Prototype
420 kV Power Transformer
Using Natural Ester Dielectric Fluid
By
Ronny Fritsche,
Uwe Rimmele, Frank Trautmann,
and Dr. Michael Schäfer
PROTOTYPE 420 KV POWER TRANSFORMER
USING NATURAL ESTER DIELECTRIC FLUID
Ronny Fritsche*, Uwe Rimmele*, Frank Trautmann*, and Michael Schäfer**
*
Siemens AG – TR Nuremburg
**
TransnetBW GmbH – Germany
Abstract
First introduced in the early 21st century, the use of alternative liquids, especially synthetic
esters, is a well-established practice with distribution transformers. Similarly, retrofit
transformers that are refilled with alternative liquids are also an accepted practice.
The story of alternative liquids began in 2004 when a medium power transformer with a rated
voltage of 238 kV and rated power of 135 MVA was filled and tested successfully with a
synthetic ester for the first time at Siemens. Then, in 2008, a medium power transformer with
rated voltage of 110 kV and rated power of 40 MVA was filled and tested successfully for the
first time with a natural ester.
In April 2013, the world’s first large power transformer filled with natural ester was successfully
tested. This transformer was developed and built by Siemens at their power transformer factory
in Nuremberg, Germany. The transformer is rated at 420 kV, uses new design criteria, and is the
largest unit using a renewable resource liquid. The rated power of this transformer is 300 MVA
with an overload condition of up to 400 MVA.
Introduction
The increasing demand for electric energy leads to huge investments to develop new primary
resources and techniques for electric energy generation and transmission. Simultaneously, the
demand for environmentally friendly energy generation and supply becomes more important for
all of us. Sustainability, biodegradability, and renewable energy are keywords in today’s global
energy sector. New markets, as well as new challenges for the entire energy supply system, are
already announced and upcoming.
Power transformers are one of the most important components of energy grid systems. These
components enable the most efficient transport of electric energy from the location where the
energy is generated to the location where the energy is needed. However, power transformers
are conventional pieces of electric equipment, due to physics concepts and materials developed
decades ago. This makes it critical to implement new materials based on renewable resources to
achieve sustainability, reliability, and safety.
It is also becoming increasingly important to develop and install electrical equipment that
provides a high level of environmental safety and reliability. Especially in regions of great
infrastructure density, where newly interlinked systems become an ideal solution, this is a major
target. A high degree of health and environmental safety will enable the installation of power
equipment in areas of high population density without increasing risk level.
2
In the case of power transformers, insulating oil is the most critical conventional material.
Consequently, the focus is to replace transformer oil with alternative liquids that enable a higher
level of sustainability and environmental safety. This paper will review the existing solutions for
alternative liquids and will spotlight the introduction of such materials to large power
transformers.
State of the Art – Insulating Liquids
The insulation liquid most commonly used in power transformers is based on mineral oil. It has
been used for decades in electrical equipment for insulation and cooling purposes because the
technical parameters, as well as the physical behavior of mineral-based oil, are well known.
Mineral oil is a non-renewable fossil resource that may be exhausted in the upcoming decades.
Consequently, there is a strong demand to replace that kind of insulation liquid with any kind of
alternative solution that enables a higher degree of sustainability.
Several types of alternative liquids are available for the market of electrical equipment. The
alternative liquids can be divided into three basic types: natural ester-based liquid, synthetic
ester-based liquid, and silicone oil. Because the properties of these types of alternative liquids
are different from those of mineral oil, as well as among themselves, it has to be clearly
investigated for which purpose these liquids will be used. The following sections will describe
the composition of various liquids and will outline some specific characteristics of each.
Mineral Oil
Conventionally applied transformer oil consists mainly of carbon and hydrogen in molecules of
different structures. There are three basic transformer oil structures: paraffinic, naphthenic, and
aromatic. Paraffinic molecules have lower thermal stability than naphthenic molecules. The
most important group is the aromatic structure group, which has a major influence on the grade
of the oil. While monoaromatics are always alkalized and generally have good electrical
properties, polyaromatics exist naturally in the mineral oil and may have a negative impact on
electrical properties (e.g., charging tendency and impulse breakdown withstand strength).
