Research On Distortion Detection And Compensation Control Strategy For Unified Power Quality Conditioner

With the continuous development of power industry, grid structure and load composition become increasingly complex, Voltage and current problems often come out in pairs in the power distribution system. If it is installed corresponding governance device for every kind of different power quality problems, treatment costs will increase significantly and the operation and maintenance is also very complicated. Therefore, the device is necessary to solve many power quality problems simultaneously for the needs of modern power system. As a power quality controlling device with comprehensive ability to compensate, Unified Power Quality Conditioner (UPQC) not only can improve the quality of power supply side, but also prevent the current harmonic from polluting the load side. Therefore, it is considered to have extensive prospect nowadays when power quality problems are more seriously concerned.Basic concepts of power quality and power quality problems in the power system are introduced firstly in this paper. The research necessity and research status of UPQC are recounted afterward. On this basis, the topology, working principle and mathematical modeling of UPQC is explained. Finally, the detection method and the control strategy, which are the important parts affecting the compensation effect of the UPQC, are discussed and investigated in detail.In the distortion detection part, the method of fourier analysis and wavelet transform in the frequency-domain are described. Then, the detection algorithm of p-q and ip-iq based on instantaneous reactive power theory is discussed,and combine with the compensation characteristics of UPQC,this thesis deduces the detecting method of voltage and current based on Park transformation in theory, which is based on instantaneous reactive power theory. It solves the problem that the compensation is not detected accurately under the circumstance of voltage distortion.Through the simulation, the validity of this detecting is verified.In the control part, this paper fistly analysed the space vector control strategy and hysteresis tracking control strategy, but unstable switching frequency causes great switching loss in the hysteresis tracking control strategy and there is interphase interference in three-phase three-wire system. Therefore, a space vector based hysteresis control strategy with fixed switching frequency for UPQC is researched in this thesis. In this control strategy, it judged the region of command voltage vector by using the sign of phase-to-phase current error vector. Compared with the traditional algorithm, detection of three-phase grid voltage is not needed. This control strategy can solve the problem that switching frequency is unstable and overcome the problem of interphase interference in three-phase three-wire system. In addition, This control strategy significantly simplifies hardware circuit and its related algorithm design. Through the MATLAB/Simulink simulation, proved that this kind of control strategy for UPQC researched in this paper is feasible and effective.Finally, this thesis has carried on the experimental verification for UPQC. The experiment platform is builded and the main circuit is designed.The program flow chart of signal detection and control algorithm is explained.