Special Issue

Polymer Materials Technology

A Study on Partial Discharge Phenomena of Winding Wires
Daisuke Muto*, Makoto Oya*, Tsuneo Aoi*, Takahiro Ueno*

Partial discharge inception voltage (PDIV) in the 50 Hz AC sine wave voltage has
been evaluated and the influence of the environmental condition and the shape of
the winding wire give on its mechanism have been considered. Also showing the effect of the
thicker insulation and the lower permittivity by using the twisted pair insulated with various materials, the validity of the PDIV estimated by the model computation has been explained. Also, it was
confirmed that the PDIV of the twisted pair at the temperature of 25-230℃ reduces as the temperature increases and it has been shown that the dominant factor is the spark voltage of the air andthe temperature dependence of the permittivity of the insulating material. Further, it has been verified that the PDIV of specimen combined with rectangular wires reduces significantly at a 95% relative humidity and that the ionic contaminant on the wire surface is one of the main factors.

ABSTRACTINTRODUCTION
1.1. INTRODUCTION
In recent years, vehicles such as hybrid vehicles (HV) andelectric vehicles (EV) powered by an electric motor havebeen rapidly growing within the framework of the environmental issues and the depletion of the energy sources.
From the viewpoint of the power performance of the vehicles and the improvement of their fuel mileage, the driving motor mounted in the vehicles requires the improvement in their power density, i.e. high output and small
size. Since the increase in voltage is an effective measureagainst these challenges, there are also vehicles that areoperated at a high voltage exceeding 500 V. In general,these motors are driven by the inverter, thereby achieving
a satisfactory control and power. However, the effect of
the surge generated due to the fast switching of theinverter on the coil insulation of the motor is a concern
and many studies on the partial discharge phenomenon
and the insulation deterioration caused by the inverter
surge, its evaluation technology and the insulating material having a partial discharge resistant have been carriedout.
Furukawa Electric and Furukawa Magnet Wire worked
quickly to strengthen the coil insulation for the high voltage of motor and the partial discharge and have developed the rectangular wires having an unprecedented
insulator structure. The developed product of our company has been adopted as a motor stator coil of compact
HV, thereby contributing to the downsizing/weight reduction of the motor and the improvement of the fuel mileageof the vehicle1).
* Research & Development Div.
Polymer Materials R&D Laboratories

In this paper, the research results of the authors about
the partial discharge phenomenon of the winding wires
and its mechanism are reported and further, ourapproach for the improvement of the partial discharge
characteristics of the winding wires is introduced.

PARTIALDISCHARGEDISCHARGE PHENOMENON
PHENOMENON
2.2. PARTIAL
OFTHE
THEWINDING
WINDING WIRES
WIRES
OF
2.1

Estimation of The Partial Discharge Inception
Voltage (PDIV)
Because the partial discharge has a strong effect to theinsulation deterioration of the coil, it is important to clarify
the mechanism. It is extremely important to capture,especially the mechanism for the partial discharge inception voltage (PDIV). In general, the PDIV of the winding
wires is estimated by the electric field strength and the
Paschen curve 2). For example, the relationship between
the electric field strength for the air gap of the twisted pairand the gap distance was evaluated by using the electricfield analysis such as the finite element method and theestimated PDIV is defined as the applied voltage when
the Paschen curve and the electric field strength coincide.
Further, Dakin et al have shown that the PDIV of the insulator which is sandwiched by the cylindrical electrodes
can be approximated by a function of the ratio of the insulation thickness and the relative permittivity 3). Referencing
these techniques, the authors have estimated the PDIV of
the conventional round wires and rectangular wires which
are applied to HV motor in recent years.
Two-dimensional computation models of the electricfield strength are shown in Figure 1. The twisted pairmodel of 1 mm conductor diameter is used for the calculation of the electric field for the round wires. The shortest
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A Study on Partial Discharge Phenomena of Winding Wires

HV(V =Va )Insulation
Conductor

GND(V =0)

Gap distance (d )HV
(V = Va )1 μm 1.0 mm

GND(V =0)

10～250 [μm]（a）Twisted pair model(Round wire)（b）Parallel plate model(Rectangular wire)

Figure 1 Computation model for the electric field.

Electric field strength[kV/mm]100

Paschen curve
V3
V2
V1

10

(Applied voltage V2 means estimated PDIV.)
1

1

10

100

1000

Gap distance (d ) [μm]Figure 2 Electric field strength vs. gap distance.

