Delta Configured Multi Busbar Sub-Module Modular Multilevel STATCOM With Partially Rated Energy Storage for Frequency Support

This paper presents a delta-configured, modular multilevel, STATCOM with integrated partially-rated energy storage (ES) and submodules (SM) based on the Multi-Busbar Sub-module (MBSM) topology. The soft-paralleling of SM capacitors using the nature of diodes leads to lower SM voltage deviation and lower circulating current to be used for SM balancing and improves the overall operation of the converter. Partially rated ES are distributedly integrated in SMs to provide ancillary service, e.g. frequency support, in addition to the classic reactive power provision. The study uses the analytical formulation of the MBSM to derive the control system and the optimal phase angle of the circulating current required for SM balancing. A comparison of different operating modes (MBSM vs emulated full-bridge SM) is presented and confirmed by simulation results. Analysis indicates that the MBSM improves the power efficiency (30.5% total losses reduction at the 1 pu reactive power set-point and 3.56% reduction at 1 pu active power set-point) compared with the conventional full-bridge SM, at the cost of doubling the number of semiconductor devices; but not the number of sub-modules.