Research On Optimization Of Model Predictive Power Control Strategy For Three-level Rectifier

The energy and power industry are very important to the development of China’s national economy.With the increase of the installed capacity of new energy sources such as photovoltaic,wind energy and hydrogen energy,it poses new challenges to the construction of new energy system.Reducing energy consumption and achieving the goal of "double carbon" are the main investment directions of the country at present.Three-level Pulse With Modulation rectifier has the characteristics of bidirectional energy flow and high power quality.It is a research hotspot in the fields of high-power AC-DC-AC power transmission,high-voltage DC power supply,energy storage and so on,and the research on its control method is of great significance.Model Predictive Control is simple to realize and has the advantages of fast response and multi-objective collaborative control,which is suitable for three-level rectifier control.In this thesis,the Neutral Point Clamped three-level rectifier is taken as the research object,and the effective control method is explored by analyzing the discrete model predictive control strategy.The main work is as follows:Firstly,the PWM rectifier is introduced,including classification,topology structure and several current control strategies.Then,taking NPC three-level PWM rectifier as the main topology,the working principle is analyzed,the mathematical model is deduced,and the instantaneous active and reactive power is calculated according to the instantaneous power theory.The traditional direct power control strategy and multi-objective model predictive power control strategy are studied.The latter discretizes the network-side current and instantaneous power through Euler formula,calculates the power prediction value,neutral point potential,switching frequency and so on,and substitutes them into the value function,calculates the value function under different switching vectors,and obtains the optimal switching vector to act on the switch tube after polling and optimization.The traditional multi-objective model predictive power control strategy has many problems,such as large calculation,uncertain weight coefficient,large power fluctuation and poor parameter robustness.Aiming at the problem of large amount of calculation in traditional control strategy,this thesis proposes a new power control strategy,which divides the space voltage vector into 12 sectors,and conducts vector preselection by judging the sector where the negative conjugate complex power error vector is under the action of zero vector.Each sector has 7 alternative vectors,including1 large vector,1 medium vector,2 small vectors and 3 zero vectors.After analyzing the capacitor voltage on the DC side,the small vectors are classified and optimized by zero vector,and the pre-selected vectors are finally reduced to four,which greatly reduces the difficulty of optimization and reduces the calculation amount in the polling process.At the same time,this thesis provides an effective solution to the problems that may exist in the control process,such as high voltage jump,delay and high switching frequency.In order to reduce the error of power reference value that may occur in the control process,this thesis puts forward a power reference value correction strategy based on the repetitive control principle,analyzes the principle of minimum power area in the power tracking control process,improves the value function,and puts forward a model predictive power control strategy with reduced harmonic content.The improved control strategy can reduce the calculation amount and ensure the neutral point potential balance,network side current harmonics and other parameters,and has good control performance.In the process of model predictive control,the accuracy of model parameters is an important index that directly affects the predictive performance of the system.Aiming at the problem of inaccurate prediction caused by parameter mismatch,this thesis analyzes the causes of model mismatch of PWM rectifier and puts forward a modelfree predictive power slope control strategy based on parameter robustness.Different from the traditional control strategy,this thesis transforms the power error in the value function into the power slope error through mathematical deformation.The power slope at k+1 is estimated by the power slope at k-2 and k-1 and the phase difference between the grid-side voltage and the PWM rectifier-side voltage.The reference value of the power slope is obtained by delay compensation calculation,and then the predicted value and the reference value of the power slope are substituted into the value function to calculate the optimal vector.Because the rectifier parameters Ls and Rs are not used in the process of predicting the power slope,the system has certain parameter robustness.When the inductance parameters are mismatched,the grid-side current is harmonic.The simulation and experiment of the control strategy proposed in this thesis show that the two improved algorithms have excellent performance in both steady-state and dynamic performance.This thesis has 63 pictures,7 tables and 74 references.