Introduction to PD Testing
khaubner@doble.com
Mob: 0417 17 8026
What is a partial discharge?
• Partial discharges (PD) are localized electrical discharges
within an dielectric insulation system, restricted to only a
part of the dielectric material, thus only partially bridging the
electrodes. So the breakdowns stay local
• The insulation may consist of solid, liquid or gaseous
materials, or any combination.
• The term “partial discharge” includes a wide group of
electrical discharge phenomena.
• Materials are differently effected by PD but discharge activity
has in general detrimental effects on the insulation material
• Partial Discharge Measurement is sensitive to overall and
localised defects
HV
Internal discharges:
• In voids or cavities within solid or liquid
materials (incl. at boundaries of different
insulation materials)
• Continuous impact of discharges in solid
dielectrics forms discharge channels (treeing)
External discharges:
• Surface discharges: At boundaries of different
insulation materials ⇒ tracking
HV
• Corona discharges: Discharges in gaseous
dielectrics when strong, inhomogeneous fields
are present
Discharges due to electrically floating potentials
(related to gaseous dielectrics)
PD – wide group of discharge phenomena
HV
Reason for PD Ignition
• Aging processes, due to
– Electrical overstress
– Mechanical overstress
– Thermal overstress
• Incorrect assembly, manufacturing
defects
When should HV plant be tested for PD
• PD measurements should be performed at the factory to
ensure correct design and manufacturing quality.
• The On-site Commissioning tests and periodic PD tests to
ensure that no transport damage has occurred and that the
was been assembled correctly.
• The On line PD monitoring provides a continuous
surveillance of discharge activity for risk assessment of
the asset (trend analysis, warnings, “nursing” of suspect
equipment etc.
Almost all HV insulation systems can be adversely affected by PD:
- Rotating Machines
- Transformers
- Cables
-Switchgear
Components
Typical Locations of PD Ignition
Cavities, interfaces
of different dielectric
properties and at
sharp electrode
edges and
protrusions
In this Perspex block a
carbonised breakdown
channel is developing from
one electrode.
The material ahead is still
insulating and high
impedance. This prevents
any significant current flow
and the tree extends only
slowly, branch by branch.
But eventually it will break
through.
Close up of 11kv CW Pump
stator winding
Before and after lab ageing
Close-up of damage in previous slide. Here the PD have eroded away
the resin leaving dry glass cloth and sheets of mica-paper
Surface Effects
PD damage due to generation of Ozone
Ozone is a chemically reactive gas that combines with other gases in air
to create Nitric Acid
Why do PD occur?
• Practical dielectric insulation often contains voids or cavities
• The voids/cavities are usually filled with a medium (i.e. gas),
often with lower breakdown strength than the main
dielectric (the breakdown strength of air is ~100 times less
than many solids)
• The electric field in a composite dielectric is distributed
according to capacitances, and the dielectric permittivity of
the medium (gas) is usually lower than the solid. Thus, an
air-filled void will have a field stress enhancement related
to the dielectric/solid permittivity εr
Example – void in solid
Cc: Capacitance of void
Cb: Capacitance of solid in series
w/void
Ca: Capacitance of the rest of the
solid
Va: Applied voltage of solid
Vc: Voltage across void
V+/V-: Inception volt. for PD in void
Groups of discharges originate from
a single void and give rise to
current pulses (pos. and neg.)
Why do PD occur?
