Voltage Modulated Direct Power Control for a Weak Grid Connected Voltage Source Inverter Based on Series Voltage Regulator

Abstract

In this paper, we design a voltage modulated direct power control (VM-DPC) for a three-phase voltage source inverter (VSI) connected to a weak grid, where the PLL system may make the system unstable if the conventional vector current control (VCC) method is applied. Compared with the conventional VCC method, the main advantage of the proposed VM-DPC method is that the PLL system is eliminated, SVR is used for providing stiff dc source so that voltage variation at inverter input side stays within specified limit. SVR uses a dual active bridge dc-dc converter followed by a full-bridge dc-dc converter. Moreover, in order to inject the rated real power to the weak grid, the VSI system should generate some certain amount of reactive power as well. An eigenvalues based analysis shows the system with the proposed method tracks its desired dynamics in the certain operating range. Note that the required power rating of the SVR is very low (say 2.7%) compared to the load demand considering 5% voltage regulation. In this work, the voltage regulator is connected at the mid-point of the grid, but it may be connected in some other locations to get optimal rating of the same. The proposed configuration is simulated in MATLAB/SIMULINK. Both simulation and experimental results match the theoretical expectations closely.