An Experimental Study of the Failure Mode of Zno Varistors Under Multiple Lightning Strokes

electronics

Article An Experimental Study of the Failure Mode of ZnO Varistors Under Multiple Lightning Strokes

Chunlong Zhang 1,2,3,4, Hongyan Xing 1,2,3,*, Pengfei Li 3,4, Chunying Li 3,4, Dongbo Lv 3,4 and Shaojie Yang 4 1 Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China; [email protected] 2 Jiangsu Key Laboratory of Meteorological Observation and Information Processing, Nanjing University of Information Science & Technology, Nanjing 210044, China 3 School of Atmospheric Physics, Nanjing University of Information Science & Technology, Nanjing 210044, China; [email protected] (P.L.); [email protected] (C.L.); [email protected] (D.L.) 4 Meteorological Disaster Prevention Technology Center of Heilongjiang Province, Harbin, 150030, China; [email protected] * Correspondence: [email protected]; Tel.: +86-025-58731370

Received: 15 January 2019; Accepted: 31 January 2019; Published: 2 February 2019

Abstract: In this study, in order to explore the failure mode of ZnO varistors under multiple lightning strokes, a ﬁve-pulse 8/20 µs nominal lightning current with pulse intervals of 50 ms was applied to ZnO varistors. Scanning electron microscopy (SEM) and X-ray diffractometry (XRD) were used to analyze the microstructure of the material. The failure processes of ZnO varistors caused by multiple lightning impulse currents were described. The performance changes of ZnO varistors after multiple lightning impulses were analyzed from both macro and micro perspectives. According to the results of this study’s experiments, the macroscopic failure mode of ZnO varistors after multiple lightning impulses involved the rapid deterioration of the electrical parameters with the increase of the number of impulse groups, until destruction occurred by side-corner cracking. The microstructural examination indicated that, after the multiple lightning strokes, the proportion of Bi in the crystal phases was altered, the grain size of the ZnO varistors became smaller, and the white intergranular phase (Bi-rich grain boundary layer) increased signiﬁcantly. The failure mechanism was thermal damage and grain boundary structure damage caused by temperature gradient thermal stress, generated by multiple lightning currents.

Keywords: failure mode; impulse current; microstructure; multiple lightning; ZnO varistors

1. Introduction The installation of surge protective devices is one of the most economical and effective means to avoid or reduce damage caused by lightning impulses in power distribution systems. The core component of a surge protective device is a ZnO varistor, which has many advantages, including a good nonlinear property, small normal leakage current, low level of residual pressure, and no follow current. ZnO varistors have been widely applied for the protection of electronic and electrical equipment from lightning [1,2]. However, after a natural lightning strike, ZnO varistors often have varying degrees of failure. An impulse test is the most direct means to study the performance of ZnO varistors. Currently, an 8/20 µs single pulse waveform had been adopted for such tests [3,4]. A host of studies have been presented regarding the performance changes that occur in ZnO varistors under single pulse lightning impulses [5–8]. However, modern lightning observations and artiﬁcially triggered lightning

Electronics 2019, 8, 172; doi:10.3390/electronics8020172 www.mdpi.com/journal/electronics Electronics 2019, 8, 172 2 of 13 acquisition data have shown that two-thirds of the lightning events in natural settings are a multi-pulse process. The statistics show that nearly 70% of cloud-to-ground lightning strikes involve from 2 and up to 20 strikes, with an average number of between 3 and 5, and a time duration between strikes of 15 ms to 150 ms. There is a significant difference between the single-pulse lightning waveform and the multiple lightning strikes of natural lightning [9,10], and the total time and energy of a multi-pulse are several times that of a single pulse. When a low-voltage distribution line is struck by a lightning, the lightning current flowing through the surge protective device generates heat in the ZnO varistor due to power loss, causing the body to heat up. When a multi-pulse lightning current and single-pulse lightning current flow through a ZnO varistor, there is a huge difference in duration and energy absorption, which inevitably leads to a difference in the temperature increase of the ZnO varistor and the final electrical performance parameters. Therefore, lightning single pulse test methods have been unable to properly simulate the damage caused by lightning [11]. Darveniza et al.[12] pointed out that multiple lightning impulse currents could potentially cause severe damage to lightning protection devices protecting the equipment in power distribution systems. Previous studies have shown that six-pulse lightning current impulses have resulted in serious damage to lightning protection components, and confident conclusions cannot currently be drawn regarding their impact according to the lightning protection test standard. Lee et al. [13,14] found that ZnO varistors degraded when subjected to multiple impulse currents, and their life mainly depended on the amplitude of the lightning surge. Haryono et al. [15,16] analyzed the damage effects to ZnO varistors undergoing multiple lightning impulse currents from the perspective of energy absorption. At the present time, the research regarding the performance of ZnO varistors undergoing multiple lightning impulse current has been mainly based on the analysis of the macroscopic electrical properties [17–19]. Qingheng Chen et al. [20] used the simulation network model to analyze the current, temperature, and thermal stress in zinc oxide varistors. The results showed that reducing the average size of the ZnO grains can significantly reduce the temperature difference inside the ZnO varistor. Thermal stress increases the impact energy absorption capacity of zinc oxide varistors. Pengfei Li et al. [21] analyzed the damage form of metal oxide under multiple lightning impulses and found that it was mainly the edge that was cracking. However, few research studies have been conducted to investigate the damage failure mechanisms under multi-pulse continuous impulses, especially regarding the changes in microstructures. Tsuboi et al. [22] believed that damage could occur in the internal components of ZnO when subjected to multiple lightning impulses, resulting in the failure of the ZnO varistor, which provided a theoretical basis for the modeling of this study. Therefore, in this study, a five-pulse current with a time interval of 50 ms was applied to the samples. The actual lightning strike to the ZnO varistors in practical applications was simulated as realistically as possible, and the macroscopic damage form and static parameter variation characteristics were monitored. At the same time, the ZnO varistors were analyzed before and after the impulse by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). It was expected that we would investigate the failure mode of the ZnO varistors during the natural lightning strikes from the perspective of macroscopic and microscopic bonding. Indeed, some experimental data regarding the performance of ZnO varistors under multiple lightning impulse currents are shown here. The failure modes of the ZnO varistors undergoing multiple lightning impulse currents are discussed on the basis of the results of thermal effect. This is believed to be particularly important for improving lightning protection and safety performances of ZnO varistors.

