NONDESTRUCTIVE TESTING IN DIAGNOSTICS OF HIGH-VOLTAGE VARISTORS

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1 XIX IMEKO World Congress Fundmentl nd Applied Metrology Septemer 6 11, 2009, Lison, Portugl NONDESTRUCTIVE TESTING IN DIAGNOSTICS OF HIGH-VOLTAGE VARISTORS Lech Hsse, Jnusz Smulko Fculty of Electronics, Telecommunictions nd Informtics, Gdnsk University of Technology Gdnsk, Polnd, lhsse@pg.gd.pl Astrct High voltge vristors tht consist of grined ZnO hve to e tested efore ssemling. The recommended method for stndrd industril testing of vristors demnds ppliction of high voltges nd intensive currents tht is inconvenient nd needs extensive power consumption. Nondestructive methods for vristor qulity nd endurnce evlution hve een proposed. They rely on the ppliction of resonnt ultrsound spectroscopy nd nonlinerity testing. The prmeters of ssessment hve een proposed nd the results of mesurements performed for severl hundreds of vristor smples re quoted. Keywords: vristor, dignostics, non-destructive testing 1. INTRODUCTION Vristors used in high-voltge electricl systems for over-voltge protection hve to e tested during mnufcturing using high-voltge (ove 400V) nd intensive current shocks. A high current density nd n intensive heting cn cuse irreversile destruction of the tested vristors [1-3]. Moreover, these testing methods require metlliztion of the vristor structures nd therefore cn e crried out t the end stge of production process. It mens tht elimintion of defective specimen needs n erlier preprtion of expensive contcts when compred to reltively smll cost of the pure ZnO mteril. We propose non-destructive methods using Resonnt Ultrsound Spectroscopy (RUS) or nonlinerity mesurement, effectuted t significntly lower power consumption in the erlier production stge efore metlliztion. Such test demnds mesurement systems tht cn precisely mesure resonnt spectrum of tested ojects in the pproprite frequency rnge or mesure Third Hrmonic Index (THI) fter stimultion of tested smple y n pproprite hrmonic signl. The defective specimens cn e then excluded from the further processing. Another nondestructive method rely on the noise spectroscopy [4]. The current-voltge chrcteristic of vristors depend on mount of grin types existing in its structure. The widespred presence of different kind grin contcts diminishes desirle nonliner vristor chrcteristics nd influences on its inhomogeneity mnifesting itself in the mechnicl properties which cn e detected y RUS. 2. VARISTOR SAMPLES The DC current-voltge chrcteristic of high-voltge vristor hs three distinguishle regions: prerekdown, rekdown nd sturtion (Fig. 1). A vristor ehves s highly resistive resistor (~10 9 Ω) t low voltge region. At rekdown region the DC current I is exponentilly proportionl to the pplied DC voltge U: I α = k U (1) The exponent α depends on type of oundry junctions tht exist etween the grins. Vristors sturte t higher voltges nd their resistnce does not exceed tens of ohms. They should exhiit s much s possile nonliner chrcteristics (high vlue of prmeter α). There re good, low nonliner nd liner junctions etween the ZnO grins. The exponent α > 30 indictes the most required grin contcts. The low nonliner contcts exhiit α 10. These vrious contcts cn e discriminted y the shpe of their voltge-current chrcteristics (Fig. 1). Fig. 1. Vristor current-voltge chrcteristics: () - generl, () - distinguished types of oundry junctions etween ZnO grins

2 A rel vristor is mixture of vrious grins nd its outcome chrcteristic depends on mount of grin types tht exist in its structure. The widespred presence of liner or wekly nonliner grin contcts decreses α nd diminishes desirle nonliner vristor chrcteristic t its reltively low voltge region, for exmple up to hundred volts. Thus, vristors qulity s nonliner element cn e evluted y degree of linerity of their DC chrcteristics in the mentioned voltge rnge. Three types of vristors hving different voltge threshold (280 V, 440 V nd 660 V) hve een mesured. All vristors hve the sme dimeter (30 mm with tolernce 0.5 mm Fig. 2) ut the higher voltge threshold ws otined for the thicker smples (3 mm, 4 mm nd 5 mm, respectively). during its scnning y mens of lser em. The imges of scnned surfces hving dimensions 50 μm 50 μm re shown in Fig. 3. Mrked differences in the grin structures etween higher nd lower qulity smples cn e seen. Fig. 2. Vristor specimens: ZnO structures fter eing fired (top row) nd with metlized contcts (ottom row); left 280 V, middle 440 V, right 660 V; - surge rrester with ZnO vristor Two tches of smples (every contining 100 specimens) for ll the mentioned ove voltge thresholds were produced for testing. There ws difference etween proportions of liner nd nonliner grin junctions presented in ech tch s result of chnges introduced rtificilly into the mteril composition. It hd impct on their DC current-voltge chrcteristics. Therefore, one of the tches could e treted s group of lower qulity vristors with higher lekge currents. Vritions in grin size of ZnO structures suggest differences lso in their mechnicl properties (for exmple stiffness, sound velocity) tht cn e identified y convenient mesurements during their friction. The pper proposes new method of qulity ssessment of ZnO structures sed on resonnt ultrsound spectroscopy (RUS) tht cn e implemented in n industril ppliction for the low cost nd short mesurement time. The structure of grins in tested specimens ws preliminry oserved y mens of n Atomic Force Microscope (AFM) with scnning proe enling to otin of 3D imges od scnned surfces. The AFM mesured n interction etween the proe nd the surfce of vristor Fig. 3. Surfces of vristor structures on 280 V structure: - higher qulity, poor qulity Mrked differences in the grin structures etween higher nd lower qulity smples cn e seen. 3. RESONANT ULTRASOUND SPECTROSCOPY RUS is known s n coustic, nondestructive technique used to study the elstic properties of vrious mterils [5]. It enles to mesure nturl frequencies of free elstic virtions of simply shped specimens. A typicl mesurement relies on scnning selected frequency rnge including the pproprite resonnces of the mesured specimens. Using this method it is possile to mesure resonnt frequencies nd frequency responses even for the smll smples (mm sized). To nlyticlly determine the nturl resonnce frequencies of the vristor discs specimens nd to prevent from detecting spurious resonnces tht cn pper due to mechnicl nd electricl coupling in the mesurement system the Finite Elements Method (FEM) modelling of the tested oject ws used. The mesured vristor discs were modelled using elstic constnts corresponding to zinc oxide. Shpes of the two lowest resonnce modes for vristor on nominl voltge 280 V re shown in Fig. 4 [7].

