Study of Sparkless Commutation of Large Currents

V. V. Ivashin, A. I. Blyudov

(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

In research and testing of electrotechnical apparatus in high-power laboratories, for the synchronized switching-on of an impulse generator, switching devices are needed that are capable of commutating currents of tens and even hundreds of kiloamperes [1, 2]. Similar tasks are faced by a commutating device that switches an impulse generator onto an inductive load, for example, the electromagnet of a charged-particle accelerator or some other object.

Earlier, sparkless ion-mechanical commutating devices were proposed for commutation of large currents [3]. These devices also make it possible to solve a more difficult problem—to switch on powerful oscillatory LC circuits at voltages on capacitor banks of (10–15) kV without the occurrence of an arc at the contacts. Such problems arise in developing power-supply circuits for various devices in accelerator technology and thermonuclear research.

The commutating device developed by us is a fast-acting contact apparatus with an oil damper, preventing vibrations of the moving parts and rebounds of the blades of socket-type contacts. The electrodynamic drive makes it possible to ensure an operating time of no more than \((3 \div 4) \cdot 10^{-3}\) sec. The weight of the apparatus is 100 kg. A sparkless switching-on system, consisting of a choke and an auxiliary valve (or group of valves) operating in parallel, makes it possible to ensure an accuracy of circuit switching-on no worse than \((10 \div 15) \cdot 10^{-6}\) sec.

In Figs. (1, 2) the circuit of the commutating device is shown, connected into the break of the commutated circuit at points A and B.

The circuit operates as follows. A control pulse from the electronic system is applied to valves \(B_1\) and \(B_2\). The load current then flows from A to B through valve \(B_1\). Through valve \(B_2\), the storage capacitance \(C_1\) of the contact drive is discharged into the drive winding \(P_1\), ensuring a closing time of contacts \(K\) of \((3 \div 4) \cdot 10^{-3}\) sec. The saturation choke \(Dr.\), after the contacts close, is remagnetized in approximately \(1 \cdot 10^{-3}\) sec, which ensures the absence of current in the circuit with the contacts until they have made good contact. After the contacts \(K\) close, the contact-block system \(BK_1\) switches over, connecting to valve \(B_2\) the coil \(P_2\) of the electrodynamic drive for opening the contacts, the winding \(O.R.\) for demagnetizing the saturation choke, and the capacitance \(C_2\), the discharge of which ensures opening of the contacts. Necess—

Fig. 1.

The required electrical strength of the coils \(P_1\) and \(P_2\) of the actuators (5 kV) was ensured by the use of glass insulation and its impregnation with epoxy compounds, which at the same time also provided their high mechanical strength. The contacts can be closed at any instant of time. If synchronized disconnection of the circuit is necessary when the current passes through zero, a circuit for capacitive artificial commutation is connected in the device in parallel with points A and B [3].

Special attention must be paid, in designing and operating the device, to the time of remagnetization of the choke \(D_r\) and to the transfer of current from the auxiliary ignitron \(V_1\) to the contacts \(K\).

Figure (1, a) shows the connection of the device into a powerful oscillatory circuit \(L_{\mathrm{n}}C\). The connection of two ignitrons \(V_1\) for parallel operation is explained by the insufficient pulse power of these devices, which may often occur in practical cases. The inclusion of the variable resistance \(r_2\) ensures normal operation of the commutating device at different voltages on the capacitor bank \(C\); when it is discharged into the load \(L_{\mathrm{n}}\), a pulse current with amplitude \(I_m\) flows in the circuit.

An analysis of the operation of the circuit over the time interval \(t_n\), when remagnetization of the choke occurs, shows that the resistance \(r_2\) must be chosen from the condition

Fig. 2.

\[ r_2=\frac{L_{\mathrm{n}}+L_a}{U_m}\cdot \frac{\psi/t_{\mathrm{p}}-U_m\frac{L_a}{L_{\mathrm{n}}+L_a}-U_0} {t_1+\dfrac{t_{\mathrm{p}}}{2}}, \tag{1} \]

where \(\psi\) is the flux linkage of the reactor;
\(t_1\) is the contact closing time;
\(t_{\mathrm{p}}\) is the reactor remagnetization time;
\(L_\varepsilon\) is the leakage inductance of the anode-divider windings;
\(U_0\) is the voltage drop across the valves \(B_1\).

