AM1

Partial Discharge Measurement and Diagnosis of High Voltage

Assets

ARIJIT MONDAL

1
Slide 1

AM1 Arijit Mondal, 08-04-2020

 AGENDA

1. Basic of PD.
2. Different coupling methods for PD Measurement
3. What we see from PD testing in transformer, rotating machines,
cables & GIS?
4. Megger Solution from PD measurements.

2
 BASICS OF PD

3
 PD definition (IEC 60270)
 Partial discharge (PD):
localized electrical discharge that only partially bridges the
insulation between conductors and which can or can not occur
adjacent to a conductor

 PD normally develop in air gaps or on insulation surfaces

HV electrode

Defects

LV electrode

4
 Different Types Of Partial discharge
 Three basic types of PD
sources:

• Internal PD Internal
PD
• Surface PD
H
• Corona PD Surface V
PD HV

 Focus on
internal discharges
Corona PD
 Physics Behind Internal PD

 Field enhancement at defect site

 PD triggering process

 What happens during a PD?

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 Field enhancement at defect site (1)
 Thus, cavities are characterized by:

• a larger field (in general) than the field inside the

dielectric

• lower electrical strength than the dielectric

 Discharges can occur in cavities without breaking

the complete insulation system.

 That is why we have, partial discharges

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 Field enhancement at defect site(DISCHARGE
SEQUENCE)
• In practice, a cavity in a material is often nearly spherical, and for such cases the
internal field strength is.
For, εr >> εrc.

• E is in the average stress in the dielectric under an applied voltage Va. During the
operation when Vc reaches breakdown value V+ of the gap t then the cavity may
break down
• The sequence of breakdown under sinusoidal alternating voltage is illustrated in Figure

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 Field enhancement at defect site(DISCHARGE SEQUENCE)
• The dotted curve shows the voltage which would
appear across the cavity if it does not break down.
As Vc reaches the value V+, a discharge takes
place, the voltage Vc collapses and the gap
extinguishes.
• The voltage across the vcavity then starts
increasing again until it reaches V+ when a new
discharge occurs. Thus, 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 Vc across it
reaches V-.
• In process, groups of discharges originate from a
single cavity and give rise to positive and negative
current pulses on increasing and decreasing of the
voltage respectively.

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 Electron avalanche
Before collision After collision
+
Positive λ
ion

- Free - -
electron

Gas
Electrons
molecul
e
If the field is sufficient to give to the
electron the necessary kinetic energy
(accelerating it along the distance λ) to
liberate others electrons : we have an
electron avalanche. (Ei≥Einc)

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 Effects after DISCHARGE SEQUENCE
• When the gas in the cavity breaks down, the surface of the insulation, provide instantaneous
cathode and anode. Some of the electrons impinge upon 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. In addition, channels and pits are
formed which elongate the insulation by the ‘edge mechanism’.
• Additional chemical degradation may result from active discharge products, and hence the net
effect is a slow erosion of the material and a consequent reduction of the breakdown strength of
the solid insulation. When the discharge occurs on the insulation surface, the erosion takes place
initially over a comparatively large area.
• The erosion roughens the surface and slowly penetrates the insulation and later on it will
increase channel propagation and ‘tree-like’ growth through the insulation is formed.
• For practical application, it is important that the dielectric strength of a system does not
deteriorate significantly over a long period of time (years).

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 VARIOUS PD QUANTITIES
Apparent Charge (q):
The apparent charge q of a partial discharge is that charge which if injected instantaneously
between the terminals of the test object, would momentarily change the voltage between its
terminals by the same amount as the partial discharge itself and is expresses in pico
coulombs. Apparent charge q (as given in the new IEC Standard 60270) is not equal to the
amount of charge locally involved at the site of the discharge and which cannot be
measured directly.
Repetition Rate (n):
It is the average number of partial discharge pulses per second measured over a selected
time. In practice, only pulses above a specified magnitude or within a specified range of
magnitudes may be considered
Specified PD Magnitude:
It is the value the PD quantity stated in standards for the given test object at a specified
voltage

