Split-Multistring Photovoltaic 3-Level NPC Inverter with Capacitor Unbalance Control Suitable for Partially Shaded Conditions

In a conventional grid connected photovoltaic system employing centralized inverter with long strings connected across dc-link, the performance of the plant in terms of power generation and efficiency is not optimal especially during partial shading and/or panel mismatch conditions. To improve the power yield during such conditions, a splitmultistring configuration employing 3Level Neutral Point Clamped (NPC) inverter is described in this paper. In this configuration, each long PV string is split into two half-strings and each half string is then connected across DC link capacitor of the NPC inverter through a boost converter. This configuration enables the maximum power generated in individual strings of shorter length to transfer to the grid and thereby produces higher yield. The decoupling nature of the boost converter that is connected with each string prohibits the transients generated in a string to have a detrimental effect on other strings. However, the unequal MPP voltage of each string due to partial shading and/or panel mismatch may cause an unbalance across dc link capacitor voltages, which in turn produces distortion in the load side and affects the quality of output power supplied to the grid. To mitigate this voltage unbalance issue, a Double Signal Pulse Width Modulation (DSPWM) based controller is adopted here that is able to balance the dc link capacitor voltages quickly. The dynamic responses of the system including grid synchronization, Total Harmonic Distortion (THD) etc. at different solar irradiation is simulated in MATLAB/Simulink and the simulation results are produced. The dc link capacitor voltage balancing scheme applied for this configuration has experimentally been verified in a 3LNPC prototype inverter developed in laboratory. Keywords—Multistring, Neutral Point Clamped (NPC) inverter, partial shading, photovoltaic system, Multilevel Inverter.