# Application of MMC-RPC in High-Speed Railway Traction Power Supply System Based on Energy Storage
###### Abstract  
In order to effectively improve the power quality and utilize railway regenerative braking energy in high-speed railway traction power supply system, this paper adopts the Modular Multilevel Converter type Railway Power Conditioner (MMC-RPC) with distributed super-capacitor (SC) energy storage (ES) scheme. Firstly, the single-phase MMC mathematical model is established, and the power compensation characteristics of MMC-RPC are derived. Secondly, the Virtual Synchronous Generator (VSG) control strategy is adopted to provide inertial support. Compared with the double closed loop (DCL) control, it has better anti-disturbance and dynamic performance. Then, based on the VSG control, a two-stage circulating current suppression strategy is proposed, in which Quasi-Proportional Complex Integral (QPCI) control is used to suppress the bridge arm circulating current, and integrated Proportional Integral-Quasi Proportional Resonant (PI-QPR) control is an improvement of the VSG control to further suppress the circulating current inside the VSG. Furthermore, the virtual DC motor (VDCM) strategy was proposed to control the charge and discharge of the distributed SC connected to MMC-RPC to recycle railway braking energy. Finally, simulation results of the VSG small signal model and the MMC-RPC simulation model on the Matlab/Simulink platform verify the effectiveness, stability of VDCM and improved VSG controls in the MMC-RPC. a +
Footnote †: journal: Applied Sciences  
connected back-to-back by two-level voltage source converters, which would generate a large number of harmonics. In addition, the voltage withstands capacity of switching devices is limited [7,8]; it needs to introduce step-down transformers [9] and filter device, which is too expensive, bulky and inefficient [2]. Therefore, this paper adopts MMC type RPC (modular multilevel converter, MMC-RPC). While solving the harmonic, negative sequence and reactive power problems, it has better quality, efficiency and reliability by increasing the number of voltage levels of each module and the number of modules in series to improve the total voltage level. For such a reason, MMC-RPC can be directly connected to the traction network without using the step-down transformers. Compared with traditional RPC, MMC-RPC has smaller harmonic components due to Carrier Phase Shift (CPS) modulation. And the switching frequency of each sub-module is low, which reduces the switching loss [10]. Furthermore, the modular structure of the MMC-RPC not only expands the capacity of the converter, but also reduces the difficulty of maintenance. To improve the control effect of the RPC, scholars have carried out much research on the topology and control strategy of the RPC. One study [11] presented a current compensation (CP) method to balance the active power, compensate the reactive power and suppress harmonics by calculating the active current, reactive current, and harmonic current references. However, it is only applicable to the traditional RPC and traction power supply systems using \(V/v\) transformers. The authors of [12] proposed a closed-loop voltage equalization control strategy based on CPS modulation for three-leg MMC-RPC, which uses the capacitor voltage balance control between sub-modules and the interphase capacitor voltage average control to obtain the average correction of capacitor voltage. However, it is still affected by the sudden change of power to lose control and stability. The authors of [13,14] adopted CP and energy balance control for the MMC-RPC system. However, it is difficult to maintain the stability of the system frequency when the traction load changes dynamically due to the lack of inertia support. The authors of [15,16] adopted a direct power control without a phase-locked loop, which improves the dynamic performance of the system.