Symmetrization of line currents in three-phase four-wire loads

Single-phase loads and non-linear three-phase loads can produce unbalanced line currents, generating a current in the neutral line as a consequence. In some cases, these neutral currents, i.e. their RMS value, can be even higher than the nominal line currents, although the neutral lines are not designed to withstand these rates. Related problems can be partially resolved by a three-phase parallel active power filter, that reduces harmonic distortion, but the harmonic distortion is just one of the reasons for unbalance. The remaining unbalances can be successfully eliminated only when a proper method for filter current reference determination is applied. The paper introduces some characteristic load conditions and proposes a suitable filter configuration. Simulation results of symmetrization are shown, based on a proven filter current reference determination method, where synchronization with the line voltage is emphasized.