Analysis of dielectric response measurement methods and dielectric properties of resin-rich insulation during processing

Abstract: The dielectric diagnostic methods ofpolarisation/depolarisation currents and recovery voltage,which are time domain methods, and capacitance and tandmeasurements at different frequencies, which is a frequencydomain method, have been studied here. In the first part of thethesis, these measurement methods are analysed and evaluated.Three different types of insulation systems are included in theevaluation, oil/paper ("high" loss), resin-rich mica tape("medium" loss) and cross-linked polyethylene, XLPE, ("low"loss). These three systems have different dielectric responsein both shape and magnitude. Conclusions are made regardingchoice of measurement method depending on the dielectricresponse of the insulation material. Examples are also given ofhow to estimate conductivity and the dielectric responsefunction in the time domain from measurements with a finitecharging period.Furthermore, relations between time domain and frequencydomain and the possibilities of Fourier transforming data fromone domain to the other are discussed. Fourier transforming isdone with a spline approximation technique, the Hamonapproximation and by fitting base functions that have analyticFourier transforms to measured data. These techniques arereviewed with special attention to the problem of estimatingdata outside the measurement window.The second part of the thesis includes studies of the changeof dielectric properties during the manufacturing stage of acomposite insulation used in high voltage rotating machines.The insulation consists of a resin-rich mica tape with wovenglass or polyester film (PET) as carrier material. The aim wasto improve the quality of the composite insulation byoptimising the heat and pressure cycle used in theproduction.A test cell has been designed and built to be able toprocess simple parallel plate samples under conditions similarto the factory process. With a maximum heating rate of 9°C/min and a maximum cooling rate of 20° C/min arbitrarytemperature paths could be programmed. All samples wereprocessed under static pressure in the MPa range.The chemical reaction during curing of the resin-rich micatape was studied with differential scanning calorimetry (DSC).A simple reaction rate model was fit to the DSC measurementsmaking it possible to calculate the degree of curing during anarbitrary temperature path.The change of dielectric response with time during curingunder different temperature paths and at a constant pressurewas monitored. Both laboratory experiments and factorymeasurements have been made and based on these measurements asimple network model is proposed to explain the measureddielectric response in terms of material structure and degreeof curing.Keywords: Dielectric response, diagnostic methods, Hamon,Fourier transform, rotating-machine, insulation, resin-rich,mica tape, curing, process, monitoring, differential scanningcalorimetry (DSC), network model.