3 Fig.6. Overll view of system The vristor ws closed into the specilly shielded round metl ox pdded with cork s n nti-virtion isoltion mteril (Fig. 7). Fig. 7. Mechnicl prts of RUS system: metlic shielded ox with cork surrounding mesured vristor, hndle of the ox Fig. 4. First two resonnce modes clculted for the 280 V vristor It should e noted tht proper identifiction of the lowest modes is prticulrly importnt for the RUS procedure proposed y Migliori[5] System for RUS Smples hve een mounted etween two trnsducers t opposite corners with miniml pressure. One trnsducer excited the smple nd the other one mesured its response. A hrmonic signl with sweeping frequency ws put into the input of the sensor exciting virtions in the tested vristor (Fig. 5 nd Fig. 6). Signls of oth sensors were mplified y two independent mplifiers; the receiving premplifier ws working s chrge mplifier (its inherent noise should e negligile). As stimuli the genertor (NI PCI 5406) ws used, nd the output signl from the receiving premplifier ws put into the NI PCI 6132 DAQ enling the signl processing up to 3 MHz/s/chnnel. The system cn mesure vristor virtions within the frequency rnge khz y sweeping the frequency with resolution elow one Hz. The whole system ws working under the LVIEW control (Fig. 8) [6]. Fig. 5 Simplified lock digrm of system for RUS of vristors Fig. 8. Front pnel of virtul instrument prepred to mesurement system control

4 The procedure enles to set the mplitude of the stimuli signl, the frequency rnge nd the step of frequency sweeping wht gives pproprite spectrum resolution Results of mesurements Tking into ccount the verge velocity of ultrsonic signl in the vristor mteril (out 8300 m/s) nd frequencies of different elstic modes, the pproprite resonnce frequencies ws chosen to select during testing good nd defected smples. It ws 175 khz, 153 khz nd 130 khz for specimens 280 V, 440 V nd 660 V, respectively. Exemplry resonnce spectr for smple 280 V no. 57 is shown in Fig. 9. Thus, the prmeter Q cn e interpreted s proportionl to n verge sound velocity within vristor structure nd the velocity is dependent on qulity (homogeneity) of vristor. The exemplry collective sttisticl results of the Q vlue for the 100 smples 440V re shown in Fig. 11 [5]. Fig. 11. Sttisticl shift of prmeter Q to the lower vlues for 440 V smples with poor qulity (440-A poor qulity smples, 440-O good qulity smples) Fig. 9. Spectrum of resonnces for smple 280-A57 Generlly, vristor resonnce frequencies hve een shifted to the lower frequency rnge for smples with poor qulity (Fig. 10). A shift of the Q vlues into the left for the defective smples cn e esily seen. It is due to the lower velocity of the signl in the poor qulity, heterogenous smples. A full seprtion etween good nd defected smples hs een chieved. Therefore the proposed prmeter Q seems to e good criterion for the selection of vristors on good nd poor qulity components. 4. TESTING OF NONLINEARITY Differences in grin structures etween high nd lower qulity smples hve impct on their DC current-voltge chrcteristics (Fig. 12). Fig. 10. Difference (only for three smples due to clrity) etween resonnce frequencies for group A (poor qulity, t the top) nd group B (good qulity, t the ottom) for the sme type of vristor structures We propose to use homogeneity prmeter Q s criterion of smple selection during industry production. The prmeter Q ws evluted on the sis of the vlues of resonnt frequency f r nd ctul dimensions of tested discshped vristor structures: 2 2 Q = f r ( w + d ) [Hz m] (2) where: w thickness of vristor, d dimeter of vristor. Fig. 12. Exemplry chrcteristics of two vristors 660 V (lin-lin scle): higher (red squres) nd poor (lue squres) We decided to mesure the THI t smll currents tht flow through the vristor excited y hrmonic signl t voltge mplitude up to 100 V. Such tests re nondestructive ut demnd mesurement system tht cn precisely mesure third hrmonic component eing t lest five orders lower thn the excittion signl. Thus, vristors qulity cn e ssessed y linerity of their DC chrcteristics [8] in this voltge rnge The mplitude of the third hrmonic U 3 ws mesured in specilly prepred setup (Fig. 13) tht consisted of hed with five contct electrodes for ech side of vristor, THI

