International Conference on Condition Monitoring and Diagnosis CMD, Peking, China, 2008 1

Abstract – This paper describes a novel dielectric diagnostic method "DIRANA" which features a fast measurement methodol-ogy and an improved analysis algorithm for determining moisture in pressboard and paper of oil-paper-insulated transformers. Dielectric response methods provide substantial advantages compared to moisture determination by conventional equilibrium diagrams, for example it is unnecessary to wait for moisture equilibrium. For the new approach, combination of time and frequency domain measurements substantially shortens the measurement time, typically to 25 % compared to a pure fre-quency domain measurement. The data analysis algorithm compares the measured dielectric properties of the actual power transformer to modeled dielectric properties. The software features a compensation for the influences of conductive aging byproducts. Conductive aging byproducts have similar dielectric properties as water; without compensation the analyzed moisture content will be too high, leading to unnecessary actions like drying. This paper also includes examples of practical onsite measurements in comparison to other measurement methods and a guide for assessing the obtained moisture content.

Index Terms – Dielectric polarization, Frequency domain spec-

troscopy, Moisture measurement, Polarization and depolarization currents, Power transformers

I. INTRODUCTION

ower transformers represent the most expensive chain links connecting generation to utilization. Due to the cost

pressure of a liberalized energy market the utilities shift maintenance from time based to condition based approaches. This development requires reliable diagnostic tools.

Water in oil paper insulations causes three dangerous ef-fects: it decreases the dielectric withstand strength, accelerates cellulose aging and causes the emission of bubbles at high temperatures. Therefore knowledge about the actual moisture concentration is required in order to decide for further actions as for example on-site drying.

State of the art for moisture measurements are equi-librium diagrams, where one tries to derive the moisture in the solid insulation (paper, pressboard) from moisture content in oil (ppm). This method fails for several reasons, where aging of oil and water titration has the major impact [1].

Karl Fischer titration suffers from moisture ingress during transportation to the laboratory, different procedures releasing water from the sample leading to unsatisfying comparability of the results [2].

Therefore dielectric diagnostic methods were developed,

which deduce moisture in paper and pressboard from dielectric properties like polarization currents and dissipation factor [3]. These methods promise to give higher accuracy and are designed for onsite moisture determination.

This paper describes a new dielectric diagnostic method that combines time and frequency domain measurements and provides reliable results also for aged transformers.

II. MEASUREMENT OF DIELECTRIC PROPERTIES

A. Dielectric Properties of Insulations The multilayer insulation of power transformers consisting

of oil and paper shows polarization and conductivity phenom-ena. Dielectric diagnostic methods measure the interfacial polarization effect, which originates from the interfaces be-tween cellulose and oil. Polarization is superimposed by the DC conductivity of cellulose and oil. The resultant total current density in frequency domain J(�) caused by a sinusoi-dal field strength E(�) can be expressed by (1), [4].

)()()()()(0

00 ωωχ

ωεσωχεωεω EjjJ

lossesresistive

componentcapacitive���

���

�

���

���

�

�

��

′′+−′+∞=�� ��� ��

�������

(1)

The imaginary part of the current density represents its ca-pacitive component caused by the high frequency part of permittivity � and the low frequency susceptibility �'. The real part includes a resistive current due to the DC conductivity �0

and a resistive current due to dielectric losses �”. The inertia of the dipoles and charge carriers moved by the electrical field cause these dielectric losses.

Moisture, temperature and conductive aging products influ-ence these phenomena. The discrimination of moisture from other effects is a key quality feature for the analysis of dielec-tric measurements.

B. Accelerated Measurement of Dielectric Properties The dielectric response of insulations can be recorded in

time domain or in frequency domain. A time domain current measurement records the charging and discharging currents of the insulation. They are also known as Polarization and Depo-larization Currents PDC.

Frequency domain measurements are derived from the old known Tangent Delta measurements with a frequency range

Maik Koch and Michael Krüger Omicron Electronics, Oberes Ried 1, A-6833 Klaus

*E-mail : maik.koch@omicron.at

A Fast and Reliable Dielectric Diagnostic Method to Determine Moisture in Power Transformers

P

International Conference on Condition Monitoring and Diagnosis CMD, Peking, China, 2008 2

much enhanced especially to low frequencies. The derived measurement method is called Frequency Domain Spectros-copy FDS.

