Synthetic Ester Transformer Fluid A Total Solution to Offshore

M&I Materials, Hibernia Way, Manchester, M32 0ZD, UK Authors email: [email protected]

Keywords Ester, Transformer, Alternative Fluids, Synthetic Ester, MIDEL 7131, Natural Ester

Summery Synthetic ester transformer fluid has the ability to substantially reduce the risk associated with operational offshore transformers. In comparison to alternative dielectrics such as cast resin, silicone fluid and natural ester, synthetic ester has proven to be the most suitable for offshore transformers. This paper presents evidence to support these claims. Introduction With many European governments committing to the generation of renewable energy, a huge amount of investment is about to be placed into offshore wind parks around European shores. Reliability of equipment is key to the success of any operational offshore wind park and this is particularly true of the electrical transformer, without which none of the generated electricity would reach the end user. Due to the criticality of its role it is becoming increasingly important to ensure that the correct transformer is chosen for both the turbines and offshore substation. This will reduce a variety of risks that a transformer can be exposed to in challenging offshore conditions. Owners, operators and engineers should not observe a transformer as a grey box that performs a mundane duty. Instead every effort should be made to understand that specifying the correct dielectric inside can have a huge impact on the validity and risk reduction of the overall wind park. This paper aims to give the reader an overview of transformer dielectric technology and hopefully provide sufficient evidence that specifying the

correct technology will ultimately lower the risk of transformer failure. Offshore Wind Park Transformers

Figure 1 Offshore Transformer Locations

Figure 1

(1) shows a basic schematic diagram of an

offshore wind farm. Electricity is generated at a low voltage within the turbine which is then stepped up via a turbine specific transformer. Dependant on how far offshore the wind park is this may be further stepped up via power transformers located on the offshore electrical substation. Future offshore substations may house as many as 3 power transformers. Due to the remote location and harsh environment increasing demands will be made upon the transformer dielectric. Brief Explanation of Transformer Dielectrics Transformer types are largely dictated by the use of dielectric insulation inside and as such it is important that end users have a clear understanding of all the available technologies. Two typical types of transformer are

1. Cast resin transformers: Use a solid epoxy resin to encapsulate the windings. An

Synthetic Ester Transformer Fluid

A Total Solution to Offshore Transformer Technology

European Offshore Wind Conference 2009, Stockholm, Sweden

J. O’Brien, M. Lashbrook, R. Martin M&I Materials (MIDEL 7131), Hibernia Way, Manchester, M32 0ZD, United Kingdom

authors email: [email protected]


obvious restriction is the inability to dissipate heat as effectively as fluid.

2. Fluid filled transformer: Can be filled with

mineral oil or alternative fluids as synthetic ester, natural ester or silicone fluid.

A dielectric as the name suggests is used to provide electrical insulation. Fluid dielectrics also offer an efficient cooling medium conducting away the heat generated by the transformer losses. Types of transformers dielectric specific to their application are shown below.

Table 1: Transformer dielectric suitability

Fluid filled transformers can include mineral oil, silicone fluid, synthetic and natural esters. Each fluids suitability will be compared later in the paper. Offshore Wind Turbine Transformers Offshore turbine transformers are located within the turbine nacelle, turbine tower or inside a specially constructed housing unit below the nacelle. Each design is dependant on the respective manufacturer and will have repercussions as to the demands placed upon the transformer and ultimately the transformer dielectric. Typical locations are shown below

Figure 3: Typical offshore turbine transformer locations

Fire Risk Assessment of Transformer Dielectrics As can be seen from the transformer locations there is a significant need to address fire safety to reduce the risk to the associated multi million pound turbine. Traditionally mineral oil is used as the dielectric of choice within standard transformers however it is known to carry a high fire risk and as such alternative dielectrics have to be considered in order to reduce this risk. According to IEC 61039

(2) fluid dielectrics can be

classified according to fire point. Table 2 below shows the fire point and resulting classification

Table 2: Classification According to IEC 61039

K Class classification is obviously a very desirable attribute and will dramatically reduce the risk of a transformer fire occurring, as the following text from IEC TS 60695-1-40 shows ‘More than 150,000 transformer containing Class K insulating liquids are in service, with an excellent

fire safety record’ As can be seen mineral oil does not satisfy the criteria of Class K less flammable fluid and should be deemed as high risk. Environmental Risk Assessment of Transformer Fluid Dielectrics As the transformers will be in a offshore location it is advisable to perform an environmental risk assessment to the potential impact of the fluid should it detank from the transformer. Both natural and synthetic esters are officially classified as ‘readily biodegradable’. This means they pass strictly controlled degradation tests carried out according to OECD methods

(3). These

test methods are internationally established and recognized. Their behaviour contrasts markedly with mineral oil and silicone fluid.


