Identify Moisture Distribution in Multilayer Oil-immersed Paper by Frequency Domain Spectroscopy

Hua-dong Peng, Ming Dong, Xue-zhou Wu, Yuan LiuState Key Laboratory of Electrical Insulation and Power

EquipmentEEC, Xi’an Jiaotong UniversityXi’an, Shaanxi 710049, China

Email: peng.huadong2011@gmail.comdongming@mail.xjtu.edu.cn

Hua-dong PengChongqing Electric Power Research Institute,

Chongqing 401123, China

Abstract—Moisture content of oil and paper is one of the most important factors influencing the dielectric behavior of insulation systems. Moisture estimation in insulating paper can provide a way to understand the real condition of insulation and helps to assess its remaining useful life. In recent years, Frequency Domain Spectroscopy (FDS) is widely used by power utilities for moisture estimation and condition assessment of oil-paperinsulation equipment, such as transformer, bushing and cables, and the estimated moisture value is an equivalent one of the whole facility. Actually, moisture distribution in the insulation is very complicated due to temperature and load variation, and failures usually happen in the weakest place with high moisture or potential defects. In this paper, some single layer papers with the same average moisture content but different moisturedistribution were prepared, and FDS measurements are performed on these samples to identify the different moisture distribution. Although the dissipation factor is used to reveal the whole insulation condition in traditional concept, the results show that different distribution has different performance in FDS curves. The curves are mainly determined by the highest moisture part, and the distribution difference displays in the low and high frequency range.

Keywords-oil-immersed paper; frequency domain spectroscopy;moisture distribution; dissipation factor; complex capacitance

I. INTRODUCTION

With the advantage of good insulation and excellent heat dissipation performance, oil-paper insulation structure is widely used in oil-immersed electric power equipment. As paper is mainly made up of fibrous structure, it is easy to get moisture absorption. What’s more, in the long-term operation process, oil-paper insulation is continuously subjected to thermal, electrical, mechanical and chemical stresses, which deteriorate the insulating property and lead to produce some by-products such as moisture, furfural, hydrocarbon, organic compound and so on [1-3]. The existence of moisture in insulation will severely reduce its electric strength, speed up itsaging rate and shorten its insulation life, so the moisture content must be controlled in a relatively dry level for high voltage equipment during its service time [4]. For a new transformer, the moisture content in solid insulation is usually below 0.5%, and the higher the voltage level is, the lower the moisture content must be limited [5]. Once a device is put into operation, its insulation will deteriorate and produce certain

amount of moisture, which then acts as catalyst for further aging [6].Therefore, estimation of moisture content is of great importance for diagnosis of insulation condition.

As an accurate and reliable condition assessment of insulation in transformers, bushings, cables, etc., frequency domain spectroscopy (FDS) has been widely used for the insulation diagnosis of electric equipment in recent years.Diagnosis with this technology is based on the fact that paper and oil change their dielectric properties when the insulation is degraded or moisture levels increases in solid materials.Although moisture estimation by FDS has been investigated by some researchers, the obtained moisture content is an equivalent value reflecting the whole condition of insulation.However, moisture is usually distributed in solid insulation due to temperature and load variation, and breakdown commonly happens in the part gathering the the vast majorityof moisture.

In this paper, some specimen with different moisture content is prepared and FDS measurements are performed in order to identify moisture distribution in multilayer oil-immersed paper.

II. EXPERIMENTATION

A. Test DeviceA sealed metal vessel was designed to perform FDS

measurements of oil-immersed paper, as shown in Fig. 1.Electrode structure is the main part of the test device, including a high-voltage electrode with a diameter of 120mm, a measurement electrode with a diameter of 100mm and a guard electrode used for avoiding surface leakage current and edge effect. The specimen is tightly fixed between high-voltage electrode and measurement electrode with uniform pressure by a spring.

B. Preparation of SpecimenIn this paper, new mineral transformer oil and Weidmann

pressboard with the thickness of 1.5 mm are used. Before the test, the pressboard was firstly dried for 48 hours under 105in a vacuum oven and then taken out to absorb moisture in the atmosphere ambient until they reached an initial water content level of 0~6% by weight. During this process, the weight of the pressboard was continuously monitored by a high-precision electronic balance. Secondly, the prepared specimen was put

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into the test device and impregnated in oil in a vacuum ambient by operating exhaust port and oil port in coordination. After that the whole device was put into a well temperature-controlled oven and kept at 60 for a week to ensure oil immersed in paper and moisture between them reaching equilibrium completely [7].

