Signal Processing and Renewable Energy

March 2021, (pp. 51-78)

ISSN: 2588-7327
eISSN: 2588-7335

A Review of Partial Discharge Signals Occurrence in Polluted High

Voltage Insulator
Hosein Fadaei1, Faramarz Faghihi*1 and Hosein Mohmmadnezhad1

1
Department of Electrical Engineering, Science and Research Branch, Islamic Azad University,
Tehran, Iran.
Received: 21-Sep-2020, Revised: 02-Dec-2020, Accepted: 03-Dec-2020.

Abstract
The application of sufficiently intense electric fields to insulators induces finite currents and then the
insulators become conductive. This phenomenon is called dielectric breakdown and known as a
fundamental non-equilibrium and nonlinear transport phenomenon in solids. Partial discharge (PD) is
a small electrical avalanche caused by locally disrupted electric fields in dielectric materialsand is
known to be one of the major factors which accelerates the degradation of electrical insulation.
Pollution is the most important cause of external electrical failure of the insulator, fol-lowed by the
operation of relays and the exit of transmission lines from the power grid. It is believed that the
flashover and partial discharge phenomena are strongly related to environmental conditions such as
temperature, humidity, pressure, pollution, and even space charges. In this review paper, we will
identify the types of insulators associated with pollution sources and present laboratory results while
defining partial discharge and its effects on equipment.

Keywords: Partial discharge, insulator, high voltage, contamination, flashover.

1. INTRODUCTION
Infected insulation behaviors were studied floor shape a conductive layer on that floor.
through researchers over the years. While the With this formation that appears, the
humidity reaches a massive level, the incidence of leakage moderns will observe.
contaminants that deposit at the insulation Leakage current increases the temperature,
and as a result, dry bands shape on the floor
of the insulation. All through the formation of
*Corresponding Authors Email: dry bands, minor discharges and arcs arise, as
faramarz.faghihi@srbiau.ac.ir a stop end result, viable insulating glide takes
area [1-4].
52 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …
A good way to simulate the phenomena discharge is caused by environmental
happening at the surface of insulators, impacts, we will have a better understanding
numerous theoretical fashions have been of environmental impacts, and HVDC
proposed, among which, Rizak version [5] insulation systems will be considered for
attempts to provide the simulation of the future design.
crucial arc length of infected insulation and The definition of PD is almost huge,
additionally the impact of dynamic voltage generally defined as a comprehensive
drop. Inside the leakage modern-day discharge or surrounded by an insulating
waveform, the important flashover voltage fabric [11]. Due to the excessive electrical
grows to be additionally investigated. Vosloo resistance of insulating substances, PDs are
and Holtzhausen [6] recently proposed a typically low density within the tool. One of
version of a circuit identical to an infected the drawbacks that have obtained a lot of
insulator based mostly on which everyday research interest is air-filled cavities,
leakage present-day measurements were generally spherical or cylindrical, surrounded
considered. In the unique case, the leakage with the aid of solid dielectric fabric. Those
contemporary waveform is saved wherein the experiments especially use the “sandwich”
almost everyday rate occurs. A theoretical method to make a cylindrical hollow which
version for predicting the flashover voltage is includes three plates with a circular hole
likewise provided [7], which includes the re- drilled within the main ab plate. The
ignition homes of dry band arcs. Within the compressed plates are then placed among the
stated fashions, the same circuit, in particular, parallels plate electrodes. The outstanding
includes a conductive layer resistor at the dielectric material used to make cylindrical
floor of the tainted insulation, which seems to cavities is called LDPE [12]. Spherical
indicate the incidence of a mainly quick arc cavities in epoxy resins [13,14] and silicone
in circulating along the vicinity [5-7]. Not rubber are made by using injecting air and
one of the above models encompasses the syringes earlier than the filtration machine
analytical mathematical evaluation of the [15].
corresponding equal circuit proposed. The present paper part of a long-term
Monitor partial discharge, as a symptom study for a Partial discharge.
of insulation deterioration, can be used to The main objectives of this article can be
improve the reliability of HV insulation. summarized as follows:
Usually, the techniques employed for PD • Search for literature in the following areas
detection are based on chemical [8], acoustic - Definition of partial discharge
[9], optical [10], and electrical which all - Types of partial discharges
involved with ultra frequency measurements. • Study of insulation classifications
The electrical measurement of PD in HVDC • Pollution types of infections
systems are widely used. Leakage current • Scientific study of the effect of pollution on
data flowing in a reactor’s insulation post, to increasing partial discharge
help to build a better understanding of the
correlation between PD. When the visible 2. PARTIAL DISCHARGE (PD)
Signal Processing and Renewable Energy, March 2021 53
Based on the IEC60270 standard, partial object under test. The pulses will be
discharge is defined as “localized electrical measured using appropriate tracking circuits
discharge that only partially bridges the that haveentered into the test circuit for
insulation between conductors and which can testing”.
or cannot occur adjacent to a conductor,”. • Discharge Inception Voltage (PDIV): “The
The IEC60270 standard also states that: starting voltage is the lowest operating
“Partial discharge is generally thought to voltage at which a PD pulse value is equal to
be the result of a concentration of local or less than a specified value”.
electrical stress on the insulation or • Discharge Extinction Voltage (PDEV):
insulation surface. In general, such “The extinction voltage Ue is the lowest
discharges appear as pulses lasting much less applied voltage when the magnitude of the
than 1 us. Therefore, more persistent forms selected PD pulse is equal to or less than the
can occur, such as so-called pulse discharges specified value”.
in gas dielectrics. Partial discharges are often • Apparent Charge (q): “The apparent charge
accompanied by the emission of sound, light, q of a PD pulse is a charge that, if injected
heat, and chemical reactions,” [16]. into a sample test circuit in a very short time
Since PD will compromise insulation between the test terminals, it does the same
materials in both physical and chemical thing on the PD current pulse meter itself.
ways, PD is important to be discussed. If PD Significant cost is usually described in pico-
issues keep happening, it might eventually coulombs (PC). The apparent load is not
cause high-voltage equipment failure. Thus, equal to the local charge at the discharge
detecting and mitigating PD problems are point, so, it cannot be measured directly”.
critical topics in the field of high-voltage • PD Pulse Repetition Rate (n): “The ratio
engineering. Based on different between the total number of PD pulses stored
characteristics, PDs can be defined as internal in a time interval and the duration of that
discharge, surface discharge, and corona. By interval. In practice, only pulses above a
internal discharge, it is meant that discharges certain value or within a certain range is
occur inside the solid insulation or at the edge considered”.
of conductors in solid/liquid insulation.
Surface discharge is a type of discharge that 3. TYPES OF PARTIAL DISCHARGE
may occur on the surface of insulation
IEC 60270 (PD measurements) defines par-
material. Corona charge refers to partial
tial discharge as ‘a localised electrical
discharge in gasses around conductors that
discharge that only in part bridges the
are removed from solid or liquid insulation.
insulation between conductors and which
Partial discharge is a complex physical
may additionally or won't occur adjoining to
process with stochastic properties [17].
a conductor’. If the insulation is
• Partial Discharge Pulse (PD pulse): In IEC
completelybridged then the insulation breaks
60270 standard, partial discharge pulse is
down. These discharges can occur internally
defined as “The current or voltage pulse can
or externally to the insulation.
be caused by a partial discharge inside the
54 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …
In line with the nature, and area of partial 3- Corona discharge - occurs at sharp edges
discharge, they can be classified into the or skinny conductors which can be
subsequent three categories [18]: connected to excessive capability or
1- internal discharge - normally happens in ground.
cavities packed with a gas known as cavities Figure 1, shows the different types of
of liquid, and solid insulation substances. discharges along with their locations.
2- surface discharge - happens on the surface
of an insulating fabric or in reference to
various insulating substances.

