energies Article
A New Method for Evaluating Moisture Content and Aging Degree of Transformer OilPaper Insulation Based on Frequency Domain Spectroscopy Guoqiang Xia 1 , Guangning Wu 1, *, Bo Gao 1 , Haojie Yin 1 and Feibao Yang 2 1 2
*
School of Electrical Engineering, Southwest Jiaotong University, Chengdu 610031, China;
[email protected] (G.X.);
[email protected] (B.G.);
[email protected] (H.Y.) Energy China HEPDI, Changsha 410007, China;
[email protected] Correspondence:
[email protected]; Tel.: +8615198026601
Received: 3 April 2017; Accepted: 8 August 2017; Published: 12 August 2017
Abstract: The condition of oilpaper insulation is closely related to the life expectancy of a transformer. The accurate results of oilpaper have not been obtained due to the impact of influencing factors. Therefore, in order to improve the evaluation accuracy of oilpaper insulation, in this paper, oilpaper samples which were prepared with different aging and moisture content were analyzed by frequency domain spectroscopy (FDS). Results show that when the moisture content is less than 2%, the range of 101 ~103 Hz is mainly affected by moisture and aging has little effect. However, with the increase of moisture content, the effect of aging degree on this band became increasingly prominent. Sm , which represents the integral value from 10−1 to 10−3 Hz, and SDP , which represents the integral value from 101 to 103 Hz, were extracted as characteristic parameters of moisture content and aging degree respectively. Compensation factors γ which represents the influence ratio of moisture on SDP and φ which represents the influence ratio of aging on Sm were introduced to compensate for the influence of moisture content and aging degree on characteristics respectively. Then, a new method was proposed to evaluate the condition of oilpaper based on compensation factors. Through this method, the influence in characteristics can be eliminated by the obtained actual compensation factors, thus distinguishing the internal influence between moisture content and aging degree on FDS. Finally, this method was verified by field test. Keywords: aging degree; characteristic parameters; compensation factor; frequency domain spectroscopy; moisture content; oilpaper insulation
1. Introduction Power transformer is one of the most expensive and important pieces of equipment in power grid [1,2]. The safety and stability of such transformers are both important and necessary in operating power systems [3]. Oilpaper insulation is an important form of power transformer insulation system; the remaining life of the transformer largely depends on the insulation status of oilpaper which is affected by thermal, mechanical, partial discharge and other aging factors in the longrunning process [4–6]. Therefore, using an accurate assessment tool to evaluate the condition of oilpaper insulation has become a popular research topic. Aging degree and moisture content (MC) are two important aspects of oilpaper insulation state assessment. The increase of moisture content will raise dielectric loss and reduce breakdown voltage of oilpaper insulation which will damage the insulation structure and affect normal operation of equipment [7,8]. The aging degree of oilpaper insulation will directly determine the service life of the transformer. When the aging degree reaches a certain extent, with the degradation of insulating paper, the breakdown voltage and mechanical properties of the insulation system will significantly reduce, Energies 2017, 10, 1195; doi:10.3390/en10081195
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thus the transformer main insulation will be unable to meet the requirements, and the insulation system will be completely ineffective. Various methods have been developed to help to evaluate the condition of the insulating paper, including the breakdown voltage, the tensile strength, the degree of polymerization and Karl Fischer coulometric titration [9,10]. More recently, Frequency Domain Spectroscopy (FDS) [11–13], Polarization and Depolarization Current (PDC) [14–16] and Recovery Voltage Measurement (RVM) [17], which are based on dielectric response, have become a hotspot for scholars to study because of easy operation and no need for sampling. Compared with PDC and RVM, FDS has the advantages of rich insulation information and strong antiinterference ability, so it has great potential in oilpaper insulation condition evaluation [18]. A lot of valuable research about oilpaper insulation condition assessment and the influence of moisture and aging on FDS measurement have been done. Blennow [19] studied measurement of moisture content in field transformer based on field experience, and presented that precautions should be taken with the results to minimize effects caused by equipment remaining connected to the transformer as well as by external conditions, while the internal relationship between aging and moisture had not be explored. Belén García and Juan Carlos Burgos’ studies [20–22] focused on moisture diffusion in oilpaper insulation, and explored the influence of temperature, thickness of cardboard, aging and other factors on moisture diffusion coefficient between oil and paper. The mechanism of factors affecting water diffusion was elaborately demonstrated, which revealed an internal relation between moisture and aging degree of paper. Studies of Jadav et al. [23] and Liao et al. [23,24] explored the influence of aging degree and moisture content of oilpaper insulation on FDS. The results showed that aging mainly affects the lowfrequency range of tanδ curves, while moisture has an effect in the whole measurement frequency domain, and the effect of moisture on tanδ curves is more prominent than that of cardboard aging. As above, although a lot research has been done related to condition assessment, FDS or internal relationship between moisture and aging, there still remains challenging problems that the assessment results of aging condition and moisture content are mostly combined rather than separated, which causes the inaccuracy of condition assessment results. Therefore, exploring effective ways to evaluate the condition of oilpaper insulation and separating the influences of aging degree and moisture content on dielectric response measurements are still challenging problems. In this study, different with the previous studies, aging degree and moisture content compensation factors are introduced to eliminate the influences on different frequency bands between aging and moisture. Therefore, problems in previous literature can be solved by compensating one factor when measuring another, and compensation factor algorithm is proposed to achieve accurate assessment on the field test based on compensation factors and characteristic parameters. In this paper, an accelerated thermal aging experiment was carried out under laboratory condition and five sets of oilpaper insulation samples were prepared with different aging degree and moisture content. Firstly, the effect of moisture content and aging degree of insulation paper on FDS curves were studied, and characteristic parameters were put forward to represent aging degree and moisture content. Then, by analyzing the combined effects of moisture and aging on FDS, this paper introduced the moisture compensation factor (MCF) and aging compensation factor (ACF), and proposed a new method that could eliminate the mutual influences between moisture content and aging degree to evaluate the condition of oilpaper insulation. 2. FDS Test Principle and Test Device 2.1. Theoretical Basis Under sinusoidal AC electric field, displacement polarization and relaxation polarization are generated in the dielectric, which brings electric conduction loss and polarization loss. Thus, the dielectric can be equivalent to complex capacitance [25]. When AC voltage U (ω) with an angular frequency of 2πf is applied to both ends of the dielectric, the throughcurrent of the dielectric is: I (ω ) = jω [C 0 (ω ) − jC 00 (ω )]U (ω )
(1)
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In Equation (1), C 0 (ω ) and C real andω )] imaginary part of complex capacitance I(00ω()ω=) is j ωthe [C '( ω ) part − jC ''( U (ω ) (1) respectively, by performing Fourier transform on Equation (1), there is: In Equation (1), C '(ω ) and C ''(ω ) is the real part and imaginary part of complex capacitance respectively, by performing Fourier on Equation (1), there is: σ0 transform + χ00 (ω ))]U (ω ) = jωC0 (ε0 (ω ) − jε00 (ω )) (2) I (ω ) = jωC0 [ε ∞ + χ0 (ω ) − j( σ 0ε 0 ω I(ω ) = j ωC 0[ε ∞ + χ '(ω ) − j(
εω
+ χ ''(ω ))]U (ω ) =j ωC 0(ε '(ω ) − j ε ''(ω ))
(2)
0 In Equation (2), C0 is geometric capacitance; χ0 (ω ) and χ00 (ω ) are real and imaginary part of the χ '( ω ) and χconstant ''(ω ) are complex In polarization respectively; ε0 capacitance; is the vacuum dielectric (εreal × 10−12 part F/m); Equation (2), C0 is geometric and imaginary of ε∞ is 0 = 8.854 0 −12 F/m); is the dielectric constant (ε0 = 8.854 is real the frequency complex polarization optical dielectricrespectively; constant; σε00 is the vacuum DC conductivity of material; ε (ω×)10 and ε00 (ωε)∞are ε '(ωloss ) and ''(ω ) are optical frequency dielectric constant; σ0 is theThe DCfrequency conductivity of material; and imaginary part of complex permittivity. domain dielectric is εdefined as:
real and imaginary part of complex permittivity. The frequency domain dielectric loss is defined as:
tan δ(ω ) = tan δ(ω ) =
σ0 00 ε00 (ω ) C 00 (ω ) σε 0 ω + χ (ω ) 0 = = ω) 0 (ω ) 0 (ω ) 0 (ω ε ∞++χ χ''( ) ε ε''( ω) C C ω ) ε 0ω ''(
ε '(ω )
=
C '(ω )
=
(3) (3)
ε ∞ + χ '(ω )
In Equation (3), σ0 /ε0 ω presents the loss of free charge movement in the material, χ00 (ω ) is the loss caused by the charge transfer polarization. In Equation (3), σ0/ε 0ω presents the loss of free charge movement in the material, χ ''(ω ) is the loss caused by the charge transfer polarization.
