PD Monitoring of MV/HV Power Cables - Supergen … Monitoring of MV/HV Power Cables ... and a...
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Transcript of PD Monitoring of MV/HV Power Cables - Supergen … Monitoring of MV/HV Power Cables ... and a...
Introduction to HVPD Ltd
Introduction to HVPD – Our global presence
• HVPD are experts in the growing industry of on-line partial discharge (OLPD) condition monitoring and condition based maintenance (CBM) of high voltage networks.
• We supply portable and permanent OLPD surveying, diagnostic test and continuous monitoring solutions, and a complimentary range of on-site services, monitoring services and training.
• Over 400 customers in 100 countries trust our technology.
Contents
• Introduction to partial discharge in power cables
• Measurement equipment
• Continuous monitoring
• Partial discharge location methods
• Case studies
Partial Discharge Detection Theory
• PDs are incepted by the high voltage applied to cable.
• PD pulses are short duration impulses (ns – µs) that propagate in both directions away from PD site between cable core and sheath.
• Signals can be detected on both the core and earth screen at terminations.
End B End A
Available Waveform Display
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Time (mSec)20191817161514131211109876543210
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MV/HV Cable Faults - Causes and Effects
Common Causes
• Poor workmanship (at cable accessories)
• Mechanical damage caused by poor installation practices (including damage to cable sheath during cable pulling and minimum cable bend radius’ being exceeded)
• Poor quality or poorly manufactured cables and cable accessories
• Aging of insulation
Effects
• Electrical trees and interfacial surface tracking
• Localised heating/moisture ingress into the cable (caused by damage to the armour/outer sheath)
• ‘Infant Mortality’ and premature failure within the firs years of operation
PD Damage to HV Cable Transformer Terminations
Tracking on 110 kV Termination
(PD detected before failure)
Failed 110 kV Termination
(same type as opposite)
Insufficient Mastic Around Connector in 33 kV Joint
Trees on 66kV paper cable
Why and When to Perform PD Testing
At Manufacture
• Quality Assurance
• Type/routine tests, e.g. IEEE/IEC standard
At Commissioning
• To check for transport damage
• To ensure the installation has been made to a good standard, for example attachment of terminations and joints to power cables
Service Life • Detection of issues that emerge over time
• Condition Based Maintenance
Spot Test: Surveying vs Diagnostic Testing
• Survey - Detection – Identify equipment with PD in network
– Simple instrumentation – can be susceptible to noise
– Low level of training
– Usually only on-line
• Diagnostic/Location Testing – Detailed test result – more advanced instruments
– Higher level of training to perform
– Off-line and On-line
Use survey to identify equipment with PD and diagnostic testing only where PD is identified
Spot Testing vs Monitoring
• Spot Test – Snapshot of the condition
– Doesn’t take into account variations with operating stress – Labour resource to perform testing
• Monitoring – Continuous evaluation of the condition
– Detect variations with operating stresses (e.g. temperature/humidity)
– Less labour requirement after set-up
– Usually deployed on critical plant or plant with high PD in spot test
Test Equipment
• Detection – Simple handheld detectors, give
indication if PD detected and its level
• Diagnostic – More advanced detector, often with
PC software
– More information – type of PD, locations
• Monitoring – Portable or permanent logging of PD – Interface to plant control system
(SCADA)
Aspects of Testing MV and HV Cables
• MV
• Equipment at primary substation and ring main units
• Cross-bonding less common
• Some tolerance to PD activity
• HV
• Strong safety motivation for sealing ends
• Equipment and substations, cable termination, cable joints
• Very little tolerance to PD activity
PD Detection – Energies for Different Points in Cable System
Corona at metal contactsElectrical charge RF Electromagnetic radiationAcoustic UltravioletOzone
Discharges on insulator surfaceElectrical charge RF Electromagnetic radiationAcoustic UltravioletOzone
Partial discharge in termination insulation systemElectrical charge RF Electromagnetic radiation (local)Acoustic (local)
Partial discharge in cable insulation or jointElectrical charge RF Electromagnetic radiation (local)Acoustic (local)
