Research On Power Balance Control Method With Phase Disposition For Cascaded H-bridge Inverter

Compared with traditional two-level converter,multilevel converter is widely used in medium-high voltage and high-power applications because of its advantages such as the better output voltage waveform,low voltage stress and small output filter.The classic multilevel topologies include neutral point clamped,flying capacitor,and cascaded H-bridge.Cascaded H-bridge converters which require the minimum number of components and do not need DC-side voltage-dividing capacitors become a hot spot of multilevel research.Because all cascaded cells are independent of each other,when the active power is transmitted,the power balance problem should be researed.Modulation strategy is a key technology of multilevel converters,which determines the output voltage,power distribution,system efficiency and other output characteristics.Carrier phase-shiftde modulation strategy can achieve power balance naturally,but the output line voltage harmonics is poor.Phase disposition modulation strategy has better output line voltage quality with lower distortion,but is unable to achieve power balance between cells.Power imbalance makes the battery discharge unbalanced,which will increase the difference of the input voltage,affect the battery life,reduce system reliability and increase maintenance costs.Therefore,the power balance research on the cascaded H-bridge converters under phase disposition modulation strategy has an important significance.This paper introduces power balance control methods for the three cells cascaded H-bridge converters.The output voltage characteristics is analyzed with Fourier series,and the expression of the amplitude of the fundamental component of the output voltage is given,which clarifies the reason of power imbalance.The improved carrier phase disposition modulation strategy based on the modulation wave control freedom is proposed.The simulation results verified the output characteristics.The new power balance control method based on carrier control freedom is proposed.The reason of power balance and its output characteristics are analyzed.The effects of power balance are analyzed and verified by simulation and experiment results.The method can achieve the balance control of power in one output cycle without changing the output voltage of the converter.