2.2. FDS Test Device 2.2. FDS Test Device
FDS test device is a threeelectrode test system which was made in laboratory, the schematic test device is a threeelectrode test system which waswere madeplaced in laboratory, the the schematic diagram FDS is shown in Figure 1. When measuring, samples between measuring diagram is shown in Figure 1. When measuring, samples were placed between the measuring electrodes and pressed with the spring. Before each measurement, the entire electrode was placed in electrodes and pressed with the spring. Before each measurement, the entire electrode was placed insulating oil which is dried and degassed. Then, in order to prevent the difference between internal in insulating oil which is dried and degassed. Then, in order to prevent the difference between and external temperature during each FDS measurement, the whole threeelectrode device was put internal and external temperature during each FDS measurement, the whole threeelectrode device ◦ into the constant box for 6 h.box Analyzer measuring FDS is IDAX300, test frequency was30 putCinto the 30 temperature °C constant temperature for 6 h.ofAnalyzer of measuring FDS is IDAX300, −3 ~103 Hz, output −3 3 range is 10 voltage peak is 200 V. test frequency range is 10 ~10 Hz, output voltage peak is 200 V.
Figure 1. The principle diagram of FDS test device.
Figure 1. The principle diagram of FDS test device.
2.3. Preparation of OilPaper Insulation Samples In this paper, 1 mm thick cardboard and 25# naphthenic mineral insulating oil were selected as experimental material. Five sets of oilpaper samples were prepared with different aging degree and moisture content under laboratory conditions. In order to control the initial state of the samples, the
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pretreatment steps are as follows: firstly, insulation papers were dried at 90 ◦ C/50 Pa vacuum box for 48 h, and insulating oil which had been degassed was heated up to 50 ◦ C; then, the transformer oil and insulation pressboard were impregnated for two days in 40 ◦ C/50 Pa vacuum box. The pretreated insulating oil and paper which were set to 20:1 ratio and were placed into the iodine bottle with some copper. The ratio of oilpaper (20:1) was utilized to simulate the ratio in the actual transformer. The bottle in the open state was put into the test chamber with temperature control accuracy of ±0.5 ◦ C; the aging temperature was set to 120 ◦ C. Several sets of oilpaper insulation samples aged 0 days, 14 days, 35 days, 55 days and 80 days were obtained. Samples with different aging degree were placed in a vacuum drying oven (110 ◦ C/50 Pa) for 24 h, so as to obtain dry samples. The average moisture content of dried samples measured by MSC Karl Fischer was 0.21%. It is worth noting that this content is lower than that of the actual new transformer [26], so the samples can be considered as dried samples. Then, the DP (degree of polymerization) of different aging time samples were tested; the measured results were shown in Table 1. Table 1. Average DP of different aging days. Aging Days
0
14
35
55
80
Average DP
1250
932
740
504
402
The moisture content of oilpaper insulation samples in the experiment were controlled at 0% to 4%. Dried oilpaper insulation samples were placed in a wet box with natural moisture absorption; moisture content was dominated by the variation of sample’s weight. Finally, at each aging time, oilpaper insulation samples were prepared with moisture contents of 0.21%, 0.98%, 1.98%, 2.97%, 3.90%. The sets of obtained samples are shown in Table 2; sample groups are represented by A0 , B0 , C0 , etc. Table 2. Sample sets with different aging days and moisture content. MC/%
0.21
0.98
1.98
2.97
3.90
1250
A0
A1
A2
A3
A4
932
B0
B1
B2
B3
B4
740
C0
C1
C2
C3
C4
504
D0
D1
D2
D3
D4
402
E0
E1
E2
E3
E4
DP
3. Experimental Results and Analysis 3.1. The Influences of Aging and Moisture on tanδ Curve Figure 2a–e shows the effect of aging on curves varied with moisture content. When moisture content is low, aging degree mainly affects the FDS at a lowfrequency range (10−1 ~10−3 Hz) and moisture affects all frequency range; this is consistent with the experimental results of the literature [27]. The main reason is that in the lowfrequency range, interface polarization of oilpaper insulation plays a leading role, but with frequency enhancing, the rate of polarization cannot keep up with the rate of the alternating electric field. However, when the frequency is higher than 10−1 Hz, steering polarization plays a leading role. (In this paper, lowfrequency range represents 10−3 ~10−1 Hz, highfrequency range represents 101 ~103 Hz).
