Introduction. To improve the power quality of a supply system, the total harmonic distortion (THD) is the most important parameter in the quantification of harmonics caused by nonlinear loads. In practice, it must be less than 5 %. The novelty of the proposed work consists in the use of a cascaded five level active filter, when the converter consisting of six H-bridge pairs, each one includes four transistors. Purpose. To increase the efficiency of this filter, two techniques for quantification of harmonic currents are proposed, first the PQ-theory which is simple but can only be used in case of a balanced grid, and second the synchronous reference frame theory (SFR-theory), which is capable of creating harmonic current not only in an unbalanced grid, but also in an unbalanced and distorted beam. Methods. Using the control techniques, the harmonic current is extracted from load current and considered as a reference. The constructed current should follow this reference. Results. The estimation of the active and reactive powers is based on the measurement of the currents crossing the load and the network voltages, these powers are used to determine the shape of the harmonic (reference) current. Using the PI regulator, the output current of the five-level inverter follows the reference current perfectly. The inverters output current is injected into the grid to eliminate harmonic currents. Practical value. In practice, the harmonic distortion rate THD is the most widely used criterion for criticizing the waveform of the currents and judging the quality of the energy involved. For currents on the source side, the THD is considered acceptable if it is less than 5 %, in our proposal the THD is 0.85 % with the PQ-theory and 2.34 % with SFR-theory, so it is optimal.