Development Of Module Cascaded Power Electronic Transformer In Distribution Network

With rapid economic growth, power grid had grown, and people demand for electric power is rising. Power transformers had been used as an extremely significant component in electric power system, and its plays an important role in energy transmission, power conversion and electrical isolation. Since the past 100 years, transformer’s disadvantages such as volume large, weight heavy, etc have not been changed. Meanwhile, the electric power system has changed deeply: the specialty of modern power network for enlargement, complication and diversification are posing greater challenge for power transformer. The increase of nonlinear load has deteriorated the power quality, and the customers required the high quality power energy simultaneously. So, how to development the power quality becomes an increasingly important issue.The burgeoning development of power electronic and control technology created a power electronic transformer(PET) with multiple functions. Except for primary functions, PET has some advantages to attract scholars such as small bulk, lower weight, controllable power quality,fault self-isolation and AC/DC coexist, etc. This paper proposed a single-phase input/3-phase output module cascaded PET, applied in distribution network, to solve the users’ influence which caused by voltage quality problems of distribution network.For a variety of quality problems appeared in the grid voltage, this study analysis was separated into two parts. The first analysis presents a control strategy without PLL which used in cascaded rectifier of high voltage side, because frequency, phase and amplitude mutations of grid voltage reduced the precision of phase locked loop(PLL). By establishing the relationship between instantaneous power, voltage and current of single-phase system in two phase static coordinate, it obtained the no PLL current control signals and voltage sharing signals of PWM rectifier to realize the control without PLL. Both the regulator and the modulation method had been proceeded detailed analysis and comparison in the meantime. Then, for the most harmful power quality problems in grid—the voltage drop depth and the short-term power outages, a scheme has been proposed, joining the DC bus interface of PET to the super capacitor energy storage system, and enabling PET the ability of crossing the short-term low voltage. The paper did a mathematical modeling analysis for PET with energy storage system, and the operation mode, control strategy of the super capacitor energy storage system had been analyzed.In order to verify the feasibility of principle and control strategy of module cascaded PET in this article, it had built a prototype platform in the laboratory, and showed the hardware design of the main circuit in detail. Then, the dual control system had been simply introduced, and finally, the primary functions of PET had been illustrated by analyzing experimental waveforms.