Solving expression (1) with respect to \(t_{\mathrm{p}}\), one can determine the reactor remagnetization time for various other circuit parameters (\(L_a\), \(r_2\), \(\psi\), etc.). In the case where the time \((t_1+t_{\mathrm{p}})\) corresponds to a small angle of the sinusoidal variation of the current of the \(L_{\mathrm{n}}C\) circuit, the current during this time increases in the circuit practically linearly:

\[ i_1=U_m\frac{t}{L_{\mathrm{n}}+L_a}. \tag{2} \]

After remagnetization, the resistance of the reactor decreases and the transfer of current begins from the circuit with valves \(B_1\) to the circuit with contacts. In the general case, the currents \(i_{B1}\) and \(i_k\) are described by complex equations. In the case where the transfer of current from \(B_1\) to \(K\) has time to occur over a time corresponding to angles \((20 \div 30)^\circ\) of the frequency \(\omega=1/\sqrt{LC}\), the current in the valve can also be determined by the simplified formula

\[ i_{B1}=\frac{k_1}{\delta}\cdot t-\left(\frac{k_1}{\delta}-k_2\right)\frac{1}{\delta}\left(1-1^{-\delta t}\right)+I_1 1^{-\delta t}, \tag{3} \]

where

\[ \delta=\frac{r_2+r_3}{L_a+L_3}; \qquad k_0=\frac{U_m}{L_{\mathrm{n}}}; \]

\[ k_1=\frac{k_0\cdot r_3}{L_a+L_3}; \qquad k_2=\frac{I_1 r_3+k_0 L_3-U_0}{L_a+L_3}. \]

Numerical calculations show that the nature of the redistribution of current between the circuit with valves \(B_1\) and the contacts \(K\) depends strongly on the value of the active resistance \(r_3\) of the reactor winding, and that there exists a critical resistance \(r_{3\mathrm{cr}}\) at which the current in valve \(B_1\) will be reduced to zero. Failure to satisfy the criticality condition leads to a substantial increase in the average value of the valve current,

which is undesirable. In the first approximation, the criticality condition is met by the expression

\[ U_0 > I_1 r_{\mathrm{zkr}} + k_0 \left[ L_{\mathrm{e}} + 3 (L_a + L_{\mathrm{e}}) \frac{r_{\mathrm{zkr}}}{r_2 + r_{\mathrm{zkr}}} \right]. \tag{4} \]

In Fig. 1b, the dashed lines show the form of typical curves of the current \(i_{B1}\) for different degrees of nonfulfillment of the criticality condition (1, 2) and when this condition is fulfilled (3).

The influence of the inductance \(L_a\) on the character of the change in the current \(i_{B1}\) in real circuits may also be substantial; therefore, in pulse devices of this type the parameters of the anode divider must be chosen only from the condition of ensuring normal ignition of the valves connected in parallel. For uniform current division, however, small active resistances must be connected in series with each valve. This makes it possible to make the windings of the anode divider small in size and with small leakage inductances.

The damping circuit \(r_4 c_4\) is calculated for the remagnetization current of the choke and ensures the absence of sparking when contact bounce occurs at the moment they are switched on.

It follows from Fig. 1b that the magnitude \(I_1\) of the current pulse through the valves depends on the operating time of the apparatus \(t_1\) and on the remagnetization time \(t_p\). When the sum of these times is \((1.5 \div 2) 10^{-3}\) sec, which is technically quite attainable, even when modern ignitrons or thyratrons are used as valves \(V_1\), such switching devices can be used as generator circuit breakers capable of making and breaking short-circuit currents of tens of thousands of amperes.

References

1. S. I. Yablonko. Modern Laboratories of Breaking Capacities, TsINTI, 1962.

2. V. P. Golubev, B. A. Sheshin. Switching apparatus for a high-power laboratory. Bulletin of the Electrical Industry, No. 5, 1958.

3. V. V. Ivashin. Inventor’s Certificate No. 155206.