Table 3: OECD 301 Biodegradation Classifications

The new standard, IEC 61039 (2008) classifies the biodegradability of dielectric fluids, tested to OECD 301 C or F, using the % of Theoretical Oxygen Demand (ThOD) removed

(4). The results of which

are shown in table

Table 4: IEC 61039 Biodegradation Classifications

To further support the environmental credentials of ester fluids they are also officially classed as ‘non-water hazardous’ according to Umweltbundesamt (UBA). Following these findings silicone fluid and mineral oil can be deemed as high risk to the environment should the fluid leech or be dispelled from the transformer. Performance Risk Assessment of Transformer Dielectrics in Offshore Locations Cooling performance All devices that use electricity give off waste heat as a by-product of their operation. Transformers are no exception. The heat generated in a transformer operation causes temperature rise in the internal structures of the transformer. This generated heat is dissipated via an air exchange within a cast resin transformer and via a fluid exchange within a fluid filled transformer. It must be considered that air is a poor cooling medium with a very low specific heat, poor thermal

conductivity and requires large flow rate to remove heat effectively. Conversely fluid is a good cooling medium with a high specific heat, good thermal conductivity and much lower flow rate to remove same amount of heat. It is therefore expected that forced cooling of a cast resin transformer is required with the addition of specific fans within the turbine. However this method of cooling can push potential corrosive sea air across the exposed windings of the cast resin transformer. According to Declercq et all

(5) ‘the only way to

protect a dry type transformer is to house it in a hermetic envelope equipped with an heat exchanger, an air treatment unit (drying/filtering) and forced ventilation. A total concept which is costly, large in dimensions, consumes energy and needs maintenance. In case the fans would refuse to work the transformer and turbine have to be shut down.’ Transformer overload, harmonic and transient withstands Power electronics within the turbine and variable wind patterns can subject the transformer windings to rapid increases of heat. It is known that aluminium is often used as the conductor within a cast resin transformer as the expansion coefficient is closer to that of epoxy resin than copper. However the expansion of aluminium is still different to that of epoxy and as such thermal cracks can appear in the insulation. These cracks represent the weak point within the insulation structure. According to Dr Volker Wasserberg

(6)

‘within cast resin insulation this has the potential to

lead to partial discharge in a concentrated area, which cannot be dissipated or diluted. This has the

potential to increase the likelihood of failure.’ Liquid transformers are much more capable of coping with partial discharge as they have the ability to self heal, i.e. will automatically send new liquid to the area to rectify the fault.

Picture 1: Faults within a cast resin transformer taken from an

off shore wind turbine(7)


Mechanical stress withstand As the wind load hits the offshore turbine it can cause dramatic vibrations in the transformer especially when situated in, or under the nacelle. To reduce the vibrations and resonance a liquid filled transformer can incorporate a corrugated tank with vertical and horizontal reinforcements. The active part of the transformer is also highly clamped to the tank restricting movement in any of the dimensions. The fluid filled dielectric has a much greater withstand to any generated movement than a solid dielectric, as found in cast resin transformers, which is much more vulnerable at these low frequency stresses. Risk Assessment of Transformer Fluid Stability High Temperature Operation As space within a nacelle or tower is at a premium it is desirable to have the turbine transformer build as small as possible. In this instance the transformer will run hotter than a conventional transformer. To allow for high temperature operations designers opt for a fluid/aramid paper insulation. Top oil temperatures can regularly reach 135°C and hot spot winding temperatures +180°C. At these temperatures mineral oil would be very close to its flash point and would be expected to experience fluid degradation in the form of sludging. Silicone, natural and synthetic esters have the ability to operate at these elevated temperatures and remain stable providing the transformer remains in a sealed form. However as these transformer are in an offshore location and with large mechanical forces exerted on the transformer there is a small possibility that the seals of the transformer can be compromised. Therefore it is prudent to consider the risk to each of these fluids should air or moisture ingress into the operating transformer. Oxygen Stability of each of the transformer fluids The rotating bomb test, according to ASTM D2112, subjects the test fluid to pressurised oxygen at elevated temperature in the presence of common catalysts. Measuring the time for the oxygen pressure to drop, to a predetermined level, gives understanding of how oxygen stable the fluids are.

