Once operational next year the 180 MW output capcity of the Robin Rigg offshore wind farm will make a valuable contribution to the UK government 15 per cent renewables target by 2015. Richard Cooke describes the electrical connection part of this offshore project and discuss the many challenges of connecting wind energy sources to the grid.

Richard Cooke, AREVA T&D, UK

Due to the increasing need for the development of renewable energy sources to address the issues around climate change, the wind energy market has grown from primarily small, medium-voltage wind farms connected to the distribution network to a greater – and growing – number of large, high-voltage farms that connect directly to the transmission grid.

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E.ON’s Robin Rigg offshore wind farm is one such project, which is under construction in the UK. AREVA T&D is working alongside E.ON Climate and Renewables, to design, construct and monitor the progress of both the offshore and onshore electrical equipment on this project.

Robin Rigg offshore wind farm project

The Robin Rigg project is one of a limited number of offshore wind farm sites currently in operation or under construction in the UK. The Robin Rigg development is part of the Round One Phase of the 1998/1999 government initiative on seabed leases. In response to this initiative, the Robin Rigg venture comprises two projects that are to be undertaken simultaneously: Robin Rigg East and Robin Rigg West. Each site has 30 turbines, which is in keeping with the limit stipulated by the UK government.

AREVA T&D already has extensive expertise in medium- and high-voltage onshore wind farm connections, from previous involvement in developments in countries like Germany, Norway and France. One of the unique aspects of the Robin Rigg undertaking is the fact that E.ON will provide an electrical system design exceeding the bare minimum of its connection agreement. Building on E.ON’s system design, AREVA T&D has helped it to go beyond the basic requirements of their connection agreement by facilitating the implementation of a system, which has an electrical output that is largely in compliance with the current UK grid code.

The Robin Rigg offshore wind farms will be located on the Robin Rigg sandbanks, 8.5 km from Balcary Point in the Solway Firth in Scotland. Cables will run southeast, away from the offshore platforms, to connect to the onshore substation near Seaton in Cumbria. Each of the two wind farms will have a 90 MW output capacity. Thus, overall, the Robin Rigg stations will provide an output capacity of 180 MW, with the ability to supply over 117 000 average homes with electricity.

Each farm will utilize four strings of turbines, supplied by Vestas, and each string will comprise of between seven or eight turbines, each be separated by a distance of 500 m. Each turbine will measure 75 m vertically, with a maximum distance between the mean sea level and the vertical blade of 125 m.

The offshore wind farm will be interconnected by underwater AC cables, which will be buried in the seabed. The sub-sea cables will then connect into the high-voltage offshore substation built by AREVA T&D, which will be located on a platform near the turbines.

This offshore substation will step up the wind farm’s voltage from 33 kV to 132 kV in order to transmit the electricity onto the national transmission grid. The company will supply six panels of 33 kV, WS gas-insulated switchgear manufactured by its factory at Regensburg, Germany, and customized by its unit in Stafford, UK. AREVA T&D’s switchgear is insulated with SF6 gas and has a robust interlocking system, with an integrated voltage detection system.

AREVA T&D will also supply a 33/132 kV 100 MVA transformer and Micom protection panels from its UK factory, and a 132 kV gas-insulated B65 disconnector from its factory in Switzerland. The B65 disconnector features a spring-operated mechanism and will protect the substation equipment. All the equipment used for the substations will be designed to withstand sea salt, rust, wind and rain, which all have the potential to have a corroding effect and damage the offshore substation.

Site work begins

AREVA T&D began site works for the Robin Rigg onshore substation in early summer 2007. This summer, mechanical and electrical installation of the plant and equipment began at the onshore substation site, as well as the commissioning and energization of the electrical system.

For the offshore substation, installation of the equipment to the platform is scheduled to begin in the late autumn of this year. The shipping of the offshore platforms out to their final positions is takeing place this summer, and E.ON is looking to bring about final wind farm commissioning in the second quarter of 2009.

The Robin Rigg development will help the UK government to increase its renewable energy sources and will also go some way towards helping Britain to reach its target of having 15 per cent of the country’s electricity production generated from renewable sources by 2015.

Technical challenges faced by renewables

The UK is one of a number of countries that are signatories of the Kyoto Protocol, and an increasing number of wind farms and other environmental sources of energy are connecting into the transmission and distribution grid. However, this new mixture of power systems provides a number of technical challenges. The well-documented variability of the output from wind farms does pose a significant challenge to their greater implementation.

However, the real issue lies in the fact that power networks were initially designed for centralized electricity generation production and not for distributed energy sources, such as wind farms. Operators of wind farms must manage their unpredictable energy production, while also ensuring that they comply with the new and emerging grid codes.

