Moisture Measurement of Transformer Oil Using Thin Film Capacitive Sensor

Tarikul Islam*, Md Firoz A. Khan1, Shakeb A. Khan* and Mathew Shaji Thomas2 *Professor, 1PhD research scholar, Electrical Engineering Department, Jamia Millia Islamia (Central University), New Delhi-

110025, India. 2B. Tech. Student, Department of Chemical Engineering, IIT Madras, Chennai, Tamil Nadu-600036, India

tislam@jmi.ac.in, firozamu@rediffmail.com, skhan3@jmi.ac.in and mat.st19@gmail.com

AbstractPresent work deals with the development of an online moisture measurement scheme in a transformer using thin film parallel electrode capacitive sensor. The fabrication of the sensor is done with a capacitive porous alumina (-Al2O3) dip coated by sol gel technique having two parallel gold electrodes on a substrate of alumina. The execution, analysis, results and applications of the scheme is discussed. The electrical characteristics of the thin film capacitive sensor with change in temperature and moisture in transformer oil is studied. The proposed scheme has the potential to determine the online moisture in transformer at various temperatures.

Keywordstransformer oil; power transformer; thin film capacitive sensor.

I. INTRODUCTION

Transformer is one of the most significant and highly priced equipment in a power system. It is very important to continuously monitor the efficient and reliable working of the transformer. The majority of power transformers use mineral oil because of its excellent dielectric, oxidation stability and cooling properties. The reduction of dielectric performance and the acceleration in aging of the mineral oil is mainly due to presence of moisture in the transformer oil. In general for each doubling of moisture concentration, the mechanical life of the insulation is reduced to half [1]. Moisture can ingress in a transformer from atmosphere or from internal sources, which is due to chemical reactions that are always active in an energized transformer. In a transformer, the paper insulation may contain much more moisture than oil. A typical 150MVA, 400KV transformer with about seven tons of paper for insulation can contain as much as 223 Kg of water. The oil volume in a typical power transformer is about 80,000 liters. Assuming a 20 ppm moisture concentration in oil, the total mass of moisture is about 2 kg, much less than in the paper [2]. Hydrogen, a natural gas is liberated when the transformer is under operation, and the ever-present oxygen in the oil, causes the natural formation of moisture. Paper insulation is the most vulnerable material to moisture in a transformer. The paper insulation has natural pores due to which it has affinity to moisture absorption as a result from the time the transformer is manufactured one has to deal with moisture [2, 4]. Added to this, paper and oil degradation produces water. Moisture accelerates the deterioration of both the insulating oil and the paper insulation, liberating more water in the process. This is a never ending cycle and once the paper insulation has been degraded, it can never (unlike the oil) be returned to its

original condition [3, 4]. At higher temperature, moisture has a tendency to migrate from oil to paper and vice versa [5]. Also the moisture solubility in the transformer oil rises with rise in temperature [6]. Because of the above mentioned reasons typical life expectancy of a transformer is about 40 years [7]. Hence to predict the future failure of the transformer, online monitoring of the oil temperature and moisture content in the transformer is very essential. Various methods of moisture measurement in transformer oil reported in various literatures including detection of dissipation factor (tan), interfacial tension, acidity and dielectric strength, which give erroneous results for moisture estimation in oil and paper. In comparison, certain mathematical formulae and equilibrium charts provide comparatively accurate results. The different mathematical models are used for the calculation of the moisture content [8]. Apart to these, there is certain equilibrium charts from which at a certain temperature, paper moisture can be estimated when oil moisture values are available. Most of the works reported are based on offline and do not give correct value of moisture level [6, 8]. In this work a simple technique has been proposed to measure real-time moisture content of transformer oil using thin film metal oxide based capacitive sensor. The sensor has been fabricated in the lab by employing a low cost sol-gel method. A simple experimental setup has been developed in laboratory. The experimental methods and experimental results are reported.

II. SENSOR FABRICATION

The structure of thin film parallel electrode capacitive sensor fabricated by sol-gel technique is shown in Fig. 1. The fabrication processes began with, properly cleaning of an alumina substrate of dimension 19mm X 19mm and then screen printing of gold electrode of dimension 16mm X 16mm using metal paste. The electrode was heated at 9000C for 1 hour. A thin film of nearly 5m thickness has been deposited on the gold electrode by dip coating method. A second macro porous electrode having dimension of 14 mm X 13 mm has been formed on the sensing film. The film was sintered in a programmable furnace with accuracy of 20C. The sample was heated at 4500C for 1 hour and 9000C for another 1 hour. The resulting film has a distribution of micro pores (

Fig. 1. Structure of thin film parallel electrode capacitive sensor.

