An Overview of the Suitability of
Vegetable Oil Dielectrics for Use in
Large Power Transformers
By
D. Martin, I. Khan, J. Dai & Z.D. Wang
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An Overview of the Suitability of Vegetable Oil Dielectrics for Use in
Large Power Transformers
Daniel Martin, Imad Khan, Jie Dai & Zhongdong Wang
University of Manchester
Abstract
It would seem a regular event that some sort of disruption affects the world’s oil supply.
Natural disasters such as hurricanes at sea and Alaskan oil spills impinge on supply while the
mere hint of conflict in certain parts of the world sends oil prices spiralling. Couple this with
ever growing world demand for oil and controversy whether oil production is past its peak
adds to the urgency to explore the use of non-fossil oils. Therefore, it can be considered of
the utmost importance to develop renewable technologies now to reduce future reliance on
fossil products. This paper presents some of the findings of the project to ascertain the
suitability of vegetable oil based dielectrics as alternatives to mineral oil in large power
transformers. So far, all of the evidence suggests that vegetable oils are indeed suitable
replacements.
Aims and Objectives of Research Project
This project has been initiated at the University of Manchester through the funding provided
by the UK Engineering and Physical Sciences Research Council and a variety of Utilities and
manufacturers involved in the power industry. These companies are AREVA T & D, EdF
Energy, National Grid, M & I Materials, Scottish Power, TJH2B and United Utilities. The
aim is to investigate whether ester oils are suitable replacements for mineral oil in large
power transformers above 132kV voltage levels. If esters are found not to be compatible with
current transformer designs, then suggest modifications to designs to permit the use of ester
oils.
Terminology
There are two types of esters available for use, one being a natural ester which has been
refined from plant materials and the second is the synthetic ester, manufactured industrially.
There are slight differences in natural and synthetic ester molecular structure which lead to
interesting physical and chemical differences.
Comparing Esters to Mineral Oil
Esters have higher flash and fire points than mineral oil making esters better suited to
transformers and essential in environments, such as underground or offshore, where fire
prevention is of high priority.
Esters are more biodegradable than mineral oil so if spillage occurs cleanup costs are
reduced. High biodegradabilities are demonstrated by esters when compared to regular
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mineral oil . FR3 is covered by the U.S. Edible Oil Regulatory Act . Midel 7131 meets the
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dual criteria for ready biodegradability as laid out in OECD 301 .
Esters are non toxic. FR3 passes the Trout Fry Acute Toxicity test and is not subject to the
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Federal Regulation of Used Oils . Midel 7131 meets the criteria of non-water hazardous set
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by the German Federal Environment Agency, Umweltbundesamt . Mineral oil contains light
3
naphthenic petroleum distillates. The International Agency for Research on Cancer regards
certain distillates as carcinogenic, however Nynas believes that very little of these distillates
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remain after refining .
Natural esters are from renewable sources. However, it is recognised that agriculture may
require a large amount of diesel to power farmyard machinery. In addition fertilizers are
required, which may require fossil fuels to produce.
Ester Use in Speciality and Distribution Transformers
Synthetic esters have been used successfully for several decades in specialty and distribution
transformers. One prominent manufacturer highlights the safety advantage of synthetic esters,
as no fires have been reported while another U.S. manufacturer indicates that over 100
Utilities are using natural esters in distribution transformers. Field studies performed using
FR3 in several distribution transformers since 1996 have concluded, so far, successful
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operation .
Ester Use in Large Power Transformers
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Recently, there have been developments in using esters in several large power transformers .
Among these are a 238kV transformer filled with a synthetic ester located in an underground
power station near Lake Malgomaj, Sweden developed by VA TECH EBG. The ester was
chosen as the site is located in an ecologically sensitive area, where the environmentally
friendly nature of esters was considered paramount. According to a policy decision by the
end user, Vattenfall AB, in future all underground stations must use esters instead of mineral
oil.