High Frequency Current Transformer (HFCT) Sensors
• Detect PD in cables and connected plant
• Wide bandwidth (from 100 kHz to 20 MHz)
• Attach to power cables at terminations and earthing links of HV equipment
• Installation inside or outside of cable box
• Temporary or permanent
HFCT Sensor Attachment to Power Cables
The HFCT sensor should be attached to intercept either the conductor PD current (i+) or the earth PD current (i-)
HFCT on Earth (i-)
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1
2
HFCT on cable with Earth
brought back through (i+)
2
HFCT around cable
(i- + i+ = 0)
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HFCT Attachment at Cross-bond Points on 132 kV Cables
PD Against Phase for Power Cables – On-line Detection
Available Waveform Display
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Available Waveform Display
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Single Core Cable Three Core Belted Cable with HFCT on Common Screen
Modern PD Detection Systems
Hardware Filtering/Amplification
Digitiser/DSOPD Data Analysis Software
PD Sensors
Trigger signal
Indicated Condition
Indicated Condition
Noise Reduction Performed with the Kronos™ Software
Raw Data PD Event
Recognition Apply Expert
De-noising Rules De-noised PD Data
1,121,068 more noise pulses are rejected.
61,924 PD pulses (0.2%) correctly identified
HVPD implements the rules on the software
21,183,018 (100%) data
point acquired over 33,941
power cycles
The Kronos™ automatically (before training) recognises
1,182,992 (6%) data points as possible PD pulses
and rejects the remainder as noise
Data Analysis
Data before analysis/noise rejection
Data after analysis/noise rejection
Data Analysis
Continuous 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
Although PD incepted by voltage, load can have effect • Mostly on PILC cables • Load variations
– Movement of oil/impregnant – Expansion of conductors
S S M T W T F S S M T W T F
PD burns in a cold cable (90% of cases): fluid shrinks, voids appear, local PD in voids
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Prozess
Datum
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PD burns in a hot cable: electrodes expand - possible movements inside accessories lead to increased field strengths in dielectrics – PD in accessories
10kV PILC Cables
Examples of PD rises to Failure
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Number above threshold 1 (start date=12/07/99)
Time (Days)16014012010080604020
Cou
nts
abov
e th
resh
old
450,000
400,000
350,000
300,000
250,000
200,000
150,000
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50,000
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Peak, ave, and No above thresholds (start date=29/11/00 Chan=18)
Time (Days)6050403020100
Sig
nal l
evel
s (m
V)
0
360,000340,000320,000
300,000280,000260,000240,000220,000200,000180,000160,000140,000120,000
100,00080,00060,00040,00020,000
On-line Cable Mapping (PD Site Location)
• Cables act as waveguides for PD pulses and as low-pass filters.
• PD pulses attenuate and disperse as they travel down the cable
• PD sensor must have a good low frequency response to detect long distance PD.
• Increasing the number of test points gives more conclusive results.
• A study was carried out using ~500 pC calibration pulses injected into 20 km, 400 kV cable to determine attenuation and measurement range – the pulse was successfully detected 20 km from the source.
PD Detection Theory PD Pulse Propagation and Attenuation
Cotton, I., O’Donnell, V,. & Christofides, N. Limitation in the application of on-line and off-line PD measurement systems CIRED 2005
Reference: S.Sutton, R. Plath and G. Shröder, “The St.Johns Wood – Elstree Experience – Testing a 20km Long 400kV XLPE-
Insulated Cable System After Installation”, Jicable 2007 - 7th International Conference on Insulated Power Cables, Paris – Versailles, 24-28 June 2007.
PD Location in Power Cables
Direct pulse Reflected pulse
ΔT L
Measurement End Remote End PD event
Direct Pulse
Reflected Pulse
PD Site Location
1001% ⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎠
⎞⎜⎝
⎛ Δ−=L
TPD
ΔT = Time difference between direct and reflected pulses. L = Cable Return Time for cable
Single-ended Cable Mapping
• Only possible for long distances when the tested cable’s far end impedance change is HIGH (e.g. if the end of the cable goes into a transformer and/or the circuit breaker at the far end of the cable is OPEN).
