Page 85 - Basic PD Theory

References

 IEEE Std 43 -
Recommended Practice for
Testing Insulation
Resistance of Rotating
Machinery

Figure 61. Equivalent circuit of the currents during an insulation resistance test

Leakage current (I L): linear current that varies with the amount of Contamination or moisture on the surface of the insulation.
It is highest on severely contaminated machines.
Capacitance current (I C): non-linear that starts high, and within seconds after application of the voltage, goes to zero. This is
also known as the charging current and is a property of the geometric capacitance of the insulation and the internal resistance of
the test instrument.
Conductance current (I G): linear current that is a property of the insulation material. For epoxy, I G is almost zero, for asphalt
and polyester it is slightly higher.
Absorption current (IA): non-linear current that is due to the energy absorbed as the covalent bonds of the organic
molecules stretch and shift due to the presence of an electric field. As an insulation system ages and its chemical
properties change, its absorption current may also increase. Some insulation systems such as shellac bonded, mica have
been known to have a decrease in absorption current with age, while others, such as polyester resin, have been known to
have a high absorption current when uncured.

Figure 62.
Types of currents for an asphalt-mica
insulation with a moderate surface
leakage current and relatively low
conduction current.

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