The Journal of CPRI,

Vol. 8, No. 1, March 2012 pp. 25–30

Investigation on Polarization – Depolarization Current Measurements in

Insulation Systems for High Voltage Motor

Tapan M Rami* and Prasanta Kundu**

This paper presents the assessment of insulation systems for high-voltage induction motor through
measurement of insulation resistance(IR), polarization index(PI) and polarization–depolarization current
or charging and discharging current. It is considered that the insulation systems are having different
types and conditions of insulation aging. It demonstrates that the IR and PI cannot be used individually
to judge insulation dryness, and the combination of IR and polarization and depolarization current
(PDC) analysis is a better technique of insulation quality assessment than the insulation resistance
alone. The PDC analysis is non-destructive dielectric testing method for determining the conductivity
and moisture content of insulation materials in high-voltage motors. On the basis of this analysis, it
is possible to take further actions like overhauling, drying process and replacement of the winding of
the motor. This paper also presents a description of PDC analysis technique with the practical and
theoretical background and some results of IR, PI and PDC measurements on High-voltage motor.

1.0 INTRODUCTION problems. Stator insulation aging and breakdown

can cause a costly, forced outage and significant
Large numbers of high-voltage motors around the loss of revenue as well as repair/replacement
world are approaching towards the end of their costs. Therefore, prevention of such outage is
design life. They are very expensive to replace; major concern for both the manufacture and
however, some of these motors are still in good the end user. To this end, there has been a lot
condition and could be used for some more of effort towards developing reliable insulation
years. One way to achieve this objective is to quality assessment techniques [1–3].
increase the time interval between maintenance
outages and reduce the time of the outage. While performing insulation resistance (IR)
Towards this end, detecting defects at an early and polarization index (PI) measurements in
stage, being able to model and predict the growth motors with modern day insulation systems, it
of such defects and integrating maintenance to is often noticed that good/acceptable IR and PI
mitigate the consequent reduction in reliability values are obtained in spite of the motor winding
are critical to ensure that the motors survive from being excessively contaminated. This is mainly
one outage to the next with the desired reliability. due to the fact that IR and PI measurements
Various industrial surveys show that problems are largely reflective of charge transport rather
initiated in the stator winding insulation are one than charge storage mechanisms. Charge storage
of the leading root causes of high-voltage motors analysis is useful since it is possible to identify
failure. It is known that up to 70 % failure of whether charge is stored in normal “traps” within
high-voltage motors results from stator insulation insulation or within contaminants that are likely

* Manager, Essar Oil Ltd., Refinery Site, 39 KM, Jamnagar-Okha Highway, Vadinar - 361305, Gujarat, India. E-mail: Tapan.Rami@essar.com
** Department of Electrical Engineering, S V National Institute of Technology, Surat - 395 007. E-mail: pk@eed.svnit.ac.in
26 The Journal of CPRI, Vol. 8, No. 1, March 2012

to be present in the insulation. The polarization a total time period that will not be less than
and depolarization current (PDC) analysis is the charging time period. The charging and
used to quantify and characterize charge storage discharging currents are plotted on a log–log
mechanisms, and therefore a reliable indicator of scale and analyzed in the times domains [3,4].
presence of contamination [4–7].
The schematic diagram of the PDC measuring
The paper has been investigating the PDC set-up is shown in Figure 1.
measurement for separation of moisture and
aging impact on motor insulation degradation.
One case study is presented, where the testing
of high-voltage motor is carried out at healthy
situation, moisture and conductive contamination
situation of insulation system. The test results are
compared to determine the condition of insulation
system of motor and to assess the healthiness of
the motor.

2.0 THEORETICAL AND

EXPERIEMNTAL DETAILS

2.1 Polarization Index (PI)

It is a ratio of insulation resistance at two different

times, typically 10 minutes and 1 minute. The one
minute reading of insulation resistance (IR) has
been successfully used by maintenance engineers
since long times. To detect the conduction FIG. 1 PDC MEASURING SETUP

problems and to judge the insulation dryness, PI

can be used individually. This is presented later The procedure for taking readings of charging
though a case study. It is well-accepted that a and discharging current of insulation is as
very bad PI values need not at all mean a high follows.
probability of insulation failure, and a very good
PI may be obtained when failure is imminent (i) The setup shown in circuit diagram (Figure 1)
[1,2,4]. in charging mode is made.
(ii) The capacitance between winding and
2.2 Polarization and Depolarization Current earth (i.e. between each phase and other
(PDC) two grounded phases) is measured in both
polarities.
The principle of PDC measurement is as
(iii) The initial leakage current in nA is measured
follows. A DC voltage of 2500 V is applied to the
with multimeter connected across 1 M Ohm
winding/windings using a highly regulated
resistor in both polarities.
electronic power supply with a stability of
<1 v/sec. The voltage is maintained for a time (iv) Now the winding is energized with IR Tester
period of not less than 1000 seconds. The current at 2.5 kV for 17 minutes and the IR values
flowing through the insulation is monitored are noted down at regular time intervals.
during this charging period. Then, the windings IR values are recorded with intervals of
are discharged through a micro ammeter and 5 secs for initial 30 sec, then with intervals
discharge currents are monitored after the initial of 10 secs for next up to 2 mins and later
winding capacitance discharge (<5 secs), over with intervals of 30 secs. Thus, immediately
The Journal of CPRI, Vol. 8, No. 1, March 2012 27