2. Experiment

2.1. Impulse Test and Waveform The multiple lightning impulse equipment used in this experiment was a 20-pulse lightning impulse current generator, as shown in Figure1. Multi-channel discharge technology was adopted to simulate the multiple lightning stroke processes. The waveform generator circuit diagram is shown Electronics 2019, 8, 172 3 of 13

ElectronicsElectronics 2019 2019, ,8 8, ,x x FOR FOR PEER PEER REVIEW REVIEW 33 ofof 1313 in Figure2, in which C is the capacitor, G is the impact gap, Rs is the resistor, and Ls is the inductor. inductor.inductor.With the With impulseWith the the testimpulse impulse end astest test the end end axis, as as the therethe axis, axis, are ther ther 10 triggeree are are 10 10 channels trigger trigger channels channels at each end.at at each each When end. end. the When When primary the the primaryprimarycurrent iscurrent current fully triggered, is is fully fully triggered, triggered, 20 high-voltage 20 20 high-voltage high-voltage pulses can pulses pulses be generated, can can be be generated, generated, and the time and and interval the the time time can interval interval be changed can can bebefrom changed changed 1 ms tofrom from 999 1 ms.1 ms ms to to 999 999 ms. ms.

FigureFigureFigure 1. 1. 1. 20-pulse 20-pulse20-pulse lightning lightning lightning impulse impulse impulse current current current generator. generator. generator.

C2C2 G2G2 RsRs 2 2 Ls2Ls2 Ls1Ls1 RsRs 1 1 G1G1 C1C1

C4C4 G4G4 RsRs 4 4 Ls4Ls4 Ls3Ls3 RsRs 3 3 G3G3 C3C3

C5C5 C6C6 G6G6 RsRs 6 6 Ls6Ls6 EUTEUT Ls5Ls5 RsRs 5 5 G5G5 UrUr IpIp C17C17 C18C18 G18G18 RsRs 18 18 Ls18Ls18 Ls17Ls17 RsRs 17 17 G17G17

C19C19 C20C20 G20G20 RsRs 20 20 Ls20Ls20 Ls19Ls19 RsRs 19 19 G19G19

FigureFigureFigure 2. 2. 2. Multiple MultipleMultiple lightning lightning lightning impulse impulse impulse curre curre currentntnt generator generator circuit circuit diagram. diagram.

InInIn this this this study, study, study, Section Section 11 1 of ofof IEC IECIEC 62305-1 62305-162305-1 Lightning LightningLightning Protection: Protection:Protection: General GeneralGeneral Rule RuleRule [23] [[23]23 ]provides provides provides a a a special special special definitiondefinitiondefinition for for multiple multiplemultiple lightning. lightning.lightning. Lightning Lightning withwith an an averageaverage average ofof of threethree threeto to to four four four impulses, impulses, impulses, and and and intervals intervals intervals of ofofapproximately approximately approximately 50 50 50 ms ms ms was was was defined defined defined as as as multiple multiple multiple lightning. lightning. lightning. Therefore, Therefore, Therefore, the the the multiple multiple multiple lightning lightning lightning strikes strikes strikes in ininthis thisthis test testtest were werewere represented representedrepresented by a groupbyby aa group ofgroup multiple ofof multiplemultiple impulse impulse currents,impulse currents, whichcurrents, included whichwhich five includedincluded consecutive fivefive consecutiveconsecutive impulseimpulse currents,currents, eacheach ofof whichwhich hadhad 8/208/20 μμss waveforms.waveforms. TheThe timetime betweenbetween twotwo consecutiveconsecutive pulses pulses was was 50 50 ms, ms, an andd the the pulse pulse amplitudes amplitudes were were the the nominal nominal discharge discharge currents currents of of the the selectedselected ZnO ZnO varistors. varistors. The The waveform waveform diagram diagram is is shown shown in in Figure Figure 3, 3, where where the the five five yellow yellow vertical vertical

