Field experiences with the world s l argest natural e ster-filled transformer EVENTS LIQUID INSULATION ABSTRACT KEYWORDS

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1 EVENTS LIQUID INSULATION ABSTRACT Electric utilities have a duty to supply electricity securely whilst being mindful of environmental needs and fire safety, especially in urban areas. Mineral oil in power transformers is the most critical conventional material used. Substituting mineral oil with bio-degradable and renewable liquid such as natural ester, provides a much higher level of safety and sustainability. This article presents monitoring results for the world s largest natural ester-filled transformer of advanced design. The results show that thus far, over nearly three years in service, the transformer has been operating perfectly, and that there are absolutely no concerns with the operation of such transformers. KEYWORDS natural ester, Envirotemp FR3, large power transformer, field results Field experiences with the world s l argest natural e ster-filled transformer TRANSFORMERS MAGAZINE Volume 3, Issue 3

2 ADVERTORIAL Regulations require from electric utilities to secure electricity supply while maximizing health and environmental safety, particularly in areas of high population density Photo courtesy of TransnetBW GmbH Envirotemp FR3 fluid monitoring results 1. Introduction Today many regulations are in place demanding from electric utility companies to consider a lot more than just security of supply when expanding the grid. While operators need to ensure technical performance capability, concerns about issues of personal safety and environmental protection are of no less importance. To be able to respond to these requirements, utilities need innovative technologies to equip themselves for the energy supply market of the future [1]. Sustainable, bio-degradable and renewable these concepts are the driving force behind the power industry today, with an impact not only on the method of energy generation, but also on the components used. Sparing use of resources and employment of renewable raw materials in place of fossil fuels wherever possible has become a must. 2. Power transformers for new requirements For power transformers, which are conventional pieces of electric equipment built according to the physics concepts and with materials developed decades ago, it is critical to implement new materials based on renewable resources to achieve sustainability, reliability, and safety. Substituting mineral oil with bio-degradable and renewable liquid such as natural ester, provides a much higher level of safety and sustainability 37

3 LIQUID INSULATION Photo courtesy of SIEMENS Photo courtesy of TransnetBW GmbH Figure 1. SIEMENS Envirotemp FR3 -fluid-filled large power transformer at dispatch from the factory (left); being commissioned at the Another concept gaining importance is the development and installation of electrical equipment able to provide a high l evel of health and environmental safety and reliability, particularly in regions of great infrastructure density where reducing the risk level has become a major goal. Insulating oil is the most critical conventional material used in power transformers. As a consequence, the need for achieving a higher level of sustainability and environmental safety has placed f ocus on natural esters as a substitute for transformer oil. In relation to the increased biodegradability and sustainability requirements, natural ester based fluids are among the most important and widely used alternative liquids. Table I. Specific data of 420 kv natural ester transformer Rated voltage Rated power Voltage ratio Cooling type Liquid Insulation level Liquid weight The world s largest power transformer filled with natural ester Envirotemp FR3 fluid, manufactured by SIEMENS, has been operating at the highest voltage level in the Germany s grid for almost three years 3. Natural ester boosts transformer performance Natural esters have been used in transformers for decades, predominantly in distribution transformers, and small and medium power transformers. However, their usage in large power high-voltage transformers has 420 kv 300 MVA 405 ± 11%/115/22 kv KDAF/KNAN Natural ester (Envirotemp FR3 fluid) 630 kv AC/1425 kv LI/1050 kv SI ~95 tons been limited up to 230 kv until recently. In April 2013, Siemens successfully tested the world s largest power transformer filled with natural ester E nvirotemp FR3 fluid, developed and built at their p ower transformer factory in Nuremberg, G ermany. This transformer is owned and operated by TransnetBW, one of the four transmission network operators in G ermany. Several articles have been published thus far on the various aspects of the transformer using FR3 fluid, comparing the properties of natural ester and mineral oil [1], describing design aspects and first service results [3], discussing advantages of using natural esters as insulating liquid in power transformers [4], and presenting aspects of electrical, thermal and mechanical design, as well as manufactur ing, testing, 38 TRANSFORMERS MAGAZINE Volume 3, Issue 3

4 Photo courtesy of TransnetBW GmbH substation (centre); and in operation (right) Over the past two and a half years in service, the transformer has undergone many tests and measurements, including gas analysis, water content, breakdown voltage and tan delta analyses of natural ester samples, and more transportation and operation of the FR3- filled transformer [5]. unit will have a longer expected life span than a similar mineral oil unit. Designed to operate at the highest voltage level in Germany s power grid, the transformer s rated voltage was specified to 380 kv, and its insulation levels meet the standard IEC The transformer also boasts a somewhat special mechanical design, using sealing equipment materials based on fluororubber, and components which were all qualified for operating in natural ester liquid. Specially designed bushings for medium voltage and wye connection 3.1. New design The world s largest transformer filled with Envirotemp FR3 fluid is rated at 420 kv, uses new design criteria, and is the largest unit using a renewable resource liquid. The rated power of this transform er is 300 MVA, with an overload condition of up to 400 MVA [2]. Its cooling performance satisfies the l imits prescribed by the standard IEC In regard to the cooling type, the transform er was designed to use forced cooling for the inner and outer cooling circuit, but also to operate up to 180 MVA load with non-forced cooling. In this design, the esters are not supposed to experience limiting top oil temperatures, so this Figure 2. Concentrations of gases dissolved in natural ester 39

