| 摘要 |
PROBLEM TO BE SOLVED: To reduce the error caused by a frequency fluctuation in the third harmonic wave extraction calculating method applied to a digital overcurrent relay. SOLUTION: When the third harmonic wave is extracted and calculated based on the sampling values gn (n=0, 1, .........) of the system voltage or current obtained by sampling the system fundamental wave frequency at every 30 deg., the sine component S and cosine component C of the third harmonic are obtained as follows: S=(1/12)[(g0-g6)-(g2-g8) +2(g4-g10)-(g6-g12)+(g8-g14)], C=(1/12)[(g1-g7)-2(g3-g9)+2(g5-g11)-(g7 -g13)]. Data after 90 deg. and the arithmetical mean are obtained in the fundamental wave in addition to the conventional calculating method, and the pulsation component appearing at the frequency two times the optional phase angle by the said frequency fluctuation is inverted in sign at 90 deg.×2=180 deg. and offset. |