Natural esters have been chosen by various Brazilian Utilities such as CELESC and
ELETRONORTE. CELESC is using a natural ester in a 138kV 30MVA mobile transformer
and two 138kV 40MVA substation transformers. ELETRONORTE has revised its
specification to use natural esters in all transformers and reactors rated up to 138 kV, and has
recently ordered a 242 kV shunt reactor filled with a natural ester from AREVA T&D as part
of a development exercise with the manufacturer. This reactor has recently completed
manufacture and test. It was one of an order for five duplicate reactors, the others being filled
with mineral oil.
In the UK EdF and AREVA T & D have developed a natural ester filled 132kV 90MVA
transformer. Paul Dyer, EDF Energy Networks Transformer Specialist, said “We take our
environmental responsibilities seriously and are proud to work together with our partners on
this exciting trial which has benefits for the environment. We hope other distribution network
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operators can learn from our example, fitting with our ambition to be a point of reference in
the electricity industry. This breakthrough is good news for the environment and presents an
opportunity for the whole electricity industry. We will be following the progress of this trial
with interest.”
Challenges in Power Transformer Design
There are a number of challenges in large transformer design which differ from that of
distribution transformers. These include higher working voltages, larger oil gaps between
conductors and using cellulose insulation, which must be impregnated with oil. Additionally,
UK type power transformers are usually free breathing and use a breather to dry the air in the
conservator. The advantage to using a free breathing tank is that there are no issues with
pressure build-up however exposure of the oil to air allows the oil to oxidise. It is possible to
seal a free breathing transformer by placing a bag in the conservator to allow for thermal
expansion of the oil while preventing contact between the oil and atmospheric oxygen. An
unseen benefit has been that oxidation of the copper inside the transformer tends to occur in
preference to the copper reacting with the sulphur forming damaging products. It is not
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recommended to use natural esters in free breathing designs due to ester oxidation .
However, synthetic esters are suitably chemically stable for use in free breathing transformers
as have been in use in such transformer designs for several decades.
Review of Dielectric Capabilities of Esters
It is important to note that due to differences in ester and mineral oil chemistries, care must
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be taken when comparing specifications and setting acceptance criteria . In addition, it is
critical to interpret a measurement result correctly when reviewing the mechanisms behind
the degradation of transformer insulation. Standards are available to advise on acceptance and
maintenance of esters, however it is somewhat of a disadvantage that BS/IEC 61099:1992
focuses on synthetic esters while ASTM D6871 considers natural esters. Therefore, it can be
difficult to compare standards.
AC and Impulse Dielectric Strength
Test standards compare the AC dielectric strengths of oils to either BS/IEC 60156 or ASTM
D1816. The effect of higher ester viscosity on the effectiveness of measuring the breakdown
voltage has been considered, and it has been concluded in the case of FR3, that the inter-test
stir time is sufficient to disperse breakdown products however recommends a longer 15
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minute stand time after pouring .
The dielectric strength of an oil can be a difficult quantity to compare due to the large number
of variables involved which can affect the breakdown voltage, such as impurities, water and
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particles. Water is often described as the number one enemy of insulation , with an oil being
of no exception. Although manufacturers and researchers provide breakdown voltage data, it
is essential to consider the effect of the moisture content. If the oil is too wet, poor dielectric
performance may be incorrectly attributed to the oil and not the water content.
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Effect of Moisture on Oil Dielectric Strength
There are two mechanisms for oil to increase in water content in a transformer. The first is
through absorption from the atmosphere, unlikely if an air drier is used or the transformer is
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sealed, and the second is ageing of the cellulose insulation creating water . It has been
concluded that it is the percentage saturation of water in the oil which affects breakdown
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voltage rather than the absolute moisture content . As esters are far more hygroscopic than
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mineral oil , moisture has less of an impact on the dielectric strength of esters than mineral
oil.
Table 1
Comparison of water solubility of transformer oils
Oil
Solubility @ 20°C (ppm)
Mineral oil
Natural ester
Synthetic ester
55
1,100
2,700
In an investigation performed at the University of Manchester, glycerol solution was used to
vary air relative humidity in a desiccator. Oil was then allowed to uptake moisture from the
air over the duration of a week. It can be seen from figure 1 that esters retain high AC
dielectric strength with increasing moisture.
