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2016 China International Conference on Electricity Distribution (CICED 2016) Xi’an, 10-13 Aug, 2016 Quantitative Assessment of moisture content in Transformer Oil-paper Insulation Based on Extended Debye model and PDC XIA Guoqiang. WU Guangning College of Electrical Engineering, Southwest Jiaotong University, Chengdu 610031, Sichuan Province, China Abstract—In order to assess the moisture content of insulating pressboard in oil-immersed transformer, and thus providing reliable guidance for insulation condition and life estimation of oil-immersed transformer, in this paper, we made four groups of oil-paper insulation samples with different moisture content under laboratory conditions and studied the relationship between pressboard and moisture content by using polarization and depolarization current (PDC) technique. The experiment result shows that PDC curve is very sensitive to the change of moisture content, and the change of moisture content mainly affects the tail of depolarization current curve; the extended Debye model parameters can be obtained from depolarization current curve, and as the moisture content increases the number of branches in Debye model increases, the largest time constant becomes large; the peak of time domain spectroscopy turn up at 1000s., the time constant branch which is longer than 1000s presents the pressboard branches of Debye model; the feature quantity Qpaper can show the internal polarization of insulating paper and has a linear fitting relationship with moisture content, so Qpaper can be used to assess the moisture content of insulating pressboard. Index Terms—Debye model, insulating paper, moisture content, oil-paper insulation, polarization and depolarization current. I. INTRODUCTION Power transformer is one of the most expensive and important equipment in power system. The condition of the transformer is very important on ensuring the safe and stable operation of power system [1]. The main insulation system consist of fiber paper, cardboard and insulating oil, its performance can degrade under the thermal, electrical, mechanical, chemical compound factors during its operation [2], [3]. Moisture is one of the most important factors. If the moisture content increase to twice as the original, the lifetime of insulation system will be halved [4], [5]. The increase of moisture content in oil will rise CICED2016 Session x Paper No dielectric loss and reduce breakdown voltage of oil-paper insulation which will damage the insulation structure and affect normal operation of equipment; water is not only the degradation product of insulating paper but also reaction catalyst, it will accelerate the degradation process of insulating materials. However, water mainly exists in insulating paper, and it has a very low proportion in oil which also can be removed through the oil filter. Therefore, the study on the assessment of the insulation pressboard moisture content is hot spot in current research. In recent years, the demand of non-destructive test methods and technical means on transformer insulation system promotes the development of diagnostic techniques based on the dielectric response. Where, Polarization/Depolarization Current (PDC), Return Voltage Method (RVM) are based on the time-domain [6]–[9], Frequency Domain Spectroscopy (FDS) is based on frequency domain[10], [11]. The traditional way to measure the moisture content of insulating paper is that the moisture of transformer oil can be measured by Karl Fischer titration, then though T.V.Oommen equilibrium curve we can obtain the moisture content of insulating paper [12], [13]. But in reality, there are many restrictions [13]. T.K.Saha presented the Extended Debye model of oil-paper insulation based on PDC and connected the relaxation time constant with branch of Debye model [14]; JIANG Xiubo analyzed the moisture of oil-paper insulation by RVM, studied the relationship between moisture content and polarization spectroscopy [10], but they did not come up with an effective feature quantity to present the moisture content; XU Yuan proposed a method to assess moisture content of oil-paper insulation based on FDS [15], but this method relative to PDC is more time-consuming. So, in this paper, in order to obtain the feature quantity of insulating paper of transformer, oil-paper insulation samples were prepared with different moisture contents under laboratory conditions. Then we measured the PDC of samples, plotted the PDC curves and time domain Spectroscopy; the parameters of Extended Debye model can be calculate though the PDC curves; finally, combining the relationship Page /5 2016 China International Conference on Electricity Distribution (CICED 2016) between oil-paper with the branch of Debye model, we proposed a feature quantity of moisture content of insulating paper. II. THEORY A. PDC principle [16] According to Dielectric Physics Theory, when a step voltage is applied on the dielectric material, dielectric will produce charging current which is called polarization current; when the voltage is removed, the short circuit current of dielectric is called depolarization current. Typical Circuit of PDC is shown in Fig.1. The relationship between polarization current and time: i p t C0U 0 0 f t The geometric capacitance C0 can be measured by power frequency capacitance method; insulation resistance R0 is decided by the ratio of measuring voltage and the sum of depolarization and polarization current in the largest measuring time. The PDC in oil-paper insulation can be seen as composed by different time constant relaxation process, so it can be modeled as the sum of a series of exponential functions. The Extended Debye model branch parameters can be obtained by formula (3) to (6). n id [ Ai exp(t / i )] id t C0U 0 f t f t tc (2) Wherein, U0 is test voltage