Fig. 1. Different Types of Discharges.

Fig. 2. Simple model of a cavity in dielectric.

Signal Processing and Renewable Energy, March 2021 55
TABLE 1. The relative permittivity and breakdown strength of HV insulating materials.
Relative Breakdown strength
Material
permittivity kV/mm
Air (atmospheric pressure) 1.006 2

Transformer Oil 2.2 28

Polyethylene 2.3 24

Polyurethane 4.0 10

Paper 3.0 9

Mica 6.0 42

Epoxy 4.7 12

3.1. Internal Discharges 3.1.1. Analogue Discharge Circuit

Inner discharges take location in inclusions Inner partial discharge behavior below
or cavities (voids) within the insulation. It voltage stress a.c. can be without problems
isn't always nearly feasible to completely described by using representing the analog
dispose of the prevalence of small faults like equivalent circuit, regularly known as the ‘
voids in an insulating material. A practical abc’ circuit [18]. Figure 2 displays a simple
insulation shape of a high voltage stator will chart of a hollow in a segment of strong
invariably incorporate small voids or insulation fabric with its equal circuit.
cavities, often due to inhomogeneities inside In figure 2, the capacitor Cc represents the
the insulating material used for the capacitance of the cavity; Cb represents the
production. Herbal resin-based coils and Capacitance of the dielectric material in
artificial resin-based totally coils contain collection with the hollow space, and Ca
represents the capacitance of the rest of the
gaseous inclusions and are formed because of
dielectric material. ‘S’ represents the spark gap
small quantities of air that grow to be trapped
which breaks down whilst the voltage across it
in the insulation during the curing and urgent reaches the void breakdown level.
levels. Table 1 shows the relative permittivity
These cavities, packed with air, and breakdown strengths of a few typical
commonly have a decrease permittivity and a high voltage insulating materials.
decrease breakdown strength in comparison The relative permittivity of air ‘εo’ is the
to the insulating fabric. This causes the same as harmony, whereas the permittivity of
sphere depth to be higher within the hollow soliddielectric (e.g. Insulated epoxy mica) which
space than within the dielectric material and is given via ‘εr’ which varies between 4 and 6.
can motive a breakdown, even beneath While the Given solid dielectric is subjected to
everyday operating stress, affecting the long high voltage (ES), the electrical strain advanced,
time integrity of the insulating fabric. across the hollow space (EC), given by using:
EC = (εr/εo).ES (1)
56 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …
where εo equals to relative permittivity of air cavity. The breakdown voltage U+ is decided
and εr equals to relative permittivity of solid with the aid of the Patches curve, which
dielectric relates the breakdown voltage of the air to the
For that reason, it's miles clean from fabricated from pressure and electrode
equation (1) and table 1 that the electrical distance [18]. While the discharge occurs, the
stress across the hollow space is appreciably voltage throughout the hollow space drops to
improved. The quantity of pressure voltage V+ (extinction voltage) and the
awareness, within the hollow space, relies release is switched off. This usually takes
upon on numerous other factors such as the place at instances less than 0.1μS. The
shape and length of the hollow space. Kang voltage does not attain zero since a residual
et al. [19] confirmed that discharge voltage will continue to be for the duration of
parameters consisting of significance, the hole.
repetition fee, imply discharge strength and After the extinction of discharges, the
suggestion drift price was higher in samples voltage throughout the hollow space
with larger cavities. Due to this fact that increases again along side the applied voltage
breaking electricity of air at a atmospheric Va. Some other discharge occurs while the
stress is about 3 kV/mm, it can be seen that voltage across the hollow space reaches U-.
discharges occur at medium to excessive This sequence of discharges repeats
voltage. numerous instances till the applied voltage Va
In keeping with figure 3, the high voltage starts offevolved to lower and the discharges
across the dielectric is denoted by Va and the cease. For this reason, several discharges are
voltage throughout the hollow space is produced in the course of the growing portion
denoted via Vc. A discharge happens when of the tremendous 1/2 cycle. A similar
the VC voltage exceeds the fantastic method takes region within the bad half of
breakdown voltage U+ (initial voltage) of the cycle in which the discharges arise while the

Fig.3. Sequence of internal discharges under a.c. voltage.

