EFFECT OF THE MAGNETIC FIELD ON THE ANODE SPOT FORMATION IN THE VACUUM ARC.

K.N.Ulyanov, V.A.Fedorov, Ye.F.Prozorov. Russian Federation State Research Centre "All-Russion Eletrotechnical Institute", Krasnokazarmennaya st. 12,

11 1250. Moscow, Russia.

Introduction The mathematical model of a high current vacuum arc based on calculation of the form of ion trajectories at inherent and external axial magnetic fields by a method of coarse particles (current pipes). It was shown at the absence of external magnetic field transmission of all ion trajectories through a crossover is impossible at currents as low as J,, = (ymcv/0.2ez)ac, (ac,= 0.82). With a higher current increase a part of the ion trajectories passes through a crossover and comes back. At J* 2J,, almost all ion trajectories cannot pass a crossover. If the return point of the ion trajectories is inside the discharge gap, the ion deficit occurs near the anode, practically all the supply voltage is applied to the anode layer and depending on the conditions an anode spot is formed or a current interruption occurs. The described in [ 11 experiments showed the anode spot was in fact formed at JXZJ,, that was an argument in favour of the offered model. The calculation of the trajectory form at the external axial uniform magnetic field, made in [l] showed at each J > J,, it is possible to choose the value of magnetic field which ensures transmission of all ion trajectories through a crossover.

Thus, by means of the axial magnetic field the ion deficit may be excluded and stationary current flow be ensured. This fact is well known and used to preclude the anode spot in the vacuum interrupters [2, 31. The mathematical model, presented in [l] allows to refine quantitative characteristics of suppressing the anode spot.

The aim of the work is to obtain quantitative experimental data on the process of suppressing the anode spot by a longitudinal magnetic field and to compare them with theoretical ones.

The experiments were carried out with the unit and the same measuring instruments, as outlined in [l]. In addition to this there was designed a system of the impulse longitudinal magnetic field which exceeds many times an inherent magnetic field of the arc current at a maximum current value. Experimental set, The experimental set included the following function blocks: discharging chamber with cylindrical electrodes and coils to produce an external magnetic field, vacuum evacuation system, discharge power supply, discharge trigger circuit, current gene- rator to create an impulse magnetic field, control panel, electrical parameter and optical recording measuring instruments.

The cathode of the discharge was made of non- stainless steel in the form of a 27 mm cylinder with plane butt surfaces. The trigger block was arranged at the center of the cathode. It was fixed at 2 mm in depth by a dielectric bush. The supplementary discharge over

the ceramics was a source of plasma initiating a breakdown of the main gap.

The anode of the discharge was a 020" copper cylinder spaced at 13 mm tkom the cathode. The axis of the electrode system was vertical, the cathode was placed at the top of the system.

The system of two coils to produce an external axial magnetic field was arranged co-axially with the electrodes. The coils were fixed to the post of the cathode unit at equal distance tkom the electrodes. Each coil contained 40 turns of the wire. The average wound diameter was 53 mm, the distance between central planes of the coils was 36 mm. The coils were connect- ed in series to the target ends, which were terminated through the dielectric at the chamber flange. The coil ensured a sufficiently uniform impulse magnetic field in the discharges region with a maximum induction of 1 T at the amplitude current value 1 kA at the coils.

The vacuum evacuation system contained two sorbtion ceolite pumps providing a preliminary evacu- ation of the chamber and electrodischarge magnetic assembly to obtain high vacuum. The used system allowed to attain an oil-free evacuation of the chamber. The vacuum evacuation system contained two sorbtion ceolite to a pressure not under

The discharge power supply consisted of a rectifier with an output voltage varied from 0 to 5 kV and oscillating circuit. The circuit included a 140 pF capacitor and tapped coil the taps of which make it possible to change amplitude and period of current fluc- tuations. A minimum value of the coil inductance was 14 pH, maximum one - 112 pH. A maximum half- period wave value of the current was 400 ps.

