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Moisture Determination by Improved On-Site Diagnostics Maik Koch and Michael Krueger

Omicron Electronics Austria

Abstract

This paper discusses approaches to measure moisture in power transformers using dielectric response methods and equilibrium diagrams. Moisture in oil-paper insulations is a vital status-property for it damages by three mechanisms: It decreases the dielectric withstand strength, accelerates cellulose aging and causes the emission of gaseous bubbles at high temperatures.

In recent years dielectric diagnostic methods have been developed, which derive moisture concentration of the solid insulation from its dielectric properties such as dissipation factor or polarization current. Dielectric response methods provide substantial advantages compared to moisture determination by conventional equilibrium diagrams, for example it is unnecessary to wait for moisture equilibrium. In this article a new method is introduced, which combines measurements in frequency and time domain, and which makes reliable diagnostics possible even for heavily aged insulations.

The second part of this paper deals with equilibrium diagrams applied for moisture determination. Since the conventional application of these diagrams leads to erroneous results, an advanced representation of equilibrium diagrams using relative moisture in oil is introduced. Relative saturation in oil and cellulose provides easy, accurate and continuous measurements and directly reflects the destructive potential of water in oil-paper-insulations.

Examples of onsite moisture determinations compare the new methods to conventional approaches and describe the assessment of moisture analysis results.

Moisture in Oil-Paper-Insulated Transformers

Power 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.

Dangerous Effects of Water Water in oil paper insulations causes three dangerous effects: it decreases the dielectric

withstand strength, accelerates cellulose aging and causes the emission of bubbles at high temperatures. Figure 1 (left) illustrates the influence of moisture relative to saturation and acidity on the dielectric withstand strength of oil [1]. Water molecules dissociate themselves and enable acids to dissociate supporting breakdown processes with charge carriers. It is worthy to mention, that breakdown voltage correlates better with moisture saturation [%] than with moisture content [ppm].

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50

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0 5 10 15 20 Relative Moisture Saturation / %

Bre

akdo

wn

Volta

ge/ k

VTAN 0,01TAN 0,10TAN 0,3TAN 0,49

75

Temperature / C50 70 90 110 130

0,1

1

10

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1000

Life

e

xpe

ctan

ce / a Dry

1%

2%3%

4%

Figure 1. Effects of water: Left: Breakdown voltage in oil depending on moisture relative to saturation and total acid number TAN [1]

Right: Expected life for solid insulation depending on moisture content and temperature [2]

Because of hydrolysis the long cellulose chains break down into smaller pieces. Water is added in this reaction, acid serves as a catalyst [2]. Thus the degree of polymerisation (DP, number of jointed glucose rings per cellulose chain) decreases from 1000-1500 to 200-400 at the end of the cellulose life span. Figure 1 (right) depicts the life expectance of paper, supposing a starting DP of 1000 and ending DP of 200.

State of the Art in Moisture Determination The decision about maintenance actions as for example on-site drying requires knowledge

about the actual moisture concentration. State of the art for moisture measurements are equilibrium 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. Furthermore 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 [4].

Therefore dielectric diagnostic methods were developed, which deduce moisture in paper and pressboard from dielectric properties of the insulation [5]. 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. The paper further presents essential improvements on equilibrium diagrams.

Measurement of Dielectric Properties

Dielectric Properties of Insulations The multilayer insulation of power transformers consisting of oil and paper shows

polarization and conductivity phenomena. Dielectric diagnostic methods measure the interfacial polarization effect, which originates from the interfaces between 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 sinusoidal field strength E() can be expressed by (1), [6].

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)()()()()(0

00

EjjJ

lossesresistivecomponentcapacitive

++=

(1)

The imaginary part of the current density represents its capacitive 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 influence these phenomena. The discrimination of moisture from other effects is a key quality feature for the analysis of dielectric measurements.

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 Depolarization Currents PDC.

Frequency domain measurements are derived from the old known Tangent Delta measurements with a frequency range much enhanced especially to low frequencies. The derived measurement method is called Frequency Domain Spectroscopy FDS.

The combination of time domain polarization current measurements with frequency domain spectroscopy [7] 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 Hz or less. For further evaluation the time domain data are transformed to frequency domain. Figure 2 (left) illustrates the combination of dielectric measurements in time and frequency domain.

