Partial Discharge Test - Switchgear

Predictive Maintenance of MV Switchgear with
Partial Discharge
R K Gupta
Principal Consulting Electrical Engineer (Power System)
Reliability Engineer
BE Electrical
Protection, Automation, PLC & SCADA, Level 3 MCSA, Colorado USA

IEC 60270 Section 3.1 Notes
“Partial discharges are in general a consequence of local electrical stress
concentrations in the insulation or on the surface of the insulation…”
..“Corona is a form of partial discharge that occurs in gaseous media around
conductors which are remote from solid or liquid insulation…”
..“Partial discharges are often accompanied by emission of sound, light, heat,
and chemical reactions…”
“Localized electrical discharge that only partially bridges the insulation
between conductors and which can or cannot occur adjacent to a conductor.”
Partial Discharge Test

IEEE Standard 400-2001 Notes
“Partial discharge measurement is an important method of assessing the
quality of the insulation of power cable system”
“A partial discharge is an electrical discharge (formation of a streamer or
arc) that does not bridge the entire space between two electrodes.”
“Partial discharges may occur in a “void…at a contaminant…or at the tip of
a well-developed water tree...”
Partial Discharge Test

Partial discharge Taxonomy
Three basic types of PD sources:
 Internal PD
 Surface PD
 Corona PD
Focus on
Internal Discharges

PD Inception Condition
 A free electron is available in the gas inside the void.
 Mechanisms that allow a free electron inside the gas:
a. Ionization by photon/gas molecule collision (background radiation)
b. Schottky emission of electrons from metal and /or dielectric surfaces
(electron injection).
 The external field, fE0, exceeds the inception field, Einc.
The free electron, accelerated by electric field, can trigger an electron
avalanche (PD).

Partial Discharge Process in Solid
Insulator /Air Insulator
 Solid insulators are manufactured to give an even distribution of electrical
stress between the conducting electrodes.
 Manufacturing processes invariably give rise to small cavities or voids in the
insulation bulk. These cavities are usually filled with a gas of lower breakdown
strength than the surrounding solid.
The permittivity of the gas is invariably lower than that of the solid insulation,
causing the field intensity in the cavity to be higher than that in the dielectric.
Therefore under the normal working stress of the insulation, the voltage across
the cavity may exceed the breakdown value and initiate electrical breakdown in
the void.

Equivalent circuit for cavity in insulator
A solid insulator of thickness d contains a
disc shaped cavity of thickness t and area
A, as shown fig. In the equivalent circuit
the capacitance Cc corresponds to the
cavity, Cb corresponds to the capacity of
the dielectric that is in series with Cc and
Ca is the capacitance of the rest of the
dielectric. Given that capacitance C, in
Farads/m2, is given by;
Where;
e0 = permittivity of free space = 8.854 x 10-12 Fm-
1
er = relative permittivity
A = area between electrodes
d = separation of electrodes

Breakdown Strength in Cavity
If we assume that the gas in the cavity (of thickness t) in figure 1 has a relative
permittivity of approximately 1, then:
And,
As Cb and Cc essentially form a capacitive divider, the voltage across the cavity,
Vc, can be expressed as;
Substituting into the above equation gives;

Electrical field strength across the cavity (Ec) is given by the equation
Given that in most circumstances t << d and er is greater than 1, it can be seen
that electrical stress in the cavity is greater than that in the surrounding
insulation. This, coupled with the fact that the breakdown strength of the gas is
likely to be significantly lower than that of the insulation, makes the gas in the
void liable to breakdown under normal working conditions.

From the equations above it can be seen that the voltage across the dielectric
at which discharge activity will initiate in the cavity, Vai, is given by;
In practice voids in solid insulators are very often approximately spherical.
In this case the field in the void is given by;
Where erc = relative permittivity of gas
in void.
When er >> erc this approximates to:

Relative permittivity and breakdown strengths of some typical high voltage
HV Insulating Material

Voltage and Current in discharging
Cavity
Each time a discharge occurs in the cavity,
charge is transferred from one side of the
cavity to the other until the potential
difference across the cavity is too small to
maintain the discharge. When the insulator
is subject to a sinusoidal alternating voltage,
charge builds up within the void as the
applied voltage increases or decreases. This
causes a series of discharges with charge
first moving in one direction, then the other.
Figure 2 shows how the voltage and current
across a cavity changes with applied voltage.