Additional molecules, such as nitrogen, oxygen, and sulfur, also naturally exist in mineral oil.
The final properties and grade of the transformer oil are dependent on the composition of the
different components. The composition of hydrocarbons in oil varies for each particular crude,
and the final amount and composition of hydrocarbons is dependent on the particular refinement
process and number of steps.
Because of its excellent thermal and insulating properties, transformer oil has been used in power
transformers for many decades. Consequently, the characteristic behavior and parameters of
transformer oil are well known. The electrical characteristics are particularly well reviewed and
are approved for the majority of large power transformer applications. There are a lot of
parameters, investigation results, and design rules published that enable the insulation design of
mineral oil filled electrical equipment. Additionally, the cooling characteristics and behavior of
flowing fluid have been thoroughly investigated and fixed in several rules and standards.
3
Finally, the service characteristics of transformer oil are well known and published in several
standards.
Gas-to-Liquid Insulating Oil
Recently, a second fossil source has entered the market of electrical equipment for use as a
cooling and insulation liquid. The so-called “gas-to-liquid” (GTL) is a fossil gas converted to
liquid form. This type of mineral oil was introduced by Shell and approved for use in power
transformers. Natural gas is converted into an oil form (GTL) using a three step process. This
GTL is suitable for making transformer oil. In the first step, methane is reacted with oxygen to
create synthesis gas. Next, the synthesis gas is converted to liquid waxy hydrocarbons. Finally,
the waxy hydrocarbons are upgraded, in a process known as hydrocracking, and then distilled
into a wide range of products. Base oil is the primary product, but transport fuels and feedstocks
for the chemical industry are also produced. During the third step of the process, the final
properties of the liquid are adjusted by the degree of distillations. Because GTL is produced
from synthesis gas, no impurities, such as sulfur, nitrogen, or other heterocyclic species, are
included. Therefore, GTL is easy to adjust to the final required properties of the insulation
liquid. In comparison to mineral oil-based transformer oil, GTL is highly resistant to
degradation and oxidation. Consequently, the aging behavior of GTL is better than that of
mineral oil. The properties of GTL must be thoroughly evaluated and compared to mineral oilbased transformer oil before it can be used in power transformers.
Silicone Fluid
Other alternative liquids to transformer oil are silicone fluids, which are specially developed for
transformer applications and are fully synthetic coolants and insulation fluids. Due to their high
ignition temperature and self-extinguishing behavior, these liquids present a lower fire hazard
than mineral oil. Thermal stability, even in the presence of air, is better with silicone fluids than
that of other liquids. However, its high viscosity at higher temperatures and poor lubrication
properties are disadvantages in transformer applications. The very low biodegradability and the
formation of jelly-like bridges of silicone-oxide in the presence of arcing are also disadvantages.
Furthermore, silicone fluids are expensive when compared to other insulation liquids. Silicone
fluids are mainly used in traction transformers and will not be considered in detail within this
paper.
Ester Fluids
In regard to the requirements for increased biodegradability and sustainability, the most
important alternative liquids are ester based. These liquids are the only alternative liquids that
are classified to be fully biodegradable. Of course, this is still dependent on the composition of
the esters, the volume of the disposed liquid, and the time allowed for biodegradation.
Esters are divided into two main classes: synthetic and natural. Synthetic esters are derived from
crude oil-based chemicals. These esters are usually the product of a polyol combined with
synthetic or natural carboxylic acids to create structures where several acid groups are bonded to
a central polyol structure. Polyol is a molecule with more than one alcohol functional group.
4
The acids used are usually saturated in the chain, giving the synthetic esters a very stable
chemical structure. The properties of synthetic esters lead to benefits for transformer insulation,
especially with respect to the thermal and environmental behavior of the equipment.
