Determination of Power Transformer Winding Natural Frequencies and Damping Factors from the Measured Voltage Transfer Functions

Abstract

The problem of damage inflicted to power transformers by high-frequency switching overvoltages occurring in 6–35 kV electric networks is solved by using protective RC circuits and detuning the windings natural frequencies from the frequencies of network voltage transient oscillations. To select the optimal parameters of protective RC circuits, simulation of transients in the cable network together with the transformer primary winding is required. To this end, it is necessary to have a reliable high-frequency model of the transformer. For verifying this model, measured values of natural frequencies and damping factors of free oscillations are required. To detune the transformer winding natural oscillation frequencies, their values have to be measured together with measuring the resonance overvoltage ratios in the transformer windings. Based on this information, the frequency ranges of network voltage transient oscillations that are dangerous for the transformer windings can be estimated. An approach to estimating the natural frequencies and damping factors based on measurements and analysis of voltage transfer functions at accessible intermediate points of power transformers windings is proposed. The article gives a practical example of estimating the natural frequencies and damping factors of free oscillations in the high-voltage winding of a dry transformer using the proposed approach, as well as by approximating transient voltages and analyzing the power transformer primary winding frequency responses.