Protective control method and apparatus

Claim 1

A protective control apparatus for sampling an AC voltage and/or current in an electric power system at a frequency which is an integral multiple of the rated frequency of said electric power system, converting said sampled value into a digital value, memorizing the digital value, and processing the memorized digital value or a synthetic value comprising: a first means for calculating a product K o S o by multiplying a constant K o and a value (said digital value or a synthetic value thereof) S o , which is obtained by a sampling at a first predetermined time which is prior to a second predetermined time by the amount of m/2 (m equals to a natural number) times the period of the rated frequency; a second means for calculating a product K l S l by multiplying a constant K l and at least a selected value among plural values S l (l equals to one of natural numbers 1-n), said selected value being obtained by multiplying (-1) l and a sampled value obtained by sampling at a third predetermined time which is prior to the first predetermined time by the amount of l/2 times the period of the rated frequency; a third means for calculating a predicted value Y m at the second predetermined time based upon a value obtained by sampling at the first predetermined time and a value at a time prior to the first predetermined time according to the following equation ##EQU35## utilizing said product K o S o obtained by said first means and said product K l S l obtained by said second means; a fourth means for calculating a variation value Δm according to the following equation Δm=X.sub.m -Y.sub.m, being based upon said value X m obtained at the second predetermined time and said predicted value Y m ; and a fifth means for rendering a protective control operation, utilizing the variation value Δm; whereby said constants K o and K l satisfy the following conditions ##EQU36## where ε is considerably less than 1, so that when said voltage or current is a sine wave of the rated frequency, said variation value Δm is expressed by the following equation |Δm|<|ε X.sub.m | and, wherein the predicted values are determined by the following equation: ##EQU37## where S k is a specific term of S l , with l being equal to k which is a natural number, or an approximation of the value given by the above equation.