Research of Current Source Converter Suitable for Energy Storage System with Batteries

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In this research, we investigate current source converters suitable for energy storage systems with batteries. First, a simplified single-phase cascade current source inverter is proposed. It has the same construction as the conventional three-phase current source inverter that is interconnected to the single-phase three-wire distribution system. Though the proposed circuit has no output transformer, the output currents can be obtained as the five-level waveforms by diverting the pole transformer in the utility system. With discussing several experimental discharging characteristics to the power grid, it is clarified that the output distortions can be decreased sufficiently. Moreover, the proposed equipments are improved so as to minimize the dc smoothing reactor. As a result, the reactor capacity can be decreased to a hundredth, from 300mH to 3.5mH.Next, several PWM control schemes for the three-phase current source inverters are investigated. We propose novel PWM control patterns, which can be simplified effectively by overlapping the control signals to the plural switching devices as long as the inverters are interconnected to the power grid. To decide respective pulse widths composing output currents, a method in proportion to the corresponding system ac voltage is presented. Several experimental discussions for the charging characteristics with the converter and the discharging ones with the inverter are discussed in detail. Then, by improving to the compound control scheme with PWM and PFM, harmonic reductions in the ac line currents can be satisfactory for the Guidelines for System Interconnection.