Breakdown Voltage and Its Influencing Factors of Thermally Aged Oil

energies

Article Breakdown Voltage and Its Inﬂuencing Factors of Thermally Aged Oil-Impregnated Paper at Pulsating DC Voltage

Jing Zhang 1, Feipeng Wang 1,*, Jian Li 1,*, Hehuan Ran 1, Xudong Li 1 and Qiang Fu 2

1 State Key Laboratory of Power Transmission Equipment & System Security and New Technology, School of Electrical Engineering, Chongqing University, Chongqing 400044, China; [email protected] (J.Z.); [email protected] (H.R.); [email protected] (X.L.) 2 Electric Power Research Institute of Guangdong Power Grid Co., Ltd., Guangzhou 510080, China; [email protected] * Correspondence: [email protected] (F.W.); [email protected] (J.L.); Tel.: +86-23-6511-1795 (F.W.); +86-23-6510-2430 (J.L.)

Academic Editors: Akhtar Kalam and Tapas Mallick Received: 27 July 2017; Accepted: 12 September 2017; Published: 15 September 2017

Abstract: Breakdown strength is an important electrical property of insulating paper. Oil-impregnated paper of various aging states was prepared. Its breakdown voltage was then measured at pulsating direct current (DC) voltage with various ripple factors, and alternating current (AC) and DC voltage for comparison, respectively. The AC breakdown voltage is the smallest, and pulsating DC (r = 1/5) breakdown voltage is the greatest before the paper reaches its end of life. A dielectric model was adopted to investigate the difference in magnitude of breakdown voltage at different voltage waveforms. Meanwhile, it was found that breakdown voltage fluctuated and even increased occasionally during the thermal aging process, and a somewhat opposite changing tendency versus aging time was observed for breakdown voltage at DC voltage and pulsating DC voltage with small ripple factors (r = 1/5 and 1/3), compared with AC voltage. The degree of polymerization (DP) and moisture content of the paper were measured, and the characteristics of the pores and cracks of the paper were obtained to investigate the possible influencing factors of breakdown voltage at different aging states. The results showed that the moisture content, oil absorption ability associated with pores and cracks of paper, and the damage to paper structure all contributed to the variation of the breakdown voltage versus aging time, while the importance of their influence differed as the aging state of paper varied.

Keywords: breakdown voltage; pulsating DC voltage; thermal aging; double-composite dielectric model; oil absorption ability; oil-impregnated paper

1. Introduction Power transformers are essential to power transmission networks [1]. As the insulating medium of power transformers, oil-impregnated paper (OIP) degrades under interaction with heat, moisture, oxygen, and acids [2,3]. Among those aging factors, heat is the main cause of insulation aging, and thus reduces its mechanical strength and electrical performance. Therefore, condition assessment of OIP using its dielectric and physicochemical characteristics is essential to the safe operation, planned maintenance, and replacement of transformers [4]. Breakdown voltage is an important property of insulating paper, and thermal aging is the main form of OIP aging. Investigation into the breakdown characteristics of thermally aged OIP is conducive to understanding the inﬂuencing mechanism of aging on its insulating performance.

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Meanwhile, as the direct current transmission system is developing rapidly, the converter transformer has attracted much attention by researchers. The valve-side windings in a converter transformer withstand the voltage of complex waveforms [5]. The turn-to-ground insulation of the valve-side wingdings withstands pulsating DC voltage [5,6], also known as combined AC-DC voltage or AC and DC combined voltages [7–10]. The breakdown characteristics of OIP at pulsating DC voltage show many differences compared with that at AC voltage [6]. Therefore, it is necessary to investigate the breakdown voltage of aged OIP at this type of voltage waveform. The breakdown voltage of OIP or oil-paper insulation has been widely studied at various voltage waveforms, including AC voltage [11,12], DC voltage [13,14], impulse voltage [12,15–18], and pulsating DC voltage [6,19,20]. In [6], the breakdown voltage of OIP at pulsating DC voltage (with a ripple factor of 1 and 1/3) was measured. It was found that the pulsating DC breakdown voltage lies between AC and DC breakdown voltage when temperature varies from 60 ◦C to 110 ◦C. Chen et al. [20] theoretically calculated the distribution of electric field strength for the oil-paper insulation when it was subjected to pulsating DC voltage. Experiments showed that the breakdown voltage reaches its maximum value when a small ratio of AC component is contained in the applied voltage, and this maximum value shows no significant difference compared with that at individual DC voltage. The papers used were all unaged insulating papers. There are few publications investigating the breakdown voltage of thermally aged paper at AC voltage [12,21,22], let alone investigation into that at pulsating DC voltage. Sun et al. [12] found the AC breakdown voltage of thermally aged OIP increases with the increased duration of thermal aging. The increase in breakdown voltage was attributed to the emergence of micro globules and the breakdown of the cellulose fibers, which improved the electric field distribution in the OIP. Liao et al. [21] measured the AC breakdown strength of oil-impregnated pressboards thermally aged in both mineral and vegetable insulating oil at 110 ◦C. The breakdown voltage of the pressboard fluctuates with the increase of thermal aging duration and no clear correlation with degree of polymerization (DP) was found. Tang et al. [22] investigated the AC breakdown voltage of thermally aged paper in both mineral and vegetable insulating oil at 90 ◦C, 110 ◦C, and 130 ◦C, and no pronounced correlation between AC breakdown voltage with the DP of the paper or thermal aging temperature was observed. The breakdown voltage firstly exhibited a small decrease but then increased during the thermal aging process. It remains to be seen, however, whether the breakdown voltage of thermally aged OIP at pulsating DC voltage exhibits the same changing tendency as that at AC voltage. This work aims at investigating the breakdown voltage of thermally aged OIP at pulsating DC voltage and the possible influencing factors. Firstly, the Kraft insulating paper was thermally aged in oil at a temperature of 120 ◦C and 130 ◦C. Next, breakdown voltage of paper of different aging states at pulsating DC voltage was measured. For comparison, identical experiments were carried out at AC and DC voltage. Lastly, the difference in magnitude of breakdown voltage at those three kinds of voltage waveforms was interpreted with the double-composite dielectric model. The moisture content of the paper and oil absorption ability associated with the pores and cracks of the paper, and the structural damage of the paper were adopted to explain the changing tendency of breakdown voltage versus aging time.

