Hierarchical Control Strategy for Effective Virtual Frequency Responses of Multiple WPPs

When renewable energy-based generations are increasing, they are subject to cause frequency stability problems in a power system. To solve this problem, various virtual inertial control methods (VICMs) have been recently developed. However, they all have dealt not with the steady-state frequency stability, but only with dynamic frequency stability issues. In particular, the wind power plants (WPPs) operated by these VICMs can only provide a similar inertial response (IR) to that by conventional synchronous generators without providing the primary frequency response (PFR). This results in lowering the settling frequency (SF) below the maximum steady-state frequency deviation (MSSD) band. This paper proposes the new hierarchical control strategy for the WPPs, which are operated by three virtual frequency response control methods (VFRCMs) to provide both IR and PFR. Firstly, the VFRCM-1 provides virtual IR. Next, the VFRCM-2 increases the output power from the WPPs to prevent the frequency drop, which can occur after the frequency nadir is arrested. Finally, the VFRCM-3 makes the WPPs possible to raise the SF above the MSSD band by providing the virtual PFR. The effectiveness of proposed hierarchical control strategy is verified by several case studies on the IEEE 39-bus test system with the high penetration of renewables.