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Comparison of HYDRAN and laboratory DGA results for
electric faults in ester transformer fluids
Jie Dai, Imadullah Khan, Z. D. Wang and I. Cotton
School of Electrical and Electronic Engineering,The University
of Manchester,
Manchester M60 1QD, UK
Abstract- Due to the properties of biodegradability and high
fire
temperature, ester fluids are becoming potential substitutes to
mineral oil
for power transformers. Laboratory based DGA (Dissolved Gas
Analysis)
techniques and the online DGA devices have been widely used on
mineral
oil filled transformers to diagnose faults. It is therefore of
importance to
understand how these techniques and devices perform with ester
fluids. In
the paper high and low energy discharges were generated in
mineral oil
and esters and the on-line readings from HYDRAN 201R were
recorded
with time and compared with the laboratory DGA results. It is
concluded
that DGA fingerprints for electrical faults in esters are
similar to those in
mineral oil. However, in the present experimental configuration,
a slower
response of HYDRAN reading was found for esters than mineral oil
due to
both the higher viscosities of esters and the limited oil
circulation.
1 Introduction
The ester fluids, with good dielectric strength, such as the
syntheticester Midel7131 and the natural ester FR3 are available in
the marketas alternatives to mineral oil. Ester fluids are
biodegradable and henceenvironmentally friendly [1, 2]. Their high
flash points and fire pointsalso make them suitable for the
transformers located in underground,offshore or urban areas for
safety reasons. Esters have already beensuccessfully used in a
great number of distribution transformers, andhave great potentials
to be applied in high voltage transformers in thefuture. However
ester filled transformers face the challenge of beinglack of
operating experience and especially the experience withcondition
monitoring and assessment, which mineral oil filledtransformer has
accumulated over years [3].
Modern high voltage transformers work with relatively
lowinsulation tolerance because of compact structure. A close
monitoringis considered important to understand the behavior of
transformers [4].DGA has been used for many years as an effective
and reliable tool todetect incipient faults in mineral oil filled
transformers. Theinformation provided by DGA analysis is extremely
important to theasset managers in electricity supply companies. It
is therefore essentialto ensure that traditional DGA analysis
techniques can still workproperly if alternative fluids are used in
power transformers. Table 1gives the types of dissolved gases
evolved during faults in a mineraloil filled transformer and their
indicative relationships with differenttypes of faults.
Former studies suggested that the fault gases or the so called
keygases were also found in esters and dissolved gas analysis
istherefore still a feasible method to monitor ester oil filled
transformers[6, 7]. However, little research covers the performance
of on-line
DGA devices in association with ester fluids. On-line
DGAmonitoring has been accepted as an efficient means for early
stagefault diagnoses, and the on-line DGA devices, such as
HYDRAN,have been widely used in mineral oil filled transformers for
years. TheHYDRAN sensor reads the ppm (part per million) value of
thedissolved gas in oil through the membrane which only allows
gases topass. The sensor mainly responds to hydrogen with 100%
efficiencyand shows low sensitivity to other gases, which is
approximately 15%to CO, 8% to C2H2 and only 1% to C2H4. Its
accuracy is said as 10%from 20OC to 40OC [10]. For high voltage
transformers, HYDRAN is
normally installed between the cooling bank and the main tank to
takethe full advantage of oil circulation. Sometimes a position on
theupper part of the transformer tank is preferred to install the
on-lineDGA monitoring devices. Although HYDRAN is unable to
providethe concentrations of all fault gases, it works reliably as
an early stagefault indicator since hydrogen is the gas which is
normally evolved inmost faults.
Table 1: Fault indicator gases [5, 6]Element Key indicator
Secondary indicator
H2 Corona Arcing, Overheated Oil
CH4 Corona, Arcing, and Overheated Oil
C2H6 Corona, Overheated Oil
C2H4Overheated
OilCorona, Arcing
C2H2Arcing Severely Overheated Oil
COOverheatedCellulose
Arcing if the fault involves cellulose
CO2Overheated Cellulose, Arcing ifthe fault involves
cellulose
O2Indicator of system leaks,over-pressurization, or changes
inpressure or temperature.
N2Indicator of system leaks,over-pressurization, or changes
inpressure or temperature.
An experiment was carried out to investigate fault gas
generation inesters with comparison to that in mineral oil and to
test theperformance of HYDRAN 201R in ester fluids. A HYDRAN
201Rsensor was connected to an oil filled test vessel, in which
partialdischarges (PD) and electric breakdown activities were
generated. Theresponses of HYDRAN towards the electrical faults in
differentinsulating fluids were recorded.
