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

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IntroductionFor 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 meta-aramid 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 DescriptionFor 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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Figure 1. Mechanical Property Comparison

Mechanical PropertiesWhen 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.

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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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 PropertiesOne 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.

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.

Figure 2

Figure 3

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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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Figure 5. Dielectric Loss and Constant Results

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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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.

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

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Moisture Absorption PropertiesThe 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.

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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 <0.5 percent moisture while it takes Kraft a little longer than 23 minutes to dry to the same level. That’s almost 4 times longer to dry. Depending on the drying requirements of the transformer manufacturer and environmental conditions in the manufacturing facility, using LFT insulation can drastically reduce drying time and cost – if not eliminate it.

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Thermal PropertiesKeeping coil hotspots cool is an important part of transformer design and operation. So 3M™ Liquid-Filled Transformer Insulation also offers advantages in thermal conductivity and stability.

For starters, the thermal conductivity of LFT insulation is about 10% higher than Kraft paper (0.24 W/(m*K) vs 0.26 W/(m*K)). A side-by-side test of 75 kVA liquid-filled transformers – one made with Kraft and the other with LFT insulation – demonstrated that the temperature at the coil surface could be reduced by ~6° C when using LFT insulation.

In addition, IEEE C57.100™ describes a method used to assign a thermal class. IEEE standard C57.154™ defines the use of a higher-temperature insulation system and materials in high-temperature transformers. The use of a higher-temperature insulation system can be a major benefit to manufacturers and utilities. Doing so can extend the life expectancy of a transformer or allow more power to be transmitted through a smaller transformer.

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Figure 8. Plots Showing Material Drying Rates

Preferred insulation system thermal classes

Thermal class Hottest spot temperature °C

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The table below is from the IEEE C57.154™ high temperature transformer standard. It shows the higher thermal classes assigned for liquid-filled transformers.

The IEEE C57.12.100™ standard was followed in testing to determine which thermal class the product achieves. First, samples were thoroughly dried in an oven before being placed in mineral oil. Then stainless steel test vessels containing the test components were put under a vacuum of 28 in Hg for degassing and saturating the insulation with oil. The test vessels were nitrogen purged before sealing.

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Next, the test vessels were placed into ovens at 185° C, 195° C, 200° C, and 210° C for thermal aging. The lowest temperature was chosen to complete the testing within a year’s time. The high temperature was chosen to be below the material component’s change of state, which would categorically change the nature of degradation.

For material aged at each temperature, the time required to decrease tensile strength to 50% of initial (unaged) strength was determined. Plotting this time to “failure” for samples at each aging temperature on a logarithmic-scaled life expectancy chart determined the thermal class.

On this chart the horizontal-axis is temperature and the vertical-axis is the expected life of the material. A curve is fit to the data and the thermal class is determined by projecting the life curve to 100,000 hours. As shown Figure 9, the 3M material curve intersects the 100,000-hour line at 155° C. Testing also was performed using Kraft paper to verify the test method and results.

Life Expectancy of Insulation Materials in Mineral Oil

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Thermal class determined where life curve intersects 100,000 hr line = 155.12° C

Figure 9. Insulation Material Life Expectancy Chart

The IEEE standard C57.100™ also calls for a new life equation to be created once the high temperature aging experiment is complete.

The life equation for 3M™ Liquid-Filled Transformer Insulation is as follows:

LIFE = EXP[(21674/(273+T))-38.763]

Based on the results shown in the life expectancy chart and the life equation, a thermal class of 155° C can be assigned to LFT insulation, which is a 50° C improvement over Kraft—and a 35° C improvement over Thermally Upgraded Kraft.

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The combined mechanical, electrical, moisture-related and thermal properties of 3M™ Liquid-Filled Transformer Insulation indicate that it can be used to reduce transformer costs in several ways even though its material cost is greater than Kraft paper.

First, the cost of drying solid insulation and transformer coils prior to placing them in oil can be greatly minimized or potentially eliminated. The level of savings will depend on how much energy and time (money) is spent on drying coils in the manufacturing process and what level of reduction can be achieved. Actual savings will have to be determined on a case-by- case basis.

Second, if the conventional 65° C average temperature rise is maintained (105° C Temperature Class), at this relatively low

Summary3M™ Liquid-Filled Transformer Insulation meets the high performance requirements of high-temperature, liquid-filled distribution transformer designs with excellent thermal stability, low moisture absorption, and long-term reliability.

