Proceedings of the International Conference on Smart Electronics and Communication (ICOSEC 2020)

IEEE Xplore Part Number: CFP20V90-ART; ISBN: 978-1-7281-5461-9

Analysis of Phase Resolved Partial Discharge Patterns

of Kraft Paper Insulation Impregnated in Transformer
Mineral oil
Mahesh Kumar K M Dr. B Ramachandra Dr. L Sanjeev Kumar
Dept. of Electrical & Electronics Dept. of Electrical & Electronics Dept. of Electrical & Electronics
Engineering Engineering Engineering
PES College of Engineering, Mandya PES College of Engineering, Mandya Sri Siddharath Institute of Technology,
Karnataka, India Karnataka, India Tumkur, Karnataka, India
maheshkm01@gmail.co m bramachandra1@gmail.co m sanjeev_ssit@yahoo.co.in

Abstract—Partial Discharge (PD) is referred as a Paraffin ic or naphthenic are the common co mpounds of
small discharge that partially bri dges the insulation over mineral oil [2]
a s mall portion of its space. PD occurs because of small
defects in insulation such as voi d i n soli d and bubble i n Power transformers have demonstrated as reliable in normal
liqui d Insulati on. The power transformer uses both soli d operation with a failure rate o f less than 2 percent per year
and li qui d insulati on in terms of Kraft paper for windi ng globally. Failures of Transformer are b roadly classified as
and oil for cooling which protects transformer for long electrical, mechanical, or thermal. The electrical failure is
life operation. The partial discharge defects in insulation influenced by the insulation degradation wh ich affects long -
need to be i dentified early in order to esti mate and term operation of transformers; Loss of mechanical strength
extend the life of transformer. In this paper parti al of the solid insulation in the windings is defined to be end
discharge patterns of pig tail s pecimen (a model of life of the transformer. [3].
transformer windi ng) is i mpregnated in li qui d insulation
(mineral oil) is discussed. The detection of PD i n The dielectric strength of oil, cellulose insulation system
dielectric li qui d required PD measuring system and depends on the duration of voltage application, the polarity
Instrument with l ow background noise. A uni que of voltage, field enhancement factor, area & shape of the
measuring setup was designed and cali brated as per IEC electrodes, degree of contamination of the oil, wh ich also
Standard. The high voltage stress is applied on the includes its temperature and pressure .[4]
pigtail speci men continuously until speci men failure;
simultaneously PD data are recorded wi th a ten second Transformer windings are separated by Dielectric materials.
del ay from one acquisition to another. The pulses appear These materials are immersed in oil for heat transfer and to
like burst in liqui d insulation are comprised of discrete act as insulator. Co mmonly used dielectric material is Kraft
vari able high frequency pulses. These pulse burst paper and is made of cellulose pulp, it has good thermal
occurrence is compared with the phase of the input wave stability, can be exposed to higher temperature. The
and the acquired data is further investigated to study its impregnated Kraft paper will be aged due to thermal stress,
behaviors. acidity and moisture. These ageing factor of Kraft paper
affects life t ime of the transformer.[5] As the insulating
Keywords— Partial Discharge, Kraft paper, Mineral property of kraft paper degrades, it becomes weak and after
oil, Pigtail specimen, Power transformer a threshold the insulation breakdown occurs.
I. INTRODUCTION
The three layer kraft paper is wrapped over the conductor
Power Transformers are very co mmon electrical device used and immersed in insulating liquid such as mineral and tested
in electrical Industries may be Generat ion, transmission, for partial discharge. The discharges are measured as per
distribution or High voltage required electrical co mpanies. IEC Standard and it looks like bursts of pulses occur over a
They are expected to function reliable and efficient for many fin ite time. This bust time interval is determined fro m the
more years. The oil quality plays an important role in cavity format ion and collapse time followed by its life
examining quality of the transformer, the characteristics of within the oil. PD behavior is characterized by a number o f
transformer oil need to examine fro m time to t ime during its events linked with the insulation cavities in solid and
course of operation. [1] Co mmonly used oil is mineral o il bubbles or contamination in the liquids [6].
which is obtained fro m refining the hydrocarbons collected
during the distillat ion of petroleu m. These oils are also used In order measure the Partial discharge in Insulation requires
in several other electrical equip ments such as circuit PD measurement standards and instruments. Continuous
breakers, capacitors, reactors and switches, helps in partial discharge (PD) data need to be measured on
extinction of arc and partial discharges. The very important insulating liquids, using suitable detector system. Fro m the
characteristics of mineral o il are associated with their literature various authors have suggested RC or RLC
dielectric strength and stability to withstand failures due to circuits are normally used for PD Detection [7]. But the
temperatures and high voltage stress. Also their permittivity magnitude of signal is very low need to be amp lified.
is an exceptional option for cooling and Insulation. Narro w band or wide band PD amplifier circuits are

