Research On APF Based On Cascaded H-bridge Converter

The active power filter (APF) is a kind of power electronic equipment that canrestrain harmonics and compensate reactive power dynamically. The applications oftwo-level topology are limited in some situations because of its low voltage operationand low volumes. Cascade topology can output higher voltage and the same level witha minimum number of components. That widens the application range of APF to highpower occasion. In this paper, in-depth research is conducted on some of the keytechnologies of single H-bridge cascaded APF.The harmonic current detection is a key technology of active power filter,because the quickness and accuracy of detection directly affect compensationperformance of the whole system. The article adopts an improved harmonic detectionmethod of sliding window iterative DFT. It combines discrete Fourier and slidingwindow iterative principle. In order to reduce calculation and be easy for digitalrealization, the method use fixed iteration factor and reconstruction of iterativeequations instead of PLL and rotation factors.The traditional space vector modulation can’t be used in single heavy cascaded Hbridge topology directly. The paper shows the equivalence relation of single heavycascaded H-bridge and combined converter, and then analyzes the principle andrealization method of modulation. It combines the advantages of equivalent higherswitch frequency under the lower switch frequency condition and high utilization rateof dc side voltage with SVPWM.In this paper, instead of the single-inductor filter, LCL-filter is adopted to act asoutput filter connecting with the grid. Passive damping method is taken to suppressthe resonance of three-order LCL system. It has such good characteristics ofamplification at low frequencies and attenuation at high frequencies. Thoughanalyzing the relationship and restriction of the LCL filter element, the detailedparameter design method is given. And then, we verify that it is suitable for singleH-bridge topology by an example.Finally, with the emulated platform of MATLAB, the designed LCL filter,harmonic detection, and sampling time staggered space vector modulation are testedthrough simulation. And parts of the theory are verified by experiments.