HIGH-VOLTAGE PULSED SWITCHING DEVICE WITH AN AVERAGE CURRENT OF 2000 A

V. V. IVASHIN, A. I. BLUDOV, A. M. ELENKIN

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

Among the various switching devices for high pulsed powers used in the practice of physical experiments, the creation of high-voltage switching devices with an average current of several thousand amperes has until now presented known difficulties.

One of the possible solutions to this problem may be the switching device being developed at the Research Institute of Nuclear Physics, Electronics, and Automation at Tomsk Polytechnic Institute for supplying the electromagnet of the “Sirius” accelerator at 1.5 GeV.

This device is implemented on the basis of known electromechanical switching circuits [1]. The parameters of the accelerator power-supply circuit are given in Table 1.

Table 1

The power-supply system of the electromagnet with the switching device under consideration is shown in Fig. 1.

The switched circuit consists of electromagnet \(L_1\), forcing choke \(D_\phi\), which reduces the rate of rise of the magnetic field during injection, and storage battery \(C_1\), supplied from powerful rectifier \(VP_1\).

Sparkless closing of contact apparatus \(KA\) is ensured by switching valve \(B_1\) and saturation choke \(D_1\). De-energizing of the contacts during opening is ensured by a circuit of capacitive artificial commutation, consisting of saturation choke \(D_2\), valves \(B_2\) and \(B_3\), and a small auxiliary capacitance \(C_2\), charged from rectifier \(VP_2\).

Since the system requires preliminary bringing into operating mode,

Fig. 1. Connection diagram of an ion-mechanical commutator in a power oscillatory circuit

then starting and normal switching-off (as well as emergency switching-off) are carried out by an additional disconnector \(P_1\), made on the basis of a VAB-28 apparatus.

Since the operating principle of ion-mechanical circuits has been described previously [1, 2], it is of interest to note only the existing features of the design embodiment of the contact apparatus and the operation of the artificial capacitive commutation circuit when supplying the load over a wide range of currents. A schematic section of the contact apparatus in the “on” position is shown in Fig. 2. The contact system proper consists of a central current-carrying rod 8, a 12-lamella socket 6, and terminals 9. The loop-shaped design of the current leads makes it possible to obtain additional electrodynamic pressing of the lamellae when switching large pulsed currents. In order to increase the speed of operation and the working voltage, closing and opening of the lamellae are carried out by means of a dielectric cylinder 5. The latter, by means of pusher 4, lever 3, and roller 2, is acted upon by cam mechanism 1. Uniform rotation of cam 1 is provided by a synchronous-reactive motor through a reduction gear (not shown). When moving upward, the dielectric cylinder, by means of its projections, spreads the lamellae apart and enters with its cone-shaped part into the contact gap, ensuring an accelerated increase in the strength of the contact interval with a small travel of the movable lamellae.

When the dielectric cylinder moves downward, the lamellae, under the action of springs 7, close the external circuit. Such a design makes it possible to obtain an operating time of up to \(3.0 \cdot 10^{-3}\) sec without vibration of the lamellae and withstands a test voltage of up to 25 kV. Synchronization of the starting instants of the auxiliary valves is linked to the contact apparatus by means of additional contacts.

Another feature of this device is the provision of sparkless opening of the contacts when the load currents vary within a specified range. As applied to various requirements, the following ways of solving this problem are possible:

Fig. 2. Schematic diagram of the design of the mechanical contact apparatus

a) changing the magnitude of the commutating capacitance or the ampere-turns of choke \(D_2\) in accordance with the magnitude of the current being commutated;

b) using a saturating choke \(D_2\) with large ampere-turns;

c) changing the moment at which commutation begins in such a way that the capacitance-recharge current is the same at different load currents;

g) changing the charging voltage of the commutating capacitance in such a way that the change in flux linkage of the saturation choke \(D_2\) produced by it does not exceed the specified limits. A combination of methods is, of course, also possible.

In the present device, in order to ensure a change in the flux linkage of choke \(D_2\) by a constant amount when different load currents are switched off, control of the voltage on the commutating capacitance is used as a function of the voltages on the storage battery or, equivalently, as a function of the maximum value of the commutating current.

The control law can be found from the following considerations. Assuming that the recharge of capacitance \(C_2\) takes place according to a linear law, the magnitude of the flux linkage produced by it can be written as

\[ \psi = \frac{1}{2}\, U_{C_2}\cdot \Delta t, \tag{1} \]

where \(U_{C_2}\) is the voltage on capacitance \(C_2\) at the beginning of recharge;
\(\Delta t\) is the recharge time of the capacitance, which can be determined from the parameters of the commutating circuit:

\[ \Delta t = -\,\frac{U_{C_2} C_2}{I_2}, \tag{2} \]

where \(I_2\) is the recharge current of the capacitance.

Substituting (2) into (1), we obtain:

\[ \psi = -\,\frac{U_{C_2}^{2} C_2}{2 I_2}. \tag{3} \]

To ensure reliable sparkless operation of the contacts, it is necessary that their opening take place within the middle of the remagnetization step of choke \(D_2\). Therefore, equating the right-hand side of expression (3) to a value equal to half the flux linkage of the choke, \(0.5\psi_s\), one can find the dependence of the voltage on the commutating capacitance on the current of the main circuit:

\[ U_{C_2} = \sqrt{\frac{\psi_s I_2}{C_2}}. \tag{4} \]

The commutating capacitance was supplied according to this nonlinear law by a device whose circuit is shown in Fig. 3. The nonlinearity is provided by connecting a nonlinear element—an electron tube—into one of the arms of the measuring divider \(ID\). In this case a pentode of type 6Zh4 was used. In practice, adjustment to the required law is performed by regulating the operating mode of the tube by means of the control and screen grids.

The error signal, equal to the difference of the voltages on the measuring arms of the dividers, after amplification by the dc amplifier \(UPT\), controls the phase shifter \(FV\) of rectifier \(VP_2\). The phase shifter is of the bridge type, active-capacitive. An electron tube is connected into one of the arms. The controlled parameter is the internal resistance of the electron tube. Rectifier \(VP_2\), with a power of about 1.5 kW, is assembled as a half-wave rectifier using a TR-6/15 thyratron.

In conclusion, it should be noted that similar commutating devices may find application in various physical installations for commutating the energy of powerful storage units at current-pulse durations of \(40\cdot 10^{-3}\) s and more.

Fig. 3. Block diagram of the controlled rectifier.

REFERENCES

1. V. V. Ivashin, G. A. Sipaylov. Contactless switching-off of large currents. Elektrotekhnika, No. 9, 1964.
2. V. V. Ivashin. Inventor’s certificate No. 155206.