A Fast Positive-Sequence Component Extraction Method With Multiple Disturbances in Unbalanced Conditions

Fast and accurate acquisition of current components is a key factor for an active power filter to realize transient control under unbalanced conditions. In this letter, a robust real-time algorithm, which rapidly separates the positive-sequence component (PSC) from multiple decaying dc (DDC) components, dc bias component, negative-sequence component, and harmonics, is proposed. To this end, first, the multiple DDC components are detected in the multiple disturbance and unbalanced grid context, by making use of the periodicity of remaining components. The dc bias can be obtained accordingly, followed by the detection of a compound signal encompassing the positive-sequence component, negative-sequence component, and harmonics. Specifically, the compound signal can be extracted precisely, based on the detected DDC and dc bias components, with one grid cycle response time, or approximately, within half grid cycle. A switching logic of the two approaches is designed to shorten the overall convergence time and improve steady-state accuracy. The PSC is then effectively extracted by constructing the virtual orthogonal signal of the compound signal and using dq-frame filtering. Compared with the existing transient control techniques, the proposed scheme guarantees one grid cycle response time and simultaneously suppresses multiple disturbances. Finally, experimental results verify the effectiveness of the proposed method.