BEST COPY AVAILABLE IEEE Region 10 Conference. Tencon 92 11th - 13th November, 1992 Melbourne. Australia

LABORATORY STUDIES OF THE EFFECTS OF MULTIPLE LIGHTNING CURRENTS ON LOW VOLTAGE ZINC OXIDE

V.4 R I STO R S M. Darveiiiza, S. Lester, and Y. Zhou Department of Electrical Engineering

University of Queensland Australia. 4072.

ABSTRACT: Multiple return st.rokes a.re a frequent. feature of the lightning ground flash. This paper examines the effects of multiple stroke lightning cur- rent,s on low voltage ZnO varistors, and provides ev- idence that sextuple 8/20ps current,s can cause dam- age not evident with standard lightning current. t,est.s.

Introduction

Electrical overstresses are the major causes of fail- ure, permanent degradation and t,emporary malfunc- tion of electronic systems and equipment. Over- stresses are i n the form of transient overvoltages and mainly originat,e froin light.ning, the swit.chiiig of ill- ductive loads, or electrost,at,ic discharge. Today's so- ciety is greatly dependent on electronic technology, and with electronic equipment that. may be life say- ing often vulnerable t,o the effects of elect,rical over- stress, protection of this technology has beconie vi- tally important.

Lightning, overall, is the most. cominon threat. t.o the reliability of electronic equipment and syst.elns. Light,ning occurs in two forms, cloud-t,o-cloud tlis- charge and cloud-to-ground discharge. Onlv cloucl- to-ground lightning causes damage t,o ground based electronic equipment and syst,eins.

Cloud-to-ground lightning occurs when a highly ionised plasma called the stepped leader grows down- wards from a charged thundercloud and is discharged 1.0 ground. Arc current. then t,ravels from t,he ground to the thundercloud and is called t,he return st.roke. After a few tens of milliseconds. another leader can travel down the conductive path and produce a sec- ond return stroke. This process can be repeated sev- eral times. The maximuin number of ret,urii st.roltes t.hat has been recorded is 26. The iiiost. comliioii number is 3 or 4, and the t.ime int.erval I x t . w c u i strokes is about 4 h S with a range of 10 to YOins [l]. These brief lapses between strokes cause light.- ning, as seen by the human eye. to flicker and ~ l i r whole event is described as a flash.

Transient overvoltages produced by liglitiiiiig can affect equipment via mains supply. telecomnlunica-

tion and data lines, grounding systems, or electro- niagnet,ic radiation. The most common method of suppressing t,ransient overvoltages, particularly in prot,ect,ing industrial equipment, is through the ap- plicat.ion of metal oxide varistors (MOVs). These de- vices are voltage dependent nonlinear resistors which have an electrical behaviour similar to back-to-back zener diodes. MOVs are commonly constructed by bonding zinc oxide grain particles in a ceramic ma- t ris. Electrodes are generally soldered to contacts t,hat. are often constructed of fired silver. The fin- ished device may then be encapsulated in a polymeric material and tested to meet set standards. MOVs eshibit. large non-linearity in current-voltage (I-V) characteristics and so provide the excellent voltage- limit,iug capabilit,ies for surge-current applications. Varist.ors function by clamping the surge voltage at a rated voltage level as the current through the device increases several orders of magnitude. This clamp- ing ahilit,y limits their current handling capability, as compared to comparably sized gas tubes which op- erat,? with a crowbar a.ction.

The main objective of this paper is to report on espriments in which multipulse lightning currents were applied to varistors to simulate the effects of mult,i-st~roke lightning currents. As noted above, manufacturers test MOVs after their construction t,o meet set standards. As prescribed in Standards [a ] , t.he tests include simulation of lightning currents in blie laboratory by application of 8/20ps impulse currents to the device under test in the laboratory. \\'hen more than one impulse is applied to the device during t,est.ing by the manufacturer, there is either a t,iine interval of 50 to 60s between successive im- pulses, or enough time is allowed between impulses to enablr t,he varistor to cool back to ambient tem- perat ure. Clearly, if lightning is observed to produce an average 3 or 4 return strokes each separated by around 40ms, then these laboratory tests are an inad- ec1uat.e indication of the ability of the MOV to with- st.and t.lie stresses from a typical lightning ground flash. If and when Standards mention multiple pulses [2. 31, it. is only in the context of degradation of life espectancy.

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To simulate the niult,i-stroke light,ning impulse cur- rents, use was made of a multipulse lightning iiiipulse generator developed by Darveniza et,. al. [4] for tests on distribution surge arresters [5]. This is capable of generating up to G impulses of 1.2/50ps voltages t,o 80kV or 8 / 2 O p currents t,o 8kA for each impulse, with t.inie intervals of 20 t,o 130111s between each im- pulse. It produces a realistic simulation in the lab- oratory of how a multiple-st.roke lightning ground flash may affect equipment. A digital stora.ge oscil- loscope (DSO) was used in conjunction with voltage dividers and resistive shunts to record tlie oscillo- grams of v( t ) and i ( t ) . The DSO was equipped 1.0 perform niatheniatical manipulations of the stored data and these were used to obtain the Joule inte- gral Ji?dt and the absorbed energy Jvi d t .