The dotted curve shows the voltage that would occur across the cavity if the
discharges did not equalize the potential difference across the cavity. As the
voltage Vc reaches the value V+, a discharge takes place and the, the voltage Vc
collapses and the discharge extinguishes. The voltage across the cavity then starts
again increasing until it reaches V+ , when a new discharge occurs. In this way
several discharges may take place during the rising part of the applied voltage.
Similarly, on decreasing the applied voltage the cavity discharges as the voltage
across it reaches V-, In this way groups of discharges are generated by a single
cavity and give rise to positive and negative current pulses on raising and
decreasing the applied voltage respectively.
Voltage and Current in discharging
Cavity

DEGRADING EFFECT IN SOLID
INSULATION
When the gas in a cavity breaks down, the opposite surfaces of the insulation
momentarily become cathode and anode. Some of the electrons hitting the
anode are sufficiently energetic to break the chemical bonds of the insulation
surface. Similarly, bombardment of the cathode by positive ions may cause
damage by increasing the surface temperature and produce local thermal
instability. These degrading effects form small channels and pits in the surface
that can elongate through the insulation. In addition to the ionic
bombardment, chemical degradation may result from active discharge
products, like O3 or NO2, formed in the air by the discharges. The net effect is
slow erosion of the insulating material and a very gradual increase in the size
of the cavity

Electrical Trees
 Electrical trees are comprised of a series of interconnecting channels or discharge
paths with diameters ranging from less than a micron to tens of microns.
 Discharge activity in voids will eventually become centered at particular sites
producing deep cavities in the surface. The cavities grow in length along the discharge
axis and the energy of discharge impinging on their tips increases. This combined with
electrical stress concentration by virtue of their point like form, produces increasingly
intense electrical fields at the tips of the discharging cavities.
 Eventually the breakdown strength of the material in the immediate vicinity of the
tip is exceeded. Breakdown follows with the evaporation, in the space of a few
nanoseconds, of a small volume of material.
 This rapid conversion launches small shock waves into the insulation. These waves
create, in time, a structure of fine cracks extending into the insulation. Their name
comes from the dendritic patterns they from in the insulation.

Electrical trees emanate from points of stress enhancement
in insulation. This can be a metal inclusion or a protrusion on
a conductor but in practice they more usually originate from
a void. The exact process by which electrical trees propagate
is still not fully understood, however, it is generally accepted
as being a combination of mechanical and thermal effects.
There are two clear stages in the development of electrical
trees under the application of an alternating voltage, the
inception period, which may be considerable and a much
shorter formative period. Eventually the tree will bridge the
insulation. Discharges continue to occur without breakdown
because space charge sets up a reverse field in the channels
to counter the field between the electrodes. During this
period the channels slowly widen. Eventually the field can no
longer be maintained in the widened channels and
catastrophic breakdown occurs, creating a very large channel
though the insulation.
Electrical Trees

Tracking
 Tracking is the formation of a permanent conducting path across an insulator surface.
Usually the conduction path results from degradation of the insulation. For tracking to occur
the insulation must be a carbon based compound.
 Most high voltage plant is situated outside. In industrial areas, insulators become
contaminated with pollution and dirt from the atmosphere. Where substations are situated
near the sea, salt very quickly covers the plant. In the presence of moisture, these
contaminating layers gives rise to leakage current over the insulator surface. This heats the
surface and through evaporation causes interruption in the moisture film. Large potential
differences are generated over the gaps in the moisture film and small sparks can bridge the
gaps. Heat from the sparks causes carbonization of the insulation and leads to the formation
of permanent carbon tracks on the surface.
Tracking as a phenomenon severely limits the use of organic insulators in outdoor
environments. The rate of tracking depends on the structure of the polymers and can be
significantly reduced by adding appropriate fillers to the polymer which inhibit carbonization.

