Reference no: EM132880178

Cable Redundancy Analysis - Northumbria University

Project Background

General information about KLOC

Kinewell Energy has developed the software package, Kinewell Layout Optimisation of Cable (KLOC), which optimises offshore windfarm inter-array cable layouts which enables cost savings in the region of 20% of the cable system capital cost over the operational lifetime of the system. The software was Highly Commended at the 2016 Innovation Awards of the Institute of Engineering and Technology (IET) in two categories; Power and Model Based Engineering.

Background information

KLOC's inter array cable layouts are currently based on two network orientations:
• Radial. The substation is connected to wind turbines via cables consecutively (below, left).
• Branched. The substation is connected to wind turbines via cables which may branch off and connect to other turbines (below, right).

Over the operational lifetime of the wind farms, some cables will likely fail. This could be due to any number of potential reasons, for example a shipping anchor might snag on a cable causing it to rip up. When a cable fails, which could be any cable in any location, circuit breakers will isolate the fault to protect the connected equipment. The number of turbines removed from service due to the fault will depend on the location and type of isolation. When a turbine is removed from service, its energy generation stops causing a loss in revenue for the operator and should be minimised.

The typical protection philosophy used by the industry at present is:
• On fault detection, the circuit breaker at the substation trips isolating the entire string
• Engineers visit the wind farm, and manually isolate the fault through an isolator within the turbine immediately prior to the fault. This allows the turbines closest to the substation to be returned to service while awaiting a full cable repair
• Engineers visit the wind farm to repair the faulted cable. This allows the full string of turbines to be brought back into service

Both visits to the wind farm take time and are not immediate; the right people, equipment and vessels must be organised and deployed within a suitable weather window. During this time, energy that would otherwise be generated and sold, is lost, causing reduced revenue to the operator. These reduced revenues are the Unavailability Losses.

Unavailability losses can be minimised by:
1) reducing the time to isolate the fault
2) reducing the number of turbines isolated when a fault occurs

Reducing the number of turbines isolated could be achieved by using ‘redundant cables' that link different strings together. Some wind farms do make use of redundant cables at the ends of radial strings to create ‘loops'. The cost of these connections rarely makes economic sense on an energy export perspective and are there for insurance purposes so that the turbine monitoring systems remain in service (without needing emergency diesel generators). They are operated ‘normally open' and thus under normal operation the loop is disconnected making the network electrically the same as a radial network.

As the industry moves to larger turbines the unavailability losses resulting from a cable failure will also get larger. This might make it economical to consider redundant cables for energy export purposes.

The costs of installing redundant cables are well understood, and do not need to be considered further. The benefits of redundant cables, however, is complex and dependent on many inter- related considerations:
• What are the different conceptual ways that redundant cables could link multiple strings together? Should the links always be at the end of the string? Should there be only one redundant link per string?
• What the different ways the protection can be set-up, and how does this impact what is in and out of service during the fault and repair cycle for each of the different ways redundant cables could be configured?
• How might the power flows change from the normal operation configuration to enable increased exports under fault conditions for each protection set-up and redundant cable configuration?
• In a fault condition, the rerouted network might mean more turbines connected to a string than it is designed for:
o Will the turbines need to be operated at reduced power to maintain cable thermal ampacity limits?
o Should all the turbines have their power reduced by the same amount or by differing amounts?
o Should larger cables with higher thermal ampacity limits be selected?

Attachment:- Cable Redundancy Analysis.rar