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Abstract

Traction transformers in the past were filled with mineral oil to provide the cooling and electrical isolation. Whilst mineral oil serves the purpose as an effective coolant and dielectric, it has a number of limitations, most notably the relatively low flash and fire point, making it relatively easy to ignite. Trains travelling through tunnels have to comply with the most strin-gent fire safety requirements – and mineral oils have notable shortcomings in terms of fire safety and maximum operating temperatures. Synthetic ester filled transformers in comparison have the ability to operate at higher temperatures and are de-signed for increased fire safety and performance. Transformer mineral oils, unlike ester fluids, are also not classified as biode-gradable, so leaks or spillages can have a detrimental effect on the environment. As a superior alternative, synthetic esters have been widely adopted for their technical advantages, increased fire safety, performance and reliability characteristics in often demanding conditions. In addition, the new draft CEN/TS45545 has also taken account of the need to standardise on fire safe electrical insulating fluids.

Zusammenfassung

In der Vergangenheit waren Bahntransformatoren mit Mineralöl gefüllt, um die Kühlung und elektrische Isolation zu gewährleisten. Während Mineralöl dabei als wirksames Kühlmittel und Dielekt-rikum dient, weist es doch eine Reihe von Einschränkungen auf, insbesondere einen relativ niedrigen Flamm- und Brennpunkt, so dass es sehr leicht entzündbar ist. Züge, die Tunnel durchfahren, unterliegen strengsten Brandschutzanforderungen, Mineralöle weisen jedoch starke Mängel hinsichtlich des Brandschutzes und der maximalen Betriebstemperatur auf. Im Vergleich dazu können mit synthetischem Ester gefüllte Transformatoren auch bei hohen Temperaturen eingesetzt werden und wurden speziell für höhere Brandschutzanforderungen entwickelt. Außerdem sind Transformator-Mineralöle, im Gegensatz zur Esterflüssig-keit, nicht als biologisch abbaubar klassifiziert, daher können Lecks oder ausgelaufene Flüssigkeit schädliche Auswirkungen auf die Umwelt haben. Als überlegene Alternative wurde syn-thetischer Ester wegen seiner technischen Überlegenheit, wie erhöhter Brandschutz, seinen besonderen Eigenschaften und seiner Zuverlässigkeit unter oft anspruchsvollen Bedingungen vielfach eingesetzt. Darüber hinaus trägt der Neuentwurf CEN/TS45545 auch der Notwendigkeit Rechnung, die Vorschriften hinsichtlich der feuerfesten elektrischen Isolierflüssigkeit zu vereinheitlichen.

High Performance and Environment Conscious Transformers Transformatoren gesteigerter Leistung mit umwelt­freundlichem Kühlmittel

Mark Lashbrook, BEng(Hons), Sabine Bowers, BSc(Hons), Manchester, Great Britain

Introduction

The importance to move passenger and freight traffic across borders had led to the creation of the so called TEN-Nets (Trans-European Networks). With trans-national traffic also comes the need for trains to technically cope with multiple operating voltages or even dual power systems. Additionally both high speed and long distance lines increasingly use high voltage AC lines. The use of 25 kV voltages when compared to lower volt-

age DC systems significantly reduces the losses in overhead cables and allows sub-stations to be spaced much further apart. This reduces capital installation costs and running costs for the railway oper-ating company. For rolling stock to run on a high voltage AC system, it is necessary to have a trans-former on board the train which takes the 25 kV overhead voltage and reduces it down to a workable voltage for the con-

verters which feed the traction and aux-iliary supplies on the train.The on-board traction transformer is a conduit through which all power to the train must flow and hence becomes a sin-gle point of weakness if not properly de-signed with the best materials. Even in systems where multiple transformers are used and distributed throughout the roll-ing stock, such as EMUs, a significant re-duction in power would occur if one of

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the transformers on the train were to fail, meaning a reduction in speed and accel-eration. Any failure would also increase stress on the other transformers, requir-ing them to cope with a higher operating temperature for a short period. Most commonly in the past, traction transformers were filled with mineral oil to provide the cooling and electrical iso-lation. Whilst mineral oil served the pur-pose as an effective coolant and dielectric, it has a number of limitations, most nota-bly the relatively low flash point, which has a number of knock on effects. Trains for example travelling through tunnels have to comply with the most stringent fire safety requirements – and mineral oils have notable shortcomings in terms of fire safety and maximum operating tempera-tures. Synthetic ester filled transformers have the ability to operate at higher tem-peratures and are designed for increased fire safety and performance. Transformer mineral oils, unlike ester fluids, are also not classified as biodegradable so leaks or spillages can have a detrimental effect on the environment. The search for a suit-able replacement fluid with superior fire safety, operating characteristics and bio-degradability has led to the introduction of synthetic esters some 30 years ago in traction transformers. Since then, syn-thetic esters have been widely adopted for their technical advantages, increased fire safety, performance and reliability characteristics in often demanding con-ditions. In addition, the new draft CEN/TS45545 – with the intention to also adopt in the TIS – has also taken account of the need to standardise on fire safe electrical insulating fluids.