• If the voltage across the void is high enough (>the
inception voltage), the field stress in the void will exceed its
dielectric strength and the voltage across it collapses/breaks
down, i.e. a partial breakdown/discharge has occurred
• The solid dielectric in series with the void will withstand the
externally applied voltage and “choke off” the PD
• The PD will re-ignite if the voltage builds up to the inception
voltage again or on voltage reversal
• The PD breaks chemical bindings in the dielectric, the solid
erodes and gets ”thinner” and eventually fails
Detection of partial discharges
• Detection is based on the energy exchanges that take place
during the discharge
Dielectric Heat
Light
losses
Electromagnetic
Chemical changes,
radiation
gases
Impulse
current pulses
Sound/
noise
Macroscopic-Physical
Effects
HV
Detection Methods
Optical Effects
(Light)
Optical
Pressure Wave
(Sound)
Mechanical
Opto-acoustic
Discharge Effects
Dielectric Losses
High Frequency
Waves
Electrical
IEC 60270
Chemical Effects
Chemical
Heat
Acoustic
HF/VHF/UHF
1. Electrical transients: The flow of charge at the defect will cause
an equivalent charge transfer within the apparatus also reflected
in the external circuit ⇒ allows effective and calibratable
detection (IEC 60270)
2. Electromagnetic radiation: PD generates high frequency
electromagn. radiation up to 1 GHz)
•
VHF/UHF sensors (inductive/capacitive sensors)
•
Spectrum analyzers
•
Skin effect currents leaving dielectric through gaskets
TEV
3. Chemical changes: Measurements of by-products/gases
• DGA in dielectric liquids
• SF6-gas
• Ozone etc.
PD Detection Methods
4. Sound/noise/vibration: Tens of kHz regime
•
Contact probes/transducers to detect/locate
internal discharges (GIS, cable accessories,
transformers, switchgear)
•
Airborne/remote detection of corona and surface
discharges: Windings, cable terminations etc.
5. Light: Ultra violet cameras to locate PD (e.g. DayCor
Corona Camera)
6. Heat: Infrared cameras/scanning to detect more intense
PD
PD Detection Methods
PD Test Methods
• Off-Line, On-site, Out of service
–
–
–
–
VLF: Very Low Frequency ≈0.1Hz
OWTS: Oscillating Wave Test System
Resonant Test (10 - 400Hz)
Power Frequency 50/60Hz
• On-Line, In-service
– Normal working voltage
– Spot Test or Continuous Monitoring
– Wideband (100kHz to 400MHz)
PD Off-Line Detection Circuit
II. TE-Messprinzipien – Elektrische TE-Messung
ƒ Measurement Impedance in
Series mit Coupling Capacitor
ƒ Measurement Impedance in
Series mit Test Object
ƒ Bridge Circuit
ƒ Coupling via Bushing Tap
Measurements options according IEC 60270
II. TE-Messprinzipien – Elektrische TE-Messung
ƒ Calibration Circuit
Calibration of test Circuit
II. TE-Messprinzipien – Elektrische TE-Messung
ƒ Narrow Band PD-Measurement
- Bandwidth 9 kHz and 30 kHz
- Centre Frequency between 50 kHz und 10 MHz
ƒ Wideband PD-Measurement
- Bandwidth typically between 40 kHz und 400 kHz (IEC60270=
<1MHz)
- On-site sometimes higher measurement frequencies are used
(typically up to 20 MHz)
PD in the Frequency Domain
typical noise spectrum
on-site
frequency spectrum of
PD pulses
frequency
characteristics of PD
measuring systems
– Filtering
- frequency selective
filtering of sinusoidal
noise
Windowing
- Software or hardware
windowing of pulse
shaped periodical noise
Gating / Masking
– subtraction of pulse
shaped stochastical noise
Synchronous
measurements
Bridge measurement
Separation of pulses
based on individual
pulse characteristics
Interference sources & suppression techniques
Record discharge parameters associated with every
individual PD pulse. Data is analysed “instantly” to
produce results.
