This paper first summarizes the working principle, advantages and disadvantages of cascaded H-bridge, NPC and FC multilevel inverters, and simulates the switching frequency, voltage stress and output harmonic content of the device with seven levels as an example. . Then the working principle and control strategy of compact multilevel inverter are studied. The main research contents include main circuit topology, operation principle, control strategy and modulation strategy, and the relationship between the number and level of devices in various topologies. Inductive summarization, the formation of mathematical expressions. A loss model of a conventional two-level, diode-clamped three-level, compact seven-level inverter was established, and the loss values ​​at the switching frequencies of 10 kHz, 20 kHz, and 50 kHz were calculated. The simulation model of single-phase compact seven-level inverter in island operation is established by the control method of voltage outer loop current inner loop. Finally, the single-phase compact seven-level inverter is built in grid-connected operation. Simulation model.
(1) The traditional two-level inverter has large electromagnetic interference, large du/dt, large equipment, high switching frequency, low inverter efficiency, and is not suitable for high-voltage systems. Multi-level inverters can be effective. solve these problems.
(2) The control of the H-bridge multi-level inverter is relatively simple, but the number of independent DC voltage sources required is large; the control of the NPC multi-level inverter is simple, and no additional independent DC power supply is needed, but more The number of clamp diodes; FC multi-level inverters have many combinations of switching states, are more flexible, and the control is more complicated. It is necessary to consider the problem of unbalanced flying capacitor voltage.
(3) Since the number of devices of the compact seven-level inverter is larger than two-level and three-level, the total conduction loss of the power device is large. When the switching frequency is low, the conduction loss is the main loss, so the loss of the compact seven-level inverter is large, and the advantage is not obvious. However, as the switching frequency increases, the switching loss becomes the main loss. At this time, the compact seven-level inverter has the smallest loss value and the advantage is obvious.
(4) Compared with the traditional multi-level inverter, the compact multi-level inverter uses the fewest components, the unique switch state, simple control, and small device size.
(5) The control strategy of the voltage outer loop current inner loop is adopted for the single-phase compact seven-level inverter island operation, so that it can output the voltage and frequency according to the given voltage and frequency reference value; The seven-level inverter is connected to the grid for simulation, which proves its feasibility and reliability when it is connected to the grid.
This paper has achieved certain results by learning the existing seven-level inverter and the new compact seven-level inverter. However, due to the limited time and author level, there is still much work to be further studied. The specific instructions are as follows:
(1) In the learning and simulation of traditional multilevel inverters, only a part of the topology is simulated by current single-loop closed-loop. Therefore, only the parameters such as the number of components, device withstand voltage, and device switching frequency can be learned and compared. At the same time, since the flying capacitor inverter can not be controlled by the traditional carrier stacking method, the improved carrier stacking method or the learning space voltage vector control method can be further studied for controlling the flying capacitor inverter.
(2) The more the inverter level is, the smaller the output harmonic content is. Therefore, the smaller the filter inductance required, the smaller the loss caused by the filter inductor. Therefore, in the loss model, the loss of the filter inductor can also be established. This can further highlight the efficiency advantages of multi-level inverters. At the same time, when performing loss comparison, it is also possible to establish a loss model of other seven-level inverters and compare them with a compact seven-level inverter.
(3) In the current simulation, only the model when the DC source is the ideal voltage source and the rectifier bridge is established, and the model of the compact seven-level inverter connected to the PV system should be continued to be applied to In the photovoltaic grid-connected system.
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