Universal earth-fault protection method for high impedance grounded power system

The main problem for any transient earth-fault protection is to extract the useful signal components from the measured residual current Jo. Many used solutions are based on either complete fundamental frequency and/or complete higher harmonic phasors. However, for the fundamental frequency current phasor of the Jo current, only the Jo1*cos(φ1) component has useful physical meaning during the EF transient, while the Jo1*sin(φ1) component is just a disturbing part which can only cause the wrong operation of the protection. At the same time, for all higher harmonics phasors of the Jo current the situation is exactly the opposite. The Joh*sin(φh) is the useful component with a clear physical meaning while the Joh*cos(φh) component is a disturbing part which can only upset the earth-fault measurement. Jn the proposed solution, only the two useful components are used. They are first integrated over short period of time deriving values proportional to the energy content of the two original signals. Only the positive or negative sign of these integrated values is then used in the design to determine the direction of the earth fault. Before being used for directionality check, the integrated value must also exceed the pre-set minimum threshold level, for security reasons.