2. Experiments

2.1. Accelerated Thermal Aging Experiments OIP of different aging states were obtained through accelerated thermal aging experiments. The naphthenic-based mineral oil Karamay 25# from Petrolchina and Kraft insulating paper from Taizhou Xinyuan Electrical Equipment Co., Ltd., China were used to conduct the experiments. The paper has a thickness of 0.13 mm and a diameter of 80 mm, respectively. To simulate the processing procedures of oil-paper in an actual transformer and to control the initial state of experimental samples, the oil and paper were separately degassed and dehydrated in a vacuum drying chamber at 50 Pa/80 ◦C for Energies 2017, 10, 1411 3 of 16

Energies 2017, 10, 1411 3 of 16 48 h. Then the paper went through an oil impregnation process for 48 h at 50 Pa/60 ◦C. Thereafter, theyof oil were to paper transferred being into 10:1 a stainlessby weight. steel Figure container, 1 illustrates with the ratiothe ofsample oil to paperprocessing being 10:1procedure by weight. for Figureaccelerated1 illustrates thermal the aging. sample Lastly, processing the sealed procedure containers for acceleratedwere put into thermal an oven, aging. which Lastly, was the heated sealed to ◦ ◦ containers120 °C or 130 were °C. put Table into an1 lists oven, the which thermal was aging heated temperatures to 120 C or and 130 correspondingC. Table1 lists aging the thermal times. agingThe temperature temperatures for and accelerated corresponding thermal aging aging times. experiment The temperature was determined for accelerated according thermal to aging the experimentfollowing two was principles. determined Firstly, according the temper to the followingature should two not principles. be too low Firstly, (say thebelow temperature 90 °C), as should a low ◦ nottemperature be too low results (say belowin a very 90 longC), as time a low to temperatureaccomplish the results thermal in a veryaging long experiment. time to accomplish Secondly, the thermaltemperature aging should experiment. not be Secondly, too high, the as temperature a high temperature should not may be tooexceed high, the as aflash high point temperature of the ◦ mayinsulating exceed oil the (say ﬂash above point 140 of ° theC). insulatingBased on the oil (saytwo aboveprinciples 140 andC). Basedthe thermal on the aging two principles temperatures and ◦ ◦ theadopted thermal by agingother temperatures researchers, adopted120 °C byand other 130 researchers,°C were finally 120 C chosen and 130 forC werethe thermal ﬁnally chosenaging forexperiment. the thermal aging experiment.

Figure 1. Sample processing procedure. Figure 1. Sample processing procedure.

Table 1. Thermal aging temperature and corresponding aging time. Table 1. Thermal aging temperature and corresponding aging time. Thermal Aging Temperatures (°C) Aging Time (Day) ◦ Thermal Aging Temperatures120 ( C) 2, 4, 7, 10, Aging 22, 35, Time 60, 120, (Day) 180 120130 0.5, 2,2, 4,4, 7,7, 10,10, 22,22, 35,35, 60,60, 120,90 180 130 0.5, 2, 4, 7, 10, 22, 35, 60, 90 At each due time, one container was removed from the aging oven to obtain aged OIP samples for theAt following each due time,test items: one container breakdown was voltage, removed mo fromisture the content, aging ovenDP, nitrogen to obtain gas aged adsorption OIP samples and desorption tests, and scanning electron microscope (SEM) test. The moisture content and DP of paper for the following test items: breakdown voltage, moisture content, DP, nitrogen gas adsorption and were measured to see if it was strongly correlated with the variation of breakdown voltage of OIP. desorption tests, and scanning electron microscope (SEM) test. The moisture content and DP of paper Measurement of the moisture content of OIP was accomplished with a Karl-Fischer coulometric were measured to see if it was strongly correlated with the variation of breakdown voltage of OIP. titrator (Metrohm moisture meter 851 and oven 885, Herisau, Switzerland), following IEC 60814. Measurement of the moisture content of OIP was accomplished with a Karl-Fischer coulometric titrator Measurement of the DP of the paper was conducted according to IEC 60450. The gas adsorption and (Metrohm moisture meter 851 and oven 885, Herisau, Switzerland), following IEC 60814. Measurement desorption test, and SEM test were conducted to investigate the mesoporous distribution, and pores of the DP of the paper was conducted according to IEC 60450. The gas adsorption and desorption test, and cracks of the paper during the thermal aging process, respectively. and SEM test were conducted to investigate the mesoporous distribution, and pores and cracks of the paper during the thermal aging process, respectively. 2.2. Experimetnal Setup for Breakdown Voltage Test 2.2. ExperimetnalFigure 2 illustrates Setup for the Breakdown experimental Voltage setup Test for the breakdown voltage test. It consists of a power source,Figure an oil2 illustrates vessel, an the electrode experimental system, setup and for some the breakdownmeasurement voltage instruments. test. It consists The power of a powersource source,included an a oilfunction vessel, generator an electrode and a system, high voltage and some power measurement amplifier (TREK instruments. 50 kV/12 ThemA). power The function source includedgenerator a provides function generatorthe required and avoltage high voltage signal, power and the ampliﬁer high voltage (TREK 50power kV/12 amplifier mA). The raises function the generatorvoltage to provides the required the required level. An voltage asymmetric signal, and electrode the high system voltage was power designed ampliﬁer [23] raises to measure the voltage the tobreakdown the required voltage level. of An OIP, asymmetric as depicted electrode in Figure system 3. was designed [23] to measure the breakdown voltage of OIP, as depicted in Figure3.

