Research On Application Of Mathematical Morphology In Harmonic Detection

Harmonic detection is one of the important research topics in the study ofpower quality problems. Beyond numerous harmonic detection methods, theharmonic current detection method based on instantaneous reactive power theory iswidely used in harmonic detection and APF because of its definite physical meaningand simple detecting circuit. In order to promote the performance of the harmonicdetection method based on instantaneous reactive power theory, this paper mainlydoes the following work:First of all, the harmonic current detection method based on instantaneousreactive power theory is introduced and analysed, including the total harmonicdetection of three-phase circuit, total harmonic detection of single-phase circuit,and specific harmonics detection(including three-phase circuit, single-phase circuit),and then the influence of the low pass filter in the detecting circuit on theirdetecting effect is analysed.Secondly, the mathematical morphology theory, especially the mathematicalmorphology filter is introduced. And in view of the structure elements selectionissue, the relationship between the filtering result and the signal sampling points percycle is analyzed, and the method of the single frequency signal incentive issimplified to get the frequency response characteristics of the morphology filter.Then, based on the basic operations of corrosion and expansion, a mathematicalexpression of structure elements’ length and the amplitude of the output signal isderived, and a quantitative relationship between the frequency responsecharacteristic and the length of the structure elements is given. This conclusionpresents useful references for the selection of the structure elements in power signalprocessing. Through the simulation results, the effectiveness of the conclusion isverified.Thirdly, the influence of the length of the structure elements for the results ofthe morphological filter is analyzed and the length of structure elements is adjustedby using the method of adding proportion coefficient of filter. And the basicoperations--corrosion and expansion are improved so as to construct the improvedmorphological filter, and the optimal proportional coefficient value and the shortestof structure elements are obtained. The improved morphological filter that usesshorter structure elements without affecting the morphology filter precision isproposed, while weakening the adverse effects caused by long structure elementsand improving the operation speed and the dynamic response performance.Finally, the improved mathematical morphology filter alternative to traditional low-pass filter is used in the harmonic detection method based on instantaneousreactive power theory compared with the detection methods using conventionallow-pass filter, simulation results show the better characteristic of the improvedmorphology filter and the effectiveness of the detection method using improvedmorphology filter.