Study On A Low-Frequency Traction Power Supply System And Its Measurement And Control Method

The Urban rail transit, which has the advantages of comfortable and speedy, safe and green, has become the optimum selection for the development of urban public transportation. It is generally known that a DC mode of traction power supply system will cause stray current, and it is difficult to extinguish the arc while a fault occurs. A low-frequency traction power supply system was studied to address the shortages above-mentioned. With low-frequency AC mode, the problems existing in single-phase 50Hz AC mode such as negative-sequence current, reactive power and harmonic were avoided. Besides, there’s no phase splitting, and the voltage losses of traction electric network are lower. To apply low-frequency AC system won’t generate stray current compared to DC system. Therefore, it won’t cause any electrochemical corrosion on metallic facilities. And DC breaker is unnecessary by using frequency converter to cooperate the feeder circuit breaker on AC side for isolating faults of traction electric network. The pulse width modulation technology was applied for regulation of voltage and frequency by converter. It makes that transfixion power supply comes true by connecting the whole traction electric network.To make the low-frequency traction power supply system in application in the future and to perfect the existing protection scheme, a distributed protection system for section power supply of traction electric network was studied. The traction electric network was powered and protected by segmenting it into sections. While a fault occurs on traction electric network, all that is needed is to isolate the fault zone to ensure uninterrupted power supply for the fault-free area, minimizing the area of outage and strengthening the reliability of the system. Besides, to compare with centralized protection, the distributed protection has the advantage of instantaneity and quickness not influenced by the quantities of sections.First of all, the principle and achieving method of new low-frequency traction power supply system was studied. The system consists of traction network and traction substation of which includes traction transformer and frequency converter. The traction transformer was powered by utility grid in three-phase transformer connections through the primary side. The secondary side of traction transformer links to frequency converter by three-phase AC switch, outputting low-frequency single phase AC to power the traction network and locomotive in transfixion power supply model. While a fault occurs, the frequency converter can works as a switch to cooperate with the feeder circuit-breaker on single phase bus to isolate the fault zone timely and effectively.Secondly, the fundamental of distributed protection system was presented in detail. The differential current protection method based on low voltage startup was proposed, and analyzed the working process of the whole distributed protection system. The models of this protection system, including traction network cable, traction substation, circuit breaker, data collection and measurement and control unit, were established under Matlab/Simulink software, and composing a complete distributed protection system model of single-line and multiple-line. To simulate a T-R short circuit fault under the model, analyzing the voltage and current value of traction network by data collection device while a short circuit fault occurs, there comes to a conclusion of while the voltage drop to 1800V it will be identified as a criterion of protection action. To analyze the real-time waveform of port voltage and current and the control command of port circuit breaker, simulating the implementation process of distributed protection system in fault condition, it verifies that applying distributed protection system to identify and isolate the faults on traction electric network is practical and effective.Finally, in order to perfect the distributed protection system, faults wave-recorded and data extraction way were introduced. The existing synchronous sampling method, data transmission mode and data processing algorithm were analyzed and come to a conclusion-applying failure triggers synchronous sampling method and full-wave Fourier data processing algorithm may be better for the application of distributed protection system.