A novel PWM modulation and hybrid control scheme for grid-connected unipolar inverters

The purpose of this paper is to provide low cost, high power quality and high efficiency solutions for grid-connected inverters. Compared with bipolar switched inverters, unipolar switched inverters have the advantages of higher efficiency due to reduced switching frequency and low iron losses of inductor in output filter. However, unipolar inverters produce large harmonic distortion due to the dead-time effects. To address this issue and achieve higher efficiency, a new PWM modulation and a hybrid current control scheme is proposed. Firstly, according to the polarity of current reference, in each leg, the gate drive signal of positive or negative switch is removed, and the dead-time insertion is eliminated. With this modulation method, the inverter circuit operations are divided into six operation modes, and their mathematical models are derived. According to this mathematical model, a hybrid control scheme is proposed that consists of a PI controller and an open-loop controller. The PI controller operates in the linear areas, and the open-loop controller operates in the nonlinear areas. Several control rules are designed to realize smooth transition between different control modes. With this control scheme, firstly, the influence of dead-time can be eliminated, and the harmonic distortion can be greatly reduced. Without the influence of switching dead-time, the dc-link voltage can be minimized that will contribute to reduced switching losses and smaller ripple current. The proposed control scheme is verified with simulation and experimental results. The experimental test is implemented in a 10 kW single-phase grid-connected inverter that uses a DSP controller TMS320F2808. The reductions of conversion losses, low harmonic distortion and low ripple current are all confirmed in the experimental system. The comparisons between the conventional control scheme and the proposed scheme are presented in this paper.