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Board, India have been tested, and analyzed using the multi regression methodology. ... technique has been used to determine the influence of transformer oil ...
Multi-Regression Based Health Assessment of Power Transformers in Consideration of Ambient Conditions Chilaka Ranga and Ashwani Kumar Chandel Department of Electrical Engineering, NIT Hamirpur, India-177005 ABSTRACT The effect of ambient conditions on oil impregnated paper insulation is detrimental for an extensive performance of transformers. In the present paper, the effect of load, moisture and ambient temperature on the transformer dielectric properties namely, dielectric strength, moisture content and degree of polymerization of paper insulation has been investigated. A technique called as multi-regression analysis has been utilized for an accurate remnant life assessment of transformers. The technique incorporates the various properties of transformers insulation along with the ambient working conditions. Oil samples collected from two different rating transformers which are operated by Himachal Pradesh State Electricity Board, India have been tested, and analyzed using the multi regression methodology. It has been observed from the analysis that the effect of moisture content on the transformers health condition is much higher than the effect of load and ambient conditions. The present approach has been found to be very accurate and more practical solution than the conventional life assessment methods. It can facilitate the operating schedule for transformers to improve their service life time.

INTRODUCTION Power transformers are the most important components of electricity transmission and distribution networks. The interruptions in power system generate different stresses including thermal, electrical, chemical and mechanical stresses inside the transformers [1]. These stresses accelerate the deterioration of the transformer insulation, and finally lead to failure of the device. Electrical stress and thermal stress are the two dominant stresses which deteriorate the transformer insulation rapidly. Electrical stress normally developed by overloads, short circuits, lightning’s etc [2]. Dielectric heating, higher ambient temperatures and unpredictable loads are the main origins for higher thermal stress [3]. These two stresses inside the transformers slow down their age, and reduce their expected life. As a result, a huge revenue loss has been faced by the utility managers and the consumers [4]. Therefore, continuous health assessment of transformers is very essential for their successful operation. In general, the health condition of power transformers is predicted from their insulating paper and oil health condition [5]. Several condition monitoring methods including breakdown voltage (BDV), interfacial tension (IFT), flash point (FP), dissolved gas analysis (DGA) etc. have been used to determine the health condition of the transformer oil [6]. Similarly, furan analysis (FA) and degree of polymerization (DP) have been used to determine the health condition of the transformer insulating paper [7]. Each of these methods determines the present health condition as well as the remnant life of the transformers. However, none of these methods does consider the effect of load, moisture and ambient temperature on the health of the transformers [6]. Hence the remnant life assessment of transformers using the above mentioned methods is not accurate. Also the ambient conditions are neither consistent nor predictable [8, 9]. Such situations necessitated further investigations which incorporate the information obtained from the various test data and the ambient conditions (load, moisture and ambient temperature) for an accurate health assessment of transformers. In the present article, the combined effect of load, moisture and ambient temperature on various dielectric properties of transformer insulation has been investigated. Further, a multi regression technique has been used to determine the influence of transformer oil temperature as well as moisture content. Two different transformers owned by Himachal Pradesh State Electricity Board (HPSEB) located at Anu substation India were tested to prove the robustness and efficacy of the present study. The present approach has been found to be an accurate and more practical solution for transformer remaining health evaluation. © 2016 Doble Engineering Company – 14th IndiaDoble Power Forum All Rights Reserved

MULTI REGRESSION ANALYSIS A statistical regression correlates two or more variables in such a manner that one variable can be predicted or explained by using information of the other variables [10]. In simple regression analysis, a single response measurement Y is related to a single predictor X for each observation as given in equation (1). The critical assumption of the model is that the conditional mean function is linear [11].

Y  a  bX

(1)

where, a is the intercept and b is the slope of X and Y . Similarly, standard multiple regression is used to evaluate the relationships between a set of independent variables and a dependent variable. In multiple regressions, all independent variables are entered into the regression equation at the same time [12]. It is computed by (2).

Y  a  b1 X1  · · ·  bn X n

(2) th

where, X 1 to X n are dependent variables and bn is the slope coefficient of n input. Multiple R and R 2 measure the strength of the relationship between the set of independent variables and the dependent variable. If the value of R 2 is less than 0.95 then it may imply an unsatisfactory fitness [10]. As its value 2 near or equal to unity, it shows the variables in the relation are having a strong co-relation. R value can be further improved by considering the number of higher order coefficients [12].