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 VARIOUS PD QUANTITIES
Partial Discharge Inception Voltage (Vi):
It is the lowest voltage at which PDs are observed in test arrangement,(in practice, lowest value at
which PD' magnitude becomes equal to or exceeds a specified low value) when the voltage applied, to
the object is gradually increased from a lower value at which no such discharges are observed.
Partial Discharge Extinction Voltage (Ve):
It is the lowest voltage at which no PDs are observed in the test arrangement, (in practice, reduced
below a specified value) when the voltage applied to the object is gradually decreased from a higher
value at which such discharges are observed.
Partial Discharge Test Voltage:
PD test voltage is a specified voltage, applied in a specified test procedure, during which the test object
should not exhibit partial discharges exceeding a specified magnitude
The Average Discharge Current (I):
It is the sum of the absolute values of the apparent charges during a certain time interval divided by
this time interval.
The Discharge Power p:
It is the average power fed into the terminals of the test object due to partial
discharges

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 The concept of PD pattern
 The information conveyed by a single PD event is
often synthesized in just two numbers:

– Phase (in relation

with the AC supply)
– Amplitude (mV or
pC)

 We need to evaluate some statistical indicator to

draw information from PD activity.

 In general, the PD pattern is evaluated to

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compress the information.

 The concept of PD pattern
 The PD pattern
represents the density of 40

discharges in the

currence
30

phase/magnitude plane

Frequency of oc
20

(third dimension). 10 240

220
200
180

el
160

nn
140

a
ch
120
240220 100

e
80

as
200180
160 60

Ph
 It is a 3D histogram
140 120 40
100 80
Magn 60 40 20
it ude c 20
h anne
l

 Usually it is represented
The polarity of
through a color map PD pulses is
also considered
in the pattern

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 Information conveyed through
the PD pattern
The PD pattern help us in distinguishing the different types of phenomena occurring
in a high voltage system
There are different kinds of discharges, some are harmful some are not.
Internal PD Surface discharge Corona discharges
can eventually trigger can induce insulation originated in the HV
electrical treeing tracking connections are not
harmful
HV
HV

HV

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 VARIOUS PD MEASUREMENT METHODS

The detection and measurement of discharges is based on the exchange of energy

transform during the discharge. These exchanges include:
− Electrical pulse currents (with some exceptions, i.e. some types of glow
discharges)
− Dielectric losses
− Electromagnetic radiation (light)
− Sound (noise)
− Increased gas pressure
− Chemical reactions
Therefore, discharge detection and measuring techniques may be based on the
observation of any of the above phenomena

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 Non-electrical methods of partial discharge
detection
Acoustic Detection:
Aural observations made in a room with low noise level may be used as a
means of detecting partial discharges.
The oldest and simplest method relies on listening to the acoustic noise from the discharge, the
‘hissing test’. This scheme has lower sensitivity and difficulties arise in distinguishing between
discharges and extraneous noise sources, particularly when tests are carried out on factory
premises.
Latest acoustical detection methods utilize ultrasonic transducers, which can be
used to localize the discharges
Visual or Optical Detection:
Visual observations can be carried out in a darkened room, after the eyes have
become adapted to the dark and, if necessary, with the aid of binoculars of large
aperture.
The use of optical techniques is limited to discharges within transparent media and
thus not applicable in most of the cases.
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 Non-electrical methods of partial discharge
detection

Chemical Detection:
The presence of partial discharges in oil or gas insulated apparatus
may be detected in some cases by the analysis of the decomposition
products dissolved in the oil or in the gas. These products accumulate
during prolonged operation, so chemical analysis may be applicable to
estimate the degradation, which has been caused by partial
discharges.

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 ELECTRICAL METHOD
The most frequently used and successful detection
methods are the electrical ones where new IEC Standard
is also related. These methods are aimed to separate the
impulse currents linked with partial discharges from any
other phenomena.
Electrical PD detection methods are based on the
appearance of a ‘PD pulse’ at the terminals of a test
object, which may be either a simple dielectric test
specimen for fundamental investigations or even a large
HV apparatus which has to undergo a PD test.
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 Different coupling methods for PD
Measurement

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 PD Measurement According To IEC-60270

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 PD Measurement According To IEC-60270

• Most PD measuring systems applied are integrated into the test circuit in accordance with schemes
which are taken from the new IEC Standard. Within these ‘straight detection circuits’, the coupling
device ‘CD’ with its input impedance ‘Zmi’ forms the input end of the measuring system
• The coupling capacitor Ck shall be of low inductance design and should exhibit a
sufficiently low level of partial discharges at the specified test voltage to allow the
measurement of the specified partial discharge magnitude.
• The high voltage supply shall have sufficiently low level of background noise to allow the specified
partial discharge magnitude to be measured at the specified test voltage. Impedance of a filter may be
introduced at high voltage to reduce background noise from the power supply.
• The main difference between these two types of PD detection circuits is related to the
way the measuring system is inserted into the circuit. Fig(a) shows the CD is at
ground potential and in series to the coupling capacitor Ck as it is usually done. Fig(b)
shows CD is in series with the test object Ca. Here, the stray capacitances of all elements of the high-
voltage side to ground potential will increase the value of Ck providing a somewhat higher sensitivity for
this. The disadvantage is the possibility of damage to the PD measuring system, if the test object fails.

