Copyright 2014 IEEE. Paper presented at 2014 IEEE conference on electrical insulation and dielectric phenomena, Des Moines, USA, October, 2014.

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1 Copyright 2014 IEEE Paper presented at 2014 IEEE conference on electrical insulation and dielectric phenomena, Des Moines, USA, October, This material is posted here with the permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of ABB s products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs-permission@ieee.org. By choosing to view this document, you agree to all provisions of the copyright laws protecting it.

2 Effect of high dielectric protrusions on the breakdown phenomena of large electrodes under positive switching impulses Liliana Arevalo R&D Department Power Systems, HVDC, ABB AB Ludvika, Sweden Dong Wu R&D Department Power Systems, HVDC, ABB AB Ludvika, Sweden Abstract To improve the insulation strength, larger electrodes are widely used on high voltage apparatus, bus terminals and interconnections. In practical applications, such large electrode may often exposed to natural conditions like rain, snow, insects and dust, among others. In order to take into account the influence of these high dielectric irregularities on the behavior of the fifty percent breakdown voltage, experimental and theoretical studies are required. This paper presents and analyses the results from a laboratory study on the effects of high dielectric protrusions, such as insects on the dielectric strength of a large electrode in an air gap. The test results indicate a drastic reduction of the dielectric strength due to the high dielectric protrusion located on the large electrode. It is theoretically demonstrated that the inception voltage of streamer is detrimentally reduced due to the local enhancement of electric field produced by the high dielectric protrusion. In addition, the unstable leader inception voltage was calculated and preliminary results indicate that the inception of unstable leader depends on the relation between the mean electric field of the set-up and the corresponding local electric field were inception of streamer takes place. Keywords breakdown voltage; high dielectric protrusion; streamer; unstable leader. I. INTRODUCTION The insulation design of high voltage apparatus and stations is determined by the switching impulse withstand levels and the creepage distance requirements, among other parameters. In order to reduce electrode surfaces stresses and to improve the voltage withstand capabilities, electrodes with large curvature radii are widely used in high voltage apparatus, bus terminals and interconnections. These arrangements to be used in indoor /outdoor stations will be subject to natural conditions such as precipitation as rain, ice and/or snow, as well as, small animals like insects e.g. flies, spiders, and any kind of bugs. Experimental investigations [1-5] have shown that the switching impulse breakdown voltage of a large electrode air gap can be seriously reduced under rain conditions, for both positive and negative polarities. Recent experimental works for large electrodes [6, 7] indicate the positive switching impulse breakdown voltage under rain conditions or with high conductive protrusions could be drastically reduced, and in some cases it could drop to the level of the breakdown voltage of a rod - plane arrangement [7]. In order to study the influence of surface dielectric irregularities on the breakdown of gaps with large electrodes experimental investigations were performed with large electrodes with spherical shaped corners. The surface irregularity was a high dielectric insect (ε r 90) located on the large high voltage electrode under positive switching impulses stress. II. EXPERIMENTAL SET-UP A. General All the tests reported in this paper were performed with switching impulse of positive polarity. The waveform was 250/2500 µs. The voltage level of the 50% breakdown probability, U 50, was obtained for each test set-up by the wellknown up-and-down procedure with 30 valid voltage applications in each test. During the test, the applied voltage and the waveform of the voltage were recoded. Two digital cameras were used to record the trajectories of the discharges. In all cases, the test object was installed with a distance of 13 meters to the nearest wall of the laboratory and variable distance to floor. The gap distance was adjusted by a crane under which the high-voltage electrode was suspended. All the test results presented here have been corrected to the standard reference atmosphere. The correction was made according to the procedure given in IEC [8]. B. Electrode arrangement The test object consisted on a quarter sphere part of a rectangular metallic shield, as illustrated in Figure 1.

Meanwhile, the breakdown without dielectric protrusion is incepted at top of the shield, at the ending point of the spherical shape, see Figure 4 Fig. 1. Metallic electrode.

The dielectric irregularity was a dead insect (fly) available in the high voltage laboratory with the following dimensions: 1.5 cm length, 0.5 cm width and 0.7 cm depth.

In the left side the electrode with dielectric protrusion and in the right side the most common starting point of breakdowns without protrusion. IV.

it is shown in Figure 5. Similar results have been observed on rain test measurements reported in [3, 5, 7] and with conductive protrusions [6, 10]. Fig. 2.

