Per-Phase Unsymmetrical Adaptive Derivative Optimized Droop for Mitigating Voltage Quality Issues of Unbalanced Islanded Microgrids

Abstract

The proliferation of unbalanced linear and nonlinear loads in microgrids degrades the voltage quality at the inverter terminals, and thus, load terminals receive unbalanced and distorted voltages. To alleviate these voltage quality issues, this paper proposes a per-phase unsymmetrical adaptive derivative optimized droop control scheme for unbalanced islanded microgrids (UIMG). The proposed controller is coupled with a per-phase unsymmetrical virtual impedance (UVI) to mitigate the unbalance of the inverter-based UIMG. Additionally, a proportional multi-resonant (PMR) controller is adopted to compensate for the voltage distortion. To improve the UIMG dynamics response while changing the loading states, derivative terms of active and reactive powers are added to the proposed control. The derivative gains are adaptively updated with the change in the microgrid loading to achieve the desired transient response. The proposed scheme is formulated as a multi-objective optimization problem to determine the per-phase unsymmetrical droop settings and UVI that fit different loading states simultaneously. Several case studies are designed to test the effectiveness of the proposed control scheme under different types of disturbances and operating conditions. The cases are conducted on the IEEE 34-bus benchmark power distribution feeder with a combination of unbalanced linear and nonlinear loads. Moreover, the proposed control is validated using real-time simulations carried out in OPAL-RT system. The results show that the proposed control scheme is capable of mitigating voltage quality issues by reducing the voltage unbalance factor and voltage distortion.