Research On DC Voltage Control Strategy Of Metro Bidirectional Substation Based On Droop Characteristics

With the economic and social development,traffic congestion and environmental pollution in large and medium-sized cities are becoming more and more serious,which has a huge impact on the life quality of each urban residents.The development of subway,light rail and other urban rail transit has become an effective measure to alleviate the problem.Metro train operation has the characteristics of short running distance between stations,high running speed and frequent starting and braking,which will produce a lot of regenerative braking energy.Now most of the regenerative braking energy is consumed through braking resistance,which is a serious waste of energy,so it is of great significance to carry out energy-saving transformation of rail transit.There are many kinds of regenerative braking energy absorption schemes.This paper discusses the scheme that the substation uses PWM converter to invert the regenerative braking energy back to the grid,and cancels the common 24 pulse rectifier.All substations use PWM converter for power supply and this kind of substation is called bidirectional substation in this paper.The bidirectional substation has the characteristics of controllable output voltage.But at present,almost all bidirectional substations use constant voltage control,and the characteristics of controllable output voltage of the Bidirectional substation have not been fully explored.There are few studies on this kind of control in the academic circle,and this paper studies it.Firstly,this paper introduces the working principle of the three-level four quadrant converter,the phase-locked loop control problem of the three-level four quadrant converter,and the solution to the problem of medium electric potential balance control technology which is unique to the NPC three-level four quadrant converter.Then,a set of three-level 50 KW level experimental platform and 2MW level experimental platform are designed,and the rationality of the design is verified by experiments.Secondly,this paper studies the parallel control method of the Metro converter.On the one hand,this paper introduces the principle of the droop control method.It applies the droop control method based on the average current compensation to the parallel control of the converter.On the other hand,this paper applies the carrier phase-shift technology to the parallel control of the converter.and then,these control methods are verified on two 50 KW level experimental platform.Finally,a control method is proposed to change the slope of the output characteristics of the substation in real time according to the power and position of the train.This method can improve the power interaction performance between trains,reduce the electrical energy of the substation which back to the grid,and ensure the power supply quality of the traction network.This paper establishes the simulation model of Metro DC traction power supply base on the actual line data.Then,K-mean clustering method is used to divide the train working conditions into 40 categories,and the concepts of near area interactive power and far area interactive power are proposed.Then the 40 categories of working conditions are simulated respectively,and the relationship between the near area and far area interactive power and the slope to achieve the ideal interactive power is concluded.Finally,this paper proposes a fuzzy control strategy,through the control experience summarized by the simulation,to control the output characteristic slope of the substation,and through the simulation verification,this fuzzy control method can improve the interaction power between trains and ensure the quality of power supply.