Electronics 2019, 8, 172 4 of 13 impulse currents, each of which had 8/20 µs waveforms. The time between two consecutive pulses Electronics 2019, 8, x FOR PEER REVIEW 4 of 13 was 50 ms, and the pulse amplitudes were the nominal discharge currents of the selected ZnO varistors. Thelines waveform represent diagramthe five pulses, is shown and in the Figure following3, where 8/20 the μs fivewaveform yellow denotes vertical the lines full represent waveform the figure five pulses,of a single and pulse. the following 8/20 µs waveform denotes the full waveform figure of a single pulse.

FigureFigure 3.3. Five-pulseFive-pulse lightninglightning waveformwaveform diagram.diagram.

2.2. Sample Preparation 2.2. Sample Preparation In this study, the various ZnO varistors that were used were provided by the same manufacturer. In this study, the various ZnO varistors that were used were provided by the same The basic parameters of these samples were nominal discharge current In = 20 kA and the maximum manufacturer. The basic parameters of these samples were nominal discharge current In = 20 kA and continuous operation voltage Uc = 385 V, and the static parameters (varistor voltage and leakage the maximum continuous operation voltage Uc = 385 V, and the static parameters (varistor voltage current) were as shown in Table1. The varistor voltage is a voltage corresponding to a current of the and leakage current) were as shown in Table 1. The varistor voltage is a voltage corresponding to a varistor of 1 mA and is a standard of a voltage whose current rapidly rises with voltage, expressed by current of the varistor of 1 mA and is a standard of a voltage whose current rapidly rises with U1mA. The leakage current refers to the current ﬂowing through the varistor at a speciﬁed temperature voltage, expressed by U1mA. The leakage current refers to the current flowing through the varistor at and maximum DC voltage, generally expressed by Iie. Under the premise of approaching U1mA, a specified temperature and maximum DC voltage, generally expressed by Iie. Under the premise of the smaller the Iie was, the better the voltage limiting the performance of the varistor would be. In this approaching U1mA, the smaller the Iie was, the better the voltage limiting the performance of the study, seven ZnO varistor blocks, with the majority approaching the static parameters, were selected varistor would be. In this study, seven ZnO varistor blocks, with the majority approaching the static as the samples and were denoted as A1 to A7. parameters, were selected as the samples and were denoted as A1 to A7.

Table 1. Changes in the electrical parameters before and after the impulse in the ZnO varistors. TABLE 1. Changes in the electrical parameters before and after the impulse in the ZnO varistors.

Initial Initial U1mA When Iie When Tmax When Groups of No.Initial Initial Iie U1mA when Iie when Tmax ◦when Groups of No. U1mA (V) Iie (µA) Failure (V) Failure (µA) Failure ( C) Impulse U1mA (V) (µA) failure (V) failure (µA) failure (°C) impulse A1 690 0 602 8.2 224 17 A1 690 0 602 8.2 224 17 A2 689 0 600 8.1 225 16 A2 A3689 690 0 0 600 604 8.17.5 222 225 15 16 A3 A4690 691 0 0.1 604 615 7.59.4 221 222 16 15 A4 A5691 688 0.1 0.1 615 605 9.48.6 216 221 16 16 A5 A6688 690 0.1 0.1 605 603 8.68.5 228 216 15 16 A7 689 0.1 602 7.8 218 16 A6 690 0.1 603 8.5 228 15 A7 689 0.1 602 7.8 218 16 2.3. Experimental Procedure 2.3. ExperimentalThe ﬂowchart Procedure of the experimental procedure is shown in Figure4. The flowchart of the experimental procedure is shown in Figure 4.