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1
10
0
10
DP=1250 DP=932 DP=740 DP=504 DP=402
0
10
1
10
1
10
tanδ
tanδ
1
10
DP=1250 DP=932 DP=740 DP=504 DP=402
2
10
2
10
3
10
3
10
3
10
2
1
10
0
10
1
10
2
10
10
3
2
10
3
10
1
10
0
10
2
10
3
10
10
f/Hz
f/Hz
(a) MC 0.21%
(b) MC 0.98%
1
10
2
10
DP=1250 DP=932 DP=740 DP=504 DP=402
0
DP=1250 DP=932 DP=740 DP=504 DP=402
1
10
0
10 tanδ
10
tanδ
1
10
1
10
1
10 2
10
2
10
3
10
3
3
10
2
10
1
10
0
1
10
10
2
10
10
3
10
3
2
10
1
10
0
10
10
f/Hz
2
10
3
10
f/Hz
(c) MC 1.98%
(d) MC 2.97% 3
10
DP=1250 DP=932 DP=740 DP=504 DP=402
2
10
1
10 tanδ
1
10
0
10
1
10
2
10
3
10
3
10
2
10
1
10
0
10
1
10
2
10
3
10
f/Hz
(e) MC 3.90% Figure 2. The tanδ curve of samples under different aging degree in set A~E. Figure 2. The tanδ curve of samples under different aging degree in set A~E.
While, when moisture content reaches a highlevel (shown in Figure 2c–e), there are some laws when moisture content reaches highlevel in (shown in Figure[23,24] 2c–e), that therethe areaging someonly laws whichWhile, are ignored in previous studies. It is aconsidered the literature which the are lowfrequency ignored in previous It the is considered the literature [23,24] the aging only affects range,studies. but with increase ofinmoisture content, the that influence of aging affects the lowfrequency range, but with the increase of moisture content, the influence of aging degree on tanδ curve increases gradually under the same moisture content. This shows that the degree on same moisture content. shows that the impact impact of tanδ agingcurve and increases moisturegradually on tanδ under is notthe only a single effect, but This an interactional process. of aging and moisture on tanδ is not only a single effect, but an interactional process. Especially in the Especially in the higher moisture content, the influence of aging on the tanδ curve will be greatly higher moisture content, the influence of aging on the tanδ curve will be greatly promoted by moisture. promoted by moisture. So, the influence between moisture and aging cannot be ignored when So, the influence betweenwere moisture and aging be for ignored whenevaluation. characteristic parameters were characteristic parameters extracted from cannot this band condition extracted from this band for condition evaluation. Figure 3 is the tanδ curve with different moisture content at the same aging degree, which can be transformed from Figure 2. According to the literature [24,27], moisture has a greater impact on the tanδ value in a highfrequency range. In the range of 10−3~10−1 Hz, comparing the tanδ curves of
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Figure 3 is1195 the tanδ curve with different moisture content at the same aging degree, which6 can Energies 2017, 10, of 15 be transformed from Figure 2. According to the literature [24,27], moisture has a greater impact on −3 ~10−1 Hz, comparing the tanδ curves the tanδ D value highfrequency In seen the range of 10 sample 0 andin Aa3 with sample D3, range. it can be that the influence of moisture and aging on tanδ is of sample D0 and Arather with sample D , it can be seen that the influence ofevaluating moisture and aging on tanδ more complicated than simple superposition [23]. Therefore, in aging degree and 3 3 ismoisture more complicated rather than simple superposition [23]. Therefore, in evaluating aging degree content, the assessment method can not only take individual factors into account while and moisture the assessment method canwhen not only take individual into ifaccount while ignoring the content, impact between the two. Because measuring moisturefactors or aging, no measures ignoring the impact between the two. Because when measuring moisture or aging, if no measures are taken to eliminate the interactions mentioned above, the measuring results are bound toare be taken to eliminate interactions mentioned above, the measuring results are bound to be disturbed disturbed by eachthe other and neither of them can be accurately evaluated. by each other and neither of them can be accurately evaluated. 2
10
2
10
0.21% 0.98% 1.98% 2.97% 3.90%
1
10
0.21% 0.98% 1.98% 2.97% 3.90%
1
10
0
0
10
tanδ
tanδ
10
1
10
1
2
10
2
10
3
10
10
3
10
3
10
2
10
1
10
0
1
10 f/Hz
2
10
3
10
3
10
10
10
2
1
10
(a) DP = 1250 0.21% 0.98% 1.98% 2.97% 3.90%
2
1
10
2
3
10
0.21% 0.98% 1.98% 2.97% 3.90%
2
1
10
0
0
10
tanδ
tanδ
10
10
10
1
10
2
10
3
10
1
10
(b) DP = 932
10
10
0
10 f/Hz
3
2
10
1
10
0
1
10 f/Hz
10
10
2
10
1
10
2
10
3 3
10
3
10
2
10
1
10
(c) DP = 740
0
10 f/Hz
1
10
2
10
3
10
(d) DP = 504 0.21% 0.98% 1.98% 2.97% 3.90%
2
10
1
10
tanδ
0
10
1
10
2
10
3
10
3
10
2
10
1
10
0
10 f/Hz
1
10
2
10
3
10
(e) DP = 402 Figure 3. The tanδ curve of samples under different moisture content in set A~E. Figure 3. The tanδ curve of samples under different moisture content in set A~E.
3.2. Extraction of Moisture and Aging Characteristic Parameters From the above analysis, it can be seen that the influence of moisture and aging on the frequency band of tanδ curve are totally different. In the range of 101~103 Hz, the tanδ curves are mainly affected by moisture content, while aging degree has little effect. For dry oilpaper
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3.2. Extraction of Moisture and Aging Characteristic Parameters Energies 2017, 10, 1195
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From the above analysis, it can be seen that the influence of moisture and aging on the frequency band of tanδ curve are totally different. In the the range of degree 101 ~103well Hz,inthe curves −1 Hz. insulation samples (Figure 2a), tanδ curves reflect aging thetanδ range of 10−3are ~10mainly affected by moisture content, while aging degree has little effect. For dry oilpaper insulation samples 1 3 Therefore, Sm (the frequency integral value of tanδ from 10 to 10 Hz) is extracted as the −3 −1 Hz. Therefore, S (Figure 2a), tanδ curves for reflect the aging degree well in the rangeand of 10 characteristic parameter characterizing the moisture content, SDP ~10 (the frequency integralm 1 3 (the frequency integral value of tanδ from 10 to 10 Hz) is extracted as the characteristic parameter value of tan δ from 10−3 to 10−1 Hz) is extracted as the characteristic parameter for evaluating the for characterizing the moisture content, and(5). SDP (the frequency integral value of tanδ from 10−3 to aging degree, as shown in Equations (4) and − 1 10 Hz) is extracted as the characteristic parameter for evaluating the aging degree, as shown in Equations (4) and (5). 103 3 (4) Sm = Z 1 10tanδ df 10 Sm = tan δ d f (4) 101
10−1
(5) SDP = Z 310−tanδ df 1 10 SDP = tan δ d f (5) 10−3 Figure 2a presents the tanδ curve corresponding to the different aging degree of the dry Figure 2a presents corresponding the different degree of the dry samples. samples. Figure 3a showsthe thetanδ tanδcurve curve of the unagedtosamples underaging different moisture content. In Figureto3aexplore shows the curve of the unaged single samples underand different moisture parameters, content. In order order thetanδ relationship between factor characteristic the to explore the relationship between single and characteristic parameters, the according characteristic SDP are fittedfactor with moisture content and aging degree to characteristic parameters Sm and parameters S and S are fitted with moisture content and aging degree according to Figures 2a and DP 3 gives the fitting equation and the fitting parameters, Figure 4 is the fitting Figures 2a andm3a. Table 3a. Table gives equation and the fitting fitting goodness parameters, Figure 4 is the fitting curve. MC and curve. MC 3and Sm,the DPfitting and SDP have excellent of index. Sm , DP and SDP have excellent fitting goodness of index. Table 3. The fitting Equation of Sm and SDP. Table 3. The fitting Equation of Sm and SDP . Fitting Parameters Fitting Equation Y = A + B*exp (C*x) MC vs.Parameters Sm m) MC = 3.874−8.093*exp Fitting Fitting Equation Y = A(−0.1411*S + B*exp (C*x) DP vs. SDP DP = 401.3 + 2113*exp (−1178*SDP) MC vs. Sm MC = 3.874−8.093*exp (−0.1411*Sm ) DP = 401.3 + 2113*exp (−1178*SDP )
DP vs. SDP
1400
Measured Values Fitting Curve
4
R2 0.9886 R2 0.9903 0.9886 0.9903
Mesured Values Fitting Curve
1200
3
2
DP
MC/%
1000
1
0
800 600 400
0
10
20
30
S m
(a) MC vs Sm
40
50
0.000
0.001
0.002
0.003
0.004
0.005
SDP
(b) DP vs SDP
Figure Figure4.4.Fitting Fittingcurves curvesbetween betweencharacteristics characteristicsand andoilpaper oilpaperstutas. stutas.
