Integration of RES in AC grids

The power quality problems are generated by the deviation from the standard values of the amplitude and frequency of the supplied voltages and currents. The causes of these variations are different and can be generated by the utility or by the end users. In the present paper we deal with the generation current and voltage harmonics by the nonlinear loads and their cancellation techniques. There are several methods for harmonic cancellation, the last and most efficient one being by direct injection of currents and/or voltages by the means of power electronic converters operating as current or voltage sources. A number of advanced solutions are presented in the present paper capable to minimize the overall harmonic distortion level. A special attention was given to weak systems harmonic cancellation for which the harmonic power flow direction has to be taken into account, as well as the use of multifrequency references and the prediction techniques for nonsinusoidal load currents