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CIGRE 2014
EXPERIENCE WITH THE FIRST 230 kV SHELL TYPE AUTOTRANSFORMER
RETRO-FILLED WITH NATURAL ESTER ON MEXICAN GRID
R. OCÓN *
Industrias IEM
R. MONTES
Mexican Utility
R. LIÑAN, A. GUZMÁN
Instituto de Investigaciones
Eléctricas
MÉXICO
SUMMARY
The improvement of performance of power transformers is a permanent care for manufacturers and
utilities since these constitute one of the most expensive and strategic components of electric power
transmission and distribution systems. Alternative fluids such as natural and synthetic esters are
increasingly being used for distribution and power transformers as a substitute of mineral oil. Some
advantages are its higher fire and flash points and because are less harmful for the environment; also it
was shown that these fluids can extend transformer insulation life.
A research project was developed between Mexican utility, IIE (Electrical Research Institute) and
transformer manufacturer in order to evaluate natural ester fluid performance in a 230 kV shell type
autotransformer, installed in a strategically location on the Mexican grid. Autotransformer name plate
data are: single phase, 25/33.33 MVA, 230/115/13.8 kV, shell type design. The autotransformer was
original built in 1978, and retro-filled in 2011 with natural ester.
The research project followed different stages, first a study of transformer retro-filled viability was
carried out, including an analysis of dielectric and thermal autotransformer performance operating
with natural ester; second the transformer condition was evaluated with field testing and then
transformer was taken to manufacturer factory in order to replace bushings, gaskets, re-design oil
preservation system and install on-line monitoring equipment. Full factory tests at 100% testing levels
was performed on HV Laboratory, with the autotransformer filled first with mineral oil and then
repeated retro-filled with natural ester, a comparison between test results was made in order to
evaluate the new operation conditions of the autotransformer. Besides, considerable studies were
conducted by IIE to investigate dielectric strength using various electrode configurations with uniform
and non-uniform electrical fields, moisture sensitivity, partial discharge, bacterial growth, thermal and
infrared characteristics, oxidation stability, material compatibility and temperature performance of the
vegetable oil used.
Finally after successful performance during factory testing, the autotransformer was shipped, installed
and energized on April 2011; since then, it has been monitoring. On-line monitoring equipment
includes optical fiber sensors located on winding leads and on load tap changer (OLTC), and on-line
dissolved gas analysis (DGA). Also, field tests like FDS (Frequency Dielectric Spectroscopy), FRA
(Frequency Response Analysis), infrared thermography and acoustic partial discharges, are routinely
performed in order to evaluate actual autotransformer condition.
* rocon@condumex.com.mx
Stray gassing formation of ethane was observed during the first months of operation, which is the
unexpected gas formation from some oils at relatively low temperatures in the 80 to 250°C range. This
is attributed to the stray gassing of FR3™ which is the natural ester used on this autotransformer
The paper presents field experience accumulated by the major Mexican utility over the last 2 years of
successful autotransformer operation filled with vegetable oil as well as discussion of design aspect
explained by the manufacturer and the IIE. Discussion about differences between dielectric test results
with mineral and vegetable oil in the autotransformer (impulse and partial discharge tests) are also
included on present paper. Main conclusion of this project is that natural esters represent a good
alternative as a substitute of mineral oil on power transformers.
KEYWORDS
Natural ester, autotransformer, shell type, retro-filled, factory testing, vegetable oil.
1. INTRODUCTION
The improvement of performance of power transformers is a permanent care for manufacturers and
utilities since these constitute one of the most expensive and strategic components of electric power
transmission and distribution systems. Alternative fluids such as natural and synthetic esters are
increasingly being used for distribution and power transformers as a substitute of mineral oil [1]. Some
advantages are its higher fire and flash points; a fire point greater than 300°C, as determined according
to ASTM D92, classifies the liquid as a less-flammable liquid. These fluids are less harmful for the
environment and it was shown that these fluids can extend transformer insulation life [2,6].
The main transformer oil functions are: as an electrical insulation, cooling media, to protect solid
insulation, and as a diagnostic tool for routine evaluation of the condition of electrical equipment over
its service life. Mineral oil has been used as an insulation system on transformers since the 1900s,
because it’s good aging behavior, low viscosity and low cost. Since the end of 1970´s natural and
synthetic esters which have higher flash and fire points than mineral oil, have been developed, and for
2010 over 200 power transformers up to 200 MVA and 242 kV, has been energized and operating
filled and retro-filled with natural esters [1]. The main reasons for replacement mineral oils are its
poor biodegradability and also the growing international demand for petroleum products, which could
lead to serious shortages as soon as the mid-21 century [3].