CORONA
Corona is a partial discharge in regards that gas
breakdown begins at a position of high electric
field but dies out as the electric field decreases
very rapidly as a function of distance from the
highly stressed position. The breakdown can die
out for two reasons;
The region of high field is too small to generate
a fully formed breakdown channel.
The field falls to such a low value, that even a
fully formed breakdown channel cannot
propagate.

Corona forms in partially ionised regions adjacent to conductors and causes a change in the electric field between the
conductor and the ground. In effect, it can be seen as an extension of the conductor. As such, it will effectively reduce
the capacitance between the conductor and ground, as their separation decreases. This causes a drop in the voltage
on the conductor, a potential difference between the conductor and the voltage source and, therefore, a current flow
from the voltage source to the conductor. The electric field in a corona is sufficiently high that when a free electron
occurs, that electron will, on average, generate more than one additional electron (and positive ion). So a corona is
full of positive and negative ions (electrons).
Thus when the field reduces to the extent that the original current ceases to flow, the electric field does not
immediately return to its previously high value. Before that can happen, the positive and negative ions must flow in
the field toward the negative and positive electrodes respectively. As the negative charges are in the form of
electrons, they can propagate sufficiently fast to contribute to the measured partial discharge signal. However
massive positive ions flow so slowly that they typically generate a very small current over a long period of time.
Thus a corona can be thought of as generating a PD signal though three mechanisms. First, the ionisation of a
channel, which tends to look like an extension of the conductor and therefore increases the capacitance of the
conductor to ground. Second rapid migration of electrons toward the positive electrode in a system where negative
charge flows as electrons. And thirdly, flow of positive ions which tends to be too slow to be detected by most PD
measuring systems. The time scale for the first two phenomena is nanoseconds to microseconds, while that for the
third phenomena can be milliseconds or more.
CORONA

Corona tends to be repetitive, as once the region is cleared of charge, it returns to the
conditions which generated it in the first place. Corona in air is sensitive to air velocity and
environmental conditions which affect space charge near the conductor.
In many gases, including air, corona generated by positive and negative voltages differ
substantially. This is due to the physical difference between negative charge carriers
(electrons) and positive charge carriers (positive ions). Electrons being light and mobile
gain kinetic energy very rapidly from an electric field, while positive ions are heavy and
much less mobile. The outside surface of molecules is made up of electrons, so violent
phenomena, which dislodge charge from a molecule, free an electron and simultaneously
create a heavy positive ion. In corona from a negative conductor, electrons propagate
away from the conductor in the direction of corona growth. Thus they can create further
electrons through molecular collisions. In corona from a positive conductor, the electrons
propagate towards the conductor and away from the direction of corona growth. In this
case, electrons are generally detached ahead of the corona tip by photons generated
within the corona.
CORONA

Water Trees
A water tree is a bush or fan like structure developing like an electrical tree, from points of
stress enhancement. Water trees cause a reduction in the insulation's breakdown stress level
which encourages breakdown. Electrical trees can, on occasion, be initiated from a water tree
speeding the breakdown process.
Water trees are more diffuse than electrical trees and
generally grow at lower electrical stresses. Two types of
water tree have been recognized according to where the
tree initiates, ‘bow-tie’ trees and ‘vented’ trees.
Bow-tie trees are initiated in the bulk of the insulating
material, often from a void, and grow towards the
conducting screens. They clearly derive their name from
the pattern they form. Vented trees grow from one of
the conducting screens into the insulation bulk

PD Testing Methods
 Online Partial Discharge Testing
Ultrasonic
Transient Earth Voltage
High Frequency Current Transformer
Electrical Method (Inductive & Capacitive)
 Offline Partial Discharge Testing
Capacitance Voltage Divider( CVD) method

