RU2198458C2 - Current-limiting circuit breaker - Google Patents

Abstract

FIELD: automatic overcurrent protection of electric circuits. SUBSTANCE: current- limiting circuit breaker has series-connected contact group and control electromagnet coil; electromagnet functions to open contact group at preset current. Superconducting current limiter connected in parallel with electromagnet coil loses its superconducting properties as soon as circuit current rises to critical value and reduces this current. If circuit current keeps rising, electromagnet operates and opens the circuit. EFFECT: enhanced operating stability due to reducing critical circuit current in advance. 2 dwg

Description

 The invention relates to the field of electrical engineering, in particular, can be used for automatic protection of electrical circuits from current overloads.

 Known devices for disconnecting electrical circuits containing a normally-closed contact group and a control electromagnet, which include in emergency situations, breaking the circuit, electromagnetic relays, starters [1]. The disadvantage of these devices is that they operate in manual mode, and also when disconnecting large currents (more than 1 kA), their electrical contacts are destroyed under the influence of an electric arc.

 The closest in technical essence to the claimed device is a device selected as a prototype — a maximum current relay containing a normally closed contact group and a series-connected coil of an electromagnet controlling the contact group. Critical current for the circuit is the actuation current of the electromagnet and disconnection of the circuit [2].

 A disadvantage of the known device is that when disconnecting large currents (more than 1 kA), the resulting electric arc destroys the contacts and they must be replaced periodically, in case of frequent trips of the device.

 The technical solution of the invention is to increase the operational stability of the tripping device due to preliminary automatic reduction of the breaking current and reducing the number of contacts.

 The specified technical result is achieved in that the device for automatically turning off the current when it reaches a critical value, containing a contact group and a serially connected coil of the control electromagnet, is equipped with an additional threshold superconducting element connected to the coil (SP limiter), the critical current of which is equal to the critical current of the electric chains. The response current of the electromagnet is higher than the rated current for the current circuit by a certain amount, selected taking into account the severity of the operating conditions of the device.

 A comparative analysis of the features set forth in the proposed technical solution with the features of the prototype shows that the claimed device differs from the prototype by the presence of a limiter connected to the SP electromagnet parallel to the coil, the critical current of which is equal to the critical current of the circuit, which is absent in this set of features, causing thereby the structural novelty of the technical solution. In addition, the use of the SP limiter allows you to reduce the breaking current to the extent determined by the design of the SP limiter, which determines the functional novelty of the proposed technical solution.

Analysis of the technical solutions known to the authors in the field of protective electrical devices and their comparison with the proposed technical solution showed that superconducting current limiters are known. However, with the introduction of the SP limiter in this connection with the rest of the circuit elements in the inventive device to increase its operational stability, new properties appear:
- in the case of an increase in current in the circuit after switching the SP of the limiter, the power allocated to it as part of the inventive device is much less than that of an autonomous SP limiter, since in the first case it is loaded before the electromagnet operates, and in the second it is constantly;
- the opening of the contacts of the proposed device occurs under the condition that after switching the SP the limiter increases the current in the circuit, otherwise the electromagnet does not work and the circuit does not turn off, the device works as a current limiter, the contacts are not destroyed by an arc;
- the device has a setting range of the operating current from I _n to I _pr The magnitude of the response current determines the breaking current and, therefore, the rigidity of the operating mode of the device.

 Thus, other, in contrast to the well-known technical solutions, properties inherent in the proposed design of the current limiter-switch, prove the presence of significant differences aimed at achieving the goal of the invention.

 In FIG. 1 shows a block diagram of a current limiter-switch device. In FIG. Figure 2 shows graphs of the time dependence of the current of the SP limiter (curve a), the coil of the electromagnet (c) and the total current in the circuit (s).

где j_кр - плотность критического тока, зависит от материала;
S - площадь поперечного сечения сверхпроводника;
l - длина сверхпроводника;
ρ_нс - удельное сопротивление материала сверхпроводника.The current limiter-switch (Fig. 1) contains a series-connected contact group 1 and an electromagnet coil 2. A superconducting current limiter 3, which is a high or low temperature superconductor, is connected in parallel with the electromagnet coil. The critical current of the SP of the limiter (I _cr ) and its resistance in the normal state (R _ns ) are set and determined by known formulas:
I _cr = j _cr • S (1)

where j _kr is the critical current density, depends on the material;
S is the cross-sectional area of the superconductor;
l is the length of the superconductor;
ρ _ns is the specific resistance of the superconductor material.

 The critical current of the SP limiter is selected equal to the critical current of the protected circuit.

где U - напряжение на устройстве.The resistance value R _nc is determined from the expression

where U is the voltage on the device.

где I_н - номинальный ток в цепи:
t - время фазового перехода сверхпроводника.The resistance of the SP limiter also determines the speed of current measurement

where I _n - rated current in the circuit:
t is the phase transition time of the superconductor.

 The device operates as follows.

While the current in the circuit does not exceed I _cr (Fig. 2), the resistance of the SP of the limiter 3 is zero and all the current flows through it. When the current reaches a value equal to the critical current (for example, in the case of a short circuit), superconductivity begins to collapse and the current through the limiter 3 drops to I _ns . At this moment (t3), the current through solenoid 2 increases to I _nm . Thus, current flows in the circuit
I _n = I _ns + I _nm (5)
If the current in the circuit remains constant, the device acts as a current limiter and the opening of the contacts does not occur (figure 2, graph 1). If the source power is not exhausted and the current in the circuit continues to increase, then when the current through the coil of the electromagnet 2 I _cf is reached, it is triggered and contact group 1 disconnects the circuit (Fig. 2, graph 2).

 Using the proposed technical solution will reduce the breaking current by several times (determined by the parameters of the SP limiter) at the same critical current, as well as reduce the number of contacts of the contact group and therefore increase the operational stability of the device.

Literature
1. I.S. Taev. Electrical control devices. M .: Higher School, 1969. - 444 p.

 2. A. S. Zhdanov, V.V. Ovchinnikov. Electromagnetic current and voltage relays. - M.: Energoizdat, 1981.

Claims (1)

 A current limiter-switch containing a normally closed contact group connected in series with a control electromagnet coil, characterized in that it is equipped with a superconducting current limiter connected in parallel with a control electromagnet coil having a switching current at which superconductivity begins to collapse equal to the critical current of the circuit, and the control coil the electromagnet has a trip current greater than the critical current of the circuit.

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