Simon De Rijcke

Impact of wind turbines equipped with doubly-fed induction generators on distance relaying

Wind generation is evolving from single wind turbines to wind power plants with an installed capacity of tens of megawatts. Therefore, wind turbines are also coupled to the transmission grid instead of the distribution grid. Problems arise during grid faults as the behaviour of wind turbines equipped with doubly-fed induction generators differs fundamentally from traditional synchronous generators. The short-circuit behaviour is vital for protection issues. A distance relaying problem in a Belgian grid topology is emphasized and studied. This study concludes that the distance relay is not performing correctly in case of three phase faults. Moreover, varying parameters such as the crowbar resistance and the wind turbine loading have a significant impact on the relay performances.

Trading Energy Yield for Frequency Regulation: Optimal Control of Kinetic Energy in Wind Farms

The burden on conventional units to regulate the system frequency increases if they are replaced due to wind farms. This paper explores up to which time scales the rotating kinetic energy in wind turbines can smooth frequency variations and assist with the regulation task. To this end, a comparison is made between a standard wind turbine controller and optimal control of wind turbines, respectively derived from causal time-domain simulations and an optimization algorithm that allows predicting. The latter algorithm is used to give a benchmark for the smoothing potential, shown by plotting the Pareto efficiency of the normalized standard deviation of frequency variability versus a normalized measure of the energy yield. Results indicate that smoothing comes with an energy loss that is determined by the energy content of power imbalances. It is shown that a wind share of 20%, within the instantaneous generation mix, can absorb frequency variations on timescales up to 100 sec while the energy loss is limited to only 2%. A higher share of wind power aggravates frequency variability. Nevertheless, in such circumstances the potential of rotating kinetic energy in wind farms increases.

Asymmetric reserve power delivered by large wind power plants

A stable and secure operation of the electricity grid is mainly achieved by contracting power generators to ancillary services in addition to their main commercial product, active power. Electricity from Renewable Energy Sources (RES-e) is today generally exempted from the participation in ancillary services. However, the increasing share integration of variable RES-e with a limited predictability has an impact on the demand and supply structure of these services. In this paper, the possibility of wind power participating in frequency control or delivering active power reserves as an ancillary service is investigated. Within this framework, technical, regulatory and economic aspects are examined and evaluated. As the specific details about the way ancillary services are contracted differ over Europe, a case-study is done for the Belgian control zone. The consequence of offering an asymmetric reserve power, with only downward regulation, is economically assessed.

Smoothing turbulence-induced power fluctuations in large wind farms by optimal control of the rotating kinetic energy of the turbines

In the current study, we use a large-eddy simulation of a wind-farm boundary layer to generate the fluctuating wind fields that are observed at different turbines in the wind farm. Using these wind fields as inputs, we focus on the development of a benchmark framework in which we explore the trade-off between high energy extraction and low variability using optimal control of multiple turbines subject to a turbulent wind field. The controls variables that are optimized are the electric torque and the pitch angles of the individual turbines over time horizons of 10 minutes. Moreover, both optimal control of individual turbines and coordinated optimal control of groups of turbines are investigated. Optimal control results are presented in terms of Pareto fronts that show optimal trade-offs between energy extraction and power smoothing. We find that power variations can be significantly reduced with limited loss of extracted energy. Moreover, coordinated control can effectively reduce fluctuations over longer time scales. For instance, considering 24 optimally coordinated turbines, variability at a time scale of 50 seconds is reduced 4 times more than the normal statistical reduction of 24 uncoordinated turbines.

Reducing power gradients in large-scale wind farms by optimal active power control

Recently, new requirements have been imposed by grid regulators on power gradients or ramping rates of wind farms. These requirements are an important concern for wind farm operators, as they introduce additional losses in farm efficiency. In the current study, we investigate the use of optimal coordinated control of rotating kinetic energy reserves to smooth power gradients in a wind farm, while maximizing power extraction. To that end, we use turbulent velocity fields obtained from large eddy simulations of a large wind farm, and set up an elementary case in which we consider the coordinated control of two turbines only. We find that optimal control of kinetic energy reserves allows to significantly reduce power variations, keeping them within requirements on power gradients. Moreover, in comparison to a per-turbine limitation of gradients with a greedy power control, we extract on average 3% more power.

Grid impact of voltage control and reactive power support by wind turbines equipped with direct-drive synchronous machines

This paper proposes voltage control and reactive power support with direct-drive synchronous machines in wind turbines during normal operation and transient events. The wind turbines are equipped with voltage and reactive power control for normal operation and undervoltage ride-through modes for voltage dips. The wind turbine contributions are evaluated both for theoretical test cases as well as for two specific locations in the Belgian grid. Simulations reveal that the preferred mode for voltage support during a voltage dip depends on the grid characteristics short-circuit power and X-R ratio. Additionally, the angle stability of both induction motor load and nearby synchronous generators can be improved by adding reactive power support by the wind turbines. From the Belgian case study, it is concluded that voltage control by wind power plants is preferred, especially in remote areas where the additional control becomes necessary to maintain operation in between voltage limits. In case of voltage dips, a coordinated control of wind power plants in the area lowers the voltage reduction.