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Properties & Benefits of 3M™ Liquid-Filled Transformer Insulation

Introduction For more than a century, the standard insulation system for liquid-filled transformers has been a combination of mineral oil and cellulose-based Kraft paper. The utility of Kraft paper and mineral oil makes sense. After all, as the primary solid insulation, Kraft paper delivers a reliably high level of mechanical strength, toughness, oil compatibility, and dielectric strength for transformer manufacturers1 – all at a very competitive price. But this solution is not perfect. Its use creates issues in transformer design and operation, which result from a combination of high moisture absorption2 (~30% at saturation), auto-accelerating hydrolysis degradation in the presence of moisture, and poor thermal stability3. To improve thermal stability, Thermally Upgraded Kraft (TUK) was introduced more than 40 years ago. It offered a 15° C higher temperature rating than normal Kraft paper, but the same basic limitations exist4 with cellulose even when thermally upgraded. Another approach used meta-aramid materials, but high costs limit their use in most liquid-filled transformer applications. To optimize cost and performance, hybrid insulation materials combining meta-aramid and cellulose have been proposed to provide incremental improvements in thermal stability. But metaaramid materials also absorb a large proportion of moisture5 and like cellulose, can require a substantial amount of dry time. A clear opportunity emerged to develop new materials that can improve the performance of solid transformer insulation – and the Electrical Markets Division of 3M Company has done so with its new, inorganic-based 3M™ Liquid-Filled Transformer Insulation (LFT Insulation). By using the unique properties of 3M™ Liquid-Filled Transformer Insulation, transformer manufacturers can create new designs that reduce costs, increase lifetime, and meet utilities’ needs for higher power density devices. With an increased resistance to hydrolysis, LFT insulation from 3M remains mechanically and electrically stable at higher temperatures such as during overload conditions. More advantages of LFT insulation include: • Excellent thermal stability with temperatures 35° C higher than TUK • Substantially less moisture absorption (~5 percent at saturation), so less time and energy are required for drying • More stable dielectric properties in the presence of moisture • Good thermal conductivity

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Properties And Benefits of 3M™ Liquid-Filled Transformer Insulation

Product Description For a transformer to function properly and reach its expected lifetime, the insulation needs to resist degradation in oil while being subjected to electrical, thermal and mechanical stresses. 3M™ Liquid-Filled Transformer Insulation is a flexible insulation material made by a wet-laid paper process. It is comprised of an organic binder, short cut fibers, and inorganic filler. In fact, the paper’s inorganic nature enables many of its beneficial properties. While Kraft paper has a long history of good performance in oil-filled transformers, LFT insulation is designed to match – and in many cases exceed – its performance in key areas. Compared to Kraft paper, LFT insulation provides: • Low moisture absorption • Stable electrical properties in the presence of moisture • Increased thermal conductivity • Higher rated IEEE thermal class of 155° C • Resistance to hydrolysis • Acceptable levels of mechanical and dielectric strength LFT insulation can be produced for layer insulation applications in thicknesses of 5, 7 and 10 mils, and also is available with a standard diamond dot pattern epoxy adhesive, to bond individual layers of a transformer coil. The LFT insulation with adhesive can be formed into rigid tubes. Using a creped version of LFT insulation, flexible tubes can be fabricated for electrical leads. For core tube and window insulation applications, low density boards of 30- to 250-mil thickness are available. Higher strength, high-density boards for more structural applications are also available from 30- to 120-mil thickness.

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Mechanical Properties When it comes to toughness, the mechanical properties of 3M™ Liquid-Filled Transformer Insulation are up to the task. With its balance of properties, LFT insulation is tough enough to hold up during coil winding processes and during the operation of a transformer, while being flexible enough to conform to the shape of the wires or other insulation components making up the transformer coil. The flexibility also allows the material to be used to fabricate tubes to protect and insulate lead wires. As seen in Figure 1, the tensile strength of LFT insulation is a little less than half of Kraft paper at a similar thickness: 30 lb/in vs. 80.3 lb/in. But look closely at LFT insulation’s tear results as presented in Figure 1 below. It is more tear-resistant than Kraft paper, while maintaining flexibility. In the crossweb direction, LFT insulation has flexibility that’s similar to Kraft, and even more flexible in the machine direction. 1400

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Figure 1. Mechanical Property Comparison

Furthermore, LFT insulation retains its flexibility throughout its life in a transformer. To demonstrate this property, a series of samples were tested following thermally accelerated aging in oil. The samples were aged then wrapped around a ¼" mandrel and examined for cracks and fractures.