Digital PD detectors
Internal PD
– always dangerous
Void in
Insulation
Tree Growth
in insulation
Sharp, Irregular
surface on
conductor
‘Floating’ metalwork
near conductors
External PD –
dangerous depending on plant
Corona from sharp
objects at high voltage
Surface
Discharges
7x Types of Partial Discharges
Discharges from field
induced situations
Corona Discharges (point at HV)
Surface Discharges
• Phase Resolved Partial Discharge (PRPD) Pattern
Analysis
Delamination - thermal ageing often caused by thermal
stress
Pattern and distribution of pulses determines the
location of the voids e.g. inside the material or
delamination from the conductor
More Surface Discharges increase in test voltage, note
low magnitude, analysis by magnitude only can
lead to false conclusions
• Phase Resolved Partial Discharge
(PRPD) Pattern Analysis
Slot Discharges
Wedge
Stator Core
Conductor
Void
Slot
Ground
Insulation
Strands
Computer Assisted PD Recognition
Data base
structure is
open for
adaptation
by the user
Typical
waveforms
for PD
Types
On-Line PD
Testing – Now
a Field
Reality
Segment Waveforms
0.03
0.02
0.01
Main segment
Example of Typical Monopolar Cable
PD Pulse on PILC 33kV Cable (-ve pulse)
0
-0.01
-0.02
(Frequency Band: 200kHz to 4MHz)
-0.03
0
1
2
3
4
5
6
7
8
9
Time (uSec)
10
11
12
13
14
Main Waveform
0.006
(Frequency Band: 4MHz to over 100MHz)
0.004
0.002
Chan 1
Example of Typical High Frequency,
Oscillatory Switchgear PD Pulse
0
-0.002
-0.004
-0.006
0.05
0.1
0.15
Examples of Typical Noise Pulses
0.2
0.25
Time (uSec)
0.3
0.35
0.4
0.45
(Switching noise at 40KHz, RF noise at 600KHz)
Segment Waveform
Segment Waveforms
Volts (mV)
0.005
0.004
0.003
0.002
0.001
0
-0.001
-0.002
-0.003
-0.004
0
35
ent
2,000
1,500
1,000
500
0
-500
-1,000
-1,500
-2,000
40
45
50
55
60
Time (uSec)
65
70
75
80
1
2
3
4
5
6 7 8 9 10 11 12 13 14
Time uSec
Ch 1
Ch 2
Ch 3
Cable
PDGold©
data
On-LinePD
PDCursors
Testing –from
Now a
Field Reality
Segment view
Volts (mV)
25
20
15
10
5
0
-5
-10
-15
-20
-25
The PD magnitude
in picoCoulombs
(pCs) is the area
under the PD pulse.
2
Risetime
3
4
Time uSec
Pulse Width
5
6
Falltime
A v a ila b le W a v e f o r m D is p la y
0 .0 3 5
0 .0 3
Available Wavef orm Display
0 .0 2 5
0 .0 2
0 .0 1 5
0 .0 1
0.012
0.008
Chan 1
0 .0 0 5
0
- 0 .0 0 5
- 0 .0 1
Chan 1
0.004
- 0 .0 1 5
- 0 .0 2
- 0 .0 2 5
- 0 .0 3
0
-0.004
-0.008
Noise
-0.012
- 0 .0 3 5
-0.016
2
4
6
8
10
T im e ( m S e c )
12
14
16
18
Time (mSec)
A vailable Wavef orm Display
0.012
A vailable Wavef orm Display
0.016
0.008
0.012
0.008
0.004
0
Chan 1
0
-0.004
-0.004
-0.008
-0.012
-0.008
-0.016
Cable PD
-0.012
-0.02
Noise
-0.024
-0.028
Time (mSec)
A vailable Wavef orm Display
A vailable Wavef orm Display
Time (mSec)
0.028
0.032
0.028
0.024
0.02
0.024
0.02
0.016
0.016
0.012
0.008
0.004
0
0.012
0.008
Chan 1
Chan 1
Chan 1
0.004
-0.004
-0.008
-0.012
-0.016
-0.02
0.004
0
-0.004
-0.008
Cable PD
-0.024
-0.028
-0.012
Noise
-0.016
-0.02
Time (mSec)
Time (mSec)
Varying waveforms from a single 50Hz power cycle period
Example shows cable and switchgear PD events plus exciter noise on two channels.