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Figure 2. The setup of breakdown voltage test at different voltage waveforms. FigureFigureFigure 2. 2.2. The TheThe setup setupsetup of ofof breakdown breakdownbreakdown voltage voltagevoltage test testtest at atat different differentdifferent voltagevoltage voltage waveforms. waveforms.

Figure 3. The electrode system. FigureFigureFigure 3. 3.3. The TheThe electrode electrodeelectrode system. system.system. 2.3. Breakdown Voltage Test 2.3.2.3.2.3. Breakdown BreakdownBreakdown Voltage VoltageVoltage Test TestTest ForFor OIPOIP ofof eacheach agingaging time,time, breakdownbreakdown voltagesvoltages ofof 1010 specimensspecimens werewere measuredmeasured toto obtainobtain 1010 ForFor OIPOIP ofof eacheach agingaging time,time, breakdownbreakdown voltagesvoltages ofof 1010 specimensspecimens werewere measuredmeasured toto obtainobtain 1010 breakdownbreakdown voltagevoltage datadata atat roomroom temperature.temperature. ForFor ththee convenienceconvenience ofof description,description, thethe peakpeak valuevalue ofof breakdownbreakdown voltagevoltage datadata atat roomroom temperature.temperature. ForFor thethe convenienceconvenience ofof description,description, thethe peakpeak valuevalue ofof thethe voltagevoltage waswas recordedrecorded andand consideredconsidered toto bebe thethe breakdownbreakdown voltagevoltage whenwhen thethe OIPOIP brokebroke down.down. thethe voltagevoltage waswas recordedrecorded andand consideredconsidered toto bebe thethe breakdownbreakdown voltagevoltage whenwhen thethe OIPOIP brokebroke down.down. TheThe positivepositive pulsatingpulsating DCDC voltagevoltage withwith variousvarious rippleripple factorsfactors waswas appliedapplied toto thethe OIPOIP duringduring thethe test,test, TheThe positivepositive pulsating DC DC voltage voltage with with various various ripple ripple factors factors was was applied applied to the to theOIP OIP during during the test, the respectively. The ripple factor r is the ratio of the amplitude of the AC component UAC to the respectively. The ripple factor r is the ratio of the amplitude of the AC component UACAC to the test,respectively. respectively. The Theripple ripple factor factor r isr theis the ratio ratio of of the the amplitude amplitude of of the the AC componentcomponent UU toto thethe magnitude of the DC component UDC, as is shown in Equation (1). Figure 4 illustrates theAC typical magnitudemagnitude of of the the DC DC component component U UDCDC, ,as as is is shown shown in in Equation Equation (1). (1). Figure Figure 4 4 illustrates illustrates the the typical typical magnitude of the DC component UDC, as is shown in Equation (1). Figure4 illustrates the typical waveformwaveform forfor pulsatingpulsating DCDC voltagevoltage withwith aa rippleripple factorfactor ofof 1.1. InIn thisthis work,work, pulsatingpulsating DCDC voltagesvoltages withwith waveformwaveform forfor pulsating DC DC voltage voltage with with a aripple ripple factor factor of of1. In 1. this In this work, work, pulsating pulsating DC voltages DC voltages with anan rr ofof 1/5,1/5, 1/3,1/3, andand 11 werewere chosenchosen toto investigatinvestigatee thethe breakdownbreakdown voltagevoltage ofof OIPOIP atat pulsatingpulsating DCDC withan r anof r1/5,of 1/5,1/3, 1/3,and and1 were 1 were chosen chosen to toinvestigat investigatee the the breakdown breakdown voltage voltage of of OIP OIP at pulsatingpulsating DCDC voltagevoltage [5].[5]. voltagevoltage [[5].5]. U AC r=UUACAC r r=r== AC (1)(1) U DC (1)(1) UUDDCCDC

Figure 4. Typical pulsating direct current (DC) voltage waveform. FigureFigureFigure 4. 4.4. Typical TypicalTypical pulsating pulsatingpulsating direct directdirect current currentcurrent (DC)(DC) (DC) voltage voltage waveform. waveform.

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3. Breakdown Voltage of Thermally Aged OIP 3. Breakdown Voltage of Thermally Aged OIP Figure 5 depicts the mean value of the breakdown voltage of OIP of different aging states with one standardFigure5 depicts deviation. the meanIn most value cases, of thethe breakdownAC breakdown voltage voltage of OIP was of the different smallest, aging and states pulsating with oneDC ( standardr = 1/5) breakdown deviation. voltage In most the cases, greatest. the AC No breakdown pronounced voltage difference was thewas smallest, found, however, and pulsating in the DCmagnitude (r = 1/5) of breakdown breakdown voltage voltage the among greatest. pulsating No pronounced DC voltage difference (r = 1/5 was and found, 1/3) and however, DC voltage. in the magnitudeThe standard of breakdowndeviation of voltage breakdown among voltage pulsating at pulsating DC voltage DC (andr = 1/5 DC andvoltage 1/3) was and greater DC voltage. than that The standardat AC voltage, deviation regardless of breakdown of the voltagethermal ataging pulsating time DCor temperature. and DC voltage Nevertheless, was greater those than thattrends at ACbecame voltage, inconspicuous regardless when of the paper thermal reached aging its time end or of temperature. life (taken as Nevertheless, DP = 150–200 those[24,25] trends in this became work), inconspicuousi.e., papers aged when for 180 paper days reached at 120 °C its and end 90 of days life at (taken 130 °C. as In DP those = 150–200 aging states, [24,25] minute in this differences work), i.e., ◦ ◦ paperswere observed aged for in 180 breakdown days at 120 voltageC and and 90 corresponding days at 130 C. standard In those deviation aging states, among minute the differencesthree types wereof voltage observed waveforms. in breakdown voltage and corresponding standard deviation among the three types of voltage waveforms.