In Figure 3, the relationship between the estimated
PDIV and the insulation thickness is shown. The relative
permittivity of the insulation layer was set to 4. In the figure, the calculated value according to the formula (1)which Dakin at al proposed was also shown.
V=163 ( t /εr ) ………(1)V : Partial discharge inception voltage[Vrms]
εr : Relative permittivity of Insulatort : Insulation thickness[μm]0.46

Estimated PDIV [Vpeak]

1500
1200
----- Equation (1)900○ Twisted pair model

600
300

□ Parallel plate model
0

50

100

150

200

Insulation thickness [μm]Figure 3 Estimated PDIV vs. insulation thickness.

In any estimation, the results that the PDIV increases
with the thicker insulation layer were obtained. This isqualitatively consistent with the empirical partial discharge phenomenon. The calculated value of formula (1)was in good agreement with the estimated value of thetwisted pair model in the region of the insulation thickness of less than 30 μm. This is consistent with the experimental result 4) that the PDIV observed in the twisted
enamel wire approximately agrees with the calculated
value using formula (1). On the other hand, when thetwisted pair model is compared with the parallel platemodel, the difference in both estimated values appeared
when the insulation thickness exceeds 40 μm. The difference became larger as the insulation thickness increases
and the result when the estimated value from the parallel
plate model is lower, was obtained. This is because thedifference in the distribution of the electric field strengthfor both models occurs if the insulation layer is thick. In
Figure 4, the difference in the distribution of the electricfield of the air gap due to the insulation thickness isshown. In the case of an insulation thickness of 30 μm,
the relationship between the gap distance and the electricfield strength for both models reconciles well, but isslightly different in the case of 100 μm and the electricfield strength values which are in contact with the
Paschen curve values for a parallel plate model are larger.
This is the reason why the PDIV for the parallel platemodel is estimated smaller than the twisted pair model.
Regarding this result, the shortest gap distance between
wires is defined as d’ and the relationship between d’ andthe PDIV is explained in addition.

Paschen curve

Va =700 [Vp]30 μm

Twisted pair model
Parallel plate model

Paschen curveElectric field strength

Gap distance ( d )

εr：4

1800

Electric field strength

distance of the air gap is set to 1 μm and the relationshipbetween the wedge-shape gap distance which is formed
between the wires and the electric field strength is evaluated. A parallel plate model consisting of combined planar section is used for the calculation of the electric fieldof the rectangular wires. A parallel plate gap of 10-250 μm
is given and the relationship between the gap distanceand the electric field strength is evaluated. In the case of
the rectangular wires, the sufficient length of the flat portion was secured in order to avoid the effect of the distortion of the electric field at the corner and the electric fieldstrength at its near center was calculated. A high speed
surface charge method is used for the calculation of theelectric field. As shown in Figure 2, the voltage which theelectric field strength of the air gap coincides with the
Paschen curve was defined as the estimated PDIV.

Va =1200 [Vp]100 μmTwisted pair model
Parallel plate model

Gap distance ( d )Gap distance ( d )（a）Thickness 30 μm（b）Thickness 100 μm

Figure 4 Difference of electric field distribution due to insulation
thickness.

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A Study on Partial Discharge Phenomena of Winding WiresIn Figure 5, the relationship between the estimated
PDIV of the twisted pair model and the parallel platemodel and d’ is shown. d’ is set in the range from 1 to120 μm, the insulation thickness is set to 30 μm and 100
μm and the relative permittivity is set to 4. From this figure, it is found that the PDIV changes depending on thed’ and becomes the smallest value (Vmin) at a certain gap
distance. Also, as the insulation thickness increases, thegap distance ( d’min) from which Vmin can be obtained,
increases. In reality, in the case of the insulation thicknessof 30 μm, Vmin= 720 V is at d’min=25 μm and in the case of
the insulation thickness of 100 μm, Vmin=1210 V is atd’min=50 μm. These numbers are same for both the twisted pair model and the parallel plate model and are notdependent on the shape of the conductor within thescope of this study. In other words, if the insulation thickness and the permittivity are same, the estimated minimum PDIV will match. This is the reason why the electricfield of the gap at d’min in the twisted pair model is nearlya uniform electric field.

Estimated PDIV [Vpeak]870

30 μm, εr：4
○ Twisted pair model□ Parallel plate model

770

d'

VTP
Vmin
670
0

d'min
30

d'

60

90

120

Gap distance ( d ') [μm]（a）Insulation thickness : 30 μm

Estimated PDIV [Vpeak]

1370

100 μm, εr：4
VTP

○ Twisted pair model□ Parallel plate model

1270

d'

d'

Vmin
1170
0

d'min
50

100

150

200

Gap distance ( d ') [μm]（b）Insulation thickness : 100 μm
Figure 5 Estimated PDIV vs. gap distance.

