A control structure for fast harmonics compensation in active filters

Shunt active filters are a means to improve power quality in distribution networks. Typically, they are connected in parallel to disturbing loads in order to reduce the injection of non sinusoidal load currents into the utility grid. The high power active filter investigated in this paper is based on a pulse-width modulation (PWM) controlled voltage source inverter. Its inner current control is realized with a dead-beat controller that allows fast tracking of stochastically fluctuating load currents. For the mitigation of stationary load current harmonics, an outer control loop is required that compensates for the persistant phase error caused by the delay of the inner loop. The outer loop developed in this paper is based on integrating oscillators tuned to the major load current harmonics. Mathematically, they are equivalent to I-controllers in rotating reference frames. Some of these oscillators are located within a closed control loop. For frequencies where the feedback would excite grid resonances they are placed in a prefilter with phase shifting elements. Since all oscillators share a common feedback full selectivity of the harmonic analysis is achieved. For every harmonic the degree of compensation can be adjusted individually. In addition to the oscillators, a direct path is provided that feeds forward the load current to the inner control loop. Thus, load current transients can be tracked with the full speed of the dead-beat controller. The direct path does not affect the harmonic analysis performed by the oscillators.