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The output current ripple of the three-level (3-L) inverter is important to the performance of the 3-L permanent magnetic synchronous motor (PMSM) drive system, but the neutral point (NP) balance for the 3-L T-type inverter is not considered in the analysis of current ripple up to this point. The NP balance algorithms change the current ripple distribution and it is necessary to develop novel methods for current ripple analysis. In this study, the sinusoidal pulse width modulation with zero-order voltage injection and virtual space vector PWM (VSV-PWM) are utilised to balance the NP potential of the 3-L inverter in the PMSM drive system. The analytical methods of current ripple under these two modulation methods are proposed and the effects of these two NP balance methods on the output current ripple are investigated, respectively. Based on the analysis of current ripple, two variable switching frequency PWM (VSF-PWM) methods considering the NP balance are proposed. The proposed VSF-PWM methods can not only reduce the switching loss but also suppress the electro-magnetic interference noise while keeping the NP potential balanced. Especially, the proposed variable switching frequency VSV-PWM (VSF-VSV-PWM) can effectively alleviate the intrinsic problem of high switching loss of VSV-PWM.

Soluble trivalent manganese solution can be used as an electrolyte for manganese redox flow battery (MRFB), due to its low cost, environmental friendliness and multiple states of charge. However, poor stability of soluble trivalent manganese solution and low trivalent manganese concentration are the disadvantages of electrolyte. To optimize the stability and conductivity of soluble trivalent manganese solution, the sulfuric acid concentration, one of the major influence factors, should be firstly improved. Two-compartment cell electrolysis technology was successfully used in this work. Moreover, to improve the concentration of Mn3+, some key factors, such as the concentration of MnSO4, current density, electrolysis time and temperature, should be further adjusted. The results demonstrated that the concentration of this produced trivalent manganese (≧ 0.3 M in 6 M H2SO4;≧ 0.4 M in 5 M H2SO4;≧ 0.5 M in 4 M H2SO4) is much higher than that have been reported. Solid product of manganese(III) sulfate was also firstly prepared. For the formation of soluble trivalent manganese, the 5 mol/L H2SO4 is the optimal acidity and 10.9 mA/cm2 is the best current density at 273 K for 3 h. Some electrochemical kinetics parameters had also been calculated (electrolyte is 5 M H2SO4 and 0.4 M MnSO4), e.g., diffusion coefficient of Mn2+ is 5.3057 × 10−6 cm2/s, transfer coefficient of electrode reaction is 0.3782, and exchange current density is 1.5662 × 10−4 A/cm2. The CV results demonstrated that the redox process of Mn(III)/Mn(II) is the quasi-reversible process.