Modular multilevel matrix converter for low frequency AC transmission

The modular multilevel matrix converter (MMMxC) applied to a low frequency ac (LFAC) transmission system is investigated. The LFAC (1 Hz ~ 20 Hz) has some advantages compared to conventional ac and dc transmission systems, such as the increase of current-carrying capacity of power cables due to reduction of capacitive charging current, and better fault protection due to existence of zero current points required for circuit breaking, respectively. For the frequency converter of the LFAC transmission system, the MMMxC that consists of a number of H-bridge cells as bidirectional switches is very promising because low frequency, high voltage and high power conversion are easily realized by connecting H-bridge cells in series; waveform distortion is improved by multilevel configuration; and current commutation is guaranteed even in very low frequency operation. In this paper, a new scheme of power and capacitor voltage control for the MMMxC is proposed and its characteristics are investigated especially under low frequency operation. In addition, experiments are carried out to verify the proposed control schemes using a 1 kW class laboratory setup.