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 How to measure PD-IEC 60270
Choosing the right filter

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 Tree-like identification of PD

3rd level (leaves):

apparatus specific defects

2nd level (branches):

characteristics of PD in internal voids (HV/LV)

1st level (root):

basic PD categories (internal, surface, corona)

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PD ON TRANSFORMERS
 Partial discharge sources in transformers

Sharp points and particles

• Fiber bridges in oil
• Surface and tracking discharge
• Gas inclusions
- Voids (bubbles in oil and solid)
- Delaminations (paper and solids)
• Humidity (indirect)

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 PD In OIL,Examples of PD pattern: Corona PD
Uncontaminated (dry) oil H2O contamination

Free metal particles

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29
 EVIDENCE OF PARTIAL DISCHARGE

The left photo shows an RBP bushing with massive traces of partial discharge (black lines).
The photo on the right shows the destruction inside a transformer
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 Contact PD,100MVA, 220/66KV

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COUPLING METHODS ON A TRANSFORMER FOR PD MEASUREMENT

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 PD measurement on HV bushings with
measuring tap
Tap 1

Tap 2

Tap 3

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 PD measurement using UHF sensor

I / Qiec E/H

IEC compliant PD
UHF PD measurement
measurement
IEC measurement UHF measurement

Dispersion Compensation current Electromagnetic field

Coupling Discrete capacitor Antenna
Frequency kHz – some MHz 100-2000 MHz
Calibration Small setups, low frequencies Magnitude and damping depends on
position of defect

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35
 HOW TO LOCALISE PD ON A TRANSFORMER
Why PD localization is necessary
 Reduction in insulating capability
is the most common reason for
transformer failures
 Partial discharges often occur
before insulation fails
 More effective planning of next
steps if precise fault location is
known

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 ACOUSTIC PD MEASUREMENT
When PD localization is
necessary
 Factory acceptance test
• PD occurs after standard
AC voltage tests on new or
repaired power transformers
• Focus on fast repair
 On-site measurement
• Typically after indication of
problems (e.g. if PD
measurement or gas-in-oil
analysis indicate PD)
• Focus on decision, if, how
and where to repair
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 Theory
Acoustic signal propagation from source to tank wall

sensor 2 sensor 1
r2(t) r1(t)

sensor 3
r3(t)
Τ2,s2
Τ3,s3
Τ1,s1
sensor i
ri(t)
x(t)
sensor 0
PD source
r0(t)
Τ0,s0
sensor n
rn(t) Τn,sn

Source: IEEE Std C57.127-2007 „IEEE Guide for the Detection and Location of Acoustic
Emissions from Partial Discharges in Oil-Immersed Power Transformers and Reactors”

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 Theory
PD signal propagation path
Sensor

Direct oil

Direct oil path

Steelwave path
Reflection path Steel Reflection
Noise

Winding PD

Transformer
tank

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 Theory
PD signal propagation speed

1600
1500
Velocity in m/s

1400
@ 150 kHz 150 kHz acoustic sensor AS150I
1300
1200
1100
1000
-40 0 40 80 120

Temperature in °C 75 kHz acoustic sensor AS75I;

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 PD MEASUREMENT ON ROTATING MACHINES

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 Introduction and overview of rotating
machines

winding
rotor
machine core
stator insulation
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 Introduction and overview of rotating
machines
Whenever a high effectiveness is required large synchronous generators are used

energy generation synchronous … 10 MW …

(power plant) generator 2 GW

HV motors asynchronous motor, … 1 MW ...

