Research On Model Predictive Control Strategy Of Cascaded Multilevel Inverter

With the rapid development of power electronics technology,high-power converters are widely used in power transmission and distribution,AC asynchronous motor speed control systems,and new energy grids.They are core equipment of static synchronous compensators(STATCOM),active power filters(APF),flexible AC transmission technology(FACTS)and AC asynchronous motor speed control device.In the field of high voltage and power,multilevel inverters have small voltage change rate du/dt and small electromagnetic interference;a large number of output levels,high power quality and low total harmonic distortion(THD);low voltage stress.But as the number of output levels increases,the control strategy of multilevel inverters is correspondingly complicated,and it is difficult to achieve multi-target control.With the widespread application of power electronics in smart grids,some malicious data injection can effectively bypass system detection making the system controller out of control and seriously affecting the normal operation of the smart grid.In response to the above issues,the following research work has been carried out in this paper:First of all,the working principle of cascaded multilevel inverter is analyzed and the mathematical model of the cascaded multilevel inverter is established.The traditional model predictive control strategy is analyzed theoretically and simulated.When the number of output voltage levels is large,the controller has a large calculation burden and does not make better use of the large number of redundant switching states of the cascaded multilevel inverter.The system has a large common mode voltage peak,and the system is not robust.It is difficult to resist the cyber security.Based on these problems,this paper proposes two improved model predictive control strategies to achieve tracking load current and suppressing common-mode voltage while reducing the calculation of the controller.The first is an improved model predictive control strategy based on the Lyapunov function.By constructing the Lyapunov function and making full use of redundant voltage vectors to redefine the cost function,the system controller calculation amount is greatly reduced and no weight factor is required.For this strategy,it can reduce the peak value of the system common mode voltage and achieve load current control.The second is a model predictive control strategy suppressing the common mode voltage.By further analyzing the characteristics of the voltage vectors of the three-level and five-level inverters,the system basic voltage vectors are reasonably selected and the optimal solution of the system cost function is derived.The control strategy is further optimized that achieves load current control and zero common-mode voltage control.Simulation studies of two improved model predictive control strategies are carried out to verify their effectiveness and correctness.Secondly,for the widespread application of power electronics in smart grids,some malicious data injections can effectively bypass system detection,causing the system controller to run out of control and seriously affecting the normal operation of the smart grid.To solve this problem,this paper further optimizes the system control strategy in an improved model predictive control strategy based on the Lyapunov function and a cascaded multilevel inverter system based on the Kalman Filter is established.The Kalman Filter is used to estimate the optimal state of the system.Then,from the perspective of cyber attacker,a secret of cyber attack that can effectively bypass Kalman Filter and detection indicators is designed.Two improved model predictive control strategies based on KNN algorithm and IF algorithm are proposed to solve the special cyber attack.Simulation studies are carried out to verify correctness and effectiveness of the cyber attack and two improved model predictive control strategies.Finally,for two common application areas of cascaded multilevel inverters,the model predictive control strategy for suppressing common mode voltage is applied to the cascaded multilevel asynchronous AC motor speed control system,which is built on the matlab/simulink platform.The simulation model of the control system verifies that the control strategy can effectively reduce the output common mode voltage of inverter and motor bearing damage while ensuring the steady state and dynamic performance of the system.Secondly,the model predictive control strategy based on IF algorithm is applied to the cascaded multilevel inverter static synchronous compensator system considering cyber security.The control system simulation model is established on the matlab/simulink platform,which verifies that the control system is still on normal operation when considering smart grid cyber security.