1. Influence of Ageing and Temperature on the Polarization/Depolarization Current Behaviour of Paper Immersed in Natural Ester Jian Hao 1, 2*, Ruijin Liao 1, Zhiqin Ma 1 and G Chen 2 1 University of Chongqing, Chongqing, China 2 University of Southampton, Southampton, UK Experimental Results and Discussion Introduction Aging Experiment and PDC Test Conclusions Transformers play an important role in providing a reliable and efficient electricity supply and are one of the most critical equipments in electric power transmission and distribution systems. The most commonly used oil in power transformers is mineral oil due to its low cost and good properties. However the performance of mineral oil starts to be limited due to environmental consideration. Natural ester insulating fluid offers fire safety, environment and insulation ageing advantages over mineral oil and is found to be suitable for the use in transformer insulation system. Transformer owners require to assess the status of the cellulose insulation in transformer non-destructively. Polarization/depolarization Current (PDC) measurement is one of the non-destructive techniques which have been used to achieve this aim. However, at the present, there are fewer publications about the PDC behaviour of natural ester-paper insulation, though the natural ester becomes more widely used in transformers. In this paper, the influence of temperature and ageing on the PDC behaviour of the paper immersed in natural ester and mineral oil were compared. jh210v@ecs.soton.ac.uk University of Southampton, Highfield, Southampton, SO17 1BJ, UK Contact details : Ageing and temperature have similar influence on the PDC behaviour of the paper wrapped with copper immersed in natural ester and in mineral oil. The paper wrapped with copper immersed in natural ester has higher polarization current value and depolarization current value than that immersed in mineral oil at the same test temperature. After aging the same length, the paper wrapped with copper aged in natural ester has higher polarization current value and depolarization current value than that aged in mineral oil at the initial seconds of PDC test. (a) mineral oil-paper insulation (b) natural ester-paper insulation Figure 4:Temperature effect on the polarization current of paper wrapped copper immersed in the new mineral oil and new natural ester Figure 1: Oil-paper insulation sample aging (a) Sample model for PDC test under 200V (b) PDC principle Figure 2: PDC measurement Figure 5:Temperature effect on the depolarization current of paper wrapped copper immersed in the new mineral oil and new natural ester (a) mineral oil-paper insulation (b) natural ester-paper insulation Figure 7: Ageing effect on the depolarization current of paper wrapped copper immersed in the mineral oil and natural ester (a) mineral oil-paper insulation (b) natural ester-paper insulation Figure 6: Aging effect on the polarization current of paper wrapped copper immersed in the mineral oil and natural ester (a) mineral oil-paper insulation (b) natural ester-paper insulation Figure 3: Principle of polarization and depolarization current C o : geometrical capacitance of the test object, U c : the step voltage (charging voltage), σ : the dc conductivity of the dielectric material, ε o : 8.852*10 -12 As/Vm is the vacuum permittivity, f(t) : the response function of the dielectric material. The moisture content of new mineral oil is 13ppm, and the new natural ester is 42ppm. At the initial aging process, the moisture content of paper wrapped with copper immersed in mineral oil and natural ester has the same value, i.e. 2%. Figure 6 and Figure 7 show that PDC current magnitudes increase with increasing aging time. After ageing the same length, the paper wrapped with copper aged in natural ester has higher polarization current value and depolarization current value than that aged in mineral oil. The moisture contents of new mineral oil and the new natural ester are 13ppm and 42ppm respectively. The moisture content of paper wrapped with copper immersed in mineral oil and natural ester are all 1.5%. Figure 4 and Figure 5 show that PDC current magnitudes increase with increasing temperature. At the beginning, the paper wrapped with copper immersed in natural ester has higher polarization current value and depolarization current value than that immersed in mineral oil at the same test temperature.