III. EXPERIMENTAL SETUP The block diagram and the photograph of the experimental set up are shown in the Fig. 2 and Fig. 3 respectively. It consists of a sealed oil container having 250ml of transformer oil, placed on hot plate. The moisture content of the oil was approximately 20 ppm at room temperature (250C). Transformer insulation paper strips impregnated with moisture was placed in the oil container in order to simulate the transformer environment. A thermometer was placed in the container to monitor the change in temperature of the transformer oil. The temperature of hot plate was controlled precisely by regulator. The nitrogen gas supplied to the oil container acts as a carrier gas which carries the moisture from the oil container. The gas containing the moisture was passed through long copper tube to the sensor chamber. The sticky oil material liberated from the oil gets trapped on the walls of the long copper tube. The sensor chamber is cylindrical having diameter of 5cm and height of 10 cm. Valves are provided to control the flow of the nitrogen gas and so the moisture of the oil. To measure the moisture in the oil container valves-1, 3 and 4 are opened and valve-2 is closed while to dry the sensor valves-2 and 4 are opened and valves 1 and 3 are closed. The fabricated thin film capacitive sensor was placed in the sensor chamber to sense the change in moisture by change of its capacitance. The leads from the electrodes of the sensor were connected to the precision impedance analyzer (Agilent 4294A).

Fig. 2. Block diagram of the experimental set up

Alumina Substrate

(19mm X 19mm)

Gold Electrode (16mm X 16mm)

Al2O3 Coating (5m thick)

Gold Electrode (14mm X 13mm)

.

Fig. 3. Photograph of the experimenta

First the fabricated sensor response has with commercial dew point meter by varcontent of dry nitrogen gas. Nitrogen moisture content has been obtained by mfrom bubbler with dry nitrogen gas. A calbeen developed between capacitance veppm. To measure the moisture content of transcontainer with paper strips has been put temperature of hot plate was controlregulator. When oil is heated, the moisturcomes out initially from paper to oil then the free space of the container. The moistuand the free space varied according to tempThe water vapor in the free space of thecarried away with dry N2 gas to the senexposed to the sensor. The electrical chasensor were recorded with the help of impe

IV. RESULTS In the beginning the sensor was exposed gas (valve 1 and 3 closed) and the mmonitored by dew point meter. The capwith the variation of moisture is showncapacitance value increases with increasechange in dissipation factor with moisturein Fig. 6. The dissipation factor increasemoisture. Fig. 7 shows the variation of increase in moisture. The phase shift increin moisture concentration. Based on the with pure moist N2 gas, the response calibrated in terms of moisture in ppm. Now dry nitrogen gas is mixed with the mfrom the oil kept in the container at diffethe capacitance variation of the senstemperature is shown in Fig. 8. The between variations of moisture with rise shown in Fig. 9. The moisture increasestemperature. The variation of dissipationangle shift with rise in temperature is shoFig. 11 respectively. Initially the electricalsensor increase gradually then above

Shaw Sensor

Thermometer

Oil Container Transformer Oil

Heater

Paper Strips Copper Tubes

al set up.

been determined rying the moisture

gas of different mixing water vapor

libration curve has ersus moisture in

former oil, the oil on hot plate. The led precisely by

re content in paper as vapor forms at

ure level in the oil perature of the oil.

e container is then nsor chamber and aracteristics of the edance analyzer.

to moist nitrogen moisture level was pacitance variation n in Fig. 5. The

e in moisture. The e content is shown s with increase in

f phase shift with eases with increase experimental data of the sensor is

moisture liberated ferent temperature, sor with rise in

calibrated curve in temperature is

s with increase in n factor and phase

wn in Fig. 10 and l parameters of the 90C parameters

increase abruptly. This abrupparameters of the sensor isconcentration of water vapodielectric constant as well as chigh temperature, the water cinsulation is liberated vigoroucontainer leading to abrupt chaas shown in Fig. 4.

Fig. 4. Evolution of water bub

Fig. 5. Capacitance change of the thinmoisture.