The new technique reduces the time duration of a measurement to 25 % compared to pure frequency domain measurements. 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 (Figure 2, right). 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.

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 at Figure 4. Dry or cold insulations require to measure down to very low frequencies, the long durations as in Figure 2 (right) 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.

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Curr

ent [n

A]

Time [s]

Trans-formation

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ctor

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e need

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ran

ge[H

z]

Figure 2. Left: combination of time and frequency domain measurements

Right: Required time duration and acquired frequency range for the different

measurement techniques

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.

Guard

CLT CHLCT

MVLV HV

Current sense 2A

Voltage source=

Current sense 1A

Inst

rum

ent

Figure 3. Schematic for a two channel measurement at a three winding transformer

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 (Figure 3). This feature halves the test duration additionally to the time and frequency domain combination.

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Analysis of Dielectric Measurements

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 conductivity. 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 (Figure 6).

high low

oil conductivity

moisture and aging of cellulose

high

high

low

low

0,001 0,01 1 100

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1000Frequency [Hz]

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insulationgeometry high

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oil conductivity

moisture and aging of cellulose

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low

low

0,001 0,01 1 100

0,001

0,01

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1000Frequency [Hz]

Diss

ipatio

n fa

ctor

insulationgeometry

Figure 4. Interpretation scheme for FDS providing discrimination between the influences of moisture, aging, oil conductivity and insulation geometry

Moisture Analysis Moisture determination bases on a comparison of the transformers 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 [8]. 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. [9].

In the moisture analysis algorithm at first the insulation temperature T from the measured dielectric response C(f) is taken and the corresponding permittivity record PB(f) from the extra- and interpolated data base. The so called XY-model combines this permittivity record PB(f) with the complex oil permittivity Oil(f). The XY-model allows for the computation of the dielectric response of a linear multi-layer-dielectric [10], where X represents the ratio of barriers to oil and Y the ratio of spacers to oil.

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The obtained modelled permittivity m(f) = tot(f) is converted into a modelled capacitance Cm(f) and then compared to the measured dielectric response C(f). The modelled capacitance Cm(f) with the best fitting to the measured capacitance C(f) gives the moisture content in cellulose and the oil conductivity of the real transformer. Figure 5 (left) depicts the programming flowchart of the new analysis algorithm.

Temperature T

Comparison

Measurement C(f)

Modelled dielectric response m(f), Cm(f)

PB(f) at temp. T

Data base (f)

=

0

2.2 oiloil j

mod_

mod_ 11

PBoil

PBtot XXYY

+

+=

Measured dielectric response

Oil(f)

Combination by

moisture content,oil conductivity

Core

Spacer

Barrier

HV-winding

LV-winding

Oil

Barriers

Spacers

Y

Oil

X

Figure 5. Left: programming flowchart of the analysis algorithm Right: representation of a cylindrical transformer insulation by the XY-model

Consideration of Conductive Aging Byproducts Aging of cellulose and oil causes conductive by-products as carboxylic acids. These acids

are deposited in the solid insulation and dissolved in oil. Their DC-conductivity increases the losses and thus imitates water.

0,001

0,01

0,1

1

10

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

Diss

ipat

ion fa

ctor 2,1% aged

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

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

Figure 6 compares the dissipation 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

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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 algorithm.

Moisture Measurement through Moisture Equilibrium

Moisture Equilibrium Moisture equilibrium bases on three conditions: thermal equilibrium (temperature),

mechanical equilibrium (e.g. pressure) and chemical equilibrium. Thermodynamic equilibrium is reached, if the macroscopic observables do not change with time and place. An equilibrium regarding the time aspect is possible during a constant load period of a transformer. Still the observables will change with place. Equilibrium for time and place can be reached only in locally limited areas, e.g. between cellulose and the surrounding oil at high temperatures and slow oil flows.

Moisture equilibrium means, that the no migration of water molecules inside materials and between oil and cellulose occurs. Moisture migrates until the water vapour pressure p gains the same value, thus differences in the moisture vapour pressure are the driving force for moisture migration, (2).