It was found that after electric faults similar types of gases
weredissolved in oil samples for mineral oil, Midel7131 and
FR3according to the laboratory DGA results. HYDRAN 201Rsuccessfully
performed as an on-line DGA indicator for ester fluidsunder
electric faults, but a certain delay in the response of
HYDRANreading was found for esters due to their relatively high
viscosities
which result in a slower oil circulation and consequently a
slowerequilibrium of gases.
2 Experimental setup
2.1 Test vessel and HYDRAN connection
The capacity of the glass test vessel is about 3 litres and a
headspace about 50ml was left above the oil surface for safety
reason. TheHYDRAN sensor was directly connected to one side of the
glass
2007 Annual Report Conference on Electrical Insulation and
Dielectric Phenomena
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vessel without any piping as shown in Fig. 1.
Fig. 1 Test vessel and HYDRAN connection
There was a distance of ~16 cm from the membrane of HYDRANsensor
to the centre of body of oil, where the electrodes were
located.Tests were done under room temperature. In order to create
a thermalcirculation, a temperature approximately 10OC higher than
theambient was maintained within the bottom oil by adding a
heating
jacket around the lower part of the test vessel.
2.2 Electric test setup
A 220V/40kV 8kVA 50Hz test transformer was used as the
voltagesource throughout the experiment. Needle electrodes were
used inboth oil gap breakdown and PD tests. The needle was
pre-processedby introducing several arcing to melt the sharp tip so
that the radius ofthe tip could remain relatively stable when used
in the tests.
a) Oil gap breakdown test: Oil gap breakdowns were achieved
byusing the needle electrode connected to the HV source leaving an
oilgap distance of 10mm above the grounded plate electrode. There
wasan over-current relay set at 3A limit on the low voltage side of
the testtransformer to control the energy during electric faults.
Theover-current relay normally operates in 20ms after the formation
of anarc.
b) PD test: It was found that a stable PD source was difficult
to
obtain from the needle to plate electrode configuration, because
thePD in oil would always propagate and cause a flashover of the
oil gap.Therefore, the following electrode arrangement was
developed toproduce a stable PD source as shown in Fig. 2.
Fig. 2 Electrode configuration for PD test
Upon the ground electrode, there was a 3mm thick pressboard
with a cylindrical electrode of floating potential sitting
above. The oilgap between the sharp needle and the floating object
was remained as0.5mm. When an electric spark happened within this
small oil gap, itwould automatically die out as the electric
breakdown built a shortcircuit between the HV electrode and the
floating object, whichthereby caused a temporary equal potential
connection andsimultaneously cleared the spark in the oil gap. The
duration of eachdischarge was so short that the current relay could
react to function. Inorder to further reduce the current during PD
tests, a 1.4 M waterresistor was connected between the test vessel
and the HV source.
The oil samples for the laboratory DGA were taken from the
bottomsampling valve of the glass vessel using syringes. During
sampling,the oil was naturally forced into glass syringes according
to the BS EN60567 standard [8]. The DGA sampling point is different
from theHYDRAN locating point, which to a certain extent inhibits
the directcomparisons of the results. This drawback will be
discussed further in
session 3.
3 Results and discussions
3.1 Oil gap breakdown test
Three breakdowns of the 10mm oil gap were made in each type
offluid with 30 seconds standing time after each breakdown. The
voltagewas manually controlled with a rising speed about 1 kV/s.
The wholeprocess took about 5 minutes. The timing of the HYDRAN
responsewas started immediately after the first breakdown. The
breakdownvoltages for the 10 mm oil gap varied from 26kV to 32kV,
and nosignificant difference was found among the breakdown voltage
valuesof the three types of fluid. Therefore, comparable energy
levels wereproduced in different fluids during breakdown tests.
Small gas bubbles were observed after each breakdown, which
travelled to the headspace. Therefore, it is suggested that a
significantamount of the gases generated during an electric fault
would bemaintained in the headspace. As the equilibrium between the
headspace and the oil volume takes some time to reach, the
HYDRANsensor actually reads the amount of gases already dissolved
in the oilat that time other than the total amount of gases being
generated.Since the mechanical impact produced by the arcing was
relativelystrong, some of the small gas bubbles generated from the
oilbreakdown were pushed 5 to 8 cm sideways from the centre. That
is,the HYDRAN membrane had a relatively shorter distance to be
incontact with the dissolved gases.