Natural esters are derived from vegetable oils. Consequently, they are the only kinds of liquids
that are primarily derived from renewable sources. Natural ester fluids are based on saturated as
well as single, double, and triple unsaturated fatty acids. Saturated fatty acids are chemically
stable but have a high viscosity. Triple unsaturated fatty acids have a lower viscosity but are
very unstable in oxidation. To reach an acceptable value of oxidation stability for natural esters,
it is necessary to add suitable antioxidants. In addition to DBPC-specific antioxidants (2.6-ditert-butyl-p-cresol) that use complex phenols, there are also amines in operation. To avoid an
unacceptable increase in liquid conductivity, the total amount of antioxidant is limited to one
percent or less. Ester fluids with a high percentage of single unsaturated fatty acids have proven
to be suitable. Depending on the natural plant and the composition of the natural ester, the
properties will differ among various types of natural esters. Therefore, it is important to
accurately define the final specification of the liquid to be used so that the composition can be
adjusted to meet transformer operational requirements.
Characteristics of the Liquid and Design Impacts
Insulation liquids within electrical power equipment have to meet several standards. The ideal
liquid represents the optimum electrical insulation and cooling performance and will be resistant
to oxidation and aging. Aside from these important features, there are a lot of other factors that
must be considered irrespective of liquid type. An evaluation will need to be completed to
determine which liquid characteristics will be sufficient for the type of electrical equipment
being used. The following is a partial list of challenges that need to be considered for use in
power transformers:
• Electrical insulation
• Cooling and transport of thermal energy
• Impregnation of solid insulations material (most likely based on cellulose)
•
Oxidation stability in case of thermal service stresses
•
Aging resistivity
•
Material compatibility and low chemical reactivity
•
Gassing tendency
•
Flash point and reaction to fire
•
Corrosion protection
Depending on customer requests, additional issues may need to be considered (e.g., biodegradability, low temperature characteristics, sustainability, and gas absorption characteristics).
None of the existing liquids can be considered the ideal solution for all of the given challenges.
Therefore, it must be clearly determined which challenges have to be tackled and which weak
points of the liquid shall be balanced.
Characteristics with Impact to Thermal Design
5
The thermal design of a power transformer depends strongly on the physical characteristics of
the insulation liquid. The kinematic viscosity, thermal capacity, and thermal conductivity are
mainly influencing the heat transport characteristic of the insulation liquid.
The hydraulic resistance of the whole transformer cooling circuit, inner and outer cooling, is
proportional to the kinematic viscosity. In a comparison of the previously mentioned liquid
alternatives, there are essential differences in the behavior of the kinematic viscosity depending
on the temperature of the considered liquid. There are significant differences among liquid
types, especially at low temperatures. This will be a key criterion for the design of power
transformers for low temperature environments. In some cases, additional measures should be
taken into account to ensure a reliable low temperature function of the liquid.
The kinematic viscosity is almost equal along the whole temperature range for mineral oil and
GTL-based transformer oil. There is no impact on the design of the transformer using GTL or
mineral oil. However, the kinematic viscosities of natural and synthetic esters are approximately
four times higher than mineral oil at typical operating temperatures. Figure 1 illustrates the
behavior of the kinematic viscosity in relation to liquid temperature for esters, mineral oils, and
GTL-based transformer oil. All given values are mean values for the different liquid types. The
graphic is illustrated in logarithmic scale because of the strong increase in viscosity at low
temperatures. Along the whole temperature range, the esters’ kinematic viscosities are higher
than those of mineral oil. This needs to be considered in the thermal design of the transformer,
especially in the cooling design of the inner cooling circuit. In particular, winding configuration
and winding insulation arrangement need to be modified for ester use. In case of the esters’
higher kinematic viscosities, pressure drop along the winding will increase significantly if oil
duct width is equal to the design for mineral oil application. Furthermore, in case of forced
cooling, the strength of pumps in relation to pipe cross section and required cooling performance
will need to be increased.
