Voltage harmonic distortion measurements using capacitive voltage transformers

This paper investigates the application of capacitive voltage transformers (CVTs) for harmonic distortion measurements. Measured and simulated frequency responses are given for the voltage transformation ratio and the transconductance ratio of the capacitive divider ground return current and the high voltage input signal of a 400 kV CVT. The frequency response of the transconductance ratio is shown to be linear and easily predictable for the harmonic frequency band, i.e. from the rated frequency of the device to approximately 2.5 kHz. This is explained with reference to the reactance of the intermediate voltage capacitor and the input impedance of the electromagnetic unit. The results are verified by simulations for a 138 kV CVT, using the equivalent circuit model and parameter values presented in a recent EPRI study. Practical application of the results for the measurement of voltage harmonic distortion is subsequently discussed.