Study Of Modularized Active Power Filter

With the development of power electronics, switching power devices such as thyristors become widely used in high-power industrial occasions' application. These non-linear loads have become the power system's main harmonic sources and seriously reduce the quality of power grid. Because of its good characteristics of the dynamic response and compensation, in the long run, high-capacity active power filter will become the main equipment for harmonic suppression and reactive power compensation in the realization of truly "green power grid".First of all, by the comparsion and discussion of the harmonic current detection method based on instantaneous reactive power theory and the harmonic current detection method based on synchronous rotating coordinate transformation, the paper analyzes their basic detection principles, characteristics and application of occasions. Through the oretical analysis, the harmonic current detection method which is based on synchronous rotating coordinate transformation has good general-purpose and high accuracy. It can also calculate the compensation current's command value under the asymmetric and distortional power supply voltage condition accurately.Next, the paper discusses the main circuit parameters' design of active power filter and designs the main circuit parameters for the APF whose capacity is 500kVA. Because current control method is directly related to the effect of active power filter compensation, a detailed analysis and comparasion of the basic principles are studied for instantaneous comparison method, triangular-carrier linear control, dual-loop voltage space vectors control method. By establishing the simulation platforms of three control methods, the paper shows that because of its high DC voltage utilization which improves the tracking performance of compensation current, the dual-loop voltage space vectors control method can get a better effect of compensation than the instantaneous value current control method and linear triangular carrier control under the fixed switching frequency.Finally, aiming at the contradictions which are between the switching device's capacity and the switching frequency in large-capacity active power filter, two aspects which are both the three-phase main circuit structure of a single module's improvment and how to combine multi-module are studied in the paper. In the improved structure of three-phase main circuit structure, not only the capacity of switching device can be reduced, but also unipolar multiplication frequency pulse width modulation can be selected in the pulse width modulation mode which can double the equivalent switching frequency and get the better effect of the compensation. Each active power filter of structure of multiple modules is not only a relative independence, but also a unified coordination which can be done in the control method to improve the device's equivalent switching frequency and obtain a better compensation characteristics.