The MAIN JOURNAL for POWER GRID SPECIALISTS in RUSSIA
3 - 6 J U N E 2 0 1 9
MADRID, SPAIN
44
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Online
Transformer Drying Solution
Effect on safety
As shown on the graph besides,
the higher the relative water satura-
tion, the lower the breakdown volt-
age (BDV) of the oil. As water mi-
grates between the solid and liquid insulation in
a transformer with changes in load and, there-
fore, temperature, so does the relative water
saturation in oil. Peaks of relative saturation are
usually observed during transformer changes
of state (high to low temperature or opposite).
Moisture reduction is therefore a key eff ort to be
made to increase safety, especially for Trans-
former with short and frequent load changes
Effect on
transformer
life expectancy
The insulation paper mechanical strength is de-
fi ned by the degree of polimerisation, also called
DP, which represents the average length of cel-
lulose chains in the paper. A new transformer typi-
cally has a DP between 1200 and 1000 while the
end of transformer life is considered when the DP
reaches 200. This degradation cannot be stopped
but its speed will depend on the water content in
paper (see picture on the left).
In CIGRE brochure D1.01.10 (2007) "Fallou
showed that the rate of degradation of the paper
at initial value of 4% water content was 20 times
greater than that at 0.5% water content."
Moisture is having a huge eff ect on the
speed of paper degradation and therefore on its
life expectancy.
THE ISSUE: Moisture is threatening the transformer
Moisture is one of the major cause of failures for power
transformers and one of the main degradation factor for
the insulation paper. It is therefore increasing the risks
of operation failures and shortening the life expectancy
of the asset.
Unfortunately, moisture can appear in a transform-
er from several external or internal sources and it has
a complex dynamic between the oil and paper inside the
transformer.
The use of silica gel breathers, sealed tanks, or
nitrogen blankets allows engineers to avoid ingress
of all or at least the most part of moisture from the
atmosphere to the transformer. However, when the
transformer is energized, the production of water in-
side the power transformer is a natural and inevitable
occurrence over time due to the depolymerization of
the cellulose paper.
0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
60
70
80
90
100 110
Br
eak
d
own v
o
lt
ag
e (kV
/2.5 mm)
% Relave saturaon (%RS)
Fig. 1. Dependency between breakdown voltage and water
content in insulating liquid*
1
10
100
1000
0
1
2
3
4
R
e
la
v
e polymeriz
a
on speed
Moisture content of insulaon paper (%)
80˚C
100C
120C
Fig. 2. Cellulose depolymerisation speed dependence on mois-
ture content in insulation paper for di
ff
erent temperatures**
* CIGRE Moisture measurement and assessment in transformer
insulation – evaluation of chemical methods and moisture capacitive
sensors, page 10
** CIGRE Moisture measurement and assessment in transformer
insulation – evaluation of chemical methods and moisture capacitive
sensors, page 14
45
THE SOLUTION: Maintaining a dry transformer
by continuous
fi
ltration
Maintaining a low level of moisture in a transformer pro-
vides signifi cant benefi ts in terms of operations and risk
as it maintains a constant high insulation level. There-
fore it is possible to load the transformer at a higher level
and to make this load vary without risking to damage the
transformer.
Also it has consequent fi nancial benefi ts since it elon-
gates the life of the asset by slowing down the paper
degradation. This degradation creates particles or even
sludges. Finally moisture is also responsible for creation
of acids in the oil. Hence keeping a low level of moisture
will reduce the maintenance costs.
Oil filtration
LFH or similar method
Online drying system
Solution type
Punctual
Punctual
Continuous
Transformer live
during process
Up to user risk assessment
(oil fl ow >500 l per hour)
NO
YES
Dries oil
YES
YES
YES
Dries paper
NO
YES
YES
Improves Breakdown
voltage
Temporarily (months)
YES
YES
Extend life
expectancy
NO
YES
YES
Dissolved Gasses
level maintained
YES
NO
YES
Operator free process
NO
NO
YES
Only continuous
fi
ltration is ef
fi
cient
to remove moisture from a transformer
As moisture is creat-
ed continuously and
as it is one of the major concern for
the transformer safety and life ex-
pectancy, it seems counter intuitive
to apply a punctual solution for this
continuous problem.
Also it is important to note that
more than 98% of the water in
a transformer is in the paper while
a very low amount is dissolved in
oil. The diff usion time of water from
paper to oil is very slow. This is why
punctual fi ltrations do not solve the
moisture issue.
200
35
60
1200
DP
Years
Life
expectancy
gain
No
water
extracon
Connuous
water
extracon
Fig. 3. Bene
fi
t of continuous online drying system
FIELD RESULTS: Installations of TRANSEC
Online Drying System in Saudi Arabia
Since more than 15 years, the TRANSEC Online Drying System technology has been installed on almost
3000 transformers worldwide. We’ll detail in the paragraphs below an installation showing the effi ciency
of this solution.
24th World Energy Congress
Special issue, September 2019
46
Installation on a transformer
for the Oil & Gas industry in Saudi Arabia
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40
45
Moi
sture PPM
Temp
eratu
re °C
Temp In
Temp out
PPM In
PPM Out
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1
2
3
4
5
6
% Moist Paper
Visit our website: www.streamer-electric.com
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A TRANSEC unit has been installed in No-
vember 2017 on 40 MVA General Electric
3 phases transformer in the Dammam region,
Saudi Arabia.
This 115kV/13.2kV transformer from the
1970s was always used at a relatively low
load which explained that moisture level in
oil never reached an alarming level. But the
level of moisture raised very rapidly in 2015-
2016 when a leakage appeared in the gasket.
In February 2016, the PPM of moisture in oil
reached 54 mg/kg; the relative humidity in
oil was 45%; the estimated water content in
the paper was 7% and the breakdown volt-
age dropped to 22 kV. At this point the trans-
former was immediately stopped and a leak
search had been initiated.
Once the leak had been found and re-
paired, the increase of moisture in the trans-
former stopped but an enormous amount was
already in the oil and paper and had to be
removed in order to operate the transformer
safely.
Using an offline solution like Low Fre-
quency Heating was out of question due to
the risk of paper shrinkage during the remov-
al of such a large volume of water. As a result
the end user decided to use an online drying
device.
Once installed (Figure 1), the TRANSEC
started removing moisture from the oil im-
mediately. It provided a positive effect on the
breakdown voltage value. In a second step
water started to get extracted from the pa-
per as the oil got dried out after a few weeks.
Over the 1st year you can see on the 2 graphs
below the PPM inflow and the water content
dropping. Within that year the water content
in paper went from 5% to about 3% (much
more acceptable level).
We can also see that during the second
year of operation the PPM values of inflow
and outflow were almost matching which is
the sign of water saturation in the Transec
unit. This is confirmed by the water content
increasing again to 4% at the end of the sec-
ond year. The saturated TRANSEC extrac-
tion cylinders have been then replaced by
new ones in order to restart the filtering pro-
cess. They had extracted about 12 litres of
water from that transformer.
To conclude, the TRANSEC device
brought the moisture level of this transformer
back to an acceptable level within one year
and still continues the extraction of moisture.
Breakdown voltage came back to a safe level
and the remaining life expectancy of the as-
set was multiplied by 2.
Р
Оригинал статьи: Online Transformer Drying Solution
THE ISSUE: Moisture is threatening the transformer. Moisture is one of the major cause of failures for power transformers and one of the main degradation factor for the insulation paper. It is therefore increasing the risks of operation failures and shortening the life expectancy of the asset.
