With the rapid development of the DC metro system, the stray current may bring new challenges to the AC power system. Especially, the stray current of the DC metro system flowing in the AC power system may cause many serious consequences such as the corrosion of the grounding grids, plenty of harmonics, and the dc bias of transformers. Therefore, it is significant to evaluate the dc current distribution in the AC power system caused by the stray current. In order to evaluate the dc current distribution, a coupled simulation model both considering the DC metro system and the AC power system is proposed in this paper. Moreover, the earth and the grounding system are both used as the coupling elements in the proposed model. The correctness of the proposed model is verified by the comparison of the calculation results and the field test data. Furthermore, based the proposed model, the dc current distribution in the AC power system is evaluated and analyzed. The results indicate that the dc current distribution is affected by the trains operating conditions and the relative position between the DC metro system and the AC power system.

Regenerative braking energy feedback system (RBEFS) can effectively feed the regenerative braking energy (RBE) of the trains back to the AC power grid. This improves the economy of the urban rail transit system greatly. The RBEFS which is utilized for both recycling the RBE and controlling the voltage of the traction network is proposed in this paper. Firstly, the structure of the medium voltage RBEFS with the higher recovery efficiency is presented, and its parameters are designed in detail. Then, the double closed-loop control strategy of the RBEFS is studied, which not only achieves unit power factor feedback but also stabilizes the voltage of traction network effectively. Finally, simulation based on MATLAB/Simulink, laboratory experiments, and the field test are used to verify the performance of the presented RBEFS. The simulation and experiments results show that the RBEFS is able to feed RBE back into the AC grid effectively and the power quality of its output current satisfies the requirements of grid-connection, as well as mitigate the voltage rise of the traction network under different startup threshold perfectly.

In DC metro systems, the stray current may cause many challenges for the nearby metal structures and the AC power systems, such as the corrosion of the buried pipes and the grounding grids of substations. Therefore, calculating the stray current and analyzing the distribution characteristics of the stray current is necessary for the design of the buried pipelines and substations. To obtain the stray current, considering the through-type structure characteristics of the return current system and the traction power supply system, a novel multiple power supply sections simulation model (multiple-section model) of stray current is proposed. In the proposed model, the train is equivalent to two dc current sources, and the dynamic results can be completed. Through comparing the rail potentials of simulation results and the field test data of the Shenzhen metro line, the proposed model is verified. And based on the proposed model of the Shenzhen metro line, the stray current distributions are obtained and analyzed. The results indicate that the stray current distribution has a certain periodicity which is approximately equal to the headway time of the train.

Failures in traction power supply system (TPSS) may cause high-speed trains to run behind the schedule or even suffer from downtime, which would directly affect the entire scheduling and cause inestimable loss. Therefore, it is crucial to evaluate the reliability of the TPSS to ensure that it can operate with high reliability. This paper proposes a method for evaluating the overall reliability of the TPSS as-sociated with high-speed railway (HSR) considering relay protection. The impact of relay protection on the reliability of the TPSS is analyzed, then the failure mode and effects analysis tables are established based on the main wiring dia-gram of the TPSS and its equipment composition to qualita-tively analyze the failure modes of primary and secondary equipment and their influence. According to the FMEA ta-bles, the fault tree model of the traction substation and cate-nary system considering relay protection is established re-spectively, and the system reliability indexes are calculated through Monte Carlo simulation. Different from existing studies, the reliability models established in this paper take into account the primary and secondary equipment of the TPSS and coupling relationship between them, and highlight the configuration of relay protection and its logic cooperating with each equipment. Through the analysis of practical ex-ample concerning the TPSS serving China's HSR, it shows that rich and direct system reliability indexes can be obtained through simulation with the proposed reliability models and evaluation method, and the impact of relay protection on sys-tem reliability can be reflected, which provides an effective means for evaluating the overall reliability of the TPSS.

With the rapid development of the dc metro system, the hazard of stray current is increasingly serious. Due to the direct grounding of the track and the lack of a stray current collector system (SCCS), the metro depot has become a weak link in the protection against the stray current. In order to evaluate and analyze the stray current distribution of the metro depot, the evaluation model of stray current in the metro depot is proposed in this article. Based on the piecewise-linearized volt-ampere characteristic, the one-way conduction device (OWCD) of the depot is modeled by a current source paralleled with a resistor. The distribution of stray current in the depot is calculated based on the nodal analysis method. The validity of the proposed evaluation model is verified by comparing the calculation results with the field data. Moreover, based on the proposed evaluation model, the distribution of stray current in the depot when the train operates at different positions is analyzed. The results show that the train in traction operation on the mainline far away from the depot is more likely to aggravate the stray current in the depot than when it is close to the depot.