17_ Koch_min risk of IT with Dielectric Response.pdf

Presentation 17.1

OMICRON electronics Asia Limited 2013 Conference on Electrical Power Equipment Diagnostics

Minimizing the Risk of Insulation Failures of Instrument Transformers by Dielectric Response Analysis Maik Koch, FKH, Switzerland Stephanie Uhrig, Martin Anglhuber, Omicron electronics, Austria

Abstract Instrument transformers in service are exposed to the harming effects of water and ageing. The increasing age is leading to a higher risk of explosions, which can damage surrounding parts and cause high follow-up costs. This paper introduces a new approach to use the dielectric response analysis for condition assessment of instrument transformers. The measurement of the dielectric response in a wide frequency range provides information about the insulation condition and especially for oil-paper/pressboard insulations about the water content in the solid insulation. The measured curve is formed by the superposition of the dielectric response of the cellulosic material and the oil. The curve can be used for the assessment of the insulation condition in various ways. Qualitative comparisons, e.g. between instrument transformers of the same type and age, might give general information which of these is in better condition. The dissipation factor and the capacitance trend can also deliver valuable data. Furthermore the water content in the solid insulation can be assessed using an automatic analysis algorithm. To confirm the validity of this assessment, measurements on several instrument transformers in service were performed. Dielectric response curves and the trend of the capacitances for new and aged instrument transformers with different water contents are discussed. Selected examples presented in this paper show, that the dielectric response is well modeled, the water content can be derived and ageing effects can be observed.

Introduction Instrument transformers are important apparatuses used in transmission and distribution networks all over the world. Being not as expensive as e.g. power transformers, instrument transformers were rather replaced than repaired. However, a defect instrument transformer can explode, what may cause extensive damages of surrounding plant sections. Nearly 50% of all major failures are caused by an internal dielectric failure 0. Since the majority of instrument transformers is oil-paper insulated, ageing of the paper material is a crucial factor for the lifetime. Paper is made of cellulose. Its ageing might lower the dielectric strength and will lead to a decreased degree of polymerization in paper. It is depending on some influencing factors, like the

temperature, present oxygen, water or acids. Especially water in the cellulosic insulation can be critical [2]. Existing water causes hydrolysis, which is producing more water as a product of this chemical reaction. Therefore water is an accelerator for ageing of the solid insulation. Typically, new oil-paper insulations have a very low water content of about 0.5% in the solid insulation. During service the water content is increasing due to e.g. leakages and hydrolysis. Above 2.2% water content the solid insulation is called moderately wet [3], what is typical for instrument transformers being in service for several years. At the physically end of life, the insulation is often wet having a water content of 3.5% or above. Even though water content in the solid insulation is no direct measurand for ageing, it is a strong indicator for the condition of the solid insulation. To avoid failures with expensive secondary damages, several tests can be performed to determine the condition of instrument transformers. The choice of tests and the testing frequency is widely varying from utility to utility. Typical maintenance strategy used prior to failure are shown in TABLE I. Unfortunately the gained information about the insulation condition is limited. Oil sampling is often avoided due to the small volume. The partial discharge measurement detects weak points in the insulation and can not give information about the overall condition. Finally the insulation resistance is helpful to find already defective insulations, but is limited for lifetime estimation.

TABLE I. MAINTENANCE STRATEGY USED PRIOR TO FAILURE FROM 3004 REPORTED FAILURES (OUT OF [4])

Maintenance strategy How often Regular visual inspection 95%

Check of oil level and/or pressure

61% Secondary voltage monitoring for

CVT 15%

Insulation resistance checks 11% DGA and/or moisture of oil 7% Thermovision inspection 4%

DF measurement at mains f

2% The measurement of dielectric properties, like the dissipation factor (tan or DF) or ca pa cita nce a t mains frequency (50/60 Hz) is often used to gain information about the insulation condition. However, several factors are influencing the dissipation factor reading at mains frequency, what causes uncertainties in the assessment. The measurement over a wide frequency range can help to distinguish between different effects for a more detailed analysis of the insulation condition.

