Experimental Study on Breakdown Characteristics of Transformer Oil Inﬂuenced by Bubbles

energies

Article Experimental Study on Breakdown Characteristics of Transformer Oil Inﬂuenced by Bubbles

Chunxu Qin 1,*, Yan He 1, Bing Shi 1, Tao Zhao 2 ID , Fangcheng Lv 2 and Xiangrui Cheng 2 1 School of Electrical Engineering, Northeast Electric Power University, No. 169 Changchun Road, Jilin 132012, China; [email protected] (Y.H.); [email protected] (B.S.) 2 Hebei Provincial Key Laboratory of Power Transmission Equipment Security Defense, North China Electric Power University, No. 619 North of Yonghua Street, Baoding 071003, China; [email protected] (T.Z.); [email protected] (F.L.); [email protected] (X.C.) * Correspondence: [email protected]

Received: 30 January 2018; Accepted: 9 March 2018; Published: 13 March 2018

Abstract: Bubbles will reduce the electric strength of transformer oil, and even result in the breakdown of the insulation. This paper has studied the breakdown voltages of transformer oil and oil-impregnated pressboard under alternating current (AC) and direct current (DC) voltages. In this paper, three types of electrodes were applied: cylinder-plan electrodes, sphere-plan electrodes, and cone-plan electrodes, and the breakdown voltages were measured in both no bubbles and bubbles. The sphere-sphere electrodes were used to study the breakdown voltage of the oil-impregnated pressboard. The results showed that under the inﬂuence of bubble, the breakdown voltage of the cylinder-plan electrode dropped the most, and the breakdown voltage of the cone-plan electrode dropped the least. The bubbles motion was the key factor of the breakdown. The discharge types of the oil-impregnated pressboard were different with bubbles, and under DC, the main discharge type was ﬂashover along the oil-impregnated pressboard, while under AC, the main discharge type was breakdown through the oil-impregnated pressboard.

Keywords: breakdown voltage; transformer oil; pressboard; bubbles; AC; DC

1. Introduction Mineral transformer oil is the main dielectric liquid for engineering application, and the oil and pressboard insulation system is widely used in high voltage electric power equipment manufacturing [1]. High-voltage DC projects are being developed in China now [2], and the AC and DC high voltages will be applied to the insulation of the converter transformer. The primary insulation of the converter transformer is also oil and pressboard insulation [3]. Bubbles will decrease the electrical strength of the oil-pressboard insulation, and will cause a partial discharge in the oil, ﬁnally resulting in the insulation breaking down due to the accumulation of the bubbles. Therefore, the bubbles are a key factor that affects the breakdown voltages of the oil-pressboard insulation [4]. Since the liquid discharge is so complex, there is no physical interpretation for all time and space stages of the development of the liquid discharge [5]. There is also no absolutely pure oil in engineering. Due to manufacturing, transportation, and other reasons, there will be some bubbles and particles in the oil. The generation progress of moisture in the mineral oil-impregnated pressboard has been studied previously. The oil-impregnated pressboard contains moisture, which will be released to the oil generally and, especially under high temperatures, the moisture is released quickly from the pressboard into the oil in the form of gas bubbles [6]. Researchers have studied the bubble-generating mechanism in oil-pressboard insulation of transformers. Heinrichs [7] discussed gas-evolving mechanisms in oil, pressboard, and combinations of oil and pressboard. Some models approximating the hot-spot conﬁguration were designed to study the critical mechanisms under conditions that were approaching

Energies 2018, 11, 634; doi:10.3390/en11030634 www.mdpi.com/journal/energies Energies 2018, 11, x FOR PEER REVIEW 2 of 11 approximatingEnergies 2018, 11, 634 the hot-spot configuration were designed to study the critical mechanisms under2 of 11 conditions that were approaching service overloads. A maximum transformer overload temperature was recommended from the results of this study. Kaufmann and McMillen also performed similar service overloads. A maximum transformer overload temperature was recommended from the studies [8]. Oommen [9] has given an initial mechanism of bubble evolution and an equation to obtain results of this study. Kaufmann and McMillen also performed similar studies [8]. Oommen [9] bubble evolution temperatures. Koch and Tenbohlen [10] have studied the formation of bubbles in has given an initial mechanism of bubble evolution and an equation to obtain bubble evolution oil-paper insulating systems. The results showed that bubbles greatly reduce the dielectric withstand temperatures. Koch and Tenbohlen [10] have studied the formation of bubbles in oil-paper insulating strength of the oil. Water accelerates the aging of the oil-paper insulation, decreases its dielectric systems. The results showed that bubbles greatly reduce the dielectric withstand strength of the oil. strength, and generates bubbles at high temperatures. The bubbles will be harmful to transformer Water accelerates the aging of the oil-paper insulation, decreases its dielectric strength, and generates operation if the water content in the pressboard is above 2%. Przybylek et al. [11–13] have studied bubbles at high temperatures. The bubbles will be harmful to transformer operation if the water that it is easier to generate the bubbles in ageing paper than in new paper at a high temperature. content in the pressboard is above 2%. Przybylek et al. [11–13] have studied that it is easier to generate Perkasa et al. [14] have studied the evolution of bubbles in vegetable oil-impregnated pressboard. the bubbles in ageing paper than in new paper at a high temperature. Perkasa et al. [14] have studied The behavior of bubbles plays an important role in the breakdown of the dielectrics. Sharbaugh the evolution of bubbles in vegetable oil-impregnated pressboard. et al. [15] suggested that breakdown occurs in the bubbles and the bubbles may be produced by the The behavior of bubbles plays an important role in the breakdown of the dielectrics. electronic progress. Ogata et al. [16] have found that the bubbles’ size will decrease with the strength Sharbaugh et al. [15] suggested that breakdown occurs in the bubbles and the bubbles may be produced of the electric field, and the generation of an electrohydrodynamic (EHD) flow of the liquid will affect by the electronic progress. Ogata et al. [16] have found that the bubbles’ size will decrease with the not only the movement of the bubble, but also the bubble evolution mechanism. Hara et al. [17] have strength of the electric field, and the generation of an electrohydrodynamic (EHD) flow of the liquid will studied the thermal bubble deformation and indicated that the breakdown voltages are close to the affect not only the movement of the bubble, but also the bubble evolution mechanism. Hara et al. [17] bubble behavior with pulse voltages. Seok et al. [16] have indicated that the bubbles’ behavior was have studied the thermal bubble deformation and indicated that the breakdown voltages are close to affected by a 60 Hz electric field and pressure, and the groove generated the bubbles’ stream. In [18], the bubble behavior with pulse voltages. Seok et al. [16] have indicated that the bubbles’ behavior was Hara and Kubuki have suggested that a partial discharge might occur in the bubble before affected by a 60 Hz electric field and pressure, and the groove generated the bubbles’ stream. In [18], breakdown, if the bubble size is as large as 0.5 mm. In [19], Hara et al. have studied that the gradient Hara and Kubuki have suggested that a partial discharge might occur in the bubble before breakdown, force and Maxwell stress strongly affect the bubble dynamics and bubble shape in the gap, and the if the bubble size is as large as 0.5 mm. In [19], Hara et al. have studied that the gradient force and electric forces that are applied on the bubble lead to a lesser effect of the bubble on the breakdown Maxwell stress strongly affect the bubble dynamics and bubble shape in the gap, and the electric forces voltage. that are applied on the bubble lead to a lesser effect of the bubble on the breakdown voltage. From the above discussion, the oil-paper insulation can generate the bubbles, which will From the above discussion, the oil-paper insulation can generate the bubbles, which will decrease decrease the electrical strength of the insulation. Most studies have been aimed at the bubble- the electrical strength of the insulation. Most studies have been aimed at the bubble-generating generating mechanism and bubble behavior in the discharge of liquid nitrogen and helium, yet, there mechanism and bubble behavior in the discharge of liquid nitrogen and helium, yet, there are few are few breakdown characteristics of transformer oil with the bubbles. Therefore, in this work, breakdown characteristics of transformer oil with the bubbles. Therefore, in this work, breakdown breakdown voltages of transformer oil and the oil-impregnated pressboard with the bubbles were voltages of transformer oil and the oil-impregnated pressboard with the bubbles were studied under studied under 50 Hz AC and positive DC power supplies. 50 Hz AC and positive DC power supplies. 2. Experimental Methods 2. Experimental Methods

