Harmonic reduction under unbalanced operating conditions of PV-connected cascaded H-bridge multilevel inverters using fault tolerant adaptive phase-shifted pulse width modulation

摘要

Balanced operating conditions of photovoltaic (PV)-connected cascaded H-bridge multilevel inverters (CHB-MLIs) require equal number of series and parallel arrays, equal amplitude modulation indices, and same environmental conditions such as temperature and irradiance for each H-bridge (cell). Unbalanced operating conditions arise when at least one of these requirements are not met and/or a DC fault occurs. Phase-shifted pulse width modulation (PS-PWM) is the most widely used technique for CHB-MLIs. However, under unbalanced operating conditions, harmonics that appear around the effective switching frequency (f(sw)) of each cell fail to eliminate each other when PS-PWM is used, and consequently, undesired harmonics are generated at the multiples of f(sw) on the output voltage of CHB-MLI. In this paper, a fault tolerant adaptive phase-shifted pulse width modulation (FTA-PS-PWM) method is proposed for any number of cells to overcome this problem. For this reason, a mathematical representation of the harmonics appearing around the multiples of f(sw) on the output voltage of CHB-MLI is suggested. In this analysis, on/off instants of the power semiconductors are investigated in frequency domain for a single carrier period. Based on derived formulations, phase angle of each cell is calculated at each sampling time using DC voltages and amplitude modulation indices of all cells in FTA-PS-PWM. In order to validate the usefulness of the proposed switching strategy, a three-phase, PV-connected CHB-MLI is established in simulation environment and a prototype of a single-phase CHB-MLI is built. It is shown that FTA-PS-PWM reduces output voltage harmonics effectively in unbalanced operating conditions including DC faults for different number of cells. Moreover, FTA-PS-PWM is also compared with previous methods in the literature and compliance of output harmonics with international standards is examined for various cases. The results reveal that FTA-PS-PWM generally provides better results in terms of individual harmonics and total harmonic distortion of output voltages of CHB-MLI and satisfies the requirements of the standards.