On-line PD testing & Diagnosis of MV & HV
Cables
Contents
Introduction to PD damage to cables
On-line PD Measurement
• Motivations
• Sensors and Measurement
• Location and Field Applications
• Monitoring
Case Studies
Conclusions
Cables
•
•
•
•
•
Paper (PILC, MIND etc.)
XLPE
EPR
PVC
3 Core and Single Core
• Cables
• Mixed cables
with transition joints
33kV XLPE 3-core cable
Right: 33kV PILC 3-core cable
Left: 11kV PILC ‘Belted; cable
33kV XLPE single-core armoured cable
PD Detection – Energies for Different Points in Cable System
Corona at metal contacts
Electrical charge RF Electromagnetic radiation
Acoustic
Ultraviolet
Ozone
Discharges on insulator surface
Electrical charge RF Electromagnetic radiation
Acoustic
Ultraviolet
Ozone
Partial discharge in termination insulation system
Electrical charge RF Electromagnetic radiation (local)
Acoustic (local)
Partial discharge in cable insulation or joint
Electrical charge RF Electromagnetic radiation (local)
Acoustic (local)
PD Damage – Incorrect Fitting of Stress Cone
Tracking on 115kV Termination (before
failure)
Failed 115kV
Termination (same
type)
PD Damage to Cables – Water Tree Conversion to
Electrical Tree in XLPE Cable
• Water trees turn into electrical trees immediately before failure
• PD testing will not detect early stages of water tree growth
Electrical
trees
Bow-tie
water
trees
On-line PD Spot-Testing
• Quick to perform
• Overview of PD activity
• MV and HV Cables, all insulation types
• Advanced diagnostic techniques for noisereduction
Continuous Monitoring
• Detect load varying and intermittent PD
(mostly on PILC)
• Detect incipient faults through changes in
pattern
• Temporary and permanent applications
PD Detection Theory
• HV Capacitor
– Placed in parallel with cable
at termination
– C = 500pF – 1nF (typically)
– C is high impedance to low
freq (i.e. 0.1 – 400 Hz) high
voltage applied to cable
– C is short circuit to high
frequency PD signals (i.e.
kHz/MHz range)
– Off-line only
Cable under test
Coupling Capacitor
Measurement
Impedance
Sensors
Sensor
HFCT
Attachment Point
Power cable earth strap/drain
wire or power cable with earth
strap/drain wire brought back
through sensor.
Current impulses from PD in
cables, cable terminations and
plant/switchgear cables are
terminated into.
Metal-clad plant housing close
to vents/seams/gaskets.
Electromagnetic radiation from
PD sites in plant that is induced
onto the plant metal housing.
Over vents in plant housing with
line of site to PD source.
Airborne acoustic (ultrasonic)
radiation from corona and
surface discharges in the plant.
TEV
Airborne
Acoustic
PD Detection Method
HFCT Sensor Attachment
Temporary Outside Cable Box
2
1
HFCT Attachment at Cross-bond Points
Circuit
Name
Cable
Type
Voltage
(kV)
Length
(m)
Return
Time
(μs)
Hotel
Maluri
Inn Taman
Maluri Y Phase
3c
XLPE
11
232
2.9
Test Date
27/07/08
Test
method
OWTS HV Capacitor
OWTS,
PD at
1.3U0
Cable under test
PD Map of Circuit
Hotel Maluri Inn - Taman Maluri - Y Phase
All
500
HFCT
450
HFCT
400
All Phases PD
OWTS Supply with
inbuilt HV Capacitor
Return
Propagation
Speed
(m/μs)
80
350
300
250
200
150
100
50
0
0
20
40
60
80
100
120
140
Location (meters)
160
180
200
220
240
On-line PD Detection Equipment
PD Detection Equipment and Methodology
Detect cable and local PD
Cable PD
• PD signals from power cable components
• HFCT detection
Local PD
• PD signals from nearby sources: cable terminations,
switchgear, transformer etc
• HFCT/TEV/Acoustic detection
Wideband Capture of PD signals, BW, 50-400MHz
• Capture all of the PD energy detected by sensor
• Capture simultaneously with power cycle and extract
impulsive signals
• Separation of PD and noise signals based on wave shape
• Continuous (RF, etc) noises removed with hardware
notch/high pass filters
Hardware Filtering/
Amplification
PD Sensors
Digitiser/DSO
Trigger signal
PD Data
Analysis
Software
HVPD On-line PD Technology
PD Detection
• HVPD-Longshot™ with PDGold© Software
• Wideband PD test system
PD Location
• HVPD Longshot™ with PDMap© Software