Figure 1
Oil breakdown voltages as functions of absolute moisture (left) and relative humidity (right)
performed to ASTM D1816 1mm gap
It can be seen from figure 2 that although the lightning impulse dielectric strengths of these
esters are less than mineral oil when dry, they are comparable when the moisture content of
the mineral oil is greater than around 10ppm.
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Figure 2
Lightning impulse voltage as functions of absolute moisture (left) and relative humidity
(right) performed to ASTM D3300
Oil Quality and Insulation Ageing
The quality of the oil directly affects the condition of the cellulose insulation as both moisture
and acid content affect the rate of cellulose degradation. It has been proposed that natural
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esters can extend the remaining life of a transformer by protecting the cellulose insulation .
It is hypothesized that natural esters reduce the rate of cellulose ageing by removing water
from the cellulose and the benignity of the compounds created during oil ageing. Acids
created during oil oxidation affect the rate of cellulosic degradation. ZTZ Service believes
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that acids formed in mineral oil are detrimental , while CPS considers that acids produced by
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esters are beneficial . This is accounted for by differences in the chemical structures of acids
formed by esters and mineral oil.
Review of Dielectric Loss and Acidity
Dielectric loss (tan į) and acidity can be used to compare chemical stability. However, esters
are generally more polar than mineral oil and different by-products are formed during
chemical reactions, therefore, care must be taken when comparing measurements. It is
important to understand the effects of the by-products resulting from chemical instability on
overall transformer performance.
Esters degrade in a different manner to mineral oil. Mineral oil oxidises in the presence of
oxygen whereas an ester can hydrolyse in the presence of water as well as oxidise. Ester
hydrolysis is viewed as beneficial as this removes water, keeping the cellulose insulation dry.
Additionally, the transesterification reaction has been proposed, where acids formed from
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natural esters bond with cellulose insulation and protect the cellulose from absorbing water .
Ester and mineral oil degradation mechanisms create acids and polar compounds, which
increase tan į and acidity, however such measurements will not provide any information on
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the nature or impact of the by-products.
Oil Degradation Through Biological Action
It is noted that although esters are biodegradable, they do not degrade inside the transformer
tank. One manufacturer has monitored free breathing transformers since 1996 and has
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concluded, that in the case of FR3, no signs of micro-organisms have been found . The
explanation proposed is that transformer tanks are too dry to permit organisation growth, and
that if water is dissolved in the ester, it is locked away from the micro-organism. If a spillage
occurs, then there is an abundance of water in the environment to allow micro-organisms to
feed on the oil.
Chemical Stability Investigations
A critique of certain, previous, investigations is that researchers may have used accelerated
ageing test methods unrealistic of a transformer environment. Such tests may have employed
either high temperatures, oxygen atmospheres or pressure; conditions that would not
normally be encountered in a power transformer. Furthermore, it is difficult to extrapolate
reaction rates from an investigation to the transformer environment due to cyclic loading,
localised hotspots and temperature variations due to oil circulation.
The aim of the investigation performed at the University of Manchester was to ascertain how,
in the presence of air, esters degrade compared to mineral oil. Samples of oil were heated at
115°C for up to 28 days in an air circulating oven with copper catalyst. The AC and lightning
impulse dielectric strengths were found as well as tan į and acidity. The dielectric strengths
of the esters show remarkable stability during 28 day ageing.
Figure 3
AC breakdown voltage (left) and Lightning impulse breakdown voltage (right) of aged oils
performed to ASTM D1816 1mm gap and ASTM 3300
The AC dielectric strength of mineral oil appears to increase first then decrease. The highest
point had the lowest moisture content of all samples of mineral oil and it is known that at the
start of thermal ageing, the dielectric capability of mineral oil can be influenced by driving
off impurities and lighter oil fractions. The mineral oil sludged, whereas no sludge was seen
in either ester, which may have affected the dielectric strength. These results infer that it may
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not be the ageing of the oil which affects transformer insulation operation, therefore, it is
important to show if the products created during oil degradation affect the rate of cellulose
ageing.