5 meter nd computer controlling the dt collection process. The hed with multiple contct electrodes enled to differentite components for which the current density ws not homogeneous in the vristor volume. The springing contct electrodes gripped firmly the vristor. Additionlly two trusses t the ottom of the hed centred position of the vristor to mke mesurement results more repetle. Fig. 13. Hed for non-metllized vristor mesurement with multiple contct electrodes tht grip the tested vristor We mesured within ll groups of the prepred vristors whether it is possile to identify their qulity y mesurement of the third hrmonic index t reltively low voltge only. Thus, the third hrmonic index ws mesured t 100 V within set of the prepred smples to settle this issue. The exponentil dependence etween the excittion signl U 1 nd the mesured third hrmonic component U 3 ws oserved. Vristors from the high qulity tches exhiited on verge slightly lower exponent round 2 when compred with the results oserved in the lower qulity tches. This outcome is in good greement with the oserved differences in DC current-voltge chrcteristics for oth types of tches. A more liner DC chrcteristic t low voltge rnge for the lower qulity tch mens lower THI component tht egins to rise fster (higher exponent) when compred with ehviour oserved for high qulity specimens. At the next step, the mesurements were mde within set of out hundred specimens for ech tch, seprtely for ll three voltge series: 280V, 440V nd 660V. The exemplry sttisticl dt for the series 280V without metlliztion hs een shown in Fig. 14. Fig.14. Distriution of third hrmonic component U 3 oserved t excittion signl of mplitude U 1 =100V within low nd high qulity vristor tches without metlliztion for series 280V The high qulity elements exhiit on verge significntly higher third hrmonic component vlue difference is up to 30%. 5. CONCLUSIONS A proposed non-destructive methods of vristor qulity nd endurnce testing (RUS nd THI mesurement) could e successively pplied for vristor specimens t the production stge efore the metlliztion of contcts. An mplitude U 1 =100 V of hrmonic excittion signl is sufficient to distinguish etween higher nd poor qulity vristors prepred for the threshold voltge 280 V. The differences in third hrmonic index vlues etween oth groups of vristors were up to 30%. Vristors series on higher voltges need n excittion signl with higher mplitude to chieve sufficiently high vriety of this prmeter however such n instrument ws not ville during our studies. A full seprtion etween good nd defected smples hs een chieved for RUS. These methods could e pplied for qulity detection of other griny mterils. Unfortuntely, noise spectroscopy [4] is very time consuming method nd ecuse of interferences in n industry environment could e pplied only in the lortory reserches. REFERENCES [1] P.R. Bueno, J.A. Vrel, E. Longo, ZnO nd relted polycrystlline compound semiconductors: An overview nd review on the voltge-dependent resistnce (non-ohmic) feture, Journl of the Europen Cermic Society, Vol. 28, pp , [2] K. P. Mrdir, T. K. Sh nd R. A. Sutton, The effects of electricl degrdtion on the microstructure of metl oxide vristor, Trnsmission nd Distriution Conference nd Exposition, vol. 1, pp , 2001 IEEE/PES. [3] M. Brtkowik, M. G. Comer nd G. D. Mhn, Filure modes nd energy sorption cpility of ZnO vristors, IEEE Trns. on Power Delivery, vol. 14, no. 1, pp , Jnury [4] L. Hsse, J. Smulko, Qulity ssessment of metl oxide vristors y noise spectroscopy, pper sumitted to XIX IMEKO World Congress, Lison, Portugl, Sept [5] A. Migliori nd J. L. Srro, Resonnt Ultrsound Spectroscopy: Applictions to Physics, Mterils Mesurements, nd Nondestructive Evlution, Wiley, [6] L. Hsse, J. Smulko, Resonnt Ultrsonic Spectroscopy in High-Voltge Vristor Dignostics, Proc. 1 st Intern. Conf. on Informtion Technology, Gdńsk, My 2008, pp [7] L. Hsse, M. Kiwilszo, J. Smulko, T. Stepinski, Qulity ssessment of ZnO structures y resonnt ultrsound spectroscopy, Insight: Non-Destructive Testing nd Condition Monitoring, vol. 51, no. 5, pp , My [8] Jinling He, Rong Zeng, Qingheng Chen et l.: Nonuniformity of electricl chrcteristics in microstructures of ZnO surge vristors, IEEE Trns. on Power Delivery, vol. 19, pp , 2004.