Besides kinematic viscosity, the specific heat capacity of alternative liquids must be considered
for thermal design of the transformer. In comparison to mineral oil, the most significant
difference in specific heat capacity can be observed in natural ester liquids. For natural esters,
the increase of specific heat with increasing temperature is stronger than those of the other
liquids. In contrast, this parameter is almost equal when comparing GTL-based transformer oil
to mineral oil. Figure 2 illustrates the specific heat capacity and the thermal conductivity of
different liquids. Because GTL-based transformer oil is almost equal to mineral oil, a single
chart for transformer oil is shown for both types. Note that thermal conductivity of natural esters
is higher compared to mineral oil over the whole temperature range.
6
10000
Synthetic Ester (Mean)
Kinematic Viscosity [mm²/s]
Naturel Ester (Mean)
Mineral Oil (Mean)
1000
GTL based transformer oil
100
10
1
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
100
Liquid temperature [°C]
Figure 1
Kinematic Viscosity as a Function of Temperature
3000
0,2500
0,2000
Thermal Conductivity [W/(m * K)]
Specific Heat Capacity [WS/(kg * K]
2500
2000
1500
1000
Natural Ester (Mean)
Synthetic Ester (Mean)
500
0,1500
0,1000
Natural Ester (Mean)
0,0500
Synthetic Ester (Mean)
Transformer oil, mineral and GTL based (Mean)
0
Transformer oil, mineral and GTL based (Mean)
0,0000
-30
-20
-10
0
10
20
30
40
50
Liquid temperature [°C]
60
70
80
90
100
-30
-20
0
20
40
60
80
100
Liquid temperature [°C]
Figure 2
Specific Heat Capacity (Left) and Thermal Conductivity (Right) as a Function of Temperature
Higher kinematic viscosity may lead to the conclusion that cooling performance of esters is
worse than that of mineral oils, but this is not necessarily true. Because of the higher specific
heat capacity and higher thermal conductivity, natural ester is able to transport a higher amount
of energy. Therefore, the cooling performance over the lifetime of the transformer can be an
advantage if the cooling design is adjusted to the natural ester characteristics.
Aging tests can be used to infer the better thermal characteristic of esters (especially natural
esters) when compared to mineral or GTL-based transformer oils. These tests investigate the
degree of polymerization (DP) of paper insulation material immersed in different liquids. Figure
3 illustrates an example of such an aging test, with typical results for a large number of aging
7
tests. The samples are aged for up to a month at 140 °C (284 °F), in both sealed and non-sealed
(open to environment) conditions. It can be observed that papers immersed in esters have a
better aging performance than mineral oil when considered with the same boundary conditions.
The drop in degree of polymerization is lower for ester than for mineral oil. This leads to
increased potential life span of an ester-insulated transformer of similar load conditions
compared to a mineral oil-insulated transformer with equal cooling performances.
In summary, for thermal and aging performance, it will be beneficial to use esters as alternative
fluids, but thermal design of the power transformer must be modified according to the specific
characteristics of the esters.
1200
Sealed condition
Degree of polymerization
1000
Open condition
800
600
400
200
0
NEW Cellulose
Mineral oil
Natural Ester
Synthetic Ester
Figure 3
Results of Aging Tests of Different Impregnated Papers
Characteristics with Impact to Insulation Design
Following the cooling task, the dielectric performance of the liquid is next most important for the
design of the electrical equipment. During the last decade, a lot of investigations were done to
characterize the dielectric performance of alternative liquids. Especially in the case of esters, a
large number of investigations were done to determine the dielectric withstand strength in
relation to different configurations and stresses.
The dielectric withstand strength of GTL-based transformer oil is almost equal to mineral oil. In
some configurations, it seems that GTL-based transformer oil will have slightly higher withstand
strength than mineral oil. However, because it has not been investigated in detail, it is still
recommended that the same design rules are used for GTL insulation design as for mineral oil
insulation design. There is no significant difference among the insulation liquids.
