Arbitrary Waveform Impedance Spectroscopy for Characterization and Diagnostics of High Voltage Insulation

Licentiate thesis, 2007

In high voltage engineering the dielectric response technique is a widely used tool both for characterization of insulation materials and for diagnostics of components such as transformers and cables. Currently available techniques are not capable of measuring on-line which would be advantageous; when characterizing a material and evaluating the effect of different voltage magnitudes, waveforms and temperatures it is desirable to be able to observe the continuous development of the dielectric properties. Present diagnostic techniques based on dielectric response need to disconnect the high voltage component from the grid in order for measurements to be performed, which is a major drawback. Apart from the inconvenience of removing a part of the power system from operation, measurements are performed with voltages and for temperatures unlike those experienced in operation which can provide misleading results. In addition, the possibility of continuous online monitoring would also provide increased opportunities for condition based maintenance (CBM). In this thesis an approach towards dielectric response measurements utilizing the harmonic content of any applied repetitive voltage is introduced. This measurement technique can utilize voltage waveforms already present in the system which makes external test voltage sources superfluous. By measuring the applied voltage and the corresponding current response, the voltage and current spectra can be obtained using the fast Fourier transform (FFT). With the use of calibrations and a detailed circuit model, the dielectric response for many harmonics can be obtained simultaneously. This approach towards dielectric response measurements has been evaluated and compared with commercial instruments operating in the frequency domain. The agreement is good and shows that the technique as implemented here has the same accuracy level as conventional techniques. In addition, some examples of applications are shown which illustrate possible studies that previous techniques were unable to perform. The technique can be used for low voltage, high voltage and online measurements, each with its own limitations and specific set of requirements necessary for obtaining accurate results.