Adoption of Best Practices for Cable Testing and Condition Monitoring In The Offshore Renewable Market by HVPD

1. Adoption of Best Practices for Cable Testing and
Condition Monitoring in the Offshore Renewables Market
Presented by:
Dr Lee Renforth
Managing Director, HVPD Ltd

2. CONTENTS
• Why do we need improved commissioning testing and
condition monitoring for subsea cables?
• Exploring the options for diagnostic testing as part of the
field acceptance/commissioning tests for subsea cables.
• What options are available for the condition monitoring
(CM) of in-service subsea cables?
• Diagnostic testing and condition monitoring Case Studies
from the Oil and Gas industry.
• How can condition monitoring technology support
condition based management (CBM) of these assets.

3. Introduction to HVPD Ltd
• HVPD are experts in the field of on-line partial discharge (OLPD) condition monitoring
technology with specific expertise in MV and HV cable monitoring.
• We have over 20 years of experience in testing of in-service MV and HV cables, switchgear,
transformers and motors/generators.
• We supply portable and permanent OLPD diagnostic test and continuous monitoring solutions,
and a complimentary range of on-site test services and training.
• Five main market sectors: Oil & Gas, Renewables, Transmission & Distribution, Shipping and
Generation.
Introduction to HVPD Ltd

4. HVPD’s Subsea Cable Clients
HVDC & HVAC Interconnector Owners – Manx 90kV 108km HVAC Interconnector
© Google

5. HVPD’s Subsea Cable Clients
Oil and Gas Operators – 33kV Cables from Deepwater Offshore Wind Turbines

6. HVPD’s Subsea Cable Clients
Offshore Wind Farm (OWF) HVAC Export Cables & 33kV Inter-array Cables

7. Subsea MV and HV Cable Faults
Common Causes
• Incorrect workmanship of the cable accessories leading to partial discharge,
electrical tracking and finally complete insulation failure.
• Mechanical damage caused by poor installation practices including damage to
the cable (from jack-up vessels, anchors, etc.) and/or poor quality cable
mechanical protection of cable joints leading to scour and mechanical stressing.
• Thermal damage caused by poor bonding of cable earthing system and/or
water ingress that leads to localised heating and thermal breakdown.

8. ‘TEAM’ Stresses for Subsea Power Cables
THERMAL
‘Thermal runaway’ problems can occur in
cables where there are high circulating
currents and local high resistance points.
ELECTRICAL
This is the No.1 cause of cable faults
occurring within the first 3 years of service,
typically due to incorrect installation of the
cable accessories.
AMBIENT
The effects of mechanical ‘wear and tear’
including ‘scour’ caused by movement of the
subsea cables with tidal and current changes.
MECHANICAL
Many subsea cable failures are caused by
mechanical damage caused by poor practice
when installing and ‘pulling-in’ the cables.

9. Introduction to Partial Discharge
Why test for partial discharge?
PD activity is an indication of an ‘incipient fault’ in HV
insulation and is widely regarded as the best ‘early
warning’ indicator of insulation deterioration.
The detection of PD at an early stage enables preventative
maintenance action to avoid unplanned outages.
What is partial discharge?
“A localised electrical discharge that only partially
bridges the insulation between conductors and which
can or can not occur adjacent to a conductor”
IEC60270 Definition

10. Most Likely Sites of PD Activity in Subsea Cables

11. 132 kV Onshore Cable Termination
Failed and Exploded Outdoor Porcelain Cable Sealing End

12. 33 kV OWF Export Cable Joint – this joint had been exhibiting
high levels of PD and was replaced and removed from service

13. 110kV Transformer Cable Terminations
Left – photo showing PD ‘scorching’, Right – photo of a failed termination
Tracking and ‘scorching’ on a 110 kV Termination
(PD detected before failure)
A Failed 110 kV Termination
(Same type as opposite)

14. Reliability Centred Maintenance (RCM) ‘Bathtub Curve’
Steady State
Failure
‘Infant Mortality’
Phase
3 Years 20-50 Years
Infant Mortality Steady State Failure
End of Life
‘Wear-out’
Time
FailureRate

15. Why and When to Perform PD Testing
New Equipment
At Manufacture
• Quality Assurance
• Type/routine tests, e.g. IEEE/IEC
standards – test to less than 5pC on the
cables
At Commissioning
• To check for transport damage
• To ensure the installation of the cable
accessories have made to a good
standard (these are the weak points in
the cable system)

16. VLF and Soak Test Commissioning Tests for 33 kV Cables
• To detect any poor workmanship and/or
installation damage with a particular
focus on the cable accessories.
• Partial Discharge (PD) and Tan Delta
(TD) diagnostic acceptance tests should
be made in combination with the VLF
voltage withstand test (from 2.0 to 3.0 U0
i.e. 38.2–57.3 kVrms for 33 kV cables).
• This test is combined with an off-line,
electrical Time Domain Reflectometry
(TDR) testing to support both future ‘PD
Mapping’ (PD site localisation) and/or
rapid fault location in the event of a cable
fault.