Ultrasonic
Electrical arcs in the air and corona effects emit sounds and ultrasounds. The basic
electrical problems that produce distinct ultrasound waves that can be detected
by Ultrasonic Testing include partial discharge, corona and tracking. Ultrasonic
measurement is most powerful on a comparative basis and will significantly
increase the reliability of correct detection of partial discharge.
Ranges of Ultrasonic detection
Center frequency 40 KHz
Bandwidth 38kHz-48kHz
Acoustic Airborne
Airborne acoustic (ultrasonic) radiation through air from corona and surface
discharges in the plant

Ultrasonic / Acoustic Airborne
Test Method
Background Noise Measurement
If any reading is made with the sensor flat on a surface (not over a vent or
air gap) this can be discounted as background noise.
AA/Ultrasonic Sensor Attachment Requirements
 There must be a clear air path (line of sight) from the sensor to the
discharge source, i.e. a vent or hole in the plant housing.
 Fully enclosed air insulated switchgear with no grills, vents, air gaps etc,
will not be suitable for Airborne Acoustic measurements.

Transient Earth Voltage
Transient Earth Voltage (TEV) PD signals
are generated by internal partial discharges
in switchgear, cable terminations, motors
and transformers. TEV signals are in a
higher frequency range of between 4MHz –
100MHz and are oscillatory in general. The
resultant PD signals are measured in dB
(decibels), as is the convention for on-line
s w i t c h g e a r t e s t i n g .

TEV measurements can be affected by background electrical noise in the
substation. Sources include:
Power electronic switching, e.g. from DC power supplies.
Corona from outdoor switchyards.
High frequency communication systems, e.g. two-way radios
TEV Test Method

The TEV sensor should be placed flat
against the metal-clad switchgear close to
vents or gaskets or seams on the metal-
clad housing. The LEDs will light to show
the measured PD level. The user MUST
hold the unit whilst in use.
It is recommended to place the TEV
sensor at multiple points on the plant for
example on the cable boxes and front and
back of each switchgear panel.
TEV Test Method

(HFCT) sensor which is clipped around
the earth strap of the cable. These
pulses are generally in the frequency
range of between 200kHz – 4MHz and
are typically mono polar in shape. The
unit measures the Cable PD pulses in
pico Coulombs (pC’s) by measuring the
charge content (area under the mono
polar pulse).

Procedure for HFCT PD Measurement
HFCT measurements can be affected by background electrical noise sources
inside and outside the substation. Sources include:
Radio frequency interference from local radio transmission
Corona from outdoor switchyards.
To measure the background noise level, attach the HFCT to the earthing
conduit of a de-energized feeder, or a nearby LV earth.

Electrical Method
Capacitive Sensor
The Capacitive Sensor measures the electric field on the cable and cable joint.
Sensors are designed to detects the quick variations of electrical fields caused by
PD. The probe is a coated metal plate that create a capacitor between the cable
and the plate. This capacitive coupler allows to read the PD activity on surface.
The PD signal is detected by a capacitive probe, then amplified by the
equipment in radio frequency bandwidth. The signal amplitude is proportional
to the discharge energy.

ULD 40 ( Ultrasonic Detector)
The ULD-40 is an ultrasonic detector designed for corona and arcing
inspections for predictive maintenance in electric utilities.
Main Applications
 Electrical Inspections: corona effect localization, arcs on
shields.
 General Mechanical Inspections: motors, compressors, gears,
bearing monitoring.
Gas, air, pressure leaks, leak detection without pressure or
vacuum.
Aerospace Sector: airplane doors and windows, air tightness

Detection of Electrical Arcs and Corona
Effects
Electrical arcs in the air and corona
effects emit sounds and ultrasounds. The
role of the ULD-40 consists of capturing
emitted ultrasounds and of translating
them into the audible range. The ULD-40
accurately pinpoints and identifies
corona effects and arcs that may be
encountered on any type of high voltage
installation simply by scanning around
the suspected area.

