Research On Proportional Integral And Model Predictive Parallel Control Method

Power electronic converters have wide application prospect,whose development is dependent not only on the progress of power semiconductor technology but also on the control technology.At present,the common control methods used in power electronics mainly include linear control,sliding mode control,repetitive control,smart control,and,et al,which have their own respective advantages and disadvantages.With the rapid development of microprocessors,it is possible for complicated digital control algorithms to apply in power electronics.As a novel digital control method,Model Predictive Control(MPC)is easy to understand for the intuitive concept,easy to accomplish for the simple control structure and able to handle multivariable,nonlinear and constrained problems,which is especially suitable for the situation of power electronic control and therefore,attracts many research attentions.However,the existing MPC control algorithms all strictly rely on the predictive model,leading to the disfect of the sensitivity to the model parameters.With model parameters mismatch,the MPC will deteriorate its control performance,cause stready state error,or even lead to the divergence of the whole control system.Therefore,the presented literatures have proposed methods to observe the model parameter online or to adopt the observer to compensate the steady state error,which is complicated to accomplish and exists many usage limitations.To solve this problem briefly,it is further proposed in the literatures to use the MPC and PI controllers at the same time in a cascade or switching structure to obtain the features of the MPC and PI simultaneously.However,these solutions are still complicated and need extra design so that they does not fully exploit the advantages of the MPC and PI controllers.For these above reasons,this article proposes the control structure in which the MPC and PI controllers are used in parallel.Through such design,the high dynamic response speed of the MPC and the robustness and steady state tracking performance of the PI can be satisfied simultaneously without extra design.The main contents of this article are:(1)The structure where the MPC and PI controllers are used in parallel is constructed to demonstrate the main problem and potential advantages.To achieve such parallel control,the main challenge is to enable MPC and PI controllers to achieve close-loop control simultaneously without control conflicts.To this end,in order to let the parallel control combine the features of both of the MPC and PI,this article proposes to regard the close-loop control system formed by the plant and the PI controller as a whole,which serves as the equivalent plant of MPC.This article introduces the derivation and calculation method of this equivalent plant in detail,and furthermore,the MPC is design on such equivalent plant to conclude the general design procedure and method of the proposed parallel control.(2)To develop the generality of the above design method,this article takes the three-phase grid-connected inverter grid-connected current control and the Buck converter inductor current control as examples,to explain the specific design processes how the parallel control strategy is applied to these two examples.According to the concrete parameters of the converters,the parallel control coefficients are also designed by bode diagram.To verify the parallel control,the simulations based on the three-phase grid-connected inverter and the Buck converter are established on the Matlab/Simulink environment,in which the parallel control,the single PI control and the single MPC are compared.The results show that the parallel control has the dynamic response performances similar to the MPC and can achieve zero steady state error similar to PI at the same time,which makes that the model parameters mismatch will only influence the dynamic performances but cause no impact to the steady state performances.(3)The parallel control is analyzed in-depth: the MPC will play a major role in the dynamic process to accelerate the dynamic response speed,and then its output will decay to zero gradually with the accumulation of the output of the PI controller.Finally,the PI controller will provide all the output during the steady state to avoid the influence caused by the model parameter miamatch.Inspired by the analysis,this article further proposes the parallel control structure of the digital MPC and the analog PI to further improve the reliablility and safety of the parallel control.Taking Buck converter as an example,this article introduces the hardware design scheme,builds a prototype,and compares parallel control with the analog PI and the digital MPC by experiments.The experimental results show that when the digital controller works normally,the parallel control can achieve the optimal control performance.When the digital controller breaks down,the converter can maintain its basic operation under the domination of only analog PI while the digital MPC is isolated from the control loop.