Study of Diamond Folds in Mass-Impregnated(MI) Cables

Abstract: High Voltage (HV) cables are designed to transmit electrical power from the source to the placewhere it is consumed. The global trend towards renewable energy sources e.g. wind farms, hy-droelectric and solar panels have led to an increase in the use of HV cables, as these sources aresituated in remote locations far from the areas where they are consumed. Therefore. there is aneed for developing efficient power transmission cables.Mass-impregnated (MI) cable is a type of HV cable which contains an electrical insulation layerof oil or mass impregnated cellulose paper. Due to its excellent dielectric properties, MI cableshas been established as a solution for high voltage electrical transmission system, which has beenused for several decades.The main area of interest in this thesis is the paper insulation in the MI cable. It is importantto ensure that the paper insulation structure is robust enough to avoid any formation of damageand/or defects due to excessive deformations during cable production, handling and installation,which might affect the overall electrical performance of the cable. During handling, the drylapped cable needs to be bent and unbent several times and there is a risk that the insulationwill develop voids due to structural defects, thereby reducing the dielectric capacity of the cable.An example of typical structural defects in MI cables is, so-called, diamond folds, i.e. a collec-tive buckling of several paper strips due to axial compressive load resembling a diamond shapedpattern.The goal of this thesis work is to contribute to the knowledge of the mechanism of diamond foldsin MI cable; the onset of diamond fold and post-buckling responses. The contributions from thecable design or production parameters are also studied. The analyses will be conducted usingnumerical simulation techniques and will be compared to results from characterization tests al-ready conducted in mechanics lab at NKT Technology consulting.The results of this thesis has given an insight about the effect of insulation design parameterssuch as registration, butt-gaps, coefficient of friction and number of layers, on the diamond foldbuckling behavior in an MI cable insulation. It is recommended that these parameters have tobe carefully controlled so as to avoid formation of diamond fold buckling in the insulation.