(mills, compressors) synchronous motor 10 MW...

standard-industry motors ... 1 kW ...

asynchronous motor
(pumps, fan, etc.) 10 MW 11 MVA hydro generator

Detail of a 50 MVA hydro generator 1000 MVA generator (Steam power plant)

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 Introduction and overview of rotating
machines
> Most common used due to its high operational safety and low maintenance

energy generation … 10 MW …
synchronous generator
(power plant) 2 GW

HV motors asynchronous motor, … 1 MW ...

(mills, compressors) synchronous motor 10 MW...

standard-industry motors ... 1 kW ...

asynchronous motor
(pumps, fan, etc.) 10 MW

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14 MVA motor in a steel factory
 Insulation of stator winding – Constructive
parts
Coil type winding Roebel bar type
winding
> Used for large motors > Used for large generators
> Coils connected in series

Source: R. Fischer, Elektrische Maschinen,

Carl Hanser Verlag 2001
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 Insulation of stator winding – Constructive
parts
Connection of the poles Stator winding
can be designed as: Packaging:

a. Loop windings
c. One-layer winding
> Low voltages and high rotating speed
b. Wave windings
d. Two-layer winding
> High voltages and low rotating speed
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 Insulation of stator winding – General
A. Covering tape
B. Spacer, coil-end bracin
C. Groundwall insulation,
mica tapes
D. Grading/ silicon carbide coating
E. Slot semiconductive coating
F. Inner semicon coating
G. Turn insulation
H. Slot wedge / seal
I. Stator core
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 Insulation of stator winding – General
The insulation is compound of: (Si,Al)2O5
MICA (Silicate):
Al or Mg INSULATO
> Mica • Muscovite (Si,Al)2
R
O5
potassium ion CONDUCTOR
> Resin • Phlogopite
(Si,Al)2
O5
Al or Mg INSULATO
> Polyethylene (Si,Al)2 R
O5

Resin Mica flakes

Polyethylen film
Coating
MICA tape Cavities

Release paper
Coating
Polyethylen film

Conductor
PD treeing:
• Tree cannot pass the barrier

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 PD measurement
New stator bar: After 350 cycles:

After 750 cycles: After 1250 cycles:

source: M. Farahani, H. Borsi; Zustandsdiagnose der Isolierung von rotierenden

Hochspannungsmaschinen durch dielektrische Antwortmessung im Frequenzbereich. ETG Köln, (2004)
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 PD SOURCES IN ROTATING MACHINES
 Voids/delaminations:
 internal cavities in the groundwall
insulation system
Embedded delamination
 Detachments between mica foils inside
the insulation.
Flat voids embedded in the insulation.
Particulary dangerous since the air
interface decreases the thermal
conductivity of the insulation system
(overheating, hot spots).
 New hydro generator bar
 Delamination in the mica insulation

Source: Brütsch et al. "Insulation Failure Mechanisms of Power Generators", DEIS July/August 2008

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 Hydro generator bar after 40 years of service

Source: Brütsch et al. "Insulation Failure Mechanisms of Power Generators", DEIS July/August 2008

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 Aged mica insulation
 The insulation contains large voids and delaminations

Source: Brütsch et al. "Insulation Failure Mechanisms of Power Generators", DEIS July/August 2008

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 PD SOURCES IN A ROTATING MACHINES

 Surface PD in the stress grading region

 observable when
 the contact resistance between
stress grading and slot coating is
too large or there is no contact at
all
 Conductive pollution affects the
stress grading region (tracking)
 Slot PD
 Observable when slot coating is too
resistive or has been abraded away

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 Visual inspection
Overhang of a hydro generator

Contamination

PD tracking

Active part of a turbo generator

Quelle: Hoof, M.;Stephan, C.-E.; Diagnose des Isolationssystems elektrischer Maschinen
aktueller Stand und Entwicklungstendenzen. ETG Köln, (2004)

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 STRESS GRADING

At the junction
of
semiconductive
and stress
grading parts in
overhang

Dust due to stress grading PD

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 SLOT PD

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 Sparking in air gaps(Bar to ground)

 Sparking between bars and

pressure finger in the region
where bars/coils leave the iron
core
 Phase to ground voltage
PD
sparking
against
pressure
finger

58
 Bar to bar PD

 Easily recognizable: horizontal bars

in the pattern.
 Phase reference helps to
distinguish between B2BPD and
sparking:
 B2BPD: phase-to-phase voltage

 Sparking: phase-to-ground
voltage

59
 Damages of stator insulation systems

Quelle: Hoof, M.;Stephan, C.-E.; Diagnose des Isolationssystems elektrischer Maschinen

aktueller Stand und Entwicklungstendenzen. ETG Köln, (2004)

