The document summarizes the findings of a Cigre study on methods for measuring moisture in transformer insulation, including the Karl Fischer titration method and capacitive sensor techniques. It describes a round robin test conducted between laboratories to evaluate the repeatability and reproducibility of the different moisture measurement methods. The study also explored using measurements from capacitive sensors and Karl Fischer titration to calculate conversion coefficients between percent relative saturation and absolute moisture content in ppm.

1. The Webinar video is available here:
https://register.gotowebinar.com/recording/998568060912407048

2. Moisture Measurement and Assessment in
Transformer Insulation – Evaluation of Chemical
Methods and Moisture Capacitive Sensors
Cigre Brochure 741
Presented by Ivanka Atanasova-Höhlein, Study Committee D1, WG D1.52
Webinar on February 06th, 2019
Remark: All references to graphs and results are to be found in the brochure and are not mentioned in the presentation

3. Table of contents
Measurement methods
Round Robin Test on Karl Fischer method and on capacitive sensors
Conclusions
Distribution of moisture between solid and liquid insulation
Sources of moisture
Evaluation of moisture
Impact of moisture
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4. Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors
External Sources
Poorly maintained or
missing dehumidifiers
Rupture in a rubber bag or gaskets.
Contamination from ambient air
during open service.
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5. Internal Sources
Ageing of Oil and Solid Insulation
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

6. Time to reach moisture equilibrium
Laminated pressboard Pressboard cylinders Insulation paper
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

7. IEC 60156
Dependence of the breakdown voltage as a function of % water
saturation in oil
Unit % RS =
dissolved amount of humidity at a certain temperature
max. amount of water at the same temperature ∗ 100
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

8. Oxygen
Humidity
Temperature
STRUCTURE OF INSULATION PAPER
Acids
FACTORS ACCELERATING AGEING
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

9. Thin film capacitive sensor Karl-Fischer
Unit = mg/kg (ppm)Unit % RS =
dissolved amount of moisture at a certain temperature
max. amount of water at the same temperature
∗ 100
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

10. 2 2 5 5 2 2 4 5 52 2 2H O SO C H N J H SO HJ C H N+ + + ® + ×
In this chemical reaction, jodine is generated electrolytically as long as the water present
in the sample completely reacts with the generated iodine.
The total jodine amount is proportional to the the water amount.
For mineral oils
described in IEC
60814.
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

12. Cigre D1.52 aimed at evaluation of both Karl-Fischer and capacitive sensor methods, therefore a Round Robin
Test has been designed and carried out for both.
Aim has been:
1) Determination of repeatability and reproducibility and comparison with existing IEC methods.
2) Evaluation of capacitive sensors on the market.
3) Possibilities and prerequisites for conversion of the %RS values to absolute values for moisture (mg/kg) and
vice versa.
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13. Conclusions:
- A very small mass of oil sample less then 1 g (instead of 5 g) is enough for the test.
This should be considered at the next revision of IEC 60814.
- The Karl Fischer method is applicable not only for mineral oils, but also for other
insulating liquids. The reproducibility of measurement depends strongly on dissolved
water in insulating liquid concentration range (approx. 50% at 10 ppm (mineral oil)
and approx. 8% at 300 ppm(ester liquids) ).
- The IEC 60814 reproducibility requirement R = 1,5 RRT X has
been fulfilled in four cases from the six RRT samples. For very low WCO value 6
ppm sample and 16 ppm mineral oil the reproducibility was higher. As also many
of well known IIS Proficiency tests during last 5 years demonstrate similar results we
conclude that the reproducibility criterion of this test method for water
concentrations lower then 30 ppm is too stringent and should be reviewed for the
future revision of standard.
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

14. Capacitive polymer sensors are widely used in monitoring of moisture in HV
equipment, as well as in automated equipment for oil processing and
refurbishment (reclaiming). The use of capacitive sensor instruments is a mature
technology and has been used since 1970’s in various applications to measure
moisture in gas phase. In late 1990’s the same technology was introduced to
measure relative moisture saturation in oils.
A capacitive moisture sensor is basically a
parallel plate capacitor . At least one of the
electrodes is permeable to water vapor and
allows water molecules to diffuse into the
dielectric polymer layer. Absorbed water
molecules increase the permittivity and this can
be measured as increased capacitance of the
sensor element. The sensor is very selective to
water and almost no interfering effects of other
molecules in oils are observed.
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

15. What should a user pay attention to?
3 sensor manufacturers agreed to send their sensors (water in oil) to
17 laboratories to carry out a RRT
Following insulating liquids have been used:
New mineral oil, aged mineral oil, new synthetic ester, new natural
ester, silicon liquid, all equilibrated to a constant humidity.
A test procedure with stirring has been set up. The measurement has
been carried out at room temperature at 3 time slots for each liquid
– immediate, 3 min and 10 min.
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