OLPD Location on Power Cables
In many on-line cases reflected PD pulses are often not visible: • Attenuation is too large to measure reflected pulses from the far end (long
cables)
• Waveforms too difficult to interpret (noisy signals)
• Teed or jointed cables
• Cables with many ring main units or switches
• Cables with no change in impedance at the far end
• Cross-bonded cable circuits – multiple reflections
Double-ended Cable Mapping (Range up to 5 km or 3 RMUs)
• Necessary when the tested cable’s far end impedance change is LOW (e.g. if the far end circuit breaker is CLOSED).
• The HVPD Portable Transponder system amplifies PD signals, allowing the HVPD Longshot™ at the other end of the cable to receive and interpret the relative arrival times of pulses at each end of the cable to give an accurate location.
• The cable earth strap must be accessible at both ends of the cable in order to perform double-ended mapping.
On-line PD Location on Power Cables Example of Usage
Without Transponder
Reflection may not be clearly visible (e.g. due to noise)
ΔT
With Transponder
ΔT ΔTtr
The large transponder pulse removes any confusion
ΔT = Time between direct and reflected PD pulses ΔTtr = Transponder time delay
On-line PD Location on Power Cables Example Results
Location (% along cable)10510095908580757065605550454035302520151050-5
All
Pha
ses
PD
(pC
)
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Time (uSec)50454035302520151050
Vol
tage
(mV
)
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0
-5
-10
-15
Direct PDPulse
Reflected PD Pulse
Transponder Pulse
Transponder Time Delay
Reflectogram showing PD and transponder pulses
PD location map for all PD pulses in cable section under test
CASE STUDY: OLPD TESTING AND CABLE MAPPING OF 33 KV XLPE CABLES IN METRO NETWORK
• OLPD testing was carried out in response to a number recent faults* of 33 kV cable joints within the customer’s network.
• The faults led to disruption of the power supply to the rail system.
• The purpose of the testing was to measure and locate any PD activity within the cables with particular focus on the cable joints.
Case Study: Introduction
* It should be noted that this was a newly installed cable system that had been in-service for just over 12 months before the faults started to occur.
• On-line Cable PD Mapping using the HVPD Longshot™ test unit and Portable transponder.
• Tests started with calibration testing with pulse injection HFCTs.
Case Study : OLPD Testing Equipment and Methodology
• Cable PD signals have been detected on Blue Phase with cross-talk (lower magnitude) on Red and Yellow phases.
• The source of PD was located to Joint Number 2 (Jt2) using the on-line PD mapping technique.
• The faulty joint on this cable was replaced and re-tested using the HVPD Longshot™ test unit to verify the repair was good
Case Study: Test Results
• Out of the 50+ circuits tested, Major PD was detected within cable accessories on the three of the circuits (6%) as shown in RED in the Table below.
• The levels of discharges detected put these 33 kV cables into RED category, “Major concern, locate PD and then repair or replace”.
Case Study: Top 20 ‘Worst Performing Circuits’
Criticality Number Circuit Comments
Peak Cable PD Level
(pC)
Local PD Level (dB)
Cumulative Cable PD Level
(nC/cycle)
OLPD Criticality (%)
Maintenance Action
1. DUB to MPS1 C2 B Phase 25888 <10 247 97.4 Major concern, locate PD and then repair or
replace.
2. ABS to AH C2 B / Y Phase 9729 <10 120 90.3 3. BUR to HCC C2 B / Y Phase 3781 <10 12.3 78.7 4. BUR to HCC C1 B / Y Phase 3245 <10 7.9 78.1 5. ABS to AH C1 B / Y Phase 2920 <10 14.4 77.4 6. NHD to QYD C2 R Phase 2849 <10 15.0 76.2 7. ALQ to AHS C2 B Phase 1733 <10 4.6 70.6 Some concern,
repeat test and regular
monitoring recommended.
