Research On High Power Flying Capacitor Three-level DC Converter And Its Control In Urban Rail Energy Storage System

Since the 19 th National Congress of the Communist Party of China,the country has thoroughly implemented the new development concept,focused on building an advanced manufacturing cluster and a modern,high-quality,comprehensive,green and energy-saving transportation network,and accelerated the construction of a strong manufacturing country and a strong transportation country.With the rapid development of the national economy,rail transit has been widely used in large,medium and small cities,and the utilization of urban rail regenerative braking energy has also attracted widespread attention in the industry.Green environmental protection,high-quality operation.The power and voltage level of the converter will determine the ability of the urban rail regenerative braking energy utilization system to absorb and release the braking energy of the train.The DC converter can realize the energy flow between the DC sides,and can solve the problem of reusing the braking energy of the train economically and effectively.This paper first summarizes the research status of regenerative braking energy utilization systems at home and abroad,and analyzes and compares different DC converter topologies in urban rail scenarios.The key technical problems of the flying capacitor-type three-level DC converter under the high-power urban rail energy storage system,the control strategy research is carried out around the output performance,and the following innovations are achieved.(1)A voltage equalization control method for flying capacitor three-level DC converter based on control variable decoupling operation is introduced.The decoupling operation can transform the multiple input multiple output system into two independent single input single output control systems,realize the independent control of the input and output variables of the flying capacitor type three-level DC converter,and solve the problem that each power device is turned on and off The problem of unbalanced flying capacitor voltage caused by the time difference,and ensure the effective and stable output of the low-voltage side voltage.(2)A multi-threshold-based flying capacitor three-level DC converter traction network side rapid voltage regulation control technology is proposed to ensure that the DC converter can be within the protection limit under different traction network voltage drops or rises.Quickly realize power output or consumption under different voltage levels.First of all,through the current inner loop control technology based on voltage feedforward,the modulation wave compensation at startup is realized,and the rapid response of the converter is improved.Secondly,the limit value protection control technology is adopted at the input end of the current inner loop to realize the autonomous discrimination of the overcharge/overdischarge protection of the converter.Finally,the voltage classification discrimination module is introduced into the voltage outer loop control to realize the multi-threshold voltage regulation and regulation on the traction network side of the flying capacitor type three-level DC converter.(3)A flying capacitor type three-level DC converter interleaved parallel control technology is proposed to meet the high-power charging and discharging requirements of urban rail energy storage systems.Firstly,the relationship between phase-to-phase current,equivalent resistance and effective duty cycle is analyzed,and a parallel current sharing control strategy based on pulse width modulation compensation is proposed.This technology can effectively avoid the low precision and insufficient design of the sensor through pulse width modulation compensation.,Parasitic impedance and other causes of unbalanced current between phases.Secondly,a multi-branch parallel control technology based on carrier phase shifting is proposed.By changing the carrier phase of each control module at the same frequency,the three-branch PWM drive signal is 120° out of phase,so that the power switches of each branch are turned on alternately to achieve low voltage.The side current ripple is reduced by 1/3 times,which effectively reduces the ripple coefficient and improves the power quality.(4)Parameter design of the main circuit of the megawatt flying capacitor three-level DC converter of the urban rail energy storage system.Completed the selection of the key components of the flying capacitor three-level DC converter unit in the DC1500V/2MW energy storage system,and set up a test system to test and verify its cyclic buck-boost function,fast charge and discharge response performance,and low-side ripple optimization.