The UK’s first high-voltage offshore substation connects the Barrow wind farm to the transmission grid, and is also an important component in helping manage wind’s contribution to the network

Phill Cartwright, Laurent Schmitt and Richard Cooke, Areva T&D, UK

Click here to enlarge image

In recent years, the wind energy market has grown from small medium-voltage wind farms connecting to the distribution network to large high-voltage parks connecting directly to the transmission grid. Areva T&D has already carried out challenging wind farm connections at medium and high voltage in Germany, Norway, Australia, India and France, offering grid connection solutions specifically designed to solve wind power quality issues, as well as providing the necessary equipment. The Barrow offshore wind farm connection, recently awarded to the company by the Kellogg Brown & Root/Vestas Celtic Wind Technology consortium, is just one of these projects.

Under the €12 million contract, Areva T&D will design, manufacture and assemble the UK’s first high-voltage offshore substation off the coast of Barrow, in the North West of England, and design the entire electrical connection between the offshore wind farm and the national power grid. The project is important for Areva T&D because the Barrow substation will be one of only a handful of offshore substations in the world, connecting one of the first wind farms at the 132 kV level in the UK.

The Barrow wind farm, being developed by Barrow Offshore Wind Limited, will be located 7 km offshore and will have 30 wind turbines with a total output capacity of up to 99 MW. The turbines will be aligned in four staggered rows 750 m apart, each composed of seven to eight turbines 500 m apart. The turbines will be 75 m high, with a 100 m blade diameter. The maximum distance between the mean sea level and the vertical blade will be 125 m.

The offshore wind farm will be interconnected by underwater AC cables, which will either be buried in the sand or laid on the seabed. The subsea cables will then connect into the high-voltage offshore substation, which will be located on a platform near the turbines.


Figure 1. Relocatable static var compensators (SVC) are part of Areva T&D’s solution
Click here to enlarge image

Areva T&D’s offshore substation will step up the wind farm’s voltage from 33 kV to 132 kV to transmit the electricity onto the transmission grid. The company will supply fourteen panels of 33 kV WS gas-insulated switchgear manufactured in its German factory in Regensburg and customized by its unit in Stafford, UK.

Areva T&D’s WS 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 120 MVA transformer and Micom protection panels from its UK factory in Stafford and a 132 kV gas-insulated B65 disconnector from its factory in Switzerland. The B65 disconnector has a spring-operated mechanism and will help protect the substation equipment.

Implementation

It will take Areva T&D around six months to build the equipment, which will be designed to withstand sea salt, rust, wind and rain, which can corrode and damage the substation. The equipment will be installed into steel modules which, after undergoing preliminary tests, will be brought to the Barrow dockyard where they will be installed onto the platform.

Once assembled and following further testing, the platform carrying the modules will be taken offshore as one system and will be hoisted onto a monopile foundation driven into the seabed. To maintain the platform’s centre of gravity, Areva T&D engineers are working with KBR’s engineers to study how to position the modules in the allocated space. To help reduce the size of the platform without compromising the operation of the substation, the equipment will be stacked with minimum space between the modules made available for maintenance operations. Areva T&D’s Technology Centre will also model the entire electrical system to ensure that its performance meets Grid Code requirements and will specify the entire electrical connection between the turbines and the transmission grid at the Heysham substation.

Electricity will be transmitted from the offshore substation via a single 132 kV collector cable 26 km long. It will be buried in 1 to 3 m of sand via trenching, jetting or ploughing. At the shore, it will connect to an onshore buried cable, which will transport the electricity 3 km to a new 132 kV substation to be jointly owned by United Utilities and Barrow Offshore Wind Ltd. The new onshore substation will also be designed and installed by Areva T&D and will have a single circuit consisting of a fixed connected reactor, circuit breaker, surge arrestors and a MiCOM protection system. Another buried 132 kV cable will connect the new substation to the Heysham substation, jointly owned by National Grid Transco and United Utilities, to carry the farm’s electricity onto the transmission network.

Supervisory control of the entire wind farm will be implemented through a SCADA system, enabling the Barrow Offshore Wind teams to monitor the park’s performance via the Internet.

Areva T&D’s solutions

The UK government’s objective – to generate 15 per cent of the country’s electricity from renewable energy sources by 2015 – means Barrow is just one example of the growing number of wind farms, plus other diverse sources of energy, connecting into the distribution and transmission grid. This mix in power systems provides technical challenges. Power networks were initially designed for central electricity generation production and not for distributed energy sources, such as wind farms. The real challenge with wind farms however is managing their unpredictable energy production and ensuring that they comply with the new and emerging grid codes.

As distributed energy sources proliferate in a liberalized market, including large wind farms, the risk of disturbing the network’s stability has increased significantly. Wind may not be available when demand requirements are high, resulting in energy imbalances, and some utilities may charge penalties. 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 meet to safeguard the grid and its customers. Power imbalances and voltage fluctuations can occur because wind is an intermittent source of energy.

Power quality and balance problems may have to be resolved using power electronics technology and energy management software solutions. The cost of power imbalances can be minimized with more accurate wind predictions, as proposed as part of Areva T&D’s e-terrawind software package.

Large wind farm control

Areva T&D’s e-terrawind offers an end-to-end IT platform covering the reporting of the wind farm’s real-time critical data from both the turbine and the electrical system. E-terrawind can also be used for a large wind farm to improve its voltage and VAR controllability so it reacts (from a network perspective) very similarly to conventional plants.

In addition, e-terrawind’s generation scheduling and optimization tools allow network operators to optimize their generation strategies. It also includes a very detailed dispatcher training simulator utilizing wind generation models, which can precisely assess the effect of the wind farm on network security.

To compensate for wind power’s upredictability, operators usually combine it with other generation such as coal or hydro. 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 instant. In this 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 the revenue opportunities for wind operators.


Figure 2. Installed technology helps define the most profitable energy mix at any instant
Click here to enlarge image

With large offshore wind farms in the UK, the British Electricity Trading and Transmission Arrangement (BETTA) market system offers generators various trading opportunities, such as long term contracts. In this case e-terrawind is integrated with Areva’s e-terramarket electronic market place systems and e-terratrade participant trading systems.

Managing intermittency is not the only challenge wind farm developers and network operators face. Power quality and grid stability issues also exist, but they can be avoided. Areva T&D can stimulate the dynamic behavior of the turbines according to the developer’s control strategy and wind speed fluctuations. It can also perform steady state and dynamic studies of the installation, from the wind turbine through to the grid connection and beyond, to provide a cost-effective and technically compliant system design that meets grid connection requirements.

Power electronics solutions

In the case of the Barrow farm, an AC connection was most applicable and cost-effective, given the short distance from the shore. But for wind farms located far from the load centre, high voltage DC (HVDC) can offer many advantages: higher transmission capacity, lower losses, precise and rapid control of the power flow and fewer disturbances during network faults.

For wind farms connected with a traditional AC connection, flexible AC transmission (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, as with Barrow, where grid connection is strong, steady-state and dynamic performance studies may show that a grid-code compliant connection can be achieved using the capabilities of wind turbines coordinated with switched reactive elements.

Some wind farms do not ride through local faults. If there is a short circuit fault near to the wind farm and the fault is cleared, the wind farm may not recover. This depends on the turbine being used. A solution can be implemented at the turbine level. Alternatively, however, a system solution can be provided by coordinated control, protection of 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 C-Statcom.