MI 3200 TeraOhm 10 kV
Measurements
16
After applying a high voltage to an insulator the electric dipoles distributed in the
insulator align themselves with the applied electrical field. This phenomenon is called
polarization. As the molecules polarize, a polarization (absorption) current lowers the
overall insulation resistance of the material.
The absorption current (I
PI
) typically collapses after a few minutes. If the overall
resistance of the material does not increase, this means that other currents (e.g. surface
leakages) dominate the overall insulation resistance.
-
PI is defined as the ratio of the measured resistances in two time slots. The most
typical ratio is 10 min value to 1 min value but this is not a rule.
-
The test is typically performed at the same voltage as the insulation resistance test.
-
If the one-minute insulation resistance is greater than 5000 M
, then this
measurement may not be valid (new modern types of insulation).
-
Oiled paper used in transformers or motors is a typical insulation material that
requires this test.
In general, insulators that are in good condition will show a “high” polarization index
while insulators that are damaged will not. Note that this rule is not always valid. Refer to
Metrel’s handbook
Insulation Testing Techniques
for more information.
General applicable values:
PI value
Tested material status
1 to 1.5
Not acceptable (older types)
2 to 4 (typically 3)
Considered as good insulation (older types)
>4(very high insulation resistance)
Modern type of (good) insulation systems
Example for minimum acceptable values for motor insulation (IEEE 43):
Class A =1.5, Class B = 2.0, Class F =2.0, Class H =2.0.
DIELECTRIC DISCHARGE
An additional effect of polarization is the recovered charge (from Cpi) after the regular
discharging of a completed test. This can also be a supplementary measurement for
evaluation of the quality of insulating material. This effect is generally found in insulating
systems with large capacitance Ciso.
The polarisation effect (described in “Polarisation Index”) causes a capacitance to form
(Cpi). Ideally this charge would dissipate immediately a voltage was removed from the
material. In practice, this is not the case.
In conjunction with the polarisation index (PI), Dialectic Discharge (DD) is another way to
check the quality and suitability of a insulation material. A material that discharges
quickly would provide a low value while a material that takes a long time to discharge will
provide a higher value (described in the table below, for more information see section
5.6).
DD value
Tested material status
> 4
bad
2 - 4
critical
< 2
good