Phasor-Based Transient Earth-Fault Protection

摘要

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 I0. 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 I0 current, only the I01*cos(Φ1) component has useful physical meaning during the ground fault transient, while the I01*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 harmonic phasors of the Io current the situation is exactly the opposite. The Ioh*sin(Φh) is the useful component with a clear physical meaning while the Ioh*cos(Φh) 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.