New Stability Concept for Primary Controlled Variable Speed Wind Turbines Considering Wind Fluctuations and Power Smoothing

With a further expansion of the wind energy sector, wind turbines (WTs) have an increasing impact on grid stability. Requirements for a grid connection are adapted continuously, especially for frequency support and the fault ride through (FRT) behavior of WTs. Besides the objectives of robust operational stability and maximum energy yield, a smooth of active power is also of interest to reach efficient load to generation balancing of the grid. With stochastic wind fluctuations, power smoothing methods gain importance, but in combination with primary power control, operation points (OPs) with the risk of operational instability occur. In these OPs with speeds below the maximum power point (MPP), a deceleration of the drive train to standstill is possible. While, so far, to ensure a stable operation especially variable droop is preferred for a WT with power controlled generator, a method to guarantee stability on conventional controller principles with constant droop is missing. Therefore, in this article, a control structure with power smoothing, constant droop, and a stabilizing limitation called MPP-limiter is presented, which ensure robust operational stability of the WT without requirements for additional safety margins. Therewith, the provision of the full primary control power is passible. Based on theoretical investigations, parameter studies, individual scenarios, and test bench results, the demand for the MPP-limiter from a stability perspective is shown. Advantages of the power smoothing and its effect on the energy yield are explained, like an increase of 6 % of total energy in the transition range between partial and full load.