Investigation of the neutral voltage in an unearthed network

Abstract

This report has been performed as a master thesis work at Forsmarks Kraftgrupp AB covering 20 university credits. The thesis investigated a problem with excessive neutral voltage at one of the internal systems in the power plant. The measurement of the neutral voltage is used as the indication of the presence of earth faults in these systems. Initially, the layout of the power plant was studied as a starting point of the analyses. Discussions were then held with various persons working at the plant who had in-depth knowledge of the operation and design of these systems. Theoretical analysis of different typical loads that are connected was done and measurements of leakage currents on the one phase outputs were carried out. The statistical analysis of the fault occurrence frequency has also been performed. The statistical analysis has not shown any significant repetitiveness; i.e. the fault does not occur at specific time intervals, nor any easily discernable cause of the faults. It has however been noticed that the faults are main occurred during one half of the year, from August to January, while in the remaining months, almost no faults were observed. The reason for the faults to occur only during autumn and winter has unfortunately not been found by this thesis. The results of the measurements show that the amplitude of the leakage currents of the onephase outputs are several times larger than the combined earth current, which shows the unsuitability of the measurement setup for this type of system. The recommendation has been made by this report that the monitoring equipment be changed.