An Introduction to
Partial Discharge Testing of Cables
Advanced Solution for On-site Diagnosis, (c) SebaKMT 2009, all rights reserved
1
Introduction and Partial Discharge (PD)
PD is an important failure mode1. because it is a common failure mode
2. usually violent- a risk to staff safety.
Breakdown Voltage (V)
1 10
1 10
1 10
1 10
Paschen Curve for Air
6
5
4
3
100
1 10
3
0.01
0.1
1
10
pd product (BAR.mm)
100
3
1 10
PD a major cause of early failure for all HV insulation
Not only Cables!
Failed 132 kV termination
The core screen
terminationsomewhat
misplaced
Should be down
here
And where is the
porcelain housing
and oil?
5
So - What is a partial discharge breakdown?
This block of Perspex had
electrodes on two faceswith a needle as one: the
second a ground plane.
ground
There is a high Electro
mechanical stress in the
Perspex around the needle
tip.
It eventually leads to a void
at the needle tip, PD and
then local breakdown in the
Perspex.
The tree then starts.
Tree Growth – a model study in Perspex
The structure of the perspex will break
down to form free carbon. So the
branches will become carbonised, and
eventually more will form and extend
through the perspex as a tree.
The material ahead is still insulating and
high impedance. This prevents any
significant current flow in the carbon
track and a power follow-through is
prevented.
 Thus the discharges are low current events and only partial
breakdowns.
 Eventually the stress at the tips will become critical and a leader will
form the puncture track.
Tree Growth
Tree Growth – a model study in Perspex
PD Definition
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.
Partial Discharge
PD occurs when insulation defects exist which produce distorted and
enhanced electric field stress
Causes of PD in insulation system:
– Voids in epoxy resins, polymers, paper
– Bubbles in liquids/oils
– Metal depositions/irregularities/contaminants
– Electrodes and insulation surfaces
– Poor terminations/loose joints
 Can also arise through:
– Poor design and manufacture
– Damage of equipment
– Poor installation processes
– General “ageing” or deterioration of materials
– Lightening strikes, overloading
Partial Discharge
PD can exist for months to years before failure
PD depends on for example:
– electrical field stress
– shape/size/deterioration level of fault condition
– environment conditions - temp and humidity
– load/current
– mechanical vibration
PD can be trended to monitor transitions in
level and severity of degradation
Common Types of PD
Corona Discharge
Cavity Discharge
Surface Discharge
Loose Connection
Lamination Discharge
Treeing
Some PD Mechanisms
For voids, PD type may be
– Streamer discharge (vapour filled void)
– Townsend discharge (avalanches)
– Small micro discharges
– Glow discharges (low pressure, diffuse, atoms
radiate)
Surface discharge – often surface streamers,
PD “tracks” – charge interacts with surface
In oil - arcs/PD produce chemical by- products
– Acetylene, ethylene etc. (DGA analysis)
Void Discharges
 Void electrical discharges - 3mm spherical cavity in epoxy resin
streamer
diffuse
Morshuis (2005)
Nakao et al. (1998)
Summary PD
PD is a symptom of degradation
Once present – it dominates as it’s
own “inherent” stress degradation
mechanism
Precursor to complete insulation
failure and breakdown
PD Damage in Cables
Terminations & Joints
Faults in Joints & Terminations are responsible for
45% all cable faults
Partial discharge is the main failure mechanism and
is due to poor stress control
Caused by
• Bad construction with continuity of the core
screen not maintained into accessory
• Poor preparation of the semicon cutback
• Thermal cycling or water entering
• Cutting tool penetrating insulation- eg, knife cuts
• Mainly Workmanship issues
17
Stress relief capacitive cone and refractive types
Diagram Key:
E
D
1 – XLPE insulation
2 – Copper conductor
3 – Stress cone silicone rubber conductive insert, electrically floating
4 – Electric field equipotential lines (diagrammatic)
5 – Region of high electrical stress and surface damage
Voltage equipotential lines – stress cone electrically floating.