New standards such as IEEE C57.154™ have set requirements for high-temperature liquid-filled transformer designs that exceed the capability of conventional transformer designs using Kraft cellulose and mineral oil insulation. LFT insulation with 155° C Relative Thermal Class rating may be used in these new high-temperature transformer designs which can enable increased transformer overload capability or reduced size.

The composition of the innovative LFT insulation material from 3M enables low moisture absorption and resists degradation in the presence of moisture that is inherent to cellulose-based insulation. This can help streamline the manufacturing process with shorter drying cycle time and increase reliability once it is deployed in the grid.

3M™ Liquid-Filled Transformer Insulation opens up new transformer design possibilities and enables manufacturers to meet the stringent requirements of next generation designs.

operating temperature for LFT insulation, the reduced moisture content and reduced moisture sensitivity could extend transformer life.

Third, by designing transformers with a higher average temperature rise (155° C Temperature Class), the power rating of an existing design can be increased, or new designs leveraging this higher temperature class for individual power ratings can be created.

In either case, transformer size and potential costs (conductor, core, oil volume, tank size, elimination of cooling fins, elimination of coolant circulation system, etc.) may be reduced after balancing power rating, efficiency, and operating temperature.

Transformer manufacturers also may gain greater abilities to meet customer demands for higher-power density applications, such as when there’s a need to increase the power rating of a pole-mounted transformer while minimizing the weight or providing increased overload capability. It also applies when more power throughput is desired in environments where there are size or footprint limitations for pad-mounted or vault-installed transformers.

Transformer Design and Lifetime Impact

3M is a trademark of 3M Company. The IEEE standards cited are trademarks of the Institute of Electrical and Electronics Engineers, Inc. (IEEE). All other trademarks are property of their respective owners.

Important NoticeAll statements, technical information, and recommendations related to 3M’s products are based on information believed to be reliable, but the accuracy or completeness is not guaranteed. Before using this product, you must evaluate it and determine if it is suitable for your intended application. You assume all risks and liability associated with such use. Any statements related to the product which are not contained in 3M’s current publications, or any contrary statements contained on your purchase order shall have no force or effect unless expressly agreed upon, in writing, by an authorized officer of 3M.

Warranty; Limited Remedy; Limited Liability. This product will be free from defects in material and manufacture at time of manufacture. 3M MAKES NO OTHER WARRANTIES INCLUDING, BUT NOT LIMITED TO, ANY IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. If this product is defective within the warranty period stated above, your exclusive remedy shall be, at 3M’s option, to replace or repair the 3M product or refund the purchase price of the 3M product. Except where prohibited by law, 3M will not be liable for any direct, indirect, special, incidental or consequential loss or damage arising from this 3M product, regardless of the legal theory asserted.

References

1. Moser, H.P.; Dahinden, V. In Transformerboard II; Graphics DOK MAN:, Zurich, 1987: pp 7, 17, 82.

2. H. Bessei and R.V. Olshausen, 2nd International Conference on Power Cables and Accessories 10 kV to 180 kV, London, 1986: pg. 119.

3. Moser, H.P.; Dahinden, V. In Transformerboard II; Graphics DOK MAN. Zurich, 1987; pp 89, 142-145.

4. Katsunori Miyagi, Etsuo Oe & Naoki Yamagata "Evaluation of Aging for Thermally Upgraded Paper in Mineral Oil, Journal of International Council on Electrical Engineering, 1:2, 181-187", 2011. DOI: 10.5370/JICEE.2011.1.2.181.

5. Bahtia, A. In "Aramid papers with Improved Dimensional stability", Proceedings of Electrical Electronics Insulation Conference, Rosemont, IL, Sept 18-21, 1995; IEEE: 1995; pp 409-410.

6. Moser, H.P.; Dahinden, V. In Transformerboard II; Graphics DOK MAN:, Zurich, 1987: p 78.

7. Moser, H.P.; Dahinden, V. In Transformerboard II; Graphics DOK MAN:, Zurich, 1987: pp 82, 157-162.

8. K.H. Holle, On the Electrical Properties of Insulating Oils: The Influence of Water on their Temperature Behavior, Thesis, Technical University of Braunschweig, Germany 1970.

9. M. Beyer, 1971. Vacuum Drying of Liquid and Paper Insulations, and its Influence on the Electrical Properties, Paper VW 2192, VDI-Bildungswerk.

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