Electronics 2019, 8, x FOR PEER REVIEW 5 of 13 Electronics 2019, 8, 172 5 of 13 Electronics 2019, 8, x FOR PEER REVIEW Initial measurements： 5 of 13

U1mA、Iie、SEM scaning &Initial XRD measurements diffraction ：

U1mA、Iie、SEM scaning & XRD diffraction A group of 5-impulse test A group of 5-impulse test

Measure surface temperatures Measure surface temperatures

U1mA > ±10% or Iie > 20μA U1mA > ±10% or direct damage? or Iie > 20μA No,after 30 or direct damage? No,afterminitues 30 minitues Yes Yes

Stop impulse Stop impulse

SEM scaning & XRD diffractionSEM scaning and analyze& XRD diffraction and analyze the results the results

Figure 4. Flowchart of the experimental procedure. Figure 4. Flowchart of the experimental procedure.

(1) Initial measurements:measurements: TheThe samplessamples areare characterized characterized with with the the U U1mA1mA,,I Iieie,, and and by by photographs (1) Initial measurements: The samples are characterized with the U1mA, Iie, and by photographs taken at the beginning of the test. We used scanning electron microscopy (SEM) and an X-ray takentaken at theat the beginning beginning of ofthe the test. test. We We used used sc scanninganning electronelectron microscopy (SEM)(SEM) andand anan X-ray X-ray diffractometerdiffractometer (XRD) (XRD) on on the the ZnO ZnO varistorvarist varistoror blocks blocks before before thethe the impulseimpulse impulse test.test. (2) Impulse(2)Impulse Impulse test:test: test: we we adjustedwe adjusted adjusted the the chargingthe charging charging voltage voltage voltage of the ofof generator thethe generatorgenerator to output toto outputoutput the demand thethe demand demand impulse impulsecurrents.impulse currents. Then, currents. multiple Then, Then, multiplelightning multiple lightning lightning impulse impulse impulse currents currents currents were applied werewere appliedapplied to the toto ZnO thethe varistors. ZnOZnO varistors. varistors. The The timeThe timebetweentime between between the application the the application application of one of group ofone one group of group impulse of of impuls impuls currentsee currents currents to a ZnO toto varistor a ZnO varistor block, and block,block, that and and of thethat that nextof of thegroup thenext ofnext group impulse group of currentsimpulseof impulse currents was currents 30 minutes; was was 30 30 minutes; with minutes; such with with long such such duration longlong durationduration of time,we ofof time,time, were we ablewe were were to return able able to to returnthereturn original to the to conditions.the original original conditions. Theconditions. process The The of process theprocess impulses of of the the impulses is impulses shown inisis shown Figureshown 5in. Figure 5.5.

Figure 5. Groups of impulse currents applied to the ZnO varistors. Figure 5. Groups of impulse currents applied to the ZnO varistors.

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(3) We measured the surface temperatures, varistor voltage U1mA, and leakage current Iie of the (3) We measured the surface temperatures, varistor voltage U1mA, and leakage current Iie of the ZnO varistors varistors after after each each impulse. impulse. Then, Then, we we checked checked the the surface surface of ofthe the ZnO ZnO varistors varistors for for flashover ﬂashover or punctureor puncture and and took took photographs photographs of the of thedamaged damaged ZnO ZnO varistors. varistors. When When the thechange change in amplitude in amplitude of the of U1mA reached beyond ±10% of the original, the Iie exceeded 20 μA, or direct damage occurred, the the U1mA reached beyond ±10% of the original, the Iie exceeded 20 µA, or direct damage occurred, ZnOthe ZnO varistors varistors were were judged judged as having as having failed. failed. Subseq Subsequently,uently, the impulse the impulse test was test ceased, was ceased, and the and data the weredata wererecorded. recorded. (4) We used scanningscanning electron microscopy (SEM) and an X-ray diffractometer (XRD) on the ZnO varistor varistor blocks blocks after after the the impulse impulse test test in orde in orderr to observe to observe the microstructural the microstructural changes changes of the of ZnO the varistors.ZnO varistors.

3. Results and and Discussion

3.1. Macroscopic Macroscopic Properties

The average level change directions ofof thethe UU1mA1mA andand I Iieie afterafter the the increases increases in in the the impulse impulse groups groups for the ZnO varistors under multiple lightning impulse currents currents can can be seen in Figure 66.. ItIt cancan bebe observed that the U1mA showedshowed a a trend trend of of decreasing-stable-de decreasing-stable-decreasingcreasing with with the the increase increase of impulse groups, and was observed to to drop drop after the first ﬁrst gr groupoup of impulses and and the the last last group group of of impulses, with an average decline rate as high as 4%.4%. The Iie all showed increasing trends with larger rising rates, with an average growth rate of 0.660.66 µμA/aA/a group. group. Following Following the the 16 16 groups groups of impulses, it was observed that the U1mA fellfell sharply, sharply, with with an an average average drop drop rate of more than 10% ofof thethe originaloriginal UU1mA1mA.. This resultedresulted inin failuresfailures occurring occurring in in the the ZnO ZnO varistors. varistors. Then, Then, after after another another group group of impulses, of impulses, the ZnO the ZnOvaristor varistor blocks blocks became became damaged. damaged.