2.1. Experiment Experimentalal System The experimental system included the power supply (50 Hz AC and DC), oscilloscope, vacuum test chamber chamber,, and high-speedhigh-speed camera, as shown in Figure 11.. WhereWhere 11 isis thethe 5050 HzHz powerpower frequencyfrequency testing transformer, transformer, 2 2 and and 5 5 are protected resistor resistors,s, 3 3 is is a HV diode, 4 4 is a HV capacitor, 6 is the potential divider,divider, 77 isis the the oscilloscope, oscilloscope, 8 8 is is a syringe,a syringe, 9 is9 ais bushing, a bushing, 10 is10 the is the electrode electrode system system that that was wasimmersed immersed in the in oilthe tank oil tank with with dimensions dimensions of 35 of cm35 cm× 20× 20 cm cm× ×20 20 cm, cm, 1111 isis thethe vacuum test chamber (the dimensionsdimensions ofof the vacuumvacuum testtest chamberchamber areare 60 60 cm cm× × 50 cm × 4040 cm), cm), and and 12 12 is is high high-speed-speed camera, camera, and the model of the high high-speed-speed camera camera is is FASTEC FASTEC-IL5,-IL5, produced by Rocky Mountain Mountain High High Speed, Speed, with a frame rate of 1000 frames per second.

2 3 5 9

1 6 4 7 12 10

8 11

Figure 1. ThThee schematic schematic diagram diagram of the experimental system system..

Energies 2018, 11, 634 3 of 11

TheEnergies test 2018 oil, 11 was, x FOR KI PEER 25X REVIEW transformer oil, and the main characteristics are shown3 of in 11 Table1. The size of the oil-impregnated pressboard is 50 mm × 35 mm, and the thickness of the pressboard is 1 mm. TheThe pressboard test oil was was KI 25X soaked transformer in the oiloil, forand morethe main than characteristics 24 h, and are then shown the oilin Table in the 1. pressboardThe reachedsize the of saturation the oil-impregnated state. From pressboard the Oommen is 50 mm curve × 35 mm [20, ],and the the water thickness content of the of pressboard the oil-impregnated is 1 mm. The pressboard was soaked in the oil for more than 24 h, and then the oil in the pressboard pressboardreached is 6%. the saturation The size ofstate. the From electrodes the Oommen is shown curve as [20], Figure the water2, and content the material of the oil- ofimpregnated the electrodes is brass. Thepressboard distribution is 6%. T ofhe thesize electricof the electrodes field will is shown influence as Figure the bubbles’2, and the movement,material of the so electrodes we designed is the cone-planbrass. electrodes The distribution to study of the the electric extremely field will non-uniform influence the field,bubbles and’ movement, as is known, so we designed in the transformer the insulation,cone- aplan more electrodes uniform to study electric the extremely field is thenon- typicaluniform field, one, and so weas is selectknown, the in the other transformer two electrode systemsinsulation, (cylinder-plan a more uniform electrodes electric and field sphere-plan is the typical electrodes) one, so we select in order the other to studytwo electrode the relative systems uniform (cylinder-plan electrodes and sphere-plan electrodes) in order to study the relative uniform electric electric fields. The sphere-sphere electrodes (a typical relative uniform electric field) was used for the fields. The sphere-sphere electrodes (a typical relative uniform electric field) was used for the investigationinvestigation of the of breakdown the breakdown voltage voltage of of the the pressboard. pressboard.

(a) (b)

Figure 2. The schematic diagram of the electrodes (mm). (a) cylinder-plan electrodes; (b) cone-plan Figure 2.electrodes;The schematic (c) sphere diagram-plan electrodes; of the and, electrodes (d) sphere (mm).-sphere (a electrodes) cylinder-plan used for electrodes; the investigation (b) cone-planof electrodes;the breakdown (c) sphere-plan voltage electrodes;of the pressboard. and, (d) sphere-sphere electrodes used for the investigation of the breakdown voltage of the pressboard. Table 1. Characteristics of the tested oils.

Dielectri Moisture ViscosityTable 1. (mmCharacteristics2/s), Flash of the tested oils. Acid Number Tan δ (90 °C, 50 Hz) c Liquid (ppm) 40 °C Point (°C) (mg∙KOH/g) DielectricKI25X Moisture≤40 10.13Viscosity 142Flash Point 0.0006Tan δ (90 ◦C, Acid0.05 Number Liquid (ppm) (mm2/s), 40 ◦C (◦C) 50 Hz) (mg·KOH/g) 2.2. TestKI25X Method ≤40 10.13 142 0.0006 0.05

2.2.1. The Breakdown of the Oil 2.2. Test Method The tests were carried out at a high voltage experimental hall which is about 40 m × 20 m × 6 m, and the test method is as follows: 2.2.1. The Breakdown of the Oil 1. Before voltage was applied, about 7 L of initial oil was put into the electrode systems, and the The testsoil’s were surface carried is 5 cm out above ata the high electrodes. voltage After experimental 30 min’ standing, hall which the bubbles is about were 40 injected m × 20by ma × 6 m, and the testsyringe method at isa speed as follows: of three bubbles per second, and the bubbles linked the two electrodes. The bubbles were injected all the time until the breakdown occurred. 1. Before2. T voltagehen, AC or was DC applied,voltages were about applied 7 L ofon initialthe electrodes oil was at putthe rate into of the voltage electrode rise of 1 systems, kV/s until and the oil’s surfacebreakdown is 5 of cm theabove oil. We thethenelectrodes. waited 0.5 h. After 30 min’ standing, the bubbles were injected by3. a syringeWe changed at a speedthe oil and of threerepeated bubbles the tests. per second, and the bubbles linked the two electrodes. The4. bubblesAll of the were discharg injectedes were all therecorded time untilby the the high breakdown-speed camera. occurred. 2. Then, AC or DC voltages were applied on the electrodes at the rate of voltage rise of 1 kV/s until breakdown of the oil. We then waited 0.5 h. 3. We changed the oil and repeated the tests. 4. All of the discharges were recorded by the high-speed camera. Energies 2018, 11, 634 4 of 11

