8 • Green & Greener News

A SUBSEA THREAT TO THE OFFSHORE RENEWABLE INSURANCE MARKETThe o�shore wind farm industry has, to date, su�ered a higher-than-expected number of medium voltage (MV) and high voltage (HV) cable faults. These represent the majority of o�shore wind farm insurance claims, with many faults occurring during the construction phase. Dr Lee Renforth, Managing Director at High Voltage Partial Discharge Ltd, examines why cable faults occur, their �nancial impact, and the case for cable condition monitoring (CM) technology.

Subsea (MV – 33kV) inter-array cables and land-sea (HV – 132 to 400kV) export cables (Figure 1) have the following characteristics:

• They can require long (and expensive) repairs if the fault is subsea (as a cable lift/repair vessel is required).

• Unplanned outages are much more expensive than scheduled, preventative maintenance interventions (typically by a factor of at least 10).

• Operators need to ensure the networks have high reliability, good maintainability, and maximum availability.

Mission-critical subsea MV and HV cables can fail due to a number of reasons, including installation errors, poor workmanship, or inadequate design and mechanical protection of the cable components for duty in the sea. Many of these failures occur at the weak points along the cables, such as the cable joints and terminations.

It is also known that a high proportion of these failures occur within the �rst three years of service life, the so-called “infant mortality” phase of the Reliability Centred Maintenance (RCM) Bathtub Curve; failure rates then remain relatively constant before escalating through the “end-of-life” phase (20 to 25 years in service). Newly-commissioned cable installations are thus presently the biggest cause for concern with regards to risk of cable failures and subsequent insurance claims.

PREDICTABILITY FOR PROFIT

Unplanned outages from cable faults can lead to signi�cant downtime and lost revenue for the operators (and their insurers). This is particularly so if a subsea cable fault occurs during the winter months, when repairs to subsea cables are not easy to arrange due to limited specialist vessel access o�shore.

Marsh • 9

O�shore wind generation remains the most expensive of the various renewable energy options. The UK’s Department of Environmental and Climate Change (DECC) has identi�ed that a reduction in the high operating and maintenance (O&M) costs of the o�shore wind farms is essential to drive down costs and to make this source of renewable electricity more a�ordable to the electricity consumer1.

MONITORING FOR IMPROVED OPERATIONAL EFFICIENCIES AND RELIABILITY

It is recommended that o�shore wind farm operators apply condition-based management (CBM) regimes to their cable networks. This will improve operational reliability, ensure a more e�ective risk management approach, and go some way in reducing the presently high O&M costs.

Prior to failure, incipient cable insulation faults can produce pre-failure characteristics, including partial discharge (PD) activity and localised heating of the cable. Recent CM technology developments mean that these pre-failure characteristics can now be continuously monitored with the cable remaining in-service. Such systems provide the operator with an early warning of incipient cable faults to support preventative maintenance interventions and avoid unplanned outages and downtime.

The advantages of applying these on-line cable CM technologies include:

• They help to moderate the risks of unplanned cable faults, outages, downtime, and claims.

• The diagnostic monitoring is made continuously, without the need for an outage, so the cable is assessed under normal (and abnormal) conditions.

• The data can be used to support CBM to predict and prevent failure through the detection of incipient cable faults.

Due to the high rate of “infant mortality” problems with these cable networks, it is recommended that condition monitoring should start with the testing and commissioning of the new MV/HV cables’ components at installation (using portable HV power sources and diagnostic testing). This testing can then be supplemented by continuous CM technology throughout the cables’ service life, with particular focus on the initial three years of service. An example of such a continuous cable CM system, to monitor an 80 x 3.6MW wind turbine array (288MW), is shown in Figure 2.

Figure 1: MV and HV network for a typical Round 2 UK 500MW+ o�shore wind farm

1 DECC has set a target of reducing the total cost of o�shore wind generation by 25%, from the present estimated levelised cost of electricity of £135/MWh to £100/MWh (for a 500MW o�shore wind farm over the expected 20-year lifetime of the asset) by 2020.

10 • Green & Greener News

CONCLUSION

The e�cacy of any CBM scheme will be largely dependent on the quality and reliability of the CM data from which resultant repair/replace/retain decisions are made. Therefore, in order to ensure e�ective CBM, there is a need for high-level, diagnostic state and condition data from the CM technology employed.

ABOUT HIGH VOLTAGE PARTIAL DISCHARGE LTDHVPD are experts in the �eld of on-line partial discharge (OLPD) condition assessment of all types of in-service, MV and HV plant from 3.3 up to 750 kV. O�ering a complete range of partial discharge test and monitoring technology and services to customers in over 90 countries, HVPD’s solutions are utilised by a cross-section of MV & HV network operators, including electricity generators, utilities, and industrial, commercial, and service partners.

www.hvpd.co.uk

Figure 2: Illustration of an 80 x 3.6MW wind turbine array (288MW) to a 132kV o�shore collector substation

2 x 132kV Cable Joint

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Onshore Substation

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KEY:

= Onshore Substation

= Wind Turbine

= 132kV Outgoing Cables

= 33kV Incoming Cables

= 132kV Cable joint

= Switchgear

= HVPD Monitor

= HFCT Sensors on Cables

= TEV Sensor on Switchgear

= Criticality

= PD Location

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“...Prior to failure, incipient cable insulation faults can produce partial discharge activity and localised heating of the cable...”