For dry insulation samples, SDP can well reflect the aging degree of insulation paper. Similarly, For dry insulation samples, SDP can well reflect the aging degree of insulation paper. Similarly, Sm Sm has a strong relationship with MC of unaged insulation paper. However, moisture has an effect has a strong relationship with MC of unaged insulation paper. However, moisture has an effect on the on the tanδ curve in the whole measurement frequency range, and the moisture content in tanδ curve in the whole measurement frequency range, and the moisture content in insulation paper insulation paper will cause the SDP increase, resulting in the evaluation of aging degree being will cause the SDP increase, resulting in the1 evaluation of aging degree being overestimated. In the overestimated. In the frequency range of 10 ~103 HZ, the tanδ curve will be affected by aging degree 1 3 frequency range of 10 ~10 HZ , the tanδ curve will be affected by aging degree in high moisture level. in high moisture level. Therefore, using the above characteristic parameters to evaluate aging and Therefore, using the above characteristic parameters to evaluate aging and moisture will be affected moisture will be affected which results in the assessment results not being completely reliable. So, which results in the assessment results not being completely reliable. So, in order to eliminate the in order to eliminate the impact, the problem will be further studied in the next section. impact, the problem will be further studied in the next section.
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3.3. Compensation Energies 2017, 10, 1195Factor of Charicteristic Parameter
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3.3.1. Moisture Compensation Factor 3.3. Compensation Factor of Charicteristic Parameter The aging degree of insulating paper mainly affects the lowfrequency range of the tanδ curve, while moisture Compensation content has anFactor influence on tanδ curve in the whole frequency range. Therefore, in 3.3.1. Moisture the evaluation of insulation aging degree, the influence caused by moisture cannot be ignored. So, The aging degree of insulating paper mainly affects the lowfrequency range of the tanδ curve, in order to improve the evaluation precision of aging degree of insulating paper, in this section, the while moisture content has an influence on tanδ curve in the whole frequency range. Therefore, in aging characteristic parameter SDP is modified to eliminate the influence caused by moisture, as the evaluation of insulation aging degree, the influence caused by moisture cannot be ignored. So, in shown in Equation (6). order to improve the evaluation precision of aging degree of insulating paper, in this section, the aging 10−1 characteristic parameter SDP is modified influence (6)in SDP = Sto 'DPeliminate − β= −3 the tanδ df − βcaused by moisture, as shown 10 Equation (6). −1
10 In Equation (6), β is the increased value SDP = S0DP − β of = SDP caused tan δ dby f −the β moisture in insulating paper (6) − 3 10 relative to the dry insulating paper. S’DP is the measured value which includes the effect of moisture content on tanδ (6), curve. Theincreased value ofvalue β is of related to thebyaging degree in and moisture content of In Equation β is the SDP caused the moisture insulating paper relative oilpaper to the dryinsulation. insulating paper. S’DP is the measured value which includes the effect of moisture content on order to value quantify effect of agingand degree, the moisture factor tanδ In curve. The of βthe is related tomoisture the agingon degree moisture content ofcompensation oilpaper insulation. (MCF) is defined as Equation (7). of The value ofon γ indicates the influence ratiocompensation of moisture content In γorder to quantify the effect moisture aging degree, the moisture factor on aging based on tanδ(7). curves in oilpaper insulation. Figure 5ratio illustrates the meaning of (MCF) γ isdegree defined as Equation The value of γ indicates the influence of moisture content on MCF B1 and B3 samples. agingwith degree based on tanδ curves in oilpaper insulation. Figure 5 illustrates the meaning of MCF with B1 and B3 samples. S'DP S β −SDPDP (7) γ = β == S0DP γ= (7) S S SDP SDP DP DP
Z
1
DP=932, MC=0.98% DP=932, MC=2.97%
10
β
S
0
tanδ
10
DP
1
10
2
10
3
10
3
10
2
10
1
10
0
10
1
10
2
10
3
10
f/Hz Figure 5. 5. Description Description of of moisture moisture compensation compensation factor. factor. Figure
Table Table 4 presents the value of MCF under different aging degrees. It It can can be be seen seen from from the MCF MCF of with thethe increase of moisture content while slowly withwith the of A~E A~Esets setsthat thatMCF MCFaugments augmentsrapidly rapidly with increase of moisture content while slowly increase of aging degree. ThisThis indicates that thethe moisture content the increase of aging degree. indicates that moisture contentininthe theinsulating insulatingpaper paperhas has aa prominent effect on on SSDP DP, and the the higher higher the themoisture moisturecontent contentis,is,the themore moreinaccurate inaccurate evaluation thethe evaluation of of aging degree of insulating paper will be. When moisture content of the insulating paper reaches aging degree of insulating paper will be. When moisture content of the insulating paper reaches 1%, 1%, DP increases to double ofdry the insulating dry insulating paper, the assessment accuracy of SDP Sincreases to double of the paper, whichwhich affectsaffects the assessment accuracy of aging aging degree. moisture content exceeds 2%, DP is invalid to evaluate degree. degree. WhenWhen moisture content exceeds 2%, SDP is Sinvalid to evaluate agingaging degree.
