Research On The Control Method Of NPC Three-level Inverter

With the rapid development of modern power electronics technology,multilevel inverter technology and its related control methods are constantly updated and developed.The advantages of multilevel inverter are small loss of switching devices,low harmonic content of output waveform and high efficiency of inverter,so it is widely used in high-voltage and highpower occasions.Diode Clamped three-level inverter(NPC)is widely used and studied because of its simple circuit topology,few power electronic devices and capacitance.In this paper,the modulation method of NPC three-level inverter,control strategy of midpoint potential unbalance,suppression strategy of common-mode voltage and control mode of speed loop and current loop when permanent magnet synchronous motor is used as the load are studied.Firstly,the simplified mathematical model of NPC three-level inverter is established in this paper,and the midpoint current flow is connected with the switching state of the three-level inverter.On this basis,the space vector pulse width modulation method is studied.In order to solve the problem of midpoint potential imbalance in NPC three-level inverters,this paper optimizes the control strategy based on traditional virtual space voltage vector pulse width modulation(VSVPWM).The specific steps are as follows: In order to simplify the calculation,the 60° coordinate system is introduced,the small sectors in the space voltage vector diagram are re-divided,the time adjustment factor and the adjustment coefficient are introduced,and the beat free control is introduced to connect the input voltage with the output current and jointly suppress the fluctuation of the midpoint potential.The simulation proves the effectiveness of the control strategy proposed in this paper.Secondly,the common-mode voltage is related to the switching state of the three-level inverter.Considering the relationship between common-mode voltage and the basic vector,the basic virtual vector is reconstructed.The basic virtual vector ensures that the common-mode voltage amplitude is Vdc/6 and the current flowing through the midpoint is 0.At the same time,the variable hysteresis control and active control are combined to optimize the VSVPWM modulation strategy,so that the amplitude of the common-mode voltage on the output side of the three-level inverter is reduced to the theoretical minimum,while the midpoint potential fluctuation of the three-level inverter is suppressed.The simulation results prove the effectiveness of the proposed control strategy.Finally,the Permanent Magnet Synchronous Motor(PMSM)is a nonlinear multi-coupling system,so the traditional PI control cannot meet the high performance requirements of PMSM.In this paper,the vector control id=0 is adopted for PMSM.The motor speed loop adopts the reaching law sliding mode control,the current loop adopts the PI control,and the vector control module adopts the optimized VSVPWM module based on the midpoint potential,so that the whole PMSM system has good rapport stability and anti-disturbance resistance,and the midpoint potential fluctuation of the three-level inverter is controlled in a small range.The simulation results prove the effectiveness of the control strategy proposed in this paper.