17. Factory Testing of Cable Systems
• Cable components tested individually.
• Cable cores should be tested both
before and then after their assembly
into the 3-core subsea cable.
• High sensitivity measurements in a
‘low-noise’ environment are required,
typically to accuracies of <5pC.
• This requires a Faraday Cage,
electromagnetically screened test room
to achieve this sensitivity.
• The Faraday Cage HV test facility
shown opposite can measure PD
activity down to 1pC.

18. Cable HV Withstand Voltage Field Acceptance Test Options
VLF (Very Low Frequency) (0.05–0.1 Hz)
example supplier: Baur - Austria, b2hv - Austria
Variable Frequency Resonant Test
Systems (RTS) (20-300Hz)
example supplier: High-Volt – Germany
Damped AC / Oscillating Wave (OWTS)
example suppliers: Seitz, SEBAkmt - Germany
24 Hour Soak Test (at U0)
No external power supply is required although
extended, continuous 24-hour OLPD monitoring is
necessary during the duration of the soak test.

19. Manufacturing
Damage
Mistake
Repair
Aging
Acceptance
Testing
Continuous
Monitoring
Transportation Installation
Factory
Testing
Power
frequency
50/60 Hz
Operation
Power frequency 50/60
Hz
From ‘Cradle to Grave’ PD Testing and Monitoring Philosophy

20. Partial Discharge Cable Mapping – PD Site Location along the cable
All
PD Map of Circuit Meols Drive - Graham Road
Location (% along cable)
10510095908580757065605550454035302520151050-5
AllPhasesPD
7,000
6,500
6,000
5,500
5,000
4,500
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
0
The locations of any defects that exhibit PD activity will be detected along the length of
the cable using the technique of time domain reflectometery (TDR).
The PD Map of the cable below shows three (3) main sites of PD activity along the cable.

21. On-line PD Testing & Monitoring to Support
Planned Maintenance Interventions
Repeat testing before the cable
supplier/jointer warranty runs out!
• It is highly recommended that an on-line
PD test is carried out before the warranty
period expires (typically only 12 months).
Continuous On-line PD (OLPD) monitoring
throughout the service life
• To detect whether PD activity has
initiated during the service life of the
cable/plant
• To support maintenance and operation
decisions, by detecting and localising any
PD activity in in-service cables
• To direct preventative maintenance
interventions.

22. CONDITION MONITORING OF IN-SERVICE
SUBSEA MV AND HV CABLES

23. On-line Condition Monitoring Options for Subsea Power Cables
THERMAL
Distributed Temperature Sensing (DTS)
using fibre optic detection technology.
ELECTRICAL
On-line Partial Discharge (OLPD), plus
sheath current and power quality monitoring .
AMBIENT
Vibration monitoring using fibre optic
detection technology.
MECHANICAL
Mechanical strain monitoring using fibre
optic detection technology.

24. Fibre-Optic Strain, Vibration and Temperature Sensing Solution
• Optoelectronic devices which measure
temperature and strain by means of optical
fibres functioning as linear sensors
• Provides real-time, dynamic temperature and
strain information along the complete length of
power cable for health monitoring.
• Can identify small hot spot locations and
localised mechanical damage without prior
installation knowledge.
• Provides accurate temperature data input for
dynamic cable rating based on actual
“installed” conditions to monitor higher power
flows through the cable.
Fibre optic
monitoring
cable
DTS/Strain
Monitoring
unit
Field
splice
box
Local
splice
box

25. Subsea 3-core Cable with Built-in Fibre-Optics
• Subsea cables can include up to 4 fibre-optic cables that can be
utilised for distributed temperature, strain and vibration sensing.
• The example below shows a fibre along the cable centreline, and 3
fibres laid up and located in the interstices between the phases.

