On Droop-based Voltage and Frequency Restoration Techniques for Islanded Microgrids

Decentralized hierarchical control techniques do not require a communication layer in their secondary control level. These control techniques reduce the vulnerability of microgrids (MGs) to cyber-attacks, reduce data losses, lessen time delays, and mitigate costs for communication infrastructure. In the islanded AC MG, droop control is profusely used at the primary level to achieve accurate power-sharing. However, it may result in steady-state voltage and frequency (V/f) deviations with varying load conditions. The secondary control layer is required to restore these deviations while maintaining droop dictated power levels. The secondary layer supports V/f control at a slower time scale, while the primary droop-based layer supports V/f tracking at a faster time scale. This paper presents a review of the decentralized secondary control systems that interact with droop based primary controllers. The V/f restoration schemes by the secondary control layer are categorized as linear and non-linear technologies. The mathematical formulations for each control scheme are presented, and a tabulated summary of all the control systems. Finally, simulations are conducted for a selected control technique from each category, and the results are presented and discussed.