Results of Laboratory Tests

Three varieties of varist.ors were used i n thr exper- iments, types commonly used in industry. Table 1 summarises the device characteristics.

The DC voltage required t.o pass a current, of ImA (often referred to as the DC reference voltage), was measured for each device before and after test.s wit.11 single and sextuple 8/20ps impulse current.s. This lmA reference is a meawre of the breakdowii volt- age of tlie device, and is used as an indication of any changes in device characteristics. The before and af- t.er I-V cha.racteristics were a.lso nieasuretl for each device.

The Staiidards [2, 31 specify various cril.eria ror failure of an MOV under test, including a short.- circuit failure mode or a. degradat,ion failurr iiiotle when the device exhibits a shift. in the varistor volt- age a t 1mA in excess of 3~10% of t.lie initial value.

The results of the tests with sextuple impulse cur- rents are summarised in Table 2. The magnitudes of the multipulse currents applied to all device* except, one (TlS3) , were of the order of 75% of t,he maxi- m u n ~ rated current. The multipulse current applied t,o T1S3 was of the order of 100% of thr n~axiniuni rated current.

Of the 8 varistors su1)jected to niult.iple ciiireiil pulses, three were destroyed by short. circuiting (T2S1, T2S3, T3S1), four had t,lieir reference D(.' voltage degraded in the range 53'70 to 98.6% ( T l S l , TlS2, T lS3 , T3S2), and one was unaffected (T2S2). In contrast, experiments made on similar varistors by applying single-impulse currei1t.s a t 75 a i d 100%. rated currents only showed minor changes io DC ref- erence voltages and i n ot.lier device chalacterisl,ics.

The application of sextuple currents of magriit tide 75% of the maximum rated current caused a largr change in tlie DC reference voltage (sometiiiirs to tlie extent of short circuiting t,he device) a i d other device characteristics. The main reason for t.liis is tlie absorption of the incoming t,ransient. pulse energy by tlie varistor. This addit.ive energy ahsorpt.ioii hy

each test. MOV is illustrated in Fig. 1. At the end of the second pulse of the sextuple, each device had already absorbed a mean of 68% more than the rated maximum energy. The additive energy absorption of G impulses was seen to lead to severe degradation of characteristics or destruction, for 7 out of the 8 devices tested.

The application of multipulse currents caused three types of physical damage; namely, swelling of the plastic encapsulation surrounding the ZnO disc, da.mage to the disc in the vicinity of the iiiet,allised electrodes, or complete shattering of the MOV. Other studies have been made of the changes caused by sextuple currents to the microstructure of the ZnO material using metallurgical techniques [GI. These indicate that high temperatures occur a t 1oca.lised spots which are capable of causing cracks iii varistor discs and gas evolution from and phase changes i n tlie ZnO materials.

As stated previously, varistors of each type were subjected to single-impulse currents of the order of 75% a n d 100% of the maximum rated current. A Type 2 varistor was also subjected to increasing cur- rent impulses up to 13.25kA - 103.85% higher than the rated maximum. A Type 3 varistor was sub- ject,ed to increasing current impulses up to 9.72kA - 49.94% higher than the rated maximum. The DC reference voltage in each case remained within *lo% of the initial value. In these tests, sufficient time was allowed between impulses for the varistors to cool to ambient temperature. These results and those ob- t.ained from the multiple current application, indi- cat.e MOVs can withstand a single 8/20ps impulse current. up to their rated current and higher, but not. inultiple currents of magnitudes of about 75% of t.lieir rat,ing. Further t,ests with sextuple currents are required to determine a withstand rating for such currents.

The I-V characteristics of the four degraded spec- iiiieiis hecame essentially linear, with resistances of t.he order of tens of ohms. T h e single device that was st.il1 operational had an I-V characteristic, within *lo%, of the initial value.

Fig. 2 illustrates the residual voltage, applied cur- rent. ahsorbed energy and dissipated power of spec- imeii T S 3 .

Discussion and Conclusions

The aut,liors believe t,he results of these experiments are represent.at,ive of the performance of ZnO varis- tors wheii subjected to multiple lightning impulse wrren1 s.

The follo~ving conclusions can be drawn:

1. Electronic equipment and systems and their low-volt age protectors may not often be subjected

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to tlie full severity of a lightning strike. However they are often exposed to a portion of t,he light.- ning currents, and these do include multiple surge currents caused by multi-stroke events, which repre- sent a t least two-thirds of all cloud- ground liglit,ning flashes.