Presentation 17.2


DIELECTRIC RESPONSE OF INSTRUMENT TRANSFORMERS Dielectric response methods have been developed to deduce water content in paper and pressboard from dielectric properties like polarization currents and dissipation factor [5]. They are used to assess the condition of power transformers, but can also be applied to other oil-paper insulations like bushings, cables or instrument transformers. Due to the wide frequency range of the dielectric response measurement it is possible to distinguish between different effects and gain information about the insulation condition itself, water content in the solid insulation or oil conductivity [3]. The setup for the dielectric response measurement is the same as for traditional dissipation factor measurement at mains frequencies. The resulting curves are similar to the single response of cellulosic material without any oil (Figure 1) [6], since the insulation itself consists mainly of paper material. The dielectric dissipation factor is decreasing with increasing frequency, usually having a minimum around 1-100 Hz. Especially at low frequencies, the slope of the curves seems to be linear. As for other oil-paper insulations, the temperature as well as the oil conductivity is influencing the dielectric response. Higher temperatures and higher oil conductivities are shifting the curves towards higher frequencies [7].

Figure 1. Dielectric response of paper at 20C with 1% (2% and

3%) water content

ASSESSMENT OF THE DIELECTRIC RESPONSE Qualitative Comparison A measured dielectric response curve can be assessed using different methods. Like shown in Figure 2 and Figure 3, a comparison between instrument transformers of the same type can give information, whether the overall condition in the solid insulation is similar or if one of them shows a stronger deterioration of the insulation. Typically the one with a worse insulation condition shows a higher dissipation factor. Reason for a worse insulation condition could be e.g. a defect sealing and resulting water ingress or a higher workload.

Figure 2 shows the dielectric response curves of four current transformers of the same type. All of them were manufactured in 1963 and having nearly identical slopes, suggesting a similar ageing behavior. The curve shape itself is very similar to the one of paper material only (Figure 1). According to the analysis (chapter 3.2), the insulations have a water content of 1.8 1.9% and an oil conductivity of 3 6 pS/m.

Figure 2. Dielectric response of four current transformers of

same type

Figure 3. Dielectric response of two current transformers of

same type

Figure 3 shows another example of two current transformers of the same type. They are having different curve shapes. Like mentioned above, the curve with a higher DF is typically in a worse condition, in this case it is having a water content of 2% in the solid insulation according to the analysis. The other current transformer has a significant lower water content of 0.6% in the solid insulation.

Figure 4. Dielectric response of instrument transformers of

different age and condition

f /Hz0.001 0.01 0.1 1.0 10.0 100

DF

0.005

0.010

0.020

0.050

0.100

0.200

0.5001.000 1%@20C

3

2

1

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0,01 0,1 1 10 100 1000

Diss

ipat

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Fact

or

Frequency / Hz

phase Aphase Bphase Cphase A-2

0,001

0,01

0,1

1

0,01 0,1 1 10 100 1000

Diss

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Fact

or

Frequency / Hz

phase Aphase B

0,001

0,01

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0,001 0,01 0,1 1 10 100 1000 10000

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Frequency / Hz

new, 0.6% wc, Cratio=1.01

medium, 2.1% wc, Cratio=1.08

old, 3.0% wc, Cratio=1.35

old, 3,5% wc, Cratio=1.55

Presentation 17.3


The principle changes of the dielectric response curves due to ageing and/or moisture ingress can be understood by comparing instrument transformers of different age and condition (Figure 4). Newly manufactured and very dry instrument transformers have a very flat response. Both, water content and ageing result in a steeper slope at lower frequencies. However, the values for the dissipation factor at mains frequencies are in the same range as for new ones. Only for the heavily aged and wet instrument transformer the dissipation factor at mains frequency is significantly enhanced.

Moisture Determination The moisture determination using the dielectric response curves is based on a comparison between the measured curve and a modeled curve (Figure 5). The curve modeling is done with help of a data base including material properties of cellulosic materials with different water contents and temperatures. Using the so called XY-model (Figure 6), a dielectric response is calculated under consideration of the insulation geometry, temperature, oil and moisture content [7]. A fitting algorithm aligns the modeled response of the data base to the measured curve of the real insulation and automatically delivers the water content of the cellulose material as well as the water saturation or the oil conductivity.