In Mexico, during 2009, utility initiated a research project in order to evaluate and gain experience
with the use of natural esters for high voltage and large power transformers. The project was carried
out by IIE (Electrical Research Institute) in conjunction with a transformer manufacturer (Industrias
IEM) which selected for retro-filled with vegetable oil, a 230 kV shell type auto-transformer installed
at a strategically location on Mexican grid. Although, autotransformer retro-filled process could be
done on site, the decision was to take the autotransformer to manufacturer factory; first because it was
necessary to replace bushings, gaskets, re-design oil preservation system and to install on-line
monitoring equipment, and second, because in this way a complete factory testing could be applied
and a better comparison between testing results with the unit filled with mineral oil and then filled
with vegetable oil could be done. Selected fluid for substituting mineral oil was FR3™ which is an
environmentally friendly coolant made from edible seed oil. It is biodegradable and non-toxic and is
classified as a natural ester fluid.
A description of the stages and acquired experiences with this project is addresses on following
paragraphs.
2. EVALUATION OF SELECTED AUTOTRANSFORMER
Selected autotransformer nameplate characteristics are: 230/115/13.8 kV, shell type design, single
phase, 60 Hz, 25/33.33 MVA, ONAN/ONAF, 55°C average winding rise, on load tap changer on HV
for ± 10% of 230 kV, 900 kV BIL at high voltage, manufactured on 1978 and located on Irapuato II
substation.
Important considerations for transformer retro-filled process includes [10,11]: Review of material
compatibility with new insulation fluid, evaluation of insulation system dielectric performance,
evaluation of transformer thermal performance, consideration for no load an on load tap changer
operation, consideration for change in capacitance, insulation power factor and insulation resistance
due to fluid properties, consideration for forced oil pumps and oil preservation system. Also the use of
dissolved gas analysis (DGA) tool for transformer diagnostic is quite different than for mineral oil,
because different gas generation rates are produced for natural ester fluids [12].
The project followed different stages, first a study and evaluation of autotransformer retro-filled
viability was carried out including material compatibility tests and a detailed analysis of dielectric and
thermal autotransformer performance, operating with natural ester.
1
Material Compatibility
Material compatibility tests followed manufacturer standards, taking samples of pressboard, paper,
core steel, gaskets, etc., placing samples in vegetable oil and heat it up to 120 °C for at least 7 days.
After this time period, oil properties changes were evaluated (interfacial tension, dielectric strength,
power factor, etc.) as well as material mechanical properties, in order to evaluate material
compatibility. From these test results a decision for changing all autotransformer gaskets was taken.
Selected material for new gaskets was hydrogenated nitrile butadiene rubber (HNBR). Above results
were also validated, testing same materials in accordance with ASTM D3455-02. Additionally,
considerable studies were conducted by IIE to investigate dielectric strength and performance of the
natural ester used [13]. For dielectric strength, various electrode configurations were used with
uniform and non uniform electrical fields (see figure 1a). Experiments about moisture sensitivity,
partial discharge, bacterial growth, thermal and infrared characteristics, oxidation stability and
temperature performance of vegetable oil were made. These studies confirmed that FR3™ is a good
alternative as a substitute of mineral oil on power transformers [13]. Figure 1b shows some results for
dielectric strength as function of moisture in oil in accordance with ASTM test methods.
(a)
(b)
Figure 1. (a) Comparison of vegetable and mineral oil dielectric strength for uniform and non uniform electrical
fields; (b) Vegetable oil dielectric strength as a function of active water content in accordance with ASTM D877
and ASTM D1816.
Dielectric Evaluation
A typical insulation system normally used in liquid-immersed transformers shall contain solid
materials for insulating the conductive parts and also a dielectric fluid for insulation and perform the
heat transfer process. These insulation materials have to withstand electrical, mechanical, chemical,
and thermal stresses for the expected life of the equipment. Natural esters have different dielectric
properties than mineral oil [7]; specifically the permittivity (dielectric constant) is quite different for
mineral oil and natural esters as well as various kraft insulation materials impregnated with these
fluids. Inside transformers, the dielectric strength of a given gap is a function of the width of the gap
[7,8]. Designer’s utilizes solid barriers to divide an oil gap into smaller gaps in order to allow a higher
average stress [8]. Electrical stress is divided between solid and liquid insulation as a function of its
dielectric constants. Also, apart from the electrical stress across the oil duct, a dielectric stress
tangential to the surface of solid insulation (creep stress) is produced and has to be calculated and
controlled by transformer designers. A detailed analysis and calculation of electrical field distribution
in the autotransformer insulation system was made using the permittivity values for different operating
conditions; filled with mineral oil and natural ester (see table 1). This calculation was performed using
a commercial finite element (FE) program. On figure 2 equipotential lines for the main
autotransformer high-low space are shown. The solid insulation barriers were considered and
appropriated materials, sources and boundary conditions were selected for 900 kV BIL.