With the aim to characterize the effect of the dielectric protrusion, tests with and without insect were carried out. 2400 2200 2000 1800 27.

TEST RESULTS Test with and without fly were performed at two different distances to the floor: 4 and 5 m. The test results are given in the table below: Gap distance [m] TABLE I.

3 Meanwhile, the breakdown without dielectric protrusion is incepted at top of the shield, at the ending point of the spherical shape, see Figure 4 Fig. 1. Metallic electrode. The corner of the electrode is a quarter of a sphere, connected with metallic bands to the rest of a rectangular geometry. The dielectric irregularity was a dead insect (fly) available in the high voltage laboratory with the following dimensions: 1.5 cm length, 0.5 cm width and 0.7 cm depth. The relative dielectric constant ε r of the insect is approximately 90. See Figure 2. Fig. 4. Point of inception of the discharge. In the left side the electrode with dielectric protrusion and in the right side the most common starting point of breakdowns without protrusion. IV. ANALYSIS OF THE RESULTS AND DISCUSSION The results of the fifty percent breakdown voltage evince that the high dielectric protrusion reduces the breakdown voltage of the electrode detrimentally, as it is shown in Figure 5. Similar results have been observed on rain test measurements reported in [3, 5, 7] and with conductive protrusions [6, 10]. Fig. 2. Dimensions of the dielectric protrusion, dead insect (fly) The insect was located in the middle point of the quarter of the sphere, i.e., point of concentration of maximum electric field, as it is illustrated in Figure 3. With the aim to characterize the effect of the dielectric protrusion, tests with and without insect were carried out % 33 % U50 [kv] With protrusion Without protrusion Rod - plane Fig. 3. Location of the insect in the electrode. III. TEST RESULTS Test with and without fly were performed at two different distances to the floor: 4 and 5 m. The test results are given in the table below: Gap distance [m] TABLE I. FIFTY PERCENT BREAKDOWN VOLTAGES Without protrusion With protrusion U 50 [kv] Std. dev. % U 50 [kv] Std. dev. % Photographic observations during testing show that the all breakdowns started at the location of the dielectric protrusion. Distance [m] Fig. 5. Fifty percent breakdown voltage vs. gap distance for the gap with and without dielectric protrusion. The continuous gray line corresponds to the well-known rod plane gap arrangement according to Paris formula [9], the green line corresponds to the U 50 without protrusion and the red line to the U 50 with the dielectric protrusion. It is clear that a dielectric protrusion as the fly can reduce the breakdown voltage of the arrangement. The U 50 reduces in a 27.7% for a gap distance of 4 m and 34.5 % for a gap distance of 5 m. The severe reduction of the breakdown voltage of the electrode with dielectric protrusions is probably due to the breakdown mechanism. The high dielectric protrusion even though is not conductive can enhance the electric field of the large high voltage electrode. Consequently, the voltage at which streamers are incepted reduces, and then the leader inception and propagation process will take place at lower voltage levels.

It is possible to observe in Figure 5, that albeit the U 50 of the gap is reduced due to the high dielectric protrusion, the U 50 values obtained are still higher than that of a rod - plane gap.

One can also notice that the longer the gap, the lower the breakdown voltage of the electrode with the dielectric protrusion. The test results show that for gap distances of 5.