Figure 5. Calculation of the water content based on comparison

of the measured dielectric response to a modeled curve

Figure 6. Representation of a cylindrical multi-layer-insulation

by the X-Y model as used for power transformers

The used XY-model is developed to model the properties of oil-paper insulations, initially for power transformers. The model takes into account the amount of barriers and spacers of the insulation. Instrument transformers consist as well of a cylindrical insulation, similar to power transformers. The main part of the insulation (70%

- 90%) is consisting of paper, enwrapping the inner conductor and therefore having similar behavior as barriers. Also oil gaps exist, as between spacers of power transformer insulations. Therefore it is assumed, that the XY-model can be applied to instrument transformers as well. However, the insulation of instrument transformers is not as ideal cylindrical as power transformers. Therefore the uncertainty of the assessment of the water content will be higher compared to power transformers. Several measurements on instrument transformers were done and the analysis algorithm was able to get a proper fitting and reasonable results, leading to the presumption that this approach is adequate for the dielectric response analysis on instrument transformers.

Figure 7. Dissipation factor for new and aged pressboard

samples at 20C, having similar water contents

Aging of cellulose and oil causes conductive byproducts as carboxylic acids. These acids are deposited in the insulation and influence the resulting dielectric response. Figure 7 compares the dissipation factor of aged material to that of new material at similar water contents. The conductive ageing products have a similar effect as higher water contents. Accordingly, this might lead to an overestimation of the water content. To avoid such an overestimation, the analysis algorithm compensates for the influence of conductive aging byproducts (like described in [3]), resulting in a more reliable result for aged insulations.

Analysis of Dissipation Factor at Different Frequencies Besides the water content the dielectric response delivers more valuable data. If single values are needed, e.g. for an asset management software, information about the dissipation factor or capacitance trend might be used. To visualize the general influence of water content and ageing, the results of more than 30 instrument transformers of different types and designs were analyzed. The condition varies from newly manufactured, medium aged in service, to heavily aged and out of service. Unfortunately no direct quantification for ageing is possible without dismantling. For measuring the degree of polymerization a paper sampling would

Measurement

Temperature

Insulation geometryXY-model

moisture content,(oil conductivity)

Comparison

(Oil)

Data base

Presentation 17.4


be necessary. Since the water content in paper is increased during lifetime due to the hydrolysis, it is used as an indicator for the insulation condition. A typical measurement providing information about the insulation condition of high voltage assets is the dissipation factor measurement at mains frequency. For all investigated instrument transformers, the dissipation factor at 50 Hz is shown in Figure 8. This value tends to increase for higher moisture contents in the solid insulation and is strongly influenced by the insulation temperature. However, the dissipation factor at 50 Hz is having very similar values (0.002 0.004) for dry insulations and insulations with a water content up to about 3%. This is reasonable, since the slope of the dissipation factor has the smallest steepness and its minimum around mains frequencies. A high measured dissipation factor of more than 0.01 would therefore indicate a high water content, what is usually a sign for bad insulation condition.

Figure 8. DF at 50 Hz and water content for various instrument transformers of different condition (temperature: 25C..30C)

Figure 9. DF at 10 mHz and water content for various

instrument transformers of different condition (temperature: 25C..30C)

The dielectric response enables to analyze also lower frequencies, e.g. the dissipation factor at 10 mHz. This value also tends to increase with increasing water content in the solid insulation (Figure 9). As it can be seen in Figure 4 as well, water content and ageing is increasing the slope of the dissipation factor curve leading to wider range of the values at 10 mHz.

Analysis of Capacitance Trend An ideal insulation has a frequency independent capacitance. However, the capacitance of a real insulation is slightly increasing towards low frequencies (Figure 10). This trend is more pronounced for aged insulations compared to new ones. The capacitance trend can be visualized in the ratio of the capacitances, e.g. at 10 mHz and 50 Hz. This value is independent of capacitance and insulation temperature. A clear tendency can be observed, that the ratio is increasing with increasing water content (wc). This finding suggests the capacitance ratio as a good value, which can be used for assessment decisions.