• Transponder Installed at far cable end
PD Monitoring
• HVPD Longshot™ (up to 48 hours)
• HVPD Mini™
On-line PD Signal Measurement
20 ms
Available Waveform Display
0.015
0.01
Chan 1
One power
cycle of raw
data from
HFCT sensor
0.005
0
-0.005
-0.01
-0.015
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
Time (mSec)
Chan 1
15 us
Segment Waveform
Curs 1
Curs 2
Segment Waveform
Segment Waveform
Ch 1
15
15
10
4
Volts (mV)
0
-5
Volts (mV)
10
5
5
0
-5
-10
-10
-15
-15
Ch 1
Ch 1
2
0
-2
-4
0
2
4
6
8
Time uSec
10
12
14
Cable PD Pulse: 16mV, 1160pC
0
2
4
6
8
Time uSec
10
12
14
Local PD Pulse: 15mV, 24dB
0
2
4
6
8
Time uSec
10
12
Noise Pulse: 6mV
14
Normal Distribution of All Events in a Single Power Cycle
1 Power Cycle
0.015
Noise Peak
Cable PD Peak
Chan 1 (V)
0.01
0.005
0
-0.005
-0.01
100
-0.015
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
Time (ms)
Segment Waveform
10
Volts (mV)
10
5
0
-5
-10
-15
0
2
4
6
Time uSec
8
10
Segment Waveform
15
1
10
4
6
8
10
12
Peak (mV)
14
16
18
20
Volts (mV)
Counts
15
5
0
-5
-10
-15
0
5
10
15
20
Time uSec
25
30
Noise and Interferences in On-line PD Measurements
Noise can be problem on-line
• Cables/plant not isolated from network
• Sensors often attached to ground
Noise sources
• Continuous: Radio/TV stations, communications
• Impulsive - Periodic
• Power electronics, e.g. inverter power supplies, UPS
• Rotating machine excitation
• Impulsive – Random
• Switching Events
• Corona
• PD on other cables/plant
Noise Source Summary
RF broadcast signals
overhead
lines
corona/
surface
discharge
outdoor
termination
switchgear bus
power
electronic
devices
corona/surface
discharge/RF
signals/PD in
outdoor
equipment
PD in
switchgear
switching
noise/
interference
switchgear earth
excitation signals/
PD from generator
PD from adjacent
feeders
noise from far
end of cable
PD from
transformers
substation earth
excitation signals/
PD from motor
G
Noise Discrimination – Hardware Filters
• Preferred over software filtering
• Remove interferences in HFCT pass-band
• Use in difficult measurement situations
Noise Discrimination – Hardware Filters
• Single Frequency RF Noise Reduction
Power Cycle
PD and noise detail
Noisy Data
Denoised Data
0.08
0.06
0.04
CH3 (V)
0.02
0.00
-0.02
-0.04
-0.06
-0.08
13.0m
13.0m
Time (s)
13.0m
13.1m
Noise Discrimination – Event Recognition
• Power Electronic Switching
0.015
Voltage (V)
0.01
Continuous amplitude
across power cycle
0.005
0
No phase pattern
-0.005
-0.01
-0.015
Segment Waveforms
2
4
6
8
10
12
Time (ms)
14
16
18
20
Main segment
0
Pulses appear as chain
16kHz repetition frequency
0.15
0.1
0.05
0
-0.05
-0.1
-0.15
0
20
40
60
80
Time (uSec)
100
120
140
PD Test and Location Applications
PD location at RMU
• Signals propagate between cables with
little attenuation on two feeder RMUs
• Reduced detection points on network – no
need to test every RMU
• Synchronous capture necessary to
determine source feeder
Measured PD pulse
Segment Waveform
Zoom of pulse start
Ch 3
Ch 4
100
50
Segment Waveform
0
-50
-100
150
C
C
100
-150
-200
0
1
2
3
4
5
6
Time uSec
7
8
9
10
Volts (mV)
Volts (mV)
200
150
50
0
-50
-100
-150
1
2
3
Time uSec
Example Test Configuration for PD Location on Circuit with RMUs
Section of Cable Undergoing Mapping Test
11kV
RMU
(Normally open)
HVPD-Longshot
RMU
RMU
(Normally closed)
(Normally closed)
Transponder
Cable Return Time, L for On-line Measurements
• Use from Off-line Tests
• Estimate from cable length and average
propagation speed for cable type
• Measure with on-line TDR (requires
impedance change at far end of cable)
PD Site Location,
  T  
PD%  1  
 100
  L 
Cables with Cross-Bonding
PD Propagation in Cross-bonded Cable Systems
• Cross-bond point is seen as change in impedance in
cable system
• Some of pulse propagates into cross-bond point and
onto all cables
• Some of pulse is reflected back down source cable
• Long-cross bond leads/coaxial cross-bond cables are
high impedance to HF PD signals
ZA
ZD
ZB
ZE
ZC
ZF
Cross-bond Propagation Example
Current
Transformers
Pulse on
