Secondary control with reduced communication requirements for accurate reactive power sharing in AC microgrids

A secondary control method is proposed for accurate reactive power sharing as well as frequency and voltage restoration in islanded AC microgrids (MGs). The proposed method consists of an MG secondary controller, local secondary controllers for distributed energy resources (DERs), and a low-bandwidth communication link for broadcasting common data from the MG secondary controller to DERs. The broadcasted data includes the MG point of common coupling voltage magnitude and a common vertical shift for frequency and voltage restoration. Local secondary controllers calculate specific shifts for the Q-V droop characteristic of each dispatchable DER and the V-Q reverse droop characteristic of each photovoltaic (PV) system, aligning their operating points with the Q-V_PCC and V_PCC-Q droop characteristics, respectively. By employing V_PCC as a common global variable, coordination of reactive power generation of all dispatchable DERs and PV systems is achieved, enabling accurate reactive power sharing. Importantly, in the proposed scheme, the required communication bandwidth and the communication burden are minor and are not increased with the number of DERs. Additionally, the DERs are relieved of the need for data transmission capability. The small signal stability of the proposed method is examined and its effectiveness is validated through Hardware-in-the-Loop (HIL) experimental results.