THIRD HARMONIC EXTRACTION ARITHMETIC METHOD

摘要

PROBLEM TO BE SOLVED: To reduce errors by frequency fluctuation or the like in the extraction arithmetic method of third harmonics applied to a digital overcurrent relay or the like. ;SOLUTION: In extracting and computing the third harmonics based on the sampling values gn (n=0, 1,...) of a system voltage or current obtained by performing sampling for every 30° of a system fundamental wave frequency, the sine component S and cosine component C of the third harmonics are respectively obtained from S3(n)=(1/6) [(g0-g6)-(g2-g8)+(g4-g10)], and C3(1)=(1/6)[(g1-g7)-(g3-g9)+(g5-g11)]. Third harmonic components are found by the square root of the arithmetic mean (1/2) [[G3 (n)]²+[G3(n+3)]²] of the square value [G3(n)]²=[S3(n)]²+[C3(n)]² of an optional timing (n) and the square value [G3(n+3)] after three sampling cycles and pulsation components by fluctuation are code-inverted by 90°×2=180° and offset.;COPYRIGHT: (C)1999,JPO