Here, let us consider the difference of the estimated
PDIV for the 2 models described before. The twisted pairmodel is applicable under the case of d’=1 in Figure 5.
The estimated PDIV is set as VTP at this time. The estimated PDIV of the parallel plate model is equal to Vmin. Table
1 shows the comparison of VTP and Vmin. The difference ofthe estimated PDIV (ΔV =VTP -Vmin) became 15 V if theinsulation thickness is 30 μm and 100 V if the insulation

thickness is 100 μm. In other words, as the insulation
thickness increases, ΔV increases. This explains the difference of the estimated PDIV in the case of the twisted
pair model and the parallel plate model shown in Figure
3. At the same time, this result is suggesting that the PDIV
which is observed in the twisted pair arranged for thewires to contact each other may not always agree withthe minimum PDIV which is estimated by the insulation
thickness and the permittivity. The determination of thegap distance between the wires is important in the evaluation and the estimation of the minimum PDIV which is
critical from a practical viewpoint.
Table 1

Difference of the estimated PDIV.

Estimated PDIV［Vpeak］
Insulation
ΔV
Thickness
V TP
V min
（=V TP −V min ）［μm］ （PDIV of twisted pair） （Minimum PDIV）
30

735

720

15

100

1310

1210

100

In this section, the PDIVs of the twisted pair model andof the parallel plate model are estimated using numericalcalculation and in the case when the insulation thickness
exceeds more than 40 μm, the difference in both the estimated values of the PDIVs occurred. It was shown thatthe reason was the difference of the distribution of theelectric field and the importance to consider the gap distance between the wires when estimating the PDIV wasdescribed. It is considered that the findings which are
obtained by this study give the beneficial information to
the design of the insulation of the thick layer winding
wires and to the evaluation of the partial discharge, especially for the purpose of the improvement of the PDIV.
2.2 Insulation Thickness and Permittivity
Dependency of PDIVAs described in the previous section, the PDIV is dependent on the insulation thickness and the permittivity.
Authors investigated these relationships using a twisted
pair of the wires coated with various insulating materials.
In Table 2, the samples which were used in the experiment are shown. The diameter of the conductor is 1 mm,
the thickness of the insulation layer(t ) is from 29 to 107 μm
and the relative permittivity(εr ) is from 2.9 to 45. For thematerial of the insulation layer, the resins of Polyamideimide(PAI), Polyether-ether-ketone(PEEK), Polyphenylenesulfide(PPS) and Polyethylene-terephthalate(PET) areused. Sample A is an enameled wire with an insulation
layer formed by baking varnished PAI resin on the conductor. Others are the extruded wires with the insulator
which is formed by extruding on the conductor. For
Sample 5, PEEK and PPS are laminated with the same
thickness. The relative permittivity is calculated from thecapacitance of the wire in the 100 Hz. 3531 Z HiTester(manufactured by HIOKI E.E. CORP.) was used for themeasurement of the capacitance. For the measurement
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A Study on Partial Discharge Phenomena of Winding Wiresof the PDIV, the twisted pair is used as the test specimen
and the measurement was performed by the following
method. The specimen is setup in a constant temperatureand humidity chamber that is controlled at 25℃ and 50%
relative humidity. The 50 Hz AC sine wave voltage wasapplied between the wires at a rate of 50 V/s and the
input voltage when the discharge charge amountexceeds 10 pC is defined as the PDIV. For the detectionof the partial discharge, KPD2050 (Manufactured by
Kikusui Electronics Corp.) was used. Five measurements
were carried repeatedly in one experiment and by using
the data in the range from the 2nd to the 5th, the average,
the maximum and the minimum of N=3 (3 experiments)
were determined.

