METHOD FOR DETECTION OF SINGLE ENDED EARTH FAULT IN A THREE-PHASE SYSTEM

摘要

A process is disclosed for recognising a unipolar earth leakage in an output line of three phase mains driven with an arc suppression coil for earthing by resonance the neutral point. The process has the following steps: (a) a first pair of measurement values (Uo1, Io1) is formed at a first moment in time (t1); (b) the displacement voltage is modified at a second moment in time (t2) and a second pair of measurement values (Uo2, Io2) is formed at the second moment in time (t2); (c) the output earth admittance (Ya) is determined from both pairs of measurement values (Uo1, Io1; Uo2, Io2), as well as an asymmetric admittance (Yu) as inner admittance of a voltage source that feeds the parallel oscillating circuit made up of the arc suppression coil and earth admittance (Ya), and the thus determined values (Ya, Yu) are stored as reference values; (d) a third pair of measurement values (Uo3, Io3) is formed at a third moment in time (t3); (e) a failure admittance (Yf) is determined from the third pair of measurement values (Uo3, Io3) as inner admittance of a voltage source that feeds a parallel oscillating circuit made up of the arc suppression coil, earth admittance (Ya) and asymmetric admittance (Yu) by using the stored reference values (Ya, Yu); and (f) an earth leakage is indicated when the failure admittance (Yf) exceeds a predetermined value (K1).