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  :: Dynamic impulse conduction in ZnO arresters
  :: °ü¸®ÀÚ 2007-06-13 17:13:27 , Á¶È¸ :123058  
  :: File download   [pdf : 1737 KB  Download: 7257]
DYNAMIC IMPULSE CONDUCTION IN ZnO ARRESTERS
 
A Huddad
Cardiff Universisy, Wales, UK
P Naylor
BICC Supertension & Subsea Cables Ltd, UK
 

ABSTRACT
In. this paper, Ilew fast-impulse test data obtained all ZnO surge arreters with a special coaxial tell cell are presented.
The rine to peak of the discharge current was observed to become shoner as Ihe magnirude was increased. An
explanation for this phenomefloll is suggested based on changing current paths within tne material. To accounI for this dynamic behavior, we propose an equivaleflf circuil which uses parallel paths wirh associared inductances and resistances to model the conduction process in the inrergranular layer.

 
l.introductton
 Since the introduction of zinc-oxide material in 1968, much research has been directed towards the
characterisation of the electrical behaviour of the material under various stress conditions. There is now an
extensive published literature on the response of the material to impulse current stresses of different shape
and amplitude. An aim of some of these investigations is to achieve an equivalent circuit representation which
would adequately simulate the observed test resuits[ 1-7].
A feature of many of the published equivalent circuits[ 1 ] is the representation of the zinc-oxide material by two
series sections; i) to account for the resistance of the zinc oxide grains and ii) to simulnte the properties of the
intergranular layers. The zinc-oxide grains are represented by a low resistance Renin wh o s~ effect is of
importance at very high impu lse discharge currents only .
The intergranular layers are represented by a paralJel resistance (RrJ - capacitance (Cjg) network, the resistance branch having a strong non-linear voltage current characteristic. For impulse currents in the kiloampere
range, the resistive curront dominates and the capacitance component can be neglectcd.
The impulse response in the low conduction regime of the material. however, s hows a significant capacitive component of the current. For impulse currents in the low-conduction regime, the materia! may be simulated by the capacitallce branch. Evidence has been published that indicates that this capacitance is also non¡¤linear. In addition to this basic representation, other components may be included such as an inductive component to represent the equivalent inductance of the metal oxide materiaVarrester body or to account for the mater ials response to stcep currents.
Much recent research on zinc-oxide surge a!Testers has conCCDlrated on the very fast transient response
c haracterisation [2-5). Due to the difficulty in obtaining reliab le test data for fast-rate-of-risc impulse currents, the
IEEE working group 3.4.11 (Application of Protective Devices Subcommittee, Surge Protective Devices
Committee)[4] has limited its efforts to the modelling of metal oxide surge arresters to current impulses with rise
times of 0.5).1s or greater. Extensive reviews have been publiShed [1.4,6] on the ZnO impulse response, voltage
measurement and equivalent circuits.
The present work is concerned with the transition from low to high conduction under fast-impulse cond ition s.
Firstly, new impro .... ed impul se tCSt data are presented, These test data were obtained using a fast transient
coaxial test module and improved voltage measurement methods[2,7J. The new test data obtained show that there
is no evidence of an overshoot on the residual voltage trace. In addition, they show, for the first time, that the
discharge current exhibits a longer time-to-peak at low amplitudes than at high a.mplitudes. Finally, an
equivalent circuit is proposed to account for these new observations. The cin::uit response is also computed and
compared with the test data.
 
2, FAST-IMPULSE TEST DA TA
2. 1. Test Pracedure
In order to cany out fast transient tests with minimumcircuit inductance a new coaxial test module has been
designed and constructed(7 J. The module is a lowinductance, test facility i!lcorporating integra] voltage
and current transducers. It allows tests to be carried out on I SkY arresters with measurements to SOkV at lkV{ns
and 5kA at IOAlns. For the very fast fronts , the source capacitors wcre arrnnged in a coaxial cOllfiguration to
minimise source inductar.ce.
The inductive effects on the measured values could be further minimised by measuring thc value of the residual
voltage at the installt of peak current when the rate of change of current is zero. In this way, a more realistic
representation of the resisti ve behaviour of the arrester can be achieved independently of the test circuit by
e liminating the effects of dildt on the measuring system, thus allowing comparison between different results.
 