source, C0 is geometric capacitance of sample, f(t) is dielectric response function, σ is dc conductivity, ε is permittivity of vacuum, tc is measuring time of polarization. (3) i 1 Ai U 0 (1) The relationship between depolarization current and time: Xi’an, 10-13 Aug, 2016 ip 1 exp(Tp / i ) Ri (4) i Ri Ci (5) U0 n U0 [ exp(t / i )] Rg i 1 Ri (6) Wherein, τi is the time constant of branch I, tp is the largest measuring time, Rg is the conductivity of samples. III. DESIGN OF EXPERIMENT ip id object U0 Precision ammeter A Fig.1 Typical Circuit of PDC measurement B. Extended Debye model Oil-paper insulation of transform is non-uniform structure which is composed of two kinds of dielectric. For a single dielectric, its polarization process can be simulated as capacitors, non-uniform structure dielectric’s polarization can be equivalent to series and parallel of resistors and capacitors. In the research content of T. K. Saha, the composite branches of oil-paper Equivalent Model had been simplified and finally the research obtained extended Debye model as show in fig.2 [14]. R0 R1 R2 Ri Rn C1 C2 Ci Cn C0 In this paper, 1mm thick cardboard and 25# naphthenic mineral insulating oil were selected as experimental material. Four group oil-paper insulation samples were prepared with different moisture contents under laboratory conditions, and the specific steps are as follows: first of all, insulation pressboard was dried at 90 ℃/ 50 Pa vacuum box for 48 hours, and transformer oil which has been degassed was heated to 50 ℃ ; then, the transformer oil and insulation pressboard were impregnated 2 days in 40 ℃ / 50 Pa vacuum box; after that the oil-paper insulation samples naturally absorbed moisture in the air, the moisture content of oil-paper sample was decided by the weight of the moisture absorption percentage in the air; finally we obtained four group oil-paper insulation samples with moisture contents 1.15%、2.04%、3.02%、4.29%. Four group samples’ PDC tests were carried out in the three-electrode measuring device, as shown in figure 3. PDC test equipment is PDC-Analyser-1MOD PDC current tester, DC voltage is 500V, measuring time is 10000s. It's worth noting that before the start of the test in each group, we need to place the entire measuring device into 60℃ incubator for 5 hours, so as to make sure that the internal and external temperature of the entire system achieve a relatively balanced. Fig.2 Extended Debye model of oil-paper insulation CICED2016 Session x Paper No Page /5 2016 China International Conference on Electricity Distribution (CICED 2016) Xi’an, 10-13 Aug, 2016 Incubator 1E-8 Fixed Knob Sample Electrical connections Electrode Polarization Current/A Temperature Probe mc mc mc mc 1E-9 1E-10 1E-11 1 . 10 b) mc mc mc mc Depolarization Current/A 4.29% 3.02% 2.10% 1.15% 1E-10 1E-11 1E-12 1 10 100 1000 10000 t/s a) Depolarization current CICED2016 Session x 1000 10000 Polarization current Fig.4 Polarization and depolarization current with A. Polarization and depolarization of samples It can be seen from Figure 4(a) that when the moisture content of oil-paper samples increases, the initial phase of the depolarization current curve did not change significantly, but still rise overall while at the end of the curve has obvious increasing trend. Reference [10] pointed out that the insulating paper main influence the tail of the depolarization current, while the insulating oil affects the beginning of the current. Moisture basically has the following effects on oil-paper insulation samples: moisture increases the migration velocity of free charge and ion inside the insulating paper; the hydrophilic ionic and groups easy to combine with moisture to be charged particles, ions deviate from the equilibrium position which led to the polarization process enhanced. Moisture is mainly presents in the insulating paper and has a very low proportion in oil. Based on the above analysis, with moisture content increases, the tail of the depolarization curves significantly increase and the initial phase of the curves rise overall. 1E-9 100 t/s Fig.3 Structure of three-electrode measuring device IV. RESULTS AND DISCUSSIONS 4.29% 3.02% 2.10% 1.15% Paper No different moisture contents As shown in figure 4(b), the polarization current curve rise with the increase of moisture, while the end of the curves approaching steady. The polarization current curve is composed of conduction current and different relaxation currents, and also moisture increases the dc conductivity. So, in the late of measure time, relaxation currents gradually decay and the polarization currents mainly reflect conduction current. B. Debye Model and Time-domain spectroscopy Reference [14] proposed a method for solving the extend Debye Model by depolarization current curve. In this paper, according to this method, we solved depolarization current curves in Figure 4(a), the specific parameters of Debye model are shown in table 1. It can be seen from the table 1 that with the increase of moisture content, the number of branches in the Debye model was rising. This is because that with the insulation performance dropped significantly, conduction current and polarization losses increased. So the number of interface polarization increase which is more complicated than low moisture content. So, the number of branches of extended Debye was increased so as to fit depolarization current curve more accurate. It also can be seen form table 1 that the largest time constant get larger with the increase, the reason as mentioned above Tab.1 Extended Debye model parameters mc/% 1.15 2.04 3.02 4.29 τ1/s — — — 1.4 A1/A — — — 5.6e-12 τ2/s — — 1.5 17.8 A2/A — — 4.89e-12 7.61e-12 τ3/s 1.5 1.3 17.1 175 A3/A 2.31e-12 5.89e-12 1.13e-11 1.36e-11 τ4/s 17.5 172 173 350 A4/A 6.68e-12 8.6e-12 2.24e-11 2.15e-11 τ5/s 170 1013 1017 1510 A5/A 1.1e-11 2.56e-11 4.85e-11 5.64e-11 