728

750

823

788

830

912

3

23.2

902

849

860

977

4

28.8

1055

994

990

1078

5

45.0

1330

1245

1220

1325

6

58.8

1501

1445

1430

1506

φ1.0 mm twisted pair
AC50 Hz
25℃/50%RH

1200

36

4.5

PAI

800

2

29

2.9

PEEK

600

3

36

3.1

PEEK

3.2

PPS

72

3.2

PEEK/PPS

6

107

3.6

PET

The testing results are shown in Table 3 and Figure 6. In
these, the estimated PDIV which is obtained by the
Paschen curve and formula (1) is also shown. The error
bar which is shown in the figure represents the maximum
and minimum value in the measured values. On the horizontal axis in the figure, the value which is twice the insulation thickness (the distance between conductors) divided by the relative permittivity was plotted. From thisexperiment, the result, that as the insulation thickness
increases and also as the relative permittivity decreases,
the PDIV increases, was obtained. As a matter of fact, it isfound that, for example, from the comparison of the sample 4 and 5, the PDIV increases with the higher insulation
thickness, and that from the comparison of sample 1 andsample 2 or 3, the PDIV increases with lower permittivity.
The observed PDIV was in good agreement with the estimated value obtained by the Paschen curve and thevalue calculated by the formula (1) and the validity of theestimation of the PDIV described in the previous section
was shown. More precisely, it can be said that the estimated value by the Paschen curve is close to the minimum observed PDIV. In this regard, it is estimated that insome cases, the PDIV is observed higher due to the insufficient supply of primary electrons. Because the formula(1) is also approximating to the measured result, it can besaid that it is effective in the simple estimation of the PDIV.
However, since the importance of knowing the minimum
value of the PDIV is high, it is considered that the estimation using the Paschen curve is more appropriate in theinsulation design of the winding wires.

Equation
（1）

830

1

46

Paschen
curve

775

1400

5

Minimum

20.0

Insulating
material

4

Average
16.0

Relative
permittivity（εr ）Φ 1 mmEstimated PDIV
［Vpeak］

2

1600

Test samples.
Conductor
diameter

2 t /εr

Measured PDIV
［Vpeak］

1

Insulation
thickness
［μm］
（t ）

sampleTest results.

samplePDIV [Vpeak]

Table 2

Table 31000

Measured PDIV
Estimated PDIV
Equation (1)
0

10

20

30

40

50

60

70

2×t/εrFigure 6 Thickness and relative permittivity dependence of
PDIV.

In this section, the measurement results of the PDIV in
50 Hz AC voltage using a twisted pair of the wires coatedwith a variety of insulating materials were shown. The
effect of the thicker insulation layer and the lower permittivity to the improvement of the PDIV was confirmed.
Further, the observed PDIV was in good agreement withthe value estimated by the Paschen curve and the electricfield of the air gap. Therefore, the validity of the estimation
using a numerical calculation was proved.
2.3 Effect of The Environment
Since the winding wires are used in a harsh environment,
it is important for practical use to clarify the effect of theenvironment on the partial discharge and its generation
mechanism 5). In this section, the author’s knowledge
about the influence of the temperature and the humidity
on the PDIV was described.
2.3.1 Temperature Dependency of The PDIV
By using the twisted pair wire coated with a variety of
insulating materials, the temperature dependence of the
PDIV was investigated. Three kinds of wires (Sample A-C)
with different insulating materials were used for the test.
Sample A is PAI which the permittivity property isimproved, sample B is PAI and sample C is PEEK. The
diameter of conductor is 1 mm and the insulation thickness is 30 μm. The twisted pair was used as the testing
specimen and the PDIV measurement was performed by
the following method. The specimen was set in a temperature controlled chamber. 50 Hz AC sine wave voltage isapplied between the conductors at a rate of 50 V/sec andthe voltage when the discharge charge amount exceeds
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A Study on Partial Discharge Phenomena of Winding Wires

100%

φ1.0 mm twisted pair
AC50 Hz90%○ Sample A□ Sample B△ Sample C

80%

70%

0

50

100

150

200

250

Temperature [℃]

Figure 7 Temperature dependence of PDIV.

It is considered that the PDIV is determined by the relationship between the spark voltage of the gas indicatedby the Paschen curve and the electric field strength in the
air gap. The spark voltage Vs of the gas in the uniform
electric field is represented by a product function of thepressure p [Torr] by gap distance d [m]. Here, it is possible to consider replacing the gas pressure p by the gas
density of ρ[kg/m3].

Vs = f ( pd )= f (ρd )……(2)In addition, the density ρ is inversely proportion to theabsolute temperature T [K] under a certain pressure fromthe equation of state of gas.

ρT = const. ..................(3)Spark voltage (Vs)

From these relationships, it is believed that Vs is lowered due to the change in the gas density at high temperature. Therefore, the change in Vs with increasing temperature is estimated and the PDIV considered was therefore
estimated using electric field analysis. In Figure 8, theestimation result of the temperature dependence of Vs isshown. The pressure is set as constant and the calculation example of the voltage between the gaps in the twisted pair is also shown in the figure. In the right region, i.e.
in the gap distance that affects the PDIV essentially, theresult that as the temperature increases, a Vs decreases
was obtained.

Temperature T1 , T2
(T1 ＜T2 )T1
T2

Calculated coil gap voltage(φ1.0 mm twisted pair model)
Gap distance ( d )Figure 8 Estimation of temperature dependence of spark
voltage.