Figure 10. Frequency dependent capacitances of various

instrument transformers depending on frequency

TABLE II. RATIO OF CAPACITANCES AT 10 MHZ AND 50 HZ FOR FOUR EXAMPLES MENTIONED IN FIGURE 4 AND FIGURE 10

Condition Water

C10mHz/C50Hz new 0.6 1.01 medium 2.1 1.08 old 3.0 1.35 old 3.5 1.55

CONCLUSION The dielectric response analysis, typically used for power transformer can also be applied to oil-paper insulated instrument transformers. The measurement of the dielectric properties over a wide frequency range enables a detailed analysis of the insulation condition, amongst others delivering information about ageing and water content:

A qualitative comparison between instrument transformers of the same type can easily be used for identifying defect units.

Using the described algorithm it is possible to estimate the water content from the dielectric response curve. The water content is typically a good indicator for ageing, since the dominant ageing mechanism in the solid insulation is hydrolysis.

The dissipation factor at 50 Hz is typically increasing with increasing water content. Also the dissipation factor at low frequencies, for example 10 mHz can be used. It is also increasing with increasing water content.

0,001

0,010

0,100

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t 50

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water content in %

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t 10

mHz

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0,001 0,1 10 1000C

in n

FFrequency / Hz

Presentation 17.5


There is a trend of an increasing capacitance at low frequencies. This trend is stronger with increasing age and/or water content. A ratio, formed of capacitances between 10 mHz and 50 Hz, can visualize the increase.

References [1] Cigr WG A3-06. "Tutorial on Reliability of High Voltage

Equipment" (held during the SC A3 High Voltage Equipment Symposium, Vienna, Austria, September 7, 2011)

[2] Sokolov et al. Moisture Equilibrium and Moisture Migration within Transformer Insulation Systems (Cigr Working Group A2.30, Technical Brochure 359, Paris 2008)

[3] M. Koch, M. Krueger. A Fast and Reliable Dielectric Diagnostic Method to Determine Moisture in Power Transformers (International Conference on Condition Monitoring and Diagnosis CMD, Peking, China, 21 - 24 April 2008)

[4] CIGR Study Committee A3. "State of the Art of Instrument Transformers" (Technical Brochure 394, Paris, 2009)

[5] S. M. Gubanski, et al. Dielectric Response Diagnoses for Transformer Windings (CIGR Task Force D1.1.14, Technical Brochure 414, Paris, 2010)

[6] M. Koch, M. Krueger, M. Puetter. "Advanced Insulation Diagnostic by Dielectric Spectroscopy" (TechCon Asia Pacific, Sydney, Australia, May 2009)

[7] M. Koch. "Reliable Moisture Determination in Power Transformers (PhD thesis, Institute of Energy Transmission and High Voltage Engineering, University of Stuttgart, Sierke Verlag Gttingen, Germany, 2008)

About the Author Maik Koch leads the Insulation Material Laboratory at the FKH (Expert Commission for HV engineering and testing) in Switzerland. He studied electrical power engineering at various German universities and graduated as a Ph.D. at the

University of Stuttgart in Germany in 2008. In 2007, he joined Omicron electronics, Austria, where he lead the Product Management. He joined FKH in 2013. His field of expertise is condition assessment of HV assets by electrical, chemical and dielectric analysis methods. He wrote more than 70 scientific papers and contributes to working groups of VDE, CIGRE, IEC and IEEE dealing with subjects such as HV testing and diagnostics, insulation ageing and on-line monitoring.

AbstractIntroductionDIELECTRIC RESPONSE OF INSTRUMENT TRANSFORMERSASSESSMENT OF THE DIELECTRIC RESPONSEQualitative ComparisonMoisture DeterminationAnalysis of Dissipation Factor at Different FrequenciesAnalysis of Capacitance Trend

CONCLUSIONReferencesAbout the Author

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