16. 1. Verification of the sensor calibration
The calibration of the sensors has been tested in air on the basis of salt
solutions with known RS% at a constant temperature (22°C) and different
equilibration time (10 min, 30 min, 60 min).
The result of the sensor reading %RS to the theoretical %RH for the sensors
1and 3 are shown on Figures below.
y = 0,9466x + 1,4006
R² = 0,999
y = 0,9771x + 0,3657
R² = 0,9993
y = 0,9876x + 0,1322
R² = 0,9994
0
10
20
30
40
50
60
70
80
0 20 40 60 80
SensorReadingRS%
Theoretical %RH
Sensor 1
y = 0,9973x - 3,1672
R² = 0,9956
y = 1,0163x - 4,0344
R² = 0,9943
y = 1,0074x - 3,539
R² = 0,9941
0
10
20
30
40
50
60
70
80
0 20 40 60 80
SensorReading%RS
Theoretical %RH
Sensor 3
10 Min
30 Min
60 Min
Sensor 3 is linear only over 10% RS and can practically
measure no value below 10% reliably.
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

17. The response time of sensor 1 has been investigated in different insulating liquids at room temperature –
in new mineral oil, in aged mineral oil , in synthetic ester and in natural ester .
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors
New mineral oil Aged mineral oil

18. %RS reading of sensor 1, 2 and 3, depending on temperature and time
- Sensor 3 is constantly significantly off
- Sensor 2 shows temperature dependency, shown by bigger offset at
higher temperature
- Sensor 1 is in line with chamber at all tested conditions
Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors
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19. Conclusions:
• Results show that the 3 tested sensors differed in their performance and
correspondingly in the relative standard deviation, although they had been
factory calibrated and had similar specification.
• Capacitive sensors have intrinsically response time 10 minutes or
even more for esters. Oil flow is absolutely necessary.
• Additional tests showed differences in the sensitivity (especially in the lower
humidity range), response, temperature dependence and compatibility.
The user must be aware of the importance of all these parameters for the sensor
behavior and require the necessary information from the manufacturer.
Capacitive thin film polymer sensors have intrinsically response time of 10 min,
which, however, is fully acceptable in continuous on-line monitoring devices.
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

20. Measurement range (moisture)
aw or
RS (%)
0-1
0-100
Measurement range (temperature °C) -xxx - +xxx
Accuracy (including non-linearity, hysteresis,
repeatability, temperature dependance) with
reference to which RS range
± xx (RS range)
Response time (90%) at 20°C (min) xxx
A capacitive sensor should be accompanied by a calibration
certificate, which should show the difference between observed
and reference humidity and temperature in the desired ranges.
Additionally, it should be specified whether the calibration is
traceable to international references or not.
For an instrument giving readings in moisture ppm, the formula used
to convert relative saturation to ppm, standard coefficients and
instructions how to change coefficients are needed.
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

21. 20
Capacitive sensors –
Relative Humidity
Result in %RS
Karl Fischer – Absolute
Humidity
Result in mg/kg ( ppm)
Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors
As the conventional mg/kg measurements have been well established and with reference data readily available for
condition monitoring, there is a motivation to convert the new measurement data of RS into
WCL (water content in liquid mg/kg equivalents) and vice versa.
From a mathematical point of view, this can be done through the relationships expressed in Equation 1 and Equation 2.
Note Equation 2 has an Arrhenius form as the moisture saturation in insulating liquid (S) is temperature (T) dependent.
The natural logarithmic form can also be expressed in its decimal logarithm form for convenience as shown in Equation 3.
=
100
×
Equation 1
= ⁄
Equation 2
= 10 ⁄
Equation 3

22. Method-2 (1 lab)
Method-2 uses both Karl-Fischer and capacitive sensor measurements of the
insulating liquids conditioned in an airtight test cell at three temperatures.
(under equilibrium conditions)
Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors
Method-3 (5 labs)
Method-3 also uses both Karl-Fischer and capacitive sensor measurements
of the insulating liquids with a simple setup and a quick conditioning process.
Method-1(1 lab)
Method-1 uses a closed chamber exposing the stirred insulation liquid to a
constant environment with pre-set %RS and temperature. The absolute
moisture content of the liquid at equilibrium state is measured through Karl-
Fischer titration. Moisture saturation coefficients are calculated based on
results obtained at multiple temperatures.
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors
Water solubility (S) versus temperature for new
mineral oil from RRT results
Water solubility (S) versus temperature for aged
mineral oil from RRT results

