Theoretical research on insulation thickness design of 66 kV AC submarine cables

With the accelerated green energy development, offshore wind power generation become more and more attractive. Currently, several types of research present that the application of array submarine cables operating at high-voltage 66 kV instead of medium-voltage 35 kV shows considerable advantages on offshore wind farm systems. Insulation thickness is one of the crucial factors for the safe and reliable operation of submarine cables. In engineering applications, a high safety margin is often maintained to ensure a low electrical field in the cable insulation. However, an excessively high insulation thickness design in submarine cables can block the heat dissipation of the conductor, lower the current-carrying capacity, and raise production costs. In this work, the theoretical design of insulation thickness for 66 kV AC submarine cable are studied based on AC and impulse breakdown test. The AC and impulse breakdown strength of XLPE insulation with various temperatures from 30°C to 90°C and thickness from 0.2 mm to 0.4 mm were carried out, respectively. The temperature coefficient under AC and impulse voltage are both obtained at around 1.10 and 1.23 by comparing the breakdown strength at 30°C and 90°C. Besides, it is found that the AC breakdown strength is increased with the temperature from 30°C to 90°C, and then decreased with the temperature further rising from 50°C to 90°C. The thickness coefficients are also obtained by fitting the AC and impulse breakdown strength of the specimens with a thickness of 0.2 mm, 0.3 mm, and 0.4 mm. Step-stress test is also carried out to calculate the life exponent and deterioration coefficient of XLPE insulation based on the inverse power model. The life exponent of the XLPE insulation is evaluated at 30°C and 90°C, which is 13.9 and 15.1, respectively. It is found that the internal maximum AC and impulse field strength that cable insulation can bear during operation are 60 kV/mm and 145 kV/mm, respectively. Finally, an appropriate insulation thickness of ~8.9 mm is recommended based on the obtained internal maximum field strength, which is considered to meet the demand for 66 kV AC submarine cable. This study could provide an experimental and theoretical reference for the thickness design of submarine cables and will be helpful for the future development of light AC submarine cables.