MMC Submodule Topology With CVSB and DCFRT Capability for Offshore Wind Farms Black-Start

This article presents an MMC improved semi-full-bridge sub-module (ISFBSM) structure with negative level output, capacitor voltage self-balancing (CVSB), and DC fault ride-through (DCFRT) capability. The ISFBSM topology can be obtained by adding two IGBTs alongwith antiparallel diode to the SFBSM. Compared with the SFBSM, the ISFBSM has a faster DC fault handling speed and more negative output levels. When the DC fault occurs, the capacitors in the ISFBSM are put into the fault circuit in series. The double reverse capacitor voltage can block the fault current faster. Moreover, the ISFBSM can output −2
u
_c
, −
u
_c
, 0,
u
_c
, and 2
u
_c
. The extra −2
u
_c
voltage level can ensure that the ISFBSM-MMC reverse DC voltage is more stable than the SFBSM-MMC when the whole wind power transmission system performs power flow reversal control. Although the hardware cost of the ISFBSM is higher than the SFBSM, the safe and stable operation of the offshore wind power transmission system should be prioritized in extremely harsh climate conditions on the sea. Compared to the SFBSM, the faster DC fault handling speed and more negative output levels of the ISFBSM make it more suitable for offshore wind power transmission in scenarios such as handling DC faults and black-start of wind farms. Then, the negative level output, capacitor voltage self-balancing, and DC fault clearing mechanism are introduced detailedly. Eventually, the MATLAB/Simulink simulation and the dSpace1202 MicrolabBox experimental results confirm the ISFBSM validity.