A new method to assess the contribution of VSC-HVDC connected wind farms to the primary frequency control of power networks

This paper introduces a new method to assess the contribution of VSC-HVDC connected wind farms to the primary frequency control (PFC) of power networks. This is a formulation that enables the estimation of the post-disturbance conditions of the power system, including its electrical frequency, after the incidence of power imbalances, where various VSC-HVDC connected wind farms may be simultaneously simulated. As opposed to a full dynamic simulation, which may be time-consuming, the developed method resorts to modelling all power system components for the PFC time frame by using enhanced steady-state power-flow models which are solved for a power system operating point that agrees well with the operating conditions of the network after the action of the PFC. Therefore, models of DFIG and PMSG are first derived by following a proposed procedure for the determination of the frequency regulation of the wind turbines, often called statism, whose value varies with the wind speed, as opposed to that of conventional power plants which is constant. Because the power reserve of the wind generators is used to counteract the frequency deviations in the AC power network, a VSC-HVDC link model is also developed which enables a free power transfer to conform to the output power of the wind farm. The proposed method is validated using a 9-bus power system comprising a 120-MW VSC-HVDC connected wind farm and whose results are compared against those computed by a full time-domain dynamic simulation, obtaining a very good agreement between the two quite distinct approaches where, for instance, the post-disturbance system frequency computed by the proposed method only differs by 0.092% with respect to that obtained by the full dynamic simulation. The IEEE 57-bus test system, slightly modified to incorporate two VSC-HVDC connected wind farms, is also simulated to show that the impact of the frequency regulation of the wind farms on the PFC may be assessed in a very quick, effective manner.