Power Electronic Converters in Advanced Co-Phase Traction Power Supply System

Siemens & Halske AG exhibited an about 550-m section of electrified railway at the Berlin Trade Exhibition held in 1879, marking the first instance of electric traction being used to pull trains in human history. In May 1881, Germany built and put into operation an electrified railway that spanned 2.45 km at Lichterfelde, on the outskirts of Berlin. This railway became the world’s first commercially operated electrified railway, ushering in a new era of electric traction for railways. In the mid-twentieth century, societal development created an increasingly strong demand for railway transport, leading to extensive construction of electrified railways (Qian 2003).

Continuous traction power supply systems that incorporate interconnected substations throughout the entire traction network are poised to become a next-generation traction power supply mode. This system fundamentally relies on various types of converters, including three-phase-to-single-phase converters (Shu et al. 2013; Liu 2010; Chen et al. 2016; Xia 2017; Li et al. 1988; He et al. 2013b; Sun et al. 2017; Li 2010). A brief overview of a continuous traction power supply system, which consists of two substations interconnected and operating in a three-phase-to-single-phase topology, is illustrated in Fig. 2.1. At Substation 1 and Substation 2, high-power power electronic converters are connected to the three-phase power grid and single-phase traction network, on both sides, respectively. These converters are capable of controlling the ports—managing voltage amplitude, frequency, and phase—which facilitates the interconnection on both ends of the three-phase power grid and the single-phase traction network. The two-way flow of energy between the traction network and the three-phase grid, achieved through power control, enables not only power supply to the loads in the traction network from the three-phase power grid but also the feedback of traction braking energy to the three-phase power grid for consumption.

A feasible architecture for continuous traction power supply systems is the three-phase-to-single-phase topology depicted in Fig. 3.1a. This design facilitates energy conversion and transmission between the three-phase power grid and the traction network. In this architecture, the converter that connects to the three-phase power grid should be configured as a three-phase AC-DC structure, utilizing a voltage source inverter (VSI) topology (Liu 2010).

The traction power supply network operates in a single-phase mode, and the converter on the traction side features a single-phase topology. Single-phase conversion circuits are widely utilized in various applications, such as power factor correction (PFC) and single-phase PWM converters (Zheng 2009; Xu 2013). To align with the traction network side, which typically operates at high voltages of up to 25 kV and supports traction load power ranging from several megavolt-amperes to tens of megavolt-amperes, most power converters for traction applications are designed with high-voltage, high-power single-phase conversion circuits. Applicable conversion circuit topologies include the single-phase two-level H-bridge, single-phase three-level H-bridge, and single-phase cascaded multilevel configurations. This chapter examines the aforementioned single-phase AC-DC conversion topologies and their operating principles. It also discusses the modulation and control strategies for typical single-phase converters, analyzes the unique characteristics of DC voltage ripple inherent to single-phase AC-DC topologies, and explores control methods to mitigate the effects of DC voltage ripple.

If all traction substations within a traction power supply system are equipped with AC-DC-AC converters, continuous power supply can be achieved along the entire railway through interconnected substations, eliminating segmentation between them. In this configuration, the AC-DC-AC converters at these substations act as loads connected to the electric power system, with their single-phase sides interconnected via the traction power supply network. This setup forms a mutually supported traction power supply microgrid that spans the entire railway. Consequently, the three-phase side of the AC-DC-AC converters in this continuous traction power supply system is linked to the electric power system, while the single-phase side connects to the traction network. Typically, this is arranged in a three-phase-to-single-phase converter topology, with AC ports on both sides connected to the active power supply network. The AC ports on either side of the converters operate in a four-quadrant mode, enabling active interconnection and control of both active and reactive power.

Modular multilevel converters (MMCs) represent one of the multilevel converter structures currently under extensive research and application (Shu et al. 2013). With advantages such as compact structure and easy extensibility, these converters have been widely studied and implemented in various fields, including high-voltage, high-power, large-capacity reactive power compensation (Ambrozic et al. 2003) and DC power transmission (Shu et al. 2011; Xing 2016). These systems are typically based on a three-phase-to-three-phase configuration (Liu 2007). However, traction networks, which generally operate in a single-phase configuration, require a single-phase bridge arm output from MMCs. Given the current lack of studies on three-phase-to-single-phase converters based on MMCs, further research is needed, particularly regarding applications in continuous traction power supply scenarios.

Oettmeier et al. (2009) and Zhang (2012a) eliminated the phase break at the substation outlet in co-phase power supply transformations of existing traction substations. While this resolved power quality issues within individual substations, the phase break at the section post between substations remains, preventing mutual energy support and thus hindering the upgrade to multi-substation continuous traction power supply systems. Therefore, an improved continuous power supply system is proposed, with the structure of a single traction substation for continuous power supply shown in Fig. 7.1. A single co-phase power supply traction substation primarily consists of a traditional traction transformer, a two-phase-to-single-phase PWM converter, and a step-up transformer. The traction transformer receives electricity from the three-phase grid and converts it into two-phase voltage. Two single-phase rectifiers are connected to the two-phase input terminals, sharing a DC bus. The rectifiers then output AC power through the single-phase inverter, which is transmitted to the traction network via the step-up transformer. The two-phase-to-single-phase PWM converter operates in four quadrants, allowing bidirectional active power flow between the three-phase grid and the single-phase traction network. The reactive energy from the traction load will not be transmitted to the three-phase grid which operates at unity power factor. This significantly addresses the power quality issues caused by locomotive loads in traditional traction power supply systems. The single-phase inverter not only supplies the reactive power demanded by the locomotive load but also provides harmonic control. Its adjustable output voltage amplitude, phase, and frequency enable continuous co-phase power supply along the entire line (Zhang 2012b; Pi et al. 2008). This approach eliminates phase breaks within and between existing traction substations, reduces the requirements for the installed capacity of individual substations, and enhances the operating efficiency of the system.