Research On Model Predictive Power Control Of Grid-connected Inverter

With a large number of renewable energy connected to the grid,the grid-connected inverter,as an important power interface to transmit electric energy to the grid,has a direct impact on the control performance of grid-connected power quality,so the research on the control strategy of grid-connected inverter has become a hot topic.The direction and amplitude of the output voltage vector predicted by the traditional single vector model can not be adjusted,which makes the grid-connected current have high harmonic content,large power pulsation,and the switching frequency is not fixed,which increases the switching loss.In order to improve the grid-connected power quality,reduce the power pulsation and reduce the switching loss,the model predictive direct power control(MPDPC)strategy for grid-connected inverters is studied in this paper.Firstly,this paper introduces the topology categories and characteristics of grid-connected inverters,chooses the voltage source inverters with higher operating efficiency as the control object,analyzes the common control strategy principles and advantages and disadvantages of grid-connected inverters,and compares them with MPDPC,which shows the superiority of MPDPC.Secondly,through the analysis of the topology and working principle of the grid-connected inverter,the mathematical models of the three-phase static coordinate system,two-phase static coordinate system and two-phase rotating coordinate system are established.The mathematical principle and operation mode of space vector pulse width modulation(SVPWM)are analyzed in detail,and the simulation model is built.The parameter calculation method of DC side voltage and AC side filter inductance of grid-connected inverter is studied.On this basis,combined with SVPWM,this paper designs single-vector MPDPC strategy and double-vector MPDPC strategy with constant switching frequency.Through analysis,compared with the single-vector MPDPC,the two-vector MPDPC expands the coverage range of the output voltage vector,but the amplitude of the output voltage vector cannot be adjusted when the two vectors are both effective vectors.It can improve the reactive power pulsation by carrying out the beat free control on the reactive power,but cannot effectively reduce the active power pulsation.Aiming at the limitation of two-vector MPDPC,a three-vector MPDPC is proposed in this paper.The strategy in each control cycle to twice the voltage vector selection,with two adjacent effective voltage vector and a zero voltage vector synthesis expect voltage vector,the output voltage vector direction and amplitude are adjustable,and at the same time,the active power and reactive power without difference frequency control,can effectively improve the grid power quality and reduce power pulse,and combining the SVPWM make fixed switching frequency,solved the problem.Finally,the corresponding simulation models of the above three control strategies are built in Matlab/Simulink software,and the design ideas of the controller algorithm are analyzed.In the case of ideal grid and unbalanced grid voltage,a series of comparative analyses are made on the three strategies by means of power abrupt change and filter inductance value change.And it was verified by RT-LAB hardware-in-the-loop simulation experiment platform.Results show that compared with single or double vector model to predict the direct power control strategy,the proposed prediction vector model direct power control strategy under the condition of ideal power grid current harmonic content is low,power fluctuation is small,has good steady-state performance,at the same time on the grid voltage unbalance situation also has good adaptability,verify the feasibility and effectiveness of the strategy.