Switching of the oscillating circuit was done by means of the discharge chamber itself after a discharge has been initiated in it by means of a triggering unit. An initiating discharge was created by a triggering circuit forming a voltage pulse with an amplitude 8 kV and energy 30 J. The circuit was initiated by means of an optical connection line. The coils forming an impulse magnetic field were supplied from the oscillating circuit comprising a 200 pF capacitor and throttle with an inductance 3080 pH.

The coil current was controlled by variation of a charging voltage of the capacitor whose maximum value was 5 kV. Switching the circuit was made by means of an ignitron through the triggering circuit, which also was controlled by the optical connection line. The duration of a semi-period of the magnetic field fluctuations was 2.5 ms, which exceeded considerably the duration of a semi-period of the discharge current fluctuations.

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0-7803-S7914/001$10.00 02000 IEEE 191h IntSymp. on Discharges and Electrical Insulation in Vacuum-Xi'an-2000 312

Diagnostic Svstem, The system was equipped with measuring circuits and optical visualization for electrical discharge parameters. Discharge currents were measured with the help of Rogovsky coil. The voltage drop across the discharge gap was measured by a voltage divider with a limitation and count down of 1:240. The measuring of the magnetic system current was carried with the help of Rogovsky coil. Signals from the belts and voltage divider were applied to the inputs of two oscilloscopes and recorded by a film.

To reduce a level of high frequency noises all measuring cables were inserted in metallic pipes and oscilloscope was placed in a grounded metallic board.

The optical visualization for dynamics of the discharge glow was made by a fast photorecorder through a window in the discharge chamber case. Photography was conducted with the use of a double- lens insert, which allowed to take 60 series exposures in a picture showing the phenomenon to be studied in development. To define the start and finishing of the discharge current waveform the photorecorder was provided with two photoelectron multiplier mounted at the place of the first and last lenses. Signals from the photoelectron multiplier in the form of marks were supplied to current or voltage waveforms indicating moments of the start and end of the photography.

The rotation frequency of the superfast photorecorder was 37500 r/m that corresponds to the photography time 190 ps at the total number of exposures 60. An interval between exposures was 3.2 p. Exposure time was 2.7 ps. Thus, the photography time during the experiments exceeded a semi-period of the discharge current, which permitted the arc glow picture to be visualized from the moment of its occurrence to quenching.

The photoelectrical technique of definition of the anode spot formation moment was based on measuring a relative intensity of the spectral lines, specific for the anode material. For this purpose a 600 line/" grating monochromator was used. The near-anode area of the discharge was projected to the input slit of the monochromator tuned to the line corresponding to an anode matter (for a copper anode h=5 10.5 nm). The photoelectron multiplier a signal from which was recorded by a storage oscilloscope was located behind the output slit. When an anode spot occurred of the evaporated anode matter was increased and this led to an increase of the glow intensity at the fixed spectral line.

Eqerimental results . The experiments were aimed at studying the influence of the longitudinal mag- netic field on characteristics of the vacuum arc and determination of the magnetic field induction allowing to suppress the anode spot at different discharge currents.

The experiments were carried out as follows. A given level of the voltage set by a control panel was supplied to the capacitor of the discharging circuit. ARer this a vacuum arc discharge was initiated in the

chamber by a triggering circuit. At the same time wave- forms of current and voltage at the chamber were recorded and a fast photography of the discharge gap was conducted to obtain dynamics of the arc glow development. If the anode spot appeared the experiment was continued with application of the axial magnetic field to the discharge gap. Its value was raised with each following impulse till the arc regime without anode spot came into existence. Then the experiment was made at another discharge current.

Impulses of the discharge current and magnetic system had the form of falling sinusoids. The processes to be studied took place during the first semi-period of the discharge current pulse Tl2, which took place for 170 p. The discharge initiation was over when the current at the coil attained its maximum value and a semi-period of fluctuation was 2500 p and therefore a change of the magnetic field for a discharging period was under 1 per cent.

The chamber was at a permanent evacuation and characteristics of the discharge were taken when a residual pressure after previous impulse was set at a level not more than 4.1 0-6 Torr.

