Correlation between current-voltage characteristics and DC field grading for dielectric liquid used in wet-mate DC connector

Subsea DC transmission and distribution system is a promising technology for powering subsea oil and gas fields with high power, long distance and ultra-deepwater depth. In subsea DC transmission and distribution, wet-mate (WM) connectors are considered as a challenging component due to the complicated electrical field distribution in the insulation system and harsh undersea wet-mating environmental conditions. In order to design reliable liquid insulation systems for subsea connectors, it is essential that the behavior of the DC-stressed liquid insulation be fully characterized for thorough understanding. In this paper, liquid conduction phenomena in synthetic ester oil are studied using the current-voltage characteristics and electric field distribution mapping based on the Kerr electro-optic effect. The correlations between the current-voltage characteristics and DC field grading among different electrode gaps and oil types were analyzed. The experimental results suggest that for applied voltage less than the saturation voltage (V_s), the conduction is dominated by ion dissociation in the bulk oil. Correspondingly, the liquid conduction is ohmic and with formation of heterocharges around the contacts giving symmetrical field distribution. At applied voltage higher than saturation voltage (V_s), this bulk conduction will transit to extrinsic dominated conduction with carriers injected through liquid-electrode interfaces. Under such carrier injection, the corresponding field distribution becomes asymmetrical. In addition to the laboratory experiments, computer simulations with an ion drift-diffusion conduction model taking into account electrode injection were carried out to validate the experimental field distribution at different electrode gaps. The effect of electrode material on the electric field distribution in the oil is investigated and implications for wet-mate connector design principles are presented.

1 Introduction

The subsea DC transmission and distribution system installed on the seabed is a promising technology for powering subsea oil and gas (O&G) fields with high power (e.g., >100 MW), from a long distance (e.g., > 100 km) and in ultra-deep-water (UDW) (depths up to 3,000 m). For a modularly stacked subsea DC transmission and distribution system, WM connectors are essential components for system field deployment, faulty component isolation and repairing. However, the design and development of WM DC connectors are challenging due to complicated electrical field distribution in the insulation system as well as the harsh subsea environment. A subsea DC connector is not yet commercially available and the research and development of this component are essential for the subsea O&G extraction and processing industry [1], [2].