 Portable expert system for detection of partial discharges
 Increase your network reliability and workers security
XDP

 It is an instrument that enables to quantify the intensity of the high
frequency electric field signal emitted by the partial discharge of a
component, and to translate it into decibels (dB)
 A decibel (dB) is a unit that expresses the intensity of a signal in
relations to a reference on the logarithmic scale
XDP

With the XDP
 Follow up on ageing process of the critical components’ insulation
 Perform Quality Control of the insulators during installation or repair
 Safety control prior to work on energize equipment

XDP Advantages
 Portable
 Verification done under normal charge of the electric network – No
service interruption
 Quick readings
 Very sensible
 Pinpoints the problem
 Our 3rd generation of PD Detector

XDP Technical Specifications
 Peak measurement value in dB relative to 15 picoCoulomb
 Peak detection with a fast numeric circuit
 Fast numeric processor (DSP) for instant analysis results
 Wave’s shape inspection in high frequency to determine PD proximity

XDP Technical Specifications…
 LCD (Liquid Crystal Display) displays wave form
and dB reading
 Easy to read, big letter casings
 Upload/download capability
 PC (Windows) interface for PD readings
analysis/trending

 Dynamic range of 40 dB
 Up to 64 readings recording capability
 Bandwidth of 300 kHz to 70 MHz
 Sampling frequency of 30 MHz
 Sampling period of 16 cycles (50 or 60 HZ)
 Built-in Real Time Clock

 Keeps in memory up to 10 PD signatures for quick PD recognition
 Quick auto-verification procedure for high reliability
 8 hour battery autonomy
 The smaller PD Expert System on the market
 203 X 114 X 51 mm (8 X 4.5 X 2 in.)
 0,86 kg (1,9 lbs)

XDP Applications
 Cable & accessories
 Joint
 Elbow
 T
 Custom
 Switchgears
 Distribution Transformers
 Etc.

The Handling Rod
The use of the Spatula, which is a surface sensor, with the 61 cm (24 in.)
handling rod does not required direct access to the conductor

The Capacitive Sensor
 The Capacitive Sensor measures the electric field on the cable and
cable joint Our Capacitive Sensor uses a flexible Spatula easily
adaptable to the cable joint form

How it works
 The XDP detects the quick variations of electrical fields caused by PD
 The probe is a coated metal plate that create a capacitor between the
cable and the plate
 This capacitive coupler allows to read the PD activity on surface
 It is moved along joints and on cable insulation jacket, in order to
detect the presence of PD activity

 The PD signal is detected by a capacitive probe, then amplified by the XDP
in radio frequency bandwidth
 The signal amplitude is proportional to the discharge energy
 The energy is displayed on the XDP in dB (relative value)

How to use the Spatula Sensor
 Check the reference value on a regular basis to make sure that the XDP
works well
 Turn off the XDP
 Connect the reference module
 Connect the Spatula Sensor
 Place the sensor on the module in a way to make it cling around it
as much as possible

Check the reference value…
 Activate the XDP
 On the numeric display, you must read the displayed value on the sensor’s
case

 This technique will allow you to detect a PD activity even in an
environment with normal to high ambient noise (12 – 20 dB)
 Always adjust the spatula the way it fit as much as possible around the
cable and cable joint
 Always use the handling rod

Inductive Probe
 Applications
 Terminations
 Shielded cables
 Cable joints with grounding braid

Quality control of a 12 dB ambient noise
splice
 Without defect
 A = 10dB
 B = 12dB
 C = 12dB
 D = 10dB
 With defect
 A = 12dB
 B = 14dB
 C = 34dB
 D = 16dB

PD activities level classification
 Level 1: Free of Partial Discharge activity. No necessary action at this time.
Retest within 12 to 18 months (< 12dB)
 Level 2: Moderate levels of Partial Discharge activity. Retest within 1 year
(12 – 20dB)
 Level 3: High level of Partial Discharge activity. Repair or replacement
required (20 – 35dB)

PD activities level classification…
 Level 4: Very high level of Partial Discharge activity (> 35dB)
 We strongly recommend to restrict the access to the area
 Repair or replacement required as soon as possible

Partial Discharge Test - Switchgear

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