Signal Processing and Renewable Energy, March 2021 57

insulation making the very last damage

system quicker [23].

3.3. Corona Discharges

Corona discharges are electrical discharges

resulting from the ionisation of the medium,
surrounding the conductor. They have a tendency
Fig. 4. Surface Discharges.
to occur around sharp factors or edges at
excessive voltage due to an excessive awareness
voltage throughout the hollow space exceeds of charge on a small floor vicinity. The inception
U-. On this manner companies of regularly voltage of corona discharges is difficult to the
taking place superb and terrible Discharges country as it's far dependent on various factors
will be produced. Each discharge will motive like surface smoothness and environmental
a present day pulse as shown in figure three, conditions which have an effect on the distance
and those can be detected through fee near the conductor. They seem faster at
electricalway [20,21]. negative than at fantastic voltage; with a.c.
voltage, they often arise throughout the terrible
1/2 cycle best [18].
3.2. Surface Discharges
In rotating machines, the stop-windings
Surface discharge can also arise while there are more susceptible to corona kind
may be a pressure element parallel to the discharges as the electric stress is intensified
dielectric surface, as proven in Figure 4. in the overhang component because of its
Strain causes depletion. They are normally featureshape. The fuel adjoining to the
located in trees, cable ends, and device insulation in the immediate place of the slot
winding coils. Floor discharge relies upon on go out breaks down and might result in the
numerous elements [22]: improvement and propagation of discharges
• Physical residences of the surroundings over end-windings [24,25]. Consequently,
wherein discharge takes region (gas, the software of strain grading structures
liquid). alongside the end windings is taken into
• Physical houses of solid dielectrics consideration critical for high voltage
(permittivity, floor resistance, machines.
conductivity).
• Distribution of electric subject within the 4. CATEGORIS OF HIGH VOLTAGE
site between electrodes. INSULATION
• Type of voltage and time of operation.
The demand for electrical energy has been
• State of surface of strong dielectrics (pollut-
increasing gradually. For transmitting a
ed, sodden).
massive amount of electricity, high voltage
Cracking is the end result of carbonation
transmission systems have been extensively
of insulation surface and carbon tracks which
used. In the transmission system, high
have a tendency to reduce electrical
voltage apparatus, such as insulators, play a
58 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …
significant role in attaining normal operation discharges activities at sharp edges.
of the power supply [26]. Moreover, the glaze is chemically inert due
Outdoor insulators are widely used in to the strong molecular bonds that form the
power transmissions and distribution porcelain material. Therefore, the insulator is
networks. They help in providing mechanical mechanically solid and has a good resistance
support to the overhead line conductors, and surface against corrosion [30, 31]. Figure 5
they isolate them electrically from the tower. shows a typical standard profile of a cap-and-
This section aims to provide an insight into pin porcelain insulator, with the main
three different insulating materials in use components of a glazed porcelain shed and
today: porcelain, glass, and polymers. metal cap-and-pin, which are all embedded
Porcelain and glass are also known as together with the mortar [32]. The insulators
ceramic insulators, and polymeric insulators are joined in series to achieve a string. The
are known as composites or non-ceramic number of units used in the string varies
insulators. Different profiles of insulators are based on the system voltage.
selected based on the environmental
conditions and the application for which they 4.2. Glass Insulators
are used, for instance, suspension, tension,
and post types. Insulators are also available Glass insulators have been used in service for
with different dimensions and designs up to 70 years, as is discussed in detail in
according to the voltage rating [27, 28]. [33]. These insulators offer great resistance
due to the thermal handling they receive.
4.1. Porcelain Insulators With this handling, insulators have enhanced
their mechanical properties compared with
Porcelain insulators have been well known in porcelain cap-and-pin insulators [27].
the power system industry for a long time and Toughened glass insulators (Figure 6) have
are still the most widely used. This inorganic the advantage of facilitating the inspection of
material has demonstrated relatively higher the electrical lines even in the case of
thermal resistance and strength regarding the breakage. Though the glass disc smashes into
degradation of the surface. Porcelain is a small parts, the mechanical strength of the
highly glazed surface [27-29]; this gives the insulator remains the same as for the
insulator better self-cleaning properties in unbroken insulator.
heavy pollution areas and helps reduce the

Fig. 5. Standard design cap-and-pin porcelain insulators.

Signal Processing and Renewable Energy, March 2021 59

Fig.6. Cap-and-Pin glass insulators.

