Adnan Fazal

The effect of composition and processing on electric characteristics of XLPE in HVDC cable applications

Polyethylene exhibits many key characteristics including low dielectric loss, high breakdown strength and good processability. Most modern extruded high voltage cables employ cross-linked polyethylene (XLPE) as the insulation material. The main advantage of XLPE is its excellent thermo-mechanical properties; it is relatively cheap and has low dielectric loss and low conductivity making it an ideal material for this application. Crosslinking enhances a number of thermo-mechanical properties such as deformation resistance at higher temperatures, tensile strength and creep properties. In comparison with lov density polyethylene (LDPE), the heat deformation characteristics of XLPE are superior and, for this reason, XLPE is currently the most common insulation material for power cables ranging from low to high voltages. This paper reports on an investigation into the development of a new XLPE formulation for use in high voltage direct current (HVDC) cable applications. Specifically, the electrical performance of two novel LDPE resins are compared with an industrial standard (reference) LDPE material. For crosslinking, dicumyl peroxide (DCP) was selected, as the decomposition temperature is high enough to prevent pre-curing during processing and to allow an efficient and rapid crosslinking at moderate temperatures. Moreover, the behavior of various systems is compared in terms of electrical breakdown performance and the influence of material composition and processing on these parameters is described.

The impacts of the temperature and electric field on the electrical characteristics in semicon-bonded XLPE insulation

HVDC cables play a vital role in the power transmission system for renewable energy and global power trade. Nowadays, the crosslinked polyethylene (XLPE) extruded cables have been widely applied in power industry due to the superior performance on the thermo-mechanical properties and dielectric properties. The low volume conductivity and the minimized space charge accumulation are the two key requirements for a reliable high voltage direct current (HVDC) cable insulation. This paper reports on the impact of temperature and electric field on the space charge behavior and DC conductivity in XLPE material for cable insulation. The samples were carefully prepared to simulate the real cable insulation structure. A layer of LDPE film mixed with DCP (dicumyl peroxide) was sandwiched between two layers of semicons (also contain crosslink agent) and then crosslinked at 200 °C to ensure the semicon layers were thermally bonded with the XLPE insulation. The crosslinked samples were then degassed in the vacuum oven with for 6 days at 80 °C. The space charge characteristics and the conductivity of the semicon-bonded XLPE samples were measured at room temperature and high temperature. The electric fields were kept at 20 kV/mm and 40 kV/mm and the influences of the electric field and the temperature on space charge dynamics and conductivity in the semicon-bonded XLPE samples are discussed.

The impacts of degassing on space charge characteristics and DC conductivity in semicon-bonded XLPE For HVDC cable applications

High voltage direct current (HVDC) cables are one of the key components for the HVDC power transmission system. HVDC cables for higher voltage level, larger power capacity and higher reliability are still desired for the power industry as part of a global power strategy. One of the major concerns related to HVDC applications is the presence of space charge within the dielectrics, which distorts the electric field distribution and contributes to accelerated ageing and consequent failure of the cable insulation. The byproducts generated from the crosslinking process during the manufacture of XLPE insulation for HVDC cable are one of the major sources of hetero-charge formation in dielectrics. Therefore, degassing is widely applied in cable manufacture for the removal of such crosslinking byproducts. In this paper, composite XLPE samples were produced that include two layers of semiconductive polymer (semicon), which are thermally bonded on the top and bottom of the XLPE insulation, to mimic a real cable manufacturing process. The samples were degassed under vacuum and elevated temperature for 1, 3 and 6 days and their space charge characteristics and conductivities were then measured at an applied field of 40 kV/mm. The influence of the degassing period and the semicon material formulation on space charge formation and distribution in the system are discussed and the findings are compared with undegassed samples.

The effect of electrode material and semicon bonding on space charge dynamics of XLPE

This study sets out to explore the space charge characteristics of high voltage direct current (HVDC) cable grade crosslinked polyethylene (XLPE) using different electrode materials and configurations, ranging from sputtered gold to thermally bonded semi-conducting materials. The existence of both homo and hetero charges in HVDC cables is well known; however, the role of semi-conducting materials and the formation of space charge is critical and required more attention. Space charge accumulation at high electric stress was monitored using the pulsed electroacoustic (PEA) technique. Experimental results have shown that charge injection processes take place in all cases and the amount of charge and polarity of the dominated injected charge has significant dependence on the electrode material under the same applied electric field.