Figure 9 shows the estimated result of the PDIV in consideration of the temperature dependence of Vs. In thisestimation, it was assumed that there is no temperaturedependence on the insulation thickness and the permittivity. In the figure, the actual measured values of sample A
to C were also shown. It was indicated that there is a tendency where the estimated PDIV, in consideration of thetemperature dependence, decreases as the temperature
increases. Qualitatively, it indicated the same tendency as
the actual measured value. However, to be precise, thedivergence was seen for both at the temperature of morethan 150℃ and the estimated value was estimated larger
than the measured value. Especially, there is noticeable
difference when compared with the sample B or C. Thatis, the difference between the insulating materials in thetemperature dependence of the PDIV cannot be
explained only by considering the temperature dependence of Vs.
Ratio of PDIV to basis (25℃)

Ratio of PDIV to basis (25℃)

10 pC is defined as the PDIV. KPD2050 (manufactured by
Kikusi Electronics Corp.) was used for the detection of thepartial discharge. The measuring temperature is from
25℃ to 230℃. After reaching the predetermined temperature, the measurement was done quickly. Five measurements were performed at each temperature and the datain the range from the 2nd to the 5th are used and theaverage value of N=3 (3 experiments) was calculated.
Figure 7 shows the temperature dependence of the
PDIV. In the figure, the retention ratio is shown in the casethat the reference (100%) is the PDIV of each sample at
25℃. From this result, it is found that there is a tendency
that as the temperature increases, the PDIV decreases
and the decrease rate differs by insulating materials. It isconsidered that the reduction of the spark voltage caused
by a change in the air density affect to the temperaturedependence of the PDIV. For example, there is an experimental result 6) that the temperature dependence of the
PDIV from room temperature to 80℃ in the air gap
between the insulation sheets using CIGRE Method -Ⅱelectrodes can be explained mainly by the relative air
density. So the temperature dependence from 25 to
230℃ observed in this experiment is considered from theviewpoint of the spark voltage of the air and the temperature dependence of the permittivity of insulating material.

100%

φ1.0 mm twisted pair90%

Estimated
value

80%

70%

Sample A
Sample B
Sample C

Measured
value
0

50

100

150

200

250

Temperature [℃]

Figure 9 Estimated PDIV with a consideration of temperaturedependence of V S .

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A Study on Partial Discharge Phenomena of Winding Wires

Relative permittivity (εr)

Next, the difference in the insulating materials is studied
from the viewpoint of the temperature dependence of thepermittivity. In Figure 10, the temperature dependence ofthe relative permittivity (εr ) of each insulating material isshown. The relative permittivity was determined by thecapacitance of the wires at 100 Hz. It can be seen fromthe figure that each insulating material has a different
temperature dependence. On the basis of the value at
25℃, the permittivity of Sample A slightly increased atmore than 150℃, the permittivity of Sample B increased
linearly and the permittivity of Sample C increased rapidly
at more than 150℃. In general, it is thought that because
the sharing voltage of the air gap increases as the permittivity increases, the PDIV decreases. In another words, it
is expected that the measured PDIV at high temperature
is affected by the increase of εr in addition to thedecrease of Vs described before.

○ Sample A□ Sample B△ Sample C

5.0

100 Hz

4.0

3.0

2.0

0

50

100

150

200

250

Temperature [℃]

Figure 10 Temperature dependence of the permittivity.

Estimated
value
90%

80%

70%
0

Measuredvalue(Sample A)
50 100 150 200 250

100%

Estimated
value
90%

80%

70%
0

Measuredvalue(Sample B)
50 100 150 200 250

Ratio of PDIV to basis (25℃)

100%

Ratio of PDIV to basis (25℃)

Ratio of PDIV to basis (25℃)

In Figure 11, the estimated PDIV based on the temperature dependence of Vs and εr is shown. In any samples,the estimated value is in good agreement with the measured value and the difference between both values
stayed less than 3%. From the above results, it was
shown that the dominant factor that characterizes theobserved temperature dependence of the PDIV is the
spark voltage of the air and the temperature dependenceof the permittivity of the insulating material.