8. MPS3 to BNS C2 R / B Phase 1337 <10 6.4 65.5 9. NHD to QYD C1 R Phase 887 <10 8.8 47.8
10. HCC to CRK C1 Y / B Phase 759 <10 2.5 39.2 11. AHS to SLD Y / R Phase 705 <10 3.1 38.5 12. STD to ABH Y Phase 238 <10 1.0 24.1
Re-test in 12 months.
13. ALR to BNS C1 B Phase 184 <10 0.9 18.6 14. ALR to BRJ No PD detected 0 <10 0 0 15. ALG to PMD No PD detected 0 <10 0 0 16. ALG to KBW No PD detected 0 <10 0 0 17. AQD to AQ2 No PD detected 0 <10 0 0 18. JDD to CRK No PD detected 0 <10 0 0 19. ODM to JDF C1 No PD detected 0 <10 0 0 20. ODM to JDF C2 No PD detected 0 <10 0 0
CASE STUDY: OLPD Testing, Location, Monitoring with Preventative Maintenance on a 33 kV Land-
Sea Offshore Wind Farm Export Cable (UK)
Case Study: Circuit Details
• 1.7 km single core XLPE land cable • 9.6/11.5 km 3 core XLPE subsea cable
33kV Switching Substation
Offshore Wind farm
Land-Subsea Cable Joints
Land Cables 3 x single core
Offshore 33kVGIS Switchgear
33kV Grid Substation
3 core Subsea Cables
Circuit 1 Circuit 2
Case Study: OLPD Test and Mapping Data
L1 L2 L3
Cable PD
Phase of Pow er Cycle (deg)360270180900
PD
Mag
nitu
de (
pC)
0
Cable PD
Phase of Pow er Cycle (deg)360270180900
PD
Mag
nitu
de (
pC)
0
Cable PD
Phase of Pow er Cycle (deg)360270180900
PD M
agni
tude
(pC
) 10,000
5,000
0
-5,000
-10,000
High levels of PD (of up to 10,000 pC / 10 nC) measured on Circuit B, Phase L3.
Location (meters) 1,600 1,400 1,200 1,000 800 600 400 200 0
Case Study: PDMap© Graph Showing PD Location
Land-sea Transition
Joint Joint Pit 7
Switching Substation
Case Study: PD Signals Before and After Joint Replacement
Joint 7 with PD removed and replacement cable section installed
Location (meters) 1,600 1,400 1,200 1,000 800 600 400 200 0
High PD detected on L3
PD Located
Lower-level sporadic PD signals from different site after joint replacement
BEFORE
AFTER
Case Study: Circuit B – Evidence of Surface Degradation Due to Bad Fitting Heatshrink Stress Control
Case Study: OLPD testing of 110 kV XLPE cables and terminations for oil refinery client (Slovakia)
Following the failure of two 110 kV transformer cable terminations, OLPD testing on the other cable terminations of same type was carried out.
Condition Assessment
Failed 110 kV Transformer Cable Termination
Test Set-up and Results
HFCT L1 Earth Strap HFCT L2 Earth Strap HFCT L3 Earth Strap
Cable PD
Phase of Pow er Cycle (deg)360270180900
PD M
agni
tude
(pC
)
0
Cable PD
Phase of Pow er Cycle (deg)360270180900
PD
Mag
nitu
de (
pC)
0
Cable PD
Phase of Pow er Cycle (deg)360270180900
PD
Mag
nitu
de (
pC)
400
300200100
0-100-200-300
-400
Significant levels of PD activity (of up to 400 pC) were detected on Phase L2.
Cable PD Segment Waveform
Time us3210
Vol
ts (
mV
)
10
5
0
-5
-10
Forensic Investigation
• L2 termination was eventually replaced 15 months after the initial tests were made.
• Investigation showed evidence of severe tracking.
• The cable had not yet failed i.e. the OLPD testing gave a very good ‘early warning’ of 15 months against the incipient fault.
