Contents I. Introduction
The paper presents control of four-leg three-level flying capacitor converter with active filtering function (AFF) used as an interface between renewable energy source (RES) (e.g. small wind turbine or photovoltaic panel) with four wire electrical grid. The most common solution is to use the three-phase three-leg converter connected to the four-wire grid by dy transformer [1], (Fig. 1), what gives possibility to fed asymmetrical load in stand-alone operation mode. Unfortunately, the mentioned dy transformer is quite large and heavy, increases costs significantly and causes additional losses. This transformer can be eliminated by application of four-leg converter (Fig. 2) [2], [3]. Thus, it gives possibility to fed asymmetrical load in stand-alone mode of operation with precise control of neutral current, as well as it can be treated as three independent single-phase converters. Additional advantages can be achieved when three-level topology is used (e.g. lower voltage on single switch, higher voltage on converter, lower total harmonic distortion, reduction of passive components) [4]. The flying capacitor (FCC) topology is most suitable in four-leg applications, where three-phase converter should be treated as three independent single-phase converters. RES connected to the grid by three-phase three-leg converter with dy transformer RES connected to the grid by three-phase four-leg converter
