Proc. of Int. Conf. on Advances in Electrical & Electronics 2010
*
Characterization of Performance of Paper-Oil Insulation System of Power Transformers in Presence of Copper Corrosion
Faheem Ahmed Khan1, J. Sundara Rajan2 Senior Member IEEE and 1
**
Mohd. Z. A. Ansari1, Member IEEE
Dept of Electrical and Electronics Engineering, Ghousia College of Engineering Ramanagaram, Karnataka, India *
2
faheem_khanb@rediffmail.com, ** zahedansari@gmail.com
R & D Management Division Central Power Research Institute Bangalore, Karnataka, India sundar@cpri.in Abstract— Corrosion of copper conductors due to sulpher in transformer oil is a serious problem affecting power and converter transformers. Though the reactive and corrosive sulpher components are of very low order in fresh transformer oils, they pose a serious problem as the transformers age. These reactive and corrosive sulpher components react with copper forming copper sulphide, which migrates from conductor surface to outer layers of paper. Since copper sulphide is conductive, it affects the voltage distribution of insulation. Experimental study has been carried out to investigate the role of copper sulphides in leakage current and breakdown of paper insulation.
filled in test cell such that the electrodes are fully immersed. One end of electrode is connected to high voltage (HV) and the other electrode is grounded. The electrodes are movable such that the gap distance between them can be varied. A variable ac voltage is applied to the electrodes through a motorized auto transformer and stepup HV transformer. A high precision ammeter was used to measure the leakage current. A. Preparation of insulation paper for test and method A set of paper samples of size 8cm x 12 cm and thickness 50 μm were prepared and thickness was ascertained to be 50 +/- 5 μm. These samples were dried in a hot air circulating oven at 80°C for 6 hours to remove moisture. The method adopted for breakdown measurements are:
Keywords- Surface discharge, transformer oil, copper corrosion, voltage distribution.
I.
INTRODUCTION
Transformers are vital parts in power systems. Analysis of failed transformers has shown that presence of high sulpher content in transformer oil has played a major role in the failure of power and converter transformers all over the world [1]. These failures are not confined to a particular weather condition. Fresh oil will have very low quantity of mercaptan and total sulpher components but the values change very rapidly during service conditions. This is influenced by various parameters that come into play during normal life of transformers [2–6]. High temperature is recognized as the main driving force but it is not always a necessary condition for failure to occur. The present experimental work carried out deals with understanding of (i) Reaction of corrosive oil with paper (ii) Formation of copper sulphide (Cu2S), (iii) Migration of Cu2S to the inner most layer of paper insulation and its influence on dielectric strength and (iv) Correlation of these factors to breakdown of transformer. II.
i. ii.
iii.
B. Simulation of Cu2S formation in oil in the laboratory 2 grams of Cu2S is added to one litre of fresh transformer oil and stirred gently so that Cu2S is mixed uniformly. This was done to simulate the problem of Cu2S in the laboratory. Similarly, concentration of Cu2S is increased to 4 g /litre, 6 g /litre, 8 g/litre, 10 g /litre, 12 g/litre and 14 g/litre and the above tests are repeated. C. Insulation Paper with different quantity of copper sulphide In this method Cu2S is applied to different layers of paper insulation simulating Cu2S formation and migration into first, second, third, fourth and inner most paper layers.
EXPERIMENTAL METHOD
Two brass hemispherical electrodes of 25 mm diameter, fixed on either sides of an acrylic cell are used for measurements. Half a litre of fresh and virgin oil is 81 © 2010 ACEEE DOI: 02.AEE.2010.01.79
Breakdown voltage was determined after recording Voltage-Current characteristics for fresh and virgin oil. Different amount of Cu2S was added to fresh and virgin oil and breakdown voltage was determined after establishing voltage-leakage current characteristics. Different quantity of Cu2S was applied to insulating paper and its effects on breakdown voltage and leakage current were determined.