Key Techniques Of Active Power Filters Based On High Precision Control

With the wide application of power electronic equipment in industry and daily life, grid harmonic pollution becomes increasingly serious. Active power filters, as power electronic equipment, can effectively solve harmonic problem. And, nowadays, it moves toward large-scale industrial application, based on the rapid development of the semiconductor and digital control technology. This paper focuses on the related techniques of active power filters, mainly including the control strategy of a three-phase four-wire shunt active filter with the dc-link composed by two capacitors connected in split, the high performance controller of the hybrid active power filter which is suitable for large capacity and high voltage application, and the high accuracy sample method in the digital control system of the active power filters.Firstly, in order to compensate any unbalanced harmonic load with a three-phase four-wire shunt active filter, this paper studies on the control strategy under the ABC coordinate frame, making each phase of the APF work independently. Based on the accurate model, the controller designs of current inner loop, maintaining voltage loop and equalizing voltage loop of DC-bus are given in details. Repetitive controller is introduced to the current loop control. It effectively improves the compensation accuracy. Simulation and laboratory results show the validity and feasibility of all the designs.Secondly, to find a high performance controller for a shunt hybrid active power filter which is suitable for large capacity and high voltage application, Repetitive controller is firstly adopted into the control of output voltage in the hybrid active power filter. It can largely improve the compensation accuracy.And design method of this controller and key parameters are studied. So as to solve the reactive over-compensation when merely using the shunt hybrid active power filter, a novel static var compensator combining hybrid active power filter and thyristor controlled reactor is presented. Simulation result in Matlab/simulink shows the feasibility of the related topology and control strategy.Finally, the high accuracy sample method in the digital control system of the active power filter devices is studied. As high power electronics equipment usually has low switching frequency, and the signal to noise ratio is indispensable in the THD calculation. Then a novel over-sampling method is presented. The theory is analyzed and some issues about the application of the new over-sampling method, taking the measurement of THD for example, are discussed. Through simulation and experiment, the data sampled by the conventional method and the new method is compared to demonstrate the advantage of the proposed method and its validity.