AirOilCellulose ppp == (2) Supposed the same temperature and pressure rules, moisture exchange can be described in

terms of relative saturation. The moisture content relative to saturation level in adjacent materials becomes equal (3). The material might be cellulose, oil, air or even a plastic.

AirOilCellulose RHRSRS == (3)

Conventional Equilibrium Diagrams It is a standard procedure for operators of power transformers to derive the moisture by

weight (%) in cellulose from the moisture by weight in oil (ppm). This approach consists of three steps: (1) Sampling of oil under service conditions, (2) Measurement of water content by Karl Fischer Titration and (3) Deriving moisture content in paper via equilibrium diagrams (e.g. [14]) from moisture in oil.

Unfortunately crucial errors effect this procedure: Sampling, transportation to the laboratory and moisture measurement by Karl Fischer

titration, Equilibrium conditions are rarely achieved (depending on temperature after

hours/days/months), A steep gradient and high uncertainty in the low moisture region compounds the accuracy, Diagrams from various literature sources lead to different results, The temperature gradient in windings (up to 30 K) causes a uneven moisture distribution, Equilibrium depends on moisture solubility in oil and moisture adsorption capacity of

cellulose.

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0

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Mo

istu

re in

ce

llulo

se / %

60C New PB60C Aged KP60C Aged PB

2,9

1,5

2,62,12

40C New KP21C New KP

60C New KP80C New KP

0

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istu

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r / %

21C Aged40C Aged60C Aged80C Aged

21C New40C New60C New80C New

21C Aged40C Aged60C Aged80C Aged

21C New40C New60C New80C New

Figure 7. Left: equilibrium diagram for moisture in Kraft paper KP and oil with additional graphs for new pressboard PB and aged kraft paper and pressboard

Right: equilibrium diagrams adapted to the moisture adsorption capacity of new Kraft paper in new oil and of thermally degraded Kraft paper in aged oil

The validity of equilibrium diagrams is restricted to the original materials that were used to establish the diagrams. Especially aging changes the moisture adsorption capacity substantially. The following Figure 7 (left) displays the graphs for equilibrium of new Kraft paper with new oil at 20, 40, 60 and 80C. Additionally for 60C it shows moisture equilibrium for new pressboard in new oil and for aged Kraft paper and aged pressboard in aged oil. Assumed the moisture content in oil is 20 ppm these curves lead to a moisture content in new paper of 2,9 %, in new pressboard of 2,6 %, in aged paper and aged oil it is 2,1 % and for aged pressboard and aged oil 1,5 %. Thus equilibrium diagrams not adapted to the specific materials are inapplicable to calculate moisture in paper from moisture in oil.

Diagrams Adapted to the Moisture Adsorption Capacity The first step to improve equilibrium diagrams is to adapt them to the water adsorption

capacity of the materials involved [3]. Diagrams as Figure 7 (right) might be used to determine the true water content in cellulose, since they are adapted to the moisture adsorption capacity of the materials. They still have the essential drawback, that their validity is restricted to the involved materials. For other materials and ageing conditions they have to be redrawn, that is, a correction is necessary for every transformer with its special materials and aging conditions. Because of this disadvantage the following subsection describes the next step to more universal equilibrium diagrams.

Measurement via Moisture Saturation of Oil In this approach instead of moisture in oil relative to weight (ppm) the relative saturation in

oil (%) is used. Additionally the diagrams are adapted to the moisture adsorption capacity of the cellulose. Based on equation (3) via moisture adsorption isotherms the moisture content in cellulose is derived from moisture relative to saturation of the surrounding oil. The advantages are: Oil aging and its influence on moisture saturation level becomes negligible, since it is

already included into the measurement of moisture saturation.

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With relative moisture on the X-axis the graphs become less temperature dependent compared to moisture by weight on this axis (Figure 8, right)

Errors due to sampling, transportation to the lab and titration are excluded. Continual, accurate measurement and easy implementation into a monitoring systems

Figure 8 (right) shows moisture by weight in thermally degraded Kraft paper as a function of moisture saturation. With thermal aging the ability of cellulose to adsorb moisture decreases. In this example a moisture saturation of 4,1 % at 47C oil temperature gives in Kraft paper 2,2 % moisture content relative to weight. The approach to use moisture saturation of oil substantially improves moisture determination in transformers; still the diagrams have to be adapted to the moisture adsorption capacity of the specific cellulose material. The next section shows a way to overcome this drawback.

