SPWM-based D-Σ Digital control for paralleled 3-φ grid-connected inverters

This paper presents an SPWM-based division-summation (D-Σ) digital control for paralleled three-phase grid-connected inverters. The proposed D-Σ control approach derives the plant, and then obtains the control gains or duty-ratio control laws 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 circulating currents. The inverter system can achieve the functions of grid connection, rectification with power factor collection, and STATCOM 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 start up and stored in the controller for scheduling loop gain cycle by cycle. Experimental results from a three-inverter system have confirmed the analysis and discussion of the proposed control approach.