(a)

(b)

Figure 5. Breakdown voltage of oil-impregnated paper (OIP) after thermal aging with one standard Figure 5. Breakdown voltage of oil-impregnated paper (OIP) after thermal aging with one standard deviation: (a) 120 °C; (b) 130 °C. deviation: (a) 120 ◦C; (b) 130 ◦C.

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EnergiesFigure 20176 presents, 10, 1411 the changing tendency of breakdown voltage against aging time at different6 of 16 voltage waveforms. Before reaching its end of life, the breakdown voltage at pulsating DC voltage Figure 6 presents the changing tendency of breakdown voltage against aging time at different stayed in a relatively high level and did not necessarily decline. Instead, it exhibited some ﬂuctuations voltage waveforms. Before reaching its end of life, the breakdown voltage at pulsating DC voltage with occasional increase versus the aging time regardless of the thermal aging temperature. The stayed in a relatively high level and did not necessarily decline. Instead, it exhibited some fluctuations ﬂuctuationwith occasional in breakdown increase voltage versus at the individual aging time DC regardless or AC voltage of the was thermal also observed. aging temperature. Nevertheless, the changingThe fluctuation tendency in breakdown of breakdown voltage voltage at individual versus aging DC timeor AC at voltage DC and was pulsating also observed. DC voltage withNevertheless, small ripple the factors changing (r = 1/5tendency and 1/3)of breakdown was opposite voltage to versus the trend aging at time AC voltage.at DC and This pulsating changing tendencyDC voltage was especially with small obvious ripple factors for papers (r = 1/5 in and the 1/3) early was or opposite middle stage to the of trend aging, at AC i.e., voltage. papers agedThis for no morechanging than tendency 35 days was at 120 especially◦C and obvious no more for than papers 22 in days the atearly 130 or◦ middleC. For example,stage of aging, in Figure i.e., papers6b, from an agingaged for time no ofmore four than days 35 days to seven at 120 days, °C and the no AC more breakdown than 22 days voltage at 130 °C. increased For example, compared in Figure with 6b, that of thefrom previous an aging aging time state,of four while days to the seven DC breakdowndays, the AC voltagebreakdown and voltage pulsating increased DC breakdown compared with voltage withthat small of ripplethe previous factors aging (r = 1/5state, and while 1/3) the decreased, DC breakdown though voltage they all and stayed pulsating at a relatively DC breakdown high level comparedvoltage withwith small that of ripple unaged factors OIPs. (r = 1/5 All and the 1/3) above decreased, tendencies though became they all inconspicuous stayed at a relatively when high the OIP level compared with that of unaged OIPs. All the above tendencies became inconspicuous when the reached its end of life. It should be noted that the changing tendencies of the AC breakdown voltage at OIP reached its end of life. It should be noted that the changing tendencies of the AC breakdown both temperatures were not in line with that obtained in reference [12], while they were similar to that voltage at both temperatures were not in line with that obtained in reference [12], while they were in referencesimilar to [ 21that]. in reference [21].

(a)

(b)

Figure 6. Breakdown voltage of OIP versus thermal aging time: (a) 120 °C; (b) 130 °C. Figure 6. Breakdown voltage of OIP versus thermal aging time: (a) 120 ◦C; (b) 130 ◦C.

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4.4. DiscussionDiscussion

4.1.4.1. DifferenceDifference inin MagnitudeMagnitude ofof BreakdownBreakdown VoltageVoltage atat DifferentDifferent VoltageVoltage WaveformsWaveforms InIn thisthis section,section, thethe double-compositedouble-composite dielectricdielectric modelmodel isis usedused toto interpretinterpret thethe differencedifference inin thethe magnitudemagnitude ofof breakdownbreakdown voltage voltage of of OIP OIP at at pulsating pulsating DC DC voltage voltage at at each each aging aging state state from from that that at ACat AC or DCor DC voltage. voltage. TheThe insulatinginsulating paper paper contains contains plenty plenty of pores of ﬁlledpores with filled gases. with After gases. degassing After and degassing impregnation, and thoseimpregnation, pores are those ﬁlled pores with insulatingare filled with oil. Theinsulating OIP can oil. be The equivalent OIP can to be a equivalent composite dielectricto a composite with two layers, as modeled in Figure7, where ε , γ , d and ε , γ , d are the equivalent permittivity, dielectric with two layers, as modeled in Figure1 1 7, 1where ε2 1, γ21, d21 and ε2, γ2, d2 are the equivalent conductivity, and thickness of the oil and paper, respectively; R , R , C , and C are their equivalent permittivity, conductivity, and thickness of the oil and paper, respectively;1 2 1 R1, R2 2, C1, and C2 are their resistance and capacitance, respectively; U and U are the voltages applied on insulating oil and equivalent resistance and capacitance, respectively;1 2 U1 and U2 are the voltages applied on insulating insulatingoil and insulating paper, respectively. paper, respectively.

(a) (b)

FigureFigure 7.7. The (a)) double-compositedouble-composite dielectricdielectric modelmodel andand ((bb)) correspondingcorresponding equivalentequivalent circuit.circuit.

The electric field in the OIP is co-determined by permittivity of oil and paper at AC voltage while The electric ﬁeld in the OIP is co-determined by permittivity of oil and paper at AC voltage while it is co-determined by the conductivity at DC voltage. Therefore, the electric field strength in oil and it is co-determined by the conductivity at DC voltage. Therefore, the electric ﬁeld strength in oil and paper, i.e., EOil and EPaper, can be obtained at pulsating DC voltage, which are as follows [19,26]: paper, i.e., EOil and EPaper, can be obtained at pulsating DC voltage, which are as follows [19,26]:  EU22  U Oil ACε DC γ (2) 21dd2  12  21 dd 122 EOil = UAC · + UDC · (2) ε2d1 + ε1d2 γ2d1 + γ1d2