18
C
poor preparation
poor
preparation
erosion traces
poor preparation
Cable Termination Area
Terminations
1. Incorrect installation/ poor workmanship
2. Manufacturing issues – poor QA
Joints
PD in cable itself
Localised defects
They reflect local defects in the cable and may or may not be the result
of external or internal damage.
They typically result in Partial Discharges leading to a complete
breakdown.
They may also relate to local conversion of water trees into electrical
trees shortly before failure although the whole cable may not be
globally effected by water treeing.
Causes for PD in PILC
Dry insulation due to thermal load
or bad impregnation
Water ingress due to lead
sheath corrosion
higher field stress
carbonised PD tracks
Paper layers
several metres
Why do PD occur?
Conventional PD Measurement
 IEC60270 Standard (Updated Standard imminent)
 Measures integrated PD current pulse to provide a measure of charge
involved in a PD
 PD magnitude normally quoted in picoCoulombs
IEC 60270 Testing
The elements of a detection system are
Test power supply
AC, DAC, VLF
a calibrator injecting known charge into the sample
A coupling capacitor to provide compensating charge,
and isolate the measurement equipment
A measuring impedance or quadropole
A detector
IEC60270 Off-line PD testing
Blocking Impedance
Calibrator
Coupling Capacitor
HV-Source
The Source may be
AC 50/60Hz, DAC
(OWTS) or VLF but
the detection circuit
remains the same
Quadripole (if not integrated in CC)
PD Measuring System
Cable
Phase-Resolved PD (PRPD) Patterns
 Plot of cycle phase against PD magnitude
 Original CIGRE patterns still often used for initial fault recognition (Natrass
1986)
 3D f-q-N (and variations) often now used
Phase-Resolved PD (PRPD) Patterns
(Porzel 2003TU Ilmenau
Partial discharge diagnosis
New generation of OWTS M series, OWTS M 28 and OWTS M 60
OWTS M 28
OWTS M 60
OWTS system
Unique is how the
voltage is generated
DAC
Here is our Standard
PD Detector Circuit as
per IEC 60270
PD Location in cables – PD-mapping
Not only the magnitude, intensity, PD inception and extinction
voltage & phase resolved patterns are important but of upmost
importance is the Location of the PD activity .
The exact location of the PD source can be determined using the
well known TDR Time Domain Reflectometery principles.
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
reflection on far end
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
triggering of PD detector and reflection on near end
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
reflection on near end
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
reflection on far end
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
near end
cable
PD detector
source of PD
far end
Location of partial discharge in a cable
Direct Impulse
near end
Second Impulse
source of PD
cable
PD detector
time = 2 • distance of PD from far end • pulse velocity
distance from near end = cable length – distance from far end
far end
Location of partial discharge in a cable
Direct Impulse
near end
source of PD
cable
PD detector
time = 2 • cable length • pulse velocity
far end
Location of PD´s by the TDR Analysis
PD on first part
of the cable
Δt
t [µs]
PD in the middle
of the cable
Δt
t [µs]
PD at the end
of the cable
Δt
t [µs]
Δt =
time difference between 1st and 2nd reflection
Field measurement
PD-pattern @ U0
After performing TDR-analysis a PDmap is obtained.