In contrast, the dielectric behavior of ester fluids differs significantly from mineral oils. The
withstand strength of ester liquids is strongly dependent on the applied test voltage, the test
8
voltage setup, as well as the kind of test voltage. Verification of the dielectric strength for the
different voltage stresses alternating current (AC), lightning impulse (LI), and switching impulse
(SI) is essential as criteria for proper insulation design of power transformers. Unfortunately,
there are still no standards available that determine testing conditions to verify the essential
dielectric properties of liquids for all test purposes and for their use in transformer equipment.
Standards like IEC 60156 test the oil breakdown on a short distance, but not across composite
insulation. Consequently, new testing had to be developed to verify the dielectric withstand
strength for ester liquid.
Impulse Stress for Alternative Liquids
The most significant difference in dielectric withstand strength of esters compared to mineral oil
occurs at impulse stresses and with inhomogeneous test arrangements. When test voltages are
increased, the ratio of withstand strength between esters and mineral oil also increases. Figure 4
illustrates breakdown voltage behavior for an inhomogeneous point-to-plate arrangement in
different kinds of insulating liquid. It is shown that with increasing electrode distance, the ratio
between related breakdown voltage of mineral oil and esters increases. Especially in the case of
natural esters, the ratio of related breakdown voltage compared to mineral oil increases
significantly. At higher impulse stresses of greater than 650 kV, the ratio also becomes
significant for slight inhomogeneous arrangements, and this disparity has to be considered within
the insulation design of transformers.
Further test results of strong inhomogeneous arrangements show that the streamer propagation is
much faster in insulation arrangements with ester. Fortunately, streamer propagation mode for
strong inhomogeneous arrangements is not of essential significance for the insulation design of
transformers. Rather, strong inhomogeneous arrangements can be avoided in power
transformers themselves (although this statement does not apply to the inhomogeneous
arrangements required for tap changer internal configurations). The dielectric strength as a
function of insulation arrangement and type of dielectric stress is of much more interest for the
power transformer’s design. Table 1 documents the most critical design areas in power
transformers and how to estimate the reduction of dielectric withstand strength in ester liquids.
9
UBD / UBD0 (Lightning Impuls Stresses)
3,5
3
2,5
2
1,5
1
Mineral oil (Mean)
Synthetic Ester (Mean)
0,5
Natural Ester (Mean)
0
15
25
35
45
50
55
65
75
Electrode distance [mm]
Figure 4
Related Breakdown Voltage of a Inhomogeneous Point-to-Plate Electrode Arrangement in
Different Insulation Liquids
Table 1
Insulation Arrangements in Power Transformers and Related Significant Stress for Design
Kind of
Arrangement
Insulation structures
inside Windings
Slight inhomogeneous
insulation arrangement
Inhomogeneous
arrangements
Critical Stress Type
Ratio of Oil to Ester
Dielectric Strength
(Approach)
LI and AC service
stresses
about 1.4 for LI stresses
LI and AC stresses
All kinds of stresses
1.35–1.2, dependent on stress
type
1.35–1.2, dependent on
degree of homogeneity
AC Stress and Partial Discharge for Alternative Liquids
In addition to the impulse stresses, the partial discharge behavior and the withstand strength at
AC stresses was also investigated for ester fluids. It was found that in the case of partial
discharges, the energy input of the discharges is much higher for ester liquids. Therefore,
inhomogeneous arrangements and non-insulated metal elements on ground potential should be
avoided if they are close to high voltage windings or leads. In particular, the areas around esterinsulated lead arrangements have to be analyzed in detail, and lead arrangements will need to be
modified compared to mineral oil design.
At AC stresses, the dielectric strength of large ester liquid gaps is reduced compared to mineral
oil gaps. Consequently, large oil gaps have to be avoided in the design of transformers with ester
10
application. This has to be considered mainly for lead arrangement design. Especially at high
AC voltage tests (395 kV and above), this is important for lead design. In some cases, the lead
design for ester application is comparable to those of High Voltage Direct Current (HVDC)
transformers.
In summary, different dielectric characteristics of esters need to be considered for insulation
design. In particular, distances between windings and grounded elements need to be adjusted
accordingly. The insulation designs of oil ducts within barriers at winding end and lead
arrangement also have to be modified, in comparison to mineral oil insulation systems.
