Grid-connected Inverter Control System Without Grid-side Voltage Sensor Based On Power Prediction

In recent years,with the increasing shortage of traditional fossil energy sources and the increasingly prominent environmental pollution problems,the efficient development and utilization of clean energy such as solar and wind energy has become the focus of attention of all countries in the world.As an important link of new energy power generation and grid connection,inverters have become one of the hotspots of scientific research by scholars from all over the world.The advantage and disadvantage of the grid-connected inverter control method play a decisive role in the performance of the grid-connected inverter system.Among many inverter control methods,the model predictive control method is considered one of the most promising inverter control methods with excellent dynamic response and flexible multi-objective constraint ability.So it is used as the main grid-connected inverter control method in this article.In addition,the traditional closed-loop control system of the grid-connected inverter usually requires at least five sensors,including two grid-connected current sensors,two grid voltage sensors,and one DC link voltage sensor.Considering the unbalanced grid voltage,the number of sensors will further increase.The use of a large number of sensors will not only increase the economic cost and reduce the power density of the equipment,but also make the grid-tied converter system vulnerable to sensor failures.Therefore,the research on grid-connected inverter systems without grid-side voltage sensors has important practical significance.This paper carries out research based on the three-phase two-level inverter model,and proposes a predictive direct power voltage sensor-less grid-connected inverter control algorithm.First,the basic principles of model predictive control of grid-connected inverters are introduced,and the two-step prediction control method is used to eliminate the influence of calculation delay on the system.Next,the basic principles of dual vector control strategy and common mode voltage suppression control strategy based on virtual voltage vector are introduced in detail,and the traditional single vector control strategy and two improved control strategies are compared and analyzed.Finally,the common mode suppression control strategy based on virtual voltage vector is used as the control method in this article.Regarding the problem of inverter’s excessive dependence on sensors,this paper replaces the grid voltage sensor with the method of estimating the grid-side voltage based on the virtual flux linkage.And use a dual low-pass filter to replace the pure integral stage in the virtual flux calculation,thereby eliminating the DC offset problem caused by the integral stage.In a voltage sensor-less control system,the system power factor is often low due to inaccurate model parameters.This paper proposes a filter inductance online detection method suitable for voltage sensor-less systems.By creating an inductance set and using mathematical models,rolling optimization and cost functions in predictive control to accurately detect inductance values,the system control performance has been effectively improved.Finally,in order to make the voltage sensor-less grid-connected inverter system more perfect to meet the needs of the inverter in practical applications,this paper also studies the control method of the inverter in the case of unbalanced grid voltage.The positive and negative sequence separation method based on the second-order generalized integrator is used to extract the positive and negative sequence components of the inverter output voltage and grid-connected current,the positive and negative sequence components of the grid voltage are calculated separately,and then the power reference value compensation method is used to achieve Three different control targets under unbalanced grid voltage conditions.Use Matlab/simulink simulation software to build a model to verify the feasibility of the above method and the accuracy of theoretical analysis.Firstly,the output results of the three model predictive control strategies were compared,and the best control strategy was selected.Then the feasibility of the voltage sensor-less control method and the filter inductance online detection method is verified through experiments and simulations.Finally,it is verified by simulation that the grid voltage estimation link can still accurately detect the grid-side voltage value under the condition of grid voltage imbalance,and the voltage sensor-less grid-connected inverter control system can achieve different control goals.