0

500

1000

1500

2000

2500

0 100 200 300

Cap

acita

nce

(pf)

Mo

Sensor Chamber

Exhaust

Impedance Analyzer

pt increase in the electrical s due to large increase in or causing large change in conductance of the sensor. At contents trapped in the paper usly in the free space of the ange in concentration of vapor

bbles between 900C to 1000C.

n film sensor with variation of

400 500 600 700 800 900

oisture (ppm)

Evolution of water bubbles

Fig. 6. Dissipation value of the thin film sensor for different moisture concentration.

Fig. 7. Change in phase angle with the variation of moisture.

Fig. 8. Capacitance change of the thin film sensor with variation of temperature.

Fig. 9. Calibration curve between Moisture (ppm) and temperature (0C)

Fig. 10. Dissipation value of the thin film sensor for different temperature.

Fig. 11. Change in phase angle with the variation of temperature.

0200400600800

100012001400160018002000

0 100 200 300 400 500 600 700 800 900

Dis

sipa

tion

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or (m

u)

Moisture (ppm)

-100

-90

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Phas

e an

gle

()

Moisture (ppm)

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30 40 50 60 70 80 90 100

Cap

acita

nce

(pf)

Temperature (0C)

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Moi

stur

e (p

pm)

Temperature (0C)

0

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200

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400

500

600

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30 40 50 60 70 80 90 100

Dis

sipa

tion

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u)

Temperature (0C)

-90

-80

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e an

gle

()

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V. CONCLUSION Present work proposes a capacitive porous alumina based moisture sensor fabricated by simple sol-gel technique for online moisture measurement scheme in a transformer. The sensor based on thin film parallel-electrode is very fast, highly reproducible, low hysteresis and mass producible. It can withstand several harsh environmental conditions. The device has been fabricated by a very simple low cost technique. The device has been successfully employed to determine the moisture content in oil in a simulated environment. The proposed scheme provides an alternative method as compared to the conventional offline techniques which has certain well known limitations. The experimental set up is also simple.

REFERENCES [1] F. M. Clark, Factors Affecting the Mechanical Deterioration of

Cellulose Insulation, Transactions of Electrical Engineering, Vol. 61, pp. 742-749, October 1942.

[2] G. Beer et al., Experimental Data on the Drying-Out of Insulation Samples and Test-Coil for Transformer, CIGRE Paper No. 135, 1966.

[3] J. Fabre and A. Pichon, Deteriorating Processes and Products of Paper in Oil. Application to Transformers, 1960 International Conference on Large High Voltage Electric System (CIGRE), Paris, France, Paper 137, 1960.

[4] H. P.Moser, Transformerboard, Special print of Scientia Electrica, translated by EHV-Weidmann Lim., St., Johnsbury, Vermont, USA, Section C, 1979.

[5] A. J. Morin, M.Zahn, and J.R. Melcher, Fluid Electrification Measurements of Transformer Pressboard/Oil Insulation in a Couette Charger, IEEE Transactions on Electricallnsulation, Vol. 26, No. 5, pp. 870-901, October 1991.

[6] Y. DUI, A. V. Mamishev, B. C. Lesieutre, M. Zahn and S. H. Kang, Moisture Solubility for Differently Conditioned Transformer Oil. IEEE Transactions on Dielectrics and Electrical Insulation Vol. 8 No. 5, pp. 805-111, October 2001

[7] T. 0. Rouse, Mineral Insulating Oil in Transformers, Electrical Insulation Magazine, Vol. 14, No. 3, pp. 6-16, May/June 1998.

[8] P. Pahlavanpour, M. Martins, and Eklund, Study of Moisture Equlibrium In Oil-Paper System With Teperature Variation, in Proc.7th Int. conf. on Properties and Application of Dielectric Materials, Nagoya, pp. 1124-1129, June 2003.

[9] T. Islam, L. Kumar, S A Khan, A novel sol-gel thin film porous alumina based capacitive sensor for measuring trace moisture in the range of 2.5 to 25 ppm, Sensors and actuators B, 173, pp. 377 384, 2012.

[10] D.D. Saha, K. K. Mistry, R.Giri, A. Guha, K. Sengupta, Dependence of moisture absorption property on solgel process of transparent nanostructured -Al2O3 ceramics, Sens. and Actuators B 109, pp. 363366, 2005.

[11] R. Fenner, E. Zdankiewicz, Micromachinised water vapor sensor, a review of sensing technologies, J. IEEE Sensors 1 (4), pp. 309-317, 2001.

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