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Properties And Benefits of 3M™ Liquid-Filled Transformer Insulation

Un-aged samples were tested and as were samples that were aged to the point where the retained tensile strength was 49%, 36%, and 35% of the initial value. In each case the 3M™ Liquid-Filled Transformer Insulation did not break or crack. Figure 2, below, shows the performance of unaged material. Figure 3 shows a sample aged to 35% initial tensile strength. While the LFT insulation shows discoloration it remains flexible without cracking.

Electrical Properties One of the key properties of layer insulation is its level of dielectric strength when saturated with an insulating liquid. Two of the most common types are mineral oil and natural or synthetic ester oil. Figure 2

In testing, Cargill Envirotemp™ FR3™ Fluid was used as the natural ester oil. The results in Figure 4 show that infusing both 3M and Kraft solid insulation types with oil significantly increases the dielectric strength of each one. When saturated, the dielectric strength is high and similar to Kraft. The result: 3M™ Liquid-Filled Transformer Insulation does not require an increase in insulation thickness when incorporated into a transformer coil design.

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Figure 4. Dielectric Strength of Insulation Materials and Systems

To determine just how well an insulation material will perform in a transformer, dielectric loss and constant are important considerations.

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When completely dried and then saturated with mineral oil or FR3 fluid, Kraft paper and 3M™ Liquid-Filled Transformer Insulation demonstrate similar properties. The dielectric loss of Kraft at 23° C and 100° C when saturated with mineral oil was measured at 0.96 percent and 9.26 percent, respectively. For LFT insulation, it was measured at 1.51 percent and 11.13 percent, respectively. The dielectric constant for Kraft at 23° C and 100° C was measured to be 3.3 and 4.3, respectively. At the same temperatures, LFT insulation was measured to be 2.8 and 4.0. Figure 5 shows this comparison and includes the same performance when saturated with FR3 oil.

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Figure 5. Dielectric Loss and Constant Results

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Properties And Benefits of 3M™ Liquid-Filled Transformer Insulation

On the other hand, when the neat insulation materials are placed in a normal environment (23° C and 50% RH), the results are very different. The dielectric constant of Kraft than in that environment experiences a moderate increase to 4.9 when measured at 23° C and 100° C. However, the dielectric loss increases significantly to over 40% at 23° C and to 60% at 100° C. The measurements made at 100° C were taken soon after placing the sample in the oven. If left to equilibrate, it would be expected that in this environment those dielectric loss values would decrease until they approached the level seen in Figure 6 in the Vacuum-Dried section. However, in a real world transformer, with Kraft’s higher saturation concentration these values are to be expected2, as moisture enters the transformer over time and as Kraft degrades and liberates water. When 3M™ Liquid-Filled Transformer Insulation is measured in the same environment, the dielectric constant measurements of 2.8 at 23° C and 3.5 at 100° C are largely unaffected by the presence of moisture. Moreover, the dielectric loss remains at a relatively low level of 5.4 percent and 8.4 percent for the same measurement process and temperatures. These results demonstrate how LFT insulation offers more stable dielectric properties than cellulose-based Kraft paper in the presence of moisture.

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Figure 6. Dielectric Loss & Constant of Neat Insulation in Dry and Humid Environments

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Moisture Absorption Properties The effect of moisture on the dielectric properties of Kraft paper is well known.8 This highlights emphasizes the importance of removing it and maintaining very low levels of moisture in a transformer’s primary insulation. The chart below compares the moisture levels of 3M™ Liquid-Filled Transformer Insulation to Kraft when equilibrated at three levels of relative humidity: 50%, 65% and 95%. It’s surprising how much moisture Kraft absorbs at 95% relative humidity – more than a fourth of its own weight. In fact, Kraft absorbs more moisture at 50% RH than LFT insulation does at 95% RH. 30

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Figure 7. Moisture Content of Neat Insulation at Different Relative Humidity Levels

Another crucial concern is how much time and energy it takes to dry insulation before it is ready for use in a transformer. Figure 8 shows how much longer it takes to dry Kraft paper than LFT insulation under identical conditions. This test was done to simulate the conditions that the insulation would experience in a transformer. The samples were prepared by conditioning them at 95% RH for several hours. Samples were collected by cutting ten 2" X 4" pieces of 10-mil thick material and stacking them together to a height of 0.10". The stacks were then dried at 150° C and the weight loss was measured over time. The curves show that it takes about 6 minutes to dry LFT insulation to a level of