VALVE-MECHANICAL APPARATUS FOR SWITCHING HIGH PULSED POWERS

V. V. Ivashin, A. I. Blyudov, A. M. Elenkin

(Presented by the scientific seminar of the departments of electrical machines and general electrical engineering)

In modern science and technology, installations in which high powers must be switched are finding ever broader application. For example, in accelerator technology and thermonuclear research it is necessary to obtain powerful pulsed magnetic fields with energies of tens of millions of joules. The currents in the windings that create these fields may reach ten thousand amperes or more. In laboratories of breaking capacities, from surge generators it is also necessary to obtain currents of tens and hundreds of thousands of amperes at generator voltages of 10 kV and more. Switching such currents with the aid of ionic valves is practically impossible because of the insufficient magnitude of the permissible average and effective current through each device and because of their comparatively low pulsed overload capacity [1].

For switching large currents in such cases, various mechanical devices have been developed that possess a high pulsed overload capacity and insignificant energy losses in the transient resistance of the contacts, which determined their small dimensions. The first mechanical contact apparatus was developed by P. L. Kapitsa for switching a surge generator on and off into an inductive load [2]. The operation of closing the contacts at zero generator emf and opening at the first passage of the load current through zero was ensured by means of a special cam mechanism rigidly connected to the generator shaft.

A double contact gap of 1 mm was traversed in \(0.3 \cdot 10^{-3}\) sec. The small arc arising at the contacts because of the impossibility of achieving exact mechanical synchronization and instantaneous opening of the contacts was extinguished by blowing. The generator voltage was 2250 V, the maximum current was 72,000 A, and the duration of the first current pulse was \(18 \cdot 10^{-3}\) sec. The switching apparatus withstood several thousand operations without serious damage. The shortcomings of the apparatus are the presence of many moving parts, the difficulty of synchronization when the operating mode is changed, the presence of an arc, and the impossibility of operation at voltages of 10–15 kV.

In [3] contact apparatuses are described that were developed for switching currents of 50 and 150 kA at voltages of 500 V and 2000 V. An electromagnetic drive was used in these apparatuses.

In [4] a design is described of an apparatus for switching on a surge generator operating in a high-power laboratory for testing high-voltage equipment. The contacts have a spring drive, are designed for currents up to 120 kA, and operate under an air pressure of 6 atm.

The intrinsic operating time is \(29 \cdot 10^{-3}\) sec; the contact speed upon closing is \(4\ \text{m/sec}\); the weight of the apparatus is about \(1.5\) tons.

In [5], various designs of closing apparatuses are described, made by foreign electrical-engineering firms for laboratories of breaking powers.

At present, mechanical switching apparatuses are finding ever broader application in the switching of large currents in installations for thermonuclear research. For example, in [6, 7] designs are described for apparatuses rated at 20 and 100 kA, intended for short-circuiting an inductance coil when the current value is close to the maximum. The pneumatic drive makes it possible here to develop speeds of up to \(50\text{–}60\ \text{m/sec}\) and to produce designs with a closing time on the order of \(1 \cdot 10^{-3}\) sec.

It should be noted that in these devices the high speed is achieved by preliminary acceleration of the moving parts, and the apparatuses perform only the function of closing high-current circuits. The total operating time of the apparatus considerably exceeds \(1 \cdot 10^{-3}\) sec.

Thus, switching apparatuses based on mechanical closing of circuits are used, as a rule, only for closing circuits at zero or nearly zero voltage. Contact opening and disconnection of the load from the power source synchronized with the current zero was accomplished only at comparatively low voltages; moreover, even in this case, disconnection was implemented only in the work of P. L. Kapitsa.

At the Research Institute of Nuclear Physics at the Tomsk Polytechnic Institute, fundamentally new devices have been developed for switching high-current pulse circuits at high voltage [8, 9, 10]. These devices are based on a combination of a fast-acting mechanical apparatus and, connected in a definite manner, controlled ionic valves, which ensure operations of arc-free closing and arc-free opening of contacts.

Fig. 2.

This paper describes one of the developed designs of an ultrafast contact apparatus capable of carrying pulse currents of tens of thousands of amperes at a voltage of 10000 V. The effective current of the contacts is 1500 amperes. To simplify the design, increase the speed of operation, and simplify control, an induction-dynamic drive was chosen for the apparatus. With an induction-dynamic drive it is easy to obtain a force of several tons, which, with a contact mass of up to \(2 \div 2.5\) kg, makes it possible to impart to them accelerations of up to several thousand \(g\) and to ensure a total operating time of no more than \((3 \div 5) \cdot 10^{-3}\) sec. Structurally, such a drive is made in the form of a multi-turn coil and a conducting disk located in the immediate vicinity of the coil in such a way that the derivative of the mutual inductance between the disk and the coil with respect to the coordinate of the necessary displacement of the disk is maximal. The disk can transmit forces to the moving contacts of the apparatus. When passing throu—

through the coil a pulse current from some source, the disk is repelled from the coil with a force

\[ f = i_1 i_2 \frac{\partial M}{\partial x}, \]

where \(i_1, i_2\) are the currents in the coil and in the disk.