PD Severity
• New equipment, 5pC typical in IEC standards –
Apparent charge measured Off-line
• On-Line results less clear
– No direct electrical connection of sensors
– Wideband detection
– Calibration difficult to achieve without an outage
Na tiona l Ge rma n (DIN VDE)
Limit va lue s for
Volta ge
PD le ve l
HV Appa ra tus
Te st Sta nda rd
Bushings
Ca pa citors
Ca ble s
Inte rna tiona l (IEC)
Publica tion
Re ma rks
300 pC (oil)
10 pC (GH)
10 pC (HP)
IEC 137 (1984)
(oil) Oil impregnated
(GH) Cast resin impregnated
(HP) Hard laminated paper
IEC 358 (1990)
Identical
IEC 885-2 (1987)
IEC 885-3 (1988)
Test procedure
Test procedure
DIN VDE 0674
Part 99/12.92
1.05 U / √ 3
1.5 U / √ 3
DIN VDE 0360
(1.1 Um )
(100 pC)
Part 3 A 1/08.83
1.1 Um / √ 3
10 pC
DIN VDE 0472
Part 513/07.82
DIN VDE 0271
/06.86
2 U0
20 pC (PVC)
IEC 840 (1988)
1.5 U0: 10 pC (VPE)
DIN VDE 0273
2 U0
5 pC (VPE)
IEC 502 (1994)
1.5 U0: 20 pC (VPE)
2 U0
5 pC (VPE)
1.5 U0: 40 pC (PVC)
/12.87
DIN VDE 0263
/02.91
Ca ble Joints
CT + PT
DIN VDE 0278
Part 1/02.91
Test procedure
DIN VDE 0278
Part 2/02.91
2 U0
20 pC (VPE)
40 pC (PVC)
Joints, Terminations
DIN VDE 0278
Part 6/02.91
2 U0
20 pC
Pluggable and screwable
encapsulated cable terminations
DIN VDE 0414
(1.1 Um )
10 pC (liquid)
Part 10/05.85
1.1 Um / √ 3
50 pC (solid)
Tra nsforme rs a nd
DIN VDE 0532
1.3 Um
300 pC
Re a ctors
Part 3/07.87
1.5 Um / √ 3
500 pC
Dry Type
Tra nsforme rs
DIN VDE 0532
Part 6/01.94
1.1 Um / √ 3
Ta p Sw itch for
Tra nsforme rs
DIN VDE 0532
Part 3/04.93
Insula te d Sw itchge a rs
up to 38 kV
Ga s-insula te d Sw itchge a rs
(GIS)
IEC 44-4 (1980)
Identical
(liquid) Liquid insulation
(solid) Solid insulation
IEC 76-3 (1980)
Identical
20 pC (GH)
IEC 76-3 (1982)
1.1 Um / √ 3
(GH) Cast resin impregnated
1.5 Um / √ 3
50 pC
IEC 214 (1989)
Identical
DIN VDE 0670
Part 7/09.88
1.1 U
1.1 U / √ 3
100 pC (HP)
10 pC (GH)
IEC 466 (1987)
Identical
DIN VDE 0670
Part 6/04.94
1.1 U
1.1 U / √ 3
IEC 298 (1990)
Harmonized
HD 18755
DIN VDE 0670
Part 8/02.94
1.1 U
1.1 U / √ 3
IEC 517 (1990)
Identical
IEC 664-1 (1992)
Test procedure technical identic
Low Volta ge Compone nts
DIN VDE 0110
Part 20/08.90
Optocouple rs
DIN VDE 0884
/08.87
1.6 UI OR M
10 pC (GH, solid)
5 pC
Table 1: Summary of national and international partial discharge test standards for HV apparatus, derived from IEC 270 respectively DIN VDE 0434