PD mapping for U <= Uo, Uo = 12kV (RMS)
C:\Dokumente und Einstellungen\putterh\Desktop\OWTS Data\Sweden\EON ES\Maleras\Maleras sample 1\1\2008-05-26-19'53'11\
L1
3.000
L2
L3
2.800
2.600
2.400
2.200
2.000
PD [pC]
1.800
PD-mapping @ U0
1.600
1.400
1.200
1.000
- Three weak spots, 500m, 1000m and
3000m
800
600
400
- PDIV @ 0.5U0
200
0
0
200
400
600
800
1.000 1.200 1.400 1.600 1.800 2.000 2.200 2.400 2.600 2.800 3.000 3.200 3.400
Location [m]
Case Studies DAC/OWTS system
LEW PD fault location, L2 , 20 kV XLPE cable
Two detected faults, termination and splice
Discharges in
a joint
Number of PD events versus location
Case Studies DAC/OWTS system
High PD Level in
L1
L2 / L3
Joint at 200 m ( 208 m)
Joint at 365 m
cable length:
660m
cable type:
XLPE
year of installation: 1997
nom. Voltage:
12/20 kV
56
Case Studies DAC/OWTS system
incomplete shrinking
cable length:
660m
cable type:
XLPE
year of installation: 1997
nom. Voltage:
12/20 kV
57
Interpretation and weight factors
PD inception voltage Ui
< Operation voltage Uo
> Operation voltage Uo
PD extinction voltage Ue
< Operation voltage Uo
> Operation voltage Uo
PD value (Magnitude)
< typical values
> Typical values
Location in the cable
section
Cable insulation
Joints or termininations
PD Mappings
Local concentration
Distibuted along the cable
PD intensity
low
high
Typical PD trending and limiting values
Cable Element
Type
Trend / Limit
Paper
up to 10,000 pC
Insulation
Joints
PE /XLPE
< 20 pC
Oil
Insulation
> 10,000 pC
Oil /Resin Insulation
Silicone / EPR Insulation
Oil
Termination
Terminations
Terminations
dry
Termination
Shrink-/
Slip-on Terminations
5,000 pC
500 to 1,000 pC
6,000 pC
3,500 pC
250 pC
On-Line PD detection
PD activity can in principle be also detected
on-line by connecting a clip-on HF-CT to the
cable screen although sensitivity is lower.
The Measurement
The LPD-Monitor:
•Can be applied in MV and HV range
•Is quick and easy to deploy, no outage
required
•Can measure 16 cables at a time
•Automatically evaluates the captured
data
•Differentiates PD from noise events and
classifies it into local and cable PD
LPD-Monitor
Partial Discharge Diagnosis (Online)
LPD-Monitor
Small PD-activity
Moderate PD-activity
Intensive PD-activity
Critical PD-activity
Table overview of
all 16 Channels.
Partial Discharge Diagnosis (Online)
LPD-Monitor
Partial Discharge Diagnosis (Online)
LPD-Monitor
On-line PD possible but with reduced sensitivity
Swgr
Panel
i+
iBlue Phase
i+
i-
iYellow Phase
i+
i-
iRed Phase
TEV
Sensor
i-
11kV Cable Box
(with split-core, high frequency current transformer (HFCT)
attached to earth bar)
HFCT around 11kV cables
(no earth connection)
HFCT’s around 33kV cable cores
(after earth screen has been ‘taken off’)
Available Waveform Display
0.012
0.008
0.004
Chan 1
Chan 1
Available Wavef orm Display
0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
-0.005
-0.01
-0.015
-0.02
-0.025
-0.03
-0.035
0
-0.004
-0.008
Noise
-0.012
-0.016
2
Available Waveform Display
4
6
8
Chan 1
10
Time (mSec)
12
14
16
Time (mSec)
18
Chan 1
Curs 1
Curs 1
Curs 2
Curs 2
0.012
Available Waveform 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
Noise
-0.02
-0.024
-0.028
Time (mSec)
Available Waveform Display
Curs 1
Curs 2
Chan 1
0.032
0.028
0.024
0.02
0.016
0.012
0.008
0.004
0
-0.004
-0.008
-0.012
-0.016
-0.02
-0.024
-0.028
Available Waveform Display
0.028
Chan 1
Time (mSec)
Curs 1
0.024
0.02
0.016
0.012
Chan 1
Chan 1
Chan 1
0.004
0.008
0.004
0
-0.004
-0.008
Cable PD
-0.012
Noise
-0.016
-0.02
Time (mSec)
Time (mSec)
Chan 1
Curs 1
Curs 2
Chan 1
Curs 1
Curs 2
Curs 2
Example shows cable and switchgear PD events plus exciter noise on two channels.
Questions?
Diagnostics,
today's knowledge,
tomorrow’s solution.