Three cases were considered: a) conventional insulation, i.e. autotransformer filled with mineral oil, b)
autotransformer filled with natural ester and solid insulation impregnated with mineral oil and c) autotransformer filled with natural ester and solid insulation impregnated with natural ester.
2
Figure 2. Equipotential lines on main high to low space for selected auto-transformer.
Table 1. Permittivity values for different cases [7].
Material
Mineral Oil
Natural Ester
Low Density pressboard (impregnated with M.O)
High Density Pressboard (impregnated with M.O)
Low Density pressboard (impregnated with N.E)
High Density Pressboard (impregnated with N.E)
Dielectric Constant (25 °C)
2.4
3.3
3.9
4.5
4.4
4.6
The stress distribution within the selected parts of the insulation system was calculated, and the
dielectric strength of oil gaps was checked against “Weidmann Reference Curve - WRC”. This
consisted in plotting the electric field along a chosen field line in the critically stressed area,
calculation of the cumulative field Ecum(x) distribution, and comparison to the WRC that is expressed
by the formula (1):
Ecum(x) = 2.3·Eo/x0.37
[kV/mm] ----------------(1)
A factor 2.3 is introduced to account for the impulse test conditions, and Eo=18 kV/mm corresponds to
the paper coated electrodes. Safety margins confirmed that there was no dielectric problem for
autotransformer filled with FR3™ fluid. A comparison for three different cases is shown on figure 3; a
base line was plotted between series (HV winding) and common voltage windings and electrical field
was calculated over it. As it is shown on figure 3, different values of dielectric constant, produces
different distribution of electrical field between solid and liquid insulation. Better distribution of
electric field, between solid and liquid insulation was found for case (b).
Thermal Evaluation
Thermal parameters of FR3™ such as its temperature dependent viscosity, thermal conductivity,
specific heat and density were used in a cooling program in order to evaluated cooling performance
with natural ester. The viscosity curve comparison with mineral oil is presented on figure 4. From
these calculations, a maximum of 3-5 °C were found for calculated average winding rise differences,
and this is mainly because the slower moving liquid compared with mineral oil. The results confirmed
that there was not cooling problem and autotransformer could operate without modification of actual
cooling equipment. With this calculation and based on actual ambient temperature conditions a
re-design of the expansion tank and preservation system was made. The new preservation system
included a rubber bag for avoiding contact between vegetable oil and surrounded air. This is very
important due to oxidation characteristics of vegetable oils [11].
3
Figure 3. Electric field distribution for modeled Figure 4. Curves of FR3™ and typical mineral oil
cases; better condition is case (b) with transformer viscosity.
filled with natural ester and solid insulation
impregnated with mineral oil.
3. FIELD AND FACTORY TESTING
Initial field testing included routine maintenance tests like DGA, turn ratio, insulation resistance,
insulation power factor, and excitation current measurement. Also other tests like frequency response
analysis (FRA) and frequency dielectric spectroscopy (FDS) were made. All testing results indicated a
good condition of autotransformer, and then it was disassembled and shipped to manufacturer factory.
Full factory tests at 100% testing levels, in accordance with IEEE standard [14] was performed on
manufacturer HV Laboratory, with autotransformer filled first with mineral oil and then repeated with
autotransformer retro-filled with vegetable oil. On load tap changer and bushings condition were also
evaluated. After testing, all bushings were on good condition but because its age, all were replaced. On
load tap changer (resistor type, located on high voltage winding) was in good condition but because it
was not original designed for operation with vegetable oil, the decision was to leave it filled with
mineral oil.
A resume of some testing results are shown on table 2. Measured winding capacitances are larger for
autotransformer filled with FR3™ as it was expected; due to the bigger value of dielectric constant.
Insulation resistance are considerable less for auto-transformer filled with FR3™ than with mineral
oil; this is because the natural properties of vegetable oil (lower resistivity). Winding Insulation power
factors were very similar as well as polarization index. With respect to thermal performance; measured
values matched well with calculation, and a maximum of 5 °C on average winding rise for tertiary and
high voltage windings were obtained.