The rate of change in acidity and tan delta is an effective indication whether or not oil
degradation is occurring as these will be affected by the products of chemical reactions. It can
be seen that the inclusion of copper increases the rate of acid production in mineral oil,
however decreased the rate of acid production in the natural ester. The copper has not
affected the synthetic ester, demonstrating good chemical stability.
Figure 4
Acidities due to ageing in air (left) and ageing in air + copper (right)
In the case of the tan delta, copper has increased the rate of polar compound production in the
natural ester. Thus, it can be concluded with the natural ester that in the presence of air and
copper, the resultant compounds are highly polar although have little or no acidity. This is
different to mineral oil as the resultant compounds are acidic, but not very polar.
Figure 5
Dielectric dissipation factors of aged oils
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CPS believes that the compounds formed by the natural ester are most likely to be ketones
and aldehydes. Without copper, which is acting as a pre-oxidation catalyst, the natural ester is
likely to be hydrolysing into weak acids. This explains why the acidity is increasing although
the tan delta is mostly constant. This would infer that if the natural ester were exposed to air,
it would not form the weak acids which are touted as beneficial to transformer cellulose
insulation. This demonstrates that esters degrade differently to mineral oil and that more
research is required to ascertain the impact of ageing by-products on the long term operation
of the transformer. The importance of this is that it adds credence to the belief that natural
esters should only be used in sealed systems. The synthetic ester appears to have been only
mildly affected, as neither acidity nor tan delta increases significantly.
o
At the operating maximum top oil temperature, normally 90 C, both mineral oil and esters
should be stable for long periods of time without much change. Chemical stability can be
assessed by the gas emitted during ageing reactions.
Oil and paper samples were sealed in glass bottles and heated uniformly in an air circulating
o
oven at temperature of 90 C for a period of time up to 14 days, and dissolved gas analysis
(DGA) was carried out. The mineral oil is Nynas Nitro 10GBN, the synthetic ester is Midel
7131 and the natural ester is FR3. The period of time is chosen arbitrarily. Table 2 compares
the concentration of fault gases of mineral oil, synthetic ester and natural ester with Kraft
o
paper at 90 C. Of the three oils, although natural ester generated the smallest volume of fault
o
gases, it generated a significant amount of ethane and hydrogen. Paper inclusion at 90 C has
caused the increase of carbon monoxide and carbon dioxide, CO and CO , as compared to
2
that of oils only. These gases are key indicators for cellulose related faults, in both mineral oil
and esters. The concentrations of CO and CO are highest in mineral oil, lower in synthetic
2
ester and the least in natural esters.
Table 2
o
DGA results of mineral oil and esters at 90 C (oil and paper)
Oil type
Gas (ppm) /Duration
Mineral oil
Control
Synthetic ester
14 days
Natural ester
Control
14 days
Control
14 days
H2
8
46
7
13
8
244
CH4
1
10
1
3
1
6
C2H6
0
2
1
0
1
116
C2H4
1
2
1
1
1
2
C2H2
1
1
1
1
1
0
CO
6
590
5
307
6
88
CO2
108
3407
45
2212
82
1354
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Cellulose ageing can be detected by the reduction in degree of polymerisation (DP). The DP
values of paper samples were analysed and the results shown in Figure 6. The DP results of
paper aged in ester indicate that the paper integrity may be preserved.
Figure 6
DP values for mineral oil and esters impregnated paper at control, 90 and 150°C
Fault Detection Using DGA Results for Alternative Oils
DGA has been used as an effective tool to detect incipient faults in mineral oil filled
transformers. Two broad categories of faults, i.e. thermal and electrical, can be detected by
DGA. The electrical fault can be further classified into low energy partial discharges and high
energy arcing. Faults could occur in the bulk of oil as well as in cellulose/oil interface. In
order to apply the DGA diagnostic method on ester filled transformers, it is necessary to
determine if the same types of fault gases are generated, and then to identify the generation
rate and the concentration of the fault gases in alternative oils, as compared with mineral oil.
Thermal Tests
o
Oil samples, with and without paper inclusion, were subjected to 200 C, the lower limit of
medium fault temperature range for one hour. The DGA results of mineral oil and esters were
obtained as shown in Table 3 on the following page.