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Table 4. Moisture compensation factor γ form set A~E. Energies 2017, 10, 1195
0.21 0.98 1.98 2.97 3.98 Table 4. Moisture compensation factor γ form set A~E. β 0 0.0019 0.0154 0.0471 0.1143
DP 1250 MC%
DP β γ β γ β γ β γ β γ
1250 932 932
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MC/%
740
740 504
504 402
402
γ
0 0.21
2.3795 0.98
19.2866 1.98
58.9871 2.97
143.1470 3.98
β
00 00 00 0 0 0 00 00 00 0 0 0
0.0028 0.0019 2.3795 2.3333 0.0028 0.0042 2.3333 2.6250 0.0042 0.0066 2.6250 0.0066 2.6400 2.6400 0.0102 0.0102 2.6154 2.6154
0.0257 0.0154 19.2866 21.4167 0.0257 0.0419 21.4167 26.1875 0.0419 0.0695 26.1875 0.0695 27.8000 27.8000 0.1322 0.1322 34.1538 34.1538
0.0827 0.0471 58.9871 68.9167 0.0827 0.1473 68.9167 92.0625 0.1473 0.2563 92.0625 0.2563 102.52 102.52 0.6275 0.6275 160.8974 160.8974
0.2402 0.1143 143.1470 200.1667 0.2402 0.3848 200.1667 240.5000 0.3848 0.7406 240.5000 0.7406 296.2400 296.2400 1.2730 1.2730 326.4103 326.4103
γ β γ β γ β γ
In In order order to to find find the the optimal optimal fitting fitting relationship relationship among among the the parameters, parameters, MCF, MCF, DP DP and and MC MC were were fitted by threedimensional interpolation. The threedimensional interpolation fitting surface is shown fitted by threedimensional interpolation. The threedimensional interpolation fitting surface is in Figurein6.Figure From the fittingthe surface, increase MCF canofbeMCF seencan when degreeaging and shown 6. From fittingthe surface, thetendency increase of tendency be aging seen when moisture content increase. Figure 6 also shows aging andthat moisture simply superimposed, degree and moisture content increase. Figure that 6 also shows agingare andnot moisture are not simply and the interaction them is more serious in is themore highserious moisture and aging degree range. superimposed, andbetween the interaction between them incontent the high moisture content and Under the premise of Under given aging degree and moisture content, moisture compensation has aging degree range. the premise of given aging degreethe and moisture content, thefactor moisture acompensation unique valuefactor whichhas cana be solved by interpolation function. Thus, using the obtained MCF γ to unique value which can be solved by interpolation function. Thus, using eliminate the characteristic SDP canthe acquire the accurate of DP. Inthe this paper, matlab interpolation the obtained MCF γ to eliminate characteristic SDPresult can acquire accurate result of DP. In this function toolbox is used for fitting andtoolbox calculation. paper, matlab interpolation function is used for fitting and calculation.
400 300
γ 200 100 0 4 3
1200 2
MC/%
1000 800
1
600
0
400
DP
Figure 6. 6. Fitting Fitting surface surface of of moisture moisture compensation compensation factor. factor. Figure
3.3.2. Aging Aging Compensation Compensation Factor Factor 3.3.2. 1~1033 Hz, when moisture content Although aging aging degree degree has has little little impact impact on ontanδ tanδininrange range10 101 ~10 Although Hz, when moisture content reaches aa highlevel, highlevel, the the effect effect of of aging aging on onthis thisband bandhas hasbecome becomeobvious. obvious. Therefore, Therefore, with with moisture moisture reaches contentincreasing, increasing,the theinfluence influencecaused causedbybyaging aging degree cannot ignored. characteristic content degree cannot bebe ignored. So,So, thethe characteristic Sm Sism is modified to eliminate the influence of aging degree. The modified S m is as shown in Equation (8). modified to eliminate the influence of aging degree. The modified Sm is as shown in Equation (8).
Sm = S 'm − λ =
103
101
tanδdf − λ
(8)
In Equation (8), λ is the increased value of Sm caused by the aging degree in insulating paper relative to the unaged insulating paper in frequency 101~103 Hz. S’m is the measured value which including the effect of aging degree on tanδ curve. The value of λ is related to the aging degree and
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Sm =
0 Sm
−λ =
Z 103 101
tan δ d f − λ
(8)
In Equation (8), λ is the increased value of Sm caused by the aging degree in insulating paper relative to the unaged insulating paper in frequency 101 ~103 Hz. S0 m is the measured value which Energies the 2017,effect 10, 1195of aging degree on tanδ curve. The value of λ is related to the aging degree 10 of 15 and including moisture content of insulating paper. In order to quantify the effect of aging degree on moisture content moisture content of insulating paper. In order to quantify the effect of aging degree on moisture of oilpaper insulation, the aging compensation factor (ACF) φ is defined as Equation (9). Figure 7 content of oilpaper insulation, the aging compensation factor (ACF) φ is defined as Equation (9). illustrates the meaning of ACF with D0 and D3 0samples. Figure 7 illustrates the meaning of ACF with D and D3 samples. φ ϕ==
S' S0 m−−SmSm λλ == m Sm SSm m m
(9)
2
10
(9)
DP=1250 MC=3% DP=402 MC=3%
1
10
λ
0
tanδ
10
Sm
1
10
2
10
3
10
3
10
2
10
1
10
0
10
1
10
2
10
3
10
f/Hz
Figure 7. Description of aging compensation factor. Figure 7. Description of aging compensation factor.
Table 5 presents the value of ACF under different moisture content. It can be seen that the ACF Table 5 presents value of ACF different content. It can be seen that the ACF increases with thethe aging degree andunder moisture contentmoisture of insulating paper. When the moisture content is less than 1%, the and maximum ACFcontent is 0.13,ofwhich has little effect on measurement of increases with the aging degree moisture insulating paper. When the moisture content moisture content. While when moisture content exceeds 2%, the overall ACF is higher than 1, the is less than 1%, the maximum ACF is 0.13, which has little effect on measurement of moisture content. maximum can reach 5. Thisexceeds indicates2%, thatthe when moisture content is than high,1,the of agingcan onreach While when moisture content overall ACF is higher theeffect maximum moisture become obvious which is consistent with previous analysis of tanδ curve in Section 3.1.1. 5. This indicates that when moisture content is high, the effect of aging on moisture become obvious
which is consistent with previous analysis of tanδ curve in Section 3.1.
Table 5. Aging compensation factor φ form set A~E.
