# Three-Phase PV CHB Inverter for a Distributed Power Generation System
## 1 Introduction  
During the last decade, photovoltaic (PV) energy conversion has become a fully mature technology, giving (in many countries) an impressive contribution to sustain the electricity demand. Its worldwide diffusion has also been supported by the increased reliability of power converters (henceforth PV inverters) which interface solar systems with the utility grid. The role of PV inverters is of utmost importance for the overall suitability of photovoltaic energy, because they manage energy exchange between solar panels and utility grid by guaranteeing (i) maximum conversion efficiency and (ii) high quality of delivered electricity. The first task is performed by means of Maximum Power Point Tracking (MPPT) algorithms that--by adjusting the switching operation of the power devices forming the converter--allow the PV system to always work with the optimum load. The second task, which consists of delivering \"pure\" sinusoidal currents with assigned parameters, is pursued by means of proper signal processing strategies. Although a wide variety [1] of PV inverter topologies have been proposed so far, all of them can be roughly classified with respect to the number of independent inputs and to the size of the PV generators supplying each of them (DC (Direct Current) side) into two main categories, adopting, respectively, \"centralized\" conversion approaches or \"distributed\" conversion approaches. By recalling that an arbitrary large PV field is always built up by interconnecting many elemental PV generators, centralized conversion schemes are those where a single-input inverter serves manyPV generators [2; 3]. This solution has indubitable strengths; indeed, it minimizes the number of power devices (thus increasing both reliability and conversion efficiency), simplifies cabling, and so on. The weaknesses depend on the fact that when mismatches among operating conditions of the PV generators occur, power production can be adversely affected; in fact, as is widely known [4; 5], the worst performing generator limits the performance of the whole photovoltaic system. In order to overcome this issue, it is possible to partition the solar field into subfields, each connected to a dedicated PV inverter. This approach can be driven up to equipping each solar panel with its own DC-DC [6; 7; 8; 9] or DC-AC (Alternate Current) converter [10; 11; 12]. In this case, the yield of the PV system increases during mismatch conditions, but is adversely affected during normal operation (when no mismatches occur), because of the power dissipation across the increased number of switching devices; moreover, the need for a high voltage gain often requires the adoption of a double-stage converter topology, with a DC-DC stage providing gain, followed by the DC-AC conversion. On the other hand, a significant effort has been paid in recent decades to improve the conversion features (e.g., efficiency, total harmonic distortion, and so on) of the grid-tied inverters (AC side). In particular, recent literature proves the effectiveness of some multilevel approaches [13; 14]. Although the appealing advantages provided by both the classical topologies, such as Neutral Point Clamped (NPC) and Flying Capacitor (FC) [15], and by the recent Crossover Switches Cell (CSC) [16] and PUC5 [17], only Cascaded H-Bridge (CHB) [15; 18] is suited for multiple isolated DC-sources [19], thus joining the best features of both distributed and centralized approaches, while implementing a multilevel conversion. In particular, Cascaded H-Bridge (CHB) multilevel PV inverters [20; 21; 22; 23; 24] are composed of several \"smaller\" (in terms of rated power) full-bridge DC-AC converters (hereafter referred as _cells_), each of them made by downsized switching devices, which are therefore less affected by dynamic and static power losses.