Abstract: This paper introduces the circuit topology of a diode-mounted (NPC) three-level inverter and adopts a space vector pulse width modulation (SVPWM) control strategy to achieve precise torque control of an asynchronous motor. The SVPWM control strategy and algorithm of the three-level inverter are studied. The SVPWM modulation algorithm of the three-level inverter is simulated and analyzed using MATLAB software, proving its correctness. This modulation scheme has good energy-saving effect when applied to a motor variable frequency speed control system. Keywords: Three-level inverter; Space vector control; SIMULINK [b][align=center]A SVPWM Control Method Based on Three Level Inverter Zhang Hong-ling, WANG Da-zhi[/align][/b] Abstract: This paper introduces the form of a diode-clamped three-level inverter and uses the SVPWM method for high toque control accuracy. It investigates the three-level inverter and the space vector pulse width modulation control method used in motors. Finally, the SVPWM modulating method of three-level inverters is simulated by MATLAB, and the correct result is proved. It has good effect in high-voltage motor control. Key word: three-level inverter; space vector control; SIMULINK; In recent years, the application of multi-level inverters in high-voltage and high-power applications has received increasing attention. Various circuit topologies and control methods have been proposed and studied, among which the three-level structure is a research hotspot. There are various topologies for three-level inverters, such as diode-clamped, leap capacitor, and independent DC power supply cascade topologies, among which the diode-clamped topology is the most widely used. It can effectively improve the withstand voltage of the speed control system, reduce output voltage harmonics and switching losses, and is widely valued in high-power applications of power systems. This paper, based on the diode topology of a three-level inverter, adopts a space voltage space vector control strategy to achieve high-performance voltage vector control with low harmonic components. 1 Three-level Inverter Topology In Figure 1 below, each phase arm has 4 IGBTs, 2 clamping diodes, and 4 reverse recovery diodes. During operation, the interlocking of diodes 1 and 3, and diodes 2 and 4 of each phase arm is always ensured, and each phase output has three levels. For example, when phase A is simultaneously turned on by S1 and S2 and turned off by S3 and S4, a positive level can be obtained at the inverter circuit output; simultaneously turning on S2 and S3 and turning off S1 and S4 results in zero output voltage; simultaneously turning on S3 and S4 and turning off S1 and S4 results in a negative level at the output. It can be seen from the circuit structure that the zero level is achieved by the combined action of S2, S3, and diodes D5 and D6. [align=center]Figure 1. Main circuit topology of a three-level inverter[/align] Therefore, from the perspective of the three-level topology, each phase can have three switching states. When using the same switching devices, a higher inverter voltage can be obtained than that of a two-level inverter. Moreover, under the same DC voltage, the three-level topology is more reliable than the two-level structure. In addition, when controlling the turn-off of the switching devices, the dead time should be set reasonably to avoid the simultaneous turn-on or turn-off of the IGBTs in the same bridge arm. 2. Space voltage vector of three-level inverter 2.1 Principle of space voltage vector If the switching variables Sa, Sb, and Sc represent the output states of each phase bridge arm, and Si (i=a,b,c) = (1,0,-1) represents the output level P, the output level O, and the output level N of the i-th phase, respectively, then the three-phase three-level inverter can output 3[sup]3[/sup] = 27 switching states. The output state is expressed as: According to the principle of equal amplitude, the stator voltage space vector can be expressed as: Substituting (1) into (2) we get: Mapping it to the plane, the space vectors corresponding to the 27 switching states of the three-level converter are shown in Figure 2. These 27 space vectors can be divided into four categories, including 6 long vectors, 6 medium vectors, 12 small vectors, and 3 zero vectors. By mapping the vector amplitudes to them, the distribution law of the three-level voltage vectors can be clearly seen. The vector diagram analysis is generally based on the principle of symmetry. As long as the 60-degree region is analyzed, the number of small triangles in the 60-degree region is 4. The voltage level of the output phase voltage of the three-level inverter from the trough to the peak is 9 levels, and the voltage level of the output line voltage from the peak to the trough is 5 levels. [align=center] Figure 2 Space voltage vector distribution diagram of the three-level inverter[/align] 2.2 The influence of the space voltage vector on the midpoint potential It can be seen from the circuit topology that as long as the midpoint current is not zero, the DC side capacitor will charge and discharge, thereby affecting the midpoint potential. Among the four basic voltage vectors, the zero vector has no effect on the midpoint potential because the three-phase potentials are equal and no current flows through it; the large vector has no effect because the midpoint does not participate in energy transfer; the medium and small vectors have effects on the midpoint potential because the midpoint current is not zero. Both the small and medium vectors affect the midpoint potential because the midpoint potential rises when current flows in and falls when current flows out. When the direction of the current in a phase is determined, the effect of the small vector state on the midpoint potential is opposite, which is why the midpoint potential balance can be adjusted by selecting the duration of the small vector state. 