Sviluppo di convertitori modulari multilivello ad alta efficienza per fonti rinnovabili

The global market for commercial and residential systems, as well as research interest in Photovoltaic (PV) systems, has been steadily increasing for years. Low- Voltage (LV) (< 1.5 kVdc) medium-power (a few hundreds of kW) PV systems have witnessed rapid growth in the global electricity market as they are widely accepted and easy to install and implement. Therefore, the high penetration of LV PV systems in electricity distribution networks requires highly efficient and reliable power conversion units. Two alternatives are being considered for PV inverters: a) Two-level with a 1.7 kV or higher semiconductors and b) three-level inverter with a 1.2 kV or lower semiconductors. Multilevel topologies, especially the three-level ones such as Neutral-Point-Clamped (NPC) and Flying Capacitor (FC) structures, have become highly competitive alternatives to the two-level hard switching inverters due to their advantages such as lower switching losses, higher power quality, smaller filters, and lower Electromagnetic Interference (EMI). The potentially usable semiconductors are: a) 650V and 1.2 kV Insulated Gate Bipolar Transistors (IGBTs); b) 650V, 900V, or 1.2 kV Silicon Carbide (SiC) Metal–Oxide–Semiconductor Field-Effect Transistors (MOSFETs), and c) Gallium Nitride (GaN) MOSFETs. Accordingly, for low voltage applications, the optimal choice is to use 650V or 1200V components. Three-level topologies are associated with a major drawback due to the inevitable leakage inductance that can cause over-voltages in the semiconductor devices and consequently limit the operation when a high current or a large variation of current (di/dt) is used. This issue becomes more critical with a load PF different from unity or when operating in the rectifier mode where the current and voltage have opposite signs. On the other hand, the standard MMC topology can solve these issues. Since each module can be assembled with two switches and a capacitor, parasitic inductance can be minimized quite easily, even with a standard and inexpensive module assembly. Moreover, the switches are always modulated and not just half-wave. As a result, the switches dissipate the same amount of loss, resulting in a more uniform temperature operation. Unfortunately, the enormous amount of capacitance and, in turn, the energy stored in module capacitors limits the use of classic MMC topology in LV applications. The main focus of this work is to evaluate the MMC for LV PV applications, reduce the MMC capacitance requirements, and propose a high efficiency, cost-effective, and compact three-phase MMC structure. This work starts with the evaluation of the MMC for a low voltage PV system, the sizing and design of the different components, and the investigation of possible modulation and control schemes. Then, an improved modulation scheme for a modified MMC structure is proposed, which is shown - analytically and experimentally - to reduce the capacitance requirement of this structure. To further reduce the capacitance requirement of the MMC, a three-level MMC topology with an optimized capacitor size is proposed. The proposed structure is designed and optimized to have relatively lower capacitance and fewer number of components than conventional MMCs. The different switching states, component design, sizing, modulation, and proposed topology control are studied. Also, capacitor voltage control is introduced to reduce the number of current sensors required. Finally, the discontinuous conduction mode is analyzed, which leads to an optimized MMC structure with low capacitance and inductance requirements.