INTRIGUING OBSERVATION ON THE BREAKDOWN TRAJECTORY OF LARGE AIR-GAPS UNDER SWITCHING IMPULSE VOLTAGES

Li Ming 1*, Dong Wu2, Urban Åström2 and Gunnar Asplund2 1 ABB AB, Corporate Research, SE-721 78 Västerås, Sweden

2 ABB AB, Power Systems, SE-771 80 Ludvika, Sweden *Email: li.ming@se.abb.com

Abstract: In an insulation arrangement with a high voltage electrode surrounded by several grounded objects, i.e. a multi-gap system, the breakdown may take place over longer gap spacing instead of the shortest one. This is especially the case when several long gaps are stressed simultaneously by switching impulse voltage. In order to facilitating the insulation design of UHVDC systems, the authors of this report have conducted laboratory studies on the multi-gap system, representing the situations inside the converter valve hall. The electrodes used there are often of a large curvature. They are located at the places surrounded by grounded objects such as the walls of the valve hall. Many interesting results have been obtained. In this paper some of the intriguing observations on the breakdown trajectories are presented.

1 INTRODUCTION

It is well know that breakdown trajectories often not take the shortest way to bridge the gap. In an insulation arrangement with a high voltage electrode surrounded by several grounded objects, i.e. a multi-gap system, the breakdown may take place over longer gap spacing instead of the shortest one. This is especially the case when several long gaps are stressed simultaneously by switching impulse voltage. Such phenomenon has already been observed and studied in 60th during the development of equipment for EHV transmission system. The breakdowns with trajectory bridging the longer gap instead the shorter one, which is often the real test object, were referred initially as “anomalous” breakdown. But, it is soon realized as rather normal [1, 2]. Today, this phenomenon has been better understood thanks to the deeper knowledge on the streamer and leader mechanism, the vast study on the gap factors of various gaps structures, and increased capability on electric field simulation, e.g. [3-6]. Many studies have contributed to the determination of the criteria on how to avoid the effects of other grounded objects on the test objects by keeping these objects out off certain distance [5]. Some of the studies were aimed at the design of the tower window of the transmission lines. Method for probability calculation has been introduced into the evaluation of the dielectric strength of the multiple gap system [7-8]. However, in such statistic method, the gap factors of the individual gap without considering the influence of other objects were used. The multi-gap system has been treated as several parallel gaps. In addition to this, most of the studies in literature were carried out with electrodes of small curvature like a rod-rod gap.

In order to facilitating the insulation design of UHVDC systems, the authors of this report have conducted laboratory studies on the multi-gap system, representing the situations inside the converter valve hall. The electrodes used there are often of a large curvature.

They are located at the places surrounded by grounded objects such as the walls of the valve hall. Many interesting results have been obtained. In this paper some of the intriguing observations on the breakdown trajectories are presented.

2 EXPERIMENTS

The experiments were carried out at the high voltage laboratory of ABB at Ludvika, Sweden. Test voltage was positive switching impulse (190/2085 µs).

The test objects were tube-sphere-plane-wall gaps with or without suspension or support insulators. A reference test with rod-plane gaps was also performed. Ground walls are regards as the important parts of the electrodes system.

Ground wall

+SI

Figure 1: An example of HV test arrangement (along the axis-line of HV connection)

Paper Published on the16th International Symposium on High Voltage Engineering, Cape Town, South Africa, 2009

Figure 1 shows an example of HV test arrangements and this is the view along the axis-line of the HV connection. Two simulation walls were made from steel net and the steel nets were welded to the floor. The two walls crossed at right angel.

Two cameras were installed for observing the flashovers. For every shot, two photographs were recorded simultaneously. The photographs shown in the paper are overlaps of photographs of breakdown trajectories.

3 OBSERVATIONS

3.1 Influence of electrode configuration Figure 2 shows the flashover trajectories in the rod-plane air-gap test, where the distance from the rod to the ground-wall was much larger than the distance to the floor. As was expected, no flashovers took the shortest direct path in the air-gap and almost all of the trajectories wandered away from the rod-plane axis.

