Experimental Investigation of an Algorithm for Monitoring the State of Instrument Voltage Transformer Insulation on a Network Segment Physical Model

Abstract

The article presents the results from investigating the power plant generator voltage circuit segment physical model made using two high-voltage voltage transformers. A possible algorithm for monitoring the state of voltage transformer insulation in the power plant generator voltage circuit, which is intended for predicting insulation malfunctions, is studied on the basis of a physical modeling approach. Malfunctions resulting from a short circuit fault in the high-voltage winding of type ZNOL single-phase instrument voltage transformers with the grounded neutral caused by gradual degradation of their cast insulation are investigated on the physical model. The short circuit fault is modeled by short-circuiting the transformer’s primary winding taps. Also, the effect the short-circuiting of turns in the primary high-voltage winding of one transformer has on the currents in the primary windings and voltages in the secondary windings of both devices is considered.

The results from experimental investigation into the electrical characteristics of the generator voltage network segment’s physical model have confirmed that there is a growth of current in the transformer primary winding at a constant output voltage at the secondary winding. This is necessary for validating the algorithm of detecting a short circuit fault development process in the high-voltage winding. It has been experimentally shown that the effect of the instrument transformer output voltage remaining constant in short-circuiting part of primary winding turns with the secondary winding operating in the no-load mode still takes place also when the voltage transformer shifts to operate under nonlinear magnetic core magnetization conditions, as was expected from the numerical simulation results.

The results from analyzing the effect the secondary winding load has on the transformer operation mode are presented. A numerical assessment of the effect the load resistance has on the instrument voltage transformer output voltage with taking into account the presence of short-circuited turns in its primary winding has shown that there is a correlation between the measured characteristic of the network phase (phase voltage) and the state of the transformer high voltage winding insulation. This correlation can supplement the list of cases with incorrect operation of relay protection devices.