\(M\) is the mutual inductance between the disk and the coil.

In the contact apparatus, a section of which is shown in Fig. 1, there are two coils of an inductive-dynamic drive—the coil 6 for closing the contacts and the coil 2 for opening them. Between the coils is placed a duralumin disk 5, which, together with the movable contacts 4, simultaneously serves as a contact bridge. When the closing coil 6 operates, the disk-bridge moves downward and the movable contacts 4 enter the socket contacts 3. When the opening coil 2 operates, the disk is thrown upward by electrodynamic forces and contacts 3 and 4 open by a double gap of 25 mm. The vertical displacement of the disk is ensured by means of two guides 8, to which is connected a system of toggle levers 9 and a specially designed double-acting oil damper 7. By the force of the springs of the toggle levers the disk is held in the upper or lower position, i.e., in the “on” and “off” position, with a force of 20 kg. The system of toggle levers reliably fixes the position of the disk-bridge during operation of the apparatus.

The guides 8 move together with the disk during its motion upward and downward. In this process, 5 mm before the end of such displacement, by means of a rocker connected to the guides, the oil damper 7 is engaged; it absorbs up to 90 percent of the kinetic energy of motion of the moving parts and thereby substantially reduces shock mechanical loads and noise during operation of the apparatus. Braking by the damper takes a time of \(1 \cdot 10^{-3}\) sec. The drive coils, the damper, the toggle levers, and the disk connected with them are fastened on a textolite plate mounted on four support insulators. The double air gap of 25 mm permits operation at voltages up to 10 kV. The apparatus can be hermetically sealed and filled with gas under pressure, which will additionally increase the dielectric strength of the contact gap. The total time for closing or opening the contacts from the moment power is applied to the drive coil is not more than \(4.5 \cdot 10^{-3}\) sec. The energy source for feeding the drive coils may be capacitors of \(50 \cdot 10^3\) μF, charged to a voltage of 6 kV, discharging into the drive coil through a controlled switching device, for example, a thyratron or a controlled spark gap.

Fig. 3.

When current flows through the socket and the disk-bridge, a force \(F_{\mathrm{sb}}\) acts on the disk, tending to move the disk upward. Fig. 2 shows a schematic representation of the electrical circuit of the contact

apparatus. To compensate for this force, a bus carrying the current of the circuit being switched is passed above the disk. The electrodynamic force \(F_k\) between the current in the disk and the current in the bus is directed opposite to the force \(F_{cb}\). The distance between the upper bus and the disk is chosen so that the condition

\[ F_k - F_{cb} = F_p > 0, \]

is satisfied; under this condition, when current flows the disk is pressed downward by the electrodynamic force \(F_p\).

The length of the lamellae at the socket contacts and the mean diameter of the sockets are chosen from the condition of additional compression of the sockets at the moment current passes through them, which increases the electrodynamic stability of the apparatus.

The control circuit of the apparatus and the auxiliary equipment are mounted in a separate control cabinet. The dimensions of the contact apparatus are \(500 \times 700 \times 800\) mm, weight 50 kg.

Fig. 4.

The apparatus may be used for switching current both from powerful capacitor banks and from pulse generators. For example, Fig. 3 shows one of the possible connection circuits of the apparatus for switching the current of a pulse generator (11). Suppose that from pulse generator 1 into load 3 it is necessary to pass a single first current pulse of maximum magnitude. For this purpose, at a phase of the generator’s e.m.f. close to zero, auxiliary valve 4 is opened and contacts 8 of the apparatus begin to close. Until the moment of their closing, the current passes from pulse generator 1 to load 3 through valve 4. After the contacts close at time \(t_1\), saturation choke 5 is rapidly remagnetized, and the current transfers from valve 4 to the contacts. After the first pulse, at time \(t_2\), when the current passes through zero, valve 7 is opened, and the current of the second half-wave, small in amplitude and opposite in sign, passes through valve 7. During the negative half-wave the contacts open; at time \(t_3\), when the current passes through zero, the valve blocks the circuit, and the switching process is thereby completed. Figure 4 shows the current curve from the pulse generator with an aperiodic component of current, where \(i_{v4}\) and \(i_{v7}\) are the currents of valves 4 and 7, and \(i_k\) is the current switched by the contact apparatus.

REFERENCES

1. Smart D. L. Some smitchin problems in thermonuclear „reseach, „Proc. Inst Electr. Engs” 1959, A-106, Suppe N 2.

2. Kapitza P. L. Further Developmeut of the Method of obtaing strong magnetic Fields. Proc Royeb Soc. 1927, vol. 115, p. 658.

3. I. V. Safonovskii, L. P. Gnedin. A switch for making and breaking a powerful electric circuit for the duration of a current half-wave, Proceedings of the Leningrad Industrial Institute, 1937, issue 1, No. 2.

4. V. P. Golubeva, B. A. Sheshin. A switching apparatus for a high-power laboratory, Vestnik elektropromyshlennosti, 1959, No. 5.

5. S. I. Yablonko. Modern laboratories of breaking powers, TsINTI, 1962.

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