4 It is possible to observe in Figure 5, that albeit the U 50 of the gap is reduced due to the high dielectric protrusion, the U 50 values obtained are still higher than that of a rod - plane gap. Gap factors of the order of 1.49 and 1.25 are still obtained with the dielectric protrusion in the gap. One can also notice that the longer the gap, the lower the breakdown voltage of the electrode with the dielectric protrusion. The test results show that for gap distances of 5.0 m, the U 50 is closer to the rod - plane gap than for the gap distance of 4.0 m. A. Streamer inception calculation The effect of dielectric protrusions can be studied theoretically by means of electrostatic calculations. The method proposed in [11, 12] was used. It consists on evaluate the background electric field arising from the high voltage source by means of FEM calculation, then whether the conditions necessary for streamer inception is satisfied at the electrode is checked [13]. If the streamer inception is fulfilled, the charge accumulated in the streamer region and the voltage level at which the condition is fulfilled are calculated. The method was applied to configurations with and without dielectric protrusion. The dielectric protrusion was represented as an ellipsoid with the dimensions of the fly and relative dielectric constant ε r of 90. The calculation is performed in per unit system p.u. The electrode is stressed with a double exponential source such a switching impulse and the surroundings of the high voltage electrode are considered as perfectly grounded. Figure 6 illustrates the results of the electric field calculation performed in FEM software. In the figure, it is pointed out the location of the maximum electric field and its magnitude. and the point of maximum electric field (1.3 V/m) when a dielectric particle as the fly is located on the surface of the electrode. Notice that all values are in p.u. For the case without protrusion, the maximum electric field is located at the top edge of the sphere (orange spot) with a magnitude of 1.11 V/m, the rest of the spherical area is stressed with an electric field of the order of 0.99 V/m. For the case with the dielectric protrusion, the highest electric field is located on the surface of the dielectric protrusion and its magnitude is 1.30 V/m. The surrounding region has an electric field magnitude of the order of 1.0 V/m. Electric field calculation demonstrates the hypothesis that the dielectric insect enhances the electric field. For this particular case the enhancement is of 30%. The local enhancement of electric field produces that the inception of streamers occurs at lower voltages than in the case without dielectric protrusions. Table II summarizes the voltage magnitudes at which the streamer inception criteria is fulfilled. The calculation indicates that the inception voltage is drastically reduced due to dielectric protrusion. The inception voltage is reduced to 46 % of its value without protrusion. It is important to outline that the streamer inception voltage for the dielectric protrusion between 4 m and 5 m distance to ground floor does not change significantly. This minor variation for two different distances to ground floor indicates that the inception of streamer depends mainly on the local electric field than on the mean electric field of the set-up. TABLE II. STREAMER INCEPTION VOLTAGES Gap distance [m] Voltage at which streamer inception criteria is fulfilled With protrusion [kv] Without protrusion [kv] Fig. 6. Electrostatic electric field at the electrode with and without dielectric protrusion. The upper figure illustrates the area with high electric field without protrusion and the magnitude and location of the point of the maximum electric field 1.11 V/m. The lower part shows the area with high electric field B. Streamer to leader transition Once streamers are incepted there is a possibility that the streamer propagates and thermalize to incept a leader discharge. It is well known from literature [13-16] that to incept an unstable leader a thermalization process is required. During the streamer propagation the gas temperature of the streamers is about 300 K, which equals to ambient temperature. The charge injected by the streamer channel supplies the energy to increase the gas temperature. If the relaxation of vibrational temperature makes that the neutral temperature continues to increase from ambient temperature to the critical value circa 1500 K, unstable leader inception could take place. A simplified criteria for the leader inception proposed by Gallimberti [13] based on a physical interpretation for the critical radius concept was used in this part of the paper. The model predicts that there is a minimum streamer charge (around 1µC) necessary for the stem to be heated above the critical temperature and for the leader channel to be launched.

