Dielectric Diagnostics of Power Transformers and Cables - Return Voltage Measurements, Theory and Practical Results

摘要

Dielectric diagnostics are an important tool to characterize ageing and degradation of hv-equipment. After a short review of the basic behaviour of materials in electric fields, the paper will discuss the basic principles of Return Voltage Measurements (RVM), especially with respect to hv-equipment that use cellulose-oil insulations: transformers and cables. The p-factor calculated from three characteristic parameters of a measured Return Voltage curve has been used successfully for paper-oil cables since nearly two decades. For transformers the same measurement procedure can be applied, but due to differences in the electrical system, the p-factor is misleading, and a different diagnostic parameter, the r-factor has to be extracted from the Return Voltage curve. Starting from the insulation geometry of a power transformer, the decisive point is the use of the two dielectrics cellulose and oil in series. This type of insulation system fits very well the conditions described by the Maxwell model for two dielectrics in series. The interpretation of RVM on the basis of the Maxwell equivalent circuit, reveals that the Dielectric Time Constants of the two dielectrics cellulose (tauB) and oil (tauO), calculated from the experimentally found Return Voltage curves, are very meaningful and powerful parameters, especially as they are characteristic for the actual dielectric properties of the insulating materials and are not influenced by the actual geometry of the measured system. The r-factor, defined as the product of the logarithms of these two parameters tauB and tauO , has proven to be an effective measure for the characterization of the degree of ageing and degradation of the insulation system of power transformers.