Study On The Comprehensive Power Quality Conditioner

Active power filter technology is the developing direction of modern power quality control technology, and have broad application prospects for harmonic pollution control and reactive power compensation. But Active Power filter (APF) has not been broadly applied in the domestic market yet due to its higher prices, large capacity and lower operating efficiency. Therefore, the study of new active power filters with high cost performance and low loss has a high theoretical and practical significance.This paper presents a new topology of three-phase Comprehensive Power Quality Conditioner (CPQC) for compensating harmonics and reactive power simultaneously in distribution systems. The proposed scheme decouples reactive power and harmonics for compensating both of them by employing cascaded STATCOM for reactive power compensation and APF for harmonics compensation.In the scheme, the STATCOM module is composed of a cascaded-multilevel converter and a tie reactance, and the harmonics APF module is connected across the tie reactance through a harmonics tie reactance. The APF module operates with low power rating, while STATCOM module withstands most of the stresses. The advantages of the scheme are as follows:(1) Because the fundamental frequency voltage of the tie reactance is smaller compared with PCC voltage, the APF converter's DC voltage can be reduced, meanwhile no fundamental frequency current flows through APF. So the scheme reduces the rating of APF from both current rating and voltage rating. (2) STATCOM operating frequency of the switching device can be greatly reduced, so the switching loss can be reduced. Combined with the advantages of APF and STATCOM, the CPQC has strong practical advantages.The paper details the CPQC's Structure and working principle, and decoupled control strategy of harmonics and reactive power. Using average switching-model and small-signal model, the accurate transfer functions of APF and STATCOM are derived. The paper is devoted to the control strategies of APF module and STATCOM module. DC voltage control is the key to stable operation of APF, the proposed strategy is drawing energy from the voltage of STATCOM tie reactance for APF's DC capacitor. The STATCOM current control strategy is feedforward decoupling control based on synchronous rotating frame. It proposes a control method that devotes itself mainly to meeting a demand of reactive power, as well as to hierarchical voltage-balancing of the multiple isolated and split dc capacitors. The control method is characterized by fast and stable voltage-balancing performance and easy expansion into a higher number of levels. Based on the small-signal model, the control system transfer functions are deduced, preparing for design of control system. The PSCAD/EMTDC simulation results confirm the structure of CPQC and effectiveness of proposed control strategies.A general method for the design of the main circuit parameters is presented.