# Three-Phase PV CHB Inverter for a Distributed Power Generation System
## 1 Introduction
Moreover [25], thanks to ability of multilevel topologies to produce \"stepped\" output voltages, CHB inverters reproduce sinusoidal waveforms better than conventional full-bridge inverters with less filter efforts, the total harmonic distortion (THD) being a function of the number of discrete voltage levels exploited to synthesize the output waveform [26]. Moreover, the modular structure of CHB inverters is naturally well-suited for distributed power conversion, because each cell can be powered by a small PV generator and perform a dedicated MPPT [20]. Despite of the aforementioned advantages, in CHB inverters, the control of electronic devices switching operation is much more demanding compared to conventional inverters. Moreover, the increased number of switching devices requires an accurate evaluation of power consumption during turn-on and turn-off transients [27], which affects the conversion efficiency. In a recent paper [28], a single-phase CHB multilevel PV inverter suitable for high granularity maximum power point tracking (up to single panel level) was presented. The innovative control strategy adopted therein allowed only one elemental converter to work in switching mode while the others were kept either \"ON\" or \"OFF\" (see next section). The proposed three-phase CHB adopts a mixed staircase-pulse-width modulation (PWM) [29; 30; 31; 32; 33] strategy that drives only one power cell at time to work in PWM, while all other cells provide a constant output voltage. Output voltages achieved by each cell can be independently adjusted so that the maximum power point (MPP) of each PV subsystem (connected to the corresponding cell of the inverter) can be individually tracked. The number of sections can coincide with the number of solar panels forming the PV system; thus, single panel MPPs can be individually adjusted, and the proposed approach can be effectively adopted for single panel distributed conversion schemes. In order to extend this modulation strategy to a three-phase system, the injection of a proper zero-sequence voltage is adopted to avoid the per-phase power imbalance due to unevenly operating PV generators (i.e., intra-phase power imbalance). In fact, the per-phase power imbalance leads to unbalanced three-phase currents, not acceptable in grid connected application. To this aim, a cascaded PI (Proportional Integral) control was implemented, comprising an outer control loop regulating the overall DC-link voltage and an inner control loop accounting for the regulation of direct and reactive grid currents by means of a multivariable PI controller [34]. This latter ensures that \(d\) and \(q\) components are almost fully decoupled, while allowing a simple implementation and assuring fast dynamics and zero steady-state error. Even though effective approaches which concern optimal control strategies of power balancing have been proposed in recent literature [35; 36; 37], the adopted choice reaches the goal of proving the compatibility of the proposed modulation strategy with traditional and commonly adopted control techniques. The paper is organized as follows: in Section 2, the proposed circuit topology is described and the mathematical model for controlling its operation is presented; in Section 3, the control strategy, along with the proposed CHB inverter implemented in Matlab/Simulink(r) are shown, and numerical results are discussed to verify the effectiveness of the proposed approach in static and dynamic conditions, even under partial shading. Conclusions are drawn in Section 4.