Control algorithms for a transformerless hybrid active filter without current sensors

This paper presents novel control algorithms for a transformerless hybrid active filter without current sensors. The hybrid active filter is composed of a shunt active filter in series with a passive filter tuned nearby 5^th harmonic frequency. This structure allows reducing the DC-Link voltage, which is very important when the connection of hybrid active filters to power grids at medium voltage level is desirable. The control algorithms employed to determine the 5^th harmonic current are derived from the instantaneous reactive power theory, pq-theory, together with a PLL circuit. Previous works introduced control algorithms for providing selective harmonic filtering with the active filter acting as a current-controlled voltage source. Dissimilarly, this work conceives the active filter operating as a voltage-controlled voltage source, besides providing selective harmonic filtering. Based on the proposed algorithms, the number of inputs is reduced and the algorithms to determine the 5^th harmonic current becomes more simple. In a comparative analysis, simulation results considering the active filter as a current-controlled voltage source and voltage-controlled voltage source, in transient and steady-state conditions, are presented.