Research On Active Common-mode Suppression Strategy Based On Dual Inverter And Application

As the conventional DC to AC power conversion equipment,voltage source two-level pulse-width modulation(PWM)inverters have been widely used in motor drive and gridconnected renewable systems.The PWM inverters have the advantages of low harmonic content and fast dynamic response.However,in practical application,the PWM inverters have some significant negative effects on the system,especially the common-mode(CM)leakage current and zero-sequence current caused by the common-mode voltage(CMV).Therefore,based on the systematically discussion for the start-of-art of CM suppression,this thesis makes a deep research on the different CM negative effects of inverter system.First of all,for traditional star-connected load,the time-domain characteristics of CMV for different inverter topologies are studied,and the feasible range of CMV elimination topologies is determined.In addition,the general spectrum analytical formula of the CMV is derived by using the double Fourier integral for the two-level multiphase inverter under the conventional PWM strategy,and the frequency-domain characteristics of CMV are evaluated in the range of wide carrier order and full modulation ratio coefficient.At the same time,the equivalent circuit models of CM stray loop for the grid-connected photovoltaic and motor drive inverter systems are established,and the characteristics of these two CM stray loops are compared and summarized,the effect range of conventional CM suppression methods is also defined.On this basis,by comparing the two inverter systems with the experiments of different topologies and modulation schemes,the correctness of the CMV characteristics and the CM equivalent circuit models are verified,it also indicates that the traditional reduced CMV modulation strategies cannot achieve the effective CM suppression,and the more effective CMV elimination scheme should be considered.According to the CMV elimination demand of topology,the dual inverter is chosen as the research object in this paper.Under the condition of the common DC bus,the AC side of the dual inverter mainly has the following combination modes: two inverters are connected to the three-phase star-connected load in parallel through the external inductances;two inverters are connected to the three-phase open-winding load in series;two inverters are directly connected to the dual three-phase load.The three modes have different application objects,and the CM suppression mechanisms are not identical.The follow-up work of this thesis focuses on the active CM suppression method of the inverter system under these three combination modes.For the mode of two inverters driving the three-phase star-connected load in parallel,considering the voltage and switching times balance between two inverters,this thesis proposes a zero-CM PWM strategy based on paralleled inverter,which can eliminate the CMV in theory.In order to solve the peak amplification problem of circulating current caused by modulation strategy,the modified modulation method by adjusting the time segment sequence before and after sector switching and the additional closed-loop control are utilized to suppress the peak value of circulating current.Aiming at the problem that the pulse-delay effect and switching delay of inverter worse the effect of CMV elimination,a general dead-time compensation strategy is proposed to reconstruct the phase-leg current by the current ripple prediction,which can improve the CMV elimination effect.Through the comparison of experimental results in three-phase grid-connected occasion,it is verified that the proposed scheme can achieve the effective CM suppression.Two inverters connected to both sides of the open-winding motor is another power supply mode.Considering the negative effect of zero-sequence current in open-winding motor drive system,the modulation strategy of unipolar SPWM combined with the phase-shift of pulse is proposed,which can make the two inverters generate the identical CMVs and eliminate the high frequency zero-sequence current caused by the CMV,while the retained double frequency effect can reduce the high-frequency current ripple of the phase current.Furthermore,the quasi PR(proportional-resonant)controller and zero vector regulation methods are used to suppress the low-frequency zero sequence current.The related experimental results verify the feasibility of the proposed method.In view of the CM suppression requirements for dual inverter driven dual three-phase motors,the dual three-phase motor scheme with uniform back electromotive force(EMF)is first proposed by the modification of the multi unit three-phase motor,which can realize the CM suppression of dual three-phase inverter system by combining the zero-CM PWM strategy of paralleled inverter with the proposed dual three-phase motor.Compared with the scheme of three-phase motor driven by parallel inverter,the parallel inductor can be cancelled and the system structure can be simplified.In addition,in order to supplement the CM suppression technology of dual three-phase inverter system,two generalized CMV elimination PWM strategies are proposed based on the principle of end-to-end arrangement of PWM signals,which can decouple the reference voltage and angle displacement for the two sets of three-phase windings,which guarantees the current balance and CMV elimination for general dual threephase motors under the conditions of parameter symmetry and asymmetry.Compared with the experimental results of the conventional schemes,the feasibility and effect of the proposed scheme are verified.