научная статья по теме THE LIFETIME OF A HIGH-CURRENT TRIGGERED VACUUM SWITCH WITH MULTI-GAP Физика

Текст научной статьи на тему «THE LIFETIME OF A HIGH-CURRENT TRIGGERED VACUUM SWITCH WITH MULTI-GAP»

ПРИБОРЫ И ТЕХНИКА ЭКСПЕРИМЕНТА, 2014, № 3, с. 63-66

ЭЛЕКТРОНИКА И РАДИОТЕХНИКА

THE LIFETIME OF A HIGH-CURRENT TRIGGERED VACUUM SWITCH WITH MULTI-GAP © 2014 t. Z. Zhou, L. Zhao, J. Chen, L. Dong

Dispatching and Control Center, State Grid Zhejiang Electric Power Company Hangzhou, 310007, China E-mail: zhouzhengyang123@sina.com Received August 16, 2013

Abstract — Triggered vacuum switch (TVS) synthesizes the merits of spark gap and vacuum technology. A TVS sample with multi-gap is described in this paper, in which the main electrode is made up of spatially interleaved rods of opposite polarity on a ring. Three pairs of rods are adopted, which can form six main gaps, promoting the high current carrying ability evidently. Due to hundreds of high current pulses, the influence of vacuum arc to the trigger pin and main electrode is inevitable, leading to the termination of lifetime of TVS, especially in the erosion of trigger pin and arc deposition on the trigger surface. In this paper, the major influence factors of lifetime are analyzed and it is concluded that the main current arc is the key factor to determine the lifetime of TVS. As well the change of trigger voltage, trigger resistance and trigger delay will predict the termination of TVS lifetime.

DOI: 10.7868/S0032816214020347

1. INTRODUCTION

A triggered vacuum switch (TVS) is one of the important switch apparatuses in the field of high pulsed power system. It uses the vacuum as the insulation and the arc extinguishing material [1—3]. It has some advantages, such as compact structure, wide range of working voltage, high current carrying ability, fast insulation recovery, etc [4—8].

The research of TVS began on 1960s. Lafferty et al did abundant work on the TVS with flat electrode [9]. In 1970s, Boxman et al did research on the trigger and work principle of TVS and laid the foundation of TVS theory [10—12]. However, the vacuum arc concentration was unavoidable when the TVS with flat electrode carried high current. In order to solve the arc concentration, the GE Company proposed a kind of electrode with multi-rod, forming many arcing gaps [13— 14]. So, the vacuum arc would burn in the multi-gap, keeping the diffusion mode in the whole arc.

Recently, the TVS with multi-gap is improved constantly in Russia, and some commodities of TVS are developed, such as RVU-43, RVU-45, RVU-47, etc [7, 15, 16]. However, the basic theory work has not enough yet. In this paper, a TVS sample with multi-gap is fabricated, and a special trigger structure with ceramic surface is designed. The influence of vacuum arc to the working lifetime of TVS when conducting high current is analyzed and the electrical characteristics for lifetime forecast are discussed.

2. TVS STRUCTURE AND EXPERIMENT PLATFORM

The basic structure of TVS with multi-gap is shown as Fig. 1, with a height of 160 mm, diameter of 115 mm and weight of 4 kg. It includes a ceramic shell, a metallic shield, main electrode with multi-rod, and a trigger pin. The metallic shield can adjust the electric field distribution, and protect the inner surface of ceramic shell. The main electrodes conduct high current, each of which is made up of 3 pairs of rods, interlacing each other and forming 6 vacuum gaps. The distance of each gap is 10 mm. The trigger pin is set in the middle of cathode and connected to the trigger source, providing the initial plasmas for the TVS.

The experimental platform was set up as Fig. 2. The C is capacitor bank of 1.860 mF, with high current of

— Ceramic shell

Main anode Main cathode Metallic shield Trigger gap

Fig. 1. Basic structure of TVS with multi-gap.

64

ZHOU h gp.

Tr

D

Ri

R2

Fig. 2. Platform of experiment.

Trigger voltage, kV 9.10

4.50

0 20 40 60 80 100 120

Trigger times

Fig. 3. Surface variety under trigger current of 150 A.

Trigger surface

Fig. 4. Deposition of metal arc on the trigger surface.

more than 100 kA and rated working voltage of 10 kV. R2 is high power resistance of 10 mQ, which is used as the load. In fact, the road includes another 4 mQ of arc resistance and connected resistance, and 0.6 ^H of inductance of the loop.

3. THE MAIN FACTORS OF WORKING LIFETIME

3.1. Trigger current

The literature [11] proposed that, when the trigger current reaches 100 A, the deposition of trigger arc to the trigger surface is serious. After trigger experiments of 50 times, the effective resistance of trigger surface reduces from the 20 MQ to the 55 kQ rapidly. The trigger surface is also ceramic in [11], and the effective distance of trigger surface is only 0.1 mm. However, the effective distance of trigger surface in this paper is 0.5 mm.