MAINTENANCE DATA OF TRANSFORMERS As everyone is aware the atmospheric conditions gradually change as per three different seasons such as winter, summer and rainy seasons. In general, in upper north region of India, December to March is to be well thought-out as winter, whereas, April to July treated as the summer and August to November as the rainy season. Monthly average ambient temperature is always alternating in nature from season to season. It results an accelerating deterioration of the transformer insulation. Therefore, the effect of annual load, moisture and ambient temperatures on transformer insulating properties has been investigated in the present study. The details of the two test case transformers are given in Table 1.

Table 1 Details of Test Case Transformers Details

Transformer 1

Transformer 2

Year of manufacture

1997

1992

Manufacture company

ECE Industries Ltd.

ECE Industries Ltd.

Installed location

Anu Sub-Station HP-India

Anu Sub-Station HP-India

Date of installation

23-08-1997

23-08-1997

MVA rating

25

25

HV/LV rating

132/33 kV

132/33 kV

HV/LV Load rating

43.73/109.34

43.73/109.34

Aging time (days)

01/01/2015 – 31/12/2015 (365 days)

01/01/2015 – 31/12/2015 (365 days)

The oil samples were drawn from these transformers at the end of every month, and then performed various diagnostic tests of the transformers. Annual load, oil temperature and winding temperature data were collected from Anu sub-station, HPSEB-India. Annual ambient temperature data of Anu sub-station © 2016 Doble Engineering Company – 14th IndiaDoble Power Forum All Rights Reserved 2-8

area was collected from an Indian weather report. Annual average winding and oil temperatures of transformers 1 and 2 are shown in Figure 1(a)-(b).

(b)

(a)

Annual average winding and oil temperatures for (a) TF-1 (b) TF-2 Figure 1 It is noticed from Figure 1 that the load and the ambient temperatures are high in summer season. It leads to higher degradation of the insulation in summer. Further, the annual load on these two transformers is shown in Figure 2. Also the average load of the two test transformers is high in summer season. Figure 3 represents the annual ambient temperature in Anu sub-station area.

(b)

(a)

Annual average (a) load and (b) ambient temperatures of test case transformers. Figure 2 As per the above discussed regression methods correlations have been obtained between the load, ambient temperature, winding temperature and oil temperature. These are given from (3) to (6).

Tw, TF -1  -14.606  0.675I L  0.3538Ta ; R2  0.96

(3)

Toil , TF -1  3061.08  71.84I L  0.414I L 2  0.313Ta  0.0035Ta 2 ; R2  0.971 Tw, TF  2  1256.9  50.735I L  11.4454Ta  0.528I L  0.0369Ta  0.236I LTa ; R  0.873 2

2

2

Toil , TF  2  493.45  19.64I L  4.9122Ta  0.203I L  0.01438Ta  0.1076I LTa ; R  0.859 2

2

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2

(4) (5) (6)

DIAGNOSTIC TESTS AND RESULTS The oil samples collected from the two transformers have been examined by BDV, moisture content, degree of polymerization tests. These tests were performed as per IEC, ASTM, IEEE recommendations, with a span of 720-745 hours. It was continued up to 8760 hours (i.e. 1 year). The significances and interpretations of these tests have been discussed in the following sub-sections.

A. BDV Test The maximum voltage that can be applied across the insulation without any electrical breakdown is called as dielectric strength [13]. Any significant reduction in the dielectric strength indicates that the oil is no longer capable of performing its vital function. ASTM D 877, ASTM D 1816, IEC 60156, BS 5730a, BS 148, BS 5874 are the various standards for BDV testing [9]. The test results of two transformers are given in Figure 4.

Breakdown voltage test results of two transformers Figure 4 B. Degree of Polymerization The mechanical properties of the transformer insulating paper can be established by the direct measurement of its tensile strength or degree of polymerization (DP) [14]. It is generally suggested that DP values between 150 and 250 represent the lower limits for end-of-life criteria for the paper insulation. If its value is below 150 then the paper has zero mechanical strength [15]. IEC 60450 is the standard test code of DP test. DP results of the two transformers correspond to every month of 2015 are given in Figure 5.

