HT PVCHECKs-PRO - Appendix - Theoretical Outline; Measurement of Polarization Index (PI); Dielectric Absorption Ratio (DAR)

PVCHECKs-PRO
EN - 60
11. APPENDIX – THEORETICAL OUTLINE
11.1. MEASUREMENT OF POLARIZATION INDEX (PI)
The purpose of this diagnostic test is to evaluate the influence of the polarization effects.
Upon the application of a high voltage to insulation, the electric dipoles distributed in the
insulation align in the direction of the applied electric field. This phenomenon is called
polarization. Because of the polarized molecules, a polarization (absorption) current
generates, which lowers the total value of insulation resistance.
Parameter PI consists in the ratio between the value of insulation resistance measured
after 1 minute and after 10 minutes. The test voltage is maintained throughout the whole
duration of the test and, at the end, the instrument provides the value of ratio:
Some reference values:
PI Value Insulation condition
<1.0 Not acceptable
from 1.0 to 2.0 Dan
g
e
r
ous
from 2.0 to 4.0 Good
> 4.0 Excellent
11.2. DIELECTRIC ABSORPTION RATIO (DAR)
Parameter DAR consists in the ratio between the value of insulation resistance measured
after 30s and after 1 minute. The test voltage is maintained throughout the whole duration
of the test and, at the end, the instrument provides the value of ratio:
Some reference values:
DAR Value Insulation condition
< 1.0 Not acceptable
from 1.0 to 1.25 Dan
g
e
r
ous
from 1.25 to 1.6 Good
> 1.6 Excellent