Recent years, multilevel inverter has been applied widely in the largecapacity electrical energy transformation of highvoltage and AC speed adjustable system. Compare with traditional twolevel inverter, the multilevel inverter has many strong points: lower voltage stress of electrical devices, higher power factor, and better harmonic characteristic. Whereas, along with the increasing of level numbers, this kind inverter appears several notable problems: 1. losses of power devices; 2. unbalancing of DCcapacitorvoltage; 3. commonmode voltage. Based on the research of diode clamped multilevel inverter, this paper focuses on the problem of capacitor voltage balancing. The main contents of this paper are as follows:1.This paper gives a brief account of some balancing methods of DCcapacitorvoltage from the hardware improving and the control strategy, and analyzes the strongpoint & shortcoming of these methods.2.According to the modulation theory of multilevel inverter, this paper researches the method of capacitor voltage modeling based on SPWM & SVPWM modulation, and analyzes the manner of fluctuation and excursion on capacitorvoltage.3.Base on the analysis of DCcapacitorvoltage unbalancing principle, a new modeling method considered of capacitor voltage balancing is proposed, a modulation strategy which can balance capacitor voltage in all range is also formulated.4.A hybrid modulation strategy is proposed by consideration the capacitor voltage balance PWM in all range and SVPWM modulation simultaneity, which can either balance capacitor voltage or improve harmonic characteristic.Finally, the summary and expectation of the research on multilevel inverter are brought out.
