Reduction of harmonic currents generated by discharge lamp systems

The paper presents a novel scheme for minimising harmonic currents generated by three-phase systems of discharge lamps. The main idea is to install, in the neutral conductor, a filter with characteristics which allows the first harmonic component to flow with minimum attenuation while higher harmonic components of the neutral current are attenuated. For filter design, a three-phase system of fluorescent lamps is substituted by a Thevenin equivalent seen from the common point of a lamp connection and the ground point of the supply system. It leads to nonlinear Thevenin equivalents when an internal impedance and an equivalent voltage are dependent on the current flowing in the neutral conductor. The equivalent circuits developed are subsequently used for filter design and simulation. Rosenbrock´s optimisation procedure is implemented to compute filter parameters. The assumed objective function aims at the impedance for the first harmonic to asymmetrical current flow and large impedance for higher harmonics to reduce harmonic distortion in the supply and neutral currents. The equivalent models developed are applied for simulation of filter working conditions in the neutral conductors. Simulations are verified by laboratory tests, showing good agreement