The moment of the anode spot occurrence was defined by three means: by occurrence of high frequen- cy noises at the waveform of the discharge voltage; by appearance of the copper lines (the material of the anode) in the arc glow spectrum and also by a fast photography. All three means gave close results.

Fast photographs of the discharge in the case of the anode spot occurrence had typical the next pattern. Upon ignition the arc a brightly glowing area was seen at the cathode, a discharge column had a sufficiently uniform glow covering a near axis part of the gap. With an increase of the discharge current the arc column started widening. At the moment prior to the spot occur- rence the glow brightness in the near-anode area was reduced that can be explained by appearance near the anode of vapours of another matter with a lower glowing capability. Following this a brightly glowing area (an anode spot) came into existence. After disassembling the camber one could see circular areas of 0 = 6 mm at the anode.

When the discharge gap was applied with the magnetic field sufficient to suppress the anode spot photographs showed a uniform glowing of the gap for the whole time of the current flow.

Comparison of the photographs of the dis- charges with waveforms of the voltage at the arc demonstrated at the moment of the anode spot initiation a high frequency noise appeared at the arc voltage. In the case of application of the magnetic field of sufficiently high value fluctuations of the arc voltage were absent. The moment of the spot appearance in photographs correlated also with an increase of the signal from the monochromator.

Experiments showed for our condition without a magnetic field the burning regime with the anode spot was realized at a discharge current J, 2 5 kA and at

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I

0 0.05 0.1 B, T 0.15

1,kA 4.5 a 1.5 d, mm 11.7 K 0.76

Fig. 1 The dependence of spot initiation time 2, vs magnetic field B. (Imax= 7 kA)

J, = 5kA the spot was in general formed in the region of the maximum current. An increase of the current amplitude led to reduction of time of the anode spot appearance t,. As before, it was initiated at the well treated surface of the anode at instantaneous values of the current J = 4+5 kA. This result is in agreement with similar data in [ 11 and value J = 2J,, = 5kA correspond- ing with the theory.

At values J > 2J,, with the discharge subjected to longitudinal magnetic field insufficient to suppress a spot time t, of the spot initiation was increased and shifted to a current maximum. At t, attained quarter of a period the anode spot was suppressed (B = B,,). Fig. 1 shows the ratio of the dimensionless spot initiation time 2,=4t$T and magnetic field induction B for the arc current with amplitude J, = 7kA. An increase of the field value above B,, always led to suppression of anode spot. The experiments were carried out at currents in the limits of 2 kA I J, I 12 kA and whenever the spot was formed application of the external longitudinal field promoted to convert the discharge into a burning regime without anode spot.

Fig.2 illustrated the relationship between mini- mum magnetic fields B,, leading to suppression of the anode spot and amplitude values of the discharge cur- rent. It is seen it is near a linear one.

Discharge channel diameter values vs the cur- rent at values B close B,, at the moment of the anode spot formation are given in Table. They are used to

6.5 9.3 10.5 10.8 2.2 3.1 3.5 3.6 10.6 10.6 16.7 14.5 0.69 0.57 1.0 0.92

0’4 J

0.0 i 0 5 i o hn,, kA 15

References [ 13 K.N.Ulyanov, A.B.Bogoslovskaya, J.I.Londer, V.V.Stepanov, V.A.Fedorov. “The mechanism of anode spot formation in a vacuum arc”, ISDEIV XIV, Santa Fe,

[2] Y.Sunaga, N.Ito, S.Yanabu, H.Awaji, H. Okumura, Y.Kanai.” Research and development on 13.8 kV 100 kA vacuum circuit-breaker with huge capacity and frequent operation”, International conf. of large high voltage electrical systems, CIGRE Report 13-04, Paris,1982, pp.1-8. [3] S.Yanabu, S.Souma, T.Tamagawa, S.Yamashita, T.Tsutumi.” Vacuum arc under an axial magnetic field and its interrupting ability ”, Proc. IEE, ~01.126, N4, April 1979, pp. 313-320.

USA, 1990, pp.279-283.

Fig. 2 The dependence of magnetic field B,, vs current discharge amplitude I,,,.

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