Study On Control Strategy Of Current Source Low Voltage Electric Drive System

Inverters can be divided into voltage source inverter(VSI)and current source inverter(CSI)according to the energy characteristics of DC side.Compared with VSI,CSI has the advantages of increasing the amplitude of the AC output voltage,improving the waveform quality of AC output and there is no need for short-circuit protection because the shoot-through state is allowed in CSI.Therefore,CSI has been applied in photovoltaic grid-connected and has potential in motor-driven applications.In this paper,the theory and control method of three-phase CSI inverters for automotive AC asynchronous motors are studied in depth.Some research results are obtained in the aspects of pulse width modulation strategy,main circuit topology,overlap time effect analysis and suppression method,design of multi-loop control system,etc.These research results not only in the academic level are important,but also have practical value.The switching signal of CSI,which is different from VSI,cannot be directly generated by comparing the modulated wave with the carrier wave.Therefore,the switching state of power switches CSI is analyzed and a two-three-value logic conversion circuit from VSI to CSI is proposed.And after deducing the formula of CSI output current harmonics,the CSI output current harmonics of four-segment SPWM,seven-segment SPWM,seven-segment SVPWM and five-segment discontinuous SVPWM modulation strategies are analyzed and compared.Finally,through the analysis the current transient process of CSI commutation process,the theoretical analytical formulas of the conduction loss and switching loss of the power switch tube are obtained,which provides the basis for theoretical analysis of CSI control method.In order to keep DC storage inductance current continuous,it is necessary to add turn-off delay to the switch drive signal of CSI.As a result,there is an overlap-time state during current commutation process,which increases harmonic distortion of output current.After analyzing the overlap-time effect during current commutation process,this paper establish the relationship between the nonlinear error of CSI output current and three-phase filter capacitor voltage vector.Then,the current error pulse signal was equivalent to continuous cycle signal according to the pulse equivalent principle.After that,the expression of harmonic component caused by overlap-time was derived based on Fourier analysis.Finally,an average current feedforward compensation strategy was proposed.Simulation and experimental results show that,even under different overlap-time,compensation strategy still can compensate fundamental wave attenuation of CSI output current effectively and reduce the distortion of low-order harmonic caused by overlap-time.In space vector modulation process for traditional three-phase CSI,DC storage inductance current is usually considered as constant and charge-discharge process is ignored during a carrier period,which could cause storage inductance current discontinuous or continuous increasing.In this paper,an improved dc-side topology of CSI was proposed,which adds inverse parallel switch on DC side energy storage inductances.Then three working mode of the storage inductance during modulation process was analyzed thoroughly.Finally,the calculating method for optimal given value of storage inductance current under each working conditions is proposed.And the SVPWM modulation strategy is designed to control storage inductance current for the improved topology.Both the simulation and experimental results show the optimal given values of storage inductance current can meet the demand of AC-side and remain stable within a certain range.Besides,load current of CSI can accurately follow the given,system has good steady and dynamic performance.Thus,the correctness and feasibility of the proposed SVPWM modulation strategy is proved.In order to realize the buck working and energy bi-direction flow of CSI asynchronous motor drive system,a voltage-current(V-I)converter is introduced in DC-side.By establishment of mathematical model of filtering capacitor and motor,deducing the steady-state mathematical relationship between the energy storage inductor current and the rotor flux linkage when the rotor flux linkage has been oriented,the calculation method of optimal given value for storage inductor current and rotor flux linkage is obtained.On this basis,a hysteretic control strategy for energy storage inductor current based on V-I converter is proposed.Then the parameters design expressions of the inductance and capacitance are derived.For the LC resonance problem caused by the LC on the AC-side,the active damping method of the capacitor voltage feedback is introduced.Finally,the analytical formulas of PI controller for current inner loop,flux outer loop and outer speed loop is proposed,which can regulate the proportion and integral parameters according to the target phase angle margin and cutoff frequency.The simulation model and experimental platform are built and have verified the feasibility and correctness of the vector control strategy of induction motor based on the minimum energy storage inductor current control strategy.