5 LIQUID INSULATION The water content, dielectric strength and tan delta indicate this is a well prepared and perfectly normally operating transformer - the DGA results back that up Figure 3. Water content in natural ester samples in operation. Notably, the presence of ethan e increased over the two and a half years in service. In comparison to the transform ers using mineral oil, ethane is generated by different natural decomposition processes taking place inside the transform er with an insulation system which is a combination of natural ester and cellulose. These differences between the two insulation systems are based on the chemical structure of the various liquids, and they are explained in detail in the IEEE DGA guide [6]. Another distinction lies in a different solubility of gases in mineral oil and natural esters. Consequently, many, but not all, normally operating transformers filled with FR3 fluid have a higher ethane content than their mineral oil counterparts. Other hydro carbon gases remained low [7]. The analysis of the moisture in the transformer oil (Figure 3) was based on the oil samples which were taken from the transformer at quarterly intervals in o rder to evaluate the transformer condition. were type tested in the Research and Test ing L aboratory of the transformer factory in Nuremberg. Cable joints with specially designed coupling capacitors were used for the low voltage connection. These capacitors were also type tested in the transformer factory in Nuremberg. 4. Experiences from the field At the time of the transformer manufacture it was agreed that monitoring of the ester-filled EHV transformer will be jointly performed by the customer and the manufacturer in the coming months and years. Now, we realize that this monitoring has yielded valuable results which contribute to the knowledge of e ster liquid behaviour Monitoring results Over the past two and a half years in service, the transformer has undergone many tests and measurements. These included gas analysis, as well as water content, breakdown voltage and tan delt a analyses of natural ester samples, and more. Dissolved gas analysis (Figure 2) was used to reveal the types of gas produced inside the transformer during its time Breakdown voltage and tan delta of natural ester samples were also regularly measured and showed no significant changes with respect to the required v alues (Figure 4). Other parameters, such as the density of natural ester sample (Figure 5) and oil viscosity, were also measured to further check the condition of the transformer. While some of these measurements are done on every third sample, all of thes e Although there have been other similar power transformers developed and put into operation since the time of the transformer manufacture, this is still the world s largest power transformer of its kind. Using the monitoring data collected over time, this article brings an overview of the behaviour of the transformer during operation. Figure 4. Breakdown voltage and tan delta of natural ester samples 40 TRANSFORMERS MAGAZINE Volume 3, Issue 3

6 All results collected from SIEMENS world s largest transformer filled with Envirotemp FR3 fluid show that so far there have been absolutely no concerns with the operation of such transformer serving not only as an alternative to mineral oil now, but also as its replacement in the future. With this in view, applying these solutions now is becoming increas ingly important in order to gain ex perience with the alternative liquids that seem to be a perfectly suitable replace ment for mineral oil. parameters help us establish that the transformer is in good condition and that the condition of the natural ester itself has no influence on its performance. The performed measurements and their results show that there have been no abnormalities in the operation of the transformer and that its performance has been perfect and according to all standard expectations Other measurements PD measurements and offline insulation resistance measurements were performed before the transformer was put in operation, while further tests and electrical measurements will be conducted in the upcoming months. As there has been no outage of the transformer for a longer period, and with all other measurements indicating a perfect condition of the transformer, there was no need for these tests before. It is important to closely monitor the behaviour of these transformers now to help us understand their long-term behaviour and provide valuable references and experience for the future, and our findings contribute to this. At the moment, IEC is drafting a standard for inservice natural esters, and this will provide an additional relevant reference. Conclusion In the early days, there were certain concerns about building a large power transformer with the use of natural ester fluid s. However, all of the findings col lected from monitoring data of the world s largest transformer of this type demonstrate that this is a transformer with a perfectly normal behaviour, causing absolutely no concern and operating according to all expectations and standards prescribed by the IEEE reference guide. Any differences in the values are acceptable according to the IEEE guide. The water content, dielectric strength and tan delta results indicate that this is a well prepared and a perfectly normally operating transformer and the DGA results support this. All these findings suggest that this solution is the future for power transformers, Bibliography: [1] Ronny Fritsche, Transformers: Natural Esters Replace Mineral Oil - Green er than ever, Electrical Monitor, April 2015 [2] Ronny Fritsche, Uwe Rimmele, Frank Trautmann, Michael Schäfer, Prototype 420 kv Power Transformer Using Natural Ester Dielectric Fluid [3] Ronny Fritsche, Ivanka Atanasova- Höhlein, Karsten Loppach, Frank Trautmann, Uwe Rimmele, Michael Schäfer, Gernot Adamietz, Large Power Transformers using Natural Insulation Liquids aspects of design and first service results [4] Ronny Fritsche, Nachhaltige Elektrische Isolierung Dank Pflanzenölen, A utomatisierungs- & Elektrotechnik, 2015 [5] Ronny Fritsche, Großtransformatoren mit natürlichen Isoliermitteln, VDE Diagnostik Tagung; November 2014 [6] C IEEE Guide for Interpretation of Gases Generated in Natural Ester and Synthetic Ester-Immersed Transformers, IEEE; 2014 [7] R2070_FR3_Dissolved_Gas_Guide v5 August 2006 Contact Sabine Bowers Business Development Manager Envirotemp Dielectric Ester Fluids Cargill - Germany sabine_bowers@cargill.com Web: Figure 5. Density of natural ester sample 41