11 EUPaper ACε1  U DC γ1 (3) EPaper = UAC · 21dd 12+ UDC · 21 dd 12 (3) ε2d1 + ε1d2 γ2d1 + γ1d2 AndAnd thethe followingfollowing relationshiprelationship cancan bebe easilyeasily obtained:obtained:

UUUUU012 +=AC + DC (4) U0 = U1 + U2 = UAC + UDC (4) For the convenience of later description, we define EOb and EPb as the dielectric breakdown strengthFor theof oil convenience and paper, respectively. of later description, we define EOb and EPb as the dielectric breakdown strengthFor ofAC oil voltage, and paper, the respectively.electric field strength is inversely proportional to their permittivities for paperFor and AC oil. voltage, At room the temperature, electric field strengthε1/ε2 is about is inversely 1/2. Therefore, proportional most to of their the electric permittivities field strength for paper is andwithstood oil. At by room insulating temperature, oil. Asε 1U/0ε 2increases,is about 1/2.EOil is Therefore, always greater most than of the E electricPaper. Because field strengthEPaper/EOil isis withstoodsmaller than by insulatingEPb/EOb, before oil. AstheU insulating0 increases, oilE Oilbreaksis always down, greater EPaper is than alwaysEPaper smaller. Because thanEPaper EPb/E. EOilOb isis smallermuch smaller than E Pbthan/EOb EPb, before, so the the oil insulatingbreaks down oil breaksprior to down, the paper,EPaper whenis always EOil reaches smaller E thanOb. EPb. EOb is muchFor smaller DC voltage, than EPb the, so electric the oil field breaks strength down prioris inve torsely the paper,proportional when E toOil theirreaches conductivitiesEOb. for oil and Forpaper. DC Without voltage, theconsidering electric fieldthe infl strengthuence isof inversely moisture proportional and other factors, to their γ conductivities1/γ2 is about 100. for oilTherefore, and paper. most Without of the electric considering filed thestre influencength is withstood of moisture by the and paper. other factors,Becauseγ E1Paper/γ2/EisOil about is greater 100. Therefore,than EPb/EOb most, as ofU0 theincreases, electric E filedPaper firstly strength reaches is withstood EPb before by theEOil paper.reaches Because EOb. TheE Paperpaper/EOil thenis greater breaks thandownE Pbprior/EOb ,to as theU0 increases,oil. BecauseEPaper EPbfirstly at DC reaches voltageEPb isbefore a lotE Oilgreaterreaches thanEOb E.Oil The, the paper corresponding then breaks downbreakdown prior to voltage the oil. exceeds Because thatEPb at DCAC voltagevoltage. is a lot greater than EOil, the corresponding breakdown voltageFor exceeds pulsating that DC at ACvoltage, voltage. Equations (2) and (3) can be rewritten as follows using Equations (1) and (4):

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For pulsating DC voltage, Equations (2) and (3) can be rewritten as follows using Equations (1) and (4): U0 rε2 γ2 U0 2r 1 EOil = · + ≈ · + (5) 1 + r ε2d1 + ε1d2 γ2d1 + γ1d2 1 + r 2d1 + d2 d1 + 100d2 U0 rε1 γ1 U0 r 100 EPaper = · + ≈ · + (6) 1 + r ε2d1 + ε1d2 γ2d1 + γ1d2 1 + r 2d1 + d2 d1 + 100d2 When r is very small, i.e., the DC component in pulsating DC voltage is dominant, which is similar to that at DC voltage—the paper breaks down prior to the oil. When r is very large, i.e., the AC component in pulsating DC voltage is dominant, which is similar to the situation for AC voltage, and the oil breaks down prior to the paper. Suppose the DC (r = 0) breakdown voltage of the OIP is UD. When r increases from 0 to infinite, Equations (7) and (8) are permanently satisfied, i.e., the value of Equation (7) is always greater than 0, while the value of Equation (8) is always smaller than 0. In other words, the electric field strength in paper decreases while that in oil increases. If the increment in voltage applied to oil is greater than the decrease in voltage applied to paper, then the breakdown voltage increases and exceed UD. Note, however, that the breakdown voltage does not necessarily increase all the time, as r increases from 0 to infinite, or the AC breakdown voltage is the greatest. It is actually the smallest. Therefore, it is obvious that an r exists for the breakdown voltage to reach a maximum value, and it is greater than the DC breakdown voltage. The maximum value is obtained when Equation (9) is satisfied.

dE 199d U Oil = 2 0 > 2 0 (7) dr (1 + r) (2d1 + d2)(d1 + 100d2)

dEPaper −199d U = 1 0 < 2 0 (8) dr (1 + r) (2d1 + d2)(d1 + 100d2) E E Oil = Ob (9) EPaper EPb In other words, the breakdown voltage reaches its maximum value when the paper and oil break down at the same time. The corresponding r can be obtained with Equations (5), (6), and (9), and it is as follows: (E /E − 100)(2d /d + 1) r ≈ Pb Ob 1 2 (10) (1 − 2EPb/EOb)(d1/d2 + 100)

It is evident from Equation (10) that r is dependent on d1, d2, EOb, and EPb. Additionally, EOb and EPb are not constants for oil and paper of different thicknesses and different voltage waveforms [20]. In other words, it depends on the interaction between d1/d2 and EPb/EOb when the breakdown voltage of OIP reaches its maximum value. In this experiment, the maximum value was observed for an r of 1/5, except the insulating paper that reached its end of life when few differences existed in the breakdown voltage at the three types of voltage waveforms.