The combination of time domain polarization current meas-urements with frequency domain spectroscopy [5] drastically reduces the test duration compared to existing techniques. Essentially, time domain measurements can be accomplished in a short time but are limited to low frequencies (typically below 1 Hz). In contrast, frequency domain measurements are feasible for high frequencies as well but take very long time at low frequencies.

The new instrument acquires data in frequency domain from 5 kHz to 0,1 Hz and in time domain from 0,1 Hz to 100 µHz or less. For further evaluation the time domain data are transformed to frequency domain. Fig. 1 illustrates the combination of dielectric measurements in time and frequency domain.

Cur

rent

[nA

]

Time [s]

Trans-formation

Frequency [Hz]

Dis

sipa

tion

fact

or

0,00010,001

1

1000

100001

100

1

Frequency [Hz]

Dis

sipa

tion

fact

or

1000

1

0,0010,1

Frequency [Hz]

Dis

sipa

tion

fact

or

1000

1

0,0010,1

Fig. 1. Combination of time and frequency domain measurements

The new technique reduces the time duration of a measure-ment to 25 % compared to pure frequency domain measure-ments. For instance, data for 1 kHz down to 0,1 mHz typically need 11 hours for a frequency domain measurement, but less than 3 hours for the new instrument (Fig. 2). A polarization and depolarization current measurement will need 5,5 h to record data from 1 s to 10000 s which corresponds to 1 Hz to 0,1 mHz.

0

2

4

6

8

10

12

14

PDC FDS DIRANA

Tim

e ne

ed[h

]

0,0001

0,001

0,01

0,1

1

10

100

1000

Freq

uenc

yra

nge

[Hz]

Fig. 2. Required time duration and acquired frequency range for the different measurement techniques

The time required for the real life measurement at a specific insulation depends on the condition of that insulation. For the succeeding analysis of moisture content the properties of the

solid insulation should become visible as explained in Fig. 4. Dry or cold insulations require to measure down to very low frequencies, the long durations as in Fig. 2 are necessary. Hot or highly conductive insulations require a much smaller frequency range of e.g. 1 kHz to 0,1 Hz, thus the duration will decrease to a few minutes. For the range of 1 kHz to 1 mHz suitable for most transformers the new system needs 25 min, whereas pure a frequency domain measurement needs 1 h 5 min.

C. Performance of a Measurement A dielectric response measurement is a three terminal

measurement that includes the output voltage, the sensed current and a guard. The guarding technique insures for an undisturbed measurement even at onsite conditions with dirty insulations or bushings. For two winding transformers, after disconnection from the network, the voltage output can be coupled with the HV winding, the current input with the LV winding and the guard with the tank.

It is unnecessary to wait until the transformer cooled down or reached moisture equilibrium.

The new instrument DIRANA features two input channels. They enable for an additional time save for example at three winding transformers. Here the output voltage can be applied on the MV winding, while the input channels are connected to the HV and LV winding (Fig. 3). So the test duration will be halved additionally to the time and frequency domain combi-nation.

Guard

CLT CHLCT

MVLV HV

Current sense 2A

Voltage source=

Current sense 1A

Inst

rum

ent

Fig. 3. Schematic for a two channel measurement at a three winding trans-former

III. ANALYSIS OF DIELECTRIC MEASUREMENTS

A. Interpretation in Frequency Domain Dielectric diagnostic methods deduce moisture in paper or

pressboard from dielectric properties like polarisation and depolarisation currents, complex capacitance and dissipation factor. Moisture strongly influences these quantities.