2.2.2. The Breakdown of the Oil-Impregnated Pressboard EnergiesThe sphere-sphere2018, 11, x FOR PEER electrodesREVIEW were selected to study the breakdown voltages4 of 11 of the oil-impregnated2.2.2. The Breakdown pressboard. of the BeforeOil-Impregnated the tests, Pressboard the oil-impregnated pressboard was put between the two electrodes and we injected the oil into the electrode system. The surface of the oil is one centimeter The sphere-sphere electrodes were selected to study the breakdown voltages of the oil- taller than the electrodes, so the entire discharge system was immersed in oil. impregnated pressboard. Before the tests, the oil-impregnated pressboard was put between the two 1. electrodesAC or DC and voltages we injected were the applied oil into on the the electrode electrodes system. and The the surface rate of of voltage the oil riseis one was centimete 1 kV/sr until taller than the electrodes, so the entire discharge system was immersed in oil. breakdown. We changed the pressboard, mixed the oil, and then waited 0.5 h. 2. 1.Using AC aor vacuum DC voltages to reduce were applied air pressure on the to electrodes 100 Pa, theand bubblesthe rate of appeared voltage rise on was the oil-impregnated1 kV/s until pressboard.breakdown. AC We or c DChanged voltages the pressboard, were applied mixed to the the oil electrodes, and then wait anded the 0.5rate h. of voltage rise was 2. Using a vacuum to reduce air pressure to 100 Pa, the bubbles appeared on the oil-impregnated 1 kV/s until breakdown of the insulation. pressboard. AC or DC voltages were applied to the electrodes and the rate of voltage rise was 1 3. We mixedkV/s until the breakdown oil, changed of the the insulation oil-impregnated. pressboard, and repeated steps 1 and 2. 4. 3.All ofWe the mixed discharges the oil, changed were recorded the oil-impregnated by the high-speed pressboard, camera. and repeated steps 1 and 2. 4. All of the discharges were recorded by the high-speed camera. 3. Results and Discussion 3. Results and Discussion 3.1. AC Breakdown Voltages of the Transformer Oil 3.1. AC Breakdown Voltages of the Transformer Oil From Figure3a, the breakdown voltages of the cone-plan electrodes without bubbles are more than theFrom breakdown Figure 3a, voltages the breakdown of the other voltages two of types the con electrodes,e-plan electrodes and the without breakdown bubbles voltages are more of the than the breakdown voltages of the other two types electrodes, and the breakdown voltages of the sphere-plan electrodes are similar to the cylinder-plan electrodes. Dissado [21] and Peppas [22] sphere-plan electrodes are similar to the cylinder-plan electrodes. Dissado [21] and Peppas [22] indicated that the statistical analyses were suitable for the dielectric breakdown. Based on these, indicated that the statistical analyses were suitable for the dielectric breakdown. Based on these, we we calculatedcalculated some statistics statistics of of the the breakdown breakdown voltages. voltages.

70 70 cone-plan sphere-plan 60 60 cylinder-plan

50 50

40 40

30 30

20 20 cone-plan 10 sphere-plan 10

cylinder-plan AC Breakdown Voltages with Bubbles/ kV AC Breakdown Voltages without Bubbles/ kV

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 The number of breakdown times The number of breakdown times (a) (b)

Figure 3. The AC breakdown voltages (effective values) of the transformer oil. (a) The AC breakdown Figure 3. The AC breakdown voltages (effective values) of the transformer oil. (a) The AC breakdown voltages of the oil without bubbles; and, (b) the AC breakdown voltages of the oil with bubbles. voltages of the oil without bubbles; and, (b) the AC breakdown voltages of the oil with bubbles. From Table 2, the average value of the breakdown voltages of the cone-plan electrodes is 60.9 kV,From the Table average2, the value average of the value breakdown of the voltage breakdowns of the voltages sphere- ofplan the electrodes cone-plan is electrodes49.9 kV, and is the 60.9 kV, the averageaverage valuevalue of of the the breakdown breakdown voltages voltages of the of thecylinder sphere-plan-plan electrodes electrodes is 48.8 is kV. 49.9 kV, and the average value of theIt can breakdown be seen that voltages the system of the with cylinder-plan the most non electrodes-uniform field is 48.8 distribution kV. had the greatest withstandIt can be seenvoltage. that As the it w systemas known, with the the breakdown most non-uniform theory of the field transformer distribution oil is hada discharge the greatest withstandbridge voltage.mechanism As, it and was according known, the to breakdownthis theory, theorythe breakdown of the transformer voltage is oildetermined is a discharge by the bridge mechanism,formation and of the according bridge. Under to this the theory,most uniform the breakdown field distribution, voltage there is is determined a corona that by will the disturb formation of thethe bridge. discharge Under bridge thepath, most and it uniform is more difficult field distribution, to form the breakdown there is path a corona than under that willthe uniform disturb the fields. Therefore, the cone-plan electrodes have the highest breakdown voltages. discharge bridge path, and it is more difficult to form the breakdown path than under the uniform From Figure 3b, the breakdown voltages of the cone-plan electrodes with bubbles are much more fields. Therefore, the cone-plan electrodes have the highest breakdown voltages. than the breakdown voltages of the other two types of electrodes, and the breakdown voltages of the sphereFrom Figure-plan electrodes3b, the breakdown are more voltagesthan the ofcylinder the cone-plan-plan electrodes. electrodes The with average bubbles value are muchof the more thanbreakdown the breakdown voltages voltages of the ofcone the-plan other electrodes two types is of49.4 electrodes, kV, the average and the value breakdown of the breakdown voltages of the sphere-planvoltages electrodesof the sphere are-plan more electrodes than the is cylinder-plan 25.3 kV and the electrodes. average value The of average the breakdown value of voltages the breakdown of voltagesthe cylinder of the- cone-planplan electrodes electrodes is 18.0 kV. is 49.4 kV, the average value of the breakdown voltages of the

Energies 2018, 11, 634 5 of 11 sphere-plan electrodes is 25.3 kV and the average value of the breakdown voltages of the cylinder-plan electrodes is 18.0 kV.

Table 2. Statistics of the AC breakdown voltages (BDV).

