Design and Implementation of a Pulse Transformer and Study on Ageing of Oil Impregnated Paper under Pulsed Stresses.

MSc thesis of Ravi Kumar

Energy consumption keeps increasing, and more than three-quarters of the EU’s greenhouse gas emissions stem from it. Hence, it is important to reduce or stop burning fossil fuels to limit the temperature rise to 1.5∘C according to the Paris agreement target. Nowadays, renewable energy sources and distributed generation are increasingly integrated into the electric power system to achieve the ambitious target of the energy transition. Renewable energy sources are inherently fluctuating in their power. Whereas the grid is stable. The interface between these two can be made smooth based on a medium-frequency DC-DC converter. The advantages of increasing frequency are high power density, significant weight and size reduction, dynamic power flow control, and power quality management. All these are possible mainly because of advancements in power electronics in the recent decade. However, these power electronics induce harmonics across insulations, which are fast rising voltages and are repetitive. The dielectric strength of insulations under these fast-rising pulses is to be understood. This thesis project studied oil impregnated paper behavior under pulsed stresses.
To test the Oil Impregnated Paper samples, a source capable of generating repeating pulses with fast rising and lower overshoots is required. This source comprises an ACDC converter that will power the H-bridge. The pulses from the H-bridge are then fed into a 1:50 pulse transformer to step up the voltage, which is then delivered across OIP (Oil Impregnated Papers) through test components that regulate the pulse’s rising time and overshoot. The process began with identifying critical factors to consider while developing a pulse generator. Following that, a random transformer prototype is created along with another transformer to extract the parameters, which are then modeled in the circuit simulator and COMSOL to predict the pulse. An algorithm was created to optimize the transformer design. Finally, this design method creates a pulse transformer that generates the necessary output.
The OIP samples are then tested at frequency 50??? and at various voltages with a rise time ?? ≈ 1.3? ?. A lifetime curve was obtained using the data of ageing tests at five field strengths. The results showed that the lifetime was reduced with the increase in frequency. At lower fields, a transition was seen, indicating a shift in the ageing process."