3 Three-level SVPWM modulation scheme 3.1 Relationship between voltage vector and flux linkage space vector The ultimate goal of requiring three-phase sinusoidal current input to an AC motor is to form a circular rotating magnetic field in the motor space, thereby generating a constant electromagnetic torque. The flux linkage trajectory is obtained by alternating different voltage space vectors; therefore, SVPWM control is also called flux linkage tracking control. When the three-phase symmetrical stator windings of an asynchronous motor are powered by a three-phase balanced sinusoidal voltage, the stator voltage can be expressed as: where are the combined space vectors of the three-phase stator voltage, current, and flux linkage, respectively; ψ_m is the amplitude of the flux linkage; and ω₁ is its rotational angular velocity. When the motor speed is not very low, the stator resistance voltage drop can be ignored, and the voltage u_s is expressed as: [align=center] Figure 3 Approximate circular flux linkage increment trajectory[/align] It can be seen that when the flux linkage amplitude ψ_m is constant, the magnitude of u_s is proportional to ω₁, and its direction is the tangent direction of the flux linkage circle. Increasing the number of inverter switching cycles can make the flux linkage approximate a circle, as shown in Figure 3. Assuming the flux linkage increment consists of six segments, each with a different voltage vector, the desired phase flux linkage increment can be obtained by using a linear combination of different voltage space vectors acting at different times. 3.2 Three-Level SVPWM Modulation [align=center] Figure 4 Space Vector Cell Division of Three-Level Inverter [/align] For a three-level inverter, the space voltage vector diagram can be divided into 6 sectors, each sector is further divided into 4 small triangular areas, for a total of 24 triangular areas. In Figure 4, Va and Vc are long vectors, Vb is a medium vector, V1 and V2 are short vectors, and V0 is a zero vector. Without considering voltage compensation, the modulation ratio m = f/50, where f is the operating frequency. Let the synthesized reference vector be V and fall within sector 1, synthesized from V1, V2, and V0, with corresponding switching state times of T1, T2, and T0, respectively. The calculation method for the reference vectors of other sectors is similar. Thus, it is necessary to determine the reference vector in 24 cases, and then use different expressions for different triangular areas to calculate the vectors involved in the synthesis and their duration of action. 4. Simulation Results As shown in Figure 5 of the simulation results, the motor current fluctuates significantly during startup, but once the motor reaches the given speed, the current stabilizes near its stable value. The output line voltage waveform of the three-level inverter is close to a sine wave with relatively small harmonic components. The flux linkage trajectory is nearly circular, meeting the requirements of space voltage space vector control. The neutral point potential fluctuates slightly around the equilibrium point, but is almost symmetrical and basically balanced. (a) Stator three-phase current waveform of the motor (b) Three-level line voltage waveform (c) Flux linkage trajectory of hysteresis control (d) Waveform of neutral point potential [align=center] Figure 5 Simulation results of three-level inverter based on SVPWM[/align] 5. Conclusion The SVPWM modulation technology based on three levels proposed in this paper effectively reduces switching losses, decreases the harmonic distortion rate on the AC side, and improves the utilization rate of the output voltage and bus voltage of the variable frequency speed control system by optimizing the switching vector and its action time. When using SVPWM control, the maximum value of the fundamental frequency of the inverter output line voltage is the DC side voltage, which is 15% higher than the output voltage of a typical SPWM inverter. References: [1] Liu Fengjun. Multilevel inverter technology and its application [M]. Beijing: Machinery Industry Press, 2007. [2] Li Yongdong, Xiao Xi, Gao Yue. High-capacity multilevel converter - principle, control and application [M]. Beijing: Science Press, 2005. [3] Ding Wenpeng, Hong Naigang. A new algorithm for judging the reference voltage vector region of a three-level inverter [J]. Journal of Anhui University, 2007, (1): 72-75. [4] Song Wenxiang, Chen Guocheng, Wu Hui. A three-level space vector modulation method with midpoint potential balance function and its implementation [J]. Proceedings of the CSEE, 2006, 26 (12): 95-100.