This book introduces the research and application of active power filter in improving power quality. Based on the principle, basic structure, application status and challenges of active filter, the influence of circuit topology, system configuration and network connection type on the compensation function of active filter is studied, including the role of fuzzy logic technology in the combined control of active filter and thyristor switched filter. A high power single-phase active filter with modern power converter is designed for harmonic control of electric locomotive. This book also includes a study of a unified power quality regulator (UPQC) for a three-phase four-wire system with the integrated controller. The design, modeling and control of UPQC are discussed in detail. The instantaneous reactive power theory is reviewed and extended to more general situations. Matlab Power System Blockset is used for numerical simulation to verify the correctness of theoretical analysis.