4.2. Possible Inﬂuencing Factors for Variation of Breakdown Voltage Versus Aging Time In this section, the variation of breakdown voltage versus aging time is analyzed, including the opposite changing tendencies between AC voltage, and pulsating DC voltage with small ripple factors (r = 1/5 and 1/3) and DC voltage. Possible inﬂuencing factors such as DP, moisture content of paper and oil absorption associated with the pores and cracks of aged paper are investigated. Energies 2017, 10, 1411 9 of 16 Energies 2017, 10, 1411 9 of 16

4.2.1. DP of Paper Figure8 8 presents presents thethe DPDP ofof thethe paper.paper. TheThe first-orderfirst-order kinetickinetic modelmodel waswas usedused toto fitfit thethe DPDP versusversus the aging time, which is shown in Equation (11)(11) asas followsfollows [[27]:27]: 1111 −=ktkt (11)(11) Pt P PPt 0 where P0 andand PPtt areare the the DP DP of of unaged unaged paper paper and and paper paper aged aged for for an an duration duration of of t, trespectively;, respectively; k isk isa constant.a constant. DP DP decreased decreased dramatically dramatically at at both both agin agingg temperatures, temperatures, whereas whereas it it decreased relatively ◦ slowly when approaching about 500. Note Note that that DP DP of of paper paper aged aged for for 180 days at 120 °C,C, and for ◦ 90 days at 130 °C,C, reached a level less than 250. No clear correlation was found, however, between DP and the breakdown voltage. Even when the DP de decreasedcreased from 1265 to less than 400, no marked reduction in breakdown voltage was observed. Therefore, Therefore, the the decrease decrease in in DP DP had had no reason to be considered for the small variation of breakdown vo voltageltage of of OIP, and in turn the breakdown voltage could not be taken as an indicator of the aging aging state state of of paper, paper, which which was was in in accordance accordance with with [22]. [22]. However, this was not applicable to the life-end st stageage of the OIP, because the DP reached a seriously small level. The The structural structural damage damage of of paper paper at at this this stage together with the high level of moisture content was significant significant enough to contribute to th thee sudden decrease of breakdown voltage. This was not in line with publication [12] [12] mainly because of the difference in the moisture level and paper structural damage.

Figure 8. DP of aged paper. Figure 8. DP of aged paper.

4.2.2. Moisture Content 4.2.2. Moisture Content Figure 9 shows the moisture content in OIP. It fluctuated during the aging process and increased Figure9 shows the moisture content in OIP. It fluctuated during the aging process and increased sharply when the paper nearly reached its end of life. Moisture content was considered to influence sharply when the paper nearly reached its end of life. Moisture content was considered to influence the breakdown strength of paper [28]. That might be the reason why the obtained results were not all the breakdown strength of paper [28]. That might be the reason why the obtained results were not all in line with those obtained in [12], as the moisture content for some of the aged papers were relatively in line with those obtained in [12], as the moisture content for some of the aged papers were relatively higher than that in [12]. However, it could not explain why some OIPs with a relative high level of higher than that in [12]. However, it could not explain why some OIPs with a relative high level of moisture content had a relatively high AC breakdown voltage. For example, for paper subject to moisture content had a relatively high AC breakdown voltage. For example, for paper subject to thermal aging for two and 60 days at 120 °C and for two, seven, and 10 days at 130 °C, the breakdown thermal aging for two and 60 days at 120 ◦C and for two, seven, and 10 days at 130 ◦C, the breakdown voltage, especially the AC breakdown voltage, did not decline much, or even increased compared voltage, especially the AC breakdown voltage, did not decline much, or even increased compared with that of unaged paper, and was even higher than some other papers whose moisture level was with that of unaged paper, and was even higher than some other papers whose moisture level was relatively low. This implies that there could be some other factors offsetting the influence of moisture relatively low. This implies that there could be some other factors offsetting the influence of moisture content on AC breakdown voltage of those thermally aged paper. content on AC breakdown voltage of those thermally aged paper.

Energies 2017, 10, 1411 10 of 16 Energies 2017, 10, 1411 10 of 16

Energies 2017, 10, 1411 10 of 16

Figure 9. Moisture content of OIP. Figure 9. Figure 9. MoistureMoisture content of of OIP. OIP. 4.2.3. Oil Absorption Ability of Paper Associated with Pores and Cracks 4.2.3.4.2.3. Oil Oil Absorption Absorption Ability Ability of of Paper Paper Associated Associated withwith Pores and and Cracks Cracks Insulating paper consists of large numbers of cellulose fibers and it has a lot of pores, the InsulatingInsulating paper paper consists consists of largeof large numbers numbers of cellulose of cellulose ﬁbers fibers and itand has it ahas lot ofa lot pores, of pores, the diameter the diameter of which ranges from tens of nanometers to thousands of nanometers. In this paper, the of whichdiameter ranges of which from ranges tens of from nanometers tens of nanometers to thousands to ofthousands nanometers. of nanomete In this paper,rs. In this the paper, mesoporous the mesoporous distribution of insulating paper was measured with nitrogen gas adsorption and distributionmesoporous of insulating distribution paper of insulating was measured paper with was nitrogenmeasured gas with adsorption nitrogen and gas desorptionadsorption method,and desorption method, and its macropores were observed with SEM to associate the oil absorption anddesorption its macropores method, were and observed its macropores with SEM were to obse associaterved with the oilSEM absorption to associate ability the withoil absorption some of the ability with some of the changing tendency of the breakdown voltage. For simple discussion and changingability tendencywith some of of the the breakdown changing tendency voltage. Forof the simple breakdown discussion voltage. and analysisFor simple in thisdiscussion paper, and papers analysis in this paper, papers aged at 130 °C are selected as an example, and are the same as those agedanalysis at 130 ◦inC this are paper, selected papers as an aged example, at 130 and °C are are selected the same as asan those example, aged and at 120are ◦theC. same as those agedaged at 120 at 120 °C. °C. The nitrogen gas adsorption and desorption measurements were conducted to examine the porous TheThe nitrogen nitrogen gas gas adsorption adsorption and and desorptiondesorption measurements were were conducted conducted to toexamine examine the the nature of the insulating paper. The pore-size distribution and pore volume were obtained following porousporous nature nature of ofthe the insulating insulating paper. paper. TheThe pore-sizepore-size didistributionstribution and and pore pore volume volume were were obtained obtained the Barrett-Joyner-Halenda (BJH) method [29], and the speciﬁc surface area was obtained with the followingfollowing the the Barrett-Joyner-Halenda Barrett-Joyner-Halenda (BJH) (BJH) methodmethod [29],[29], andand the the specific specific surface surface area area was was obtained obtained Brunauer-Emmett-Teller (BET) method [30]. Figure 10 shows the typical nitrogen gas adsorption and withwith the the Brunauer-Emmett-Teller Brunauer-Emmett-Teller (BET)(BET) methodmethod [30]. FigureFigure 10 10 shows shows the the typical typical nitrogen nitrogen gas gas desorption isotherm of insulating paper. adsorptionadsorption and and desorption desorption isotherm isotherm of of insulating insulating paper.

