in Figure
Insulation resistance test 9.4. For instance, after 30 seconds of the applied voltage
this resistance may be too small, of the order of only a
Insulation resistance of the windings is a measure to fraction of the final value. As a rough guide, the insulation
assess the condition of insulation and its suitability for resistance of a dry and normal winding may also continue
conducting a high-voltage test or for energizing the to rise in the first few minutes after application of the
machine. A low reading may suggest damage to the test voltage before it stabilizes. Normally it may take
insulation, faulty drying or impregnation or absorption 10-15 minutes to reach a steady state. If the windings
of moisture. The insulation resistance may be measured are wet or dirty the steady condition may be achieved
according to the procedure laid down in IEEE 43 between within one or two minutes of application of test voltage.
the open windings and between windings and the frame These test results are then compared with similar test
by employing a direct-reading ohm meter (meggar). The data obtained from the manufacturer on similar windings
ohm meter may be hand-operated or power-driven or carried out during manufacture. If the manufacturer’s
equipped with its own d.c. source. 500 V d.c. is the original test results are available, the results obtained at
recommended test voltage for an insulation resistance site can be quickly compared and the condition of the
test for all voltage ratings, as noted later. The recommended insulation assessed easily and accurately. If the test facility
minimum insulation resistance of the machine is obtained
by the following empirical formula: to obtain test results at 1 minute and 10 minutes is not
available, the results may also be obtained for 15 seconds
R, = kV+ 1 (9.1) and 60 seconds and a graph plotted as shown in Figure
where 9.5(a) to determine the polarization index.
R, = recommended minimum insulation resistance in From these insulation tests several test data for 1 minute
MR (meg ohms) of the entire machine windings, and 10 minutes can be obtained during the process of
at 40°C or 1 MR per 1000 V plus 1 MQ, and heating over a few hours and a graph plotted for time
kV = rated machine voltage in kV versus insulation resistance. This graph is of great assistance
in determining the state of the windings before energizing.
The winding insulation resistance to be used for com- Heating or cleaning-up of the windings should continue
parison with the recommended minimum value (R,) is until the polarization index matches the reference value
the observed insulation resistance obtained by applying
a d.c. voltage to the entire windings for one minute,
corrected to 40°C. In practice, motors having insulation
resistance readings as high as ten to a hundred times the
minimum recommended value R, are not uncommon.
At site, when commissioning a new or an existing
motor after a long shutdown, it must have a minimum 1000
800
insulation level according to equation (9.1). An 11 kV 600
motor, for instance, must have a minimum insulation of
12 MQ. In normal practice, it is observed that when first 1 400
measured the resistance reading may show more than 3
the minimum value and may mislead the operator, while $200
the winding condition may not be adequate for a high- 0
voltage test or an actual operation. One must therefore 24
ensure that the winding condition is suitable before the .$ 100
machine is put into operation. For this purpose, the F! 80
C
polarization index (PI), which is determined from the .s 60
insulation test data only as noted below, is a useful pointer. 9
? 40
It must be evaluated at site while conducting the insulation I
test then compared with the manufacturer’s reference
data for the machine to assess the condition of insulation 20
at site and its suitability for operation. This is usually a
site test, but to establish a reference record of the machine, 10 Time (minutes) -
it is also carried out at the works on the completed machine 0.1 0.2 0.4 0.7 1.0 2.0 4.0 7.0 10.0
and test records furnished to the user.
Polarization index Figure 9.4 Variation of insulation resistance with time for
This is the ratio of 10 minute to 1 minute insulation class B insulation