Estimation of Capacitive Voltage Transformer Parameters from Secondary Terminal Measurements

Harmonic waveform distortion is an increasingly important consideration in the context of quality of supply in high voltage transmission systems. In high voltage transmission systems, voltage waveforms are predominantly measured using capacitive voltage transformers. These transducers, however, exhibit limited bandwidth. A number of equivalent circuit models have been proposed in literature for modelling the frequency response properties of capacitive voltage transformers. The associated measurement methodologies for determining the relevant circuit parameters predominantly rely on measurements where the excitation source is applied on the high voltage side of the device. This paper proposes a novel procedure for determining parameter values for a wideband equivalent circuit model of a capacitive voltage transformer, using the measured frequency response properties of the secondary input impedance. The resonant sub-circuits associated with the frequency response of the secondary input impedance function are identified and the mathematical relationships between the resonant frequencies and the model parameters are deduced. Estimated parameter values and measured and simulated frequency responses of the transformation ratio and secondary input impedance are given for a 400 kV measuring grade capacitive voltage transformer. It is shown that the simulated frequency responses of the transformation ratio and secondary input impedance of a simplified model, using the parameter values obtained by the method, exhibit excellent correlation with the measured results. The main area of application of the results is in modelling the harmonic frequency response of capacitive voltage transformers for steady-state conditions.