SSEPD: ANM enables 200% increase in renewables contributions for SSEPD
The Shetland power grid operates as an electrical island, with no interconnection to the main UK grid. Stability constraints, caused by an imbalance between generation and demand, have severely limited the amount of renewable generation that can connect to the grid despite the vast renewable resources available. Scottish and Southern Electricity Power Distribution (SSEPD) has deployed an Active Network Management system to coordinate a range of Distributed Energy Resources (DER) to better balance the grid, overcome the stability constraints, and allow more renewable generation to connect. This ground-breaking project has allowed an estimated additional 8-12 MW of renewables to connect, representing a 200% increase in renewables contribution, whilst reducing CO2 emissions through displacing thermal generation.
The Shetland power grid operates at 33 kV and below. The 33 kV network consists mostly of overhead lines on land with subsea cables connecting the islands. Demand varies between 11 and 45.5 MW and is concentrated in the main town of Lerwick. Lerwick Power Station (LPS) has diesel generators with sufficient capacity to meet the total system load but they are reaching the end of their life. The other main generator is at the Sullom Voe Oil Terminal (SVT) which meets its own load and exports surplus power to the public grid. The SVT gas turbines are the main source of spinning reserve and while they are in service the system is relatively stable. Sudden changes in output from intermittent sources of power, such as wind farms, can cause problems, particularly at times of low demand or if SVT is not connected. The wind resources on Shetland are excellent (Burradale wind farm records a capacity factor of over 50%) and there are also significant wave and tidal resources. However, power system stability is a major concern and has prevented the connection of additional renewable sources. The possibility of sudden loss of power, either due to changes in wind conditions or a network fault, means that it would not be possible to keep frequency within limits, which are already relaxed on Shetland.
Balancing the grid has been addressed to some degree through detailed setting of timed residential heating demand, but it still presents challenges that are exacerbated by the variability of renewable sources. On some 11 kV and low voltage circuits, heating loads are timed to ensure diversity and prevent overloads. Power flow and voltage constraints on parts of the network also limit operational flexibility. The concentration of demand around Lerwick means that a certain amount must be met from LPS or Burradale, restricting the export from SVT at times. These challenges are typical for small or large island systems and Microgrids that are looking to incorporate higher penetration of DER.
Smarter Grid Solutions implemented an Active Network Management system to better schedule, coordinate and control a wide range of new distributed energy resources, including:
- Domestic demand management of storage heating and electric immersion heaters for hot water in hundreds of homes. The average home represents around 8 kW of electrical demand and 60 kWh of thermal energy storage.
- Wind farms and micro-turbines ranging from kWs to MWs which have their output restricted if necessary due to stability, power flow or voltage constraints.
- An electro-chemical energy storage device using advanced lead acid technology to assist with system balancing and system stability.
- Forecasting of network constraints based on demand and generation forecasts.
- System balancing by scheduling power consumption and production for all distributed energy resources with an objective of maximising renewable contribution to the grid.
- Real time balancing to override schedules in response to unforeseen events, changes in conditions and short term variations.
- Management of system stability to identify configurations that may result in unacceptable oscillatory behaviour and set operating limits, including frequency response characteristics.
The ANM system is integrated with systems including SCADA, DMS and data historians, and makes use of multiple communications channels to interact with the DER including broadcast radio, point-to-point radio and fiber optic.
By actively managing both local grid constraints and system-wide stability through the fast and autonomous coordination of renewable generation, energy storage, industrial demand and domestic demand, an estimated additional 8-12 MW of renewable energy generation can be connected to the existing Shetland grid. 8-12 MW represents a 200% increase in renewable energy contribution, and a reduction in CO2 emissions through displacing thermal generation. The planned replacement of the LPS will provide capacity to extend the contribution of renewables under the control of the ANM system even further.