Research On Active Discharge Control Strategy Of Bus Capacitor For Electric Vehicles

In order to cope with the increasingly severe energy shortage and environmental pollution problems,electric vehicles with advantages such as energy saving and environmental protection have received extensive attention.In order to improve the cruising range and charging speed of electric vehicles,high-voltage drive systems are widely used in electric vehicles.However,the electric vehicle based on the high-voltage drive system has the risk of electric shock to passengers in the event of an emergency such as a collision.Therefore,it is of great significance to study the active discharge technology of bus capacitors in emergency situations such as electric vehicle collisions to improve the electrical safety of electric vehicles and promote the development of electric vehicles.At present,the main drive system of electric vehicles usually discharges the bus capacitors by connecting a bleeder circuit in parallel on the busbar.The method of active discharge through the main drive motor winding of the electric vehicle has the advantages of low cost,high reliability and small size.Therefore,this paper takes the permanent magnet synchronous motor for electric vehicles as the research object,and studies and discusses the active discharge control strategy of electric vehicle bus capacitors.The specific research contents are as follows:Firstly,the standards and regulations related to passenger injury and collision safety in the case of electric vehicle collision are summarized,and the research status of electric vehicle bus capacitor discharge at home and abroad in emergency situations such as collision is reviewed.Then,the mathematical models of PMSM and PWM power converter are established,and the operating characteristics of PMSM under power generation and motoring states are analyzed.On this basis,a mathematical model for the discharge method of the bleeder resistor is established,and the parameters of the bleeder resistor are designed and analyzed.The discharge method is experimentally verified on a permanent magnet synchronous motor drive platform for automobiles.Secondly,in order to save the complicated and bulky bleeder resistor,the method of using the motor winding to discharge is studied.In order to quickly reduce the bus voltage to a safe voltage after an emergency such as a collision occurs,a bus capacitor discharge method based on an extended stage observer(ESO)is proposed.The method is divided into a fast discharge stage and a constant voltage discharge stage.In the fast discharge stage,the bus voltage is rapidly reduced to a safe voltage,and then in the constant voltage discharge stage,the total power loss of the system is estimated through ESO and feedforward compensation,so as to The bus voltage is stabilized at a safe voltage.The discharge method realizes the rapid reduction of the busbar voltage to a safe level through the motor winding,and realizes the decoupling of the rotational speed and the busbar voltage,and has strong robustness to parameter changes.Thirdly,when the speed is higher than the speed which can be quickly reduced to the safe voltage,the bus voltage cannot be reduced to the safe voltage by applying the maximum safe current to the d-axis,resulting in the failure of the ESO-based bus capacitor discharge method.In order to solve this problem,a direct current control strategy for bus capacitor discharge based on the maximum copper loss of motor windings is proposed.Firstly,the energy model under the direct control of dq-axis current is established,and the influence of dq-axis current on the energy flow of the system is analyzed.Then,the d,q-axis current trajectory under the constraints is obtained by solving the dq-axis current limit circle when the winding copper consumption is the largest,the voltage limit elliptical trajectory when the speed drops,and the power trajectory when the regenerative braking power is less than the winding copper consumption power.Then,the winding copper under current control consumes the most power.The direct current control strategy of the bus capacitor discharge based on the maximum copper power consumption of the motor winding not only improves the discharge power of the motor winding,shortens the discharge time,but also does not cause voltage surges and other phenomena.Finally,in view of the limited discharge capacity of the motor winding discharge method,when the speed is higher than the maximum dischargeable speed of the winding,the dischargeable energy of the winding will be less than the bus voltage and reduce to the safe voltage to release the energy,resulting in the discharge time exceeding the specified time.A hybrid discharge method of bus capacitors based on Adaptive sliding mode power control(ASMPC)is proposed.In order to make full use of the discharge capacity of the bleeder resistor,the discharge power of the bleeder resistor is always kept at the maximum power during the discharge process.In order to improve the dynamic performance of discharge power tracking,a power error adaptive reaching law is proposed and analyzed.The discharge method based on ASMPC can not only improve the dynamic tracking performance of discharge power,but also reduce the chattering of the output of the sliding mode power controller,which solves the problem of limited discharge capacity of pure motor windings.In order to facilitate the application of the proposed discharge method in engineering practice,the proposed winding-based discharge method is compared and analyzed,and a principle for selecting a discharge strategy considering engineering needs and electric vehicle power system parameters is given,which is convenient for engineers designing reliable and cost-effective discharge systems for electric vehicles.