Electrode Systems Cone-Plan Sphere-Plan Cylinder-Plan Energies 2018, 11, x FOR PEER REVIEWMean BDV (kV) 49.4 25.3 18 5 of 11 Std. Deviation (kV) 3.8 5.5 2.3 With bubbles TableMin 2. BDV Statistics (kV) of the AC breakdown 40 voltages (BDV) 16.8. 13.4 ElectrodeMax Systems BDV0 (kV)Cone 53-Plan Sphere-Plan 32.8 Cylinder-Plan 20.8 MeanMean BDV BDV (kV) (kV) 49.4 60.9 25.3 49.918 48.8 Std.Std. Deviation Deviation (kV) (kV) 3.86.1 5.5 9.22.3 5.1 WithoutWith bubbles bubbles MinMin BDV BDV (kV) (kV) 40 47 16.8 32.513.4 40 MaxMax BDV BDV0 (kV) (kV) 53 68 32.8 5920.8 57.1 Mean BDV (kV) 60.9 49.9 48.8 Std. Deviation (kV) 6.1 9.2 5.1 From theWithout above, bubbles with the influence of the bubbles, the breakdown voltages of the cone-plan Min BDV (kV) 47 32.5 40 electrodes have a 18.9% decreaseMax on BDV average; (kV) the bubbles68 have led59 to a 49.1%57.1 decrease in the value of the average breakdown voltage for the sphere-plan electrodes; and, the bubbles have also led to a 62.3% decreaseFrom the in theabove, value with of the average influence breakdown of the bubbles, voltage the for breakdown the cylinder-plan voltages of electrodes. the cone-plan The results showedelectrodes that the have bubbles a 18.9% would decrease decrease on average; the the breakdown bubbles have voltages led to a 49.1 for% all decrease of the in electrodes, the value of and the reductionthe ofaverage the cylinder-plan breakdown voltage electrodes for the wassphere the-plan most electrodes; serious. and Meanwhile,, the bubbles the have cone-plan also led to electrodes a 62.3% decrease in the value of average breakdown voltage for the cylinder-plan electrodes. The had theresults minimal showed effects. that the bubbles would decrease the breakdown voltages for all of the electrodes, and Thethe electricreduction strength of the cyli andnder electric-plan electrodes field distribution was the most affect serious the. M breakdowneanwhile, the voltagescone-plan and the progresseselectrodes of the had discharge. the minimal The effects. three electrodes have different electric distributions: the electric fields of theThe cylinder-plan electric strength electrodes and electric and field the sphere-plandistribution affect electrodes the breakdown are more voltages uniform and thanthe that of the cone-planprogresses electrodes. of the discharge. When The the three bubbles electrodes were have generated different from electric the distributions: central of the the planelectric electrode, fields of the cylinder-plan electrodes and the sphere-plan electrodes are more uniform than that of and they formed a bridge that linked the two electrodes, and then the AC high voltage was applied on the cone-plan electrodes. When the bubbles were generated from the central of the plan electrode, the electrodes.and they formed If there a wasbridge a uniformthat linked electric the two fieldelectrodes, between and then the the two AC electrodes, high voltage there was wasapplied no corona that canon disturb the electrodes. the bubbles If there bridge, was a uniform and the electric discharge field between would the take two place electrodes, in the there bubbles was no bridge, corona and that would causethat can the disturb breakdown the bubbles of thebridge, two and electrodes. the discharge However, would take if there place was in the a non-uniformbubbles bridge, electric and field betweenthat the would two electrodes,cause the breakdown then the of corona the two would electrodes. take However, place near if there the highestwas a non electric-uniform field electric area, which wouldfield disturb between the bubblesthe two electrodes, and the lengththen the ofcorona the bridgewould take would place get near extended, the highest then electric the field discharge area, path which would disturb the bubbles and the length of the bridge would get extended, then the discharge in the non-uniform electric field would be longer than that of the uniform electric field, the progress path in the non-uniform electric field would be longer than that of the uniform electric field, the of whichprogress is shown of which as Figure is shown4. The as Figure breakdown 4. The breakdown voltages of voltages the non-uniform of the non-uniform electric electric field sawfield a more minor effectsaw a more than min thator of effect the uniformthan that of electric the uniform field. electric It can befield. concluded It can be concluded that the electricthat the fieldelectric can affect the movementsfield can affect of the the bubbles movements and of change the bubbles the and discharge change the path, discharge which path, directly which affected directly theaffected breakdown voltagesthe of breakdown the two electrodes. voltages of Thus, the two the breakdownelectrodes. Thus, voltages the breakdown of the cone-plan voltages electrodes of the cone had-plan a minimal effect, andelectrodes the breakdown had a minimal voltages effect, and of the the otherbreakdown two typesvoltages of of electrodes the other two had types more of seriouselectrodes effects had by the more serious effects by the bubbles. Since the uniformity of the cylinder-plan electrodes’ electric field bubbles.was Since close the to the uniformity sphere-plan of theelectrodes, cylinder-plan the reductions electrodes’ of the breakdown electric field voltages was close of the to two the types sphere-plan electrodes,electrodes the reductions are close. of the breakdown voltages of the two types electrodes are close.

FigureFigure 4. The 4. bubbles The bubbles ofthe of the cone-plan cone-plan electrodes, electrodes, where where 1 are 1 are the theinitial initial bubbles; bubbles; 2 are the 2 arebubbles the bubbles affected by the electric field. The left picture is the schematic diagram and the right one is the real affected by the electric ﬁeld. The left picture is the schematic diagram and the right one is the real photo of the bubbles affected by the electric field. photo of the bubbles affected by the electric ﬁeld.

Energies 2018, 11, 634 6 of 11 Energies 2018, 11, x FOR PEER REVIEW 6 of 11

3.23.2.. Positive Positive DC DC Breakdown Breakdown Voltages Voltages of of the the Transformer Transformer Oil Oil FromFrom Figure Figure 5a,5a, the breakdown voltages of of the three types of electrodes without bubbles were differentdifferent from from that that of of the AC conditions. From TableTable3 ,3, the the average average value value of of the the breakdown breakdown voltages voltages of ofthe the cone-plan cone-plan electrodes electrodes is is 45.7 45.7 kV, kV, the the average average value value of theof th breakdowne breakdown voltages voltages of theof the sphere-plan sphere- planelectrodes electrodes is 56.7 is kV,56.7 and kV the, and average the average value of value the breakdown of the breakdown voltages voltages of the cylinder-plan of the cylinder electrodes-plan electrodesis 49.5 kV. is 49.5 kV.

70 70 sphere-plan cone-plan 60 60 cylinder-plan

50 50

40 40

30 30

20 20 cone-plan 10 sphere-plan 10 cylinder-plan DC Breakdown Voltages with Bubbles/ kV DC Breakdown Voltages without Bubbles/ kV

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 1 The number of breakdown times The number of breakdown times (a) (b)

Figure 5. The positive DC breakdown voltages of the transformer oil. (a) The DC breakdown voltages Figure 5. The positive DC breakdown voltages of the transformer oil. (a) The DC breakdown voltages of the oil without bubbles; (b) the DC breakdown voltages of the oil with bubbles. of the oil without bubbles; (b) the DC breakdown voltages of the oil with bubbles.