690 11 U (V) 680 1mA 10 I (uA) ie 670 9

660 8

650 7

640 6

630 5

620 4

610 3

600 2

590 1

580 0 0123456789101112131415

Figure 6. Varistor voltage UU1mA1mA andand leakage leakage current current I Iieie variationvariation diagrams diagrams of of the the ZnO ZnO varistors varistors under multiple lightning impulse currents.

As shown in Table 11,, underunder thethe ﬁve-pulsefive-pulse lightninglightning currents,currents, thethe ZnOZnO varistorvaristor blocksblocks A1A1 toto A7A7 was able to withstandwithstand anan averageaverage ofof 1616 groupsgroups ofof impulses.impulses. WhenWhen thethe UU1mA1mA hadhad fallen fallen one one to two groups ofof impulses.impulses. Figure Figure7b 7b shows shows the the damage damage forms forms of the of ZnO the varistorZnO varistor block A2block during A2 during its failure, its failure,which presentedwhich presented as a side-corner as a side-corner cracking cracking along along with with patch patch collapse collapse [24 [24].]. As As shown shown inin FigureFigure 77c,c, the damage forms of of the ZnO varist varistoror blocks A1 to A4 in the different codes were observed to be highly consistent.

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(a)(a) (b)(b) (c) (c)

FigureFigure 7. 7. The The appearance appearance statestate diagramdiagram of of the the ZnO ZnO varistor varistorsvaristors sbefore before and and after after the the multiple multiple lightning lightning impulseimpulseimpulse currents. currents. currents. ( (a(aa)) )The TheThe state statestate diagram diagram ofof of SampleSample Sample A2 A2 A2 before before impulses; impulses; (b ( )b Damage)) Damage state state diagram diagram of of SampleSample A2; A2; ( c()c )Damage Damage diagrams diagrams of Samples A1 A1 to to A4. A4.

3.2.3.2. Microscopic Microscopic Microscopic Properties Properties Figure 8 details the SEM images of the ZnO varistors prior to the impulses, with a resolution of Figure 88 detailsdetails the SEM images of of the the ZnO ZnO varistors varistors prior prior to to the the impulses, impulses, with with a resolution a resolution of 10 μµm. It can be seen from the figure that the internal structure of the ZnO varistor was mainly 10of 10μm.m. It can It can be beseen seen from from the the figure ﬁgure that that the the inte internalrnal structure structure of of the the ZnO ZnO varistor varistor was was mainly composed of four types of gray matter, marked as +1, +2, +3, and +4. Through this study’s EDS composed of four types of graygray matter,matter, markedmarked asas +1,+1, +2,+2, +3,+3, andand +4.+4. Through Through this this study’s EDS analysis, it was determined that the main components of +1 in the white area were O, Zn, and Bi; analysis, it it was determined that that the the main main components components of of +1 +1 in in the the white white area area were were O, O, Zn, Zn, and and Bi; Bi; thosethose of of +2 +2 inin thethe gray area area were were O O and and Zn; Zn; those those of of+3 +3at the at thewhite-gray white-gray junction junction were wereO, Zn, O, and Zn, Bi, and thoseand ofthose +2 in of the +4 grayin the area deep were black O andregion Zn; were those O of and +3 atZn. the The white-gray proportion junction of elements were inO, differentZn, and Bi, andBi, and those those of +4 of +4in inthe the deep deep black black region region were were O O and and Zn. Zn. The The proportion proportion of of elements elements in in different regionsregions is is shown shown in in FigureFigure8 8b.b. ItIt cancan bebe seenseen that the main main components components of of the the ZnO ZnO varistor varistor are are zinc zinc regionsoxide andis shown small amountsin Figure of 8b. Bi Itcompounds. can be seen The that gray the area main in thecomponents figure indicates of the the ZnO zinc varistor oxide grain, are zinc oxide and small amounts of Bi compounds. The gray area in the ﬁgure indicates the zinc oxide grain, oxideand andthe white small area amounts indicates of Bi the compounds. rich-Bi phase The around gray areathe zinc in the oxide figure grain. indicates It was foundthe zinc that oxide the cellgrain, and the white area indicates the rich-Bi phase around the zinc oxide grain. It was found that the cell anddistributions the white areaof the indicates ZnO varistor the rich-Bi were phasenot unifo aroundrm, andthe zincthe rich-Bi oxide grain.phase Itwas was concentrated found that thein a cell distributions of the ZnO varistor were not uniform, and the rich-Bi phase was concentrated in a certain distributionscertain region. of Mapthe ZnOanalysis varistor at the were+1 position not unifo of therm, ZnO and varistors the rich-Bi is shown phase in Figurewas concentrated 8c. in a certainregion. region. Map analysis Map analysis at the +1 at positionthe +1 position of the ZnO of the varistors ZnO varistors is shown is shown in Figure in8 Figurec. 8c.

