Single-phase transformerless power conditioner based on a two-leg of a nine-switch converter

摘要

An interesting solution for problems associated with system voltage, such as sags and swells, as well as those associated with current such as low power factor and harmonic distortion is the unified power quality conditioner. However, one of its drawbacks is the high number of semiconductor devices since its conventional topology presents two full-bridge converters in a back-to-back configuration. Although the six-switch two-leg converter has a feature of presenting two sets of single-phase output terminals, the same as two full-bridge converters, but with a reduced number of switches (six, instead of eight), it has never been used in single-phase power conditioning systems. Therefore, the aim of this paper is to propose a single-phase transformerless power conditioner based on the six-switch two-leg converter. In this solution, there are two sets of single-phase outputs, each set behaving as an independent converter. The top unit of the converter is connected in parallel to the load by means of a series LC filter tuned to a specific frequency, working as a hybrid power filter and being responsible for the dc-link voltage regulation and the compensation of current harmonics generated by the non-linear load. The bottom unit works as a dynamic voltage restorer and it is responsible for compensating harmonics, sags or swells in the grid voltage, in such way that the load receives a sinusoidal voltage waveform. The challenges of the single-phase transformerless power conditioner that is proposed in this paper are different of the three-phase systems based on nine-switch converter previously proposed in literature. Dealing with the circulating current makes mathematical model and the modulation strategy completely different and, therefore, all equations have been developed for this specific single-phase power conditioner application. The proposed power conditioner presents four main advantages when compared with other configurations available in literature: (a) it is a transformerless solution; (b) it has a reduced number of switches; (c) it has a reduced dc-link voltage and rated power because of its hybrid power filter characteristics; (d) it does not have to handle with dc-link voltage unbalance. Experimental results demonstrate the validity and the effectiveness of the proposed topology.