Secondary Frequency and Voltage Control in Microgrids with dVOC-Based Inverters

In this paper, we address the problem of frequency and voltage control in microgrids in which generators and loads are interfaced via grid-forming (GFM) inverters. In our setting, the output voltage and frequency of the inverters is determined by a primary control scheme realized through a control strategy referred to as dispatchable virtual oscillator control (dVOC). This type of GFM primary control is known to stabilize system frequency and voltage magnitudes, but it is not capable of regulating them to their nominal values. To address this issue, we propose secondary frequency and voltage control schemes, both of which rely on integral control. The secondary frequency control scheme is centralized and similar in nature to that utilized in bulk power systems, whereas the secondary voltage control is completely decentralized. The operation of the proposed secondary frequency and voltage controls is illustrated via numerical simulations.