Researh On The Control Strategy Of Three-phase T-type Inverter

With the development of photovoltaic industry,photovoltaic power generation is becoming larger and larger and centralization is closer.Photovoltaic grid connected generation is the main trend of photovoltaic power generation in the future.As the core device of photovoltaic grid connected,the performance and efficiency of photovoltaic inverter can improve the efficiency and reliability of the whole system,which has important research significance.Traditional two level inverter is limited by switch voltage,and its power and conversion efficiency gradually can not meet the requirements of PV system to inverter.Compared with the two level inverter,the multilevel inverter has the advantages of low output voltage harmonic content,high waveform quality,small switch stress and so on.It is more suitable to be used as a photovoltaic grid connected inverter.With the continuous development of switching technology and the continuous progress of digital control technology,we believe that multilevel inverter will be more widely applied in the future.The T type three level topology has been developing rapidly in recent years.It is an improvement of diode clamp topology,which solves the problems of the traditional topology switching voltage unevenly and too many devices.Therefore,this paper has studied the T inverter.This paper first introduces the topology of the T inverter,analyzes and compares the advantages and disadvantages of the T type three level topology compared with other three level topology,and analyzes the corresponding relationship between the switch state and the output of the T topology by analyzing the switching process of the T type three level topology,and analyzes the causes of the common mode current.The common mode current problem of PV inverter is solved from two aspects of hardware and software.The cause of the imbalance of the three level point potential is analyzed,and a vector control algorithm that can simultaneously solve the midpoint potential balance and suppress the common mode current is proposed.By establishing the mathematical model of the T type three level inverter,the connection between the switch state and the space voltage vector is established,and the implementation of the three level SVPWM modulation algorithm is described in detail.On the premise of guaranteeing the quality of the output waveform,a modulation strategy which can reduce the common mode current and facilitate the middle point potential hysteresis control is proposed by reasonably arranging the action sequence and the action time of the small vector.The effectiveness of the algorithm is verified by the simulation experiment.In order to achieve grid connected current control of T type three level inverter,a double closed loop control structure of voltage outer loop and current inner loop is proposed in this paper.By modeling the dynamic process of the inverter,the coordinate transformation and feedforward decoupling are carried out,the output current control block diagram in the two phase rotating coordinate system is obtained.The external voltage loop and the current inner loop PI regulator are designed,and the effectiveness of the double closed loop control method is verified by the simulation.