2.2. Effect of Current ShApe
Tests were carried out applying impulses of similar amplitude but of different waveshapc in order to determine the effects on the voltage-current characteristics of the arrester. Two capacitor banks with a similar capacitance value but of different internal inductances were used in order to produce the desired current impulse shapes. It was obser ved that after the voltage front (-4Ons) the shapes of the resul ting residual voltage waveforms are quite dissimilar, the difference becoming more notablc at higher dischargc currents .
Because the residual voltage has not yet reached its penk value according to the voltage-current curve, it is
continuing to increase at a slower ratc t.han the initial jump due to the high non-linearity of the material.
The arrester branch inductance produces a voltage fall which is proportional to the ralC of change of currem which
become smaller towards the current.peak vlaue.
The residual voltage at peak current was found to vary lillie for the twO sources for similar discharge current
amplitudes, suggesting a unique resislance-vo ltage curve for the zinc-oxide material for this range of fast fronts.
 
2.3. Effect of Current Amplitude
Tests at ir.creasing charging voltage of the capacitance bank were conducted in order to check the voltage-current
characteristics of various distribution z.inc-oxide surge arresters. Figures 1.a to 1.d show voltage and current records measured on a 15kV rated surgc arrester.
These tests revealed, for the first time, thalthe time_to_ peak of the current decreases as the amplitude of the
current increases. Furthermore, it can be seen that the voltage traces do not show the initial voltage overshoot
previously_associated with z.inc-oxide tests even for these voltage, risetimes of not more than 400s. In the case of
Ihe Jow-inductance source, it was fowtd that the time-tocurrent peak decreases from about 2.SIfS at a peak
current of 100A to approximately 1.5 us at 5kA.
Figure 2 shows the relationship between time-to-curre nt peak and the amplitude of the cur-ent peak for three
ar resters of different manufac tUIe (arresters A, B and C).
It can be seen that the time to current peak reaches a constant value (-1.5jJs) when the arresters are operating
in the high-conduction regime (above -lkA). In Ihis region, most of the intergranular layers have broken down forming many current paths through the m!uerial. 1t is in this region that the resistance of the zinc- oxide grains becomes the main limiting (actor for conduction.
In contrast, the low-conduction regime shows time-Iocurrent peaks ranging from aboui 1.6).15 for CUTTent
amplitudes of approximately 500A to lime-to-peaks in the order of 5~lS for currents less Ihan l00A.
 
3. STATIC YOLTACE-CURRENT CHARACTERISTIC OF ZlNC-OXlDE SURGE ARRESTERS
   The V-I characteristic may be divided into three regions; the pre-breakdown region, the breakdown region and the upturn region. The pre-breakdown region of the characteristic is determined from direct or powerfrequency
voltages. The amplitude of the applied voltage is such that the resulting current is usually less than 10mA. Cominued application of current amplitudes greater than this value can result in excessive heat dissipation which may lead to premature ageing and thermal runaway of the arrester. Consequently, for characterisation where currents are in the hundreds of amperes and into the kilo-.lffipere range, impulse currents are applied. For current up to about 500A, switching impulses may be used. For characterisation in the kiloampere range, lightning or fast impulses are used because of their lower energy content.
 
4. M ULTIPLE-CURRENT-PATHS CONCEPT
 The experimental results (Figs. 1 and 2) show that zinc¡¤ oxide arrcsters have a dynamic vo][age-currem characteristic. The salient features of this dynamic behaviour are; i) a dependence of currcnl time-la-peak on current amplitude and ii) a residual voltage reaching ils peak value before the discharge CUTTent reaches its peak.
The time-to-current peak is seen to decrease to a minimum as the current amplitude increases. The simple
standard representation of zinc-oxide surge arresters cannot account for the decrease seen in the time-IO-peak
of the discharge current. A constant inductive element would produce a shorter time-ta-peak at a lower current
than at a higher current, which is the opposite of the measured effect. A non-linear inductance could be used
to simulate this effect, but it is difficult to determine its parameters from measurements. An alternative simpler
approach is to consider the development of discrete current paths through the zinc-oxide material, with the number of paths increasing as the impulse voltage amplitude increases. The low current (when the level of impulse voltage is low) will flow through the zinc oxide taking a path where inter granular layers are the easiest to break down. This path may not be the most direct path through the material, but as the level of the impulse voltage increases the intergranular layers which could not preViously be broken down by the lower voltage amplitude are now bridged. This results in the current paths increasing in number and becoming more direct.
To simulate this physical relationship would ideally require a circuit containing a large number of paths. The paths would have differing characteristics to simulate this current growth as the level of the applied impulse increased. However, for simulation purposes a model containing two parallel paths is proposed. It was found that the model gave good correlation with the laboratory test data.
 