Graph 1: RBOT results of transformer fluid

(8)

The results presented in graph 1 clearly show that synthetic ester is a far more oxygen stable dielectric than mineral oil or natural ester. As natural ester performed very poorly in this test further studies were made with the fluid to understand the chemical reactions that occur during the oxidation process. Research performed at M&I Materials internal laboratory shows that excessive oxidation of a natural ester can result in gels forming as shown in picture 2.

Picture 2: Oxidation of a natural ester

(9)

Natural ester represents a high risk should any of the seals around the transformer be compromised. In contrast the risk of oxidation when using a synthetic ester is low. Moisture tolerance of each of the dielectric fluids Should a transformer seal be compromised as explained above it is important we consider the effect of moisture on each of the dielectrics. Below are the moisture saturation curves for each of the fluids.


Graph 2: Moisture Saturation Curves

(10)

Synthetic ester has the highest saturation limit of all the transformer fluids. As the fluid is used primarily as a dielectric it is important to understand how moisture affects the dielectric rigidity. Results are shown in graph 3 below,

Graph 3: Absolute moisture ‘v’ Breakdown Strength

(11)

Considering graph 2 and 3 it can be concluded that synthetic ester is the most moisture tolerant of all the fluids and still remains dielectrically strong even in the presence of high absolute values of moisture. Offshore Transformer Substations For offshore substation transformers, at voltages of greater than 100kV and with ratings in excess of 100MVA, the only commercially available option is a fluid filled transformer. Cast resin transformers are limited in rating to around 40MVA at the time of writing and this is unlikely to increase significantly in the foreseeable future. Silicone fluid is not suitable for these high voltage transformers as it more prone to electrical instability at voltages above 36kV

(6).

Natural and synthetic esters offer a viable alternative to mineral oil in this power range. Indeed synthetic ester has been used as the dielectric medium inside a 238kV power transformer since 2004 with exemplarily

performance. However as discussed earlier in the paper natural ester will have to operate in a sealed transformer. This presents the possibility of the seal being compromised and as maintenance would be very difficult it is advisable to insure against this with the use of a synthetic ester. Conclusion With the evidence presented it is possible to use the following scoring system to asses which dielectric fluid will present the least risk to the offshore wind park

It can be seen that cast resin dielectrics offer a greater operational risk in comparison to fluid dielectrics. As such the following graphs make risk comparisons of the fluid only.


Using the scoring system it can be seen that synthetic ester has the lowest total and hence presents the lowest risk of dielectric failure within an offshore wind park. About M&I Materials M&I Materials Limited is dedicated to manufacturing Specialist Materials for Industry and Science and is the driving force behind a portfolio of successful brands including APIEZON

®,

METROSIL®, MIDEL

® and WOLFMET

®.

Midel is a fire safe and environmentally friendly transformer fluid that provides excellent service in thousands of sealed and free-breathing systems around the globe.

References

1. www.wind-energy-the-facts.org 2. IEC 61039 Classification of insulating

liquids 3. ‘Ready Biodegradation of MIDEL

transformer fluids’ Cantest 2007 M&I archive document

4. IEC 61039 Classification of Insulating Liquids 2008

5. ‘Transformers for offshore multi-megawatt turbines: Discussions on specifications, safety and environment’ - European Offshore Wind 2007

6. ‘Step-up transformers for wind energy plants’ – Dr. Volker Wasserberg

7. ‘Cracks in Siemens Geafol Transformers used in Middelgrunden Windturbine Park’ - Karsten Borch Gylding

8. M&I Materials internal technical report TD3203

9. M&I Materials internal technical report TD3255

10. ‘Fundamental investigations on the influence of temperature and water content on the electrical behavior of fluid impregnated insulating paper’- Dumke et all

11. ‘Ester Transformer Fluids for Environmental Protection, Improved Fire Safety, and Operational Reliability’ - Dr Russell Martin

Documents

Synthetic Ester Transformer Fluid A Total Solution to Offshore