Nonetheless, glass has a lower melting Fig. 7. Polymeric insulators.

point than porcelain, making it more
susceptible to surface erosion owing to the insulators that have good mechanical and
heat produced by surface discharges [27]. electrical properties compared with porcelain
Thus, problems occur in contaminated and glass insulators. In 1970, the first
environments where channelling of the glass generation of commercial polymeric
surface, particularly the undersides, insulators was introduced [34, 35]. With this
undermines the mechanical strength of the inception, developments of composite or
unit [33]. non-ceramic insulators increased rapidly
The flashover performance of ceramic [36].
(glass and porcelain) insulators under Currently, polymeric insulators are being
polluted conditions is substantially reduced, used in various applications, which include
and a larger number of units is required housings, bushings, and simple insulators.
resulting in a heavy insulator string. This is due to the numerous advantages that
Therefore, to reduce the size of high voltage they have over porcelain and glass insulators,
apparatus systems, the use of polymeric namely, reduced weight, excellent electrical
insulators have been introduced. These strength, easy handling and transportation,
insulators are light in weight and have a reduced breakage, and improved resistance to
water-repelling surface, which ensures vandalism; recently, they also have improved
excellent performance in polluted contamination performance. Nonetheless,
environments. despite these advantages, they have a
limitation in the form of aging [37-39], that
4.3. Polymeric Insulator is, degradation, which has a negative
Due to the increased demands for electricity, influence on their performance and makes
it is becoming important to raise the them liable to damage.
capabilities of power transmission and
distribution networks, which has led 5. TYPES OF POLLUTION
industrialists to develop and design lighter Contaminated materials can be divided into
two groups:
60 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …
- When solid contamination with insoluble The type of pollution varies according to the
particles settles on the surface of the conditions of different types of pollution
insulators, this deposit becomes a conductor sources, which are as follows:
Desert - These areas naturally contain
when wet. This contamination can be well
solutes that dissolve in water. In these areas,
determined through measurement methods due to irregular rainfall and the type of
such as (ESDD / NSDD). Often related to pollution, self-cleaning is not very effective.
remote areas, deserts or areas with industrial Sandstorms and winds, even during the rainy
pollution. This type of pollution also occurs season, can contaminate the surface of
in a series of coastal areas that form a salt insulators.
layer on the insulator surface and is Beaches - Polluted bill in these areas
contains solutes with a high solubility rate in
moistened by dew, moisture, fog or drizzle.
water and very small amounts of insoluble
This type of pollution is classified into two matter. And over a longer period can occur
groups: through the deposition of salt particles caused
Soluble contamination: The media layer by wind.
is formed on the surface of the insulator. The Industrial - The contaminated layer may
soluble contamination itself is divided into contain conductive particles such as coal,
highly soluble solutes, and low soluble metal particles and soluble particles such as
solutes. Soluble contamination is measured SOX, NOX in gases from industrial
by the ESDD salt deposition density method installations or particles that are difficult to
in mg / cm2. dissolve such as cement and gypsum.
Insoluble contamination: In this type of Agriculture - Pollution layer in these
contamination, the contaminants form a solid areas includes various solutes with fast or
layer for the media layer. Examples of this slow dissolution such as chemicals used for
type of pollution are dust, sand, soil, and oil. chemical products, bird droppings or solutes
Insoluble contamination is measured in the soil [42,43].
regarding NSDD insoluble sediment density
in mg / cm2. Insoluble contamination may 6. RESULTS PD AND FLASHOVER
include conductive contamination (such as EXPERIMENTS
metal particles).
- When the liquid electrolyte settles on the To investigate the hypothesis discussed, a
surface of the insulators with very fine downscaled insulator testing unit inside a fog
particles or insoluble particles. These chamber was built at the University of
contaminants can be well determined through Manitoba's McMath High Voltage lab. By
measurement methods such as conduction or adjusting the environmental parameters
output current. This type of pollution is (temperature and humidity) and injecting free
mostly related to coastal areas where saline charges to the surface of the insulator, we
or conductive water settles on the surface of tired to recreate an environment, similar to
insulators. In some areas, both types of that at the substation, when we observed
contamination may appear on the surface of partial discharge [44]. Ultra-sonic and heated
insulators [40,41]. humidifiers were used to adjust the relative
humidity levels and the temperature inside
5.1. Material of Pollution the fog chamber. A needle corona ring was
Signal Processing and Renewable Energy, March 2021 61

TABLE 2. Details of the insulators samples used for PD and ash-over experiments.

Insulator Outer Core Total Outer Material

Type material Material Diameter Thickness
Fiber glass reinforced
FRP hollow/foam filled 3.2 cm 3.0 mm
epoxy resin
Fiber glass Reinforced
SIR Silicone rubber 3.0 cm 3.0 mm
epoxy resin

used to apply the load to the surface of the the grooves of two smooth round metal
test specimen [45]. Free of charge for electrodes on a stick. Figure 9 shows the
simulating and modeling was the cost of the laboratory equipment used to measure
HVDC transmission line, which was leakage current and the electric field around
captured and trapped at the insulation level. the test unit.
Copper strips and two sharp objects are Figure 10 shows the circuit diagram of
introduced to create a triple points connection the laboratory setup used for investigating the
between air, insulation, and simulated partial discharge performance of different
electrodes for modeling the joint corona, insulation samples under different conditions.
created by the sharp edge of the anvil. The Two 150kV, 12mA Glassman HVDC
details of the experimental setup,
methodology and procedure are
demonstrated and discussed in the following
sections [46].

6.1. Experimental Setup

Fiber glass reinforced epoxy resin (FRP),
Silicone rubber (SIR) insulator samples were
used in the experiments. The samples were
all 13 cm long. The details of each sample are
shown in Table 2.
The FRP sample is from a Chance
universal hot stick tool. The silicone samples
are from silicone covered FRP rods for use as
a modular system of suspension type
insulators. The relative permittivity of FRP
and silicone rubber was assumed as 4.0 and
3.5. Below in Figure 8, a picture of the
samples is shown.
The effective length of the sample is 11 Fig. 8. Insulator samples used in PD and
cm, because 1 cm on each side is placed in flashover experiments.
62 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …

Fig. 9. PD and flashover experiments setups.

