Detuned filters for controlled HVDC power supplies

Controlled high voltage DC power supplies generate substantial line current harmonics as well as deteriorate Input Power Factor (IPF) during their normal course of operation. Detuned filters are incorporated to take care of these input parameters, avoiding the possibility of parallel resonances with the source. Series limiting inductors are intentionally kept in the primary side of each power supply to minimise higher order harmonics as well as to remove completely uncharacteristic harmonics generated by them, thereby improving the distortion component of power factor and leaving the scope for improving the total IPF up to near unity. The more the value of these series limiting inductors, the higher the attenuation of line current harmonics reducing higher order harmonics appreciably, which helps keeping the voltage Total Harmonics Distortion (THD) within the acceptable limit. Detail simulations have been carried out at various operating points to estimate input line harmonics and IPFs at full load, for all possible input variations and crucial voltage and current waveforms are presented in this paper. The component values of these detuned filters as well as series limiting inductors are suitably chosen and the input voltage THD below 4 % and IPF better than 0.97 are achieved, for all possible variations of input supply voltage, there by achieving the performance of detuned filter comparable to equivalent active filter. This arrangement is effective in improving the IPF of controlled converters supplying to fixed load, but fed by widely varying input voltages, because with change in the input voltages, the kVAR supplied by the filter bunch changes and closely matches to the kVAR requirements. Several experiments were carried out under actual operating conditions of these power supplies and close match between simulated and experimental results were also established.