(a)

Figure 8. Cont.

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80 O 66.68 70 zn 60 56.54 55.68 bi 49.24 48.98 50 43.46 44.32 40 27.73 30 20 Percentage of atomic % atomic of Percentage 10 3.59 1.77 0 十1 十2 十3 十4 Position

(b)

(c)

FigureFigure 8. ( a8.) SEM(a) SEM images images of theof the ZnO ZnO varistors varistors (resolution (resolution of 10of µ10m); μm); (b) ( EDSb) EDS analysis analysis results results of theof the ZnOZnO varistors. varistors. (+1, (+1, +2, +2, +3, +3, +4); +4); (c) ( Mapc) Map analysis analysis at theat the +1 +1 position position of of the the ZnO ZnO varistors. varistors.

FigureFigure9a,b 9a,b shows shows the XRD the diffractionXRD diffraction pattern ofpattern the ZnO of varistors,the ZnO beforevaristors, and afterbefore the and impulses. after the It canimpulses. be seen It incan the be ﬁgure seen in that the the figure crystal that phase the crystal compositions phase compositions mainly include mainly four include parts: four zincite, parts: syn;zincite, manganese syn; manganese oxide; bismuth oxide; oxide; bismuth and oxide; bismite, and syn. bismite, Through syn. theThrough analysis the of analysis the ﬁgures, of the it figures, was determinedit was determined that the proportion that the proportion of Bi in the of crystalBi in th phasese crystal before phases the before impulse the testimpulse was 0.9%,test was 0.8%, 0.9%, and0.8%, 0.6%, and respectively. 0.6%, respectively. The proportion The proportion after the impulse after the test wasimpulse 0.0%, test 1.0%, was and 0.0%, 1.3%, 1.0%, respectively. and 1.3%, Therespectively. proportion ofThe Bi proportion in the crystal of phasesBi in the has crystal been phases converted. has been converted.

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Counts B 20000 Zincite, syn 84.9 % Manganese Oxide 12.9 % Manganese Bismuth Oxide 0.9 % 10000 Bismuth Oxide 0.8 % Bismite, syn 0.6 %

0 20 30 40 50 60 70 2θ/(°)

Peak List

04-008-7114; Zn O; Zincite, syn; Hexagonal

04-006-0218; Mn3 O4; Cubic

04-001-9929; Mn0.5 Bi12.5 O20; Cubic

04-008-4464; Bi2 O3; Tetragonal

04-011-1986; Bi2 O3; Bismite, syn; Monoclinic

(a)

Counts C 20000 Zincite, syn 84.4 % Manganese Oxide 13.1 % Manganese Bismuth Oxide 0.0 % Bismuth Oxide 1.0 % 10000 Bismite, syn 1.3 %

0 20 30 40 2θ/(°) 50 60 70

Peak List

04-008-7114; Zn O; Zincite, syn; Hexagonal

04-006-0218; Mn3 O4; Cubic

04-001-9929; Mn0.5 Bi12.5 O20; Cubic

04-008-4464; Bi2 O3; Tetragonal

04-011-1986; Bi2 O3; Bismite, syn; Monoclinic

(b)

FigureFigure 9.9.XRD XRD diffractiondiffraction resultsresults ofof thethe ZnOZnO varistors varistors before before ( a(a)) and and after after ( b(b)) the the impulses. impulses.

FigureFigure 10 10a,ba,b illustrate illustrate the the SEM SEM images images of of the the ZnO ZnO varistors, varistors, before before and and after after the the impulses, impulses, with with a resolutiona resolution of 80of 80µm. μm. It wasIt was observed observed that that after after the the impulses, impuls thees, the grain grain sizes sizes became became smaller, smaller, while while the whitethe white areas areas increased. increased. The differences The differences in the microstructuresin the microstructures of the materials of the materials in the ZnO in varistorsthe ZnO indicatevaristors that indicate the multiple that the lighting multiple impulse lighting currents impulse led to thecurrents ZnO grains led to becoming the ZnO smaller, grains along becoming with thesmaller, grain along boundary with growth. the grain boundary growth.

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(a) (b)

Figure 10. SEM images of the ZnO varistors (resolution of 80 μµm). ( a) Before the impulses. ( (b)) After After thethe impulses. impulses.