FromFrom Figure Figure 5b,5b, the breakdown voltages of of the the cylinder cylinder-plan-plan electrodes electrodes with with bubbles bubbles are are much much lowerlower than than the the breakdown breakdown voltages voltages of the otherother twotwo typestypes ofofelectrodes, electrodes, and and the the breakdown breakdown voltages voltages of ofthe the sphere-plan sphere-plan electrodes electrodes are are close close to thatto that of theof the cone-plan cone-plan electrodes. electrodes. From From Table Table3, the 3, average the average value valueof the of breakdown the breakdown voltages voltages of the cone-planof the cone electrodes-plan electrodes is 42.4 kV, is the42.4 average kV, the value average of the value breakdown of the breakdownvoltages of voltages the sphere-plan of the sphere electrodes-plan iselectrodes 43.0 kV, andis 43.0 the kV average, and the value average of the value breakdown of the breakdown voltages of voltagesthe cylinder-plan of the cylinder electrodes-plan iselectrodes 21.0 kV. is 21.0 kV. FromFrom the the above, above, with with the the influence influence of of the the bubbles, bubbles, the the breakdown breakdown voltages voltages of of the the cone cone-plan-plan electrodeselectrodes have have a a7.2 7.2%% decrea decreasese on on average; average; the the bubbles bubbles have have led led to toa 24.1 a 24.1%% decrease decrease in the in thevalue value of Tofhe The average average breakdown breakdown voltage voltage for forthe thesphere sphere-plan-plan electrodes; electrodes; and and also also the thebubbles bubbles have have led led to toa 66.9a 66.9%% decrease decrease in the in thevalue value of T ofhe Theaverage average breakdown breakdown voltage voltage for the for cylinder the cylinder-plan-plan electrodes. electrodes. The resultsThe results showed showed that thatunder under a DC a DCvoltage voltage,, the thebubbles bubbles would would also also decrease decrease the the breakdown breakdown voltages voltages forfor all all the the electrodes, electrodes, and and the the reduction reduction of of the the cylinder cylinder-plan-plan electrodes electrodes’’ breakdown breakdown voltages voltages were were the the mostmost serious, serious, and and the the cone cone-plan-plan electrodes electrodes had had minimal minimal effects effects.. H However,owever, the the sphere sphere-plan-plan electrodes electrodes sawsaw a a min minoror reduction reduction than than that that of of the the AC AC condition. condition. FromFrom the the above, above, the the breakdown breakdown voltages voltages’’ decrease decrease of of the the cylinder cylinder-plan-plan electrodes electrodes under under positive positive DCDC voltage voltage were were similar similar to to that that under under AC AC voltage, voltage, ho however,wever, the the decreases decreases of of the the breakdown breakdown voltages voltages ofof the the sphere sphere-plan-plan and and cone cone-plan-plan electrodes electrodes are are much much smaller smaller than than th thoseose of of the the AC AC conditions. conditions. FromFrom Section Section 3.1 3.1, ,it it is is can can be be seen seen that that the the discharge discharge progress progress was was influenced influenced by by the the movements movements ofof the the bubbles bubbles:: if ifthe the bubbles bubbles were were easy easy to link to link the thetwo two electrodes, electrodes, the breakdown the breakdown voltage voltage would would see asee sharp a sharp decrease decrease.. Under Under the DC the electric DC electric field, field,the bubbles the bubbles were wereeasy to easy charge, to charge, and they and would they would then obtainthen obtain an electric an electric force with force constant with constant direction direction that made that the made bubbles the bubbles move faster move than faster under than the under AC condition.the AC condition. For that, Forthe that,bubbles the would bubbles move would from move the fromplan theelectrode plan electrode to the other to the electrode other electrode with a higherwith a speed higher under speed the under DC thecondition, DC condition, and this and progress this progress made madeit difficult it difficult for the for bubbles the bubbles to link to the link twothe twoelectrodes, electrodes, especially especially in the in thenon non-uniform-uniform electric electric fields, fields, like like the the electric electric field field of of the the cone cone-plan-plan electrodes,electrodes, the the existence existence of of the the corona corona would would make make the the bubbles bubbles’’ bridge bridge more more difficult difficult to to form. form. FFromrom Figure Figure 4,4, the the bubbles bubbles in in path path 2 2 with with high high speed speed were were easy easy to to s spreadpread to to other other regions regions in in the the oil,oil, which which would would make make the the bubbles bubbles difficult difficult to to link link the the electrodes, electrodes, so sothe the breakdown breakdown voltages voltages had had a mainimal minimal reduction reduction,, and and in in[15] [15, the], the authors authors made made a asimilar similar conclusion conclusion,, that that the the strong strong electric electric field field prevented the bubbles from flowing steadily. From Figure 6a, the central of the two electrodes formed a uniform electric field, and there was no corona and disturbance, the bubbles would move from the

Energies 2018, 11, 634 7 of 11

Energies 2018, 11, x FOR PEER REVIEW 7 of 11 prevented the bubbles from flowing steadily. From Figure6a, the central of the two electrodes formed centa uniformer of the electric plan electrode field, and to therethe cent waser noof the corona cylinder and electrode disturbance, directly, the bubbles like path would 1, and move there fromwas littlethe center possibility of the to plan form electrode path 2 under to the the center uniform of the electric cylinder field. electrode From directly,Figure 6b, like the path bubbles 1, and of there the spherewas little-plan possibility electrodes to also form spread path 2easily under to the other uniform areas electricin the oil field. from From the Figureedge of6 b,the the sphere, bubbles so ofit wasthe sphere-planalso difficult electrodes to form the also bubbles spread easilyto link to the other two areas electrodes, in the oil and from the the decrease edge of of the breakdown sphere, so voltagesit was also was difficult minor tocompared form the to bubbles the AC to linkcondition. the two However, electrodes, in andthe thecase decrease of the ofcylinder breakdown-plan electrodes,voltages was the minor decrease compared of the breakdown to the AC condition. voltages However,under the inDC the condition case of the was cylinder-plan approximately electrodes, equal tothe that decrease under ofthe the AC breakdown condition. voltages under the DC condition was approximately equal to that under the AC condition. Table 3. Statistics of the AC breakdown voltages (BDV). Table 3. Statistics of the AC breakdown voltages (BDV). Electrode Systems Cone-Plan Sphere-Plan Cylinder-Plan Electrode SystemsMean BDV (kV) Cone-Plan42.4 Sphere-Plan43 Cylinder-Plan21.1 Std. Deviation (kV) 2.7 6.2 4.4 With bubbles Mean BDV (kV) 42.4 43 21.1 Std.Min Deviation BDV (kV) (kV) 372.7 30 6.2 12 4.4 With bubbles MaxMin BDV BDV (kV) (kV) 47 37 49 3027 12 MeanMax BDV BDV (kV) 45.7 47 56.7 49 49.5 27 Std.Mean Deviation BDV (kV) (kV) 5.7 45.7 6.3 56.7 8.0 49.5 Without bubbles Std. Deviation (kV) 5.7 6.3 8.0 Without bubbles Min BDV (kV) 36 48 39 MaxMin BDV BDV (kV) (kV) 42.4 36 43 4821.1 39 Max BDV (kV) 42.4 43 21.1

