90. Analysis of Circulating Currents in a Modular Multilevel Converter-Static Synchronous Compensator

A new modulation method named CPPS is proposed in this chapter; besides, in order to identify the basic components and analytical expressions of arm current in a modular multilevel converter-based static synchronous compensator (MMC-STATCOM), an analysis is made with a mathematical method based on instantaneous power conservation under the modulation mode of CPPS. It proves the existence of the second circulating current and gives its analytical expressions as well as the relevant submodule (SM) capacitor voltage ripple. The analysis implies that the second circulation current cannot be eliminated by controllers. A three-phase MMC-STATCOM model with nine voltage levels applied to a 10-kV system is realized to verify the effectiveness of the proposed modulation method and circulating current model under the capacitor voltage-balancing control method. In addition, the result also shows that the amplitude of the second circulating current is primarily associated with the arm inductance, and the circulating current will decrease as the arm inductance is increasing.