Investigation On Current Detection And Control Technology Of Shunt Active Power Filter Used For Intensively Nonlinear Loads

Wide application of high power electrical facilities with intensively nonliear characteristics leads to serious harmonics pollution in industrial fields, and thereby the power quality issues are attracting unprecedented attention in the world including China. The research on detection, compensation and control of harmonic and reactive power currents began in the 1980s and very important progress has been made in many related subdisciplines. The active power filter (APF), one of the research hotpots in power quality field as well as the most significant and effective technological methodology, is being widely investigated to compensate both the harmonics and the reactive power currents. Specifically regarding to the application environment with instantaneous and random changing of both the voltage and current under the condition of intensively nonlinear loads, the detecting method and control methodology of harmonic and reactive power currents are systematically studied in this thesis through theoretical analysis, simulation modeling and experimental verifications.The traditional detecting methods of harmonic and reactive power currents have actually shown limitations in concrete applications especially for the intensively nonlinear loads. Based on configuration of a virtual three-phase symmetrical system and comprehensive utilization of both the p-q and i_p-i_q methods, a new effective methodology is further developed in the thesis to accurately extract the instantaneous positive-sequence active currents of base-frequency for any three-phase systems. According to the proposed algorithm, any time instant can be chosen as the starting point for current compensation, regardless of the phase information of the three-phase voltages or currents. The algorithm is applicable to three-phase circuits in any operational conditions, and gives a feasible solution to the instantaneous reactive power and harmonics compensation regarding intensively nonlinear and/or quasi-stochastic loads especially with severely distorted supply voltages. Both computer simulation and experiment results demonstrate the accuracy and feasibility of the new algorithm.The above algorithm is extended for further applications, which can effectively realize extraction and decomposition of the active or reactive components of either positive-sequence or negative-sequence currents with respect to any order harmonics. The extended algorithm can also be applied in many other electrical engineering fields of monitoring technology. Additionally, refering to the scheme of orthogonal mathematical transform in three-phase currents detection, a novel method is put forward as to detect the harmonic and reactive power currents in single-phase power systems. The method is realized by searching the proportion rate between bus voltage and the active current of base-frequency to eventually obtain the active current signal, without recurring to the phase lock loop (PLL) element for easy implementation.The main issue with the shunt APF control system is to achieve real-time tracking control of the compensation currents accurately. With regard to the instantaneous changing characteristics of intensively nonlinear loads, slide mode control theory is introduced into the current tracking control scheme for shunt APF in the thesis. Based on adaptive parameter variations of the reaching law, a new adaptive discrete-time slide mode control strategy is proposed in this thesis, and the invariance property, stability and chattering phenomenon of the control system are analyzed and verified in details. Through optimization of the control parameters, the reaching rate to the switching surface can be adaptively adjusted as to achieve flexible real-time current control in the shunt APF. The control characteristics of the proposed strategy are compared with the traditional hysteresis control techniques, and simulation results are given accordingly.As for the current coupling issue of shunt APF in three-phase three-wire circuitry, the thesis indicates that the neutral point voltage will directly affect the tracking control behavior of output currents in the shunt APF when the three-phase voltages or loads are not symmetrically given. A discrete-time mathematical model in theα-βcoordinate system for three-phase three-wire shunt APF is derived through orthogonal transform. Further, the impact of the neutral point voltage on current control is eliminated and decoupling of the three-phase currents is finally achieved. Combined with the instantaneous harmonic and reactive power currents detection and the adaptive discrete-time slide mode control strategy, an integrated current control methodology is developed. Simulation results have verified the feasibility and effectiveness of the control strategy.Experimental platforms for the shunt APF are established by incorporating the methodologies previously proposed in the thesis. With regard to theoretical analysis as mentioned above, experimental studies have been implemented under various operational conditions specifically for intensively nonlinear loads with voltage distortion. The experiments positively demonstrate that, the designed prototype renders preferable operational performance, and can effectively achieve real-time tracking compensation of the harmonic and reactive currents. The results coincide well with the theoretical analysis and have also verified the validity of the research conclusions.The investigation being carried out in this thesis has further developed the theoretical basis and control methodolgy for the shunt APF, which is significant and useful for prototype design and engineering applications.