Adoption of Best Practices for Cable Testing and Condition Monitoring in the Offshore Renewables Market

Presented by:Dr Lee Renforth

Managing Director, HVPD Ltd

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.

Introduction to HVPD Ltd

• HVPD are experts in the field of on-line partial discharge (OLPD) condition monitoringtechnology 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 andGeneration.

Introduction to HVPD Ltd

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

© Google

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

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

Subsea MV and HV Cable FaultsCommon 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.

‘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.

ELECTRICALThis is the No.1 cause of cable faultsoccurring within the first 3 years of service,typically due to incorrect installation of thecable accessories.

AMBIENTThe effects of mechanical ‘wear and tear’including ‘scour’ caused by movement of thesubsea cables with tidal and current changes.

MECHANICALMany subsea cable failures are caused bymechanical damage caused by poor practicewhen installing and ‘pulling-in’ the cables.

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

Most Likely Sites of PD Activity in Subsea Cables

132 kV Onshore Cable TerminationFailed and Exploded Outdoor Porcelain Cable Sealing End

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

110kV Transformer Cable TerminationsLeft – 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)

Reliability Centred Maintenance (RCM) ‘Bathtub Curv e’

Steady State Failure

‘Infant Mortality’ Phase

3 Years 20-50 Years

Infant Mortality Steady State FailureEnd of Life ‘Wear-out’

Time

Fai

lure

Rat

e

Why and When to Perform PD TestingNew Equipment

At Manufacture

• Quality Assurance

• Type/routine tests, e.g. IEEE/IECstandards – test to less than 5pC on thecables

At Commissioning

• To check for transport damage

• To ensure the installation of the cableaccessories have made to a goodstandard (these are the weak points inthe cable system)

VLF and Soak Test Commissioning Tests for 33 kV Cab les

• To detect any poor workmanship and/orinstallation damage with a particularfocus on the cable accessories.

• Partial Discharge (PD) and Tan Delta(TD) diagnostic acceptance tests shouldbe made in combination with the VLFvoltage withstand test (from 2.0 to 3.0 U0i.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 ‘PDMapping’ (PD site localisation) and/orrapid fault location in the event of a cablefault.

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.

Cable HV Withstand Voltage Field Acceptance Test Op tions

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.

ManufacturingD

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Test

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Transportation Installation

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Power frequency 50/60 Hz

OperationPower frequency 50/60

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From ‘Cradle to Grave’ PD Testing and Monitoring Ph ilosophy

Partial Discharge Cable Mapping – PD Site Location a long the cable

All

PD Map of Circuit Meols Drive - Graham Road

Location (% along cable)10510095908580757065605550454035302520151050-5

All

Pha

ses

PD

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.

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 hasinitiated during the service life of thecable/plant

• To support maintenance and operationdecisions, by detecting and localising anyPD activity in in-service cables

• To direct preventative maintenanceinterventions.

CONDITION MONITORING OF IN-SERVICE SUBSEA MV AND HV CABLES

On-line Condition Monitoring Options for Subsea Pow er Cables

THERMALDistributed Temperature Sensing (DTS)using fibre optic detection technology.

ELECTRICALOn-line Partial Discharge (OLPD), plussheath current and power quality monitoring .

AMBIENTVibration monitoring using fibre opticdetection technology.

MECHANICALMechanical strain monitoring using fibreoptic detection technology.

Fibre-Optic Strain, Vibration and Temperature Sensi ng 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/StrainMonitoring

unit

Field splice box

Localsplice box

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.

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.

Offshore High Voltage Network Monitoring System

OHVMS Cable Condition MonitoringFeatures 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

DynamicDe-Rating

OHVMS ‘Holistic’ MV/HV Cable Condition Monitoring S ystem

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.

OHVMS SMART-Quadplex ™ Sensor Location In the Turbine Base

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.

CASE STUDY 1: ON-LINE PARTIAL DISCHARGE (OLPD) TESTING, LOCATION, MONITORING WITH

PREVENTATIVE MAINTENANCE ON A 33 KV OFFSHORE WIND FARM EXPORT CABLE

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

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

Mag

nitu

de (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 .

Location (meters)1,6001,4001,2001,0008006004002000

PDMap© Graph Showing PD Location

Land-sea Transition

JointJoint Pit 7

Switching Substation

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

Circuit B – Evidence of Surface Tracking and Degrada tion due to Poorly-Fitted Heatshrink Stress Control

CASE STUDY 2: INSTALLATION OF AN OLPD CONDITION MONITORING SYSTEM FOR A 400 KV

ONSHORE GRID CONNECTION CABLE

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.

Sensor Locations at 400 kV Transformer Terminations

HV Cable OLPD Monitor System Drawing

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

GAS PLATFORM

Case Study 2: OLPD Testing and Location on a Deepwa ter Offshore Wind Turbine connected to an Oil & Gas Pla tform

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.

Test 1 – OLPD Test at 33 kV Switchgear on Platform

Results from Test 1 at 33 kV Switchgear on the Plat form

• 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 activitywas at the far end of the Turbine A feeder cable.

• OLPD Cable Mapping was recommended.

Test 2 – PD Test at the Wind Turbine’s 33 kV Switchg ear

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.

CASE STUDY 4: A COMPLETE 33 KV CABLE NETWORK OLPD SURVEY AND ANALYSIS TO

SUPPORT CONDITION BASED MAINTENANCE (CBM)DUBAI METRO, DUBAI, UAE

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

• 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

• 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

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

• 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 CommentsPeak 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 97.4Major concern, locate PD and then repair or

replace.

2. ABS to AH C2 B / Y Phase 9729 <10 120 90.33. BUR to HCC C2 B / Y Phase 3781 <10 12.3 78.74. BUR to HCC C1 B / Y Phase 3245 <10 7.9 78.15. ABS to AH C1 B / Y Phase 2920 <10 14.4 77.46. NHD to QYD C2 R Phase 2849 <10 15.0 76.27. 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 65.59. NHD to QYD C1 R Phase 887 <10 8.8 47.810. HCC to CRK C1 Y / B Phase 759 <10 2.5 39.211. AHS to SLD Y / R Phase 705 <10 3.1 38.512. 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 18.614. ALR to BRJ No PD detected 0 <10 0 015. ALG to PMD No PD detected 0 <10 0 016. ALG to KBW No PD detected 0 <10 0 017. AQD to AQ2 No PD detected 0 <10 0 018. JDD to CRK No PD detected 0 <10 0 019. ODM to JDF C1 No PD detected 0 <10 0 020. ODM to JDF C2 No PD detected 0 <10 0 0

CONCLUSIONS

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.

End of Presentation

Thank you for your time

Q&A?