From the above analyses, it was difficult for the bubbles to link the two electrodes in the non- uniformFrom electric the abovefield under analyses, the DC it condition, was difficult so the for breakdown the bubbles voltages to link of the the two cone electrodes-plan electrodes in the werenon-uniform minimally electric affected field by the under bubbles. the DC However, condition, under so the force breakdown of the DC voltages electric of field, the there cone-plan was littleelectrodes effect wereon the minimally formation affected of the by bubbles the bubbles. in the However, uniformunder electric the field, force so of thethe DCdescent electric of field,the breakdownthere was little voltage effect of on the the cylinder formation-plan of electrodes the bubbles under in the the uniform DC condition electric was field, approximately so the descent equal of the tobreakdown that under voltage the AC of condition the cylinder-plan. electrodes under the DC condition was approximately equal to that under the AC condition.

(a) (b)

Figure 6. The bubbles of the cylinder-plan and sphere-plan electrodes, where 1 is the initial bubbles; Figure 6. The bubbles of the cylinder-plan and sphere-plan electrodes, where 1 is the initial bubbles; 2 shows the bubbles affected by the corona; (a) the bubbles of the cylinder-plan electrodes; and, (b) 2 shows the bubbles affected by the corona; (a) the bubbles of the cylinder-plan electrodes; and, the bubbles of the sphere-plan electrodes. (b) the bubbles of the sphere-plan electrodes.

3.3. The Breakdown Voltages of the Oil-Impregnated Pressboard 3.3. The Breakdown Voltages of the Oil-Impregnated Pressboard From Figure 7a, under positive DC voltage, the breakdown voltages of the oil-impregnated From Figure7a, under positive DC voltage, the breakdown voltages of the oil-impregnated pressboard have decreased by the bubbles produced by the oil-impregnated pressboard. From Table pressboard have decreased by the bubbles produced by the oil-impregnated pressboard. From Table4, 4, the average value of the breakdown voltages of the oil-impregnated pressboard without bubbles the average value of the breakdown voltages of the oil-impregnated pressboard without bubbles was was 29.5 kV, and the average breakdown voltage with bubbles was 15.1 kV. This means that with the 29.5 kV, and the average breakdown voltage with bubbles was 15.1 kV. This means that with the influence of the bubbles, the breakdown voltages of oil-impregnated pressboard have a 48.9% inﬂuence of the bubbles, the breakdown voltages of oil-impregnated pressboard have a 48.9% decrease, decrease, on average. on average. From Figure 7b, under AC voltage, the breakdown voltages of the oil-impregnated pressboard From Figure7b, under AC voltage, the breakdown voltages of the oil-impregnated pressboard have decreased by the bubbles that are produced by the oil-impregnated pressboard. In Table 4, the have decreased by the bubbles that are produced by the oil-impregnated pressboard. In Table4, average breakdown voltages of the oil-impregnated pressboard without bubbles was 22.1 kV, and the average breakdown voltage with bubbles was 16.8 kV. This means that with the influence of the bubbles, the breakdown voltages of oil-impregnated pressboard have a 25% decrease, on average.

Energies 2018, 11, 634 8 of 11 the average breakdown voltages of the oil-impregnated pressboard without bubbles was 22.1 kV, and the average breakdown voltage with bubbles was 16.8 kV. This means that with the inﬂuence of Energiesthe bubbles, 2018, 11, the x FOR breakdown PEER REVIEW voltages of oil-impregnated pressboard have a 25% decrease, on average.8 of 11

No Bubbles 30 30 With Bubbles

25 25

20 20

15 15

10 10 AC Breakdown Voltages / kV DC Breakdown Voltages / kV No Bubbles 5 5 with Bubbles

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 The number of breakdown times The number of breakdown times (a) (b)

Figure 7. The breakdown voltages of the oil-impregnated pressboard. (a) The DC breakdown voltages; Figure 7. The breakdown voltages of the oil-impregnated pressboard. (a) The DC breakdown voltages; and (b) the AC breakdown voltages (effective values). and (b) the AC breakdown voltages (effective values).

Table 4. Statistics of the oil-impregnated pressboard breakdown voltages (BDV). Table 4. Statistics of the oil-impregnated pressboard breakdown voltages (BDV). Electrode Systems AC AC* DC Electrode SystemsMean BDV (kV) AC16.6 AC*19 15.1 DC MeanStd. BDV Deviation (kV) (kV) 16.6 4.0 3.1 19 3.5 15.1 With bubblesStd. Deviation (kV) 4.0 3.1 3.5 With bubbles Min BDV (kV) 11.3 16 8.7 Min BDV (kV) 11.3 16 8.7 Max BDV (kV) 23.5 23.5 21.5 Max BDV (kV) 23.5 23.5 21.5 Mean BDV (kV) 22.8 28 29.5 Mean BDV (kV) 22.8 28 29.5 Std. DeviationStd. Deviation (kV) (kV)6.6 6.6 2.3 2.3 2.9 2.9 WithoutWithout bubbles bubbles Min BDVMin (kV) BDV (kV) 13.213.2 29.1 29.1 25 25 Max BDVMax (kV) BDV (kV) 3030 16 16 33 33

However,However, it it can can be be seen seen that that there there was was a a sharp sharp decline decline of of the the breakdown breakdown voltages voltages after after about about ten ten dischargedischarges,s, in in total total.. T Thehe breakdown breakdown voltages voltages of of the the oil oil-impregnated-impregnated pressboard pressboard with with bubbles bubbles have have nono obvious obvious reduction reduction whe whenn compared compared with with the the breakdown breakdown voltages voltages without without bubbles. bubbles. Thus, Thus, the the data data beforebefore the the sharp sharp decline decline were selected againagain toto calculatecalculate thethe averageaverage breakdown breakdown voltages, voltages, shown shown as asAC* AC* (in (in Table Table4), the 4), average the average value value without without bubbles bubbles was 28 kV,was and 28 thekV, value and withthe value bubbles with was bubb 19.8les kV. wasThere 19.8 was kV. a 29.4%There decrease was a 29.4% of the decrease average breakdownof the average voltage breakdown under the voltage influence under of the the bubbles influence that ofwere the producedbubbles that by thewere oil-impregnated produced by pressboard.the oil-impregnated pressboard. TheThe AC breakdownbreakdown voltages voltages showed showed a sharp a sharp decline decline after aboutafter nineabout or tennine breakdown or ten breakdown discharges. dischaThe breakdownrges. The breakdown progress damaged progress the damaged oil-impregnated the oil-impregnated pressboard andpressboard the oil, andand somethe oil, fiber, and carbon, some fiber,and other carbon products, and other can products decrease thecan electric decrease strength the electric of the strength oil. Under of the ACoil. Under condition, the AC the breakdowncondition, theprogress breakdown had multiple progress discharges had multiple (shown discharges as Figure (shown8), and as Figu the oilre 8), and and the the pressboard oil and the decomposed pressboard decomposedinto other chemical into other compositions chemical compositions that would decrease that would the electric decrease strength the electric of the oil,strength and that of the the oil,compositions and that the had compositions a cumulative had effect. a cumulative It can be concluded effect. It thatcan thebe concluded cumulative that effect the in cumulative our test will effectplay anin importantour test will role play after an about important 10 AC breakdownrole after about discharges. 10 AC Onbreakdown the other discharges. hand, under On the the DC othercondition, hand, the under DC discharge the DC condition, only had one the discharge DC discharge (seen inonly Figure had8 ),one and discharge this caused (seen slight in damageFigure 8),to theand pressboard this caused and slight the oil,damage and the to repeated the pressboard breakdown and tests the canoil, changeand the the repeated morphology breakdown of water testsin oil, c whichan change make the breakdownmorphology voltage of water of transformer in oil, which oils increase. make the In [ 23breakdown], they reached voltage a similar of transformerconclusion. Thus,oils increase. the DC breakdown In [23], they voltages reached are a larger similar for conclusion. the last tests Thus, than the the first. DC breakdown voltages are larger for the last tests than the first.