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Table 3
o
DGA results of mineral oil and esters at 200 C (oil only)
Oil type
Mineral oil
Synthetic ester
Natural ester
Gas (ppm) /Duration
Control
1 hour
Control
1 hour
Control
1 hour
H
5
21
7
8
8
17
1
95
0
16
1
7
0
48
0
4
2
177
1
9
1
3
1
4
*
0
2
CH
4
CH
2
CH
2
CH
6
4
*
5
0
0
CO
1
18
148
9
CO2
73
1006
111
2
2
74
6
6
68
521
82
914
* C H in control samples is considered as affected by measurement accuracy
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2
Figure 7 shows the relative percentages of dissolved combustible gases in mineral oil and
esters with and without paper inclusion. The relative percentages of methane, ethane and
o
ethylene become noticeable for mineral and synthetic ester at 200 C, whereas the relative
percentage of ethane in natural ester is the highest of the six gases.
Figure 7
o
Relative percentages of dissolved combustible gases for mineral oil and esters at 200 C with
and without paper inclusion
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Electrical Tests
Arcing Tests. The needle to plate electrode configuration, with an oil gap distance of
15mm, was adopted. Table 4 shows the normalized DGA results of 20 breakdowns of mineral
oil and esters. A total of 20 breakdowns were executed having a one minute interval between
successive two breakdowns. Acetylene is a key gas produced during arcing, and is a primary
indicator for this type of high energy fault. Hydrogen and ethylene are also evident in
significant amounts. Although the same energy arcing occurred in the three oils, acetylene
concentration in mineral oil is about 5 to 10 times higher than that found in esters. synthetic
ester has the lowest concentration of gases. Acetylene, hydrogen and ethylene are in high
concentrations in mineral oil and are in the lowest concentrations in natural ester.
Table 4
Normalized DGA results for arcing in mineral oil and esters
Gas (ppm) / Oil type
Mineral oil
Synthetic ester
Natural ester
H2
901
97
191
CH4
145
9
14
C2H6
24
2
10
C2H4
270
26
63
C2H2
1540
126
280
CO
6
37
51
Figure 8
Relative percentages of dissolved combustible gases for mineral oil and esters as a result of
20 breakdowns
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Partial Discharge (PD) Tests . The PD test circuit differs from the arcing circuit only by
the adding of a water resistor between the high voltage sources to the oil test vessel.
Table 5 shows the normalised DGA results of three types of fluid under the conditions of PD
activity for 1 hour. Hydrogen is the key indicator for low energy discharge. Hydrogen is
found in the highest concentration in mineral oil and is the lowest in synthetic ester.
Table 5
Normalized DGA results for 1 hour PD activity in mineral oil and esters
Gas/Oils
H
2
CH
4
CH
2
CH
2
CH
2
6
4
2
CO
Mineral
20
Synthetic ester
5
Natural ester
23
2
2
2
0
0
1
2
0
2
2
0
2
2
2
8
Although the molecular structures of esters differ from that of mineral oil, the profile of
thermal fault indicating gases is the same for both types of fluid. The cleavage and
recombination of molecular fragments split from esters seems to give rise to lower fault gas
concentrations as compared to those found in mineral oil. Esters seem to be more stable under
medium temperature range thermal fault conditions.
However, natural ester generates a significant amount of ethane under thermal faults, and this
may identify ethane as a key indicator of thermal faults in natural ester.
In electrical faults, Acetylene is the key gas for indicating high energy arcing and hydrogen is
the key gas for indicating low energy partial discharges. Under the same electric faults, esters
generate faults gases 5 to 10 times less than mineral oil.
Interaction Between Esters and Cellulose
At the voltage level of 132kV and above, the transformer insulation system consists of oil and
oil-impregnated cellulose, and the life of transformer is mainly dominated by the cellulose
insulation. To apply esters in large transformers, ester impregnated solid insulation should be
proven to have comparable dielectric strength to mineral oil impregnated solid insulation.
