Studies On The DC Space Charge Characteristics Of Oil-Paper Insulation Materials

Along with the fast development of our domestic economy, the growth rate in electric power consumption is more than 14% in these three years. China will build 15 UHVDC transmission projects and become the country who holds the most UHVDC transmission projects, longest transmission lines, and largest transmitting capacities until 2020. The fast expanding in HVDC networks require higher secured and sTable DC power equipments. As the main insulation material of some key DC power equipments, oil-paper is very important for the operation security. The space charge behaviors of oil-paper will affect its dielectric strength, cause the formation of discharge channels and damage its insulating properties. Therefore, the research on the space charge behaviors of oil-paper is vital important for the operation security of power grid and the improvement on the physical chemical properties and electric performances of insulation materials.So, in this paper, based on the researches of space charge in polymeric materials, a systematic study on DC space charge behaviors of oil-paper insulation using the pulsed electroacoustic (PEA) method are performed by considering the practical operation conditions. The charge injection, movement and aggregation under different conditions such as different DC voltages, test temperatures, paper layers and aging degrees were analyzed. The regular patterns and mechanisms of charge injection amount, space charge mobility and charge distribution were compared. The distribution of trapping level inside oil-paper was calculated by using the PEA test results based on the mechanism of the Isothermal Decay Current. Moreover, using the AB initio molecular dynamics, the influence of water and acids molecules to the degradation of cellulose were simulated. And then, six cellulose defect models were built using Material Studio for the first time, and the trapping level information were acquired bused on the density functional theory of quantum chemistry. The main achievements are as follows,①For the first time, the regular patterns and mechanisms of charge injection, movement and aggregation under different conditions such as different DC voltages, test temperatures, paper layers and aging degrees were realized.Homo-charge injection happened in all the test conditions. The positive charges gathered in the interfaces of different layers, but the position for charge gathering was influenced by test temperatures and aging degrees. The test voltage affects the amount of charge injection and gathering, the test temperature affects the charge velocity, and thermal aging accelerates charge injection and gathering. The decay of space charge inside oil-paper presents a exponential rule.②The microstructures topography studies indicates that the surface roughness of insulation paper after thermal aging increased clearly, charged particles in the surface of paper increased, and the ability of charge collection of paper enhanced.③The exponential increase (decrease) equations of total space charge amount changes with time are acquired for the first time, y =±A ? e ( ?t/τ)+ B, which laid a foundation for the prediction and simulation of space charge in the oil-paper insulation.④After the characteristic extraction mechanism analysis, the variation laws of some significant parameters such as total charge amount, charge apparent mobility, trap level were obtained. The research results indicates that, once getting the charge decay curve by PEA and the decay time constant, the trap level distribution in the oil-paper can be acquired.⑤By using the molecular simulation method, six cellulose models with defects based on the simulation results of cellulose degradation which taking the influence of water and acids into consideration, were constructed for the first time. The trapping level information were acquired according to the density functional theory of quantum chemistry. This paper suggests that it is a good method to strengthen the protection on hydrogen bonds in the second and sixth positions of glucose monomer, in order to increase the number of deep traps and enhance the electrical insulation properties of cellulose insulation paper.