Fire Safety

Fire safety is one of the most critical factors for materials in the rail industry and the introduction of the new CEN/TS 45545 technical specification means that a Europe wide approach to fire stand-ards is being adopted. In order to com-ply with the requirements of this specifi-cation train builders and operators need to find materials which in turn comply. One crucial requirement of the CEN/TS

45545 specification is for K-class fluids to be used in electrical equipment for cool-ing and electrical isolation. K-class means that the fluid must have a fire point of >300 °C when measured using the Cleve-land Open Cup method ISO 2592. This ef-fectively rules out mineral oil as a choice in future rolling stock projects in Europe if designers want to ensure compliance. The fire points of common transformer fluids utilised in traction transformers is shown in Table 1.

Another aspect of fire safety is the move towards more AC traction systems in un-derground railways, in this case a large number of trains could be operating with traction transformers in underground lo-cations. It is possible to imagine a sce-nario where these AC underground trains will have four or more transformers, each containing around 1 tonne of insulating liquid. If there are 10 trains in a large un-derground station there is the potential to have 40 tonnes of heated transformer flu-id in a confined location. In this example it is crucial that the fluid used provides excellent fire safety. Experiments have

been conducted comparing the ignition resistance of mineral oil with synthetic ester to demonstrate the difficulty of ig-niting a K-class fluid. In the pan fire test it takes only 4 minutes for the mineral oil to ignite, in comparison after 70 minutes the synthetic ester has still not ignited. So it is clear that to improve ignition re-sistance it is crucial to choose a K-class fluid, the choice then becomes which K-class fluid to select, in order to have the safest solution. Fire point and igni-tion resistance is not the only aspect of fire safety, it is also important to consid-er the smoke behaviour of different so-lutions in the extremely unlikely event that they are ignited. In this case there is a desire to have a fluid which gives the lowest smoke density, with no harm-ful emissions, especially if the train is to be travelling through tunnels or under-ground where smoke has the potential to collect. Silicone liquids are well known to produce airborne silica particles when combusted, this type of fine airborne sili-ca powder has the potential to block res-pirators and hamper the rescue efforts of the emergency services. Smoke produc-tion testing on a number of transform-er insulating liquids was carried out by Exova Warrington, a well respected fire test laboratory for the rail industry [1]. A comparison of the smoke density for dif-ferent transformer fluids is presented in Fig. 1. It can be seen from the smoke den-sity recorded that synthetic ester gives the

Table 1: Typical fire point of dielec-tric fluids

Fluid Type Fire PointMineral Oil 170 °CSynthetic Ester 316 °CSilicone Liquid >350 °C

Figure 1: Smoke density comparison

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lowest smoke density of the three fluids. It also does not produce the fine airborne silica powder that occurs with burning silicone liquid.

High temperature operation

Even if mineral oil is used in the trans-former, where CEN/TS 45545 does not apply, it is limited to a maximum tem-perature of 100 °C for safety reasons as the fluid cannot be used any closer to the flash point. Using high temperature solid insulation can allow higher hot spot tem-peratures in the transformer, but this gain is small in comparison to what is possi-ble with a fluid capable of operating at elevated temperatures. There are multi-ple reasons why a high temperature fluid is desirable, in order to reduce transform-er size, or increase output power without increasing size.

Hotel Power

Modern rolling stock puts many more de-mands on the transformer than the rolling stock in the past. Where in trains of yes-terday the transformer just supplied en-ergy for movement, signalling and light-ing it now has to deal with a multitude of loads. Passengers are asking for a more comfortable, sophisticated travelling ex-perience on the railways and this requires more power from the transformer. The energy requirements of equipment other than that necessary for propulsion and signalling is often termed “hotel power”, this can include power outlets, lighting, WiFi networks, catering facilities, video screens etc. This can easily become as big a burden on the transformer as the drive power when all the necessary sys-tems are factored in.

