Power enhancement of single-phase transformer-less grid-connected cascaded half-bridge diode clamped inverter under partial shaded photovoltaic

摘要

This study presents a power-enhanced cascaded half-bridge (HB) diode clamped transformer-less multilevel inverter for grid-connected large-scale photovoltaic (PV) system addressing the problem of unequal power in isolated diode clamped (DC)-link due to partial shading. The HB diode clamped has the feasibility of increasing double times the DC-link voltage compared with the conventional cascaded h-bridge to enhance the inverter's power rating without compromising the count and peak inverse voltage of switching devices. Under partial shading conditions, the DC-link of individual HB suffers from different voltages and leads to unbalanced capacitor voltages. The proposed control scheme stabilizes the DC-link voltage of individual HB, extracts maximum active power from PV, and injects the same into the grid. The injected grid current total harmonic distortion satisfies the IEC 61727 and IEEE 1547 standards. PV parasitic capacitance in the transformer-less grid-connected system leads to leakage current. In the present work, LC-filters are placed among the power cells to bring these currents below 30 mA as per German standards VDE V 0126-1-1. For the planned inverter, a mathematical model using the switching function theory has been derived. The inverter's performance has been tested under partial shading conditions by using the MatLab/Simulink platform and experimentally verified with the software-in-the-loop real-time digital simulator OPAL-RT 5700.