Efficiency enhancement scheme of cascaded multilevel grid-connected inverter and its improvement to eliminate effect of non-ideal grid conditions

• Propose efficiency enhancement scheme with adjusting carrier frequency.
• Analyze the effect of non-ideal grid on the EPLL.
• Analyze the effect of periodic ripple of grid amplitude on the efficiency enhancement scheme.
• Propose Hybrid filter based EPLL to eliminate the effect of non-ideal grid.

In this paper, an online efficiency enhancement scheme of a cascaded multilevel grid-connected inverter for renewable energy generation systems is researched in detail. Taking the special grid current standard as the limitation condition, an efficiency enhancement scheme with adjusting carrier frequency in real time is proposed. Furthermore, the effect of the harmonics and DC offset of the tied grid on the enhanced phase locked loop (EPLL) is analyzed and a hybrid filter based EPLL is proposed to eliminate the former effect. With the improved EPLL, the amplitude, phase angle and frequency of grid are all estimated correctly and the performance of the efficiency enhancement scheme is maintained even under non-ideal grid conditions. Detailed experimental results validate the accuracy and practicability of the improved EPLL and efficiency enhancement scheme.