There are two types of existing SPWM multilevel inverters: one is a clamped multilevel inverter (including diode clamping, capacitor clamping and hybrid clamping multilevel inverters), which are typically The control method is the carrier triangular wave stacked SPWM control method; the other is the H-bridge cascaded multi-level inverter, and its typical control method is the carrier triangular wave phase-shifted SPWM control method. The improvement effect of the two multilevel inverters on the output voltage waveform and the double Fourier series expression of the output voltage are the same. These two types of multi-level inverters have two common shortcomings: one is that there are many switching devices used, 2 (m-1) (m is the number of levels), the main circuit is complex, and the cost is high; Inversion, cascade superposition and SPWM control are all performed on the inverter and its inverter switches, so a dead zone must be set, and each inverter switch must be controlled by SPWM to make the total effective of the inverter. The number of switching is 2 (m-1) fa (fa is the switching frequency), the switching loss is large, and the inverter efficiency is low. The way to overcome these two shortcomings is to develop a new independent SPWM DC power cascade superposition multi-level inverter. The central idea of its research and development is to move the cascade superposition and SPWM control from the inverter to the DC power supply, and perform SPWM control and cascade superposition on the DC power supply to achieve the purpose of reducing the number of switching devices. That is to say, a control switch is added to each independent DC power supply of the cascaded superposition multi-level inverter, so as to perform SPWM control and cascaded superposition control of the DC power supply, so that the cascaded superposition and SPWM control can be combined with This is accomplished by splitting the inverter and moving to the DC power source. The independent SPWM DC power cascade superposition multi-level inverter made in this way does not need to set the dead zone, and its number of switches is 1/2 (m-1) + 4 (m is the number of levels. ). When the number of levels m>3, the number of switches of the DC power cascaded multilevel inverter 1/2(m-1)+4 will be much smaller than that of the H bridge cascaded multilevel inverter. The number of switches of the inverter is 2 (m-1), which can greatly reduce the number of switches, and at the same time, the total effective switching times of the inverter can be reduced from 2 (m-1) fs to 1/2 ( m-1) fs, which can reduce the switching loss by 75%, greatly improve the inverter efficiency, and greatly reduce the cost.
Moving the cascaded superposition and SPWM control to an independent DC power supply is actually a technical reform and circuit simplification of the cascaded superposition multilevel inverter. This reform and simplification brings many advantages to cascaded multilevel inverters, as shown below.
① The switching devices used in the cascaded multi-level inverter are greatly reduced. For example, for an 11-level cascaded multi-level inverter, five 2H bridges are used for each phase to be directly cascaded in series, and a total of 60 IGBT switches are required. When the independent SPWM DC power supply cascade superimposed circuit is used, it is enough to use 15 IGBTs and 12 GTOs, a total of 27 switching tubes, so that the total number of switching devices can be reduced by 55%, and cheap GTO switches can also be used. devices, greatly reducing the cost.
②The number of equivalent switches is the least, which is only about 1/4 of the number of switches of the 2H bridge cascaded and superimposed multi-level inverter, which also greatly reduces the switching loss and greatly improves the inverter efficiency.
③ The inverter switching device can automatically work in the ZVS state, so inexpensive low-frequency switching devices such as GTO or SCR can be used.
④ The control method is simple, and all levels can be controlled by SPWM.
⑤ The use of SPWM control on the DC power supply can eliminate some fundamental harmonics; the cascade superposition can eliminate some carrier waves and carrier harmonics, and the effect of improving the output voltage waveform is good. This technique can be applied to multi-level inverters with output voltage levels greater than 7, so that the output voltage waveform is as close to a sine wave as possible. When the number of cascading stacks is N=5, it can achieve a perfect degree of no harmonics, and the AC output filter can be reduced or eliminated, thereby improving the dynamic response speed of the inverter.
⑥ Low cost, high efficiency, small size and low quality.
⑦It is most suitable for multi-level inverters with level m=7~15.
⑧ Adding the 3rd harmonic or zero-sequence harmonic to the sine modulation wave can realize the optimized SPWM control of the switching frequency.
To sum up, the cascade-stacked multilevel inverter of the independent SPWM DC power supply is a kind of excellent multilevel inverter with great prospects in the UPS power supply.