Space Charge Behaviour of Mineral Oil/Kraft Paper Insulation with Different Ageing Conditions Jian Hao 1, 2*, G Chen 2, Ruijin Liao 1 and Wei Li 1 1 University of Chongqing, Chongqing, China 2 University of Southampton, Southampton, UK Experimental Results and Discussion Introduction Figure 1: The PEA measurement equipment system Experimental Setup and Electrode System Conclusions Oil-paper insulation system is widely used in power transformers and cables. The excellent dielectric properties of oil-paper insulation play an important role in the reliable operation of power equipment. However, oil-paper insulation degrades under a combined stress of thermal (the most important factor), electrical, mechanical, and chemical stresses during routine operations, which has significant effect on dielectric properties of oil-paper insulation. It has been found that space charge in oil-paper insulation has a close relation to its electrical performance. In this paper, space charge behaviours of oil-paper insulation sample with three different ageing conditions (aged for 0, 35 and 77 days) were investigated using the pulsed electroacoustic (PEA) technique. The influence of ageing on the space charge dynamics behaviour was analysed. University of Southampton, Highfield, Southampton, SO17 1BJ, UK Contact details :  The more deterioration of the oil impregnated paper, the larger the total amount of space charge injected into the oil impregnated paper sample, and the slower dissipation rate of space charge in the oil impregnated paper sample. (b) (a) 1 copper electrode 2 semiconductor 3 oil–paper insulation sample (one layer) 4 Aluminum electrode 5 PVDF 6 PMMA 7 Aluminum electrode 8 capacitance 9 pulse source 10 DC-high voltage source 11 resistance 12 amplifier 13 Lecory WavePro 7200Aoscillograph 14 computer (a) Voltage-on 20kV/mm (b) Decay Figure 2: Space charge distribution of oil-paper insulation samples after aging for 0 day (a) Voltage-on 20kV/mm (b) Decay (a) Voltage-on 20kV/mm (b) Decay (a)Voltage-on 20kV/mm Figure 5: Relationship between the total charge amount of oil-paper insulation sample and testing time Figure 3: Space charge distribution of oil-paper insulation samples after aging for 35 days Figure 4: Space charge distribution of oil-paper insulation samples after aging for 77 days (b) Decay sample Constant  R2R2 AB Aging 0 day Aging 35 days Aging 77 days Table 1: Fitting parameters of total charge amount (y) and decay time (x) according to in the decay process  At the beginning of voltage-on, charge injection into the oil impregnated paper takes place but the rate of injection depends on ageing conditions. Positive charges accumulate in the middle of oil impregnated paper. A peak of positive charge appears initially and then subsides during voltage-on process.  The more deterioration of the oil impregnated paper, the higher the positive charge peak appears in the sample during voltage-on process.  There is homo-charge injection at both electrodes. It seems that positive charges dominate the bulk while the negative charges adjacent to the cathode.  The charge decay rate is fast at the beginning but becomes much slower with measurement time.  The charge decay rate becomes much slower in the aged oil impregnated paper.  The more deterioration of the oil impregnated paper, the slower dissipation rate of space charge in the oil impregnated paper sample.  The more deterioration of the oil impregnated paper, the larger the total amount of space charge injected into the oil impregnated paper sample. where ρ (x) is the charge density, d is the thickness of the oil impregnated paper, S is the area of the electrode.  There is an exponential function between the total amount of charge and the decay time in the decay process. The more deterioration of the oil impregnated paper, the higher the decay time constant.  : decay time constant; R 2 : fitting coefficient.