26. Offshore High Voltage Network Monitoring System
• OHVMS – ‘holistic’ HV/MV network condition monitoring (CM) system for offshore
cable networks and connected plant (switchgear and transformers).
• The Condition Monitoring (CM) data is used to provide predictive, ‘early warnings’
against ‘incipient’ insulation faults.
• The system helps to avoid unplanned outages, supports preventative
maintenance and reduces the high O&M costs of the OWF electrical networks.

27. Offshore High Voltage Network Monitoring System

28. OHVMS Cable Condition Monitoring
Features and Benefits
OHVMS
HOLISTIC
MONITORING
SYSTEM
Power
Quality
Partial
Discharge
Earth
Faults
Loading
Sheath
Currents
Ambient
Conditions
Intelligent
Diagnosis
System Health
Alarms
System
Stability
Alarms
Condition-
Based
Maintenance
SMART Grid
Integration
Dynamic
De-Rating

29. OHVMS ‘Holistic’ MV/HV Cable Condition Monitoring System
Example – 80 x 3.6MW Turbine Array
An OHVMS monitoring hub (MH1) is
located at the offshore substation
platform (OSP) to monitor:
• 33 kV switchgear
• 33/132 kV transformers
• 33 kV incoming cable ‘strings’
from the turbine arrays
Turbine Monitor Nodes (TMN01–
TMN21) are positioned at strategic
locations (every 3rd turbine) across
the turbine array, to provide complete
network coverage.

30. OHVMS SMART-Quadplex™ Sensor Location In the Turbine Base

31. OHVMS SMART-Quadplex™ Sensor Location In the Turbine Nacelle
• A combined electrical state and condition assessment of the health of the
network is provided using SMART-Quadplex™ sensors.
• The sensors and OHVMS monitor can be installed either at the base of the
turbine or in the nacelle, depending on the turbine design.

32. CASE STUDY 1: ON-LINE PARTIAL DISCHARGE
(OLPD) TESTING, LOCATION, MONITORING WITH
PREVENTATIVE MAINTENANCE ON A 33 KV
OFFSHORE WIND FARM EXPORT CABLE

33. Case Study 1: Export Cable Circuit Details
• 1.7 km single core XLPE land cable
• 9.6/11.5 km 3-core XLPE subsea cable

34. OLPD Test and Mapping Data
L1 L2 L3
Cable PD
Phase of Pow er Cycle (deg)
360270180900
PDMagnitude(pC)
0
Cable PD
Phase of Pow er Cycle (deg)
360270180900
PDMagnitude(pC)
0
Cable PD
Phase of Pow er Cycle (deg)
360270180900
PDMagnitude(pC)
10,000
5,000
0
-5,000
-10,000
High levels of PD (of up to 10,000 pC / 10 nC) were
measured from the onshore substation on Circuit
B, Phase L3.

35. Location (meters)
1,6001,4001,2001,0008006004002000
PDMap© Graph Showing PD Location
Land-sea
Transition
Joint
Joint Pit 7
Switching
Substation

36. PD Signals Before and After Joint Replacement
Joint 7 with PD
removed and
replacement cable
section installed
Location (meters)
1,6001,4001,2001,0008006004002000
High PD detected on L3
PD Located
Lower-level sporadic PD
signals from different site
after joint replacement
BEFORE
AFTER

37. Circuit B – Evidence of Surface Tracking and Degradation due
to Poorly-Fitted Heatshrink Stress Control

38. CASE STUDY 2: INSTALLATION OF AN OLPD
CONDITION MONITORING SYSTEM FOR A 400 KV
ONSHORE GRID CONNECTION CABLE

39. Continuous OLPD Insulation Condition Monitoring
In-service Equipment
Condition Monitoring throughout the
service life of the cable.
• Continuous OLPD monitoring of the
insulation condition of the cable
network throughout it’s service life.
• Data from the technology supports
Condition-Based Management
(CBM) of critical cable networks.
• Provides increased security and
reliability of electricity supply from
offshore renewables generation.
• Helps to reduce O&M costs through
the avoidance of faults and
unplanned outages.