Measurement and Implementation of Moisture Saturation in Cellulose Moisture saturation is a critical factor that determines the amount of water available for

interactions with materials. The destructive effects of water in power transformers are a decreased breakdown strength of oil, accelerated aging of cellulose and bubble formation. Water molecules that are available for interactions with materials cause all these destructive effects. This is not the case for molecules that are strongly bound, e.g. by hydrogen bonds to OH-groups of cellulose molecules forming a monolayer. Just water relative to weight, measured by Karl Fischer titration, reflects the bound and less active water as well. Moisture relative to saturation - not relative to weight - determines the available water for destructive effects. This approach gives the following advantages: Neither oil nor paper aging effects the validity Conversion via equilibrium charts unnecessary Direct relation to the destructive impacts of water Continual, accurate measurement and easy implementation into a monitoring systems

01.06.2003 05.06.2003 09.06.2003 13.06.2003 17.06.200305

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Mo

istu

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tpap

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/ %

Aged KP 21CAged KP 40CAged KP 60CAged KP 80C

2,2

4,1

Figure 8. Left: top oil temperature, relative saturation in oil and in cellulose measured resp. calculated by an online monitoring system

Right: determination of water content in thermally degraded Kraft paper via a moisture adsorption isotherm

Figure 8 (left) illustrates the application of a relative saturation measurement using a capacitive probe in a power transformer equipped with an online monitoring system. The load factor influences the top oil temperature, which follows in diffusion processes changing the

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relative saturation in oil. A long term average equilibrates the relative saturation in oil with the relative saturation of the surrounding cellulose and comes to 4,1 %. Using a moisture isotherm as Figure 8 (right) one can derive the moisture by weight in cellulose as well, that would be 2,2 % in this case.

Application at Onsite Measurements

Here the example of a heavily aged transformer dedicated for scrapping should be considered. Such a case delivers a good opportunity to compare different approaches to assess moisture content. Mineral oil type Shell Diala 6KX having a neutralization number of 0,49 mg KOH/g oil from 1954 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 %). Figure 9 depicts the measurement data of the dielectric measurements. The fast decaying currents in time domain and the high dissipation factor in frequency domain indicate a highly conductive insulation.

Figure 10 (left) compares the moisture results of the different measurement 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 conductive 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).

1E-07

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IpolIdep

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

Diss

ipat

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Fact

or

Figure 9. Left: polarisation currents measured between HV- and LV-winding

Right: dissipation factor as measured between HV- and LV-winding

In an oil sample the moisture saturation was measured directly onsite and also the moisture content in ppm by Karl Fischer titration in a laboratory. Via an moisture sorption isotherm 11 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 [14] determined a too high moisture content of 6,0 % (PPM), proving that equilibrium diagrams not adopted to aging state and moisture adsorption capacity cannot reliably evaluate moisture in power transformers.

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2,6

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sture

co

nte

nt[%

]

KFT FDS DiraPDC PPM RS Dielectricresponse

Oil sample Papersample

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istu

re co

nte

nt [%

]

Moisture saturation [%]0

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10 20 30

21C

80C

Moderately wet

Dry

Wet, > 30 %extremelywet

Moistu

re

conta

minatio

n

Figure 10. Left: moisture content in the solid insulation as obtained from various measurement methods Right: moisture sorption isotherm for a cellulose material relating moisture saturation to moisture content [3]

with categories according to IEC 60422 [13]

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.

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 molecules 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 importance and future operation of the transformer. Figure 10 (right) shows the relationship between moisture content and moisture saturation which helps to assess the analyzed results.

Summary

This paper discusses approaches to measure moisture in power transformers using dielectric response methods and equilibrium diagrams.

Dielectric diagnostic methods deduce moisture in the solid insulation from dielectric properties like polarisation depolarisation currents and dissipation factor vs. frequency.

A new instrument Dirana combines time domain (PDC) and frequency domain (FDS) measurements and thus substantially shortens the measurement duration.

A new software was developed which bases on a new data pool, measured at new and aged pressboard with various moisture contents and oil impregnation.

