This case study explains Vattenfall's R&D work on the island of Gotland. Vattenfall studied the potential of large-scale energy storage to improve security of supply and host more wind power, deployed test-equipment for evaluating monitoring and self-healing networks, and a novel local marketplace for flexibility and system services as part of a comprehensive smart-grid.
Gotland, a Swedish island in the Baltic Sea, mostly relies on wind power for its electricity as well as a single connection to the mainland. The island lies 200km south of Stockholm and is home to over 57,000 people. During the last 15 years the development of wind power on Gotland has been hugely successful. With excellent wind speeds, and its open position 90km from the mainland, electricity generation now often exceeds demand requiring the island to export power.
As the installed wind generation capacity increased to over 180 MW it outstripped demand making export events commonplace. The island had an aging high voltage DC connection with mainland Sweden, which wasn’t originally built for switching between import and export, and often caused a total power failure across the whole of Gotland.
The challenge of managing increasing levels of distributed renewable energy generation is now a key hurdle in many countries. Ensuring security of supply whilst providing an open platform for renewable energy developers to connect is a key challenge for network operators.
In 2011 the Swedish Energy Authority tasked Vattenfall with addressing Gotland’s energy requirements and, over a six year period, the company tested and deployed sensors and breakers in parts of the islands grid for a modern, reliable smart grid with a road-map for future development.
The development plan for Gotland included an objective to generate 2.5 TWh of electricity per year, equivalent to approximately 600 MW of new wind power construction. The main challenge for the development of Gotland’s grid involved delivering increased network capacity for the growth in wind power, whilst increasing security of supply and reducing the number of total power outages.
Vattenfall identified that improving security of supply could be achieved by disconnecting Gotland from the HVDC connection to the mainland, and running the local grid in island operation mode during short outages. The technical requirements mainly consisted of managing the grids’ stability during times the mainland connection was partly or entirely out of operation. Wear and tear of the HVDC installation would also be reduced over its remaining service life at the same time.
Part of the proposed solution for Gotland’s electricity grid involved an energy storage system to work alongside the variable wind power generation and local electricity demand.
The proposed task of the energy storage system is mainly to bridge short-term faults, by means of frequency control, or for short-term island operation. During normal operation the energy store is used for three purposes in the following order of priority:
In the event of a fault in any of the cables on the island there was a risk of an imbalance between generation and consumption, which could cause a total power outage. In order to reduce the risk of an imbalance an energy storage system that could supply a capacity output in the range of 25–50 MW, with an energy storage supply of 25 MWh to cover the time required to remedy a fault, was proposed. The energy storage facility, in conjunction with variable wind energy generation and controllable electricity consumption would create an opportunity for an open marketplace for energy services.
Vattenfall also introduced monitoring on the low voltage grid to provide real-time energy updates to customers. Reclosers were installed as an alternative to new grid reinforcements, which isolate sections of a power line that is experiencing a fault or overload and minimise the number of customers without service.
Security of supply was also increased by the incorporation of remote controlled breakers, managed via a distribution management system (DMS) which assess faults, searches for alternative routes within the network and then actions the required changes to restore power, providing a self-healing network.
In order to double the renewable electricity capacity on Gotland, the existing 70kV electricity grid on the island would have needed to be strengthened and replaced with a higher voltage, 130kV grid. Upgrading the entire grid on the island would have been prohibitively expensive so alternative solutions were required.
As a first step to incorporate more wind power into Gotland’s grid an additional 80MW of renewable generation was made possible by introducing active network management of the grid, bringing the maximum export capacity up to 130MW. Full redundancy was maintained, since the HVDC-grid consists of two independent 130 MW connections, and active control of the grid avoided the increased costs of building a larger energy storage facility.
In order to cater for growing demand, it would be possible to increase wind generation by a further 70MW with the addition of a larger 50MW energy storage facility. But, any system faults at this level of power consumption would require energy production to be managed through frequency control in order to maintain a balance between electricity generation and consumption. If a cable is disconnected in the event of a fault, an energy storage system of 25MW can handle demands for 95% of the hours in the year. With a 50MW energy storage, demands could be managed over 99% of the hours in the year.
Local energy marketplaces introduce dynamic pricing and encourage energy generators and consumers to regulate their production and consumption to use the grid more efficiently and to improve power quality. A local energy marketplace on Gotland is being designed to create a flexible solution to balance supply and demand on the island in both normal and disrupted operation.
The marketplace, planned as part of the Horizon 2020 project, CoordiNet, is expected to demonstrate how flexibility and system services can be bought. A peer to peer market is also being tested between the windpower producers and energy consumers, minimising the need for windpower to be reduced.
Mapping of potential flexibility for electricity consumption on Gotland shows that there are several customer segments that could contribute. Large industrial units that consume a lot of energy such as, ports, stone crushers and fans, could be coordinated via the local market.
The marketplace is also expected to increase security of supply as flexible customers will become a dynamic extension of the energy store, by varying their supply and demand. The full potential is not entirely clear and further benefits are expected to be realised, once a marketplace exists, for both grid operator and customers. Customers can reduce their costs by reviewing their processes and adapting them to utilise the electricity grid when it is least under pressure. Plus, there is even an opportunity for generators to obtain income from the marketplace where there is a shortage of capacity.