2. Multipulse currents caused more damage than st.andartl single-impulse current. test,s. 7 out. of 8 varistors failed the multiple current test at. 75% of maximum rated current. Of these, 3 were destroyed by short,-circuit and were physically damaged, while another 3 exhibited gross changes in their I-V char- acteristics. Devices subjected to single impulse cur- re1it.s (in one case 103.85% great,er than t.he rat,ed maximum) survived. The multipulse tests used are represeiitative of the surges caused by multiplest,roke lightning in the field, and it would seem that the atl- equacy of the standard impulse tests current.ly spec- ified for low voltage AC power circuits should bc rc- assessed.

New Orleans Aug/Sept 1989, paper 47.07, 5pp.

[5] M . Darveniza and D.R. Mercer, The Effects of Multiple-Stroke Lightning Currents on Distribution Surge Arresters, IEEE PES, paper submitted Jan. 1992.

[GI R.A. Sargent, G.L. Dunlop, and M. Darv- eniza, Effect,s of Multiple Current Pulses on the Mi- crostructure and Electrical Properties of Zinc Oxide Varistors, paper 267, 3rd Int. Conf. Props and Appl. of Diel. Matts (ICPADMSl), Tokyo, July 1991, IEEE Pub 91CH2937-1, Vol. 2, pp1120-3 (en- larged version accepted for IEEE Trans. Elect. Ins.).

Further work needs to be carried out. to est.ablish the magnitude of tlie mult.iple current. impulses as a percentage of tlie ma.ximuni rated current. t.liat. ail MOV can withstand without severe degradation.

The results of this research ha5 demonstrat.ed t lie detrimental effect of multipulse lightning currents on ZnO va.ristors, and it. is clear t,hat. similar siudics should also be made on ot,lier types of surge p r e tect.ive devices. This work is i n progress, and will perhaps result in future improvements i n the prot,er- tion of electronic devices and syst,ems.

Acknowledgement.s

The work reported in this pa.per was carried out. i n the High Voltage Laboratories at, the L1niversit.y of Queensland, using facilities funded in part by t,Iw Australian Research Committee (ARC). Thr Aua- tralian Telecommunication and Electronics Research Board (ATERB), and tlie Aust,ralian Electricity Sui)- ply Industry Research Board (AESIRB).

References

111 M.A. Uman, Light.ning. McGraw Hi l l . 1984

[a] IEEE CG2.33-1982. St.andard Test Slicrifica- tion for Varistor Surge-Protective Devices (ANSI). a i d IEEE CG2.11-1987, Standards for R4e1.al-Ositlr Surge Arresters for AC Po\tzer C:ircuits ( A X S I ) . .

[3] IEEE (262.41-1980, Guide for Surge \olt.ages in Low-Voltage AC Power Circuits (ANSI).

[4] M. Darveniza, C.J. Andrews. D.R. Mercer and T.M. Parnell, A Multipulse Impulse Gencrat.or, Sixth Int. Symp. on High Volt,age Eng. (ISH 89),

3 94

MOV Type

I (8/20Cr5) I Type 1 I 14mm I 150VrnI8 I 240V I 455 I 4.5kA I 3

Maximum Voltage Maximum Maximum Number Tested Diameter Operating a t lmA Energy Discharge with multiple

Voltage (lO/lOOps) Current impulses

Type 2

Type 3

Table 1: MOV charact.erist.ics (includitig single iinpulse energy and current ratings).

200vdc 2Omtn 150V,.,,,, 212-268V 8OJ 6.5kA 3

20111111 275V,,,, 389-473V 140J 6.5kA 2 200vdc

369Vdc

MOV Type and Specimen TYPE 1:

(TlS1) Multiple, 75%

(TlS2)

Reference voltage Reference volt,age Physical before applied stress after applied stress inspection

No visible

Device split into 3 sections. b;.e, = 235v I.;.,, = 13v defects.

Multiple, 75% (TlS3)

Multiple, 10074 TYPE 2:

Table 2: ImA DC reference volt.ages of MOVs beforr ant1 after t.esting with inultipulse currents at 75 or 100% of rated discliargr rrirrent. (refer Table 1).

Note: T l S l inrans RIO\ , ' ' lype 1 Specimen 1, etc.

VVe, = 232V = 23.9v 1 leg remained attached. '

b;.c, = 211V 1;.

Figure 2: Digital Storage Oscilloecope records for multiple impulse currenta applied to device T2s3.

(a) 6 8/20pr impulse currents, 4.4 to 4.7kA. (b) 1mpul.e reaidud voltages, 560 to 605V. (c) Dwipated power, peak 2.9MW. (d) Energy absorbed ( l v i dt) , peak 275J.

Timebaee suppressed (S) where shown, otherwise 5ps/div.

450

400

350

300

150

100

50

0 1 2 3 4 5 6

Impulse Number

Figure 1: Accumulated Energy Absorbed by Varistors During Successive Impubes.

Note: The results for T2S1, T2S2. and T2S3 are very cloee together.