DC Characterization of Silicon Insulation material
Satish Buddhawar (TU Delft - Electrical Engineering, Mathematics and Computer Science)
A. R. Rodrigo Mor – Mentor
Peter Vaessen – Graduation committee member
Milos Cvetkovic – Coach
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Abstract
The first commercial High Voltage Direct Current (HVDC) transmission link was installed in 1954 between the mainland of Sweden and the island of Gotland. Since then, there is continuous and remarkable development in the HVDC technology making it an efficient way of transmitting bulk power over large distances. Due to the transition towards low carbon electricity, more renewable energy sources are being integrated into the grid. This trend has increased the use of Direct Current (DC) in the electrical networks at low voltage levels.
In the past few decades, space charge phenomenon in HVDC insulation have been investigated. Many techniques have been developed for studying and understanding the space charge phenomenon in the HVDC insulation. Considering the growth of DC in electrical networks, further research into these techniques and development of novel dielectric materials suitable for DC is also progressing at a quick pace.
Earlier studies on space charge phenomenon were performed mainly on Polyethylene, Epoxy, Polymethyl methacrylate and Polycarbonate insulation materials. These studies have indicated that the space charge starts accumulating beyond a threshold value of the electric field and has a strong dependency on temperature as well as the electric field.
Silicon based insulation is also known as silicones. It is a highly stable and fire-resistant fluid. It is used in cable joints, traction transformers and increasingly in compact transformers where higher than normal temperatures are expected. However, it is known that when equipment is used with DC, they are expected to suffer strongly from space charge accumulation. As a result, considerable modifications in the electric field distribution with respect to the Laplacian field occurs, especially in case of voltage polarity inversion. This may cause insulation degradation and premature breakdown.
The conductivity behaviour and the electric field threshold for space charge accumulation are two of the most important parameters for the design of insulation systems under DC conditions. By limiting the electric field in insulation system below the threshold value, space charge accumulation can be minimised.
Reusing the existing AC cable joints under DC conditions could save considerable time as well as money in realising new DC networks. For that, conductivity behaviour and electric field threshold values of the insulation material should be investigated.
The main goal of this thesis is to investigate and characterise silicon insulation material with regards to its conductivity behaviour and the electric field threshold for space charge formation. This study is a first step towards the feasibility of using silicon fluid-based AC cable joints under DC conditions. The silicon insulation samples in liquid as well as cured(solid) state are subjected to conduction current measurements and space charge measurements using pulsed electroacoustic method.
Simulations are performed on a cable joint model in COMSOL Multiphysics. Presence of cured silicon layer results in a divergent current density. This resulted in high field concentration in cured silicon layer.