Adaptive Power Control Strategy for Smart Droop-Based Grid-Connected Inverters

Abstract

Grid-connected inverters play an important role in the integration of renewable energy sources such as solar and wind. However, due to the unneglectable grid impedance value seen by the inverters at the point of common coupling (PCC), especially in the weaks and resistive low voltage distribution networks, there is an inherent strong coupling between active and reactive power flow. This power coupling causes significant power quality problems including 1) voltage fluctuation of the common AC bus resulted from high penetration of intermittent renewable generation systems, 2) non-optimal control of neither power flow nor power factor of the delivered power by inverters to the common AC bus, and 3) unintended/uncompensated transmission losses, where the flow of active power through the transmission/distribution lines will cause unintended reverse reactive power flow from the grid-side and vice versa with the reactive power flow. To solve these issues, this paper proposes an adaptive mechanism for droop-based grid-connected inverters to decouple the power flow by compensating the associated unintended active and reactive power losses flowing through the transmission line (or any desired segment of it). This control strategy relies on modifying the power command provided to the frequency and voltage droop loops by considering the effects of both the transmission line resistance and inductance components on the power flow between the inverter and the grid. It uses only the local current and voltage measurements to first perform an online estimation of the transmission line resistance and inductance and then to calculate the proposed adaptive power terms. The performance of the proposed control is validated in MATLAB/Simulink and HIL experiment for a 350 kW droop-based grid-connected inverter system. The proposed control strategy can be utilized to provide ancillary services to the grid such as accurate frequency and voltage support at the location of interest.