The dissipation factor plotted via frequency shows a typical s-shaped curve. With increasing moisture content, temperature or aging the curve shifts towards higher frequencies. Moisture influences the low and the high frequency parts. The middle part of the curve with the steep gradient reflects oil conductiv-

International Conference on Condition Monitoring and Diagnosis CMD, Peking, China, 2008 3

ity. Insulation geometry conditions determine the "hump" left of the steep gradient. In order to determine the moisture content of the insulation, the measurement should also provide data left of the "hump", where the properties of the solid insulation dominate (Fig. 4).

high

low

oil conductivity

moisture and aging of cellulose

high

high

low

low

0,001 0,01 1 100

0,001

0,01

0,1

1

1000Frequency [Hz]

Dis

sipa

tion

fact

or

insulationgeometry high

low

oil conductivity

moisture and aging of cellulose

high

high

low

low

0,001 0,01 1 100

0,001

0,01

0,1

1

1000Frequency [Hz]

Dis

sipa

tion

fact

or

insulationgeometry

Fig. 4. Interpretation scheme for frequency domain data providing discrimina-tion between the influences of various physical phenomena

B. Moisture Analysis Moisture determination bases on a comparison of the trans-

formers dielectric response to a modelled dielectric response. A fitting algorithm rearranges the modelled dielectric response and delivers moisture content and oil conductivity.

A reliable moisture analysis of onsite measurements bases on an exact data pool for the modeled dielectric response. The data pool constitutes of measurements on new pressboard at various temperatures, moisture contents and oils used for impregnation [6]. The dielectric properties of aged pressboard were investigated as well in order to compensate for the influence of aging. To ensure the reliability of the achieved data base the results were compared to previous investigations, e.g. [7].

Measurement Data base

Temperature

Oil XY-model

moisture content,oil conductivity

Comparison

Fig. 5. Programming flowchart of the analysis algorithm

C. Consideration of Conductive Aging Byproducts Aging of cellulose and oil causes conductive byproducts as

carboxylic acids. These acids are deposited in the solid insula-tion and dissolved in oil. Their DC-conductivity increases the losses and thus “imitates” water. Fig. 6 compares the dissipa-tion factor of aged material to that of new material. At similar moisture content the losses in aged materials are much higher than in the new material. Accordingly, a moisture analysis algorithm that does not compensate for conductive aging products will overestimate moisture content. This may mislead

to unnecessary drying of transformers. The newly developed software compensates for the influence of conductive aging byproducts, resulting in a more reliable moisture analysis at aged transformers. Still more practical measurements will be used to improve the functionality of the compensation algo-rithm.

0,001

0,01

0,1

1

10

1E-04 0,001 0,01 0,1 1 10 100 1000 Frequency [Hz]

Dis

sipa

tion

fact

or 2,1% aged1,2% aged2,0% new0,8% new

Fig. 6. Dissipation factor for new and aged pressboard samples having similar moisture contents measured at 20°C insulation temperature

IV. APPLICATION AT ONSITE MEASUREMENTS

Generally the dielectric response of all kinds of insulation systems can be measured; power transformers, bushings, paper-mass insulated cables, generators and motors. However the software is designed for moisture determination at oil-paper-insulated power transformers only.

Here the example of a heavily aged transformer dedicated for scrapping should be considered. Mineral oil type Shell Diala 6KX having a neutralization number of 0,49 mg KOH/g oil filled the transformer. This high value together with an oil conductivity of 900 pS/m indicates a strong influence of conductive aging products and therefore a progressed aging state. Paper samples were taken after measuring the dielectric properties (polarization and depolarization currents, frequency domain spectroscopy) and oil sampling (moisture content in ppm, moisture saturation in %).

Fig. 7 compares the moisture results of the different meas-urement and analysis approaches. Karl Fischer titration at paper samples came to 2,6 % moisture by weight (KFT). The analysis results of the dielectric response methods differ from each other: Two algorithms had no compensation for conduc-tive aging products and determined 3,8 and 4 % moisture by weight (FDS, PDC). The new analysis software having a compensation for conductive aging products indicates 2,9 % moisture relative to weight (Dira).

In an oil sample the moisture saturation was measured di-rectly onsite and also the moisture content in ppm by Karl Fischer titration in a laboratory. Via an moisture sorption isotherm [9] the relative saturation reading lead to 2,5 % moisture in cellulose (RS), which well agrees with the paper samples and the dielectric response analysis compensating for conductive aging products. An equilibrium chart based on moisture content in oil in ppm [10] determined a too high moisture content of 6,0 % (PPM), proving that equilibrium diagrams not adopted to aging state and moisture adsorption

International Conference on Condition Monitoring and Diagnosis CMD, Peking, China, 2008 4

capacity cannot reliably evaluate moisture in power transform-ers [1].

