Research On Thermal Aging Monitoring And Fast Prediction On The Life Of XLPE

With more and more intensive interests focused on how to helpnuclear power plants (NPPs) continue to safely operate beyond theiroriginal40-year old, the cables as integral part of NPPs will never beneglected. Cross-linked polyethylene (XLPE), with its excellent dielectricstrength, low dielectric permittivity and loss factor, good dimensionalstability, solvent resistance, and thermo-mechanical behavior, is now oneof the widely used polymer base materials for cable insulation in NPPs.Therefore, the knowledge of the XLPE cable insulation is crucial for thesafety assessment of NPPs.Degradation of XLPE cable insulation has been recognized as one ofthe main causes of premature failure of a wide variety of plants. Althoughdegradation of cable insulation is a multifactor phenomenon in NPPs,thermal aging is a severe problem that should not be neglected. Especially,thermal oxidative degradation produces low molecular weight andoxygenated products, which have bad effects on thermal, mechanical andelectrical properties of the material. During thermal aging, severalstructural changes occur such as variation in crystallinity, chain scissionsand variation in heat of fusion and in melting point.At present, a lot of work has been done both in the theoretical andexperimental studies on the degradation of the XLPE used as powertransmission and distribution cables, but three problems still need to besolved: thermal aging mechanism; effective and suitable parameters forthermal aging monitor and life prediction; fast life prediction method. Inthis work, in accordance with the standard of UL, cross linked polyethylene was thermal aged under high temperature150oC and enoughfresh air to simulate the actual thermal aging in NPPs.11samplesincluding fresh XLPE with different aging time were gotten. Theexperimental time was long to70days.In the first part, it was aimed to research thermal aging mechanismand effective parameters for thermal aging monitor and life prediction.9different measurements were used to characterize the thermal agingbehavior of XLPE: FT-IR Spectrum,Equilibrium Swelling Experiment,SEM Microscope, Differential Scanning Calorimetric, Tensile Test, PI Test,High Frequency Dielectric spectroscopy and Breakdown strengthmeasurement. The thermal aging mechanism and16effective parameterswere found.In the second part, it was aimed to fast life prediction with ThermalGravimetric (TG). This part could be operated independently. Fresh andthermal aged samples were thermal decomposed in N2and air. Thermalaging mechanism also could be gotten only with TG. What is the mostimportant achievement in this part is to propose the thermal aging processof the first stage of the end life aged XLPE (336h) could equivalent to theactual slow thermal aging process. That’s to say, the activation energy ofthis stage could be used to calculate the service life of XLPE in a lowertemperature. The result of XLPE was expected to give a guideline for fastlife prediction of other more complicated materials.