Fig. 10. PD and flashover test circuit diagram.

sources are used separately to generate current transducers are installed separately on
positive and negative DC voltage. A 300 kΩ the system's ground wire to monitor leakage
resistor is connected in series with the sample current [47,48]. One is a CT-C1-S wideband
to limit the current of the capacitors during current transformer; the other is a customized
flashover. PDIV voltage is controlled by a Rogowski Coil. A TEV sensor from HVPD
multimeter by dividing the resistance- with 115mV/V sensitivity is installed on the
capacitive voltage of the mixture from a ratio ground to monitor the electric field distributions.
of 1000: 1 and a maximum of 225 kV with a The oscilloscope was programmed in a
measurement accuracy of 0.1% DC. Two controlled memory mode with a 10G sample
Signal Processing and Renewable Energy, March 2021 63
per second acquisition rate to record the were converted into absolute humidity and
signals from the CT, Rogowski Coil, and the used to keep all test conditions the same [51-
PD sensor. Each segment is 1μs long, 53].
including 400ns pre-triggered and 600ns
post-triggered data. The oscilloscope trigger 6.1.2 Methodology Used for Surface
was programmed at the falling edge at -270 Charging
mV [47]. Two different charging methods were
investigated to investigate the load
6.1.1. Methodology Testing for Under characteristics in insulated samples. The first
Humidity Conditions
method creates spatial loads in the
A fog chamber made by a transparent plastic surrounding air for the first time through a
shield was introduced to simulate the humid corona-free discharge source called a needle
environment in the HVDC substation. By corona ring. This protected a 3D-published
changing the humidity inside the fog polyethylene frame on which 27 needles have
chamber, we were able to study PD and been mounted. Lengthy needles 6.4 cm at the
flashover characteristics under different margin of the frame with a distance of 1 cm
humidity levels [49]. The PDIV and were distributed between both adjoining
breakdown voltage was expected to have needle tip. The needles were distributed
significant variation due to the influence of
asymmetrically across the patterned floor and
humidity on charge density and the electric
the separation distance among the needle tip
field around the energized electrode. The
and the sample surface became maintained
temperature, humidity, and pressure inside
the fog chamber were monitored by a throughout 5 mm inhibition. The two
BOSCHE BME280 sensor connected with an electrodes were held on the ground as shown
Arduino Utno microcontroller. Two in Figure 11,. The needle ring was tangled
humidifiers were controlled by the micro- and could be easily opened for installation
controller to maintain the humidity level and removal. The needle ring in Figure 4.5 is
inside the chamber [50]. The fog chamber shown closed and open on top of each other
was activated 20 minutes before the PD and [54].
flashover experiments and the humidity level The second charging method become
inside the chamber was set to 5% above the conducted to charge to the area alongside the
desired test value so that the humidi_er did sample surface through direct touch between
not need to operate during the test. After the the patterns and the DC voltage supply. This
experiment was complete, the chamber was method results inside the electrodes with DC
left open overnight to dissipate the ionized
voltage by way of giving energy to one of the
gas inside the chamber. The test was carried
electrodes, in order that the alternative
out and repeated on 3 different relative
electrode is stored in base mode. Charging on
humidity levels: dry conditions (20%RH),
normal room conditions (50% RH), and the floor of insulation utilizing draining
humid conditions (80% RH). Since the corona, from close to-strength gadgets or
temperature inside the lab was different direct contact with electricity gadgets,
during the tests, the relative humidity levels
64 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …
enables load-precipitation to manage at the
insulation surface [55,56].
Copper strips and two distorted nails
were introduced to create a triple-point
junction between air, the insulator and the
electrodes. This setup is shown in Figure 13.
Tests were carried out without copper strips,
with copper strips at both ends, with copper
only at the ground or energized side and with
copper strips and nails at both ends [57].
(a) Corona Ring Closed
6.2. Experiment Procedure

6.2.1. PD and Flashover Test Under

Humid Condition

All samples had been cleaned before the test

to make certain that the surface become easy,
dry, and free of any strain. The simplest way
to neutralize the weight is to soak a saturated
fabric in distilled water, and wrap it across
the pattern for two min. FRP and SIR samples
are then washed with isopropyl alcohol to
clean the surface to ensure that no (b) Corona Ring Open
contaminants or conductive residues stay on Fig. 12. Corona ring of needles used for
it [49]. applying non-contact space charge.

