PHASOR-BASED TRANSIENT EARTH-FAULT PROTECTION

The main problem for any transient earth-fault (i.e. ground-fault) protection in high-impedance grounded system is to extract the useful signal components from the measured residual current I-0. 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 I-0 current, only the I-01*cos(phi(1)) component has useful physical meaning during the ground fault transient, while the I-01*sin(phi(1)) component is just a disturbing part which can only cause the wrong operation of the protection. At the same time, for all higher harmonic phasors of the I-0 current the situation is exactly the opposite. The I-0h*sin(phi(h)) is the useful component with a clear physical meaning while the I-0h*cos(phi(h)) component is a disturbing part which can only upset the ground-fault measurement. In the proposed solution, only the two useful phasor 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 ground fault. Before being used for directionality check, the integrated value must also exceed the pre-set minimum threshold level, for security reasons. Another important property of these two quantities is that they will be approximately equal to zero during all operating conditions of the protected network except during the transient period of the ground-fault and consequently they can be then easily detected.