Power Quality Analysis For High Speed Railway Traciton Power Supply System Under Regenerative Braking Condition

As a high-power and single-phase rectifying load, the high-speed EMU possesses the characteristics of impact, asymmetry and nonlinearity. Its fast motion property results in a wide range of influence of the traction load on the electric power system and easily gives rise to complaints about the power quality and obstacles to power supply. In the meantime, the primary objective of construction of a high-speed railway is to guarantee its safe and reliable operation. However, a high-speed train is featured by higher running speed, larger weight and higher power compared with a normal-speed train, so its required braking force for braking and generated energy from braking are much more than those of the normal-speed train as well, which requires the high-speed EMU to possess a good braking performance. From the angle of energy conversion, the regenerative braking technology provides the braking force required for high-speed train braking; moreover, in the regenerative braking state, the high-speed train serves as a power generating unit to return the electric energy to the traction power supply system and this process is just opposite to the traction state. Based on the current design of traction power supply system for electrified railway, the effects of the regenerative braking function on the design of traction power supply system mostly include current calculation, selection of transformer capacity, selection of cross section of contact wire, calculation of voltage loss of feeding section and influences on the electric power system, etc. This poses higher requirements on the design of traction power supply system. As a result, analytical studies of the effects of the high-speed train on the power quality of traction power supply system in the regenerative braking state are of great significance for solving the traction power supply quality problems such as voltage fluctuation, negative sequence and harmonic influence of high-speed railway and guaranteeing the sufficiently effective use of regenerative braking energy.The paper firstly introduces the structure and power supply mode of the traction power supply system for high-speed railway. On this basis, the three-phase Norton model of external power supply, equivalent model of traction transformer and autotransformer, model of chain circuit of multi-conductor transmission line for traction network and mathematical model of parallel static load of induction motor for high-speed EMU are established. At the same time, the regenerative braking theory of high-speed train is analyzed. It can be known that the regenerative braking achieves the purpose of braking mostly by means of electric traction equipment and store the energy recovered in the braking process or return such energy to the power network for reuse. It can be seen through simulation that the current phase of high-speed EMU in the traction condition is opposite to that in the regenerative braking condition, which indicates that the regenerative braking condition of high-speed EMU is an opposite process of the traction condition and that the energy flows in the traction power supply system are just opposite.For the purpose of analytical study of the problem of traction network voltage rise probably resulting from entry of energy into the system through the traction transformer in the regenerative braking condition, the paper establishes a traction power supply system model on the basis of the structure of traction power supply system for high-speed railway and by taking the substation, autotransformer (AT) post, sub-section post, section post and locomotive position as the splitting plane to split the all-parallel AT traction network, and studies the traction network flow calculation method based on "high-speed EMU - traction network" coupling by taking the EMU as a constant power load.The calculation and analysis results of examples in different conditions show that the given traction network flow calculation method is characterized by quick convergence and high accuracy and applicable to analyzing the power supply capacity and power quality in different positions of traction network and different operating conditions of EMU, that the voltage of the contact wire where the EMU is located rises when it operates in the regenerative braking condition but the influence on the voltage of positive wire is small, that the rail voltage is directly proportional to the earthing resistance of AT center tap and the operating power of EMU, and that when the traction and regenerative braking conditions exist at the same time, the electric energy generated from regenerative braking of EMU is preferentially provided to the EMU operating in the traction condition within the range of the same feeding section while the remaining electric energy returns to the public power network or is stored in the traction substation.A harmonic flow calculation method applicable to the traction power supply system is presented on the basis of establishing the harmonic models of all-parallel AT traction network and CRH2 EMU. The harmonic flow calculation and analysis results of the traction network in the case of single and double EMU operation show that this calculation method which takes full account of the coupling relation between the harmonic voltage and the harmonic current can produce more accurate harmonic flow calculations of the traction power supply system for high-speed railway and is applicable to the harmonic flow calculation of the traction network with single harmonic source, multiple harmonic sources and background harmonic, that this calculation method can be used to not only analyze the harmonic flow of traction network for high-speed railway but also accurately reflect the harmonic resonance phenomenon of traction network, thereby providing a reference for the analysis and control of harmonic resonance of traction power supply system for high-speed railway, and that this calculation method can be used for harmonic analysis and assessment of the traction power supply systems under different train diagrams but the dynamic flow calculation method under the dynamics-based change of EMU running speed and operating conditions is to be further studied.In addition, the simulation analysis and measured data analysis of the harmonic characteristics and its propagation of the traction power supply system for high-speed EMU in the traction and regenerative braking conditions show that a greater harmonic distortion will be caused in the traction power supply system when the high-speed EMU operates in the regenerative braking condition but the harmonic amplification trend is irrelevant to the operating condition of EMU, that when the position of high-speed EMU is fixed, in either the traction condition or the regenerative braking condition, the harmonic amplification trends of traction network and high voltage side of traction substation are basically identical, and that when the frequency of a certain order of harmonic matches the structure of traction network, the mutual excitation of harmonic voltage and current will lead to the occurrence of harmonic resonance phenomenon.For the purpose of study of the three-phase current unbalance problem of electric power system caused by high-speed EMU acting as a high-power single-phase nonlinear load, the probability model of traction substation load and negative-sequence current is established according to the stochastic process through theoretical analysis of the negative-sequence current of V/v-wired traction transformer to analyze and simulate the probability distribution of negative-sequence current by means of statistical inferences, and simulation analysis and measured data verification are performed to obtain the influence relation between the negative-sequence current and power factor of the traction power supply system for high-speed railway in the V/v wiring mode as well as the main distribution of degree of current unbalance. The results show that the probability model of traction substation load and negative-sequence current possesses strong suitability and applicability and can truly simulate the high-speed railway operation characteristics and negative-sequence current distribution.In summary, on the basis of wide reading and studies of domestic and foreign theories and application literatures on power quality, especially study reports on power quality of electrified and high-speed railways, the paper takes as the main study subject the power quality of traction power supply system for high-speed EMU in the regenerative braking condition and carries out, by establishing the mathematical analysis model of power supply system and high-speed EMU for high-speed railway, analytical studies of the flow calculation method of traction power supply system for high-speed railway, for which the regenerative braking condition is taken into account, the harmonic generation mechanism, propagation and penetration characteristics of high-speed EMU in the regenerative braking condition, and the negative-sequence problem arising from the high-speed EMU in different operating conditions.