On Estimation of Moisture Content in Mass Impregnated Distribution Cables

Sammanfattning: Mass impregnated paper insulated cables in distributionnetworks (6-33 kV) nowadays are gradually being replaced byXLPE insulated cables, yet, the percentage of mass impregnatedcables in distribution networks is still considerable. Thisreplacement is a long-term strategy, where right timing,properly made investments, etc. play crucial role. Therefore,the evaluation of present condition of mass impregnated cablesis of great importance for distribution network companies.Among factors influencing the performance of oil impregnatedpaper insulation, moisture is one of the most common causes ofdeterioration of the insulation properties and prematureageing. Most common cause of increase in moisture content isdue to ageing (natural, thermal) of cellulose, however, leakagein the lead sheath or insufficient sealing of joints andterminations must be mentioned as well. The estimation of themoisture content is an important assessment of the insulationcondition.Fundamental concepts of dielectric response, polarisationmechanisms, and their representation in both time and frequencydomains as well as the most common diagnostic methods form thetheoretical background of the thesis. The emphasis is directedtowards non-destructive methods, in particular, frequencydomain spectroscopy for assessment of mass impregnated cableinsulation.Experimental part of this project consists of extensivestudies of the properties of insulating paper from two fieldaged mass impregnated 10 kV cables. The dissimilarity in theirage and design bringing about the differences in theirdielectric responses are pointed out. Similar behaviour is alsoobserved for other field aged mass impregnated cables. Thechemical characteristics of the paper samples from both cablesare described and discussed.The near-infrared (NIR) spectra measured on the papersamples having different moisture contents were used to createa model for estimation of moisture content in unknown papersamples. The method of application of near-infraredspectroscopy for moisture estimation in oil impregnated paperis described.Minimum value of loss tangent is found being a parametershowing good relation to the moisture content in impregnatedpaper. On this basis a model for estimation of moisture contentfrom the respective frequency response was developed. Theobserved relations between moisture content and respective timedomain response are also discussed. The study of water ingressin mass impregnated cable is presented. The continuousfrequency domain measurements were used to model the diffusionprocess in the cable insulation. Actual moisture contents weremeasured for the purpose of model verification and adjustment.Moreover, a model describing dynamics of water ingress isdescribed.