Real time optimisation of an active filter's performance and applications to the power system

Abstract

Power system loads have traditionally drawn a sinusoidal current. Due to recent advances in power electronics, many loads now draw a nonsinusoidal or distorted current from the supply. Regulations limit the level of distortion larger loads may draw, but small loads are excluded. However, the smaller loads collectively create a distortion problem which needs to be compensated for. To reduce the distortion drawn from the power system, a shunt active filter has been developed. Computer simulations which have been performed, demonstrate that the active filter can compensate for distortive load currents, resulting in near sinusoidal supply currents. Single and three phase analogue controlled active filters have been constructed to compensate for harmonic and phase displaced current distortion. The three phase active filter can also balance the supply currents for unbalanced harmonic loads. For different load types, the power amplifier switching frequency and DC bus voltage affect the active filter's efficiency and ability to reduce the supply current distortion. A digital controller has been developed which allows the active filter to operate at an optimum level of supply current and efficiency, independent of the load type. A financially based savings calculation has been developed to determine the relationship between distortion reduction and operational efficiency in order to perform this optimisation. The optimum operating point was determined using a simplex optimisation algorithm which climbs the calculated savings surface to achieve maximum savings. The ability of the optimisation algorithm to find the point of maximum savings and to adapt to load changes has been demonstrated. The optimisation algorithm has been extended to include phase displacement compensation in the calculation of the maximum savings point. A novel application of the active filter, where passive filters can not be used, is presented. This involves providing compensation for nonlinear loads operating from a weak, variable frequency AC system. The ability of the active filter to compensate for both the generator's current and voltage distortion is demonstrated. An initial investigation of a resonant link active filter to reduce the switching losses is also presented.