5. PROPOSED EQUIVALEJT CIRCUIT
  he proposed equivalent circuit is shown in Figure 3. It comprises two series sections; one to represent the
resistance of the zinc-oxide grains (Rgrai,,) and the selfinductance (Lbcd) due to the physical size of the arrester
body and a parallel network to represent he properties of the intergranular layers. One branch of the network
carries the high amplitude discharge current, so that the branch has a highly non-linear resistance Rg and a low
value inductance Lc:. The second branch has a linear resistance R~ and a higher value inductance Ld to
account for the delay in low-current fronts and the multiple --current path concept. A capacitive element Cg
to represent the arrester shunt capacitance was also included in the equivalent network.
 
6. CIRCUIT SIMULATION
  The laboratory test circuit incorporating the equivalent circuit representation of the arrester described in the
previous section is represented in Figure 4. The circuit parameters were determined from laboratory
measurements or where applicable obtained from manufactures data. The stray components were estimated on the basis of the test circuit physical arrangement.
Figure 5 shows voltage and current oscillograms from the circuit simulations representing the residual voltage
of the arrester at the point of measurement (R:ccL,~J and the diSCharge current through the arrester. Examination
of the records show thai the current shapes give good agreement with lhose obtained in the laboratory (Figwe
I). Both Ihe labormory lest resulls and .he simulations show simi lar lime-to-currenl peaks for a similar current
amplitude, Furthermore, for current amplitudes in excess of J leA the res idual voltage al currenl peak shows good
agreement between laboratory tesl results and simulations .
 
7 . CONCLUSIONS
  The laboralory tests showed thai the time-la-peak of the arresler discharge current is dependent upon the
amplitude of the current. For low amplitude currents, the arrester exhibits longer time-Io-peaks than at higher
amplhudes.
  Since a single non-linear resistance function cannot reproduce the dependence of curren! time -to-peak on
current amplitude, an equivalent circuit for zinc oxide has been proposed to simulate the observations made in
the laboratory, namely the effect of a decreasing time-tocurrent peak as me-amplitude of the c urrent increases.
The equivalent circuit is based upon the assumption that multiple current paths are formed through the zinc oxide
and that the number of paths increases and the path lengths decrease as the current amplitude increases. The
results from the simulations show agreement with those obtained in the laboratory.
 
REFERENCES
1 . Haddad A., Elayyan as.B., German D.M. and Waters R.T.: nO surge arrester elements with mixed direct and 50 Hz voltages, lEE Proceedings, Part A, VoU 38, No.5, pp.265-272, 1991.
2. Haddad A., Nayl or P., Metwal ly 1., German D.M., Waters R. T.: An Improved Non-rnductive Impulse Voltage Measurement Technique for ZOO Surge Arresters, IEEE Trans. on Power Delivery, Vol.lO, No.2, pp.778 - 784,1995.
3. Schmidt W., Meppclink 1., Richter B., Feser K., . Kehl L. and Qiu D.: Behaviour of MO-surge arrester blocks to fast transients, IEEE TraRs. onPower Delivery, Vol.4, No:l , pp.292-300, 1989.
4. IEEE working group 3.4.11.: Modelling of metal oxide surge arresters, Transactions on Power Delivery, Vol. 7, No. I., pp. 302-309, 1992.
5. Kim I., Funabashi T., Sasaki R. , Hagiwara T. , Kobayashi M.: Study of ZnO arrester model for steep front wave, IEEE trans. On Power Delivery, VoL I I, No.2, pp.835-84 I, 1996.
6. Haddad A., Naylor P.: Finte Element computation of capacitance networks in multiple electrode systems: application to ZnO surge arresters, lEE Proc. Science, Measurement and Technology, Vol 14S.No.4 pp.l 29-135, 1998.
7. Haddad, A., Naylor, P., German, D.M., Waters, R.T.: A fast transient test module for Zno surge arresters, Meas. Sci. Techno!. , Vol. 6, pp560-570, 1995.
 
A CKNOWLEDGEMENT
The authors thank Profe ssor RT Waters and DM German for their assistance and useful technical discussions.
 
ADDRESS
Electrical Division , School of Engineer ing,
Cardiff Uni versity, PO Box 687,
Cardiff eF2 3m, Wales, UK.
TeL +44 12228759(>1
Fa x; +44 1222874735
S-MAII..: HADDAD@CP.AC,UK
  :: File download   [pdf : 1737 KB  Download: 7257]
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