Voltage Measurement Errors as a result of Multiple VT Groundings

Abstract

On 12 December 2007 a distance relay tripped erroneously by zone 1 for a ground fault in an adjacent line. RTDS simulations and theoretical calculations proved that the relay had operated according to its design parameters and that the settings of the units involved were adequate. Further analysis of the fault recordings showed that the currents followed a single-phase to earth fault pattern, whereas the voltages did not follow any known pattern for the conventional types of faults. In the first part of the paper a theoretical analysis of the problems involved with grounding the VTs in multiple locations is presented. First, the nature of the impedance between different grounding points is studied. Next, the voltage gradient during an earth fault is shown and the sequence voltages affected by a fault involving earth are determined. The deviation from the true value of the faulted phase voltage is also theoretically expounded. The relaying units affected by the multiple groundings are listed and their possible misoperations presented. To end the procedure to determine that such a problem exists and the ways to confirm it are presented along with the calculations that have to be done to determine the performance of the relays involved. In the second part of the paper the analysis of a real case is presented. The theoretical background expounded in the first part of this work is applied to the above mentioned fault and it is shown that, had only been one VT grounding, the relay would have operated correctly.