Kevin Rapp

Aging of paper insulation in natural ester dielectric fluid

Aging of transformer insulation paper in natural ester (vegetable oil) dielectric fluid is compared to that in conventional transformer oil. Sealed steel aging vessels containing copper, aluminum, thermally upgraded paper, and dielectric fluid (mineral oil and natural ester) were aged at 130, 150 and 170/spl deg/C for 500, 1000, 2000 and 4000 hours. Paper degradation after aging is determined using paper tensile strength and degree of polymerization measurements. The paper in natural ester aged at slower rates than did paper in conventional transformer oil, taking 5-8 times longer to reach end-of-life. Results obtained for mineral oil samples are similar to predictions based on IEEE transformer loading guide calculations. Mechanisms for the slower aging rate are proposed.

Interaction mechanisms of natural ester dielectric fluid and Kraft paper

Sealed tube accelerated aging studies demonstrate a slower aging rate for cellulose insulation in natural (vegetable oil) ester dielectric fluid compared to the rate in conventional transformer oil. The interactions of natural ester fluid and cellulose insulation resulting in increased paper life are described by two interrelated chemical reaction mechanisms. Compared to the conventional transformer oil/Kraft paper system, the natural ester fluids greater affinity for water shifts more water from the paper into the fluid in order to maintain equilibrium. The natural ester fluid reacts via the primary mechanism of hydrolysis to consume dissolved water in the fluid, shifting further the paper/fluid equilibrium to further dry the paper and produce free fatty acids. These fatty acids serve as reactants in the secondary mechanism of transesterification to modify the cellulose structure. The change in cellulose structure is verified using infrared analysis.

Aging of Kraft paper in natural ester dielectric fluid

Kraft transformer insulation paper aged in natural ester (vegetable oil) dielectric fluid was compared to identical paper aged in conventional transformer mineral oil. Sealed steel aging tubes containing copper, aluminum, Kraft paper, and dielectric fluid (mineral oil and natural ester) were aged at 150/spl deg/C for 500, 1000, 2000, and 4000 hours. The extent of paper degradation after aging was determined using paper tensile strength, paper degree of polymerization, and furanic compounds in the aged fluid. Water contents of fluids and paper were compared. Paper aged in conventional transformer oil degraded at a significantly faster rate than in natural ester dielectric fluid. Paper in mineral oil reached three criteria for IEEE end-of-life (50% retained tensile strength, 25% retained tensile strength, and degree of polymerization of 200) within the first 1000 hours. After 4000 hours of aging, paper in natural ester did not degrade to any of the IEEE end-of-life criteria. At 4,000 hours, the paper aged in natural ester retained about 55% of the original tensile strength and a degree of polymerization of about 280. Paper aged in conventional transformer oil degraded to the same values in about 315 and 390 hours, respectively-an order of magnitude faster. The reduced paper-aging rate in natural ester is primarily attributed to the fluid maintaining the paper in a very dry state.

Aging of paper insulation retrofilled with natural ester dielectric fluid

The aging rate of transformer insulation Kraft paper is much slower in natural ester (vegetable oil) dielectric fluid than in conventional transformer oil. This study investigates the effect that replacing transformer oil with natural ester fluid (retrofilling) has on the aging rate of thermally upgraded (65/spl deg/C rise) paper initially aged in transformer oil. Sealed steel aging vessels containing copper, aluminum, dried thermally upgraded Kraft paper, and dielectric fluid (transformer oil or natural ester) were aged at 160 and 170/spl deg/C for 250, 500, 750, 1000, 1500, and 3000 hours. Half of the transformer oil systems were retrofilled with natural ester fluid after initial aging times of 750 and 250 hours at 160 and 170/spl deg/C, respectively. Paper degradation after aging is determined using paper tensile strength and degree of polymerization measurements. After replacing the transformer oil with natural ester, the aging rate of the paper initially aged in transformer oil showed an abrupt change to the reduced aging rate for paper in a natural ester.

Natural Ester Dielectric Fluid Development

Since the early 1980s, Cooper Power Systems has been actively involved in exploring and developing ester-based dielectric fluids. Introduced in 1984, our first commercialized ester was a synthetic polyol ester, developed primarily as an environmentally acceptable Askarel substitute. Although its technical performance is very good, the cost is prohibitive for most applications. The desirable properties of the polyol ester spurred exploration into other, more cost-effective, ester chemistries. This led to the evaluation of a natural (vegetable oil) ester dielectric coolant having many of the same performance advantages of synthetic esters, but much more economical. The major disadvantages of the natural esters are their inherent susceptibility to oxidation and higher pour point. We undertook a massive natural ester research and development program beginning in the early 1990s. Significant improvement low temperature flow was achieved. Oxidation inhibitors together with proper method-of-use overcome the oxidation stability issues. In many ways the natural esters perform better than the less-flammable fluids they replace, and offer significant advantages for applications where naphthenic mineral oils are traditionally applied. Although initially developed for distribution transformers, application in medium and large power transformers is becoming more common. This paper summarizes our laboratory and field experience