The samples to be tested were mounted

on the wooden structure, as per Section 6.1.
Two identical 0.5 cm wide copper strips with
bent nails were attached to both ends of the
sample. The test structure was sealed inside
a transparent fog chamber with two ultra-
sonic humidiers inside. The temperature,
humidity, and pressure were monitor by a
BOSCHE BME280 sensor. The humidity test
was carried out and repeated at 3 different
relative humidity levels: 20%,50%, and 80%.
Fig. 11. Needles ring to introduce surface
After the relative humidity inside the
charges.
chamber reached the desired level, the circuit
was groundless and the voltage increased at
Signal Processing and Renewable Energy, March 2021 65
an approximate speed of 500V/s until a at negative 500mv on the falling edge with
partial discharge occurred. The PDIV was segment windows of 100 μs (40 μs pre-
recorded with a multimeter while the leakage triggered and 60us post-triggered data). The
current from the sample to the ground and the tests terminated after 10 flashovers occurred.
associated electric field were continuously Samples were replaced if there was any
monitored on an oscilloscope. Next, we significant damage on the surface.
continued raising the voltage at the same rate Degradation of the FPR samples was
until 15 to 20kV above the PDIV. When the experienced after 4 to 5 tests (40 - 50
PD appearance frequency was significantly flashovers) [49].
higher than regular conditions, we began
recording the leakage current from the 6.2.2. PD and Flashover Test with Surface
sample to the ground and its associated Charge
electric field. The oscilloscope was
All samples were free of floor fees earlier
programmed to use segment memory mode
than going for walks during the experiments.
and triggered at negative 270 mv on the
This is done by removing the pattern of a
falling edge, with a 10G samples per second
piece of moistened material (distilled water)
acquisition rate. Each segment was 1 μs long
for two minutes by eliminating any residual
include with 400ns pre-triggered and 600ns
storage costs, and then wiping the surface
post-triggered data. Typically, we recorded
with isopropyl alcohol. Extra care became
128 to 256 segments before we moved to the
given now not to rub the floor because the
flashover test [51].
friction from wiping can cause significant
After the PD test, we kept raising the
floor price [55]. Alternatively, the surface
voltage at the same rate until the flashover
was rinsed and dabbed gently. The ring of
occurred.
needles became established symmetrically
The flashover leakage current was
around the pattern in the desired place, then
recorded on the same oscilloscope, triggered
the needles have been energized with a 20kV
DC voltage of each polarity. The length of the
implemented voltage to the needles was 10
minutes. Afterward, the voltage supply
become became off, grounded, and the ring
of needles changed into cautiously eliminated
- without touching the insulator. After that,
the voltage source became the base, and the
needle loop was carefully removed without
touching the insulation. After injecting a
space charge using a needle loop, the
flashover circuit was groundless and the
Fig. 13. SIR Sample with Pre-stressing setup. voltage increased at an approximate speed of
500V/s until a partial discharge occurred. The
PDIV, PD signal recording procedure
66 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …
followed the same steps as per Section 4.2.1 dielectric barrier, it replaces the floor fee that
[56]. is opposite to the discharge area, reduces the
ionization price, and as a result, shuts off the
6.3. Simulating Partial Discharge Activity discharge [58].
To simulate PD pastime, you need data
In fashionable, two situations need to be
about the availability of seed load and electric
frequented so that it will start the evacuation.
area in a disorder. Consequently, it's far
First, there is an electric powered discipline
necessary to provide an easy pattern of how
high sufficient in a huge sufficient region.
discharge influences these processes. PD
Second, an preliminary seed electron is to be
pastime models generally present the
had. The electric field of the electron hurries
subsequent simplification assumptions:
up the seed to such a volume that impact
1. Despite the fact that the onset and
collision ionization can take region. Collision
release of the release rely upon on the
ionization is the method through which a
strength of the electrical area over a given
high-power electron strikes a neutral
region, it is also assumed that the electric area
molecule and ionizes it to supply one electron
inside the cavities in PD structures is sort of
and every other high-quality ion. If the
uniform, or that the PD is controlled as lots as
electrical discipline is excessive sufficient,
the electric area of an unmarried unit. If this
this system maintains as a chain response
quantity is greater than the primary
with the increasing quantity of charge kinds
components, it's far assumed that the electric
referred to as electron avalanches.
area is enough for discharge.
Townsend's evacuation is composed of a
sequence of electronic Bahmans. So, one can
2. PD is efficiently modeled as a section
be self-sustaining, secondary methods which
exchange in surface price density at an empty
include the release of the weight of the steel
boundary [58] or in a vacuum conductor and
surfaces are irradiation. If the compression of
works to lessen the intracranial electric
the wide variety of loaded species is high
powered field to a few residual iscipline [60].
sufficient, the nearby context produced by
3. Comparable assumptions are made
way of the rainfall species will begin to
about electron era methods, all of which
protect towards the internal avalanche, which
might be managed by means of a single
could cause a brand-new discharge
amount of electric area.
mechanism. This is due to the fact that even
The governing equations for the
though the inner avalanche protects itself, it
beginning subject, the residual subject, and
nonetheless reasons an increase outside that
the electron generation are offered in
area. If this discipline is high sufficient, it
Niemeyer's work. They have been largely
could produce an ionizing wave, generally
unchanged in the use of other authors and
referred to as a contemporary conductor. The
taken into consideration pd interest basically
forward velocity of the nozzle exceeds the
when simulated. In the subsequent sections,
speed of the electron drift, causing discharges
an evaluation of the origin of those equations
to occur within the nanosecond c program
language period. Whilst PD turns into a
Signal Processing and Renewable Energy, March 2021 67
is completed, and using these cash statistics using the fact that the experiment was
on their software in PD modeling is offered. performed at atmospheric pressure 𝑝 = 𝑝0 =
101325 Pa, it follows that
6.3.1. Initial Electric Field
Vspark = (C/ƤO)ƤL + D/ƤO½√Ƥ𝐿 (4)
PD pastime models use an inception field,
usually denoted as 𝐸inc, to determine whether Setting the inception field equal to the
or not the electrical subject is enough for a ‘sparkover’ field
discharge to take region. The inception 𝑉𝑠𝑝𝑎𝑟𝑘
Einc = (5)
subject for an air gap that is typically used 𝐿

inside the PD activity literature is added in it follows that the constants 𝐴 and 𝐵 in (2) can
[61]. be written as
𝐵
Einc = Aρ (1 + ) (2) C
A = Ƥ0 = 24.2 V Pa-1 m-1 (6)
(𝜌𝐿)½

