| Multi-level technology has betterly solved the contradiction between the switching frequency and the capacity of the power devices. By increasing the number of the output voltage levels, the du/dt of the power devices and the total harmonic distortion(THD) and the switching losses can be reduced, at the same time, the equivalent switching frequency of the power device &converter is increased. The high-voltage&high-power output voltage can be gained by using the low-frequency&low-voltage&low-power devices which are inexpensive. At present, multi-level technology has been widely used in the high-voltage direct current (HVDC) transmission system, flexible AC transmission & distribution system(FACTS), high-voltage&high-power AC motor speed control system with frequency conversion , the grid-connection of new energy power generation and so on.Dynamic voltage restorer(DVR) is the recent emergence and another important member of the FACTS. It is in series with the system voltage and the sensitive load. By this way, the load voltage sag which caused by the bus voltage disturbance can be compensated. So, DVR is one of the most economical and effective device to solve the power quality problem which caused by voltage sags. The capcity of DVR can be increased and the THD can be improved significantly by combining the multilevel technology and DVR.In this paper, multi-level technology has been deeply researched,the new multi-level DVR is designed and the simulation analysis has been done. At last, a diode clamped three level experimental prototype has been realized. the main research results are as follows:For the characteristics of the current multi-level inverter, the carrier stacked modulation, the carrier phase-shifted modulation and a new hybrid cascade multi-level modulation strategy have been analysed and simulated. A kind of software for the carrier phase-shifted modulation which based on the asymmetry rule sampling algorithmic is designed by using the TMS320F2812DSP. The experimental results shows that the algorithm is fast and approaching sinusoidal better. It has lay the foundation for the rapid development of the multi-level technology.The multi-level inverter spectrums analysis have been done for different number of modulos under different frequency switch. The simulation shows that, cascade multi-level inverter's equivalent frequency is increased. So the output voltage harmonic content can been reduced by raising the number of modules. as a basis of the analysis , a kind of filter has been designed which considered the frequency, power factor and filter parameters choice . the simulation have been done to prove that it is reasonable.This paper designed he 6kV/5MVA cascade eleven-level transformerless DVR by integrating the cascade multi-level and the carrier phase-shifted modulation technologies. Main circuit topology and the close-loop controller which involved the single loop and double loop feedback have been designed in detail. The double loop feedback employ two feedback loops with a slow outer voltalge loop and a fast inner current loop. Voltage sag at a random time simulations has been done when multi-level DVR under the control of any control strategy. It is show that the system damping ratio and stability and error can be regulated by adjusting relevant parameters.On this basis of the theoretic analysis and simulation study, a new three level 800V/±150kVA experimental prototype has been realized. in the paper, three level invter principle and parameters choice has been introduced in detail, the carrier stacked modulation has been improved for the software design, the driver has been designed by using 2SD315A,and the experimental prototype has been installed and debugged. After that, the dynamic&simulation experiment platform has been designed and the no-load test experiments, load test experiments, over-voltage of IGBT experiments and grid-connected experiments have been completed. The experimental results proves that the device have a good performance and can be used in the main circuit topology of 380V-1000V system in FACTS.
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