Performance and Flexibility

Trans European transport networks also need trains that can stick to timetables and extra performance offers improved flexibility. The ability to run the train fast-er, or accelerate more quickly means that

delays can be made up on route and the timetable planners also have more flexi-bility if extra performance is available. If theoretically a journey from one location to another takes 24 minutes, the time ta-ble planner will need to allow 26 minutes to account for delays and turnaround time etc. If the train can run faster then the probability of missing this timetable is re-duced and this becomes even more cru-cial for trains operating across networks where the impact of delays can be magni-fied. Another knock on benefit is that roll-ing stock fleets can be reduced, if journey times between critical stations on the net-work can be cut. This extra performance cannot come at the cost of extra weight or size, as this impinges on the efficiency of the train, so solutions that keep the size of the transformer the same, while deliv-ering gains in power output are necessary. By selecting the correct high temperature transformer insulating liquid increases in power output from the transformer can be achieved, while keeping weight low.

Location of Transformers

Another aspect that comes into play in modern trains is the need to package transformers to optimise train design. Transformer OEM’s now offer transform-ers that can be mounted beneath the car-riages, or even in the roof of the train to allow multi-mode operation. An example

of where this has been employed is in the latest Regio 2N EMU’s which have inte-grated roof mounted transformers which use synthetic ester as the cooling fluid [2]. These transformers needed to be lighter and more compact than the previous gen-eration and this was achieved by clever packaging and the use of high tempera-ture insulation materials. When factoring in all the above consid-erations high temperature insulation sys-tems come into play in the transformer. By upgrading to a fluid and solid insula-tion that can handle higher temperatures it is possible to get more power from the same size and weight transformer. Based on the higher fire point, a trans-former filled with K-class fluid can be run hotter, allowing the train to accel-erate faster and travel at higher speeds, while supplying all the auxiliary systems and all for the same weight. The selection of the best fluid includes looking at the cooling performance, the fluid must act as an effective medium for transferring heat from the windings out to the cooling equipment. Synthetic ester is an effective coolant when operating at higher temper-atures, owing to high thermal conductivity and heat capacity. Silicone liquid is also a K-class fluid, but provides less efficient cooling at operating temperatures.The cooling performance of a fluid in a pumped system can be approximated by looking at the heat transfer coefficient given by the formula below [3].

Figure 2: Heat transfer comparison

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xC

Vp≈

3

.

Where x is the heat transfer coefficient, Cp is the specific heat capacity, is the thermal conductivity and v is the kine-matic viscosity. By applying this formula to the thermal property data for fluids [4–5] the graph in Fig. 2 shows a comparison of the heat transfer coefficient for silicone and syn-thetic ester across a range of tempera-ture. It can be seen that when operating at high temperatures, such as under ac-celeration, synthetic ester has far greater heat transfer efficiency, moving heat from the winding hot spots out to the cooling equipment. This means either a cooling running transformer, or a smaller trans-former for the same temperature rise. This is also important in the case where there are problems with the cooling sys-tem, for example when snow clogs ven-tilation ducts. The most efficient coolant allows the transformer to run for longer with less than ideal cooling air flow.

Environmentally friendly

Impact on the environment is something that is becoming more widely understood and the need to specify environmentally friendly products is becoming greater. In terms of harm to the environment one key measure is biodegradability, with non-

toxic, Readily Biodegradable products having little chance of any persistence or effect if they leak or are spilled. To be classified as Readily Biodegradable a fluid must pass a stringent test, one ex-ample of this being the widely recognised OECD 301 standard. This test is conduct-ed over 28 days, with the fluid combined with a solution containing microbes com-monly found in nature. In order to meet the highest level of biodegradability the fluid must achieve at least 60 % biodeg-radation in 28 days and more crucially must degrade from 10 % to 60 % in less than 10 days. A comparison of the bio-degradation of the common transformer fluids for traction is shown in (Fig. 3) [6]. This clearly shows that the only fluid used in rolling stock which meets the Readi-ly Biodegradable requirement is synthet-ic ester, mineral oil degrades much more slowly and silicone liquid is barely bio-degradable. The potential environmental impact of a spillage of mineral oil or sil-icone liquid is one crucial reason these fluids are falling out of favour for traction transformers in Europe. It is important to note that the fact that synthetic ester degrades rapidly in the en-vironment does not mean that it degrades quickly in the transformer. The processes that lead to biodegradation are very dif-ferent to those that lead to fluid ageing in operation. Biodegradation requires the presence of microbes which break down the fluid and a suitable environment for

them to live in, usually this means free water being available and temperatures kept to suitable level. In a transformer system the environment is far too dry and hot to sustain the microbes and they would die off very quickly, meaning the fluid will not biodegrade in normal oper-ation. In fact synthetic esters are very ro-bust fluids that can give very long service life in transformer applications.

