Abstract
This paper presents an SPWM-based division-summation (D-Σ) digital control for paralleled three-phase grid-connected inverters. This study first derives the plant, and then design the controller to cancel the variation effects of dc voltage, switching period and inductance. With the D-Σ digital control, each inverter can track sinusoidal reference current of each phase independently, eliminating circulation currents. The inverter system can achieve the functions of grid connection and rectification with power factor correction by taking into account wide filter-inductance variation and grid-voltage distortion, reducing core size significantly. The control laws for achieving the desired functions are derived in detail and they are expressed in general forms for readily software programming. In the design and implementation, the inductances corresponding to various inductor currents were measured at the start-up and stored in the controller for scheduling loop gain cycle by cycle. Additionally, this study adopts master-slave control, the master inverter is responsible for DC-bus voltage regulation, while the slave ones track the current command calculated by the DC-bus current and the number of modules to achieve equal current distribution. The major contributions of this research can be summarized as follows. First, the control law derived based on D-Σ digital control principle can take care of inductance and grid voltage variations. Secondly, the proposed control strategies can suppress circulating current without using zero-sequence current injection. Thirdly, slave inverters use the DC-bus current and the number of modules to calculate the current command for achieving voltage regulation and equal current distribution. Finally, these paralleled three-phase bi-directional inverters have been implemented and tested. Simulated and experimental results have shown that equal current distribution and suppression of circulating current can be achieved with the proposed control scheme.