Lightning impulse testing of natural ester fluid gaps and insulation interfaces

A significant amount of lightning impulse breakdown and withstand testing has been accomplished in mineral oil to understand this important electrical characteristic for transformer insulation system design. To be considered as viable insulating fluids for high voltage equipment, alternative dielectric liquids should have similar lightning impulse characteristics as compared to mineral oil to provide the clearances necessary for common dielectric design. This paper reviews the testing for establishing the lightning impulse breakdown characteristics of natural ester fluids relative to mineral oil test results. The key variables of the testing included: various oil gap and solid insulation creep distances, electrode configurations, electrical stress characteristics, and solid insulation surfaces. The fluid gaps ranged from 3 mm to 55 mm. The electrode configurations included quasi-uniform in oil gaps, with some attached to pressboard, and nonhomogeneous contacts in oil gaps combined with a phenolic interface. The range in gap distance was selected to reflect the range commonly used in liquid insulated transformer core/coil designs. The electrical stresses used included 1.2 × 50 ¿s lightning impulses of both positive and negative polarity. The solid insulation materials consisted of high density pressboard, Kraft paper and a high density phenolic composite. It is concluded that the impulse breakdown voltage of the natural ester fluid is similar to mineral oil for the oil gaps and electrode configurations tested.

Behavior of ester dielectric fluids near the pour point

The pour points of dielectric fluids based on natural esters are not reliable indicators of their fluidity at cold temperatures. A 25-day isothermal test was used to determine the time to solidification of several natural ester fluids and a synthetic ester at -15/spl deg/C. The results are compared to ASTM D97 pour point measurements. Natural ester dielectric fluid properties measured before and after a 120 hour freeze/thaw cycle showed no changes occurred due to solidification. Finally, the performance of both a solidified natural ester and a high fire point high molecular weight hydrocarbon was compared to that of conventional mineral oil by energizing at full rated load transformers held at an ambient temperature of -30/spl deg/C.

Long gap breakdown of natural ester fluid

Dielectric breakdown values were measured in 50– 150 mm gaps in natural ester fluid and mineral oil. The tests included 50/60 Hz and 1.2×50 µs lightning impulse using negative and positive polarities. The voltage levels attained by natural ester fluid compare closely with mineral oil and would support the use of natural ester fluid in transformers through power insulation classes.

Natural ester dielectric fluid development update

Since the early 1980s, Cooper Power Systems has been actively involved in exploring and developing ester-based dielectric fluids. Introduced in 1984, our first commercialized ester was a synthetic polyol ester, developed primarily as an environmentally acceptable Askarel substitute. Although its technical performance is very good, the cost is prohibitive for most applications. The desirable properties of the polyol ester spurred exploration into other, more cost-effective, ester chemistries. This led to the evaluation of a natural (vegetable oil) ester dielectric coolant having many of the same performance advantages of synthetic esters, but much more economical. The major disadvantages of the natural esters are their inherent susceptibility to oxidation and higher pour point. We undertook a massive natural ester research and development program beginning in the early 1990s. Significant improvement low temperature flow was achieved. Oxidation inhibitors together with proper method-of-use overcome the oxidation stability issues. In many ways the natural esters perform better than the less-flammable fluids they replace, and offer significant advantages for applications where naphthenic mineral oils are traditionally applied. Although initially developed for distribution transformers, application in medium and large power transformers is becoming more common. This paper summarizes our laboratory and field experience and is an update of work presented in 2006.

Transformer insulation dry out as a result of retrofilling with natural ester fluid

Transformer cellulose insulation generates moisture as it degrades, primarily dependent on temperature. As moisture levels increase in paper/liquid systems, dielectric performance decreases. Moisture in cellulose insulation combined with higher temperatures accelerates the cellulose aging process. Multiple accelerated life tests in the lab demonstrated that the aging rate of cellulose can be significantly slowed when impregnated with natural ester fluid compared to mineral oil. The reduction and suppression of moisture buildup in cellulose is the primary reason for the reduced aging rate. This paper reviews chemical mechanisms involved based on laboratory studies and field data from fourteen power transformers retrofilled with natural ester fluid. The data provides support that insulation dry out can result from moisture migration and hydrolysis after retrofilling transformers with natural ester fluid. The concepts of moisture migration, hydrolysis and dry out are beneficial for new transformers as well and should work to maintain like-new dry insulation conditions.