Aging Effect on Characteristics of Oil Impregnated Insulation Paper for Power Transformers

This paper deals with experimental investigations of the electrical and physical properties of oil impregnated insulation paper for power transformers at different temperatures. The ac breakdown voltage, tensile strength and water content of insulation papers impregnated in mineral oil for different time periods were investigated. The effect of insulation paper thickness on the electrical and mechanical properties has also been studied. The results showed that the breakdown voltage and the tensile strength decreased with increasing the time of immersion of insulation paper in oil at room temperature, at 5℃ and at -12℃. Also, the thermal aging effect on the characteristic of insulation paper has been studied. It was found that high temperatures affect the breakdown voltage and the tensile strength to a great extent.

• Insulation Paper; Power Transformers; Aging; Breakdown Voltage; Tensile Strength; Water Content

1. Introduction

Transformers play an important role in providing a reliable and efficient electricity supply and represent the most critical equipments in electric power transmission and distribution systems [1,2]. In the early days, only a few insulation materials were known for low temperature superconducting power applications [3]. Paper was one of the first insulation materials used in high voltage technology [4]. Since winding techniques were common in the cable industry, kraft paper was used in transformers and high capacity cables. The thermal contraction of paper is low enough and elastic elongation at low temperatures is reasonable without leading to mechanical problems. Moreover, paper is very cheap compared to plastic tapes. Recently, other materials have been used in prototypes specifically for large scale high temperature superconductivity (HTS) applications such as transformers [5, 6]. However, paper can still be used in HTS transformers [7].

Oil filled power transformers have a composite liquidsolid insulation structure, where the oil-impregnated papers are used for both mechanical and electrical purposes. Oil impregnated papers gradually age due to thermal stress, moisture and acidity; and their degradation affects the lifetime of the power transformers [8,9].

In this study, the aim is to examine how aging affects the mechanical, electrical and physical characteristics of oil impregnated papers. The study presents experimental results of oil impregnated paper breakdown strength under ac (50 Hz) voltage. Also, physical and mechanical measurements have been investigated namely: moisture content and tensile strength.

2. Experimental

Mineral oil impregnated insulation kraft papers with various thicknesses (0.2, 0.3 and 0.4 mm) have been examined in this study. Two different sets were carried out. In the first set, samples were impregnated in mineral oil for 21, 35, 56 and 77 days at temperatures: 23˚C (Room Temp.), 5˚C and −12˚C. In the second set, the samples were subjected to thermal aging conditions by using an oven. The testing temperatures were controlled and ranged from 110˚C to 170˚C. Three different thermal aging intervals have been selected (48, 72 and 96 hours). Analysis of electrical, mechanical and physical properties of oil impregnated papers was carried out.

2.1. AC Breakdown Voltage of Oil Impregnated Paper

The ac breakdown voltage was measured according to ASTM D 149 using two electrodes which were 25 mm diameter brass cylinder types. A single phase auto transformer having a 5 kVA—100 kV—50 Hz, Terco type HV 9105, has been used. The oil impregnated paper was cut into circular samples with a diameter of 42 mm. For each test, five oil impregnated paper samples were testedand the average value was calculated. The voltage was applied at a rise rate of 2 kV/s until breakdown occurs.

2.2. Water Content of Oil Impregnated Paper [ 10 ]

An accurate weight of the air dried sample (W₂) is heated at 105˚C till constant weight and then reweighed (W₁). The moisture content was calculated using the formula: % Water content = (W₂ – W₁) × 100/W₁.

2.3. Tensile Strength of Oil Impregnated Paper

It is estimated according to the German Standard Method [11] on LLOYD LR 10K instrument. A paper strip of 15 mm wide and 15 cm length was clamped at each end between the two jaws, and an increasing load was applied causing the clamps to move apart until the strip breaks at a certain force. Tensile Strength = Force (N)/Paper thickness × width (mm²).

3. Results and Discussion

In the last decades, dielectric testing techniques are being used and investigated as potential tools for condition assessment of oil-paper insulation. From fields and laboratory investigations, these techniques were found to be highly operating conditions (water content, aging, temperature, etc.) dependent.

3.1. Effect of Low Temperatures on Properties of Oil Impregnated Papers

3.1.1. Effect on AC Breakdown Voltage

The experimental results of ac breakdown voltage for different thicknesses (0.2, 0.3 and 0.4 mm) of oil impregnated papers immersed at various temperatures (RT, 5˚C and −12˚C) up to 77 days are shown in Figures 1-3.

Figure 1 represents the relationship between time of immersing insulation papers (21, 35, 56 and 77 days) with thickness 0.2 mm and breakdown voltages at 23˚C (RT), 5˚C and −12˚C, respectively.

It can be seen from this figure that, as the time of immersed papers in oil increased, the breakdown voltage decreased for all tested temperatures. The value of breakdown of the blank insulation paper is 13.11 kV. The breakdown voltages were 12.14, 11.92, 11.41 and 11.26 kV for times 21, 35, 56 and 77 days, respectively, at room temperature (RT) 23˚C. By decreasing the temperature to 5˚C, the breakdown voltages were 12.31, 12.26, 11.84 and 11.69 kV for times 21, 35, 56 and 77 days, respectively; whereas at −12˚C, the breakdown voltages were 12.64, 12.51, 12.25 and 11.98 kV for the same time intervals.

Figure 2 illustrates the same relationship for impregnating paper having 0.3 mm thickness. The results took the same trend as for samples having 0.2 mm, but with

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