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| Power Flow Calculation Method and Power Distribution Characteristics Analysis of 24 kV Flexible Direct Current Traction Power System |
| Liu Yunjiang, Hu Haitao, Yang Xiaowei, Hu Hai, Zhu Xiaojuan |
| School of Electrical Engineering Southwest Jiaotong University Chengdu 611756 China |
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Abstract Since Spanish scholars proposed 24 kV flexible direct current (DC) traction power system (TPS) in 2014. Relevant papers have reported the the power supply structure of the new TPS, the schemes of operation control scheme, and stability. To fully understand the system operation principle, it is urgent to discuss the power supply characteristics of the system. Power flow calculation is of great significance to study the power distribution characteristics of 24 kV flexible DC TPS. In this system, hierarchical control leads to more flexible and complex power flow distribution of flexible DC traction network, however existing methods rarely systematically consider the impact of complex coordinated control between traction substations on power flow calculation. Therefore, the traditional power flow algorithm of DC TPS is no longer fully applicable. To address these issues, this paper proposed a sequential linear power flow method and Proportional-Integral controller (SLPFM-PI) alternating iterative method to calculate traction network power flow considering hierarchical control.
Firstly, the “controller-source-network-train” integrated power flow calculation model was established. Then, the interaction between power flow calculation and controller's iteration was analyzed, accordingly designing a calculation flow chart of 24 kV flexible DC TPS power flow algorithm. Finally, the Matlab/Simulink model of 24 kV DC TPS was built, simulation results verified the correctness of proposed power flow calculation method.
Based on the power flow calculation, the power supply distance of 24 kV flexible DC traction substation was calculated with the limiting conditions of locomotive operating voltage and catenary current capacity, and the differences of the power distribution characteristics of the system under different control schemes was discussed. In discussion, three control schemes were analysed, which can be depicted as scheme one: traditional droop control, scheme two: droop control and regulate average voltage of all traction substations to rated value, scheme three: droop control, regulate average voltage of all traction substations to rated value, and regulate equal output power of traction substations. In case one, set speed of locomotives are 350 km/h, rated power is 8 MW, departure interval is 6 min, recording the voltage and current of each node of traction network during the whole process of first locomotive starts from the head of the traction network to the end of the traction network. During the calculation, the power supply distance L is increased from 80 km to 180 km with the step of 5 km. Under different L, the locomotive shall operate according to the running chart and working conditions set above. when L is different, the minimum value of catenary voltage, and the maximum value of catenary current during the whole process of locomotives operation were recorded. Then, determine the longest power supply distance of the substations meeting the above two constraints when the system adopts different control schemes. In another case, under the same power supply distance L and train operation conditions, the power supply characteristics of the TPS with different control schemes were compared, including minimum voltage of traction network, maximun rail potential, maximun stray current, and power supply efficiency.
Through theoretical analysis and power flow calculation results, the following conclusions can be drawn: (1) The proposed power flow calculation method can accurately calculate the traction network power flow under the different control schemes. (2) According to the normal working voltage of locomotive and the current capacity limit of catenary. Under three control schemes, the maximum power supply distance of 24 kV flexible DC TPS is 135 km, 140 km, and 100 km respectively. (3) Inter region power transmission will bring additional power loss, increase network voltage fluctuation, rail potential and stray current, therefore, the system control scheme should minimize the inter region flow of power in the system. For an actual running line, if the line is short and high traffic density, adopting control scheme three will not cause inter region power transmission of locomotive power. In this case, the advantage of scheme three can be fully used to improve capacity utilization rate of the traction substation. For a long railway line, scheme one or two is a better choice.
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Received: 16 January 2022
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2023, Vol. 38 