Depending on the power level and the insulation level of the transformer, this may lead to an
increase in the physical dimensions of the transformer by up to 10 percent. Specifically in large
power transformers, the dielectric characteristics of ester fluids will have a significant influence
on the insulation structure.
Characteristics with Impact to Mechanical Design and Processes
With respect to insulating liquids, the most important issues for mechanical design and
processing are oxidation behavior of the liquid, kinematic viscosity, expansion coefficient,
material compatibility, and pour point of the liquid.
Impregnation Issues for Alternative Liquids
The kinematic viscosity strongly influences the impregnation of the solid insulation material
based on cellulose. The viscosity of ester liquids is four times higher than that of mineral oil in
the range of normal processing temperatures (about 70 °C (158 °F)). Therefore, the mechanical
design of large insulation structures as well the transformer impregnation process itself has to be
adjusted. For example, design of large wooden and pressboard elements needs to be
reconsidered when using esters. Installation of drying and impregnation holes is suggested to
speed up the impregnation process. Impregnation tests of pressboard imply that it is necessary to
extend impregnation time if the same boundary conditions (such as liquid temperature) are
maintained.
Figure 5 illustrates the impregnation behavior of laminated pressboard LB3.1.A in different
liquids and as a function of time. The impregnation depth related to impregnation volume is
shown. It can clearly be observed that it takes at least two times longer to totally impregnate the
laminated board with ester. The liquid temperature for both specimens is equal. Natural and
synthetic esters behave similarly because of equal kinematic viscosity behavior. In order to
speed up the process of impregnation, the ester liquid should be processed at slightly higher
temperatures. However, liquid temperature has to be limited depending on the composition of
the ester liquid. Especially in the case of natural esters, additives will be removed at higher
temperatures in vacuum condition.
To balance the process for successful transformer application, it will be useful to extend the time
of impregnation and to increase ester liquid temperature slightly. This combination will achieve
a sufficient impregnation of solid insulation material.
11
1,2
Standardised impregnation volume of specimen
1,0
0,8
0,6
0,4
Mineral oil at 60 °C
0,2
Ester liquid at 60°C
0,0
0
48
96
144
192
240
288
336
384
432
480
Impregnation time [h]
Figure 5
Impregnation Behavior of Pressboard Specimen in Different Liquids; Impregnation Depth is
Related to Volume as a Function of Impregnation Time
As described previously, the kinematic viscosity must be considered in the design of the inner
and outer cooling circuit. Specifically, pumps have to be able to carry a higher viscose liquid,
and this will result also in higher power consumption of the pump. Consequently, the whole
control system has to be adjusted.
Oxidation Stability
The oxidation stability is a weak point of natural esters. In the case of higher temperatures and
liquid exposure to environmental air, the oxidation of esters increases significantly. Measures in
transformer design have to be implemented to avoid exposure to the environment. Different
options are available, such as hermetic (or sealed-tank) design of the transformer with nitrogen
cushion for the liquid expansion. An additional option is to accommodate expansion with a
conservator that is sealed by a special rubber bag. The amount of oil and the mechanical
construction of the unit will determine the best final solution. It is important that the ester
liquid’s different expansion coefficient is considered for the conservator design.
Material Compatibility
Because esters have different chemical characteristics, it is important to check material
compatibility of the liquid to solid materials used in transformers, including metal parts. The
tests have to be conducted according to existing standards, both internal and external. In
particular, materials used for sealing purposes, like rubber, need to be investigated. Furthermore,
all components need to be checked by the supplier or by the manufacturer for compatibility for
ester application. Bushings, cable joints, and tap changers have to be approved for ester
12
application, not only because of material compatibility, but also because these components need
to be qualified for dielectric strength and cooling purpose in ester liquid.