The new analysis algorithm compares measurements from a transformer to modelled dielectric responses, obtained from the so called XY-model. The new software features weighting of low frequency data and an extended XY-model.

The analysis algorithm compensates for the influence of conductive aging by-products.

The new software was successfully utilized for onsite measurements in comparison to other measurement and analysis methods.

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The conventional application of equilibrium diagrams to derive moisture in cellulose (%) from moisture in oil (ppm) is effected by substantial errors.

To exclude the interference due to oil aging the moisture in oil relative to saturation level (%) is more appropriated instead of moisture in oil in ppm.

One step forward constitutes the use of moisture relative to saturation in oil and cellulose. This measure is easy, continually and accurate measurable.

References

1. M. Koch, S. Tenbohlen: The Breakdown Voltage of Insulation Oil under the Influence of Humidity, Acidity, Particles and Pressure, International Conference on Advances in Processing, Testing and Application of Dielectric Materials APTADM, 26.-28.09.2007, Wroclaw.

2. L. E. Lundgaard, W. Hansen, D. Linhjell, T. J. Painter: Aging of oil-impregnated paper in power transformers, IEEE Transactions on Power Delivery, Jan. 2004 Volume: 19, Issue 1, p. 230- 239.

3. M. Koch, S. Tenbohlen, T. Stirl: Advanced Online Moisture Measurements in Power Transformers CMD 2006 International Conference on Condition Monitoring and Diagnosis, Changwon, Korea, 2006

4. M. Koch, S. Tenbohlen, I. Hoehlein and J. Blennow: Reliability and Improvements of Water Titration by the Karl Fischer Technique Proceedings of the XVth International Symposium on High Voltage Engineering, ISH, Ljubljana, Slovenia, 2007

5. 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 CIGR Task Force 15.01, Technical Brochure 254, Paris, 2004

6. 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 / October 2003

7. H. Borsi, E. Gockenbach, M. Krueger Method and Device for Measuring a Dielectric Response of an Electrical Insulation System European Patent EP1729139

8. M. Koch, S. Tenbohlen, M. Krger and A. Kraetge: A Comparative Test and Consequent Improvements on Dielectric Response Methods Proceedings of the XVth International Symposium on High Voltage Engineering, ISH, Ljubljana, Slovenia, 2007

9. C. Ekanayake: Diagnosis of Moisture in Transformer Insulation, Ph.D. dissertation, Dep. of Materials and Manufacturing Technology, Chalmers University of Technology, 2006

10. U. Gafvert, G. Frimpong, and J. Fuhr: Modelling of dielectric measurements on power transformers", Proc. 37th Session Large High Voltage Electric Systems (CIGRE), paper 103, Paris, France, 1998

11. M. Koch, S. Tenbohlen, D. Giselbrecht, C. Homagk, T. Leibfried: Onsite, Online and Post Mortem Insulation Diagnostics at Power Transformers, Cigr SC A2 & D1 Colloquium, Brugge, Belgium 2007

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12. T. V. Oommen: Moisture Equilibrium Charts for Transformer Insulation Drying Practice IEEE Transaction on Power Apparatus and Systems, Vol. PAS-103, No. 10, Oct. 1984, pp. 3063-3067

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

14. Y. Du, M. Zahn, et al. A Review of Moisture Equilibrium in Transformer Paper-Oil Systems IEEE Electrical Insulation Magazine, Vol. 15, No. 1, pp. 11-20, January-February 1999

Biography

Maik Koch is a product manager at OMICRON Austria. Before that he worked as a research assistant at the University of Stuttgart in Germany where he graduated as a Doctor of Philosophy. His fields of research were ageing and moisture determination in oil paper insulated power transformers using chemical and dielectric analysis methods. He received a diploma for electrical engineering at the Technical University of Cottbus in Germany. He has written about twenty scientific papers and collaborates in Cigr working groups.

Michael Krueger is head of the OMICRON competence center primary testing. He studied electrical engineering at the University of Aachen (RWTH) and the University of Kaiserslautern (Germany) and graduated in 1976 (Dipl.-Ing.). In 1990 he received the Dr. techn. from the University of Vienna. Michael Krueger has 30 years experience in high voltage engineering and insulation diagnosis. He is member of VDE, Cigr and IEEE.