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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors
Decimal logarithm Natural logarithm
Insulating liquid A10 B10 Ae Be
New mineral oil 7,09 1574 16,33 3625
Aged mineral oil 6,96 1508 16,03 3473
Synthetic Ester 5,42 629 12,48 1449
Natural ester 5,33 693 12,27 1596
Silicone Liquid 5,93 1069 13,66 2462
= × Equation 1
= ⁄
Equation 2
= 10 ⁄
Equation 3
Examples: 10% RS at 20°C means
New mineral oil – 5 mg/kg
Aged mineral oil – 7 mg/kg
Silicon fluid – 19 mg/kg
Natural ester – 93 mg/kg
Synthetic ester – 190 mg/kg

25. Cigre WG D1.52 evaluated several methods for calculating of A and B
coefficients which will theoretically allow the conversion of Karl Fischer values
in mg/kg into % saturation and vice versa. Statistical evaluation of results
shows that the uncertainty of measurement in case of Karl Fischer and
capacitive sensors will reflect on the uncertainty of A and B values.
• Reproducible A and B values can be derived only with sensors showing no
temperature dependence, having a quick response and an adequate
sensitivity ≤5% and compatible in the corresponding liquid .
• It is recommended at least three temperatures should be chosen, whenever
possible the %RS reading should be aimed in the range of 5% to 75%.
• The comparison of the water solubility of new and aged mineral oil over
the usual temperature interval shows that aged mineral oils have higher
saturation levels than new mineral oils.
• Temperature dependence of saturation levels are different for different
types of oils. Synthetic ester>natural ester>silicone oil>mineral oil
• A tool to calculate A and B based on the experimental results is provided.
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

26. 25
Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors
Installation of a capacitive moisture sensor at a
300 MVA 380/110 kV transformer filled with
natural ester

27. Knowledge on seasonal moisture variation with temperature
Seasonal moisture variation with temperature at
20 MVA ONAN network transformer
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

28. POSSIBLE EVALUATION CRITERIA FOR MOISTURE IN TERMS OF %RS
This allows to use 20%RS as an early alert for evaluating moisture in oil by means of capacitive sensors at service
operating temperatures. The proposed evaluation is valid if the sensor measures the real oil temperature and
humidity in the tank.
In case integrated capacitive sensors in a monitoring equipment are being used, especially in tempered environment,
a calculation model provided by the manufacturer may be used.
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BDV versus %RS aggregated from literature data
Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

29. An example of relative saturation changes as a result of temperature changes
Typical for moist insulation is a broad hysteresis curve and values for %RS > 20
0
5
10
15
20
25
30
35
40
0 10 20 30 40 50 60 70 80 90
%Saturation
Temperature (°C)
% Saturation only oil
% Saturation wet insulation
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30. Hysteresis loops of %RS dependence of temperature in transformers
with different moisture household.
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Risk increased:
lower BDV due to moisture >20%RS
No risk:
high BDV, moisture <20%RS
Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

31. New equipment for continuous monitoring gives the opportunity for
new evaluation methods
- Temperature vs %RS hysteresis curves show reliably the moisture saturation in oil
and thus can be used for an evaluation of the moisture household of electrical
equipment.
- For evaluation of moisture by means of capacitive sensors following should be
taken into account :
- Position of the capacitive sensor – directly in oil flow
- Position of the capacitive sensor - representative for the transformer oil
temperature directly or in a temperature controlled environment (sometimes
the case in an integrated monitoring system).
- If the sensor records the actual transformer oil temperature the measured
data can be used directly for the assessment of moisture, otherwise a
calculation model may be needed.
- Evaluation criteria by means of limits for %RS and the broadness of the
hysteresis curves.
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Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors

32. Atanasova-Hoehlein Ivanka
Marugan Marielle
Liland Knut Brede
Čuček Biljana
Gradnik Timotej
Leivo Senja
Prevost Thomas
Davydov Valery
Shkolnik Anatoli
Grisaru Marius
Koncan-Gradnik Maja
Beauchemin Claude
Kryczynski Krzysztof
Liu Qiang
Agren Patrik
Siodla Krzysztof
Li Jian
Roizman Oleg
Darian Leonid
Mihajlovic Draginja
Van Peteghem Julie
Dreier Lars
Przybylek Piotr
Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors
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34. Copyright © 2018
This tutorial has been prepared based upon
the work of CIGRE and its Working Groups.
If it is used in total or in part, proper
reference and credit should be given to
CIGRE.
Disclaimer notice
“CIGRE gives no warranty or assurance
about the contents of this publication, nor
does it accept any responsibility, as to the
accuracy or exhaustiveness of the
information. All implied warranties and
conditions are excluded to the maximum
extent permitted by law”.
Copyright &
Disclaimer notice
Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical Methods and Moisture Capacitive Sensors
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