Figure 10. Typical nitrogen adsorption and desorption isotherm of insulating paper. Figure 10. Typical nitrogen adsorption and desorption isotherm of insulating paper. The FigureBJH analyses 10. Typical showed nitrogen that adsorption the insulating and desorp papertion exhibited isotherm no of pore insulating sizes paper.of less than 15 nanometers,The BJH analysesand there showedwere various that most the probable insulating pore paper sizes exhibitedfor papers of no different pore sizes aging of states, less as than 15 nanometers,shownThe BJH in Figure analyses and 11. there For showed were OIPs variouswith that aging the most timeinsulating probable of 0, 2, 7, porepaper 10, 22, sizes exhibited and for 90 papersdays, no they pore of differenthad sizes marked of aging less probable states, than as15 shownnanometers,pore in sizes, Figure and and 11 there the. For corresponding were OIPs various with aging peakmost time value probable of of 0, 2,d Vpo 7,/d 10,reD sizeswas 22, andgreater for 90papers days,than ofthat they different of had other marked aging aged probablestates,OIPs, as poreshownwhich sizes, in Figureindicates and the 11. they correspondingFor had OIPs large with pore aging peak size time value and ofvolume, of 0, d2,V 7,/d which10,D 22,was is and consistent greater 90 days, than with they that the had ofresults othermarked presented aged probable OIPs, whichporein sizes,Figure indicates and 12. The theythe BETcorresponding had specific large pore surface peak size area andvalue of volume, insu of dlatingV/d whichD paper was is ofgreater consistent different than aging with that thestates of resultsother is presented aged presented OIPs, which indicates they had large pore size and volume, which is consistent with the results presented

in Figure 12. The BET specific surface area of insulating paper of different aging states is presented

Energies 2017, 10, 1411 11 of 16

Energies 2017, 10, 1411 11 of 16 in Figure 12. The BET speciﬁc surface area of insulating paper of different aging states is presented in FigureEnergies 13201713.. , It 10 exhibited, 1411 a ﬂuctuationfluctuation during the thermal aging process, following almost 11 the11 of of same16 16 variation tendency with that ofof porepore volume.volume. inin FigureFigure 13. It exhibited aa fluctuationfluctuation duringduring the the thermal thermal aging aging process, process, following following almost almost the the same same variation tendency with thatthat ofof porepore volume.volume.

Figure 11. Pore distribution of paper thermally aged for various durations at 130 °C. Figure 11. Pore distribution of paper thermally aged for various durations at 130 ◦C. Figure 11. PorePore distributiondistribution of of paper paper thermall thermallyy aged aged for for various various durations durations at at 130 130 °C. °C.

Figure 12. Pore volume of paper thermally aged for various durations at 130 °C.◦ FigureFigure 12. 12. Pore Pore volume volume of of paper paper thermally thermally agedag ageded forfor variousvarious durations at at 130 130 °C. °C.C.

Figure 13. Specific surface area of paper thermally aged for various durations at 130 °C. Figure 13. Specific surface area of paper thermally aged for various durations at 130 °C. Figure 13. Speciﬁc surface area of paper thermally aged for various durations at 130 ◦C. Figure 13. Specific surface area of paper thermally aged for various durations at 130 °C.

Energies 2017, 10, 1411 12 of 16

Energies 2017, 10, 1411 12 of 16 Figure 14 shows typical SEM images of insulating paper thermally aged for various durations at 130 ◦C.Figure Macropores 14 shows or typical cracks SEM with images a diameter of insulating of several paper micrometers thermally began aged tofor appear various in durations insulating at paper130 °C. after Macropores 22 days or of cracks thermal with aging. a diameter The diameter of several of micrometers pores and cracks began exhibited to appear an in increasinginsulating tendencypaper after along 22 withdays theof thermal increase ofaging. thermal The agingdiamet time.er of pores and cracks exhibited an increasing tendency along with the increase of thermal aging time.

(a) (b)

(e) (f)

Figure 14. Scanning electron microscopy (SEM) images of paper thermally aged for various durations Figure 14. Scanning electron microscopy (SEM) images of paper thermally aged for various durations at 130 °C. (a) 0 day; (b) 10 days; (c) 22 days; (d) 35 days; (e) 60 days; (f) 90 days. at 130 ◦C. (a) 0 day; (b) 10 days; (c) 22 days; (d) 35 days; (e) 60 days; (f) 90 days.

