Offline Space Charge Measurements for HVDC Cable Insulation Samples

Abstract

There is a rapid increase in the off-shore wind farms and the interconnection between countries which results in an increased energy transfer over longer distances. High voltage DC (HVDC) cables can transfer this energy in a reduced cost and efficient way in contrast with high voltage AC (HVAC) along with many other benefits. For the HVDC cables extruded cross linked polyethylene (XLPE) insulation is preferred over the mass impregnated paper oil cable due to advantages such as low costs, simplicity, no use of oil and high operating temperatures.
One of the main degradation mechanism for such insulation type is weak electrical conduction of charges due to the material conductivity. Due to a local inhomogeneity of the insulation the ow of these charges is non-uniform. This results in the charge owing inside of a region not equal to the charge owing out of that region which causes a local charge build up. These charges are known as space charges. These create region(s) of high electrical stress which can result in partial breakdown or in some cases complete breakdown of the insulation material as well. Hence, space charge accumulation in cable insulation material governs the failure behaviour and life time of the cable.
This thesis look into the possibility of carrying out an off-line space charge measurements for real-sized HVDC polymeric cable insulation samples using the electrostatic probe technique. The first objective is to develop a calibrated sample using which it is possible to carry out the space charge measurement. Secondly, the possibility to convert the measurements obtained from the sample into the space charge distribution is looked into. A calibrated sample (prototype) is prepared and both the objectives are evaluated using it.