Abstract:With the rapid development of electrified railways in China, neutral sections and power quality problems have affected the daily operation of the traction power supply system. Fortunately, the advanced traction power supply system based on power electronics technology provides an opportunity to solve the above problems. The new hybrid advanced traction power supply system (HATPSS), based on the traction transformer and two-phase to single-phase (2AC-AC) converter, is gaining traction due to its high equipment utilization and low-cost advantages. However, as a weak link in HATPSS, the 2AC-AC converter can cause problems under port faults, such as negative sequence currents, power loss, and even system instability. In addition, due to the particularity of the HATPSS architecture and the diversity of control objectives, the traditional redundant backup scheme and emergency management have poor applicability. Therefore, considering the existing equipment's emergency operation potential and the 2AC-AC converter's topological advantages, the emergency management and control and fault recovery strategies are proposed respectively under the input/output fault conditions of the 2AC-AC converter. First, according to different fault locations and operation characteristics in the 2AC-AC converter, the fault type can be divided into two conditions: type I-output port γ fault, type Ⅱ-input port α or β fault. Under type Ⅰ-output port γ fault, the uα of the traction transformer’s secondary side can be connected to the traction network to ensure power supply for the traction load. Then, by controlling the power flow between port α and port β, the two-phase power of the secondary side can be balanced to suppress the negative-sequence current. Under type Ⅱ-input port α or β fault, the uα or uβ can be connected to the traction network according to the fault location of the input port to ensure power supply for the traction load. Adjusting the output power of port γ allows for suppressing negative sequence currents. The fault recovery strategy is initiated once the faulty port is repaired, and the HATPSS returns to healthy operation. During the process of emergency control and fault recovery, the rated power operation of HATPSS and negative sequence current suppression are realized through coordinating the existing devices of the system, such as traction transformer, grid-connected switch, two-phase-single-phase converter, and healthy traction station. To further verify the feasibility and effectiveness of the proposed emergency control and fault recovery strategies, a 20 MW simulation model and 6 kW experimental platform of HATPSS are built. Finally, the following conclusions can be drawn from the simulation and experimental analysis: (1) The proposed emergency control strategies are realized using resources, such as contact switches, traction transformers, and 2AC-AC converters, without additional hardware investment cost. (2) Under input or output faults, the fault traction station's 100% output power supply can be ensured to improve the reliability of HATPSS effectively. (3) During the process of emergency operation and fault recovery, the fault traction station remains connected to the network, and only a transient negative-sequence current is on the power grid side, effectively improving the system's power quality. The proposed emergency control idea can also be applied to modular multilevel and high- frequency isolation architectures.
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