DP Table 5. Aging compensation factor φ form set A~ E. 1250 932 740 504 402 MC/% λ 0 0.1504 0.2425 0.5618 0.6318 DP 1250 932 740 504 402 0.21 MC/% φ 0 0.0268 0.0433 0.0904 0.1127 λλ 0.1504 0.2425 0.5618 00 0.3464 0.4979 0.7367 0.6318 0.9507 0.21 0.98 00 0.0472 0.0678 0.1004 0.1127 0.1295 φφ 0.0268 0.0433 0.0904 00 0.1333 1.5339 2.2930 0.9507 3.2919 λλ 0.3464 0.4979 0.7367 1.97 0.98 φ 0 0.0141 0.1624 0.2427 0.3485 φ 0 0.0472 0.0678 0.1004 0.1295 λ 0 4.3835 9.6196 18.3301 29.9635 λ 0 0.1333 1.5339 2.2930 3.2919 2.98 1.97 φ 0 0.2639 0.5792 1.1036 1.8040 φ 0 0.0141 0.1624 0.2427 0.3485 λ 0 29.1500 68.9005 136.4022 250.3395 3.90 λφ 4.3835 9.6196 18.3301 00 0.6469 1.5290 3.0269 29.9635 5.5554 2.98 φ
0
0.2639
0.5792
1.1036
1.8040
Similar to Section 3.3.1,λ MFC, DP MC were fitted by threedimensional interpolation. The 0 and 29.1500 68.9005 136.4022 250.3395 3.90 threedimensional interpolation fitting surface is shown in Figure 8. The fitting surface reflects the φ 0 0.6469 1.5290 3.0269 5.5554 relationship among the parameters. It can be clearly seen that the influence ratio of aging degree is not obvious in regions with low moisture content (under 2%), while ACF increases rapidly in the high moisture range (over 2 %). Under the premise of given aging degree and moisture content, the aging compensation factor has a unique value, which can be solved by interpolation function. Thus,
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Similar to Section 3.1, MFC, DP and MC were fitted by threedimensional interpolation. The threedimensional interpolation fitting surface is shown in Figure 8. The fitting surface reflects the relationship among the parameters. It can be clearly seen that the influence ratio of aging degree is not obvious in regions with low moisture content (under 2%), while ACF increases rapidly in the high moisture range (over 2 %). Under the premise of given aging degree and moisture content, Energies 2017, 10, 1195 11 ofthe 15 aging compensation factor has a unique value, which can be solved by interpolation function. Thus, the accurate accurateresult resultof of moisture content can be acquired aftertheusing the ACF obtained ACF φ to moisture content can be acquired after using obtained φ to compensate compensate the characteristic S m . the characteristic Sm .
6 5 4
φ 3 2 1 0 400
4 600
3
800
DP
2
1000 1200
1 0
MC/％
Figure Figure 8. Fitting Fitting surface surface of of aging aging compensation compensation factor.
4. 4. A A New New Approach Approach to to Assessing Assessing Aging Aging Degree Degree of of OilPaper OilPaper Insulation Insulation The abovecompensation compensation factors eliminate one element aging and when moisture when The above factors can can eliminate one element of agingofand moisture evaluating evaluating the other, but the disadvantage is that the specific values of factors need to be obtained the other, but the disadvantage is that the specific values of factors need to be obtained (MC value or (MC value However, or DP value). it is to often to get anyofprecise value of the two. Therefore, DP value). it is However, often difficult get difficult any precise value the two. Therefore, an algorithm is an algorithm is proposed to solve this problem, specific steps are as follows: proposed to solve this problem, specific steps are as follows: (1) Firstly, the tanδ curve of oilpaper insulation sample is measured, and SDP, Sm are calculated by (1) Firstly, the tanδ curve of oilpaper insulation sample is measured, and SDP , Sm are calculated by tanδ curve. According to fitting Equation in Table 3, moisture content can be obtained, and the tanδ curve. According to fitting Equation in Table 3, moisture content can be obtained, and the results are recorded as MC0. As the impact of moisture on aging measurement is noteworthy, results are recorded as MC . As the impact of moisture on aging measurement is noteworthy, according to DL/T9842005 0
[28], the according to can DL/T9842005 transformeroperating insulationyears, agingand guidelines> the range of DP be obtained
j−1
j−1
i
i−1
i−1
the DP and MC used as evaluation repeat 3. This method usescan thebecompensation factor results. surface Otherwise, to approximate thesteps true2,compensation factor step by step. Finally, the effect of moisture or aging on the measurement in SDP and Sm is eliminated, and the accurate evaluation results are obtained according to Section 3.2. The flow chart of compensation factor method is shown in Figure 9.
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This method uses the compensation factor surface to approximate the true compensation factor step by step. Finally, the effect of moisture or aging on the measurement in SDP and Sm is eliminated, and the accurate evaluation results are obtained according to Section 3.2. The flow chart of Energies 2017, 10, 1195 12 of 15 compensation factor method is shown in Figure 9.
Figure Theflow flow condition assessment of oilpaper insulation by Compensation factor Figure 9.9. The for for condition assessment of oilpaper insulation by Compensation factor algorithm. algorithm.
5. Field Test 5. Field Test In order to verify the feasibility of the compensation factor method to evaluate aging degree In order to verify the feasibility of the compensation factor method to evaluate aging degree and moisture content in the main insulation of field transformers, in this section, two 110 kV and moisture content in the main insulation of field transformers, in this section, two 110 kV doublewinding transformers with different operating times in different regions (Sichuan, China) doublewinding transformers with different operating times in different regions (Sichuan, China) Power Grid Company were used as examples to evaluate the main insulation status. The specific Power Grid Company were used as examples to evaluate the main insulation status. The specific information of transformers for field test is shown in Table 6. information of transformers for field test is shown in Table 6. Table 6. Specific information of field transformers. Table 6. Specific information of field transformers. Number
Number 1# 1# 2# 2#
Voltage Level
Voltage Level 110 kV kV 110 110 kV 110 kV
Rated Capacity
Rated Capacity 40,000 kVA 40,000 kVA 31,500 kVA 31,500 kVA
Put into Operation Time
Put into Operation Time 2008.02 2008.02 1997.09 1997.09
Testing Oil Temperature
Testing Oil Temperature 26 ◦ C 26 °C 28 ◦ C 28 °C
IDAX300 IDAX300 dielectric dielectric response response analyzer analyzer was was selected selected as as field field test test instrument, instrument, the the test test frequency frequency −33to was set in the range of 10− to10 103 3Hz Hzand andthe thetest testvoltage voltagepeak peakwas wasset setto to200V. 200V. At At the the beginning beginning of ◦ C. test, top oil oiltemperature temperatureofof1#1# transformer was oil temperature 2# transformer transformer was 26 ◦26 C, °C, top top oil temperature of 2#of transformer was 28was ◦ 28 The fluctuation oftemperature the oil temperature of transformers the two transformers was 2less 2 °C the The°C. fluctuation of the oil of the two was less than C than during theduring whole test. whole test.toAccording to shift temperature shiftXY method XY model of oilpaper insulation [24,29], in the According temperature method and model and of oilpaper insulation in the references references [24,29], the transformers FDS curve ofhad fieldbeen transformers had been adjusted shift,curves so as the FDS curve of field adjusted by temperature shift,by sotemperature as to obtain tanδ to tanδ curves at reference 30 °C, at obtain reference temperature 30 ◦ C, astemperature shown in Figure 10.as shown in Figure 10.
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10
1# Transformer 2# Transformer 0
tanδ
10
1
10
2
10
3
10
3
10
2
10
1
10
0
10
1
10
2
3
10
10
f/Hz Figure Figure 10. 10. The The tanδ tanδ curves curves of of field field transformers. transformers.
