Cascaded partial core resonant transformers.
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
High voltage field testing of electrical insulation can necessitate the use of bulky test equipment and large power supplies. This is due to the significant insulation capacitance presented by some types of high voltage machinery. For these applications, test equipment should be portable and have some way of compensating for the load capacitance. Partial core resonant transformers (PCRTXs) were developed as a solution to this problem. These are light weight transformers with a core consisting of a single limb which can be inductively tuned to resonate with the load capacitance at power frequency. These transformers have been designed for medium voltage applications such as high potential testing of generator stators. This thesis explores the concept of connecting multiple PCRTXs in cascade to generate higher voltages to enable portable field testing of high voltage cables and gas insulated switchgear.
An existing two winding model is extended to predict the performance of three winding PCRTXs. A new equivalent circuit model is developed to represent multiple PCRTXs connected in cascade and validated by measurements conducted on existing prototypes. New challenges presented by the cascaded arrangement are explored including a method for tuning multiple stages, primary winding current distribution and load voltage distribution across stages. The limitations of cascaded PCRTXs are investigated and it is concluded that designs with more than three stages become impractical due to increased losses and a reduction in the resonant load capacitance.
A suite of flexible software design tools is developed to make the design process user friendly. A constrained particle swarm optimisation algorithm is applied to compare the relative benefits of cascaded and single PCRTXs. Simulated cascaded test kits with varying numbers of stages are optimised for weight to meet the same design specification. The relationship between the number of stages and the optimal stage weight is presented.
A new two stage cascaded PCRTX test kit is designed and built to generate a 66 kV test voltage. New construction methods are trialled and evaluated including radially laminated core sections to reduce losses and fibreglass inter-layer winding insulation with vacuum resin infusion. The transformer is thoroughly tested and numerous design strengths and areas for improvement are identified. An existing PCRTX is added as a third stage to energise a 343 nF load to 100 kV whilst drawing 60.6 A from the supply.