In this section, the measurement result of the temperature dependence of the PDIV of the twisted pair using 50
Hz AC sine wave voltage was shown. It was confirmedthat the PDIV reduces with increasing the temperatureand it is concluded that the dominant factor is the temperature dependence of the spark voltage and the permittivity of the insulating materials. It is thought that the knowledge which was obtained by this study is useful specifically for the understanding of the partial discharge phenomenon of the winding wires which is used at high temperature environment such as in HV motor and in thedesign of insulator.
2.3.2 Humidity Dependency of The PDIV
The authors evaluated the humidity dependence of the
PDIV in the 50 Hz AC voltage using the rectangular wires.
Figure 12 shows the test specimen and the experimental
method. The test specimen was prepared by contacting
the flat face of two rectangular wires and fixing them by
the resin clips. An air gap of parallel and plate shape
between the wires is formed by sandwiching insulating
spacer of 50 μm thickness. The effective length of the test
specimen is 100 mm. The insulation thickness is 40 μm
and the materials are Polyamide-imide(PAI) andPolyphenylene-sulfide (PPS). The measurement of the
PDIV was conducted by the following method. The test
specimen is placed in a temperature and humidity controlled chamber and the AC sine wave voltage is applied.
The voltage when the discharge charge amount exceeds
10 pC is defined as the PDIV. The frequency of the voltage is 50 Hz and the increase rate of the voltage is 50
V/s. For the detection of the partial discharge, KPD2050(Kikusui Electrons Corp.) was used. The measurement
temperature is 25℃ and the relative humidity is between
50 and 95%. After stabilizing at a certain condition themeasurement was taken quickly. Five measurements
were taken repeatedly at each relative humidity. The datafrom in the range from the 2nd to the 5th are used andthe average value of N=3 (3 experiments) was calculated.

PAI, PPS (Thickness 40 μm)

100%

Estimated
value

Rectangular wire

90%

80%

70%
0

Insulating spacer (Thickness 50 μm)(Enlarged view)
Test specimen

PD tester

Measuredvalue(Sample C)

50 Hz AC voltage

50 100 150 200 250

Temperature [℃]

Temperature [℃]

Temperature [℃]（a）Sample A（b）Sample B（c）Sample C

Figure 11 Estimated PDIV in consideration of the temperaturedependency of Vs and ε.

100 mmTemperature & Humidity
chamber (25℃/50～95%RH)

Figure 12 Specimen and testing method.

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A Study on Partial Discharge Phenomena of Winding WiresIn Figure 13, the humidity dependence of the PDIV
when the rectangular wire is used is shown. In the figure,the retention ratio in the case that the reference (100%) is
the PDIV at 25℃/50% relative humidity is shown. In any
cases, the humidity dependence was confirmed and thePDIV was reduced significantly at 95% relative humidity.
The retention ratio at 95% relative humidity was 80% in
the case of PPS and was 74% in the case of PAI. In Figure
14, the difference in the behavior of the partial discharge
due to the relative humidity is shown. The discharge
charge amount when the partial discharge started wasabout 100 pC at 50% relative humidity, but it was 10 pC at
95% relative humidity. In addition to the reduction of the
PDIV, it is apparent that the discharge charge amountwhen the partial discharge starts is small.

Ratio of ＰＤＩ
Ｖ to basis(25℃/50%RH)

110%

Rectangular wires
AC50 Hz, 25℃

100%
90%
80%

○ Polyphenylene Sulfide△ Polyamide-Imide

70%
60%
30

40

50

60

70

80

90

100

Relative humidity [%]Figure 13 Relative humidity dependence of PDIV.

PD charge [pC]

PD charge [pC]

100

100

10

10
Applied voltage（a）50%RH

Applied voltage（b）95%RH

Figure 14 The difference of partial discharge behavior due torelative humidity.

The factor behind the reduction of the PDIV under thehigh humidity conditions is considered. With respect tothe temperature dependence of the PDIV, many experimental results and models which explain it were reported.
For example, they are the increase of the permittivity due
to the water absorption of the insulation layer 7), thedecrease of the insulation property of the air due to theincrease of water content, the enhancement of the electricfield due to the water droplet on the surface of the insulator 8) and the effect of the contaminants 9). Among them,
the investigation was performed in the PPS rectangularwires with respect to the possibility of the increase of thepermittivity of the insulation layer. As a result, the changeof the permittivity of PPS after 24 hours water soaking