Referring to DL/T9842005, 1# Referring to DL/T9842005, transformer has served for 10 years, which is in the middle of aging; 2# transformer has served for 1# transformer has served for 10 years, which is in the middle of aging; 2# transformer has 18 years, the main insulation DP less than 500 which is in the late aging stage. So, the initial value of served for 18 years, the main insulation DP less than 500 which is in the late aging stage. So, the the 1# transformer was chosen as DP0 = 700, and the initial value of the 2# transformer was chosen initial value of the 1# transformer was chosen as DP0 = 700, and the initial value of the 2# transformer as DP0 = 500. Table 7 shows the evaluation results of the compensation factor method for the was chosen as DP0 = 500. Table 7 shows the evaluation results of the compensation factor method insulation state of field transformers. 1# transformer status evaluation results are DP = 771.9, MC = for the insulation state of field transformers. 1# transformer status evaluation results are DP = 771.9, 1.015%; 2# transformer evaluation results are DP = 446.7, MC = 1.644%. The MC results are similar MC = 1.015%; 2# transformer evaluation results are DP = 446.7, MC = 1.644%. The MC results are to IDAX300’s results, and shown in Table 7. Since it is not possible to sample the field transformer, similar to IDAX300’s results, and shown in Table 7. Since it is not possible to sample the field so the exact value of DP is unknown, but the DP results of compensation factor method are in a transformer, so the exact value of DP is unknown, but the DP results of compensation factor method reasonable range. are in a reasonable range. Table factor. Table 7. 7. Field Field transformer transformer insulation insulation state state evaluation evaluation results results based based on on compensation compensation factor. Transformer Number Transformer Number Characteristics Quantity Characteristics Quantity
Steps Steps
Results IDAX300
Results
IDAX300
1# S’DP = 6.43e3 1# S0 DP = 6.43 DP 0 =× 70010−3 DP 1 = 714.3901 DP 0 = 700 DP 735.5002 DP12== 714.3901 DP 749.5591 DP23== 735.5002 DP 760.4752 DP34== 749.5591 DP4 = 760.4752 DP5 = 768.7620 DP5 = 768.7620 771.9256 DP6== 771.9256 DP 6 DP = 771.9 DP = 771.9 
S‘m = 7.8692 0 = 7.8692 MCS0 m = 1.2078% MC 1 == MC 1.2078% 0 0.9944% MC 0.9944% MC 2 1= = 1.0005% MC 1.0005% MC 3 2= = 1.0058% MC = 1.0058% MC 4 3= 1.0094% MC = 1.0094% MC5 4= 1.0123% MC5 = 1.0123% MC 6 = 1.0153% MC 6 = 1.0153% MC = 1.015% MC = 1.015% MC = 1.10% MC = 1.10%
2# S’DP = 0.0741 2# S‘m = 11.3092 0 SDP S0 m0 = 2.2331% 11.3092 DP0 = = 0.0741 500 MC DPDP 1 = 459.8575 MC 1 = 1.7165% = 500 MC = 0 0 2.2331% DP21==452.8577 459.8575 1.7165% DP MC12 == 1.6754% DP32==448.0983 452.8577 MC23 == 1.6497% 1.6754% DP MC DP43==446.7295 448.0983 MC34 == 1.6439% 1.6497% DP MC DP4 = 446.7295
MC4 = 1.6439%
DP = 446.7 DP = 446.7 
MC = 1.644% MC = 1.644% MC = 1.59%

MC = 1.59%
6. Conclusions 6. Conclusions In this study, Oilpaper insulation samples were prepared through an accelerated thermal this After study,aging, Oilpaper insulation were through acceleratedThen, thermal aging agingIntest. samples were samples dried and theprepared DP of them wereanmeasured. different test. Aftercontent aging, samples dried andobtained the DP ofunder them each were aging measured. Then, moisture moisture samples were (0%~4%) were degree withdifferent natural moisture content samples (0%~4%) were obtained under each aging degree with natural moisture absorption. absorption. The dielectric loss (tanδ) curves of oilpaper insulation under different conditions were The dielectric losson (tanδ) of oilpaper insulation under conditions were measured based measured based the curves FDS measurement. By analysis of different curves, the characteristic parameters of on the FDS measurement. By analysis of curves, the characteristic parameters of aging and moisture aging and moisture were proposed, and the compensation factor and compensation factor were proposed, and the compensation and compensation factor evaluation algorithm were put evaluation algorithm were put forwardfactor for compensating the influence on characteristic parameters forward for compensating the influence on characteristic parameters and accuracy of evaluating the and accuracy of evaluating the condition of oilpaper insulation. condition insulation. From of theoilpaper tests, when the moisture content is less than 2%, the range of 101~103 Hz is mainly affected by moisture content and aging degree has little effect. While when moisture content
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From the tests, when the moisture content is less than 2%, the range of 101 ~103 Hz is mainly affected by moisture content and aging degree has little effect. While when moisture content exceeds 2%, the effect of aging degree on this band became increasingly prominent. In conclusion, consistent with previously published literatures, aging degree mainly affects lowfrequency range (10−1 ~10−3 Hz) of tanδ curve and moisture mainly affects highfrequency range (101 ~103 Hz) of the tanδ curve, and meanwhile the effects of aging and moisture on highfrequency or lowfrequency bands cannot be ignored. The effects of aging and moisture on the FDS curve interact with each other. When moisture content is higher or aging is more serious, the degree of mutual interference is even greater. Moreover, the integral values Sm (101 ~103 Hz) and SDP (10−3 ~10−1 Hz) were proposed as characteristic parameters of moisture content and aging degree respectively, which have excellent fitting relationship with moisture and aging degree of oilpaper insulation. The compensation factor φ was proposed for compensating the effect of aging on Sm and the compensation factor γ was proposed for compensating the effect of moisture on SDP . The compensation factors can eliminate one element of aging and moisture when evaluating the other. Finally, a new method was proposed to evaluate the oilpaper insulation condition based on the aging and moisture content compensation factors, which can eliminate the influence between aging and moisture, and its validity were preliminarily verified by field transformers. Acknowledgments: This work was financially supported by Key Research and Development Program of China (No. 2017YFB0902704) and StateKey Laboratory of Traction Power (No. 2017TPL_Z03). Author Contributions: Guoqiang Xia and Guangning Wu contributed to all parts of the process of this study: developing the methodology, designing the numerical experiments, and writing the paper; Bo Gao performed the numerical experiments; Feibao Yang conducted the physical experiment; Guoqiang Xia analyzed the data. Haojie Yin revised the paper. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3. 4.
5.
6. 7.
8. 9. 10.
