A Railway Power Conditioner Using Direct AC-AC Modular Multilevel Converter

Abstract

With the rapid development of high-speed electrified railway, power quality problems in the traction power grid have received increasing attention. This paper thus proposes a railway power conditioner (RPC) using direct AC-AC modular multilevel converter (MMC) to cope with the power quality problems in traction power grid, such as negative-sequence current, reactive power, and harmonics. The proposed direct AC-AC MMC consists of six arms, and each arm is formed by several identical half-bridge submodules. Compared with modular multilevel matrix converter (M3C), not only the direct AC-AC MMC can realize the direct AC-AC conversion without DC-link, but also it can include DC-link according to the application requirements, which can dramatically extend its application scenarios. Moreover, the operation control of the direct AC-AC MMC is simpler than that of M3C, especially under the equal-frequency conversion for the railway power conditioner application. The topology and mathematical models are firstly presented in this paper. Next, the comprehensive control scheme including the compensation reference generation, capacitor voltage balancing control, and circulating current suppression control is put forward as well. Finally, the simulation results are displayed to verify the feasibility and performance of the proposed RPC based on direct AC-AC MMC and corresponding control strategy.