3.3. Failure Failure Mechanism Mechanism The ZnO varistors have the appearance of layers of insulation coating. When impulse current passes through thethe ZnOZnO varistors, varistors, the the pulse pulse intervals intervals are are ms ms in length,in length, and and the internalthe internal heat heat of the of ZnO the ZnOvaristors varistors will be will almost be instantaneouslyalmost instantaneously concentrated, concentrated, accompanied accompanied by a sharp by rise a insharp temperature. rise in temperature.In terms of the In energy terms convection of the energy and exchange,convection this and process exchange, can be this seen process as an adiabatic can be temperatureseen as an adiabaticrise. At the temperature time that the rise. failures At the occurred time that in the ZnOfailures varistors, occurred the in surface the ZnO temperature varistors, hadthe risensurface to ◦ ◦ temperatureover 200 C, had and risen the average to over temperature200 °C, and the was aver 223ageC, temperature as shown in was Table 2231. °C, According as shown to in Equation Table 1. According(1), in the cases to Equation of the injected (1), in energythe cases equivalent, of the injected the temperature energy equivalent, increases the of thetemperature ZnO varistors increases units ofwith the the ZnO intake varistors of energy units could with bethe directly intake determinedof energy could by the be thermal directly physical determined property by the parameters thermal physicalρ and Cp. property However, parameters during the ρproduction and Cp. However, process of during the ZnO the varistors, production absolute process material of the mixing ZnO varistors,uniformity absolute could notmaterial be achieved. mixing Asuniformity can be seen could from not the be SEMachieved. images As ofcan the be ZnO seen varistor from the blocks, SEM imagesthe microstructures of the ZnO show varistor inhomogeneities blocks, the ofmicrostr the internaluctures material show distribution,inhomogeneities which of were the observedinternal materialto have leddistribution, to signiﬁcant which differences were observed in the thermal to have physical led to properties significant of differences different parts in the of thethermal same physicalZnO varistors. properties For example,of different inhomogeneities parts of the existedsame ZnO in the varistors. thermal For physical example, properties inhomogeneities of the ZnO existedvaristor in blocks. the thermal In this physical study, t wasproperties determined of the thatZnO the varistorρ and blocks. Cp differences In this study, in different t was determined parts led to thatdifferent the ρ adiabatic and Cp temperaturedifferences in increases. differentFurthermore, parts led to thedifferent infrared adiabatic imaging temperature measurement increases. results Furthermore,showed different the temperatureinfrared imaging increases measurement in different re locationssults showed of the ZnOdifferent varistor temperature blocks after increases absorbing in differentthe impulse locations energy. of Figure the ZnO 11 shows varistor the blocks temperature after absorbing distribution the diagram impulse ofenergy. the ZnO Figure varistor 11 shows blocks theA1 andtemperature A4 following distribution the impulses. diagram It can of the be seenZnO in va theristor ﬁgure blocks that A1 there and were A4 following considerable the differences impulses. Itobserved can be seen in the in temperature the figure that increases there were in different considerable parts. Thedifferences energy atobserved the local in hot the spot temperature occurred increasestoo late to in be different passed parts. around, The which energy led at to the a largelocal temperaturehot spot occurred gradient too betweenlate to be the passed hotspot around, and whichthe surrounding led to a large area. temperature Thermal stress gradient to the between temperature the hot gradient spot and occurred the surrounding in the interior area. of Thermal the ZnO stressvaristor to block.the temperature When the thermalgradient stress occurred reached in athe certain interior value, of the it caused ZnO varistor burst damage block. toWhen the ZnO the thermalvaristor stress block [reached25]. a certain value, it caused burst damage to the ZnO varistor block [25].

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(a) (b)

FigureFigure 11. 11. TemperatureTemperature distribution map of thethe ZnO varistor after impulses. (a) Varistor blockblock A1.A1. ((b)) Varistor block A4.

