I. ATANASOVA-HÖHLEIN, Convenor DE
P. AGREN FI C. BEAUCHEMIN CA
B. ČUCEK SI L. DARIAN RU
V. DAVIDOV AU L. DREIER CH
T. GRADNIK SI M. GRISARU IL
M. KONČAN-GRADNIK SI K. KRYCZYNSKI PL
S. LEIVO FI J. LI CN
K. BREDE LILAND NO Q. LIU UK
M. MARUGAN FR D. MIHAJLOVIC SR
T. PREVOST US P. PRZYBYŁEK PL
P. RAVILA FI O. ROIZMAN AU
A. SHKOLNIK IL K. SIODŁA PL
M. DAN AU
J. VAN PETEGHEM BE
B. NOIRHOMME CA
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MOISTURE MEASUREMENT AND
ASSESSMENT IN TRANSFORMER
INSULATION – EVALUATION OF CHEMICAL
METHODS AND MOISTURE CAPACITIVE
ISBN : 978-2-85873-443-6
MOISTURE MEASUREMENT AND ASSESSMENT IN TRANSFORMER INSULATION – EVALUATION OF CHEMICAL METHODS AND MOISTURE CAPACITIVE SENSORS
In the last decade, capacitive polymer sensors have been increasingly used to evaluate moisture in power transformers.
Experience shows that effective integration of moisture sensors into on-line diagnostic systems requires proper
placement of the sensor, gathering an adequate measurement data set and evaluation of the measured data by
comparison to historical values in order to obtain a correct moisture profile. Availability of continuous moisture-in-
transformer measurements and its relation to temperature opens up new diagnostic possibilities in comparison to
conventional Karl Fischer spot value measurements. This brochure is a continuation of the work of Cigre Brochure 349
“Moisture equilibrium and moisture migration within transformer insulation systems”, 2008 and allows new insights into
the distribution of moisture between insulating liquids and solid insulation.
Main topics of the brochure are:
- OVERVIEW AND STATE OF THE ART OF MOISTURE MEASUREMENT BY MEANS OF CHEMICAL
METHODS (KARL FISCHER) AND POLYMER CAPACITIVE SENSORS
Chapter 1 deals with an overview of the most important characteristics of polymer capacitive sensors like sensitivity and
accuracy, stability and selectivity, response time, calibration, adjustment as well as the influence of these on the
measurement performance and uncertainty sources.
- CRITICAL REVIEW OF EXISTING STANDARDS FOR THE MEASUREMENT OF MOISTURE
The Working Group performed an extensive Round Robin Test (described in Chapter 2) for water determination by Karl
Fischer titration and by means of capacitive moisture sensors in different types of insulating liquids – new and service
aged mineral oil, synthetic and natural ester and silicone insulating liquid. The methods for the determination of moisture
in the solid insulation have been also evaluated. The results of the Round Robin Test with the corresponding statistical
evaluation allow the estimation of measurement uncertainties, the comparison between different sensors, as well as
recommendations for future standard revisions.
- POSSIBILITIES AND RESTRICTIONS FOR CONVERTIBILITY BETWEEN ABSOLUTE WATER
CONTENT DETERMINED USING KARL FISCHER TITRATION METHOD AND RELATIVE
SATURATION MEASURED USING 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 mg/kg equivalents and
vice versa. Conversion of moisture measurements between the capacitive sensors and Karl Fischer titration depends on
the moisture saturation curve of the insulation liquid. In a Round Robin Test the possibility and uncertainties of such
conversions for different insulating liquids have been evaluated (Chapter 3).
- MOISTURE DISTRIBUTION BETWEEN LIQUID AND SOLID INSULATION
A short description of the indirect methods used for the determination of moisture in the solid insulation, especially of
the methods based on evaluation of moisture equilibrium curves is done in Chapter 4. The influence of the structure
of solid and liquid insulation and its condition on the uncertainties of the results is elaborated.
- APPLICATION OF THIN FILM POLYMER CAPACITIVE SENSORS FOR ON-LINE MONITORING
OF OIL FILLED ELECTRICAL EQUIPMENT
Moisture determination of the insulating liquid is a routine measurement for transformers and related electrical
equipment used to infer the cellulosic insulation moisture. Experience shows that effective integration of moisture
sensors into on-line diagnostic systems involves the following elements to obtain a proper moisture profile of a
transformer: apropriate placement of the sensor, gathering an adequate measurement dataset and evaluation of the
measured data by comparison to historical values.
It is shown that it is possible to derive evaluation criteria for on-line monitoring of moisture in electrical equipment
based on the correlation between dielectric strength and relative moisture saturation in a dielectric liquid. In addition,
the relative saturation/temperature hysteresis plot is found to be an effective way to indicate the wetness of transformer
insulation based on practical examples in service (Chapter 5).
Availability of continuous moisture-in-transformer measurement by means of capacitive sensors and its relation to
temperature opens up new diagnostic possibilities in comparison to conventional KF spot measurements.