Guarding against power imbalance

Because of the proliferation of distributed energy sources in a liberalized market, combined with the challenges associated with large wind farms, the potential risk to the stability of the network has increased significantly. As a consequence, utilities have established minimum connection requirements in terms of system operating constraints, power quality and supply stability that each independent power producer must comply with in order to safeguard the grid and its customers.

Unlike traditional sources of electrical generation, wind power is unpredictable and intermittent which causes challenges when converting wind-produced electricity into a high quality energy source ready to be brought onto the grid. The experts at AREVA T&D recognize the importance of such power imbalance issues, and have identified ways in which the cost impact can be minimized with the use of more accurate wind predictions.

Wind farm control solutions

AREVA T&D’s e-terrawind offers an end-to-end IT platform solution covering the reporting of the wind farm’s real-time critical data from both the turbine and the electrical system. The system can also be used for large wind farms to improve their voltage and VAR controllability, and so making them react – from a network prospective – in a similar manner to conventional generation plants.

E-terrawind also brings innovative generation scheduling and optimization tools, which allow network operators – or generation portfolio players in a deregulated environment – to optimize the efficiency of their overall generation strategies. For network operators it also includes a very detailed dispatcher training simulator utilizing wind generation models that can precisely assess the impact of the wind farm on their network security.

A 132 kV compact air insulated substation (CAIS) linked to an onshore wind farm
Source: Vestas
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To compensate for wind power’s unpredictable electricity production in deregulated environments, wind farm operators usually combine wind-generated power with other generation capacities, such as coal or hydropower. With higher costs and environmental constraints, the challenge is to achieve better controllability and energy optimization to define the most profitable energy mix at any moment in time. In this particular case, e-terrawind extends to the level of an adapted trading functionality, including integration of weather-based generation forecast, generation portfolio scheduling and dispatch monitoring, providing interfaces to the trading market places and maximizing revenue opportunities for wind operators.

Another scenario to be considered is the emerging large offshore wind farm projects such as Robin Rigg. Here, the BETA market system offers generation players various trading opportunities between long-term contracts, day-ahead energy or in combination with other generation of industrial load assets. In this particular case, e-terrawind is totally integrated with AREVA T&D’s e-terramarket electronic market place systems and e-terratrade participant trading systems.

Network Consulting

Managing wind energy’s intermittent nature is not the only challenge faced by wind farm developers and network operators. Issues regarding power quality and grid stability also exist, but these are largely avoidable. By carrying out simulation studies and modelling at the beginning of a project, engineers can help determine ways to ensure a secure and stable connection of a wind farm into the grid that is in compliance with grid codes.

AREVA T&D is utilizing it’s expertise in land-based substations to contruct the 132 kV CAIS substation for the UK’s Robin Rigg wind farm
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AREVA T&D can simulate the respective dynamic behaviour of the turbines/wind farm according to the developer’s control strategy and wind speed fluctuations. Furthermore, both steady-state and dynamic studies of the installation from the wind turbine through to the grid connection and beyond can be performed to provide a cost-effective and technically compliant system design that meets the new and emerging grid connection requirements.

Power Electronics Solutions

Power electronics solutions such as ‘high voltage direct current’ (HVDC) schemes or ‘flexible AC transmission systems’ (FACTS) applications can also provide effective grid compliant solutions when connecting onshore and offshore wind farms to the power network.

For wind farms located close to the load centre, an AC connection may be the most applicable and cost-effective strategy. However, for wind farms located further from the load centre, HVDC solutions can offer many advantages ranging from lower capital cost and higher transmission capacity to lower losses, precise and rapid control of power flow and fewer disturbances during network faults. AREVA T&D’s HVDC solutions are in operation around the world and have recently been implemented in the Konti-Skan project linking the power systems of Denmark and Sweden.

For wind farms connected with a traditional AC connection, FACTS solutions, such as static VAR compensators (SVC) represent effective power quality solutions, and ensure that wind energy contributes to the steady-state and dynamic performance of the AC system and meets grid connection standards and requirements. However, where the grid connection is relatively strong, steady-state and dynamic performance studies can show that a grid code compliant connection can be achieved using the inherent capabilities of wind turbines in coordination with switched reactive elements.

Some wind farms do not ride through a local fault scenario. For example, if there is a short circuit fault near to the wind farm and the fault is cleared, the wind farm itself may not recover. This is primarily dependent on the type of turbine being used. A solution to this problem can be implemented at the turbine level.

However, a system solution can be provided by coordinated control and protection of both the wind turbine and a form of compact dynamic reactive power support provided at the substation. Such dynamic reactive power support can be provided by the modular product, the D€-STATCOM.

Supporting the offshore wind industry

AREVA T&D, through its system studies, support a wide range of systems and products and can provide grid connection solutions that meet the new and emerging grid connection codes anywhere in the world. Through its energy management systems, AREVA T&D provides the capability for wind farms to be integrated into the power system and its associated market systems.