A Model Assisted Testing Scheme for Modular Multilevel Converter

Systems based on the modular multilevel converter (MMC) concept can be used for a variety of applications including high voltage direct current transmission and flexible alternating current transmission system devices for power system control. In these applications, both the manufacturer and operator are keen to understand the stress conditions that the submodules are subject to. It is important to test for reliability of the submodules as a function of the design and control variables. This paper proposes a model assisted submodule testing scheme for an MMC that can be used in any of the possible applications. The submodules can be thoroughly tested individually before the complete MMC is built. Experimental measurements have shown the validity and accuracy of the proposed method with close attention given to the synchronization between the signals. The current (with dc bias and harmonic components) passing through the submodule, the switching sequence, and the capacitor voltage are shown to be identical to what the submodule would be subjected to when installed in a complete MMC. This test method is capable of facilitating research and development exercises through examining the electromagnetic and electrothermal characteristics.