2,6

3,8

2,9

4,0

6,0

2,5

0

1

2

3

4

5

6

Moi

stur

e co

nten

t[%

]

KFT FDS DiraPDC PPM RS Dielectricresponse

Oil sample Papersample

Fig. 7. Moisture content in the solid insulation as obtained from various measurement methods

To conclude, the findings at this very aged transformer show, that a compensation for aging products is necessary both for the measurement based on moisture equilibrium and them based on dielectric properties.

V. ASSESSMENT OF MOISTURE RESULTS

IEC 60422 categorizes moisture saturations of more than 6 % as "moderately wet", which is equivalent to a moisture content of approximately 2,2 %. In this area the water mole-cules become more and more active, causing the dangerous effects of water. At this level, maintenance actions such as drying should be considered taking into account the impor-tance and future operation of the transformer. Fig. 8 shows the relationship between moisture content and moisture saturation which helps to assess the analyzed results.

Moi

stur

e co

nten

t [%

]

Moisture saturation [%]

0

1

2

3

4

5

10 20 30

21°C

80°C

Moderately wet

Dry

Wet, > 30 %extremelywet

Moisture

contamination

Fig. 8. Moisture sorption isotherm for a cellulose material relating moisture saturation to moisture content [1] with categories according to IEC 60422 [11]

VI. REFERENCES [1] M. Koch “Improved Determination of Moisture in Oil-Paper-Insulations

by Specialised Moisture Equilibrium Charts” Proceedings of the XIVth International Symposium on High Voltage Engineering, p. 508, Bei-jing, China, 2005

[2] M. Koch, S. Tenbohlen, I. Hoehlein and J. Blennow: “Reliability and Improvements of Water Titration by the Karl Fischer Technique” Pro-ceedings of the XVth International Symposium on High Voltage Engi-neering, ISH, Ljubljana, Slovenia, 2007

[3] S. M. Gubanski, P. Boss, G. Csepes, V.D. Houhanessian, J. Filippini, P. Guuinic, U. Gafvert, V. Karius, J. Lapworth, G. Urbani, P. Werelius, W. S. Zaengl: “Dielectric Response Methods for Diagnostics of Power

Transformers” CIGRÉ Task Force 15.01, Technical Brochure 254, Paris, 2004

[4] W. S. Zaengl “Dielectric Spectroscopy in Time and Frequency Domain for HV Power Equipment, Part I: Theoretical Considerations” IEEE Electrical Insulation Magazine, Vol 19, No. 5 pp.5-18, September / Oc-tober 2003

[5] H. Borsi, E. Gockenbach, M. Krueger “Method and Device for Measur-ing a Dielectric Response of an Electrical Insulation System” European Patent EP1729139

[6] M. Koch, S. Tenbohlen, M. Krüger and A. Kraetge: “A Comparative Test and Consequent Improvements on Dielectric Response Methods” Proceedings of the XVth International Symposium on High Voltage En-gineering, ISH, Ljubljana, Slovenia, 2007

[7] C. Ekanayake: “Diagnosis of Moisture in Transformer Insulation”, Ph.D. dissertation, Dep. of Materials and Manufacturing Technology, Chalmers University of Technology, 2006

[8] U. Gafvert, G. Frimpong, and J. Fuhr: “Modelling of dielectric meas-urements on power transformers", Proc. 37th Session “Large High Voltage Electric Systems” (CIGRE), paper 103, Paris, France, 1998

[9] M. Koch, S. Tenbohlen, D. Giselbrecht, C. Homagk, T. Leibfried: “Onsite, Online and Post Mortem Insulation Diagnostics at Power Transformers”, Cigré SC A2 & D1 Colloquium, Brugge, Belgium 2007

[10] T. V. Oommen: “Moisture Equilibrium Charts for Transformer Insula-tion Drying Practice” IEEE Transaction on Power Apparatus and Sys-tems, Vol. PAS-103, No. 10, Oct. 1984, pp. 3063-3067

[11] B. Pahlavanpour, M. Eklund K. Sundqvist "Revised IEC Standard for Maintenance of In-Service Insulating Oil" Weidmann Third Annual Technical Conference, Sacramento, USA. 2004