Recently, modular multilevel converters (MMC) have gained more and more interest for Medium Voltage inverter applications. However, voltage ripple on submodule capacitors limits their application range as motor drives at low frequency and high torque demand. The flying-capacitor passive cross-connected arms MMC (FC-MMC) allows to solve this issue with a combination of high-frequency voltage and current injection to effectively control the capacitor voltage and ensure safe and reliable converter operation. However, in this topology, the pre-charge of the flying-capacitor and submodule (SM) bus capacitors are major issues to address, since resonance occurs between the sub-arm inductor and flying-capacitor, which can lead to uncontrolled phase current and even converter damage. This paper describes a novel pre-charge/start-up control strategy, which avoids uncontrolled oscillations. With respect to previous proposals, this strategy does not require a dedicated source (other than the SM capacitor itself) for the power supply of SM controller and gate drivers. Moreover, the method is robust with respect to possible capacitance mismatch between the dc bus of different SMs. Its effectiveness is validated considering the case of a FC-MMC based permanent magnet synchronous machine drive. The proposed strategy also guarantees that the machine current during the start-up procedure remains small enough, in order to ensure the uncontrolled torque generated remains negligible.