outgoing side
of CB, L1
L1
Input signal,
L1
50Ω
L2
50Ω
50Ω
50Ω
Cross Bond Point
Chan 2
L3
Available Waveform 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
Chan 1
50Ω
Outgoing Side
Injection Side
Pulse Injection Test Set-up
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
-0.01
-0.02
-0.03
-0.04
-0.05
-0.06
-0.07
Outgoing scale
50% of
Injection scale
Reflection from
CB
0
1
Time (uSec)
Chan 1
Chan 2
Curs 1
2
Curs 2
Procedure for Testing Cross-bonded Cable System
Detection
• Sequentially test at terminations and all CB-points
N = 2 + number of joints
Test 1
Test 2
Test 3
Test N
Continuous PD Monitoring
Continuous PD Monitoring Aspects
Detect cyclic changes in activity
• Load varying activity on PILC cables
• Humidity related activity from surface discharges
Detect changes that relate to incipient faults
• Gradual rise
• Sudden rise
• Sudden drop
Carried out on: key circuits, circuits with suspected
cyclic PD changes, circuits with high spot-test
results
PD and Load Relations
CT PD Activity (Pc/Cycle)
• Although PD incepted by voltage, load can have effect
• Mostly on PILC cables
• Load variations
– Movement of oil/impregnant
– Expansion of conductors
Cumlative PD Activity (CT)
10,000
Time
T
F
F
06/01/2011
00:00
W
T
05/01/2011
00:00
T
W
04/01/2011
00:00
M
T
03/01/2011
00:00
S
M
02/01/2011
00:00
30/12/2010
00:00
S
S
31/12/2010
00:00
S
0
01/01/2011
00:00
5,000
Time
Lower Richmond
23/03/2010
00:00
21/03/2010
00:00
19/03/2010
00:00
Time
17/03/2010
00:00
15/03/2010
00:00
13/03/2010
00:00
11/03/2010
00:00
09/03/2010
00:00
06/03/2010
00:00
07/03/2010
00:00
CT PD Activity (Pc/Cycle)
23/03/2010
00:00
21/03/2010
00:00
19/03/2010
00:00
17/03/2010
00:00
15/03/2010
00:00
13/03/2010
00:00
11/03/2010
00:00
09/03/2010
00:00
06/03/2010
00:00
07/03/2010
00:00
CT PD Activity (Pc/Cycle)
Examples of Monitoring
• Simultaneous change in PD with monitors on 11kV network
Cumlative PD Activity (CT)
10,000
5,000
0
Bemish Road
Cumlative PD Activity (CT)
10,000
5,000
0
On-line PD Severity Evaluation
PD Level Guidelines
• To determine the true severity the PD is posing
to the cable, the following should be taken into
account
– Cable accessories (joints and terminations) have a
higher tolerance to PD than the cable insulation
– Mixed circuits: Paper/PILC has a much higher
tolerance to PD than XLPE
PD Level Guidelines
Example of Normalised Distribution of Off-line PD
Levels Measured in 33kV Paper and XLPE Cables
Mackinlay, R. & Walton, C.
Some advances in PD
monitoring for high voltage
cables
MV Paper Cables: Asset or
Liability?(Digest No.
1998/290), IEE Colloquium
on, 1998
Case Study: On-line PDMapping
TNB-D PPU – Central Spectrum
May 2006
PDGold Test: Central Spectrum
PD Magnitude (pC)
Cable PD
2,000
1,000
0
-1,000
-2,000
0
90 180 270 360
Phase of Pow er Cycle (deg)
Cable PD Segment Waveform
Volts (mV)
40
20
0
-20
-40
0
2
4
6
8
10
Time us
12
14
PDGold Test: Central Spectrum
PD Magnitude (pC)
Cable PD
2,000
1,000
0
-1,000
-2,000
0
90 180 270 360
Phase of Pow er Cycle (deg)
Cable PD Segment Waveform
Install transponder at Substation B due to lower noise level
20
Volts (mV)
•
10
0
-10
-20
0
2
4
6
8
10
Time us
12
14
Transponder at Central Spectrum
Detection
HFCT
Trigger
Unit
Pulse
Generator
Injection
HFCT
Voltage (mV)
Transponder Waveform Measured at PPU
Waveform data in Time
15
Chan 1 (mV
Cur 1
Cur 2
Cur 3
Cur 5
Transponder
Injected Pulse
10
Reflected Pulse
5
0
-5
Direct Pulse
Transponder
time delay
-10
-15
-20
0
10
20
30
40
50
60
Time (uSec)
70
80
90
PDMap Waveform
PD Map of Circuit
main substation to central spectrum
All
2,000
1,800
All Phases PD
1,600
1,400
1,200
1,000
800
600
400
200
0
0
•
500
1,000
1,500
2,000
2,500
Location (meters)
Result: 3x PD Sites found on Cable
3,000
3,500
4,000
Conclusions
Conclusions
• HFCT sensors allow sensitive
measurements to be made at multiple
points in the cable system
• On-line PD location can relatively quickly
identify weak points for timely remedial
action
• Solutions available for complex circuits
such as with cross-bonding