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 Online PD Tests: Bandwidth Issues
High Frequency PD energy Small PD detection bandwidth:
Low frequency
• Good sensitivity to “far” PD
1
• Similar sensitivity to “far” and “near”
PD (mixing)
L • Less sensitive to Xtalk and noise

Large PD detection bandwidth:

• Larger sensitivity to “near” PD
(most stressed coils)
Most stressed
coils • May be smaller sensitivity to “far”
PD (depend on lower cut off
frequency)
Windings are lossy transmission
• More sensitive to Xtalk
lines.
• Lower cutoff frequency and large
Semicon layers can attenuate
bandwidth is the best solution
strongly high frequency components
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COUPLING METHODS IN ROTATING MACHINES

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Coupling Capacitors

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 Online PD Tests: Coupler Issues
• Larger C values imply:
– Larger sensitivity (“abc” circuit)
• Larger sensitivity to the lowest frequency components of the PD
signal 1e+1

Careful!
1e+0 Inadequate suppression at industrial freq.

Transfer function
1e-1

C 1e-2

1e-3 100 pF, 50  (fc=31 MHz)

1 nF, 50  (fc=3.1 MHz)
10 nF, 50  (fc=318 kHz)
R VPD 1e-4 1 nF 100 nF, 50  (fc=31.8 kHz)
1 uF, 50  (fc=3.18 kHz)

1e-5
0.0001 0.001 0.01 0.1 1 10 100 1000 10000

fcutoff 1/2RC
Frequency (MHz)

1 kHz 10 kHz
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 PD Measurement On Cables

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 PD MEASUREMENT ON CABLES

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 Coupling Methods For Cable PD Testing

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Onsite Cable Testing

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 PD Measurement On GIS

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 PD IN GIS

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 PD In GIS

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 GIS Internal Insulation Defect Type Diagram

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 PD IN GIS

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 PD Signal Propagation

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 PD Signal Propagation

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 PD Signal Propagation

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 UHF Sensors

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 UHF Sensors

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 UHF Sensors

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 Acoustic Measurement

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 Loose & Floating Elements

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 Moving Particles

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 Protrusion

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 Voids

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 PD MESUREMENT SYSTEM WE CAN OFFER

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1.For Laboratory Testing Purpose : ICM
System & ICM Compact .

2. For Oniste Testing : ICM System.

3. For PD localisation on Transformer : AIA

compact .

4.For Online continou Monitoring for

Transformer & rotating machines : ICM
MONITOR
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 ICM SYSTEM –High End Measuring System
• Advanced state of the art PD & TD
measurement and analysis tool
• High end signal pre- and postprocessing
• Highest modularity and robustness
• Simultaneous real time acquisition on up
to10 input channels
• Measurements under AC and DC
• Integrated acoustic PD location
functions
• Integrated cable fault location feature
• All in one measurement system
.

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 AIA COMPACT FEATURES
• Inputs: 4 or 12 channels (multiplexed)
• Each channel freely configurable
• Automatic sensor detection
• Acoustic freq. range: 20kHz – 200kHz
• UHF and HF frequency conversion:
100MHz – 1.8GHz -> 40kHz – 800kHz
2 – 20MHz -> -> 40kHz – 800kHz
• Interfacing via software available
• Line and ext. sync (10 - 520Hz)
• Speaker & headphone output
 GIS Solution –Portable Monitoring System

The GIS monitor is a portable unit for partial

discharge measurements on gas-insulated
switchgear (GIS) caused by hopping particles,
floating potentials, cracks in insulators or
spacers, or other degradation in the insulation
system. The instrument offers parallel real time
PD acquisition on up to eight channels. Any UHF
signal can be detected and digitized within micro
seconds. To eliminate disturbance signals from
the measurement, the instrument can be
connected to a disturbance antenna that
provides a gating signal

89
 Questions?
Power on
At Megger, we understand that keeping the power on is essential for the success
of your business. That is why we are dedicated to creating, designing and
manufacturing safe, reliable, easy-to-use portable test equipment backed by world-
leading support and expertise.

We can assist your acceptance, commissioning and maintenance testing for

predictive, diagnostic or routine purposes. By working closely with electrical
utilities, standards bodies and technical institutions, we contribute to the
dependability and advancement of the electrical supply industry.

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