Application of RVM technique to study the polarisation and depolarisation characteristics associated with thermal ageing of epoxy mica composite
The Recovery voltage measurement technique was used to investigate the polarization processes in the dielectric to assess the insulation ageing. Two different types of insulation systems are evaluated, resin poor and resin rich mica composites. This paper presents and discusses the data on polarization and depolarization characteristics of thermally degraded epoxy mica composites. Beyond polarization spectrum, additional spectra slope spectrum and peak time spectrum of Recovery voltage measurement technique were used. The combined spectrum: a plot of initial slope (V/s) against Max. Recovered voltage, Vr(V) showed a clear peak in the form of “bun” and “nose” distinctly which probably could give more information on the bigger polar species and the polarization phenomena. Multiple peaks are seen in case of thermally aged samples which clearly exhibit the degradation phenomena.
Polarization and depolarization current of linear low density polyethylene-natural rubber nanocomposite subjected to electrical tracking and moisture
Polymeric nanocomposites are widely used for high voltage outdoor 
insulating application due to their efficient electrical performance. Recently, SiO2, 
TiO2 and MMT nanofillers are used as fillers because there are listed as the main 
nanofiller commonly used in electrical engineering due to the increases of effective 
activation energy. Natural rubber (NR) is used because the nature of the interphase is 
found to affect viscoelasticity and it develops several interphases with the Linear 
Low-Density Polyethylene (LLDPE) matrix. This thesis presents the outcome of an 
experimental study which has been carried out to determine the conductivity level 
and dielectric response function of the LLDPE-NR compound, filled with different 
amount of SiO2, TiO2 and MMT nanofillers using Polarization and Depolarization 
Current (PDC) measurement technique. LLDPE and NR with the ratio composition 
of 80:20 were selected as a base polymer. The PDC testing was done on samples at 
various conditions: without any defect (normal condition), wet condition (moisture 
absorption) and after electrical tracking effect condition based on BS EN 
60587:2007 standards. One of the problems associated with outdoor polymeric 
insulators is the tracking of the surface which can directly influence the reliability of 
the insulator. The amount of water content can be used to monitor the dielectric 
quality of insulation and as an indicator of possible deterioration for outdoor 
degradation. Besides PDC, surface morphology analysis using FESEM was also 
conducted to study the changes in physical structure which can explain the 
conductivity and response function. This research found that an addition of certain 
weight percentage of nanofiller and NR into the LLDPE improved conductivity level 
of the insulator. LLDPE-NR/TiO2 at 5 wt% has become the best sample in terms of 
the lowest conductivity in normal and under electrical tracking effect. Besides, this 
sample has the fastest time response under electrical tracking and moisture 
condition. This investigation has successfully identified the PDC pattern, 
conductivity and dielectric response function of LLDPE-NR nanocomposite. Results 
show that different compositions as well as the surface physical conditions affect the 
PDC measurement results.
Analysis of dielectric response measurement methods and dielectric properties of resin-rich insulation during processing
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.