SummaryAs previously announced, DuPont has been conducting long-term thermal aging studies of new Nomex® 910 engineered cellulose insulation. These studies are based on the recently revised IEEE C57.100™-2011, the “IEEE Standard Test Procedure for Thermal Evaluation of Insulation Systems for Liquid Immersed Power and Distribution Transformers.”

After more than 10 months of aging work, DuPont has established that the thermal performance of new Nomex® 910 insulation is at least 10°C better than the industry standard thermally upgraded kraft paper (TUK) in mineral oil systems.

These studies also indicate that new Nomex® 910 insulation may provide an additional 10°C performance enhancement in natural ester liquids. Based on Table 1 of the new IEEE C57.154™-2012, this performance enhancement translates to a system Thermal Class of 130 in mineral oil and 140 in natural ester liquids.

BackgroundResearchers at DuPont have developed a unique technology that combines Nomex® aramid ingredients with cellulose, using the superior thermal capability of Nomex® to improve the thermal capability of the combined product. Because the majority of the insulation is made from cellulose, the thermal improvement comes at a small increase in the price of the insulation, providing users with an attractive cost/performance ratio.

The introduction and final issuance of the two new IEEE standards noted above have helped lead the way to a new portfolio of insulation materials for the liquid transformer industry. Nomex® 910 insulation is the first of those to be introduced by DuPont.

In what may be the first published report of aging results based on the new IEEE C57.100™-2011, an extensive evaluation was conducted of the industry standard system and new candidate systems using Nomex® 910 insulation. These reported results are based on sealed tube aging of

more than 100 aging cells, involving at least six different system combinations, including systems with mineral oil and biodegradable liquids.

Due to the unpredictability of the thermal capability of this new material, DuPont conducted aging tests at seven different temperatures vs. the minimum three temperatures required by the standard.

Sealed tube aging of the industry standard system of TUK and mineral oil at three temperatures resulted in an end-of-life target of approximately 16.3% of the original tensile strength. This system has an industry accepted thermal index of 110°C when projected to 180,000 hours and 120°C when projected to 65,000 hours.

Using this base point, the individual cell combinations were aged from four to six timeframes, with failure points beyond the base point. The data were then interpolated for each system temperature to arrive at the end-of-life times at the 16.3% target. Once these data were obtained, the life (Arrhenius) curve was then plotted with end-of-life vs. temperature. These curves were then projected to 180,000 hours to arrive at the thermal index of each system.

ConclusionsThe Arrhenius curve for the Nomex® 910 insulation and mineral oil system is shown in Figure 1. The curve projects to a thermal index of 120°C at 180,000 hours for a thermal class of 130°C.

1/Temp (1/K)

DuPont™ Nomex® 910 in Mineral Oil

0

2

4

6

10

8

0.0021 0.00230.00215 0.0022 0.00225 0.00235 0.0024

y = 16980x - 31.073R2 = 0.94545

Ln L

ife (h

ours

)

Figure 1.

DuPont™ Nomex® 910 Engineered Cellulose Insulation Thermal Aging Analysis

of Nomex® 910 insulation were at least as good as those of the 7-mil (0.18-mm) thickness of Nomex® 910 insulation paper previously tested in the distribution transformer ratio. The interpolated time to 16.3% tensile strength at 180°C was nearly 50% higher (611 vs. 414 hours) in this aging experiment (see Figure 3). Based on these results, DuPont believes this shows at least equivalent performance.

Future StudiesAdditional thermal aging analysis of Nomex® 910 insulation in several biodegradable liquids is in progress. Due to the superior thermal capability of Nomex® 910 insulation and the enhancement of the liquids over mineral oil, the end-of-life of these systems has far exceeded DuPont projections when the aging studies began. As a result, more testing at higher temperatures will be required to determine the thermal index in a reasonable amount of time.

An example of this is shown in the results from testing Nomex® 910 insulation in FR3® natural ester liquid. The calculated life of Nomex® 910 insulation in FR3® at 195°C was 1,120 hours, which is more than five times the life in mineral oil. In addition to aging at 195°C, DuPont also conducted aging at 180°C and 165°C. In both of these cases, the end-of-life target of 16.3% was not reached, even after more than twice the life of Nomex® 910 insulation in mineral oil.

Assuming the same Arrhenius slope, and considering the doubling factor of material life in 7°C increments, this initial data suggests that the thermal index of Nomex® 910 insulation is at least 10°C greater in FR3® than in mineral oil, and probably much more. This results in at least a 20°C increase over the standard system of TUK in mineral oil.

For a comparison of these results to those for the industry-proven system (control), refer to Figure 2.

This initial study was based on the distribution system material ratios found in Table B.1 of the IEEE C57.100™-2011. A 7-mil (0.18-mm) thickness of Nomex® 910 insulation was used in this study because this thickness is representative of a common layer insulation used in the distribution industry.

To verify these results with a 3-mil (0.08-mm) thickness of Nomex® 910 insulation (a thickness that would be more commonly used as wire wrap insulation in power transformers), DuPont performed a single-point aging test at 180°C, using the sealed tube material ratios representative of a power system.

Despite the fact that more solid material is in the same volume of liquid, the aging results with the 3-mil thickness

1/Temp (1/K)

Nomex® 910 Industry-Proven System

DuPont™ Nomex® 910 in Mineral Oil

Ln L

ife (h

ours

)

4

6

5

7

9

8

10

0.0021 0.00230.00215 0.0022 0.00225 0.00235 0.0024

y = 14993x - 27.048

y = 16980x - 31.073R2 = 0.94545

Figure 2.

Sealed tube materials ratios

Transformer Type

Material Power Distribution

Insulating Liquid 200 cm3 200 cm3

0.05 to 0.10 mm conductor insulation

6.4 cm3

0.13 to 0.38 mm layer insulation

11.2 cm3

1.00 to 3.00 mm low-density pressboard

1.2 cm3

2.00 to 8.00 mm high-density pressboard

16.4 cm3

Ratio–liquid to solid 8.8 to 1 16.3 to 1

HoursPower - 3 mil Dist - 7 mil

Perc

ent T

ensi

le S

tren

gth

0

50

100

150

0 400100 200 300 500 600 700 800 900

DuPont™ Nomex® 910 in Mineral Oil - 180˚ C

Figure 3.

Reprinted with permission IEEE, Copyright 2011. All rights reserved.

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Product safety information is available upon request. This information corresponds to our current knowledge on the subject. It is offered solely to provide possible suggestions for your own experimentation. It is not intended, however, to substitute for any testing you may need to conduct to determine for yourself the suitability of our products for your particular purposes. This information may be subject to revision as new knowledge and experience become available. Since we cannot anticipate all variations in actual end-use conditions, DuPont makes no warranties and assumes no liability whatsoever in connection with any use of this information. Nothing in this publication is to be considered as a license to operate under or a recommendation to infringe upon any trademark or patent right.

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