where 𝐴 and 𝐵 are constants, 𝐿 is the gap D

B=Ƥ = 8.6 V Pa½ m½ (7)
length and 𝑝 is the air pressure. In spite of the 𝑂 ½𝐴

tremendous utilization of (2) it does not these values for A and B are provided in [63].
continually seem that its origins, consisting It is of interest to note that the values of 𝐴
of the theoretical and experimental evidence used in [64], used subsequently bya number
in the back of it, is well known. In this of authors, is 25.2 V Pa-1 m-1. This appears to
section, a derivation of (2) is provided, be a typographical error; however, this is not
including a summary of the original experi- of particular significance as the error it
intellectual investigations used to decide It introduces is approximately 5%.
and a dialogue of its applicability to PD From (2) it is clear that regardless of gap
structures. length 𝐿 the electric field must still exceed 𝐴𝑝
The empirical proof at the back of (2) is a in order for discharges to take place. In [65]
sequence of experiments undertaken within 𝐴𝑝 is referred to as a limiting field, more
the 1960s with the aid of researchers at the commonly it is defined as the critical field
college of Strathclyde [62]. Those 𝐸cr. Below the critical field, attachment
experiments consisted of measuring the processes dominate ionisation processes and
‘Sparkover’ (a time period synonymous with charge multiplication cannot take place. In
breakdown) voltage Of parallel plate [61] 𝐴 is rewritten as (𝐸/𝑝)cr to demonstrate
electrodes in the air at atmospheric pressure
that it is a constant of proportionality between
and humidity. The experimental records may
the critical field and the air pressure. As an
be outfitted by means of the Curve.
aside at atmospheric pressure 𝐸cr = (𝐸/𝑝)cr 𝑝
Vspark = CL + D√L (3) = 2.45 kV/mm which is a typical value for the
breakdown strength of long air gaps.
where 𝐶 = 2.449 kV/mm and 𝐷 = 2.09 It must be mentioned that the experiments
kV/mm. As the breakdown voltage is a in keeping with fashioned through
function of 𝑝𝐿 from Paschen's law, and Strathclyde, [62], have been conducted
68 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …
beneath a uniform field among metal (2) is derived for gaps containing air. It's
electrodes for air gaps among 3. Five cm to miles probably that discharge interest will
17 cm. unexpectedly get rid of the oxygen and form
Furthermore, the authors took remarkable by products on the void floor resulting in a
care to smooth the electrodes to hold them void containing usually nitrogen. It is argued
free from dust, and indeed discovered an in [68] that the gaseous products fashioned
unacceptable scatter whilst the electrodes due to discharge interest can have pretty
have been positioned in an open ecosystem, similar ionisation characteristics, even
ensuing in the measurements being executed though this isn't completely convincing.
in an enclosure. The ‘Sparkover’ voltage, Furthermore, (2) assumes that the electric
acquired in these conditions, isn't necessarily field inside a void is uniform. By way of
going to be applicable to PD experiments. merchandise on the void surface, along with
One simple motive is that the air gaps used oxalic acid crystals in LDPE [69], should
within the experiment are certainly too big. In produce high discipline factors main to
many PD experiments, voids are fabricated divergent fields.
with air gaps less than five mm; As turned into noted previously, PD
consequently, while using (2), a researcher is activity models interpret 𝐸inc as the minimal
performing a tremendous extrapolation out of electric powered subject required for a
the range of the statistics that its miles discharge to take area inside a disorder. This
derived from. can seem to correspond with the PD inception
Some other difficulty is that the voltage (PDIV), that's determined as the
Sparkover experiment has changed into voltage at which PD activity starts offevolved
finished among metal electrodes. In PD [67,70]. But 𝐸inc is used in models to simulate
experiments, voids are generally bounded PD hobby, in other words it's far the electric
through the insulation material. This fabric area required for disprices to take location for
may nicely have very exceptional secondary the duration of a length after the primary
emission characteristics to metals, which can discharge. The interpretation inside a PD
have an effect on the inception field. activity model is consequently that 𝐸inc is
Similarly, greater, the complexity of electric area that corresponds to the PD
fabricating voids in dielectric fabric might extinction voltage (PDEV), the minimal
also cause modifications in humidity and voltage required to sustain PD interest.
contaminate the void surface, and as Considering 𝐸inc as the electric discipline
previously discussed, (2) changed into corresponding to PDEV has been proven to
decided from cautiously controlled be pretty rea-sonable for a few experimental
experiments. Every other key factor is that records [70-72].
during [63-67], (2) is believed to hold over A possible purpose for why the PDIV
many tiers of PD interest, with the stress exceeds PDEV is that, in voids surrounded by
altered as a loose parameter such that the dielectric cloth, the mechanism through
model suits measured records. This isn't which seed price is available for the first PD
always specifically accurate, firstly because may also require a higher electric discipline.
Signal Processing and Renewable Energy, March 2021 69
This may no longer be the case for voids To clarify the angles of the effect of
bounded by way of metal electrodes, which contamination on the increase in partial
have been used to determine (2) because of discharge, a completely healthy and clean
their differing secondary emission insulator was first injected with voltage. In
characteristics. the next step, the contamination was sprayed
Discharges in PD experiments may with the above-mentioned compound on the
additionally and regularly take location in surface of the insulator and re-injection of
environments which differ from the careful voltage was performed [73,74].
managed situations used to decide (2). These For a healthy insulator in two cases
motives aren't sufficient to say that (2) can't mentioned, the results of the numbers
be used when analyzing PD systems. But it recorded on the oscilloscope are as shown in
must be realized that deviations from it are Tables 3 and 4.
not unexpected, and they have to be In addition, the creep current waveform
interpreted as an approximation relevant to recorded on the oscilloscope for these two
the initial stages of PD activity rather than a states was equal to Figures 14 and 15.
physical regulation [72]. Although the amplitude of the current in
these two modes are not very different, with
7. PRACTICAL STUDY OF THE precision in the revealed current shapes, it is
EFFECT OF CONTAMINATION ON easy to confirm the ohmmeter property of the
THE INCREASE OF PARTIAL observed voltage source on the insulator
DISCHARGE ON THE INSULATOR surface in the contaminated state. While the
SURFACE current waveform on the surface A is healthy,
The type of contamination used in this clean insulator is approximately 90 degrees
experiment was as follows: a mixture of 40 g pre-phase, but the amount of pre-phase in the
of annul and 20 g of salt dissolved in one liter creep current observed on the surface of a
of distilled water. This solution was contaminated healthy insulator is about 30
equivalent to light contamination on the degrees. The same behavior in both insulators
surface of the insulator. is the waveform which deviates from the