Figure 8. The breakdown progress of the cylinder-plan electrodes in the oil.

Energies 2018, 11, x FOR PEER REVIEW 8 of 11

No Bubbles 30 30 With Bubbles

25 25

20 20

15 15

10 10 AC Breakdown Voltages / kV DC Breakdown Voltages / kV No Bubbles 5 5 with Bubbles

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 The number of breakdown times The number of breakdown times (a) (b)

Figure 7. The breakdown voltages of the oil-impregnated pressboard. (a) The DC breakdown voltages; and (b) the AC breakdown voltages (effective values).

Table 4. Statistics of the oil-impregnated pressboard breakdown voltages (BDV).

Electrode Systems AC AC* DC Mean BDV (kV) 16.6 19 15.1 Std. Deviation (kV) 4.0 3.1 3.5 With bubbles Min BDV (kV) 11.3 16 8.7 Max BDV (kV) 23.5 23.5 21.5 Mean BDV (kV) 22.8 28 29.5 Std. Deviation (kV) 6.6 2.3 2.9 Without bubbles Min BDV (kV) 13.2 29.1 25 Max BDV (kV) 30 16 33

However, it can be seen that there was a sharp decline of the breakdown voltages after about ten discharges, in total. The breakdown voltages of the oil-impregnated pressboard with bubbles have no obvious reduction when compared with the breakdown voltages without bubbles. Thus, the data before the sharp decline were selected again to calculate the average breakdown voltages, shown as AC* (in Table 4), the average value without bubbles was 28 kV, and the value with bubbles was 19.8 kV. There was a 29.4% decrease of the average breakdown voltage under the influence of the bubbles that were produced by the oil-impregnated pressboard. The AC breakdown voltages showed a sharp decline after about nine or ten breakdown discharges. The breakdown progress damaged the oil-impregnated pressboard and the oil, and some fiber, carbon, and other products can decrease the electric strength of the oil. Under the AC condition, the breakdown progress had multiple discharges (shown as Figure 8), and the oil and the pressboard decomposed into other chemical compositions that would decrease the electric strength of the oil, and that the compositions had a cumulative effect. It can be concluded that the cumulative effect in our test will play an important role after about 10 AC breakdown discharges. On the other hand, under the DC condition, the DC discharge only had one discharge (seen in Figure 8), and this caused slight damage to the pressboard and the oil, and the repeated breakdown tests can change the morphology of water in oil, which make the breakdown voltage of transformerEnergies 2018, 11 oils, 634 increase. In [23], they reached a similar conclusion. Thus, the DC breakdown9 of 11 voltages are larger for the last tests than the first.

FigureFigure 8. 8. TheThe breakdown breakdown progress progress of of the the cylinder cylinder-plan-plan electrodes electrodes in in the the oil oil..

Table5 shows the discharge forms of the oil-impregnated pressboard under the DC and AC electric fields. The breakdown meant that when the discharge occurred, the oil impregnated pressboard was broken down by the high voltage, and, meanwhile, the flashover meant that when the discharge occurred, the oil-impregnated pressboard was not broken down, but the discharge occurred along the oil-impregnated pressboard, only breaking down the bubbles and oil. From Table5, under the AC voltage, bubbles or no bubbles, the discharge forms were mainly breakdown, and under the DC condition, the discharge forms with no bubbles were breakdown. However, the main discharge forms with bubbles under the DC condition have changed into flashover.

Table 5. The discharge forms of the oil-impregnated pressboard under AC and DC electric ﬁelds.

Voltages AC DC Test No. No Bubbles Bubbles No Bubbles Bubbles 1 Breakdown Breakdown Breakdown Flashover 2 Breakdown Breakdown Breakdown Flashover 3 Breakdown Breakdown Breakdown Flashover 4 Breakdown Breakdown Breakdown Flashover 5 Breakdown Breakdown Breakdown Flashover 6 Breakdown Flashover Breakdown Flashover 7 Breakdown Breakdown Breakdown Flashover 8 Breakdown Breakdown Breakdown Flashover 9 Breakdown Breakdown Breakdown Breakdown 10 Breakdown Breakdown Breakdown Flashover

From the above results, with the influence of bubbles, the descent of the breakdown voltages under DC voltage was much more than that under AC voltage; this is because the main discharge forms under DC voltage with bubbles were flashover, and that had a minor breakdown voltage. In DC, the discharge almost takes place in the bubbles and the insulation cannot be recovered, thus the bubble will move up to the top surface of the oil along the paper, so the discharge occurred in the bubbles along the paper surface, and the flashover was formed. However, in AC, on one hand, the insulation of the bubbles can be recovered after breakdown, so the discharge will be shut down when the bubbles move toward up to the top, while, on the other hand, the oil-impregnated paper has a greater dielectric loss and produce more heat, which will reduce the insulation strength of the paper. Additionally, in AC, the pressboard is a complex insulation, there are oil and air in the paper, and there will be a higher electric strength on them, thus making them easier to breakdown, but this is not a problem in DC, and the authors in [24] share the same opinion. Therefore, the discharge form of the pressboard is always that of breakdown in AC.

