Research On Harmonic Suppression Of Circulating Current And Low-frequency Operation For Modular Multilevel Converters

With the wide application of high power energy conversion, multilevel converters are rapidly developed in recent decades. As a new multilevel topology, modular multilevel converter (MMC) gathers increasing research interests, due to its advantage over traditional multilevel converters. This paper focuses on the harmonics in circulating currents of MMC, and huge voltage ripples in submodule capacitors under low-frequency operation. Main contributions are summarized as follows:Dc/ac and ac/dc power conversions are most popular applications of MMC. In such MMCs, the complex interactions between arm currents and capacitor voltages through switching actions result in various harmonics. Based on working principles of MMC, this paper analyzes the interacting process and formation mechanism of these harmonics. It is shown that circulating current contains second-and other even-order harmonics, while capacitor voltage contains harmonics from fundamental-to higher-order ones. In upper-and low arms, even-order harmonics keep same in phase while odd-order harmonics are opposite in phase. These phenomena lay theoretical basis for the following study.Even-order harmonics in circulating currents (mainly second-order component) will introduce extra power loss, increase current stress of power devices, and even cause instability during transients. Traditional methods for circulating current harmonic suppression have problems such as limited harmonic rejection capability and complex implementation. This paper presents a plug-in repetitive control scheme to solve the problem. It is suitable for multiple harmonics suppression, easy to implement, and applicable for both single-phase and three-phase MMCs. Compared with commonly used parallel repetitive control, it combines the high dynamics of PI controller and good steady-state harmonic suppression of the repetitive controller, and minimizes the interference between the two controllers. Simulation and experimental results on a MMC inverter prove the validity of the proposed control scheme. In addition, total second-order power in capacitors doesn’t obviously increase, which strengthens practical value of the solution.When a MMC operates at low frequencies, the submodule (SM) capacitors will suffer from huge voltage fluctuations. This problem must be overcome before MMCs find wider applications in medium voltage (MV) drives that require a wide speed range. The established solution for suppressing the voltage fluctuation have been injecting high frequency common-mode voltage and circulating current into each phase leg. This inevitably increases current stress of the power devices, which is not welcome in high power applications. Based on the fact that the pulsating powers of the upper-and lower arms are opposite in phase, this paper aims to eliminate the voltage fluctuation via added power channels between the upper-and lower arms of each phase. These power channels are constructed by dual half-bridge (DHB) circuits. By adjusting the phase shift angle between the output voltages of two half bridges, low-frequency ripple powers are transferred by high switching frequency operation. Leakage inductance of the isolated transformer plays important role in power transfer. The guidelines for designing parameters of isolated transformer are proposed, based on requirements of power transfer ability, transformer current limit and control resolution. Simulations and experiments of MMC ac drives with power channels are carried out, proving that with proposed solution, capacitor voltage ripples are greatly reduced without increasing arm current pressure. Therefore the critical threat for MMC’s application to MV drives is overcome.There are residual second-order ripples in capacitor voltages, which are in same phase for upper-and lower-arms. To further decrease voltage pressure, an optimization scheme for low-frequency operation of MMC is presented. Mathematical model of MMC in double line-frequency, negative sequence coordinate is built. Second-order voltage ripples are eliminated by closed-loop control of second-order capacitor energy as well as circulating current. Experimental comparisons indicate that with proposed optimization scheme, capacitor voltage ripple is minimized, while arm current pressure virtually unchanged.