TABLE 3. Results of AC voltage injection in healthy and clean ceramic insulator mode.
Creep distance Creep flow Applied voltage Equipment
ploss(w) Equipment status
(cm) (uA) (ac kv) name
Dry and Two-piece
3 48 80 20 Healthy
clean insulator

TABLE 4. Results of AC voltage injection in a healthy and contaminated ceramic insulator.

Creep distance Creep flow Applied voltage Equipment
ploss(w) Equipment name
(cm) (uA) (ac kv) status
Wet and Two-iece
20 48 390 20 Healthy
pollution insulator
70 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …

Fig. 14. Leakage flow in dry and clean state on the surface of a healthy ceramic insulator.

Fig. 15. Wet and contaminated leakage current on the surface of a healthy ceramic insulator.

TABLE 5. Results of AC voltage injection in the case of ceramic insulators with partial fracture of the
umbrella and contaminated.
Creep distance Creep flow Applied Equipment
ploss(w) Equipment status
(cm) )uA( voltage (ac kv) name
Wet and Minor Two-piece
22.4 30.5 1120 20
pollution fracture insulator

sinusoidal state when dry. There are much Next, an insulator with a partial fracture
sharper distortions on it. The cause of this of the umbrella was examined. For this case,
event can be considered the shorter life and the contamination was first sprayed on the
more frequent partial discharge sparks in that surface of the insulator and then voltage was
state [75,76].
Signal Processing and Renewable Energy, March 2021 71
injected. The results obtained from the In the next case, a ceramic insulator with
oscilloscope are shown in Table 5. a major fracture in the umbrella, which has
As can be seen, the creep current in this increased the creep distance to 27.5 cm, was
case has increased sharply and almost tripled. injected with voltage and the results were
However, the creep interval has decreased by recorded in Table 6.
only 34%. The current waveform is A simulated image of a creep flow
completely deviated from the sinusoidal state waveform on the surface of a two-piece
and in 1/8, 8/3, 5/8 and of each cycle, large insulator with a low-fringed umbrella and
distortions are observed due to the proximity part of the lower edge of the insulator where
of the partial discharge to the complete the insulator needle is broken is shown in
discharge. Figure 17.
The impedance waveform can be felt on factor in determining the components of
the set in this case according to Figure 16. the creep current waveform is the creep
The average of the diagram is out of position, distance, which in this case is minimized
and a small width from the origin is observed from the insulator tested. However, the
in the whole diagram which indicates ohmic pollution conditions, which are fully
losses and ohmic resistance during discharge consistent with the environmental conditions
on the insulator surface with the above of the insulators installed on the power
conditions. distribution network, can greatly help

Fig. 16. Impedance waveform on the surface of a ceramic insulator with a contaminated partial
fracture.

TABLE 6. Results of AC voltage injection in the case of ceramic insulator with sensitive fracture of
the umbrella and contaminated.
Creep Creep Applied
distance flow voltage(ac Equipment
ploss(w) (cm) )uA( kv) Equipment status name
Wet and Perceived Two-piece
50 27.5 2500 20 pollution fracture insulator
72 Fadaei, Faghihi, Mohmmadnezhad. A Review of partial Discharge …

Fig. 17. Wet and contaminated leakage current on the surface of a ceramic insulator with a noticeable
fracture.

increase the growth of partial discharges in the creep distance can be significantly
severely defective insulators [75]. increased. It can be clearly proved that if the
In this case, when the creep distance has surface of the insulator is covered with wet
reached 55% of the standard value, in the contamination, if we skip many distortions
presence of wet contamination on the surface that are observed, we have reached a very
of the insulator, the current with a high linear resistance with a very good
approximation show the ohmic property for approximation [79-82].
the impedance observed on the insulator
surface. As the broken umbrella grows, the 8. CONCLUTION
impedance waveform moves toward the
Currently, the application of a type of
waveform of an ohmic resistor with a larger
insulation is widely considered that has good
true ohmic magnitude [76,77].
insulating properties, anti-fungal change
In the case of contamination on the
performance and resistance. It is heat
surface of these damaged insulators, even the
resistant but has an effective coefficient creep
current waveform deviates from the
distance, a simple production process, and a
symmetric state, and in the positive cycle, it
low cost. Although the transmission line is
travels more period and amplitude, which
more and more used by glass insulators and
does not correspond to the performance of a
composite insulators, Chinese insulators in
linear resistor, and the positive peak is
the short term cannot be replaced completely.
approximately 1.5 times the negative peak.
As it was observed, the main factor in
Is. In this case, the current and voltage
increasing the creep current on the insulator
waveform are almost in phase [78].
surface is its creep distance, but the
Although it was mentioned that the
occurrence of contamination on the insulator
impedance waveform on the insulator
surface has a significant effect on the growth
surface, especially when the insulator surface
of this increase. It also causes the observed
is clean and dry, is almost pure capacitor, but
impedance behavior to change from absolute
in the case of contamination and humidity,
Signal Processing and Renewable Energy, March 2021 73
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