Research on a Transformerless Hybrid Filter Capable of Direct Connection to Medium-Voltage Distribution Systems

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The aim of this research is to develop a hybrid active-passive filter capable of direct connection to medium-voltage utility and industrial power systems without transformers. This hybrid filter is intended for harmonic current compensation of a medium-voltage adjustable-speed motor drive for energy savings in pumps and blowers. Such a motor drive requires no regenerative braking, thus allowing the motor drive to use a three-diode rectifier as the front end. This research leads to the following findings:1. The investigators have done feasibility study and conceptual design of a 6.6-kV transformerless hybrid filter suitable for a motor drive with a three-phase diode rectifier. This hybrid filter is characterized by series connection of a three-phase passive filter tuned In the seventh-harmonic frequency with a three-phase small-rated active filter using a three-level diode-clamped PWM converter without transformer.2. The investigators have achieved theoretical analysis of harmonic curre … More nt mitigation by the hybrid filter, with focus on the required volt-ampere ratings of the passive filter and the active filter Their ratings have a tradeoff in terms of harmonic current mitigation. Computer simulation has been carried out, considering the effects of control delay and the so-called "dead time." The theoretical analysis has been compared with the computer simulation. As a result, the validity of the theoretical analysis has been confirmed, and a limitation of the analysis has been revealed.3. Taking into account the above-mentioned theoretical analysis and computer simulation, the investigators have deigned, constructed and tested a three-phase 400-V 15-kW downscaled system, where a three-level diode-clamped PWM converter has been used for the active filter. What is a major concern in introducing the three-level PWM converter into the hybrid filter is to keep the dc mean voltages of the two split dc capacitors balanced not only in steady states but also in transient states. The investigators have proposed a voltage-balancing control method for the hybrid filter The effectiveness and viability of the method has been verified by the experiments using the 400-V 15-kW system. The current THD (total harmonic distortion) in the supply side was less than 5%, and each harmonic current component was less than 3%, including up to 40-th harmonic components. Both the THD value and harmonic components perfectly meet the Japanese harmonic guidelines.4. Considerations on the 6.6-kV transformerless hybrid filter that will be put into practical use in the near future have been discussed on the basis of the results from the theoretical analysis, computer simulation and experiment. Less