5 The streamer evolution to unstable leader will depend on the amount of charge (1µC) able to thermalize the streamer steam on an unstable leader channel. However, if the charge generated by the first streamer is not enough to thermalize the streamer, the streamer should be able to continue propagating until the charge reaches the magnitude of equal or larger than 1 µc. Experimental and theoretical results [13, 14, 17] indicated that the streamer requires an average electric field of 450 kv/m to propagate. Consequently, the voltage source and the streamer zone might be able to keep such an average electric field in front of the streamer region, for the discharge to develop. Applying the criteria described in previous paragraphs, Table III shows the voltage level at which unstable leader inception criteria is fulfilled. The results show that there is a difference of ~10% for the voltage at which unstable leader inception criteria is fulfilled, between 4 and 5 m gap distance. For distance to floor of 4 m, the unstable leader inception happens direct after the streamer inception, i.e., the charge generated for the first streamer inception was enough to thermalize the channel and incept an unstable leader. However, for a distance to ground floor of 5 m, the first streamer was not enough to generate at least 1 µc to thermalize the channel. The charge generated during the streamer propagation was able to fulfil the criteria. TABLE III. UNSTABLE LEADER INCEPTION VOLTAGES Gap distance [m] Voltage at which unstable leader inception criteria is fulfilled With protrusion [kv] Base on the few results obtained here, it is possible to suggest that the stage of inception of an unstable leader depends on the relation between the mean electric field produced by the electrode arrangement and the local electric field enhancement produced by the high dielectric protrusion. However, other phenomena as tortuous of the leader channel, atmospheric conditions, and irregularities on the ground electrode, among others will determine the propagation of the leader-streamer and finally the breakdown voltage. V. CONCLUSIONS Experimental and theoretical studies demonstrate that high dielectric protrusions can reduce drastically the breakdown voltage of a large electrode ground arrangement under switching impulses. It is theoretically proved that the dielectric protrusion enhances the local electric field and consequently the streamer inception happened at lower voltages levels than the electrode arrangement without protrusion. The unstable leader inception was evaluated based on physical principles of the discharge and results suggest that the inception of unstable leaders depends on the relation between the mean electric field and the local electric field enhancement generated by the high dielectric protrusion. REFERENCES [1] E. Comellini. Possible improvements in EHV overhead line insulation to switching surges. IEEE Transactions on Power Apparatus and Systems, Vol. PAS-90, 1971, pp [2] H.M. Schneider, F. J. Turner. Switching surge flash over characteristics of long sphere-plane gaps for UHV station design. IEEE Transactions on Power Apparatus and Systems, Vol. PAS-94, N 2, March/April 1975, pp [3] F. Rizk, Influence of rain on switching impulse sparkover voltage of large electrode air gaps. IEEE Transactions on Power Apparatus and Systems, Vol. PAS-95, N 4, July/August 1976, pp [4] G. Gallet, B. Hutzler, J. Riu. Analysis of the switching impulse strength of phase to phase air gaps. IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, N 2, March/April 1978, pp [5] F. Rizk, Effect of large electrodes on sparkover characteristics of air gaps and station insulators. IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, N 4, July/August 1978, pp [6] D. Wu, L. Arevalo, M. Li, M. Larsson. Switching impulse test of large sphere-plane gaps with protrusion on large spheres. ISH th international symposium on high voltage engineering, pp. 519, August 2013, Seoul, Korea [7] L. Arevalo, D. Wu, Influence of rain on the switching impulse breakdown behaviour of post insulators with large electrode. Auckland CIGRE Symposium "Best practice in generation, transmission and distribution in a changing environment" Paper 243, September 2013, Auckland, New Zealand. [8] IEC. IEC High voltage test techniques Part 1: General definitions and test requirements. [9] L. Paris, R. Cortina: Switching and lightning impulse discharge characteristics of large air gaps and long insulating strings IEEE TPAS, Vol. PAS-87, No. 4, pp , April 1968 [10] C. Menemenlis, G. Harbec, J. F. Grenon: Switching-impulse corona inception and breakdown of large high-voltage electrodes in air IEEE TPAS, Vol. PAS-97, No. 6, pp , Nov/Dec 197 [11] L. Arevalo, V. Cooray, D. Wu, B. Jacobson. A new static calculation of the streamer region for long spark gaps. Journal of electrostatics 70 pp [12] L Arevalo, D. Wu, B. Jacobson. A consistent approach to estimate the breakdown voltage of high voltage electrodes under positive switching impulses. Journal of Applied Physics 114, to 14. August [13] I. Gallimberti, The mechanism of the long spark formation Journal of Physics Coll, 40 (C7), pp , 1979 [14] N. Aleksandrov, E. Bazelyan, E. Konchakov, Plasma parameters in the channel of a long leader in air, Plasma physics. Rep., 27, pp , [15] E. Bazelyan, Y. Raizer, N. Aleksandrov, The effect of reduced air density on streamer to leader transition and on properties of long positive leader, Journal of physics D Applied physics, 40 (14), , 2007 [16] E. Marode, The glow to arc transition in Electrical breakdown and discharges in gases. Macroscopic Discharges, vol. B, edited by E Kunhardt and L Luessesn, pp , Plenum Press, New York, 1983 [17] Les Renardières Group. Research on long gap discharges at Les Renardières. Electra N

The leader propagation velocity in long air gaps

The leader propagation velocity in long air gaps Liliana Arevalo, Dong Wu Research and development ABB Power Grids, HVDC Ludvika, Sweden Liliana.Arevalo@se.abb.com Pasan Hettiarachchi, Vernon Cooray, André