Effects of paper aged states on the space charge behavior in oil-paper insulation

Oil-paper insulation has been widely used in power transformers for quite a long time because of its good performance. The paper degrades under the combined stresses of thermal, electrical, and chemical stresses during routine operations, reducing the degree of polymerization (DP) of paper and with it, the mechanical and electrical strength. Space charge in dielectric materials has a close relation to the electrical performance of a material. Therefore, in this work, the effect of paper aged states, i.e. different DP values, on the space charge behavior in oil impregnated paper insulation is investigated and the Pulsed Electro-Acoustic (PEA) method is used for the space charge distribution measurement. In order to achieve this purpose, all paper samples are impregnated by filtered, degassed and dried new transformer oil and all PEA measurements are performed at the same temperature. The experimental results show that, when the external DC voltage is applied on the oil-paper sample, homo-charge injection occurs for all samples. The samples with different aging states exhibit quite different space charge characteristics for both voltage-on and voltage-off condition. With the aged states more serious, higher maximum charge density peak value exists in both cathode and anode vicinity for voltage-on condition. In contrary, it exhibits lower maximum charge density peak value for voltage-off condition. It also indicates that the charge injection phenomenon is more obvious in aged oil-paper sample. For voltage-off condition, aged oil-paper sample is beneficial to the space charge decay.