| As an important power conversion device,the voltage source inverter(VSI)is widely used in the fields of shipping,aviation,renewable power generation and uninterruptible power supply.Facing the critical loads,the VSI needs to deal with the quick start and stop,load changes and the load side short-circuit fault.In order to provide the power of high quality,the VSI should first achieve the closed-loop control of the output voltage.The closed-loop controller should ensure the performance of stable and accurate command tracking,meanwhile,it also needs to have a design procedure of convenient and efficient.Besides the reference of the output voltage in the closed-loop control,the output voltage of the VSI would also be influenced by the load current.As a disturbance input of the VSI,the load current would lead to a voltage drop of the output voltage due to the output impedance of the VSI,thereby causing the degradation of the performance of the VSI.Therefore,it is also necessary to achieve disturbance suppression.In addition to the normal operation,in order to ensure the reliability of the power supply,the VSI should also need to have the ability of fault ride-through when there is a short-circuit fault at the load side.Based on the background and demands above,this paper would focus on the close-loop control of the output voltage for the VSI,the adjustment of the output impedance of the VSI and its application in the disturbance suppression as well as the control of fault ride-through under the short-circuit fault in the load side.The details of this paper are briefly illustrated as follows:(1)In the closed-loop control of the VSI,the Proportional-resonant(PR)control is widely used in the control of the output voltage since it can track the sinusoidal reference directly in the stationary frame without steady-state error.In order to solve the difficulty of multi-parameter design for the PR control structure,as well as improve the dynamic performance of command tracking,based on the reduced-order characteristic of the conventional second-order resonant controller in the discrete-time domain,the method of selective pole placement and cancellation for PR control design used in VSI is proposed in this paper.Based on the proposed method,all of the control parameters can be calculated by placing the selected closed-loop poles and zeros in desired positions,trial-and-error is avoided,and the design procedure is simplified.With the closed-loop poles and zeros that are placed in the desired positions,the stability is guaranteed,and the performance of the PR control structure is improved.(2)Consider that the load is connected to the VSI most of the time,in order to get a better dynamic and steady-state performance when the load is connected,the output impedance of the VSI should be reduced by the load current feedforward control and achieve disturbance suppression.Considering the accurate control performance of the controller that is designed directly in the discrete-time domain,the discrete-time model based multiple paths full feedforward control for the disturbance suppression of the VSI is proposed in this paper.With multiple feedforward paths,the difficulty in the realization of load current full feedforward control is reduced,and the proposed method also has better versatility.In addition,before the implementation of the feedforward controller that is derived theoretically,the issues related to the realizability and performance of this controller are discussed,which include the handling of the advanced operator,the ringing phenomenon and the dynamic process of the transformer.With a lower output impedance at the fundamental frequency and the low-order harmonic frequencies,satisfactory performance is achieved under different kinds of load.(3)In terms of the existing load current feedforward control that is used to achieve disturbance suppression,due to the variation of the frequency characteristic between the actual implemented feedforward controller and the ideal feedforward controller,the output impedance of the VSI cannot be further reduced,especially the output impedance at the low-order harmonic frequencies,thereby limiting the improvement of the disturbance suppression under nonlinear load.In order to further improve the performance of the VSI under nonlinear load condition,the method of optimization adjustment of output impedance at selected frequencies for the VSI is proposed in this paper.In the proposed method,multiple resonant controllers are added to the load current feedforward control,meanwhile,the interaction among the resonant controllers is considered and handled specially.The variation of the frequency characteristic between the actual implemented feedforward controller and the ideal feedforward controller is reduced at the selected frequencies.The output impedance of the VSI at the selected frequencies is further reduced,and the performance under nonlinear load condition is improved.(4)The load current feedforward controller can be considered as a virtual impedance of the VSI,it can not only reduce the output impedance of the VSI to achieve disturbance suppression,but also can increase the output impedance of the VSI to achieve fault ride-through under the short-circuit fault in the load side.Based on this idea and the method of optimization adjustment of output impedance at selected frequencies,the method of fault ride-through under the short-circuit fault based on adaptive virtual resistance is proposed in this paper.The proposed method is still the output voltage control of the VSI,specifically,in order to achieve the fault ride-through under the short-circuit fault at the load side,the output voltage of the VSI is reduced by the voltage drop caused by the virtual resistance.At the initial stage of short-circuit fault at the load side,the current limiting control can be achieved by the virtual resistance without the result of fault diagnosis and the calculation of the current reference.Meanwhile,the closed-loop control of the maximum output current of the fault phase can be achieved because of the self-adjustment of the adaptive virtual resistance.During the whole process of fault ride-through under the short-circuit fault,the VSI still operates under the voltage control mode without limiting the output of all the controllers,therefore,the output current is not distorted severely.In addition,the proposed method still can fulfill the disturbance suppression during the fault ride-through,therefore,the output voltage recovers quickly without the phenomenon of overvoltage after the short-circuit fault is removed. |
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