Impregnation of Solid Insulation with Ester Fluids
Solid insulations including paper, pressboard, and blocks, need to be impregnated by
insulation oil to have better dielectric properties. There are three variables that would affect
the impregnation result: the pressure, the impregnation time under vacuum and the viscosity
of the fluid. Ester based fluids have higher viscosities than conventional mineral oil, which
raises an issue whether the impregnation by the ester would take much longer time than the
mineral oil.
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Impregnation of both paper and pressboard brings no technical problem due to their thin
thickness. Experimental tests in laboratory show that paper with thickness less than 0.5mm
o
can be fully impregnated by an ester under 15mbar vacuum at 60 C within 12 hours, whereas
3 mm thick pressboard would need 48 hours.
Impregnation of laminated blocks, either pressboard blocks or wood blocks, brings more
engineering challenges because of their greater thickness, the more viscous fluid and
consequently a longer impregnation time. Some comparative laboratory experiments were
carried out in the laboratory to study the impregnation process of blocks by mineral oil and
ester fluid. Increasing the temperature of oil can reduce its viscosity and thereby shorten the
impregnation time, still the exact temperature and time need to be determined for transformer
manufacture.
Figure 9
Viscosities vs. temperature
Figure 10
Laminated block impregnation by ester
As shown in figure 9, the viscosity of natural ester is much higher than that of mineral oil and
the viscosity of oil decreases quickly as the temperature increases. The viscosity of natural
o
o
ester at 60 C is approximately the same as the viscosity of mineral oil at 20 C. In order to
reveal the effect of temperature upon the impregnation of blocks in esters, two Weidmann
o
o
pressboard blocks were impregnated under 20 C and 80 C respectively. The dimensions of
the laminated block used in this test are 101.6×101.6×34.3 mm (4×4×1.35 inch) having a hole
with diameter of 12.7 mm (0.5 inch) in the centre, and four side faces of block were sealed by
epoxy resin. The reason of fabricating blocks in this way is to simulate a real impregnation
condition of laminated blocks. In transformers, the supporting blocks are normally drilled
with holes to help impregnation, at the same time the mechanical strength of laminated
blocks would not be compromised. The distance between two adjacent holes is 101.6 mm (4
inches), which means that the oil need to travel 2 inches to achieve full impregnation.
As shown in figure 10, there was remarkable difference between the impregnation under
o
o
20 C and 80 C. At first 48 hours, both block samples have similar impregnation
speedDŽHowever the impact of viscosity on impregnation appeared later. Ester has
o
o
approximately 6 times greater viscosity under 20 C than under 80 C, yet the impregnation
o
o
volume at 80 C is only doubled the volume at 20 C. Increasing the impregnation temperature
helps to facilitate the impregnation but not proportionally.
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Figure 11
o
Pressboard blocks impregnated by mineral oil and ester fluid (10mbar, 65 C)
Figure 11 shows the comparative impregnation speed of pressboard block by nature ester and
mineral oil. The pressboard blocks with dimension of 248×110×45 mm were used. Although
o
natural ester at 65 C is 3-4 times more viscous than mineral oil, the difference of
impregnation volume after 72 hours is negligible between mineral oil and nature ester.
High viscosity of ester based fluids requires longer time for impregnation than mineral oil.
Fortunately, it was found that the impregnation time was not proportion to viscosity as stated
before, and ester impregnates solid insulation faster than expectation.
Dielectric Capability of Ester-Impregnated Cellulose
Paper insulation in a transformer normally takes the electrical stress between turn to turn
under ac operating voltage and impulse surges. The designed electric stresses onto the paper
insulation need to be lower than the dielectric strength and a proper safety margin should be
maintained.
Figure 12
Dielectric strength comparison of different oil impregnated paper
3
(Paper density= 0.93 g/cm ; moisture< 0.2%)
As shown in figure 12, the dielectric strengths of oil impregnated paper decrease as the
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thickness of paper increases; and within controlled moisture level, ester impregnated paper
shows comparable dielectric strength to mineral oil impregnated paper. Similar to pressboard,
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paper with less density would have lower ac breakdown voltages . Laboratory test results
3
show that layer insulation paper with density of 0.75g/cm has lower dielectric strength than
3
paper with density of 0.93 g/cm . Nevertheless, the test results also indicate that the ester
impregnated layer insulation paper has comparable dielectric strength to mineral oil
impregnated paper.