HYDRAN readings:Normally, the HYDRAN monitor shows an initial
reading around 10ppm. When an initial increase of 5 ppm was
observed, it was recordedas the beginning of the response of HYDRAN
sensor to the dissolved
gas. The reading of HYDRAN was periodically recorded as shown
inFig. 3. When the reading stopped increasing, two oil samples
weretaken immediately from the bottom sampling valve of the test
vesselfor laboratory DGA.
0
50
100
150
200
250
1 10 100
Time (minute)
PPMv
alue
Mineral oil
Midel7131
FR3
Fig.3 Response of HYDRAN after oil breakdown tests
As shown in Fig.3 and Table 2, the HYDRAN sensor started tohave
increasing readings immediately after the 3 breakdowns inmineral
oil but a certain time delay was found in esters. Although thegas
diffusion in a liquid is not dependent on the viscosity of
liquidaccording to henrys law [9], this experiment indicates that
HYDRANtends to respond slower in high viscosity fluids. As the
speed of thekinetic movement of a fluid is highly dependent on its
viscosity and a
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relatively higher viscosity brings more resistance during
thecirculation, the oil circulation in a viscous fluid tends to be
relativelyslower, and the time for the dissolved gases in the high
viscosity estersto be in contact with the membrane of HYDRAN is
thereforeincreased.
Table 2. Performance of HYDRAN after breakdown tests
Fluid type Mineraloil Midel7131 FR3
Response delay (minute) 5 9 14
Time to reach maximumreading (minute)
40 53 74
Maximum reading (ppm) 207 129 208
Consequently, there was a delay for the HYDRAN to pick up theppm
value of dissolved gases in Midel7131 and FR3. On the otherhand the
forced oil circulation can significantly accelerate theequilibrium
of gases in the oil.
The maximum readings from mineral oil and FR3 after thebreakdown
test were similar, while the gases dissolved in Midel7131appeared
about 50% less than the other two fluids as shown in table 2.
DGA resultsTable 3 and Fig. 4 show the average concentrations of
dissolved
gases in two oil samples. The laboratory measurement error
expectedduring the DGA measurements is 10% for the 50ml oil
samplestaken in these tests.
Table 3. DGA results from oil gap breakdown testGases(ppm)
Mineral oil Midel 7131 FR3
H2 96 49 105
CH4 22 6 9
C2H6 4 2 2
C2H4 32 18 39
C2H2 157 89 175
CO 5 31 33
0
20
40
60
80
100
120
140
160180
200
H2 CO CH4 C2H2 C2H4 C2H6
Gasconcentration(ppm)
Mineral oil
Midel7131FR3
Fig. 4 Comparison of dissolved gas concentrations between
different fluids
after breakdown tests
Acetylene (C2H2) is the primary indicator for high
energydischarges [5, 6] and it is supported by the DGA results that
acetylenehad the highest concentration for all three types of
fluid. DGA ofmineral oil and FR3 are in close agreement with each
other, whereasMidel7131 has the lowest concentration of dissolved
gases.
Based on the sensitivities of the HYDRAN sensor to different
gases,the expected readings from HYDRAN 201R could be calculated
usingthe laboratory DGA results:- Expected HYDRAN reading = 100%
H2+ 15% CO + 8% C2H2 +1% C2H4 (1).
A comparison was made between the HYDRAN readings and
thelaboratory DGA results as shown in Fig. 5. The HYDRAN
sensorreadings are approximately two times higher than the expected
values,
and this trend was found in three types of fluids.
0
25
50
75
100
125
150
175
200
225
250
Mineral o il Mide l7131 FR3
Gasconce
ntration(ppm)
Calculated valueHYDRAN reading
Fig.5. HYDRAN readings versus calculated ppm values from
laboratory DGA
results for breakdown tests
HYDRAN is normally calibrated to illustrate the
realconcentration of the gases in the fluid so that the reading and
thecalculated value by (1) would be expected the same. The
differenceshown in Fig. 5 may be due to the different gas
concentration betweenthe side of the vessel where the HYDRAN sensor
was mounted to thebottom where the oil samples were taken. The
produced gas bubblesrise upwards and while travelling they
dissolved into the oil in the
upper part of oil body.
3.2 Partial discharge test
In order to produce a comparable energy level of partial
dischargeactivity in three types of fluids, 15kV voltage was
applied on the sharpelectrode in each PD test for 5 minutes. Small
sparks in the 0.5mm oilgap were constantly observed. The
oscilloscope monitoring thevoltage and discharge activities showed
that there were one to fourdischarges in every power cycle.
