Research On The Control System Of The New Metro Bidirectional Converter

Urban rail transit usually uses 12 or 24 pulse diode rectification for power supply.The diode rectification structure is simple,safe,reliable and efficient,but energy can only flow in one direction.The energy generated during the frequent braking of the vehicle requires additional resistance 、energy storage device,Energy feedback device or other devices.The bidirectional converter realizes the bidirectional flow of energy and has become one of the directions to replace the diode rectifier of the urban rail traction power supply.Relying on the project "The Metro Bidirectional converter" commissioned by the enterprise,this paper conducts research on a new type of PWM bidirectional converter from two aspects:modulation algorithm and control algorithm.First,mathematical model analysis.Aiming at the three-phase two-level main circuit topology of the bidirectional converter,the time domain and s domain mathematical models under the abc,αβ,dq three coordinate systems are established,and the instantaneous power models under the abc,αβ,dq three coordinate systems are established correspondingly.The relationship between the physical quantities of the converter is clarified,and it can be seen that the power frequency AC quantity can be converted into a DC quantity under the coordinate system,which is convenient for the design of the controller using classic control theory,and it is also the coordinate system selected by the subsequent controller.Second,the modulation strategy research.The capacity of the bidirectional converter is usually megawatt power level and the switching frequency is limited,so SHEPWM and CHMWPM modulation algorithms are selected.Starting from the principles of the two modulation algorithms and using Matlab to solve the switching angle and harmonic analysis at low switching frequencies(<1kHz).The secondary winding of the three-winding phaseshifting transformer can be used to cancel the 6k±1(k is odd number)harmonic characteristics,and the SHEPWM which eliminates the harmonics at intervals is proposed.Analyzed and compared the voltage utilization and switching loss of SHEPWM,SVPWM and SPWM,and quantitatively calculated that SHEPWM can reduce the switching loss by approximately 17%at a switching frequency of 550 Hz.Third,control strategy research.The classic voltage and current double closed loop and AC grid voltage feedforward,inductance cross decoupling and other control methods are selected to adjust the voltage loop and current loop PI parameters,and power feedforward control is added to the current inner loop to improve the system’s dynamic response speed.For the parallel operation of two bidirectional converters,the load average current feedforward control is adopted,and the current imbalance on the DC side is only 0.42%.Finally,the modulation algorithm and control algorithm were verified on the 7.5MW subway bidirectional converters mutual experiment platform.Under the switching frequency of 750 Hz,the rated current is running,and the current harmonic of the 35 kV side is 4.6%<5%.The efficiency of the converter is 98.7%>98%,which meets the design requirements.