Ester Impregnated Pressboard
Oil impregnated pressboard is widely used between transformer windings as oil barriers for
breaking up large oil gaps and acting as mechanical support. Withstand voltage tests on
mineral oil or ester impregnated pressboards were carried out with different electrode
geometries. It was found that direct breakdown of pressboard itself rarely happened; it is the
failure of the weaker component of oil/pressboard interface, that gradually damages the
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cellulose surface, known as creep discharge , and finally causes ‘breakdown’ of pressboard.
Considering this, the breakdown field strength calculated as breakdown voltage divided by
thickness of pressboard ‘cannot be regarded as the true strength but only as a mean apparent
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strength of the transformer board’ .
AC Stress Test (ester-pressboard vs. mineral-pressboard)
Mineral oil impregnated pressboard and natural ester impregnated pressboard were tested
using partial sphere electrodes (see figure 13) under ac voltages. During tests, the ac voltage
was raised up to 75kV with increasing speed of 0.5kV/s.
Figure 13
Partial sphere electrodes
Mineral oil impregnated pressboard
Ester impregnated pressboard
Figure 14
Pressboard sample after test (moisture <0.5%)
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Considerable surface discharges occurred in the oil wedge of mineral oil impregnated
pressboard and caused breakdown after 5 test cycles, while the natural ester impregnated
pressboard withstood the maximum voltage of 75kV with out surface discharges. Figure 14
shows the pressboard samples after test.
Although the results are in favour of the ester impregnated pressboard, the above-mentioned
pressboard breakdown test is not comparable in different oil medium. The lower permittivity
ratio of ester-impregnated pressboard to ester fluid results in less stress taken by oil wedge,
which prevented creep discharge initiation on ester impregnated pressboard. Lower
permittivity ratio of oil-impregnated pressboard to oil is beneficial in oil-pressboard-oil
system, since less stress will be distributed in oil duct or oil wedge, which is helpful to
prevent the occurrence of the creep discharge.
Summary
This paper has presented an overview of the research carried out at the University of
Manchester on alternative oils, and so far based on the evidence in this paper has found that
esters are likely to be suitable replacements for mineral oil in large power transformers.
However, it is noted that this is an ongoing activity with further research being required to
study the behaviour of esters in areas such as large oil gaps.
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Biography
Daniel Martin was awarded the degree of Electronic and Electrical Engineering with study
abroad in Germany by the University of Brighton, UK, in 2000. In 2000 he joined Racal
Electronics, which went on to form the international electronics company Thales, working on
communication and aircraft systems. In 2004 he left Thales to pursue his PhD in high voltage
technologies at the University of Manchester, UK, and is a recipient of a EPSRC CASE
scholarship.
Imad Khan received a BEng (Hons) degree in Electrical Engineering from the National
University of Science and Technology, Pakistan in 2004. He is currently a PhD student at the
Electrical Energy and Power Systems Group at the University of Manchester. His research
interests include Alternative transformer insulation, Electric stress analysis using FEM and
Dissolved gas analysis. He is a Student Member of the Institution of Engineering and
Technology.
Jie Dai received a BSc degree in Electronic Engineering from the University of Electronic
Science and Technology of China (UESTC) in 2003 and an MSc degree in Electrical Power
Engineering from the University of Manchester in 2005. He is currently a PhD student at the
Electrical Energy and Power Systems Group at the University of Manchester and carrying out
research on transformer solid insulation.
Zhongdong Wang received a BEng and a MEng degree in High Voltage Engineering from
Tsinghua University of Beijing in 1991 and 1993, and a PhD. in Electrical Engineering from
UMIST in 1999. Since 2000 Dr. Wang has been a Lecturer at the Electrical Energy and
Power Systems Group at the University of Manchester. Her current research interests include
condition monitoring, transients’ simulation, transformer insulation ageing and alternative
insulation materials.
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References
[1] T.V Oommen, C.C. Claiborne, J.T. Mullen “Biodegradable Electrical Insulation Fluids”,
proceedings of the 1997 Electrical Insulation Conference, Pages 465 – 468
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