Droop assignment algorithm for the inertial control of a DFIG-based wind power plant for supporting the grid frequency

In a wind power plant (WPP) consisting of multiple wind generators (WGs), the wind speed of WGs at the downstream side decreases due to the wake effect, and thus their rotor speeds are smaller than those of the upstream WGs because of an MPPT operation. Therefore, WGs in a WPP have different amount of the kinetic energy stored in their rotating masses. This paper proposes a droop assignment algorithm for the inertial control of a doubly-fed induction generator-based WPP to support the grid frequency during the large disturbance. This paper assumes that a WG in the WPP has two supplementary loops for the inertial control, i.e. the frequency deviation loop and the rate of change of frequency (ROCOF) loop. In order to give more contribution on supporting the frequency stability by temporarily releasing more kinetic energy, the proposed algorithm assigns the droop of each WG depending on the rotor speed, while the same gains of the ROCOF loop are set for all WGs. In addition, the wake wind speed arriving at the WG is calculated by considering the wind direction and cumulative impacts of multiple shadowing. The performance of the algorithm was investigated under various wind conditions using an EMTP simulator. The results clearly indicate that the algorithm successfully improves the frequency nadir because WGs with higher wind speeds temporarily releases more kinetic energy.