after applying the test voltage, the readings

will have to be taken fast.
(v) After charging for 17 minutes, the circuit is
connected in discharging mode so that the IR
tester is not in circuit and motor terminals are
connected to discharging circuit. However,
during discharging, the 1 M ohm resistor is
shorted for initial 5 sec to discharge high
initial current. After 5 sec, the shorting link
of 1 M ohm resistor is opened and the mV
readings on multimeter connected across 1
M ohm resistor are noted.
(vi) The above procedures are repeated for FIG. 2(A) INSULATION RESISTANCE VS TIME
each individual cases in new and healthy, CHARACTERISTICS FOR CASE 1
moisturize and conduction condition.
For each case study in this contribution,
plots of insulation resistance versus time
are accompanied by the PDC analysis with
the charging current and discharging current
versus time are presented. These are for
better understanding of the characteristics
of each aging type [6–7].

3.0 RESULTS AND DISCUSSIONS

3.1 Case 1 New and Good Insulation

Condition
Figures 2(a) and (b) present dielectric response
results for three diagnostic parameters: IR, PI
and charging and discharging currents of the FIG. 2(B) CHARGING AND DISCHARGING CURRENT
VS TIME CHARACTERISTICS FOR CASE 1
motor. It is considered as good insulation because
following characteristics.
(a) The shape of insulation resistance for a good 3.2 Case 2 Moisturize Insulation Condition
and dry motor is straight and increases with Moisturizing of insulation is done by steam flow
time. The value of insulation resistance inside the insulation winding. Here, moisture
should be high and PI should be more than is mainly adsorbed in the insulation, without
one for good and dry machine. Here, IR surface humidity, free water or other conductive
value is more than 10 M ohm and PI is more contaminants (conduction).
than 4, which shows a good result.
(b) When the insulation system is good and dry, The dielectric response results, in this case, are
the polarization current and depolarization shown in Figures 3(a) and (b).
current are nearly equal for about one-tenth
of the charging time as shown in Figure 2. The value of insulation resistance is in Mega ohm
Both currents are straight in log-log scale. range and comparable with the good insulation
The magnitude of PDCA depends mostly on condition. Hence, PI and IR are not sensitive
the capacitance and insulation condition. to moisture in adsorbed state. In this situation,
28 The Journal of CPRI, Vol. 8, No. 1, March 2012

PI cannot be used alone to judge insulation

dryness.

The shape of PDC characteristics (as shown in

Figure 3), is very close and similar to case 1,
but it is found that the current amplitude of this
stator is substantially higher, i.e. 0.0015 A in
comparison to 0.0012 A.

FIG. 4(A) INSULATION RESISTANCE VS TIME

CHARACTERISTICS FOR CASE 3

FIG. 3(A) INSULATION RESISTANCE VS TIME CHAR-

ACTERISTICS FOR CASE 2

FIG. 4(B) CHARGING AND DISCHARGING CURRENT

VS TIME CHARACTERISTICS FOR CASE 3

Figure 4(a) shows that there are no increase of

IR values between 15 and 60s and a significant
fluctuation is observed in IR values. The values
of insulation resistance are low compared with
the moisturize condition. This result shows that
the higher value of 1 min. IR does not always
mean safe for operation.
FIG. 3(B) CHARGING AND DISCHARGING CURRENT
VS TIME CHARACTERISTICS FOR CASE 2
Figure 4(b) shows that the deviation of charging
and discharging current. This is a very highly
distorted and unusual shape of charging and
3.3 Case 3 Moisturize with Conduction of
discharging current, may be because of the much
Insulation
deteriorated insulation condition of motor. Unlike
Figures 4(a) and (b) show the dielectric response moisture in adsorbed state, surface humidity or
results of moisture and conductive contaminants free water or conduction problems influence the
especially carbon dust. PDCA shape.
The Journal of CPRI, Vol. 8, No. 1, March 2012 29

4.0 CONCLUSIONS REFERENCES

[1] IEEE Std.43-2000. IEEE Recommended

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Many thanks to Essar Oil Ltd. for sparing the depolarization current (PDC) technique on
high-voltage motor for investigation of insulation. fault and trouble analysis of stator insulation”,
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SVNIT, Surat, for providing support for this Gyeongju, Korea, pp. 79–87, October 24,
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