Charging station as an active filter for harmonics compensation of smart grid

The change from central generation to distributed generation lead to an increased use of frequency converters in the low-voltage distribution system. Converters of photovoltaic plants or a charging station for electric vehicles mostly have some remaining power that should be used to improve the grid voltage quality by harmonics or reactive power compensation. This paper presents the functional extension of a charging station used as a shunt active filter. The inverter of the charging station operates as variable impedance for each individual harmonic frequency and can therefore reduce the harmonic voltages of the low-voltage grid. The control of the inverter is based on the sliding discrete Fourier transform to synchronize the compensation current with the phase of harmonic voltage at the charging station connection point. With dynamic impedance smaller than the low-voltage grid, the inverter collects harmonic currents from local non-linear loads and reduces harmonic voltages of the grid. The charging station can dynamically adjust each harmonic admittance complying with the harmonic voltage and the grid impedance at the charging station connection point. The operation principle is explained, and the effectiveness of the harmonics compensation has been verified by computer simulation and experimentally on a prototype.