A Study of a Three-Phase Four-Wire Shunt Active Power Filter for Harmonics Mitigation

The electrical power quality in three phase four wire systems, feeding nonlinear loads, can be improved by using active power filters (APF) in order to mitigate the total harmonic distortion. Harmonic currents and voltages produced by single phase, non-linear loads, which are connected phase to neutral in a three phase, four wire system, are third order, zero sequence harmonics. These third order, zero sequence harmonic currents, unlike positive and negative sequence harmonic currents, do not cancel but add up arithmetically at the neutral bus, and can be greater than the current of each of the phases. The danger that arises is an excessive overheating of the neutral, since there is no circuit breaker in this conductor, which limits the current, as occurs with the phase conductors. An excessive current in the neutral can also cause an excessive voltage difference between the neutral conductor and ground. This paper presents the theory and design of the control system used to mitigate harmonic problems in three-phase four-wire systems based on a split capacitor topology. Due to split capacitor topology has midpoint between the capacitors, this point can be used for the neutral return path, and thus, the entire neutral current flows through the DC bus capacitors reducing the number of switching devices. Simulation results are shown to validate the proposed procedures.