Figure 2: Photograph of the flashover trajectories in the rod-plane air-gap test

Figure 3: Photograph of the flashover trajectories in the large sphere-plane are-gap test

The gap distances in the test shown in Figure 3 were the same as those in Figure 2, but the HV electrode was changed into a large sphere. The flashover trajectories in this test showed some differences from those in Figure 2. The trajectories wandered away from the sphere-plane axis already from the starting point of the flashover at sphere surface.

3.2 Influence of ground wall As the distance between the sphere and the wall decreased, the photographs of the flashover trajectories are shown in Figure 4a and 4b taken by camera No. 1 and No. 2, respectively. From the figures, it can be seen that the some flashover trajectories abandoned its original direction and struck finally on the ground floor or wall.

Figure 4a: Camera No.1 - photograph of the flashover trajectories in the large sphere-plane are-gap test

Figure 4b: Camera No. 2 - photograph of the flashover trajectories in the large sphere-plane are-gap test

The ratio of flashovers to the wall and the floor depends sensitively on the ratio distances of the sphere to the wall and floor.

3.3 Influence of base frame The distances of the sphere to the wall and the floor for the tests shown in Figure 5 and 6 were the same. The difference was that a steel base frame was introduced in the test in Figure 6. Distribution of the flashover trajectories in the gaps are listed in Table 1. No significant changes on 50% breakdown voltages of the air-gap systems were observed due to introduction of a base frame into the sphere-plane gaps.

Figure 5: Photograph of the flashover trajectories in the large sphere- plane air-gap test

Figure 6: Photograph of the flashover trajectories in the large sphere-base frame-plane air-gap test

Table 1: Percentage distribution of flashover trajectories in the gaps without and with base frame

Percentage of flashovers Flashover trajectory

Test in Fig. 5 Test in Fig. 6

Sphere-floor 20% 7%

Sphere-wall 80% 60%

Sphere-base frame 30%

3.4 Influence of insulators The distances of the sphere to the wall and the floor for the tests shown in Figure 7 and 8 were the same. The difference was that one test was plain air gaps, while in another test, a support insulator introduced into one of the air gaps, i.e. the gap between sphere and floor.

For cases with the insulator, the flashover trajectories bridging the sphere and floor can be divided into two types: striking direct at the floor through the free air and jumping from one flange to another before attaching on the floor, as shown in Figure 8.

Figure 7: Photograph of the flashover trajectories in the large sphere- plane air-gap test

Figure 8: Photograph of the flashover trajectories in the porcelain post insulator test with the large sphere as a HV electrode

The test arrangements shown in Figure 9 and 10 were roughly the same as the test in Figure 8, but a small change was made in both tests. For the test shown in Figure 9, the bottom section of the insulator was short-circuited. For the test shown Figure 10, a small toroid was installed up the first flange of the insulator. It was observed that the directions of flashovers were therefore changed.

Table 2 shows the percentage distribution of flashover trajectories in the gaps without and with insulator. It is clear that the flashover propagation directions were sensitively affected by the installation of the insulator.

Figure 9: Photograph of the flashover trajectories in the porcelain post insulator test with bottom section of the insulator short-circuited

Figure 10: Photograph of the flashover trajectories in the porcelain post insulator test with a toroid at the first flange

Table 2: Percentage distribution of flashover trajectories in the gaps without and with insulator

Percentage of flashovers Flashover trajectory Test in

Fig. 7 Test in Fig. 8

Test in Fig. 9

Test in Fig. 10

Sphere-floor 0 % 23% 6% 33%

Sphere-insulator-

floor 31% 25% 7%

Sphere-wall 100% 46% 69% 60%

3.5 Influence of insulator-base frame The difference of the impacts of insulator and insulator-base frame on the flashover trajectories can be seen from the tests results shown in Figure 11 and 12. In both tests, the insulator lengths and the air-gap distances between the sphere and the wall are the same. The difference was that in one test, an insulator stood on the floor (Figure 11), while in another test, the insulator stood on base frame (Figure 12).

