# Application of a High-Power Reversible Converter in a Hybrid Traction Power Supply System
###### Abstract  
A high-power reversible converter can achieve a variety of functions, such as recovering regenerative braking energy, expanding traction power capacity, and improving an alternating current (AC) grid power factor. A new hybrid traction power supply scheme, which consists of a high-power reversible converter and two 12-pulse diode rectifiers, is proposed. A droop control method based on load current feed-forward is adopted to realize the load distribution between the reversible converter and the existing 12-pulse diode rectifiers. The direct current (DC) short-circuit characteristics of the reversible converter is studied, then the relationship between the peak fault current and the circuit parameters is obtained from theoretical calculations and validated by computer simulation. The first two sets of 2 MW reversible converters have been successfully applied in Beijing Metro Line 10, the proposed hybrid application scheme and coordinated control strategy are verified, and 11.15% of average energy-savings is reached.  
urban railway transit; traction power system; regenerative braking; reversible converter; direct current short-circuit +
Footnote †: journal: Applied Physics Letters  
_Applied Physics Letters_  
space, which will increase the cost of substation construction, and the utilization rate of the inverter is not very high.  
It will be a good idea to introduce high-power reversible converters into the traction power supply system of urban rail transit. Not only will braking energy be recovered, but traction power supply can also be realized by the reversible converter. Thus, the capacity of diode rectifiers can be reduced, or one of the two existing 12-pulse rectifiers may be replaced by a reversible converter for the purpose of saving space.  
Generally, the DC output characteristic of the reversible converter is constant, which can be realized by a simple voltage closed-loop. However, because it needs to work with the diode rectifier units in parallel, in order to share the traction load, the reversible converter must have a similar voltage droop characteristic as the diode rectifier. Droop control methods are commonly used in DC microgrids [22; 23] and other distribution power systems [24; 25; 26]. Few studies have paid attention to the DC voltage control of the reversible converter used in traction power supply systems.  
In the field of urban rail transit, the existing literature is mainly aimed at the steady state and transient short-circuit characteristics of the diode rectifiers [27; 28]. A mathematical model of the 12-pulse rectifier is established, and the approximate expression of the short-circuit current is deduced at the outlet. The accuracy of the calculation method was verified by modeling and simulation. However, the DC short-circuit characteristics of the reversible converter are different from the diode rectifier. The DC short-circuit fault of the grid-connected inverter used in wind power generation systems was studied in the literature [29; 30], which can be used as an important reference for this paper.  
This paper aims to introduce the high-power reversible converter into the traction power supply system of urban rail transit, and lay a foundation for further application. The following sections are organized as follows: A hybrid traction power supply system composed of a diode rectifier and reversible converter is proposed in Section 2. The DC output characteristic of the 12-pulse rectifier is introduced and a droop control method based on load current feed-forward is proposed for the reversible converter in Section 3. In order to provide guidance for system protection, the DC short-circuit fault of the reversible converter is studied, the peak short current and rise time are both calculated, which agree with the simulation results in Section 4. The effectiveness of the coordination control strategy is verified by a field test at Beijing Metro Line 10 in Section 5. Finally, conclusions are made in Section 6.