C. Test ArrangementFirst, FDS measurements are performed on single-layer oil-

paper pressboard (OIP) with different moisture content. Second, two or three single-layer OIP are combined into a whole one with the same average moisture content but different distribution in thickness, and also FDS measurements are carried out on them. Each measurement is performed at 40and voltage applied on the specimen is 200V peak value. In the test, the whole device was maintained at 40 for no less than 12 hours to decrease the effect of moisture migration between oil and paper due to temperature variation.

Figure 1. Schemetic drawing of the measuring system

III. EXPERIMENTALRESULTS

A. Single-layer Oil-immersed PressboardFig. 2 and Fig. 3 show the dissipation factor and complex

capacitance of single-layer OIP with different moisture content, respectively. The curve showed considerable variations with moisture content. In the whole frequency range, tanδ value increases with moisture increasing in pressboard. The reason may be that the strongly polar water molecule enhanced intensity of polarization and conductivity of pressboard, which increase the loss of the insulation material together. Besides, at lower moisture contents (no more than 3%), a visible minimum tanδ value can be observed and it shifts to higher frequency with moisture increase. Some researchers such as Saha [6] and Gubanski [8] have pointed out that there is a linear correlation relationship between natural logarithm of minimum tanδ valueof FDS and moisture content. It provides a way to estimate the moisture content in solid insulation materials and is helpful to the assessment and diagnosis of insulation condition. However, the minimum tanδ value disappears with an increase of moisture content, so the empirical relationship has some limitation. At highest moisture content (more than 4%), a visible dissipation peak is observed, which indicates that some polarization type plays a leading role in this frequency range and the moisture content may influence the range. As the

dissipation peak appears in 10-3Hz to 10-1Hz, the most possible polarization is interfacial polarization between oil andpressboard.

In Fig. 3, the real capacitance nearly keeps constant in the frequency range from 0.01Hz to 1000Hz. However, the phenomenon of the low frequency dispersion becomes apparent at lower frequencies, especially for pressboard with 6% moisture content. What’s more, the real and imaginary capacitance begins to cross with moisture content increasing, and it can be a characterization of moisture content.

Figure 2. Dissipation factor of single-layer OIP with different moisture content plotted against frequency at 40

Figure 3. Complexcapacitance of single-layer OIP with different moisture content plotted against frequency at 40

B. Double-layer Oil-immersed PressboardFig. 4 and Fig. 5 show the dissipation factor and complex

capacitance of double-layer OIP with average 2% moisture content, respectively. Although the average moisture content is the same, the whole specimen has different moisture distribution in thickness. In Fig. 4, the non-uniformity of specimen dry+4% is greater than that of others, and tanδ valueappears different tendency in the low frequency and high frequency range. The tanδ-f curve of specimen dry+4% also has a peak appearing at 0.1Hz, which indicates different moisture distribution in the specimen. In Fig. 5, the real capacitance changes little with moisture content changing. The imaginary one presents the loss of insulation, so the shape of curve is similar to tanδ-f curve.

FDS testinstrument

Oven

Hi

Lo

GuardOil port

Exhaustport

Epoxy resin board

Oil

Pressurespring

ElectrodePressboard

Metalcontainer

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Figure 4. Dissipation factor of double-layer OIP with average 2%moisture contentplotted against frequency at 40

Figure 5. Complexcapacitance of double-layer OIP withaverage 2% moisture contentplotted against frequency at 40

Fig. 6 and Fig. 7 show the dissipation factor and complex capacitance of double-layer OIP with average 4% moisture content, respectively. Similar to Fig. 4, the greater the non-uniformity of moisture distribution is, the bigger tanδ value in the high frequency is. Both specimens has a peak tanδ value in the curve, and the corresponding frequency shifts to higher frequencies with heavier non-uniformity, which may be used as a characteristic quantity to indicate the non-uniformity of moisture distribution when we estimate average moisture content in OIP by FDS. In Fig. 7, the real capacitance also presents remarkable difference with different moisture distribution.