Life cycle and maintenance reduction

The life of a fluid in a transformer sys-tem is usually governed by it’s ability to maintain a high breakdown voltage, low viscosity and also limit the production of acids through ageing. Much of the ex-perience to date in traction transformers has been with mineral oil and users un-derstand the limitations of this fluid in terms of lifetime. Synthetic esters have been tested in the laboratory in a num-ber of ways and have demonstrated that they have superior longevity, when com-pared to mineral oils. One such study took a sample of synthetic ester and mineral oil, ageing them side by side in open ves-sels, to represent a breathing system [7]. The fluids were kept at a temperature of 130 °C for a period of 1 440 hours, after this various parameters were measured including viscosity and breakdown volt-age. Fig. 4 and Fig. 5 show a compari-son of the results for synthetic ester and mineral oil. The results of this study clearly show that the synthetic ester is more stable when aged in at an aggressive temperature and this means that it can give a longer ser-vice life, without needing any filtering or replacement. Viscosity is not increased with synthetic ester and the high break-down voltage is maintained, unlike min-eral oil under similar conditions. The other advantage to synthetic ester is that it does not produce the sludge of-ten associated with oxidised mineral oil. Based on the robust nature of synthetic ester one large railway operator has de-cided to utilise the fluid in all their new rolling stock, in preference to mineral oil. Although fire safety was a consid-Figure 3: Biodegradation of transformer fluids

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eration in this decision the main driver was a large reduction in maintenance by having a fluid that does not require fre-quent attention.

Field experience

The longevity of synthetic ester has also been demonstrated over years of service in a num-ber of different traction ap-plications. The acid value of the fluid has been used as a key indicator of the condi-tion and how close the fluid is to needing some attention. Typically a guidance limit of

2.0 mg KOH/g is put on the acid value for synthetic ester, based on the recommen-dations of IEC 61203. Table 2 shows the results of in service fluid testing for three fleets of traction transformers. User A is using the transformers in com-muter trains, which undergo very extreme load variations with harsh acceleration between stops. The ambient temperature

of the users location can vary from well below zero in the winter to greater than thirty degrees Celsius in summer. User B and C’s transformers are somewhat more lightly loaded, with less harsh variations in the operating load. Even in the most extreme loading case of user A the fluid has an acid value which is well below the guidance limit after ten years of continu-ous service, with the rolling stock having covered in excess of half a million miles in some cases. The fluid is not the only material in the transformer and in fact is not the most critical material in terms of the overall life of the transformer. The fluid can be rela-tively easily replaced, however the paper insulation is a key material that cannot be exchanged readily and in effect the life of the paper becomes the life of the trans-former. One key advantage of synthetic ester is its ability to absorb more mois-ture without any reduction of the break-down voltage, this means that the ester has the potential to keep the paper insula-tion drier than it would be in mineral oil. Moisture content is directly linked to the ageing rate of cellulose paper insulation, so if the paper can be kept drier then the ageing rate will be substantially reduced. A study conducted by Weidmann Tech-nology has demonstrated this effect, by using the degree of polymerisation as an indicator of paper ageing. In one of the trials cellulose pressboard was immersed in the different fluids at 150 °C for a peri-od of 4 months [8]. The intention of the experiment was to significantly age the pressboard and fluids under extreme con-ditions in order to determine differences in performance. Figure 6 shows the re-sults for the degree of polymerisation. A DP of 200 is usually considered to be end of life for cellulose paper insulation, by this point the paper has lost much of its tensile strength.

These results demonstrate the ability of ester to pre-serve the paper and extend the lifetime of the trans-former. In these experiments the acid value of the ester fluid was pushed up to lev-els well above the recom-mendation of 2.0 mg KOH/g,

Table 2: Synthetic ester acid value results from in service transformers

User No. of Transformers Approximate Age Average Acid Value (years) (mg KOH/g) A 30 10 0.44 B approx. 200 2–5 0.04 C 20 5 0.076 D 1 16 0.58

Figure 4: Viscosity in open ageing study

Figure 5: Breakdown voltage in open ageing study

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reaching up to 4.0 mg KOH/g by the end of the experiment. Despite this there was no detrimental effect on the paper, in fact the paper retained far more ten-sile strength than the paper immersed in mineral oil, even though the acid value of the fluid was higher in the ester. This

is due to the type of acids produced in esters, which are more predominantly long chain acids, these are less harmful to paper than the short chain acids pro-duced by mineral oil.