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integrated with matching impedance to obtain the total to be carried out during the course of experiments without a
charge transfer in p ico coulo mb to have overall d ischarge breakdown of the specimen. It sets the maximu m voltage
event. The detectors allows capturing of discrete high needed to be handled in the experiment. The study is carried
frequency PD pulse burst signal [8]. Various investigators out in a glass cell of 750 ml capacity. The electrodes are
have investigated partial discharge distribution in liquid mounted on a horizontal axis with 2.5 mm gap as per IEC
dielectrics under non-uniform and uniform field conditions Standard 60156 [12]. The cell is thoroughly cleaned and
using Multi Channel analyzer. The authors have reported rinsed with the test oil. The gap between the electrodes is set
MCA is a helpful tool in PD measurement and its analysis to an accuracy of ± 0.01 mm by means of thickness gauge.
improves data acquisitions. [9] PD Breakdown voltage test setup is as shown in figure

PD measurements are important diagnostic tools to monitor

the insulation condition. The system mainly consists of
measuring impedance wh ich is used to decouple the PD
Current pulses from the coupling capacitor & these pulses
are amplified by designing suitable amplifier circuit fo r
analysis with PD measuring circuit. The measuring circuit is
observed in high bandwidth with good sampling rate Digital
oscilloscope. The Partial d ischarge phase resolved patterns
are captured in high frequency digitizer system [10].

II. EXPERIMENTAL PREPARATIO N

A. Preparation of Sample for Testing Fig. 2 Oil Test Unit for BDV

2) Test cell setup for PD

Actual test cell prepared is as shown in figure 3.
Borosilicate glass container with pigtail specimen inserted
in mineral oil for PD experiments with one end for High
Voltage end and another end for ground connection.

Fig.1, Pigtail Specimen

The power transformer insulations winding condition is

simu lated in the laboratory. The conductor used for the
winding is suitable selected and cut in a suitable manner in
order to rep licate field conditions of transformer winding.
Two copper conductors are connected back to back with
each of 9mm width, 3.4mm thickness are arranged like a Fig. 3 Test Cell Design
pigtail specimen [11] as shown in figure 1. It has three
portions two bent portion of 15mm on either side o f D. Experimental Setup
conductor and straight portion of length 100mm at the
center this will be at an angle of almost 30̊ with the
horizontal. Three layer kraft papers is wound on the copper
conductor. The Two conductors are wrapped tightly by
connecting PTFE tape as show in fig.1. One end is
connected to High voltage side and another end is connected
ground/earth
.
B. Mineral Oil
EHV grade vacuum imp regnated Clear, free fro m sediments
and Suspended matter transformer oil [confirming to IS -
335:1993 R 2010 /IEC-296 (class I) (1982)] is procured
fro m a reputed manufacturer in a single batch of all
experiments. The moisture content of transformer oil is
removed by placing it in a specially designed oven at a
temperature of 130ºC for 8 hours a day to totally 8 days
continuously.

C. Test Cell Setup

Fig.4 Experimental setup block diagram
1) Oil BDV test setup.
2) Test cell setup for PD.
The straight detection method is used for the measurement
The Breakdown voltage (BDV) determination is a test at
which the oil breaks down when subjected to AC electric of PD [13]. The schematic diagram is shown in Fig 4. PD
detection setup is developed to carrying out
field. This gives an idea about the amount of voltage needed
experimentation.

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High voltage transformer used is 10 kVA/ 100 kV fo r III. Results
applying a voltage to the test sample. High voltage is
controlled through electronically controlled control unit The experimental results are discussed in three sections
which houses auto transformer to vary the input voltage. PD x Breakdown Vo ltage (BDV) of transformer oil (M ineral
free Coupling capacitor of value 1000PF is used for oil).
coupling the PD pulses to matching impedance circuit x PD Inception and Extinction voltages of Paper covered
designed using parallel RLC circuit. Su itable protection is copper conductor Insulation pig-tail specimens inserted
provided to measuring impedance through isolation in mineral oil.
transformer, M OV and neon lamp. The output of RLC x Partial discharge measurements of mineral oils with pig-
circuit is amplified using narrow band amplifier with tail specimen
frequency ranging from 6KHz to 500KHz. Output of the
amp lifier is observed in Dig ital storage oscilloscope (DSO) A. Oil Breakdown Voltage
and to analyze the data PD output fro m amplifier is given to The test cell is filled with required quantity of
High speed digitizer system designed using NI-PCI Card to uncontaminated transformer o il without forming any bubble
study Phase resolved partial discharge patterns of the pigtail formation. The applicat ion of voltage was as per IEC
specimen inserted in mineral o il. System is designed as per 60156/ 95 at the rate of 2kV/s till breakdown of oil for ten
IEC Standard 60270[14] trials. Th is experimental procedure was repeated for
different liquid insulating mediums.
Figure.5 Show picture of all the equipment fo r BDV Test Results of 10 repeated Experiments
experimentation housed in faradays cage with very lo w
background noise of less than 5pc. 1 2 3 4 5 6 7 8 9 10
31 32 32 29 32 30 32 32 32 32
Figure.6 show measured PD signal through Digital
oscilloscope and NI digitizer card connected PC
Average of ten repeated experiment breakdown voltage is
taken as 32kV.

