Control of Swedish wind power plants meeting future grid codes in a changing power market

Abstract: Climate change is an issue that can no longer be ignored and actions are taken all over the world in order to prevent the effects from becoming too severe. The energy system is transitioning towards more renewable energy sources and wind power has an important role in this development. The penetration of wind power in a lot of countries all over the world has increased rapidly during the last decade, and the effects of this can be seen when looking at the balance of transmission systems. Wind power has traditionally been viewed as a marginal resource and has therefore experienced limited requirements when it comes to helping out with maintaining the system balanced. But now that wind power is increasing its share in the power system, WPPs (Wind Power Plants) also need to contribute to keeping the balance. Modern WPPs have a lot of abilities for active and reactive power control, but there are still questions to be answered when considering the best way to manage this control, and how the interaction of WPPs, grid owners and market shall occur. This master thesis was performed on behalf of E.ON Wind Sweden AB in Malmö, and the division of Industrial Electrical Engineering and Automation at Lund University, Faculty of Engineering. The purpose of the thesis is to investigate how control of WPPs can be applied in order to meet future TSO (Transmission System Operator) requirements and a changing market, with an extra focus on the Swedish situation. Two main cases of control are studied: 1) the possibility of reactive power control contributing to the local voltage balance, and 2) how WTGs (Wind Turbine Generator) within a WPP can be coordinated during curtailment making the process more efficient considering wake effects. The cases are studied through simulations using the computer programs DIgSILENT PowerFactory, and MS Office Excel. The first study concludes that modern WPPs are capable of providing reactive power, contributing to the local voltage balance. But this will occur within certain limits. The recommendation is that a reactive power service shall be sold to the grid owner, providing an extra income for the WPP owner and allowing the grid owner to use the WPP as a resource in the network. These kinds of services are also more likely to be seen in the future, when a new power system is developed which is able to adapt to an increased amount of renewable energy. This new power system needs to be smart and efficient, using all available assets in the best possible way. Being the first to introduce this service will result in a leading position within the market of such services. The second study provides two main strategies for coordination of WTGs within a WPP during curtailment. Which strategy to choose depends on the wind speed. At lower wind speeds it is recommended to start by curtailing the first row of the WPP letting more wind pass through to the back rows with the main goal of avoiding unplanned turbine stoppages. At higher wind speeds the strategy is to curtail the WPP starting at the back row in order to avoid additional fatigue on the turbines caused by increased turbulence and to make the control as fast and as accurate as possible. This is a simplified way of describing the strategies; since they also depend on parameters such as the extent of the curtailment, the wind direction, and the amount of turbines available. If the curtailment reuired is extensive during high wind speeds, it is recommended to curtail the WPP equally in order to avoid pushing single turbines too much when stresses on the structure are already high. Such control can be implemented with current technology, but developing an algorithm which coordinates this specific process would improve the control. The coordination strategies can be used today, but they will prove even more useful for future large WPPs that will need to perform curtailment, either due to grid code requirements, or even more likely due to acting on the regulating power market.