stayed in about 3% increase in comparison to the value
when it was left at the room temperature. The decrease of
the PDIV due to the change in the permittivity is estimatedas 1- 2% and it cannot explain the observed experimental
result. Therefore, with reference to the case reported todate, paying attention to the surface condition, the predicted effect of the surface contaminants especially on
the electric property was investigated. At first, the substance adhering to the surface of the wire which is used
for the measurement of the PDIV was identified by electron probe microanalyzer (EPMA). As the result, Na, K
and Cl were detected. It is believed that these wereadhering when the wires were touched by bare hand during sampling and etc. In the case that these are on thesurface of insulator, it is known that they affect the electric
conductivity of the surface in the wet state. For example,
there are a number of case reports10) regarding the insulation property of the insulator for the high-voltage powerlines under the sea salt fouling environment. So, thehypothesis that the contaminants (adhering of Na, K, Cl
and etc.) of the surface touched by bare hand affect the
PDIV under high humidity environmental condition wasmade and it was verified by evaluating the temperaturedependence of the PDIV using the wire for which the surface is cleaned and the adhered substance is removed
and the wire which was touched by bare hand evenly
after cleaning. The cleaning was performed by wiping off
the moisture using clean rag after soaking the wires in thepure water. Note that EPMA testing confirmed that thecontaminants described before were removed.
Figure 15 shows the humidity dependence of the PDIVin the case that the surface of the wire is cleaned by thepure water and in the case that the surface is touched bybare hand again. The insulation layer is PPS resin. In thecase that the surface is washed by pure water, the clear
humidity dependence was not observed and the PDIV isalmost constant. On the other hand, in the case that thewire is touched by bare hand after washing, the humidity
dependence was shown again and the PDIV reduced.
From these results, it was believed that the contaminantsof the wire surface, probably the presence of the contaminants containing Na, K, and Cl, strongly affected thehumidity dependence of the PDIV. For the mechanism
where the PDIV reduces under the high humidity environment, the increase of the air gap electric field due to theincrease of the electric conductivity on the wire surface
can be considered. In addition, because the decrease of
the PDIV could not be recognized by washing the wire
surface, it can be said that the effect of the change in the
spark voltage due to the increase of the moisture content
in the air did not appear clearly.

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A Study on Partial Discharge Phenomena of Winding Wires

Ratio of ＰＤＩ
Ｖ to basis(25℃/50%RH)

120%
100%

Polymer layer

PPS insulated rectangular wires
AC50 Hz, 25℃

Enamel layer

80%
60%
40%
30

Conductor

Cleaned wire
○ Contaminated wire
40

50

60

70

80

90

100

Relative humidity [%]Figure 16 Developed structure of rectangular winding wire.

Figure 15 Effect of surface contaminants on the humidity
dependence of the PDIV.

This section describes the measurement results of thehumidity dependence of the PDIV at 50 Hz AC voltage
using the specimen where two rectangular wires arearranged in parallel as shown in Figure 12. It was indicated that the possibility that the ionic contaminants present
in the wire surface reduce the measured PDIV under highhumidity environment is high. As described above, a variety of impact factors are considered for the humidity
dependence of the PDIV and it is considered that moredetailed study is required to elucidate the phenomenon.

In addition, the development of the enamel resin containing the air bubbles is being developed to reduce thepermittivity of the insulator. By using this technology, thepermittivity of the insulation layer can be reduced significantly. In Figure 17, the relationship between the enamelwire permittivity with PAI insulating material containing the
air bubbles and the porosity. The permittivity can be
reduced with the higher porosity. This technology is
expected to be deployed to the other applications as wellas the winding wires.

Structure of enamel layer

EFFORTTOWARDTOWARD HIGH
HIGH PDIV
PDIV WINDING
3.3. EFFORTWINDINGWIRE
WIRE
While hybrid vehicles(HV) and Electric Vehicles(EV) are
growing rapidly, our company quickly worked on the highvoltage of these driving motors and the strengthening of
the insulator for the partial discharge. The rectangular
winding wire having unprecedented structure has beendeveloped.
In Figure 16, the structure of the rectangular winding
wire for HV driving motor is shown. By introducing therectangular conductor, high space factor is realized andalso by increasing the insulation thickness, high PDIV
was secured. The insulation layer has two layers structure
and consists of an enamel layer and a thermoplastic resin
layer provided on the outer periphery. Thick insulation
which is difficult in the conventional enamel wires has
been achieved with a high quality. For the application of
the thermoplastic resin, the material which has excellent
mechanical property, heat resistance and oil resistant infull consideration of the processing of the winding wires
and the in-vehicle environment was selected. The excellent PDIV and workability of the developed rectangular
winding wires support the technology revolution of a
motor stator such as the abolition of the inter-layer insulation paper and the reduction of the coil end and contribute to the downsizing/weight reduction of the motor andthe improvement of the fuel mileage.

Relative permittivity

5.0
4.0
3.0
2.0
1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Porosity

Figure 17 Relative permittivity of PAI vs. Porosity.