B. Inception and Extinction voltage of pig-tail specimen

The discharge inception voltage (Vinc) is obtained by
gradually increasing applied voltage till the discharges
begin to appear. The voltage is maintained for one minute
and the value of the voltage at which discharges occur
contentiously for one minute is taken as the inception
voltage (Vinc). Voltage is then gradually reduced and the
voltage at which the pulses just disappear is taken as the
extinction voltage (Vex).

Vinc in kV Vex in kV
2.7 2.5

B. Phase-resolved Partial discharge(PRPD) of pig-tail

specimens in Mineral oil
1. Control panel to vary voltage 4. Test cell The PRPD technique makes use of the charge, charge
2. HV Transformer 5. DSO
magnitude, Nu mber of pulses with respect to phase of
3. Coupling transformer 6. PC
PD occurrences [15]. PD records are collected fro m
7. Amplifier circuit impregnated pig-tail specimen for t ransformer o ils
using special designed data acquisition system for
Fig. 5, Actual PD Setup in laboratory
Partial d ischarge measurement. The results are stored in
the computer in the form of files. These files are
analyzed direct ly using the analyzing system designed
in NI Lab View software and final output is used for
discussion. The experiments were performed at 1.4
times inception voltage. High voltage stress at 1.4 times
the inception is continued until specimen failure. Data
was captured over the duration of one second
contentiously allowing a settling t ime of 10 seconds
between the records. The data files are very large and
Fig.6 PD Pulse burst observed in DSO and captured using only typical file records are presented in each case. The
PC Software experimental results are presented in the follo wing
sections. Totally 171 data are collected over duration o f
2 hour 40 minutes. Obtained data are represented in
PRPD Pattern for further study

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 1. Phase- charge distribution (Φ–q) The fig.10 indicates low, med iu m, moderate and high values
of charges which correspond to 0-25, 25- 50, 50-75 and 75-
100 percentile charge level magnitude distributions
respectively shown in fig.6 corresponding data in table.1
The low level charges initial starts from 51.5 Pc slowly
reduces to 46.75 over the middle durat ion and gradually
increases as the specimen breakdown or end region. The
higher level charges have shown only small variation in
magnitude because the oil is uncontaminated there is no
huge changes in high level charges. The average level
charge initially increases and gradually decreases with
respect to time. Also at Fig.7 and Fig.8 of phase vs charge
are almost identical. Fig. 9 indicates the charge magnitudes
are decreasing indicating that specimen is at the verge o f
breakdown.
Fig 7,at the beginning of Experiment
2. Phase-number of counts (Φ–N)

Fig.11, Phase vs. Number of counts in positive and

Fig. 8, at the middle of Experiment negative half cycle

Table.2:
Phase vs No. of counts
Start Middle End
75 percentile 75.75 63 3.25
Maximum 3576 3730 4083

Percentile Variation for Phase Vs. Counts

The Fig.11 shows variations of phase-number of counts

distribution of pig -tail specimens for Mineral oil. Table 2
shows the number of counts variation for start, middle and
Fig. 9, at the end of Experiment end of experiments. Minimu m, 25 percentile and median
counts are very less only maximu m counts are present with
few 75 percentile counts.

3. Count Vs. charge

Fig. 10. Percentile variations of charge with respect to time

P-Q for Mineral oil

Start Middle End
25 percentile 51.5 46.75 106.5
median 357.5 363.5 320.5
75percentile 447 442 430 Fig.12, Charge in pc vs No. of counts
maximu m 581 569 592
Fig.12 shows variations of charge-nu mber of counts
Table.1: Distribution of Pulses According to Percentile distribution of pig-tail specimen. The maximu m nu mber o f

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The PD pulses appeared like a burst. The repetition rate of
PD Pulses increases with increases of applied voltage above
inception. With oil impregnated insulation system pd recur
regularly over applied voltage power frequency. Continuous
exposure of mineral o il for partial discharge over long
duration or long term leads to insulation degradation.

ACKNO WLEDGMENT
This research work was carried out at Center of
Excellence in High Vo ltage Insulation Laboratory, a project
laboratory developed with financial support from Karnataka
Council of Technological Upgradation (KCTU) and PES
College of Engineering, Mandya.

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