Common-Mode Characterization of a Modular Multilevel Converter in a Megawatt-scale Medium-Voltage DC Test Bed

Abstract

An undeniable innovation of future naval medium voltage dc (MVDC) power systems is the use of power electronic converters. The switching events from these devices can produce undesirable effects, such as common-mode (CM) currents, i.e., ground leakage currents, through coupling of the power system and the ship's hull or between system components. Characterization of these megawatt-scale CM drivers and their impacts are of broad and current interest to the naval community. This paper presents investigation into the CM characteristics of a megawatt-scale MVDC testbed, using a modular multi-level converter (MMC) acting as a standalone ac to dc rectifier. The MMC is the only CM voltage (CMV) source in the test setup. A CM equivalent circuit of the test setup is created and validated against experimental results. Tests show that the experimental CMV is consistent with the simulated CMV. Yet, the CMV frequency spectra features vary with different dc voltage references and base switching frequencies. Comparison of the CM current (CMC), calculated by way of the equivalent circuit, is consistent with measured CMC.