Low Temperature Operation
In some global regions, there are special environmental conditions, especially with respect to
temperature profile. In the case of natural esters, low temperatures application can be critical
because esters have a higher pour point than mineral oil (about -21 °C. or -5 °F). However,
solidification of esters begins at approximately -10 °C (14 °F), so it is important to consider this
in the design of the transformer. Many transformer specifications request lowest ambient
temperatures of lower than -25 °C (-13 °F). In this case, the evaluation of the yearly temperature
profile will be necessary. If the time period of low temperatures is rather short, some simple
measures can be taken to avoid solidification of the fluid. A broad variance of alternatives is
available to master the challenge of natural ester solidification, but it has to be determined which
alternative will be the optimum from case to case. For example, operating regulations can be
established for the transformer service condition in case of ambient low temperature (for
example, short-term low ambient temperature can be mastered by control of the outer cooling
circuit.) There are also mechanical alternatives that will enable a re-heating of the liquid, if the
ambient temperature is below the pour point for a longer period.
Characteristics with Impact to Environment and Facilities
The most important characteristics of alternative fluids are the environmental properties.
Biodegradability is one of the most significant benefits of natural, as well as synthetic, esters.
Natural esters are known to be up to 100 percent biodegradable, depending on the composition of
the natural esters. Synthetic esters are up to 90 percent biodegradable. Mineral oil is nondegradable within an acceptable time period, and just 10 percent of the composition of mineral
oil will be naturally defragmented. Table 2 shows the most important characteristics of the
liquids in regard to environmental health and safety. Because GTL-based transformer oil is
specified according to IEC 60296, its properties are almost identical to those of mineral oil.
Consequently, no extra data is given for GTL in Table 2.
The high degree of biodegradability of esters enables a financial benefit for facility investment,
because no separate containment facility needs to be installed to prevent pollution of the
environment in the event of leakage.
13
Table 2
Typical Environmental Characteristics of Liquids
Property
(Mean Values)
Mineral Oil
(IEC 60296)
Synthetic
Ester
(IEC 61099 )
Natural Ester
Fire point (°C)
°C
170
325
360
Flash point (°C)
°C
160
275
325
Pour Point (°C)
°C
-50
-60
-21
Biodegradability
-
Non
biodegradability
80–95%
Up to 100%
Water hazard class
-
Yes
None
None
Raw material
-
Crude oil
Chemical
Intermediate
Good
Renewable
Resource
*1
*1
Non biodegradability within an adequate time period, which should be below several decades
With respect to safety, it is critical to examine the flash point and fire point of liquids.
According to Table 2, esters have a great advantage compared to mineral oil.
To illustrate the benefit of esters in regard to flash and fire points, a rough estimation for energy
consumption in the case of heating the liquid from 70 °C (158 °F) to fire point was done.
Because the result is only a rough calculation, the absolute values are not of interest, but rather
the ratio between mineral oil and ester liquid is considered. With Equation 1, energy
consumption was calculated by consideration of the liquid density (ρ), the volume (V), the
specific heat capacity (c), the fire point (TFire), and the starting temperature (Tliquid) of the liquid.
For a one-liter liquid volume, approximately 2.9 times energy consumption was estimated for
ester as compared to mineral oil. This result is influenced by the heat capacity of the liquid as
well as the fire point.
Consequently, the flash and fire point characteristics of esters could potentially have a huge
impact on the fire protection classification of the facility where the transformer is installed. It
may be possible for requirements of the fire extinguishing system to be reduced, which would
decrease the overall cost of the facility.
W = ρ ⋅ c ⋅ V ⋅ (TFire − Tliquid )
(1)
Furthermore, this higher energy consumption lowers the risk of fire and explosion in the event of
an electrical equipment failure. Because there is a higher energy consumption needed for ester
fluid combustion in the event of an electrical failure, it will take longer for the ester-filled
equipment to start burning. This combustion time may be long enough to permit de-energization
14
of the failure by the protection devices. This may make ester-filled transformers preferable in
highly populated locations where safety is of particular importance.
The Practice of Alternative Liquid Application in Power Transformers
First introduced in the early 21st century, the use of alternative liquids, especially synthetic
esters, is a well-established practice with distribution transformers. Similarly, retrofit
transformers that are refilled with alternative liquids are also an accepted practice.