Figure 6. Dissipation factor of double-layer OIP with average 4% moisture content plotted against frequency at 40

Figure 7. Complexcapacitance of double-layer OIP withaverage 4% moisture content plotted against frequency at 40

C. Multilayer Oil-immersed PressboardFig. 8 and Fig. 9 show the dissipation factor and complex

capacitance of three-layer OIP with average 1% moisture content, respectively. As the average moisture content is not too big, no obvious difference is observed in tanδ-f curveexcept in the low frequency range. Although the non-uniformity of moisture distribution in specimen dry+dry+3% is greater, the interfacial polarization is not as strong as specimen dry+1%+3%, which has two different moisture distribution interfaces, so the tanδ value is smaller in 10-3Hz~0.1Hz. In high frequency range, the interfacial polarization couldn’t keep up with the change of electric field, and the tanδ value nearly keeps the same.

Figure 8. Dissipation factor of three-layer OIP with average 1% moisture content plotted against frequency at 40

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Figure 9. Complexcapacitance of three-layer OIP withaverage 1% moisture content plotted against frequency at 40

Fig. 10 and Fig. 11 show the dissipation factor and complex capacitance of three-layer OIP with average 2% moisture content, respectively. The tendency of tanδ-f curve is similar to double layer OIP. A visible tanδ peak is observed and the difference of tanδ in different frequency range presents the moisture distribution in pressboard. In Fig. 11, no distinct difference is reflected in the real capacitance curve. The average moisture content of pressboard presents close real capacitance and discrepant imaginary one.

Figure 10. Dissipation factor of three-layer OIP with average 2% moisture content plotted against frequency at 40

Figure 11. Complexcapacitance of three-layer OIP withaverage 2% moisture content plotted against frequency at 40

IV. CONCLUTION

Moisture is an important factor influencing the dielectric response when performed measurements on the oil-paper insulation systems. Understanding its effects on the dielectric properties is helpful for maintenance staff to make decisions.Usually, moisture estimation by FDS gives an equivalent value to indicate the whole insulation condition. This paper attempts to identify the moisture distribution in multiple-layer oil-immersed pressboard and some qualitative can be concluded as follows:

Different distribution has different performance in FDS curves. The curves are mainly determined by the highest moisture part, and the distribution difference displays in the low and high frequency range.

The greater the non-uniformity of moisture distribution is, the bigger tanδ value in the high frequency is, and the tanδ-f curve begins to appear a maximum peak, the real and imaginary capacitance begins to cross with moisture increasing.

REFERENCES

[1] A.M.Emsley, X.Xiao, R.J.Heywood and M. Ali,“Degradation of cellulosic insulation in power transformers. Part 2: Formation of furan products in insulating oil”, IEE Proceedings Science, Measurement and Technology, Vol. 147, No. 3, pp. 110-114, 2000.

[2] L.E.Lundgaard, W.Hansen, D.Linhjell and T.J. Painter,“Aging of oil-impregnated paper in power transformers”, IEEE Transactions on Power Delivery, Vol. 19, No. 1, pp. 230-239, 2004.

[3] T.A.Prevost, T.V.Oommen,“Cellulose insulation in oil-filled power transformers: Part I-History and development”, IEEE Electrical Insulation Magazine, Vol. 22, No. 1, pp. 28-35, 2006.

[4] T.V.Oommen. “Moisture equilibrium charts for transformer insulation drying practice”, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-103, No. 10, pp. 3062-3067, 1984.

[5] R.J.Heywood, A.M.Emsley, M.Ali,“Degradation of cellulosic insulation in power transformers. I. Factors affecting the measurement of theaverage viscometric degree of polymerisation of new and aged electrical papers”, IEE Proceedings Science, Measurement and Technology, Vol. 147, No. 2, pp. 86-90, 2000.

[6] R. Neimanis, T.K. Saha and R. Eriksson, ”Determination of moisture content in mass impregnated cable insulation using low frequency dielectric spectroscopy”, IEEE Power Engineering Society Summer Meeting, Vol. 1, pp. 463-468, 2000.

[7] Y. Du, M. Zahn, B.C. Lesieutre, A.V. Mamishev and S.R. Lindgren, “Moisture Equilibrium in Transformer paper-oil systems”, IEEE Electrical Insulation Magazine, Vol. 15, No. 1, pp. 11-20, 1999.

[8] S.Gubanski, P.Boss, G.Csepes,V.Houhanessian,J.Filippini,P.Guuinic,U.Gäfvert,V.Karius, J.Lapworth and G.Urbani, “Dielectric response methods for diagnostics of power transformers”,IEEE Electrical Insulation Magazine, Vol. 19, No. 3, pp. 12-18, 2003.

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