Experience of end users

Since the earliest adoption of synthetic ester in traction transformers, train oper-ators across Europe and US have not re-ported a need to replace or maintain any of the fluid and have not reported any op-erational issues related to the fluid. The use of synthetic ester in rolling stock has continued to show rapidly accelerat-ing growth, including lively interest and adoption in locations around the world such as India and China, US and Russia. New trains featuring synthetic ester trans-formers such as the Desiro for Scotrail in the UK, the ALP45DP for New Jersey Transit (USA) in the US or the new Vec-tron, have become common introductions by industry leading rolling stock manu-facturers based on transformer technol-ogy from the market leaders.

Conclusion

Modern train builders want to deliver rolling stock which is faster, more effi-cient, provides a multitude of electrical systems and is not heavier than existing rolling stock. In order to do this the on-

board transformer must run hotter, but by specifying the correct materials in the on board traction transformer more perfor-mance can be gained for the same over-all size and weight. Synthetic ester is the only fluid which can boast that it offers the full performance package. It is environmentally friendly, with a high fire point and fire safety as an integral part of the fluid. Synthetic ester also delivers excellent high temperature performance, paper life extension, and excellent long term reliability; no other single fluid can offer this complete per-formance range. Mineral oils offer low cost, but are not CEN/TS 45545 compli-ant and cannot be used at high temper-atures. Whilst silicone fluids offer a de-gree of fire safety, they are not environ-mentally friendly. Synthetic ester can operate safely under a range of very challenging conditions. It can be used in breathing transformers as it is oxidation stable and is also very mois-ture tolerant meaning that even in the un-likely event that the drier fails or is not maintained the fluid will not form emul-sions with water. In addition long expe-

rience has demonstrated that it can reli-ably used at elevated temperatures over extended periods in service. – A 193 –(Keywords: Electric motive power units, components)(Sources of figures: M & I Materials Ltd.)

References[1] Bodycote Warrington Fire.: Determination

of the opacity of Smoke in a non renewed atmosphere, 2007.

[2] ABB Communications.: ABB transformer in-novation is a first for regional double-deck train. 26th May 2011.

[3] Waddington, F.B.: New dielectric fluids for power engineering application. GEC J. Sci. Technology, Vol. 49, No. 1, pp. 18–22, 1983.

[4] MIDEL 7131 thermal properties data, www.midel.com/thermal-properties.htm, 2011.

[5] Serben, K.: Ready Biodegradation of MIDEL 7131 and MIDEL eN. Cantest 2007.

[6] Tenbohlen, S.; Koch, M.: Aging Performance and Moisture Solubility of Vegetable Oils for Power Trans¬formers. IEEE Transactions on Power Delivery, Vol. 25, No. 2, April 2010, pp. 825–830.

[7] Gasser, H.P.: Alterung von Pressspan in ver-schiedenen Isolierflüssigkeiten, TLM-2011, June 2011.

Figure 6: DP results of paper ageing experimentMark Lashbrook, BEng(Hons) Electrical and Electronic Engi-neering, Loughborough Uni-versity, UK Member, IET, Mem-ber CIGRE NGN. 1995 Fujitsu Microelectronics, UK, Equip-ment Engineer. 2000 Trikon Technologies, UK , Global Customer Support Engineer,

2004 Intel, Ireland, Senior Process Engineer. Since 2007 Development Engineer at M&I Materials, Manchester, UK. Technical Development and Application Innovations for ester based trans-former fluids. Research projects looking at the application of alternative fluids in high voltage transformers.Adress of the Author: M & I Materials Ltd., Hibernia Way, Manchester M32 OZD, Great Britain.E-Mail: marklashbrook@mimaterials.com

Sabine Bowers BSc(Hons) in Management & Sciences from the University of Manchester Institute of Science and Tech-nology, Manchester, UK. Nach dem Studium 1989 Eintritt bei Ford Motor Co, Köln, Produkt-marketing und Vertrieb. 1997 Eintritt bei ICL Fujitsu, Man-

chester, UK als Produktentwicklungsleiter Sun Solaris Rechner. Seit 2003 Globaler Business De-velopment Manager Synthetischer Ester für Trak-tionsanwendungen bei M&I Materials, Manches-ter, UK Beitrāge zu Forschungsarbeiten mit Ester Kühlflüssigkeiten in Traktions- und Hochtemper-aturanwendungen.E-Mail: marklashbrook@mimaterials.com