A Parallel Matrix Converter System

This paper proposes a large-scale matrix converter system which connects two main circuits in parallel and in which the carriers operating each main circuit have a phase difference of 180 degrees. A matrix converter needs an LC filter, not only because it acts as a conventional filter which reduces high frequency ripples caused by the converter switching, but also because the capacitor of the LC filter acts as a virtual voltage source. Therefore, this filter must be connected at the input side. The proposed system can miniaturize the LC filter because the harmonics of the current flowing into the LC filter contain only the even number harmonics of the carrier and the cut-off frequency of the LC filter can be raised. For example, when the cut-off frequency is doubled, the inductor or the capacitor of the LC filter can be reduced to 1/4. The principle of raising the filter current´s frequency is verified theoretically using simulation. Furthermore, in the proposed system, the input current waveforms become similar to those of a 3-level converter and the output voltage waveforms become similar to those of a 5-level converter. Therefore, the distortions of the input currents and the output voltages become less than with a conventional single matrix converter, and the common mode voltage, which causes electromagnetic interference (EMI), can also be decreased. On the other hand, the proposed system needs an additional inductance to reduce a cross current that flows between the parallel main circuits. However, it is shown that the peak value of the cross current can be kept low by applying a control to reduce it, if a large motor is used and the motor current increases