The changes on autotransformer capacitances affect winding impulse voltage distribution; although
the changes in the dielectric constant affect both the series capacitance and the ground capacitance of
the windings, and the change on the ratio of these capacitances are not significant and the overall
impulse voltage distribution is expected to be quite similar for transformer filled with mineral and
vegetable oil [10]; on this specific case, for high voltage and neutral terminals, the measured full
impulse waveforms were very similar; but for low voltage and tertiary terminals, measured impulse
waveforms have appreciable differences, and voltages were oscillating in different way for
autotransformer filled with mineral and vegetable oil. This is because on this specific shell type
design, low voltage coils are closer to the core and low voltage winding ground capacitances was more
affected than winding series capacitances. Figure 5 shows the full impulse waveforms comparison for
both conditions. Although autotransformer passed successfully all dielectric testing, it is recommended
to verify impulse voltage distribution for high voltage transformers that will be retro-filled.
4
Table 2. Test results for autotransformer, H =High voltage (series) windings, X = low voltage (common)
winding, and Y= tertiary winding, T= ground.
(a)
(b)
Figure 5. Full impulse voltage waveform for: a) high voltage terminal H1, b) low voltage terminal X1.
During induce voltage test, partial discharge measurement was performed in accordance with IEEE
standard [14]. For autotransformer filled with mineral oil a value of 275 pC was obtained after one
hour test. For autotransformer retro-filled, a value of 700 pC was measured on similar testing
conditions; it is suspected that different values are due to differences during drying process and/or due
to streamer propagation differences, but at this time more research has to be done with this respect.
On figure 6 a frequency response comparison between autotransformer filled with mineral and
vegetable oil is presented. The curves are displaces between each other, mainly due to the differences
of dielectric constant values for mineral and vegetable oil.
4. SHIPPING, INSTALLATION AND ENERGIZATION
After successful performance during factory testing, autotransformer was shipped, installed and
energized on April 2011. FR3 oil was shipped in a special tank-car sealed with nitrogen (5 PSI) for
avoiding contact with oxygen. During final vacuum filling process the rate of filling was controlled
5
(around 1800 L/hr) to avoid bubbling formation and a waiting time before energization of around 36
hours was established. Temperature of fluid during filling process was around 50 °C.
Monitoring equipments (DGA monitor and temperature fiber optical sensors) were also installed on
autotransformer and calibrated.
(a)
(b)
Figure 6. Autotransformer frequency response curves for: a) common winding and b) series winding.
5. ON SITE MONITORING
On-line monitoring equipment, which includes fiber optical sensors, was located on winding leads and
on load tap changer (LTC). Temperatures are continually measured. Periodic field tests like FDS
(frequency dielectric spectroscopy), FRA (Frequency response analysis), infrared thermography (see
figure 7) and acoustic partial discharges, are routinely performed in order to evaluate actual autotransformer condition. Stray gassing formation of ethane was observed during the first months of
operation (see figure 8), which is the unexpected gas formation from some oils at relatively low
temperatures in the 80 to 250°C range. This is attributed to the stray gassing of FR3™ which is the
natural ester used on this autotransformer and these are not considered a fault or a concern with the
autotransformer [15].
Figure 7. Autotransformer Infrared thermography and a view of auto-transformer on Irapuato II substation
6
Figure 8. Ethane gas concentration during first months of autotransformer operation; ethane stabilized at
around 100 ppm after 15 days from energization.
6. CONCLUSIONS.
Supervision and diagnostics of transformers are very important part of the strategic approach for risk
analysis. On this specific research project, autotransformer has successfully operated since it was
energized, and is continually monitoring. Experience obtained will be very useful for utility and
manufacturers because it was demonstrated that natural esters represent a good alternative as a
substitute of mineral oil on power transformers up to 230 KV; and in the near future a plan for retrofill existing, and to manufacture new, high voltage and large power transformer is expected.
Since the development of alternative fluids for electrical applications began, researchers, producers,
transformer manufacturers and utilities have been investigating the physical and electrical properties
of vegetable oils. Vegetable oil-cellulose insulation aging rate extends the range of application beyond
that of mineral oil-cellulose systems; this is one of the better advantages of using ester fluids.
7. ACKNOWLEDGEMENTS
We would like to send our sincerely gratitude to all technical staff from utility, IIE, IEM and
especially to Mr. Alvaro Cancino for their contributions for the present project.
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