Space Charge Measurement on XLPE Cable for HVDC Transmission using PEA Method

The use of cross-linked polyethylene (XLPE) cables for High Voltage Alternating Current (HVAC) transmission has been successful in the world. However polymer insulated cables for High Voltage Direct Current (HVDC) transmission are still in development due to space charge build-up that occurs in cables under DC stress. Knowledge must be gained on the processes of charge build-up, including nature, kinetics and amount of accumulated charges, and on the consequences of charge accumulation regarding field distribution in the insulation. One way to determine the space charge distribution in XLPE cables for HVDC transmission non-destructively is the Pulse Electro Acoustic (PEA) method, a direct method for measuring space charge distribution in dielectric materials. The PEA method was chosen because it is suitable for a wide range of sample geometry (thin and thick samples), and is available for cable geometry. The present contribution concerns space charge distribution measurements in sections of HVDC model cables. To account for the situation of cables in service, measurements were performed with a temperature gradient across the insulation thickness, and this could be achieved by heating through the cable conductor. Measurements performed on XLPE insulated model cables with 4.5 mm insulation thickness were realized with -40 kV DC applied for 150 minutes. Injecting a current of 200A in the conductor produced theoretically a temperature gradient of 8 degrees C between the core and the outer face of the cable, with a surface temperature of the cable of 50 degrees C. A deconvolution procedure, wherein attenuation, dispersion and cylindrical geometry are accounted for, was used to recover space charge profiles at different times under stress. It is shown that considerable field redistribution occurs within the cable, compared to the Laplacian field distribution predicted in cylindrical geometry, due in part to the conductivity gradient associated with the thermal gradient, and for the other part to charge build-up occurring within the cable.