TheThe adiabatic adiabatic temperature temperature increases increases of of the the ZnO ZnO varistor varistor units units after after the the absorption absorption of of energy energy WW (unit:(unit: J) J) were were as as follows follows [25]: [25]: ∆𝑊 ∆W ∆T = (1) ∆𝑇 ∆V ρc (1) ∆𝑉𝜌𝑐 p InIn Figure Figure 11,11, Δ∆V isis the the volume volume of of the the ZnO ZnO varistor varistor units; units; ρρ isis the the proportion proportion of the ZnO ZnO varistors, 3 3 with anan averageaverage of of 5600 5600 kg/m kg/m;; and andC pCisp is the the constant-pressure constant-pressure heat heat capacity capacity of the of ZnOthe ZnO varistor varistor units, units,which which was approximately was approximately 500 J/kg 500◦ J/kgC at °C 20 ◦atC. 20 °C. IfIf the temperature increases of the adjacent two unitsunits werewere ∆ΔTT11 andand Δ∆T22, ,respectively, respectively, then then the the thermalthermal stress stress ff underunder the the effects effects of of the the temperature temperature gradient gradient was as follows [25]: [25]: 𝐸𝑎 𝑓 （∆𝑇 ∆𝑇） (2) 1𝜇Ea f = (∆T1 − ∆T2) (2) where E is the elastic modulus; a denotes the1 − thermalµ expansion coefficient; and μ is the Poisson’s ratio. When the thermal stress f produced under the action of the temperature gradient exceeded the where E is the elastic modulus; a denotes the thermal expansion coefficient; and µ is the Poisson’s ratio. thermal stress threshold during the rupture failure of the ZnO varistors material, it could potentially When the thermal stress f produced under the action of the temperature gradient exceeded the thermal lead to the destruction of the ZnO varistors. stress threshold during the rupture failure of the ZnO varistors material, it could potentially lead to The experimental phenomenon in which the varistor voltage presented a declining trend could the destruction of the ZnO varistors. be explained by the microstructure characteristics of the ZnO varistors. As shown in Figure 10, the The experimental phenomenon in which the varistor voltage presented a declining trend could SEM images confirmed that the interiors of the ZnO varistors were composed of many zinc-oxide be explained by the microstructure characteristics of the ZnO varistors. As shown in Figure 10, grains, and the distances between various grains were fixed. Meanwhile, transverse and the SEM images confirmed that the interiors of the ZnO varistors were composed of many zinc-oxide longitudinal capacitances were formed between the grains, which displayed a vertical distribution. grains, and the distances between various grains were fixed. Meanwhile, transverse and longitudinal The microanalysis showed that the grain sizes of the ZnO varistors became smaller, while the capacitances were formed between the grains, which displayed a vertical distribution. area of the grain boundary layer increased, which led to the increases in the capacitance of the grain The microanalysis showed that the grain sizes of the ZnO varistors became smaller, while the boundary layer as follows: area of the grain boundary layer increased, which𝜀𝑆 led𝑄 to the increases in the capacitance of the grain boundary layer as follows: 𝐶 (3) 4𝜋𝑘𝑑 𝑉 εS Q In Equation (3) [26], C represents Cthe= capacitance;= ε is the dielectric constant; S is the(3) 4πkd V cross-sectional area; k denotes the electrostatic force constant; d is the anode-to-cathode distance; Q In Equation (3) [26], C represents the capacitance; ε is the dielectric constant; S is the cross-sectional represents the charge quantity; and V is the voltage. area; k denotes the electrostatic force constant; d is the anode-to-cathode distance; Q represents the It can be seen from the above equation that, as the number of impulse groups increased, the charge quantity; and V is the voltage. capacitance of the grain boundary layer increased, while the varistor voltage gradually decreased. It can be seen from the above equation that, as the number of impulse groups increased, In the ZnO varistors under the multiple lightning impulse current, the leakage current value the capacitance of the grain boundary layer increased, while the varistor voltage gradually decreased. gradually increased. This was due to the fact that after several impulse groups, the ZnO varistors In the ZnO varistors under the multiple lightning impulse current, the leakage current value absorbed the impulse energy, and the temperatures rose. This sped up the rates of ion migration, gradually increased. This was due to the fact that after several impulse groups, the ZnO varistors resulting in the leakage current values displaying a growing trend.

Electronics 2019, 8, 172 12 of 13 absorbed the impulse energy, and the temperatures rose. This sped up the rates of ion migration, resulting in the leakage current values displaying a growing trend.

4. Conclusions In this study, experiments were conducted to investigate the microstructure and macroscopic properties of ZnO varistors under multiple lightning impulse currents. (1) The macroscopic failure mode of ZnO varistors under multiple lightning impulses could be described as the rapid deterioration of the electrical parameters with the increase of the number of impulse groups before destruction occurred by side-corner bursting. Under a ﬁve-pulse lightning current, the ZnO varistors were able to withstand an average of 16 groups of impulses. (2) The microstructural examination indicated that, after the multiple lightning strokes, the proportion of Bi in the crystal phases was converted, the grain size of the ZnO varistors became smaller, and the white intergranular phase (Bi-rich grain boundary layer) increased signiﬁcantly. (3) The ZnO varistors undergoing multiple lightning impulse currents presented adiabatic temperature increases. It was observed that, due to the unevenness of the material, the ZnO varistors displayed local temperature increases after absorbing impulse heat. The failure mechanism was thermal damage and grain boundary structure damage caused by the temperature gradient thermal stress generated by multiple lightning currents.

Author Contributions: Data curation, C.L.; Investigation, H.X.; Methodology, D.L. and S.Y.; Writing—original draft, C.Z. and P.L. Funding: This research was funded by the National Nature Science Foundation of China, NSFC Grants No. 61671248, Open Foundation of Jiangsu Key Laboratory of Meteorological Observation and Information Processing, KDXS1603, and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and The APC was funded by Heilongjiang Provincial Meteorological Bureau Scientific Research Project, HQ2015005. Acknowledgments: The authors are thankful to the anonymous reviewers for their valuable comments. Conflicts of Interest: The authors declare no conflict of interest.

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