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    MOISTURE MEASUREMENT AND ASSESSMENT IN TRANSFORMER INSULATION –

    EVALUATION OF CHEMICAL METHODS AND MOISTURE CAPACITIVE SENSORS

    WORKING GROUP

    D1.52

    AUGUST 2018

  • Members

    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

    Corresponding Members

    M. DAN AU

    J. VAN PETEGHEM BE

    B. NOIRHOMME CA

    WG D1.52

    Copyright © 2018

    “All rights to this Technical Brochure are retained by CIGRE. It is strictly prohibited to reproduce or provide this publication in any form or by any means to any third party. Only CIGRE Collective Members companies are allowed to store their copy on their internal intranet or other company network provided access is restricted to their own employees. No part of this publication may be reproduced or utilized without permission from 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”.

    WG XX.XXpany network provided access is restricted to their own employees. No part of this publication may be

    reproduced or utilized without permission from 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

    MOISTURE MEASUREMENT AND ASSESSMENT IN TRANSFORMER INSULATION – EVALUATION OF CHEMICAL METHODS AND MOISTURE CAPACITIVE SENSORS

    ISBN : 978-2-85873-443-6

  • MOISTURE MEASUREMENT AND ASSESSMENT IN TRANSFORMER INSULATION – EVALUATION OF CHEMICAL METHODS AND MOISTURE CAPACITIVE SENSORS

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    EXECUTIVE SUMMARY

    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.

  • 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

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    TABLE OF CONTENTS

    EXECUTIVE SUMMARY ................................................................................................................................................. 3

    1 . IMPORTANCE, INFLUENCE AND MEASUREMENT OF MOISTURE ..................................................... 9

    1.1 IMPORTANCE AND INFLUENCE OF MOISTURE ................................................................................................. 9

    1.2 KARL FISCHER MEASUREMENTS - PRINCIPLE AND INTERNATIONAL STANDARDS ................................ 14 1.2.1 Karl Fischer titration technique principle ......................................................................................................................... 14 1.2.2 Existing international standards for KF water content measurements in various substrates ................................. 15

    1.3 MEASUREMENT OF MOISTURE WITH CAPACITIVE SENSOR ...................................................................... 17 1.3.1 Metal oxide capacitive sensors ......................................................................................................................................... 17 1.3.2 Operating principle of aluminium oxide moisture sensor ............................................................................................ 17 1.3.3 Capacitive thin film polymer sensors ................................................................................................................................ 17 1.3.4 Measurement performance and uncertainty sources of capacitive thin film polymer sensors ............................. 19 1.3.5 Sensitivity and accuracy ...................................................................................................................................................... 19 1.3.6 Stability and selectivity ....................................................................................................................................................... 20 1.3.7 Response time ........................................................................................................................................................................ 21 1.3.8 Calibration of capacitive polymer sensors ...............................................................................................