| Frequent starting and stopping of metro vehicles produces a large amount of regenerative braking energy.The energy of the 12(24)pulse rectifier unit currently used in the subway cannot absorb this part of the regenerative braking energy.The bidirectional converter can convert alternating current to direct current,and it can also convert direct current to alternating current.Therefore,the application of bidirectional converters in traction power supply systems is one of the research directions for regenerative braking energy utilization.This article relies on the scientific research project "Metro Traction Power Supply Bidirectional Converter" to carry out the research and development of high-power bidirectional converter.Firstly,the advantages and disadvantages of the existing energy-consuming,energystorage,and energy-fed regenerative braking energy processing schemes are analyzed.Aiming at the problems of low capacity,poor reliability and high loss of traditional bidirectional converters,a bidirectional converter using IGBT anti-parallel diodes to achieve diode uncontrolled rectification and PWM mode to achieve regenerative braking energy feedback is proposed.When the train is traction,the bidirectional converter is in the rectifying mode,blocking the IGBT pulse,making it in the off state,and using the diode rectifier circuit composed of anti-parallel diodes to transmit the electric energy of the AC grid to the DC traction network;When the train is braking,the regenerative braking energy generated will increase the voltage of the DC traction network,and the bidirectional converter works in the PWM inverter mode to feed the electric energy of the DC traction network back to the AC grid.This project requires a single bidirectional converter with a rated capacity of 1.25 MW,a complete set of bidirectional converter devices with 2.5MW,and requires 3 times the overload capacity.In order to reduce the loss during the inversion of the bidirectional converter,the harmonic elimination scheme of SHEPWM and three-winding phase-shifting transformer is adopted,so that when the IGBT switching frequency is only 550 Hz,the grid-side current THD<5% can be achieved by using only inductive filtering.The mathematical model of the converter is established for the inverter working condition of the bidirectional converter.On this basis,the main components of the main circuit,such as DC support capacitors,AC inductors,and power devices,were selected.The control system adopts double closed-loop control mode of voltage outer loop and current inner loop.The modulation method adopts SHEPWM modulation with elimination of harmonic interval.In order to reduce the influence of harmonics on the stability of the control system,a voltage and current preprocessing method is proposed based on the idea of harmonic cancellation of phaseshifting transformers.This method can filter out 6k±1(k=1,3,5...)harmonics,and has little effect on the fundamental wave.The rectification and inverter conditions of the bidirectional converter were simulated in MATLAB/Simulink,and the correctness of the parameter design and control algorithm was verified.Subsequently,the control program of the bidirectional converter prototype was written,and the two prototypes were used to build and feed each other experimental platform to verify the system scheme of the bidirectional converter.Under 800 kW power experimental conditions,it is verified that the harmonic elimination scheme of SHEPWM with phaseshifting transformer can achieve the goal of grid-side current THD less than 5%.At the same time,it is found that the temperature of the transformer rises too fast.In order to solve the temperature rise problem of transformer,a harmonic elimination scheme with SHEPWM and high-pass filter was proposed,and a 2MW power experiment was carried out.The results show that the new solution can solve the temperature rise problem,while the inverter efficiency,power factor,and power quality are all Can meet project requirements. |
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