J. Luksich

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

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.

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.

Review of Kraft paper/natural ester fluid insulation system aging

We examine the results of recent Kraft paper/natural ester fluid insulation system sealed tube aging studies. The initial rates of cellulose degradation of Kraft and thermally upgraded Kraft in mineral oil or natural esters are similar. Subsequent degradation in natural ester occurs at a slower rate. This appears to be true for both plain and thermally upgraded Kraft. Based on early sealed tube aging results, the insulation unit life of the natural ester/thermally upgraded Kraft insulation system is estimated to be 130°C. Accelerated aging of single phase distribution transformers filled with natural ester also show a reduction using a thermally upgraded Kraft/natural ester insulation system, meeting criteria for 120°C hottest spot temperature operation.

Optimal sizing of solar energy transformers using natural ester fluid

As a result of the increasing number of nation-states that are adopting aggressive Renewable & Alternative Energy Portfolios, the solar energy market is nearly doubling year over year. With this aggressive growth comes even greater scrutiny when sizing solar farms. There is often great incentive to get the most utilization out of the equipment. An often over looked piece of equipment on these farms is the liquid filled AC padmount transformer. Given the green nature of the solar market, every Cooper Power Systems Envirotran™ Solar transformer contains the non-toxic, biodegradable Envirotemp™ FR3™ fluid, made from renewable seed oils. Equal in importance to its biodegradability, this natural ester fluid substantially extends the life of the cellulosic insulation in the transformer, while allowing periods of overloading of the transformer. The high fire point of natural ester fluid provides a safer transformer for solar applications compared to mineral oil, especially during periods of high ambient temperature and unit overloading. This work presents how using the inherent properties of natural ester fluid in conjunction with defined inverter load profile and site environmental conditions can help optimize transformers for Solar PV applications.

Accelerated aging of aramid insulation in various dielectric liquids

Aramid insulation and other transformer construction materials were aged in mineral oil, high molecular weight hydrocarbon, natural and synthetic esters, and silicone dielectric liquids. The initial aging sequence used aging temperatures of 130, 150 and 170 °C for 1000, 2000 and 5000 hours in all liquids but natural ester. A subsequent sequence used temperatures of 210, 230 and 250 °C for 500, 1000, 2000 and 5000 hours using all liquids but mineral oil. We find that the decrease in aramid tensile strength is greatest in synthetic ester and least in silicone. The aramid dielectric strength has the greatest decrease in synthetic ester and least in high molecular weight hydrocarbon and silicone. Synthetic and natural ester liquids degrade significantly at 210 °C and higher. The dielectric strength of silicone decreases significantly at 210 °C, as does its flash point at 230 °C. The synthetic ester acids were found to be highly corrosive to core steel.