4. Conclusions The presence of the bubble will reduce the breakdown voltages of the oil-paper insulation. From the above discussion, the work may be concluded as follows: In AC and DC, the bubbles decreased the breakdown voltage of the cylinder-plan electrodes the most among the three electrodes systems, and the mean values have dropped more than 60%, whereas the cone-plan electrodes have the least decline. That is say the bubbles have the minimum effect on the non-uniform electric field. This is due to the movement of the bubbles and the corona will prevent the bubbles’ discharge path. We also found that for the corona preventing the fibers and particles to form Energies 2018, 11, 634 10 of 11 the bridge, in AC, the cone-plan electrodes have the largest breakdown voltages in the oil without bubbles. In DC, the decrease of the breakdown voltages of the cone-plan and sphere-plan electrodes influenced by the bubbles were much less than that in AC. For the oil-impregnated pressboard, the decrease of the breakdown voltage under DC voltage was more than that under AC voltage. In our work, the main discharge form is flashover in DC, however, the main discharge form is breakdown through the pressboard. The phenomenon is determined by the discharge mechanisms in DC and AC. The discharge is the single discharge in DC, which cannot destroy the oil-paper insulation, but it can change the morphology of water in oil, and which can cause the BDV increase. Meanwhile, the AC breakdown is a kind of multiple high intensity discharge, which can seriously destroy the pressboard and oil, and the cumulative effect of fibrinogen degradation products will reduce the BDV.

Author Contributions: Chunxu Qin, Tao Zhao and Fangcheng Lv conceived and designed the experiments; Yan He, Bing Shi and Xiangrui Cheng performed the experiments; Chunxu Qin and Tao Zhao analyzed the data; Fangcheng Lv contributed reagents/materials/analysis tools; Chunxu Qin wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Liao, R.; Ying, L.; Zheng, H.; Ma, Z. Reviews on oil-paper insulation thermal aging in power transformers. Trans. China Elctrotech. Soc. 2012, 27, 1–12. 2. Yin-biao, S. Present status and prospect of HVDC transmission in China. High Volt. Eng. 2004, 30, 1–2. 3. You-liang, S.; Qing-pu, W.; Wen-ping, L. R&D ofconverter transformer for ±800 kV HVDC power transmission project. Electr. Equip. 2006, 7, 17–20. 4. Tao, Z.; Yunpeng, L.; Fangcheng, L.; Ruihong, D. Study on dynamics of the bubble in transformer oil under non-uniform electric field. IET Sci. Meas. Technol. 2016, 10, 498–504. [CrossRef] 5. Ushakov, V.Y.; Klimkin, V.F.; Korobeynikov, S.M. Impulse Breakdown of Liquids, 1st ed.; Springer: Berlin/Heidelberg, German; GmbH & Co. K: Berlin, German, 2007; ISBN 9783642091858. 6. Heinrichs, F.W. Bubble formation in power transformer windings at overload temperatures. IEEE Trans. Power Appar. Syst. 1979, PAS-98, 1576–1582. [CrossRef] 7. Kaufmann, G.H.; McMillen, C.J. Gas bubble studies and impulse tests on distribution transformers during loading-above nameplate rating. IEEE Trans. Power Appar. Syst. 1983, PAS-102, 2531–2542. [CrossRef] 8. Oommen, T.V.; Lindgren, S.R. Bubble evolution from transformer overload. Transm. Distrib. Conf. Expo. 2001, 1, 137–142. 9. Koch, M.; Tenbohlen, S. Evolution of bubbles in oil-pressboard insulation influenced by material quality and ageing. IET Electr. Power Appl. 2011, 5, 168–174. [CrossRef] 10. Przybylek, P. The influence of cellulose insulation aging degree on its water sorption properties and bubble evolution. IEEE Trans. Dielectr. Electr. Insul. 2010, 17, 906–912. [CrossRef] 11. Przybylek, P.; Nadolny, Z.; Moscicka-Grzesiak, H. Bubble effect as a consequence of dielectric losses in cellulose insulation. IEEE Trans. Dielectr. Electr. Insul. 2010, 17, 913–919. [CrossRef] 12. Przybylek, P.; Moranda, H.; Walczak, K.; Moscicka-Grzesiak, H. Can the bubble effect occur in an oil-impregnated pressboard bushing. IEEE Trans. Dielectr. Electr. Insul. 2012, 19, 1879–1883. [CrossRef] 13. Perkasa, C.Y.; Nick, L.; Tadeusz, C.; Jaury, W.; Daniel, M. A Comparison of the Formation of Bubbles and Water Droplets in Vegetable and Mineral Oil Impregnated Transformer pressboard. IEEE Trans. Dielectr. Electr. Insul. 2014, 21, 2111–2118. [CrossRef] 14. Sharbaugh, H.; Devins, J.S.; Rzad, S.J. Progress in the field of electric breakdown in dielectric liquids. IEEE Trans. Electr. Insul. 1978, 13, 249–276. [CrossRef] 15. Ogata, S.; Shigehara, K.; Yoshida, T.; Shinohara, H. Small bubble formation by using strong non-uniform electric field. IEEE Trans. Ind. Appl. 1980, IA-16, 766–770. [CrossRef] 16. Hara, M.; Koishihara, H.; Saita, K. Breakdown behavior of cryogenic liquids in the presence of thermal bubbles under ramped voltage. IEEE Trans. Dielectr. Electr. Insul. 1991, 26, 685–691. [CrossRef] 17. Seok, B.Y.; Tamuro, N.; Hara, M. A study of thermal bubble behavior in the simulated electrode system of HT superconducting coils. IEEE Trans. Dielectr. Electr. Insul. 1999, 6, 109–116. [CrossRef] Energies 2018, 11, 634 11 of 11

18. Hara, M.; Kubuki, M. Effect of thermally induced bubbles on the electrical breakdown characteristics of liquid nitrogen. Proc. IEE 1990, 137, 209–216. 19. Hara, M.; Wang, Z.; Saito, H. Thermal Bubble Breakdown in Liquid Nitrogen under Nonuniform Fields. IEEE Trans Dielectr. Electr. Insul. 1994, 4, 709–716. [CrossRef] 20. Gan, D.; Liu, F.; Liu, P.; Hu, C. Calculation Method of Oil-Paper Insulation Moisture Content Based on Oil-Paper Moisture Equilibrum of Transformer. Transformer 2009, 8, 21–24. 21. Dissado, L.A. Theoretical basis for the statistics of dielectric breakdown. J. Phys. D Appl. Phys. 1990, 23, 1582–1591. [CrossRef] 22. Peppas, G.D.; Charalampakos, V.P.; Pyrgioti, E.C.; Danikas, M.G.; Bakandritsos, A.; Gonos, I.F. Statistical investigation of AC breakdown voltage of nanoﬂuids compared with mineral and natural ester oil. IET Sci. Meas. Technol. 2016, 10, 644–652. [CrossRef] 23. You, Z.; Jun, J.; Yingting, L.; Hao, W.; Yuzhen, L.; Chengrong, L. Statistical Analysis of AC Breakdown Voltage for Transformer Oil. Insul. Mater. 2015, 48, 73–77. 24. Wang, Y.; Wei, X.; Chen, Q. Breakdown Characteristics of Oil-pressboard Insulation in Converter Transformer under Composite Electric Field and Its Test Method. High Volt. Eng. 2011, 37, 2005–2011.

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