DP test results of two transformers Figure 5

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C. Moisture Test An increase in the moisture content causes reduction in the insulating properties of the transformer oil [16]. It may result in dielectric breakdown. Many transformers contain cellulose-based insulating paper in the windings. Excessive moisture content in the transformers results breakdown of the paper insulation. Ultimately, it results in the loss of the insulation performance. Figure 6 shows the variation of moisture content in the two test transformers during 2015-2016.

Moisture content present in two transformers Figure 6 Based on the above test results of two transformers, the remnant life of the insulation as per the various properties is determined using simple regression analysis. These are given in Table 2.

Table 2 Remnant life to reach failure level Dielectric property BDV DP

Complete deteriorated value [9] 25 250

Remnant time to reach deteriorated level Transformer 1

Transformer 2

4553 7234

6214 1742

LIFE ESTIMATION USING MULTI-REGRESSION ANALYSIS A multi-regression analysis has been used to find the combined effect of moisture and oil temperature on transformer properties. It gives the expected life based upon particular oil temperature and moisture level. 2 R value fitted to unity to make the strong strength of correlation between the variables. From Table 3, it has been noticed that the effect of load and ambient temperatures within limited ranges is almost constant. These ambient conditions do not have higher impact on the insulating properties. Further, the effect of moisture on BDV and DP is determined. The obtained relations are shown in Table 4.

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Table 3 Remnant life based on ambient conditions, to reach failure level. IL

Tambient 25 30 35 40 45 50 25 30 35 40 45 50 25 30 35 40 45 50 25 30 35 40 45 50

0.5 IL

0.75 IL

IL

1.25 IL

Toil 24 26 28 29 31 32 50 52 53 55 56 58 76 77 79 80 82 84 101 103 105 106 108 109

BDV- time TF-1 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576 4576

TF-2 6217 6217 6217 6217 6217 6217 6217 6217 6217 6217 6217 6217 6216 6216 6216 6216 6216 6216 6216 6216 6216 6216 6216 6216

DP-Time TF-1 7220 7220 7220 7220 7221 7221 7224 7224 7225 7225 7225 7225 7228 7229 7229 7229 7229 7230 7233 7233 7233 7234 7234 7234

TF-2 2066 2054 2041 2028 2016 2003 1858 1845 1833 1820 1807 1795 1649 1637 1624 1612 1599 1587 1441 1428 1416 1403 1391 1378

Table 4 Effect of moisture content on various dielectric properties Transformer 1 2

Output BDV DP BDV DP

Intercept 112.09 1204.64 112.03 3523.32

Moisture -2.33 -18.18 -2.63 -166.75

It has been found from Table 5 that the effect of moisture content is high on dielectric strength of transformer insulation as compared to their other properties. Finally, it has been found from the analysis that the effect of moisture content is very high on all insulating properties of transformer as compared to that of load and ambient temperature. At the end it is suggested from the analysis that the transformers should operate without moisture content to improve its health. It results a huge revenue save for the utilities as well as customers.

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Table 5 Remaining life of transformers based on different moisture levels, to reach failure level Moisture 10 15 20 25 30 35 40

BDV TF-1 88.79 77.14 65.49 53.84 42.19 30.54 18.89

DP TF-2 85.73 72.58 59.43 46.28 33.13 19.98 6.83

TF-1 1022.84 931.94 841.04 750.14 659.24 568.34 477.44

TF-2 946.5 893.6 719 701 682.4 614.6 536.9

CONCLUSION This paper analyzed the effect of load, moisture and ambient temperatures on various properties of solid and liquid insulations. Load data as well as oil and winding temperatures data have been collected from its substation. Test samples of two transformers have been collected every month and conducted various diagnosis tests including BDV, moisture and degree of polymerization. Further, correlation analysis has been done between various dependent and independent variables by using linear regression analysis as well as multi-regression analysis with a string strength of correlation factor. From multi-regression analysis, the combined effect of existing moisture level on its BDV and other properties has been evaluated. Furthermore, the remaining life of insulation at any oil temperature and moisture content has been estimated using MRA. Proposed methodology is very accurate and can be easily implemented even by an inexperienced person to compute the condition of the transformer. Subsequently, appropriate remedial actions to improve the health of the transformer can be initiated.