The story of alternative liquids began in 2004 when a medium power transformer with a rated
voltage of 238 kV and a rated power of 135 MVA was filled and tested successfully with a
synthetic ester for the first time at Siemens. Several projects followed throughout the years.
Then, in 2008, a medium power transformer with a rated voltage of 110 kV and a rated power of
40 MVA was filled and tested successfully for the first time with a natural ester.
The first power transformers with GTL-based insulation fluid were introduced in 2012. This
liquid is approved for power transformers up to 600 MVA rated power and 500 kV rated
insulation voltage.
In April 2013, the world’s first large power transformer filled with natural ester was successfully
tested. This transformer was developed and built by Siemens at their power transformer factory
in Nuremberg, Germany. This transformer used new design criteria and is the largest unit using
a renewable resource liquid. Table 3 documents the important specifications of that transformer.
Figure 6 is a picture of the unit in the test bay of the transformer factory.
The rated power of this transformer is 300 MVA, with overload condition up to 400 MVA. For
the cooling type, forced cooling for the inner and outer cooling circuit was used, but it was also
specified to run for 180 MVA load conditions with non-forced cooling. The cooling performance
was designed according to the limits given in standard IEC 60076-2. The potential of the esters
to experience limiting top oil temperatures was not encountered in this design. Therefore, this
unit will have a longer expected life span than a similar mineral oil unit.
This transformer will operate in the highest voltage level in the power grid of Germany, so the
rated voltage was specified to 380 kV, and the insulation levels were specified according to
standard IEC 60076-3. As previously discussed, the design rules for insulation design had to be
modified. Several experiments were done to determine the spread between mineral oil
composite insulation arrangement and ester composite insulation arrangement. Existing design
rules for homogeneous and slightly inhomogeneous arrangements, as well as design rules for
different kinds of stresses (e.g., AC, LI, and SI), were adjusted to the empirically evaluated
withstand strength of natural esters. All of the insulation modifications confirmed the success of
this extra high voltage (EHV) transformer. Also, the special partial discharge requirement given
in transformer specification was fulfilled. A partial discharge level of below 10 pico coulombs
(pC) at 1.1 of rated voltage was requested and successfully achieved in a factory test.
Of course, the mechanical design of this transformer is also special in some respects. In
particular, sealing equipment materials based on fluororubber were used. All components were
15
qualified to operate in natural ester liquid. The bushings for medium voltage and wye
connection were specially designed by the supplier and type tested in the Research and Testing
Laboratory of the transformer factory in Nuremberg by the end of 2012. For the low voltage
connection, cable joints are used with special coupling capacitors. These capacitors were also
specially designed by the supplier and type tested in the transformer factory in Nuremberg.
For processing of the transformer, special treatment equipment was necessary. Therefore, an
existing treatment device was renewed and updated with materials compatible to ester oil.
Additionally, the heating device for the ester was upgraded to reach a liquid temperature greater
than 70 °C (158 °F). This equipment was used for factory and on-site treatment. After
commissioning, the equipment was decontaminated and used again for mineral oil. The
impregnation time was extended – at least two times longer, as compared to mineral oil.
The customer and the manufacturer agreed to perform a joint monitoring of the ester-filled EHV
transformer in the upcoming months and years. The results will be used to increase knowledge
with respect to ester liquid behavior because state-of-the-art operational knowledge of esters is
presently limited to results available from smaller transformers.
The operation of this unit will guide further development and testing and is a pioneering feat for
the future of large transformer development.
Table 3
Specific Data of 420 kV Natural Ester Transformer
Character
Rated Voltage
Rated Power
Ratio
Cooling Type
Liquid
Insulation Level
Liquid Weight
Rate
420 kV
300 MVA
405 ± 11%/115/22 kV
KDAF/KNAN
Natural Ester
630 kV AC/1425 kV LI/1050 kV SI
~95 tons
16
Figure 6
420 kV Natural Ester Transformer in Test Bay, Nuremberg 2013-04
17
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