Lundgaard, L.; Hansen, W.; Ingebrigtsen, S. Ageing of mineral oil impregnated cellulose by acid catalysis. IEEE Trans. Dielectr. Electr. Insul. 2008, 15, 540–546. [CrossRef] Pradhan, M. Assessment of the status of insulation during thermal stress accelerated experiments on transformer prototypes. IEEE Trans. Dielectr. Electr. Insul. 2006, 13, 227–237. [CrossRef] N’cho, J.S.; Fofana, I.; Hadjadj, Y.; Beroual, A. Review of physicochemicalbased diagnostic techniques for assessing insulation condition in aged transformers. Energies 2016, 9, 367. [CrossRef] Okabe, S.; Ueta, G.; Tsuboi, T. Investigation of aging degradation status of insulating elements in oilimmersed transformer and its diagnostic method based on field measurement data. IEEE Trans. Dielectr. Electr. Insul. 2013, 20, 346–355. [CrossRef] Betie, A.; Meghnefi, F.; Fofana, I.; Yeo, Z.; Ezzaidi, H. On the feasibility of aging and moisture of oil impregnated paper insulation discrimination from dielectric response measurements. IEEE Trans. Dielectr. Electr. Insul. 2015, 22, 2176–2184. [CrossRef] Liao, R.j.; Yang, L.j.; Li, J.; Grzybowski, S. Aging condition assessment of transformer oilpaper insulation model based on partial discharge analysis. IEEE Trans. Dielectr. Electr. Insul. 2011, 18, 303–311. [CrossRef] Saha, T.K.; Purkait, P. Understanding the impacts of moisture and thermal aging on transformer’s insulation by dielectric response and molecular weight measurements. IEEE Trans. Dielectr. Electr. Insul. 2008, 15, 568–582. [CrossRef] Wang, Y.; Xiao, K.; Chen, B.; Li, Y. Study of the Impact of Initial Moisture Content in Oil Impregnated Insulation Paper on Thermal Aging Rate of Condenser Bushing. Energies 2015, 8, 14298–14310. [CrossRef] Zou, J.; Chen, W.; Wan, F.; Fan, Z.; Du, L. Raman Spectral Characteristics of OilPaper Insulation and Its Application to Ageing Stage Assessment of OilImmersed Transformers. Energies 2016, 9, 946. [CrossRef] Fofana, I.; Bouaicha, A.; Farzaneh, M. Aging characterization of transformer oilpressboard insulation using modern diagnostic techniques. Eur. Trans. Electr. Power Eng. 2011, 21, 1110–1127. [CrossRef]
Energies 2017, 10, 1195
11.
12.
13.
14. 15.
16. 17. 18.
19.
20. 21. 22. 23. 24. 25. 26. 27. 28. 29.
15 of 15
Fofana, I.; Hemmatjou, H.; Meghnefi, F.; Farzaneh, M.; Setayeshmehr, A.; Borsi, H.; Gockenbach, E. On the frequency domain dielectric response of oilpaper insulation at low temperatures. IEEE Trans. Dielectr. Electr. Insul. 2010, 17, 805–813. [CrossRef] Pradhan, M.K.; Yew, K.J.H. Experimental investigation of insulation parameters affecting power transformer condition assessment using frequency domain spectroscopy. IEEE Trans. Dielectr. Electr. Insul. 2012, 19, 1851–1859. [CrossRef] Seytashmehr, A.; Fofana, I.; Eichler, C.; Akbari, A.; Borsi, H.; Gockenbach, E. Dielectric spectroscopic measurements on transformer oilpaper insulation under controlled laboratory conditions. IEEE Trans. Dielectr. Electr. Insul. 2008, 15, 1100–1111. [CrossRef] Fofana, I.; Hemmatjou, H.; Meghnefi, F. Effect of thermal transient on the polarization and depolarization current measurements of oilpaper insulation. IEEE Trans. Dielectr. Electr. Insul. 2011, 18, 513–520. [CrossRef] Xia, G.Q.; Wu, G.N. Quantitative assessment of moisture content in transformer oilpaper insulation based on extended Debye model and PDC. In Proceedings of the 2016 China International Conference on Electricity Distribution (CICED), Xi’an, China, 10–13 August 2016; pp. 1–5. Flora, S.D.; Rajan, J.S. Assessment of paperoil insulation under copper corrosion using polarization and depolarization current measurements. IEEE Trans. Dielectr. Electr. Insul. 2016, 23, 1523–1533. [CrossRef] Wang, Y.; Gong, S.; Grzybowski, S. Reliability Evaluation Method for OilPaper Insulation in Power Transformers. Energies 2011, 4, 1362–1375. [CrossRef] Li, S.B.; Wu, G.N.; Gao, B.; Hao, C.J.; Xin, D.L.; Yin, X.B. Interpretation of DGA for transformer fault diagnosis with complementary SaEELM and arctangent transform. IEEE Trans. Dielectr. Electr. Insul. 2016, 23, 586–595. [CrossRef] Blennow, J.; Ekanayake, C.; Walczak, K.; Garcia, B.; Gubanski, S.M. Field experiences with measurements of dielectric response in frequency domain for power transformer diagnostics. IEEE Trans. Power Deliv. 2006, 21, 681–688. [CrossRef] Garcia, B.; Burgos, J.C.; Alonso, A.M.; Sanz, J. A moistureinoil model for power transformer monitoringPart I: Theoretical foundation. IEEE Trans. Power Deliv. 2005, 20, 1417–1422. [CrossRef] Villarroel, R.; Garcia, D.F.; Garcia, B.; Burgos, J.C. Diffusion coefficient in transformer pressboard insulation part 2: Mineral oil impregnated. IEEE Trans. Dielectr. Electr. Insul. 2014, 21, 394–402. [CrossRef] García, D.F.; García, B.; Burgos, J.C. Determination of moisture diffusion coefficient for oilimpregnated Kraftpaper insulation. Int. J. Electr. Power Energy Syst. 2013, 53, 279–286. [CrossRef] Jadav, R.B.; Ekanayake, C.; Saha, T.K. Understanding the impact of moisture and ageing of transformer insulation on frequency domain spectroscopy. IEEE Trans. Dielectr. Electr. Insul. 2014, 21, 369–379. [CrossRef] Liao, R.; Hao, J.; Chen, G.; Yang, L. Quantitative analysis of ageing condition of oilpaper insulation by frequency domain spectroscopy. IEEE Trans. Dielectr. Electr. Insul. 2012, 19, 821–830. [CrossRef] Saha, T.K.; Purkait, P.; Müller, F. Deriving an equivalent circuit of transformers insulation for understanding the dielectric response measurements. IEEE Trans. Power Deliv. 2006, 20, 149–157. [CrossRef] Hu, Q.F. Transformer Test Technology; China Electric Power Press: Beijing, China, 2010; pp. 56–113. Sokolov, V.; Koch, M. Moisture equilibrium and moisture migration within transformer insulation systems. Cigre Brochure 2008, 349, 1–52. DL/T 9842005, Guide for the Diagnosis of Insulation Aging in OilImmersed Power Transformer; CNDL: Beijing, China, 2006. Cheng, J.; Robalino, D.; Werelius, P.; Ohlen, M. Improvements of the transformer insulation XY model including effect of contamination. In Proceedings of the 2012 IEEE International Symposium on Electrical Insulation, San Juan, PR, USA, 10–13 June 2012; pp. 169–174. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).