Rom controlled protective relay system for power transmission network

Abstract

A digital protective relay system includes a read only memory (ROM) in which a program for executing calculations on a judgment formula for a predetermined protective relay function is stored. Switching of the judgement formula, from one form (e.g., over current protection) to another form (e.g., undervoltage protection), is facilitated by a setting unit and an operation mode switching unit which permit changing of judgement formula constants without changing the ROM program. The setting is used to input desired relay operation threshold values. The operation mode switching unit is used to specify the kind and sign of constants contained in a predetermined term in the judgment formula and also the kind of inequality in the formula. A digital data processing unit performs protective relay calculations by applying input data obtained from the power transmission system, which data has been pre-processed by an A/D converter and related circuitry and stored in a data memory, to a relay operation decision formula determined by the data stored in the ROM, the setting unit, and the operation mode switching unit.