The increased oil absorption associated with the pores and cracks of paper had different effects on theThe breakdown increased oilvoltage absorption of OIP associatedat DC and withAC voltage, the pores which and cracksis discussed of paper below. had different effects on the breakdown voltage of OIP at DC and AC voltage, which is discussed below. As is well-known, the relation between the electric field strength E(l) and the voltage applied to As is well-known, the relation between the electric ﬁeld strength E and the voltage applied to the OIP sample U0 can be expressed as [12]: (l) the OIP sample U0 can be expressed as [12]: UEdl0  l (12) Z  U0 = E(l)dl (12) where l is the distance at the direction of electric field. For simplicity, EOil and EPaper are considered to be uniform. When the applied voltage is AC voltage, the relationship between the both can be written as [22]:

EOil  2  (13) EPaper 1

Energies 2017, 10, 1411 13 of 16

where l is the distance at the direction of electric ﬁeld. For simplicity, EOil and EPaper are considered to be uniform. When the applied voltage is AC voltage, the relationship between the both can be written as [22]: E ε Oil = 2 (13) EPaper ε1 Combining (12) and (13) yields the electric ﬁeld in oil and paper as [12]:

U0 EOil = (14) ε1 ε1 1 − lo + l ε2 ε2

U0 EPaper = (15) ε2 − 1 lo + l ε1 where lo is the total equivalent thickness of oil along the electric field. For the case that more oil is absorbed by the paper, i.e., the increased lo, it causes the decrease of electric field strength, this should obviously create better electric field distribution in the OIP. Consequently, the breakdown voltage increases. In the case of applying a DC voltage, (13) can be rewritten as [6]:

E γ Oil = 2 (16) EPaper γ1

Combining (12) and (16), yields the electric ﬁeld in oil and paper as [12]:

U0 Eoil = (17) γ1 γ1 1 − lo + l γ2 γ2

U0 EPaper = (18) γ2 − 1 lo + l γ1

When more oil is contained in the paper, i.e., lo is increased, the electric field in oil and paper increase resultantly. This leads to a decreased breakdown voltage. It is therefore seen that the application of DC voltage make the breakdown voltage develop in an opposite fashion to that of AC voltage. For the OIP samples aged for less than 35 days, the increased pore volume and specific surface area were expected to have a higher capacity of oil absorption. This would result in quite different AC and DC breakdown voltage values upon the case that more oil would be absorbed into the insulating paper. This should account for that the AC breakdown voltage of OIPs aged for two, seven, 10, and 22 days did not drop much, or even increased a bit compared with that of unaged OIPs, although the moisture content in some of those OIP samples remained relatively high at around 1.8%. However, as the oil absorbed had opposite effects on breakdown voltage at AC and DC voltage, the changing tendencies of DC voltage and pulsating DC voltage with small ripple factors (r = 1/5 and 1/3) were different from that of AC voltage. For example, from an aging time of four days to seven days, the AC breakdown voltage increased compared with that of its previous aging state while the DC breakdown voltage and pulsating DC breakdown voltage with small ripple factors (r = 1/5 and 1/3) decreased. For the OIP samples aged for 35 and 60 days, mesopores pore size and volume in insulating paper were not the dominant factors influencing breakdown voltage. Instead, the influence of macropores or cracks of cellulose fibers were considered, as shown in Figure 14. They were filled with oil, improving the electric field distribution in OIP samples at AC voltage. Damage to paper structure might reduce the breakdown voltage, whereas the improvement in electric field distribution somewhat balanced this effect. Consequently, the AC breakdown voltage showed limited reduction compared with that of Energies 2017, 10, 1411 14 of 16 unaged OIPs. However, the breakdown voltage at pulsating DC with small ripple factors (r = 1/5 and 1/3) and DC voltage exhibited a greater decrease compared with that of unaged OIPs, as the oil in paper had adverse effects on it. For the OIP samples aged for 90 days, DP of paper was very small (around 160) and the structure of paper was seriously damaged in this state. Therefore, the breakdown voltage reduced sharply, regardless of the pores and cracks that were filled with oil. In this aging state, the damage to paper structure became the dominant factor influencing the breakdown voltage. It should also be noted that the moisture content was very high (about 2%) at this aging state, which also contributed to the decrease of breakdown voltage.

5. Conclusions This paper investigates the breakdown voltage and its influencing factors of thermally aged OIP at pulsating DC voltage. The main conclusions are summarized as follows. Breakdown voltage at pulsating DC voltage and DC voltage was far greater and had a greater standard deviation than AC breakdown voltage. It was co-determined by the electric field distribution in paper and oil, and their breakdown strengths. The analysis results by the double-composite dielectric model showed that a ripple factor r existed for the breakdown voltage to reach its maximum value. The difference in magnitude of breakdown voltage among pulsating DC (r = 1/5 and 1/3) and DC voltage, however, was not so pronounced. The breakdown voltage of aged OIP at pulsating DC voltage fluctuated during the thermal aging process as the AC breakdown voltage did, but showed a somewhat opposite changing tendency, especially for the breakdown voltage at pulsating DC voltage with small ripple factors (r = 1/5 and 1/3) in the early and middle stage of aging. The moisture content, oil absorption ability associated with pores and cracks of paper, and the damage to paper structure all contributed to the variation of the breakdown voltage versus aging time with different weights of importance as the aging state of paper varied. For OIPs in the early or middle stage of aging, the variation in breakdown voltage versus aging time was considered to be influenced by moisture content and the oil absorption related to the mesopore volume of paper. The oil filled the pores and improved the electric field distribution at AC voltage, and thus had a positive effect on the AC breakdown voltage, while it had an adverse effect on the DC breakdown voltage. Therefore, the breakdown voltage at pulsating DC voltage with small ripple factors (r = 1/5 and 1/3) and DC voltage showed an opposite changing tendency compared with that at AC voltage. For OIPs in the middle-late stage of aging, the macropores or cracks between cellulose fibers rather than the mesopores were considered to have influenced the breakdown voltage of OIP together with moisture content and structural damage of paper. For OIPs that reached its end of life, the damage to the paper structure and the high level of moisture were the dominant influencing factors. Both factors resulted in the sharp decrease of breakdown voltage at the three types of voltage waveforms.

Acknowledgments: The authors thank the National Natural Science Foundation of China (Grant No. 51425702) and the 111 Project of China (Grant No. B08036) for the financial support provided. Author Contributions: The research work presented in this paper was collaboratively conducted by all authors. Feipeng Wang and Jian Li designed the experiment; Jing Zhang carried out the experiments with Hehuan Ran and Xudong Li; Jing Zhang analyzed the test results with Feipeng Wang and Qiang Fu; Jing Zhang wrote this paper; Feipeng Wang revised the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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