40. Sensor Locations at 400 kV Transformer Terminations

41. HV Cable OLPD Monitor System Drawing

42. CASE STUDY 3: OLPD TESTING AND
LOCATION ON A DEEPWATER OFFSHORE
WIND TURBINE CONNECTED TO AN OIL &
GAS PLATFORM

43. Case Study 2: OLPD Testing and Location on a Deepwater
Offshore Wind Turbine connected to an Oil & Gas Platform

44. Background
• Two deep-water wind turbines supply power exclusively to an oil production platform
situated around 2km away.
• Two on-line PD tests were performed to assess the condition of the 33kV cables from
two turbines to the oil & gas platform.

45. Test 1 – OLPD Test at 33 kV Switchgear on Platform

46. Results from Test 1 at 33 kV Switchgear on the Platform
• High PD activity (in excess
of 6,000pC+) was detected
on the Turbine A Feeder.
• Analysis of the PD pulse
data suggested that the
source of the PD activity
was at the far end of the
Turbine A feeder cable.
• OLPD Cable Mapping was
recommended.

47. Test 2 – PD Test at the Wind Turbine’s 33 kV Switchgear

48. PD Location at Wind Turbine A
• Test identified the PD source to the cable joint at the top of the
tower close to the 33 kV transformer.
• Measurement of PD pulses showed the source of the PD being
at 52m from the switchgear at Test Point 2.
• The cause of the discharge was meachanical stress due to
insufficient support of the cable joint from the weight of the free-
hanging cable.

49. CASE STUDY 4: A COMPLETE 33 KV CABLE
NETWORK OLPD SURVEY AND ANALYSIS TO
SUPPORT CONDITION BASED MAINTENANCE (CBM)
DUBAI METRO, DUBAI, UAE

50. On-line Partial Discharge (OLPD) test and cable
mapping survey of the customer’s 33 kV cable
network was carried out by HVPD engineers
using the HVPD Longshot™ diagnostic test
system.
This testing was carried out in response to a
number catastrophic failures of 33 kV cable joints
within their network which had led to disruption of
the power supply to the Metropolitan 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.
It can be noted that this was a recently installed cable system that had been in-
service for just over 12 months before the faults started to occur.
After four 33kV cable joint faults in 2 months, the client requested a complete
OLPD survey of the network to detect any ‘incipient’ faults on the network.
Background

51. • On-line Cable PD Mapping using the HVPD Longshot™ test unit and Portable
Transponder technology was used to carry out an on-line condition assessment of
complete 33kV cable network.
• Tests started with calibration testing with pulse injection HFCTs, followed by OLPD
measurements and then cable mapping tests.
OLPD Testing to Support CBM of a 33kV Cable Network

52. • Cable PD signals of
6,000pC+ were detected on
the Blue Phase with some
‘cross-talk’ (lower magnitude)
on the Red and Yellow
phases.
• The source of PD was
located to Joint Number 2
(Jt2) using the cable
mapping test technique.
• The faulty joint on this cable
was replaced and re-tested
using the HVPD Longshot™
test unit to verify the repair
was good.
33kV Cable Network Test Results I

53. 33kV Cable Network Test Results II
Examples of PD located on two 33kV circuits
Left – PD Located on Red Phase Joint No.1, Right – PD located on Red Phase, Joint No.2

54. • The network consisted of 104x 33kV that were circuits tested
• High Levels of PD were detected in cable joints on the six of the circuits (6%) as shown in
RED in the Table below, Condition Category, “Major concern, locate PD and then repair”.
• A further five circuits (5%) were in the Orange/Yellow Categor, these were also repaired.
Top 20 ‘Worst Performing 33kV 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

55. CONCLUSIONS

56. Conclusions
• The increasing installation rate of installation of offshore wind farms (OWFs)
in Europe, combined with the high MV and HV cable fault rates reported to
date, has led to a ‘market need’ for better MV/HV cable condition
monitoring (CM) technology.
• Offshore wind farm subsea cable owners need to also consider the use of
diagnostic testing during cable HV withstand/commissioning tests.
• It is proposed that any CM system employed should combine thermal,
electrical, ambient and mechanical monitoring to cover all four of the ‘TEAM’
stresses that effect the reliable operation of the cables.
• The purpose of any CM system is to provide an ‘early warning’ of ‘incipient’
cable insulation faults to enable preventative maintenance interventions to
avoid unplanned outages.
• A move towards Condition Based Management (CBM) of the cable
networks (using data from ‘holistic’ CM technologies) is seen as the key to
reducing the presently high O&M costs to achieve DECC’s target of a 25%
reduction in Levelised Cost of Electricity (LCOE) by 2020.

57. End of Presentation
Thank you for your time
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