The Effect of Temperature Cycling and Transients on the Dielectric Properties of Transformer Impregnated Insulation

This master’s thesis project is a part of the Sinergie project (sponsored by Agentschap NL, 2007-2012) which is dedicated to analyze the functionality of the grid of the future. The European Union has advanced plans to decarbonize the power sector onwards the year 2050 and therefore the use of renewable energy sources is expected to increase. The distributed energy generation from, for example, wind turbines and solar cells are not constant during a day as the wind speeds and incoming solar radiation vary. Therefore an increase in power transport between different regions throughout Europe is expected. During operation, transformers heat up or cool down depending on the varying loads. These variations in load depend mostly on the time of the day, as consumers require more power at specific times of the day. The high electric field strengths, which are a result of peaking power transport, combined with these thermal cycles have their impact on transformer insulation. The grid of the future will still be partly composed of today’s components and asset management strategies will be used to replace components, such as transformers, just before their end of life. The aforementioned renewable energy sources are often connected to the grid via power electronic converters. These convertors introduce transient spikes in the grid. Therefore the effect of both temperature cycling and transients on transformer insulation are investigated in this research. Chapter 2 includes a literature study on transformers. Transformers are essential parts of the electric grid. They can be found at the generation, transmission and distribution level. During operation, degradation factors influence the state of the transformer insulation. The insulation experiences environmental ageing factors such as pyrolysis, oxidation and hydrolysis. Chapter 3 shows the measurements which have been performed. Three series of experiments have been performed on transformer paper samples with a thickness of 0.060mm. These series consist of AC breakdown tests, time to breakdown tests and tangents delta measurements. The breakdown voltage increases from 3.11kV to 3.59kV (11V/? C) when the temperature is increased from 16.6? C to 60? C. Up to 80? C, the breakdown voltage decreases to 3.12kV (23.5V/? C) when temperature is increased to 80? C. The time to breakdown is determined at 2.91kV and the time 63.2% of the samples fail increases from 48.4 to 82.1 and 147 hours when temperature is increased from 40? C to 60? C and 80? C, respectively. In the third series, transient spikes of 1kV superimposed on a 2.22kV AC waveform and samples are stressed for 22 hours. The transients are applied in a range from 1kHz up to 10kHz and it can be observed that the tangents delta increases with 6.4% compared to stressing with 2.22kV AC only. The results of the main experiments are interpreted in Chapter 4. Based on this interpretation, a model is made that can be used to calculate the accelerated ageing factors of the insulation and to incorporate the interaction between oil and paper. Measurement data should be used to calculate the loss of life due to daily loading cycles. Two loading scenarios are compared and it is shown that when renewable energy sources increase transport and introduce transients in the grid, ageing of transformers is accelerated.