Research On Extraction Method Of Main Natural Frequency

For EHV long-distance transmission line, the effect of its large distributed capacitance is becoming increasingly prominent. The natural frequency component and other transient components which appear when fault occurs on a long-distance transmission line will have a great impact on protections based on power frequency component. The study found that natural frequency component contains rich fault information, so, it is feasible to constitute rapid action relay protection principle by using natural frequency characteristics when fault occurs on a transmission line. To realize the relay protection principle based on natural frequency, the key is extracting the natural frequency component quickly and accurately.This paper firstly describes the concept of natural frequency, theoretically develops the general expression of main natural frequency and discusses the factors affecting the natural frequency. Four common frequency analysis algorithms (FFT, Root-MUSIC, Prony, HHT) are introduced and these algorithms’accuracy, efficiency and other characteristics are compared and analyzed. On that basis, an extraction method of main natural frequency based on EEMD are researched. By simulating in PSCAD and comparing with the above algorithms, the algorithm’s efficiency has improved a lot in frequency analysis, so, it can be used to extract the main natural frequency quickly. Then, the factors that affect the extraction method of main natural frequency based on EEMD are researched and analyzed.Finally, the characteristics of main natural frequency are described and EHV long-distance transmission line protection criterion which based on the main natural frequency difference is given out. Then, a refined protection criterion that based on the main natural frequency difference is introduced. This protection criterion combines efficiency and accuracy by floating threshold, taking accuracy into consideration while improving the efficiency at the same time.