A trigger current of 150 A was designed in this paper. When the main gap voltage is zero, the trend of

trigger voltage is shown as Fig. 3 after 100 times tests. From the curve fitting, it is got

y = 5.74x°.°435. (1)

Here, the trigger voltage shows the change of resistance of trigger surface. When trigger voltage rise, the resistance of trigger surface is augmented.

When main voltage is zero, the influence of trigger current to the deposition of trigger surface is little. Maybe the trigger current of 150 A is still small relatively, due to the distance of 0.5 mm, which is larger than 0.1 mm in [11]. The current density along the distance of trigger surface is only 150/0.5 = 300 A/mm, according to the distance of 0.5 mm.

3.2. Trigger voltage

The trigger voltage is due to the insulation of trigger surface. If the resistance of trigger surface is very small, the trigger voltage is clamped down on some tens of Volts, although the trigger source can supply high trigger voltage, such as 30 kV. So, the trigger voltage is not the influence factor of working lifetime. It is just a kind of outward manifestation of the trigger lifetime.

If the new sample of TVS works on the small main current or zero main voltage, the high electric field produced by the trigger voltage can get rid of the burn of trigger surface. So, the trigger voltage will rise slightly. According to the curve fitting of Fig. 3, when x = 10000, y = 8.5 kV If the dispersivity here is 2 kV, the highest trigger voltage will reach 10 kV more or less.

3.3. Main current

Firstly, the deposition of main current arc is inevitable. Because the trigger surface is set on the center of main cathode, the discharge of main gap is bound to influence the trigger surface. It was reported that, the deposition on a trigger surface of 0.5 mm would be serious when the main current was higher than 10 kA [17].

A sample was dissected after hundreds of times when TVS conducted tens of kA. It is shown as Fig. 4. The trigger surface has been deposited on a layer of metal film. The detect report shows that, the trigger surface has abundant of copper and chromium besides the ceramic.

The deposition of metal arc is a long process and the main current is under the rated current. So, the distance of trigger surface decreases very slowly. Only when the distance is very low and the trigger current becomes the leakage current mainly, the TVS will fails to be switched, due to the lack of initial plasmas.

Secondly, the trigger pin is eroded by the high main current. When the current is higher than the rated current, such as 220 kA, the great arc will erode the side of trigger surface, which will lead the effective distance to be longer. The trigger voltage will rise to be more than 10 kV, even 20 kV. The merits of trigger surface are lost,

THE LIFETIME OF A HIGH-CURRENT TRIGGERED VACUUM SWITCH

65

Fig. 5. Characteristics of trigger voltage and current.

and the TVS will fail to be switched, because the pulse voltage of trigger source is not high enough.

Because the cathode spots of initial plasmas will be set on the cathode, especially on the side of trigger surface connected to the cathode, it is easy to erode the trigger surface. When the main gap is conducted, the high di/dt of main current accelerates the electron emission of the initial cathode spots, even the concentration of initial cathode spots, which will lead to one and one craters.

It is destructive to the trigger surface. So, in order to get higher main current, the trigger structure and materials should be improved.

Lastly, the main electrodes will be eroded by the high current. The high current will erode the main electrodes and lead the fail of TVS, which is common in the TVS with flat electrode. However, the arcing area of TVS with multi-gap is the double of TVS with flat electrode almost, and the arcing in multi-channel will keep the arc in the diffusion mode, so the current carry ability of TVS is improved greatly.

The experiment showed that, the high current of 220 kA/400 ^s had not destroyed the main gap of TVS sample. The main gap was recovered to be 28 kV and could continue to carry current. The rated current of the sample is 120 kA as designed, so it has a large margin in the practical application. The high currents leave one and one burn marks on the surface of main electrodes, especially the anode. However, the burn marks are random. After repeated work of TVS, the burn marks will be as thick as huckleberries and keep the whole electrode in good condition. In other words, the structure of multi-gap will keep the electrode from concentrated erosion. So, the electrode has a long working lifetime. According to the erosion rate of electrode, the lifetime of electrode can reach 104 times.

The working lifetime of TVS is due to the trigger surface lifetime, not the electrode lifetime. And lifetime of trigger surface is due to the deposition of vac-

Trigger voltage, kV 15" ......

% 12 24 36 46 60 72

Trigger times

Fig. 6. Decrease of trigger voltage under 100 kA.

uum arc. So, the rated lifetime of TVS will be determined by the decrease of resistance of trigger surface, as far as the resistance is zero almost. If that, the trigger voltage is zero almost and cannot generate enough initial plasma. The trigger of TVS will be i

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