Dielectric Response and Partial Discharge Diagnostics of Insulation Systems by Utilizing High Voltage Impulses

In this thesis, power system transients are considered as an opportunity for development of online diagnostics of power components and specifically the insulation systems of power transformers and bushings. Failures of power components and outages can lead to a huge economical loss in the electric power grid. Therefore reliable operation of power grid requires on-line monitoring and diagnostics of power components. Power transformers and their bushings are crucial components in a power transmission system and their insulation degradation may lead to catastrophic failures. Several methods exist for diagnostics of these components. However, some of them can only be done offline in maintenance periods, and the existing on-line methods generally provide less information, especially on the internal solid insulation parts. A new technique for on-line dielectric response measurement of power transformer bushings is proposed which utilizes natural transients in the power system, such as lightning and switching surges, as stimuli. Laboratory investigations are done on implementation of the proposed technique. Measurement considerations, data acquisition and processing involved in achievement of reasonable accuracy in the Dielectric Response (DR) are presented. Capability of the technique in tracking of the degradation signatures such as moisture content in the insulation has been evaluated and it has shown a good level of accuracy by being compared to the Frequency Domain Spectroscopy (FDS). The proposed technique is tested on the service-aged 150 kV bushings and feasibility of the technique for monitoring of dielectric properties of power transformer bushings has been assessed; the results are promising for the technique to be used in the real application. Partial Discharges (PD) behavior under transients has been also studied for different materials in this project. PD behavior of different defects, at different insulation condition, responding to the overvoltage transients in form of superimposed impulses on ac voltages was investigated and it was perceived how their distinctive response and the interpretation of that, can be useful for their identification. Recordings of the voltage signals and associated PD measurements during and after the incidence of a high voltage transient are of potential use for condition assessment of the insulation of power transformers and their bushings, as a form of online diagnostics that considers how the ac PD is affected by the transient. Besides the conventional materials, surface ac PD behavior of modified papers under the superimposed impulses has been studied. New developed dielectric nanocomposites with improved electrical properties have shown the possibility for enhancement of their PD resistance. Therefore, surface PD properties of modified paper with silica and zinc oxide nanoparticles under the impulses have been assessed in this project. Proper type and optimum concentration level of nanoparticles in the paper are the factors that lead to the improvement of PD behavior in the modified paper under overvoltage transients.