Assessment of Power System Equipment Insulation Based on Distorted Excitation Voltage

Electrical insulation plays a critical role in high voltage power system equipment. The presence of electrical, thermal, and mechanical stresses imposed when they are in operation for a long time cause gradual degradation of the insulation. Therefore, regular condition monitoring and diagnostic testing of power system equipment are of paramount importance for the reliable operation of electricity supply networks and systems. The dielectric dissipation factor (DDF) measurement is one of the most common techniques for insulation assessment. From a traditional perspective, a pure sinusoidal voltage is used for excitation in the testing. However, the grid voltage nowadays in reality is often distorted with a waveform having multiple harmonic components. Generally, there are distorted voltages and currents generated due to the presence of non-linear equipment or components in the system. Thus, testing under distorted voltage with harmonics provides a more realistic diagnostic measurement as compared to traditional AC sinusoidal high voltage testing. This dissertation investigates the impact of harmonically distorted excitation on the dielectric dissipation factor of high voltage power equipment. A practical measurement method based on distorted excitation is proposed and tested on a reference capacitor-resistor test object. A theoretical and mathematical model is developed to quantify the impact of distortion on the DDF measured in contrast to the case of non-distorted excitation. It is established that for the same total RMS magnitude of the applied excitation, the DDF decreases with the increasing harmonic proportion in the applied voltage waveform. For validation, laboratory experiments and computer simulations were carried out, and data obtained were compared with the analytical results. The proposed technique is then tested on some real high voltage components (33kV dry-type current transformers). The results confirm the monotonically decreasing trend, but the pattern is more complex. The dielectric dissipation factor mathematical and electrical circuit model is implemented based on the polarisation loss. The theoretical formulation is implemented in a computer simulation using MATLAB Simulink to validate the results. In summary, the thesis provides useful diagnostic insights on the characteristics of the dielectric dissipation factor measurement under distorted excitation