HYDRAN readingsTiny gas bubbles were stably and smoothly
generated during the
continuous partial discharges, which means the dissolved gases
had alonger distance, i.e. 16 cm to reach the membrane of
HYDRANcompared to the breakdown test. Fig. 5 shows the increased
readings
of HYDRAN with time after the PD test.
0
50
100
150
200
1 10 100 1000
Time (minute)
PPMv
alue
Mineral oil
Midel7131FR3
Fig. 5 Response of HYDRAN after PD tests
Table 4. Performance of HYDRAN after PD tests
Fluid typeMineral
oilMidel7131
FR3
Response delay (minute) 5 14 30Time to reach maximum
reading (minute)30 126 420
Maximum reading (ppm) 147 71 72
Table 4 shows that the HYDRAN sensor responds considerablyslower
for esters, especially for FR3, as compared to mineral oil. Alonger
delay of response by HYDRAN reading in the PD test than in
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the breakdown test may be caused by the longer distance for the
oilwith dissolved gas to circulate. The low energy discharge has
littlemechanical impact to push gas bubbles horizontally closer to
theHYDRAN membrane.
DGA resultsTable 5 and Fig. 6 show the dissolved gas
concentrations in the oil
samples tested by the laboratory DGA.
Table 5. DGA results for partial discharge testsGases(ppm)
Mineral oil MIDEL 7131 FR3
H2 88 28 70
CH4 15 4 7
C2H6 1 1 1
C2H4 15 7 15
C2H2 75 20 60
CO 7 24 22
0
10
20
30
40
50
60
70
80
90
100
H2 CO CH4 C2H2 C2H4 C2H6
Gasconcentration(ppm)
Mineral oil
Midel7131
FR3
Fig. 6 Comparison of dissolved gas concentrations between
different fluids
after PD tests
Because the partial discharges generated in the experimental
setupwere not the cold corona type discharge, a large amount of
acetylenewas still produced. However, different from the breakdown
tests, thistype of low energy discharge generates relatively more
hydrogen in
proportion to acetylene. In the PD test, Midel7131 was found
withmuch lower dissolved gas concentrations, and FR3
showedcomparable results to mineral oil.
0
20
40
60
80
100
120
140
160
Mineral oil Midel7131 FR3
Gasconcentration(ppm) Calculated value
HYDRAN reading
Fig.7 HYDRAN readings versus calculated values from laboratory
DGAresults for PD tests
The comparison between the maximum HYDRAN readings and
theexpected values calculated from the laboratory DGA results by
(1) forthe three types of fluid was shown in Fig. 7.
For mineral oil and Midel7131, the maximum readings obtainedfrom
the HYDRAN sensor were about two times higher than theexpected
values, which was similar to the situation of breakdown
tests.However the maximum reading from HYDRAN in FR3 is in
thesimilar range as the calculated value. It took 7 hours for the
HYDRAN
reading to reach the maximum in FR3 and this long waiting
timeallows the dissolved gases in FR3 to reach a better equilibrium
in thebody of oil and a relatively higher gas concentration was
achieved inthe bottom area.
4 Conclusions
Esters have similar DGA characteristics to mineral oil for
electricfaults, and the DGA diagnosis continues to act as a
feasible means toindicate the health of ester oil filled
transformer, since key gases werefound in mineral oil as well as
ester oils after electric faults throughthe laboratory based
DGA.
The dissolved gas concentrations in FR3 after breakdown
andpartial discharges was found similar to mineral oil, while
Midel7131having lower gas concentrations for both tests.
Although HYDRAN successfully read the dissolved gases,
thequantitative comparison between the HYDRAN reading and the
DGAresults show the difference and this difference could be caused
by thedifferent sampling positions. A time delay of HYDRANs reading
wasobserved with esters due to the high viscosity of ester and the
limitedoil circulation in the experiment setup. In real situation,
ester filledtransformers operate under a relatively higher
temperature with load,
which helps to reduce the viscosity of ester and achieve a
better oilcirculation through either natural or forced oil flow.
Neverthelessfurther study on the HYDRANs sensitivities to dissolved
gases inesters is required.
Acknowledgement
The authors wish to thank AREVA T & D, EdF Energy, M &
IMaterials, National Grid, Scottish Power, TJ|H2b analytical
servicesand United Utilities for their financial support to form
the researchconsortium on Alternative fluids for large power
transformers at TheUniversity of Manchester. The authors are
grateful to the help givenby Michael Webb from MW Test Equipment
for supplying theHYDRAN 201R.
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