SUMMARY
4.4. SUMMARY
By a numerical computation and an actual measurementof the partial discharge inception voltage(PDIV), the influence which the structure of the winding wires and theenvironmental condition give to the partial discharge
property was investigated and the mechanism was considered. At first, the PDIVs of the twisted pair model andthe parallel plate model are estimated by the Paschen
curve and the numerical computation of the air gap electric field. As the result, when the insulation layer exceeds
40 μm, the difference of the estimated value for bothcases appears and it was indicated that this is due to thedifference of the electric field distribution of the air gap. It
was suggested that there is a possibility that the observed
PDIV in the twisted pair which the wires arranged in contact each other does not always agree with the minimum
value estimated by the insulation thickness and the permittivity and it was described that the consideration of thegap distance is important when estimating and evaluating
the minimum PDIV which is important in practical use.
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A Study on Partial Discharge Phenomena of Winding Wires

Next, as a result of the PDIV being measured in the 50 Hz
AC voltage using the wires of the twisted pair, it was confirmed that the estimated value and the measured value
are in good agreement. From these, the validity of the
PDIV estimation by a numerical computation and theeffect of the thicker insulation layer and the lower permittivity are indicated quantitatively. Further, the effect thetemperature and humidity environmental condition give to
the PDIV when 50 Hz AC voltage is applied, was investigated. The PDIV of the twisted pair at 25-230℃ shows theclear temperature dependence and reduces with higher
temperature. It was indicated that this dominant factor
characterizing the temperature dependence is the spark
voltage and the temperature dependence of the permittivity of the insulating material. Also, it was indicated that the
PDIV of the rectangular wires arranged in contact reduces
significantly and this is due to the ionic contaminants on
the wire surface. In the end, regarding our effort toimprove the partial discharge property of the winding
wire, the rectangular winding wire for the HV driving
motor and the technology to reduce the permittivity of theenamel wires was introduced.

CONCLUSION
5.5. CONCLUSION
In this paper, the research results by the authors about
the partial discharge phenomenon of the winding wires
are reported mainly for the experimental result and thenumerical computation. It is considered that the obtained
knowledge gives the useful information for the evaluationand the understanding of the partial discharge of thewinding wires used under the high voltage and a harshenvironment, especially for the driving motor of the HV
and EV, and the design of insulating materials.

REFERENCE
1) Takasaki, Kamiya, Mizutani, Kanaiwa, Kato, Umeda; “The development of the stator motor for compact hybrid car”, 2012 JSAE
Annual Congress (Spring) EV, HEV System Ⅲ-①, (2012), 5. (inJapanese)
2) Hitoshi Okubo, Yonghu Lu and Naoki Hayakawa : “Partial discharge
characteristics of inverter-fed motor coil samples under ac andsurge voltage conditions”, 2003 Annual Report. Conference on
Electrical Insulation and Dielectric Phenomena, (2003), 589.
3) T. W. Dakin, H. M. Philofsky and W. C. Divens : “Effect of Electric
Discharges on the Breakdown of Solid Insulation”, AIEE Part I:
Communication and Electronics, 73(1954), 155.
4) Shimamoto : “The composite acceleration test and reliability evaluation technology of electronic device and component” (1998),143.
(in Japanese)
5) Wakimoto, Takahashi, Koda, Takizawa, Ishida : “About the highvoltage insulation of the EV/HEV motor- Impact on the partial discharge inception voltage in consideration of the vehicle environment - ”
		 DENSO Technical Review, 16 (2011), 68. (in Japanese)
6) Koyama, Kurimoto, Murakami, Nagao: “Temperature and humidity
dependence of the partial discharge inception voltage in CIGRE
Method-II electrode using an insulating sheet", the Institute ofElectrical Engineers Annual Conference Proceedings 2012, (2012),
72. (in Japanese)
7) Y. Kikuchi, T. Murata, Y. Uozumi, N. Fukumoto, M. Nagata,Y.
Wakimoto, T. Yoshimitsu : “Effects of Ambient Humidity andTemperature on Partial Discharge Characteristics of Conventionaland Nanocomposite Enameled Magnet Wires”, IEEE Trans. Diel.
Elec. Insul., 15 (2008), 1617.
8) M. Fenger, G.C. Stone : “Investigation of the effect of humidity on
partial discharge activity in stator windings”, Proc. of 7thInternational Conference on Properties and Applications onDielectric Materials, 3 (2003), 1080.
9) Koyama, Yokomi, Kurimoto, Murakami, Nagao : “The influence ofthe humidity and the contaminants on the partial discharge inception voltage of CIGRE Method-II electrode system” Handout for
The Institute of Electrical Engineers of dielectric and insulating
materials research, DEI-13-045, (2013 ), 27. (in Japanese)10) Kobayashi, Matsuzaki, Arashitani, Kimata: “The development of
the composite insulator for overhead lines, No. 2” Furukawa
Review, No. 21, (April 2002)

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