抄録
In this paper, we propose a new effective method for improving waveforms of output voltages in a double connected voltage source inverter. This method is based on applying two different input DC voltages which change to two levels periodically and differ each other from half cycle in phase, as DC power supplies into the inverter apparatus. The operations of two 6-phase bridge inverter circuits and the configurations of two output transformers, connected delta-delta and delta-star, are exactly the same as the conventional 12-step inverter.
First is this paper, the principle of the proposed method is described, and the optimum input voltage waveforms are decided theoretically by minimizing the total harmonic distortion factor of the output voltages in this inverter apparatus. Then we suggest that the calculated input voltages with two levels can be realized by dividing those into two DC voltage supplies whose voltage ratio is 2:1 and adding four bi-directional switching elements. Also, the capacities of the DC input sources and the output transformers are calculated.
As a result, we can make clear that although the construction of the proposed inverter is very simple, the output voltage waveshapes of the inverter are almost the same as those of the 24-step inverter. The experimental discussions are also given.
Moreover, a voltage control method for applying it to uninterruptible power supply is discussed. In addition, it is shown that the waveforms of output voltage in the inverter apparatus are improved, similar to the conventional 36-step or 48-step inverter when the DC voltage source is divided into 3 or 4 supply parts.
