A Comprehensive Analysis of Voltage to Ground of Floating Submodule for MMC

Modular multilevel converter (MMC), which are widely used in VSC-HVDC transmission systems, is usually composed of numerous submodules (SMs) whose voltage are floating in respect to ground. During the commutation transient, high dv/dt for SMs will cause common-mode current to flow into the ground through parasitic capacitors, resulting in conducted EMI. Due to the large number of SMs for MMC, the SMs at different positions have different voltage to the ground, and the voltage jump to the ground of the SMs are also affected by the operating conditions. Therefore, this paper comprehensively analysis the change rules of voltage to ground for SMs under different operating conditions and identify the sensitive SMs with respect to dv/dt. The relevant simulation results can verify the analysis results for the position of the sensitive SMs and the voltage jump rules for SMs under different operating conditions. Furthermore, a double-pulse test of SM is implemented to verify that the common-mode current is conducted when the SM is switching.