AKNOWLEDGEMENT Authors would like to thank the authorities of TEQIP–II of NIT Hamirpur India for providing the financial support with grant number NIT/HMR/TEQIP–II/Research & Develpoment –19/2015/2157–63. Authors are also thankful to the Himachal Pradesh State Electricity Board (HPSEB)–India for providing the transformer oil samples, and the authorities of TIFAC–CORE Centre of NIT Hamirpur India for providing the necessary facilities to perform the experiments of the present research.

REFERENCES [1]

[2] [3] [4] [5] [6]

[7] [8]

R. Villarroel, D. F. Garcia, “Moisture diffusion coefficients of transformer pressboard insulation impregnated with natural esters,” IEEE Trans. on Diele. and Electr. Insul., vol. 22, pp. 581-589, 2015. G. K. Frimpong, T.V. Ommen, R. Asano, “A survey of aging characteristics of cellulose insulation in natural ester and mineral oil,” IEEE Electr. Insul. Mag., vol. 27, pp. 36-48, 2011. R. Karthik, T. Raja, T. Sudhakar, “Deterioration of solid insulation for thermal degradation of transformer oil,” Central Euro. Jour. of Engi., vol. 3, pp. 226-232, 2013. C. Neusel, et al., “Thickness dependence of the break down strength: Analysis of the dielectric and mechanical failure,” Jour. of the Eur. Cer. Soci., vol. 35, no. 1, pp. 113-123, 2015. A.T. Mohamed, et al., “Experimental enhancement for dielectric strength of polyethylene insulation materials using cost-fewer nano-particles,” Elect. Pow. and Ener. Sys., vol. 64, pp. 469–475, 2015. H. de Faria, et al., “A review of monitoring methods for predictive maintenance of electric power transformers based on dissolved gas analysis,” Rene. and Sust. Ener. Rev., vol. 46, pp. 201–209, 2015. D. Urquiza, et al., “Statistical study on the reference values of furanic compounds in power transformers,” IEEE Elect. Insul. Mag., vol. 31, no. 4, pp. 15-23, 2015. P. Preetha and M. Joy Thomas, “Life estimation of electro-thermally stressed epoxy nanocomposites,” IEEE Trans. on Dielec. and Elect. Insul., vol. 21, no. 3, pp. 1154-1160, 2014. © 2016 Doble Engineering Company – 14th IndiaDoble Power Forum All Rights Reserved 7-8

[9] [10] [11] [12] [13]

[14]

[15]

[16]

W.G. Li, et al., “Comparison between the DC and AC breakdown characteristics of dielectric sheets in liquid nitrogen,” IEEE Trans. on Appl. Sup., vol. 24, no. 6, 2014. M. Hamzeh, B. Vahidi, et al., “Reliability evaluation of distribution transformers with high penetration of distributed generation,” Elect. Pow. and Energy Sys., vol. 73, pp.163–169, 2015. S. Tang, et al., “Reliability modelling of power transformers with maintenance outage,” Syst. Sci. & Cont. Eng., vol. 2, pp. 316–324, 2014. S. Tang, et al., “Reliability modelling of power transformers with maintenance outage,” Sys. Sci. & Cont. Engg., 2014. M. L. Coulibaly, et al., “Aging behaviour of cellulosic materials in presence of mineral oil and ester liquids under various conditions,” IEEE Trans. on Dielec. and Elect. Insul., vol. 20, no. 6, pp. 19711976, 2013. T. Leibfried, et al., “Post-mortem investigation of power transformers—Profile of degree of polymerization and correlation with furan concentration in the oil,” IEEE Trans. on Pow. Del., vol. 28, no. 2, pp. 886-893, April 2013. K. Franciszek, et al., “HPLC method for determination of biologically active epoxy-transformers of Treosulfan in human plasma: Pharmacokinetic application,” Jour. of Pharm. and Biom. Anal., vol. 62, pp. 105-113, 2012. D. F. Garcia, et al., “Determination of moisture diffusion coefficient for oil-impregnated Kraft-paper insulation,” Elect. Pow. and Energy Sys., vol. 53, pp. 279–286, 2013.

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