Method for detection of high-impedance ground faults in a medium-voltage network

摘要

The present invention relates to a method for detection of high-impedance ground faults in a medium-voltage network. According to the method, the degree of unsymmetry (ki) and/or the line-to-ground admittance (YtEi) as well as the zero-sequence voltage (U0) of each sending end (i) are determined. According to the invention, for the values of the line-to-ground admittance (YtEi) and the degree of unsymmetry (ki) of each sending end are determined a reference value (YtEiref and kiref) on the basis of measurement information obtained by means of an artificial deviation of the neutral voltage performed in a reference connection status, in a memory are stored as reference values the values of the line-to-ground admittance (YtEref) and the degree of unsymmetry (kiref) of each sending end (i), as well as the normal-connection status values of the zero-sequence voltage (U0ref) and the zero-sequence currents (I0refi) of the sending ends and the zero-sequence current (I0refs) of the feeding power source, the zero-sequence voltage (U0) is monitored at least essentially continuously and, if said zero-sequence voltage (U0) changes by more than a predetermined limit difference ( DELTA U0as), for each one of the sending ends (i) are computed new values of line-to-ground admittance (YtEi) and degree of unsymmetry (ki), the most recently computed values of the line-to-ground admittance (YtEi) are compared with the reference values (YtEiref) and from the comparison is determined whether the difference therebetween exceeds the inaccuracy ( DELTA YtEi) of the measurement technique used, whereby if the comparison gives a value greater than said measurement inaccuracy ( DELTA YtEi), it is checked for instance on the basis of the change in the entire network's summed line-to-ground admittance, which is computable from zero-sequence current of the feeding power source, whether a change has occurred in the connection status of the network end, if so, the most recently measured values of the line-to-ground admittance (YtEi) and degree of unsymmetry (ki) are stored as new reference values, while, if no change has occurred in the network connection status, a ground fault is indicated.