Page /5 2016 China International Conference on Electricity Distribution (CICED 2016) id(t)·t/(A·s) τ6/s 1012 1590 1593 2870 A6/A 1.3e-11 4.75e-11 1.1e-10 1.2e-10 Figure 6 is oil-paper samples’ time domain dielectric spectroscopy, each curve in figure 6 is substantially the same trend and at the time of 1000 seconds curves emerged the first peak. Reference [17], [18] pointed out that time domain dielectric spectroscopy contains all types of polarization of oil-paper insulation. Each kind of spectral peak represents a type of polarization, the peak’s value represents intensity of polarization and the spectral peak time reflects the response speed. In theory, the time domain dielectric spectrum should contain many peaks, but the intensity and response time of depolarization processes are always different. More intense and longer constant time polarization process will cover the small, finally the time domain dielectric spectroscopy only showed the peak of 1000s which is the widest range and largest polarization. Reference [14] point out that the oil mainly effect the branches of low time constant, while the condition of insulating paper influence the biggest time constant branches. So, though the above analysis we can obtain that the time constant which is longer than 1000s present the pressboard branches, they can reflect the polarization characteristics of pressboard under different moisture content. But through the depolarization current curve and the time domain dielectric spectra can not obtain the quantitative relation with moisture content of insulating-paper, so it need to be further processed. 4.0x10 -8 3.0x10 -8 2.0x10 -8 1.0x10 -8 mc mc mc mc 4.29% 3.02% 2.10% 1.15% So in order to find out feature quantity of pressboard moisture content, we selected the branches of pressboard form the extended Debye model branches to study the relationship between moisture and pressboard. According to [19], this paper proposes a new feature quantity -- Qpaper which presents the depolarization charge quantity of pressboard, as shown in equation (6), (7). Qpaper 10 100 1000 10000 1 50 10000 CICED2016 Session x Paper No (6) n1 t paper n2 (7) i n1 measured values fitting curve t/s 40 Qpaper/10-8 Fig.6 The time domain dielectric spectroscopy of different moisture content samples C. Feature quantity of Insulation Paper Depolarization current curve is closely related to moisture content of oil-paper insulation. When we study the moisture content of pressboard, although the proportion of moisture content in oil is small, it is also necessary to exclude the influence of the factors in oil. According to (3)-(4), despite the time constant is very low, it will affect the depolarization current curves and the integration results. And in reality, besides moisture, the transformer oil also contains a lot of acid and other polar molecules produced by the aging of cellulose. These factors will have an impact on the results. n2 Ai exp(t / i )dt In order to exclude the impact of oil on the integration of formula (7), we selected the time constant branches which are greater than tpaper (time constant of paper) branches, the value of tpaper can obtain from the figure 6. Table 3 is the Qpaper of different moisture contents, it can be seen that Qpaper with the increase of moisture content gradually rised. This is because that water is highly polar molecular, the increase of moisture content makes the number of interface polarization of pressboard rise. Tab.2 Qpaper of samples with different moisture contents mc/% 1.15 2.04 3.02 4.29 Qpaper/e-8C 1.3142 10.124 22.406 41.871 In order to study the relationship between the moisture content of pressboard and Qpaper, in this paper, the moisture content of pressboard was fitted with Qpaper, fitting results shown in Table 3. The results showed that fitting Optimization is 0.98655, so the Qpaper of pressboard and moisture content have a good linear relationship. Therefore, the Qpaper can be used as the characteristic parameter of moisture content of pressboard. Tab.3 Relationship between moisture and Qpaper Fitting formula R2 Y=-15.15055+12.98261x 0.98655 0.0 1 Xi’an, 10-13 Aug, 2016 30 20 10 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 mc/% Fig.7 Fitting curve between moisture contents and Qpaper of oil-paper Page /5 2016 China International Conference on Electricity Distribution (CICED 2016) V. CONCLUSION In this paper, different moisture contents of oil-paper insulation samples were prepared, and carried out the PDC experiment. Though the Effects of moisture content of samples on the different parts of the Extended Debye model and the relationship between time domain dielectric spectroscopy and Debye model branches, finally we extracted feature quantity of moisture content of pressboard. The conclusions are as follows: (1) The branches’ parameters of extended Debye model can be obtained from depolarization curves, insulating oil and pressboard affect different time constant branches of Debye model; moisture content mainly influence the tail of the depolarization current curve, and with the increase of moisture content, the tail of depolarization current curve rises obviously. (2) The branch number of Debye model is rising and the biggest time constant branch also get larger with the increase of moisture content; the peak value of time domain dielectric spectroscopy appears at the time of 1000s which presents the intensity of polarization of pressboard. (3) We put forward a new feature quantity ----Qpaper. Qpaper can exclude impact factors in oil and other aging products which may affects the measurement result; Qpaper has a good linear relationship with the moisture content, so it can be used as feature quantity of moisture content of pressboard. 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Research area: Power Equipment Condition Assessment. . Page /5