Table 3 shows the percentage distribution of flashover trajectories in the gaps with insulator and insulator-base frame. For the test with base frame, 100% flashover trajectories bridged the air-gap between the sphere and the ground wall.

Figure 11: Photograph of the flashover trajectories in the test with porcelain post insulator on the floor

Figure 12: Photograph of the flashover trajectories in the test with porcelain post insulator on the base frame

Table 3: Percentage distribution of flashover trajectories in the gaps with insulator and insulator-base frame

Percentage of flashovers Flashover trajectory

Test in Fig. 11

Test in Fig. 12

Sphere-floor 93% 0%

Sphere-wall 7% 100%

Sphere-base frame 0%

Figure 13: Photograph of the flashover trajectories in the porcelain post insulator test with the large sphere as a HV electrode

As the insulator length and air-gap distance between the sphere and the wall were decreased, as shown in Figure 13, the flashover trajectories bridged all of the gaps, i.e. sphere-floor, sphere-wall and sphere-base frame. The percentage distribution of flashover trajectories in the test is shown in Table 4.

Table 4: Percentage distribution of flashover trajectories in the gaps insulator-base frame

Percentage of flashovers Flashover trajectory

Test in Fig. 13

Sphere-floor 13%

Sphere-wall 25%

Sphere-base frame 62%

4 DISCUSSIONS AND CONCLUSION

It is well established in literature that the discharge trajectories can not follow exactly the background E-field-lines due to the distortion of local E-field by space charges. The randomness of avalanches developing at the head of the streamers changes the propagation direction of streamers. For long air gaps, since the voltage-drop in the leader-channel is low (0.5 kV/m), flashover trajectories may propagation along a longer path in a multi-gap system.

In such a multi-gap system, it was quite often that a flashover trajectory running towards one ground electrode turned suddenly in half way to bridge another ground electrode. The interactions between different electrodes are evident. Therefore, the module for parallel independent gaps could not be utilized to

evaluate the dielectric strength of such a multi-gap system.

The introduction of insulators into the multi-gap system further complicated the situations. The flanges with floating potentials make the distribution of the electric field uncertain. Slight changes in the gap, such as to install a small toroid, may cause significant changes the distribution of the trajectory routes.

In such a multi-gap system, all the clearances of different lengths are “critical” in certain levels. It becomes difficult to identify “the critical clearance”. This phenomena needs to be taking into account during the evaluation of the test results. It becomes also difficult to make atmospheric correction for the test result.

In some cases, the authors could predict roughly on where the flashover trajectory would go, while in many other cases, the outcomes were surprise. As a practical issue in the design of UHVDC substations, it is difficult to identify a safe area where discharge will not hit.

In such a multi-gap system, the size of the electrodes, the surface conditions of the electrodes may all have effects on the test results. Studies are still on the way by the authors. Hopefully new results will be available for publication in the near future.

5 ACKNOWLEDGEMENT

The author would like to acknowledge the contributions by Per Karlsson, Stefan Andersson on the test set-up. The author would like to acknowledge the contributions by Bengt Jonsson, Roland Johansson, Thomas Holmgren, Linus Jern and other engineers at ABB transformers for the excellent work on carrying out the laboratory tests.

6 REFERENCES

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[3] L. Paris and R. Cortina: Switching and lightning impulse discharge characteristics of large air gaps and long insulator strings, IEEE PAS-87, No. 4, 1968

[4] E. J. Los: A model for study of switching breakdown of long air gaps, IEEE PAS-97, No. 6, 1978

[5] Cigre: Evaluation of switching impulse strength of external insulation, Electra 94, 1984

[6] P. Domens, et al.: Propagation of the positive streamer-leader system in a 16.7 m rod-plane gap, J. Phys. D: Appl. Phys. 24, 1991

[7] D. B. Watson, et al.: Impulse flashover trajectory in air in nonuniform fields, IEEE, EI-28, No.2, 1993

[8] T. SHindo and T. Suzuki: A method of predicting anomalous flashovers, IEEE Power Delivery, Vol.10, No.3, 1995