CN101199092A - Method of switching a circuit and controlling a circuit breaker - Google Patents

Abstract

The invention relates to a switching circuit with at least one circuit breaker (M1, _M2 ), resistor (RL), thyristor (Th) or triac (TR ) and a plurality of means (R1, Di, V3) for triggering said thyristor (Th) or said triac (TR) depending on the voltage drop across said circuit breaker (M1, M2) ), wherein said resistor (RL) and said thyristor (Th) or said triac (TR) are connected to each other in a connection across said at least one circuit breaker (M1, M2) .

Description

Method of switching a circuit and controlling a circuit breaker

technical field

The invention relates to a switching circuit, in particular to the application of the switching circuit in a switching device of a low-voltage equipment. Furthermore, the invention relates to a method for controlling at least one circuit breaker for switching a load, in particular an inductive load.

Background technique

An ideal switching device in a low-voltage installation is one that, in addition to its normal switching function, also breaks a short circuit in the case of a circuit breaker. Since a short circuit can occur at any time and needs to be limited and/or opened as soon as possible, the next time the current crosses zero, the short circuit cannot be effectively opened without pre-limiting measures. To be precise, at least one current limitation must be performed directly.

In mechanical switching devices, a countervoltage with a current-limiting effect is generally generated by the switching arc. With a corresponding design (cooling) of the interrupter, the arc is extinguished at the next current zero crossing. In electronic switching devices without mechanical normally open contacts, there is no such switching arc. The current can only be cut off immediately if the energy stored in the inductor can be dissipated. Although the inductance of the load does not play a role in the short circuit condition, there is still a large amount of energy stored in the inductance of the power supply. If the current is cut off quickly at this time, a high overvoltage will be caused, which will damage the circuit breaker.

Contents of the invention

It is an object of the invention to provide a particularly suitable switching circuit and a method which are particularly suitable for controlling at least one circuit breaker for switching loads.

The switching circuit according to the invention is achieved by the features of claim 1 . The subclaims of claim 1 relate to advantageous refinements and developments.

The invention is based on the consideration that although overvoltages can be limited with varistors (i.e. voltage-dependent resistors) or RC circuits, by so-called "clamping" the power semiconductors are controlled exactly to a voltage which remains below the maximum limit value The state can also achieve the purpose of limiting overvoltage. In this case, however, the energy stored in the power semiconductors is converted and becomes an additional load on the power semiconductors.

Accordingly, the present invention provides a switching circuit comprising at least one circuit breaker for switching a load. The "load" here mainly refers to inductive loads, such as coils, motors or wire inductance. Since the inductive impedance of the power supply also typically occurs when switching resistive or capacitive loads, the invention can also be advantageously applied to such loads.

The switching circuit also has a resistor, preferably an ohmic resistor in principle, so that the parasitic inductance of the actual resistance is negligible.

In addition, the switching circuit has a thyristor or a triac. These semiconductor devices are characterized in that they have to be triggered by means of a gate voltage and then automatically stay in the conductive state after being triggered.

The switching circuit thus has means for triggering the thyristor or the triac as a function of the voltage drop across the at least one circuit breaker. For this purpose, the component is operatively connected to the circuit breaker and to the gate of the thyristor or the gate of the triac. Such member is preferably one or several electronic components connected to the above-mentioned grid and circuit breaker.

In the switching circuit, the resistor and the thyristor or triac are connected in a connection across the at least one circuit breaker. "Bridging" means that current can flow through either the circuit breaker or the thyristor or triac, depending on the specific switching state of the circuit breaker and thyristor or triac.

According to an advantageous embodiment, said at least one circuit breaker is directly connected to the inductive load. According to another advantageous embodiment, the switching circuit is designed to switch both the positive half-wave and the negative half-wave of the alternating current.

According to an advantageous further feature, the circuit breaker is connected to a control logic circuit. The control logic can be, for example, a gate logic, a microcontroller, or an application specific switching circuit (ASIC), each equipped with drive elements if necessary. The thyristor triggering or triac triggering achieved by the components is in particular independent of the control logic and its control signals.

To this end, the control logic will open the circuit breaker in the event of a short circuit. To do this, a potential is applied to the circuit breaker such that, for example, the circuit breaker becomes a high resistance device. Wherein, the short-circuit condition is a fault condition deviated from the normal working condition, that is, the current flowing through the circuit breaker exceeds the allowable limit value. For this purpose, the control logic preferably has measuring means for determining the short-circuit current flowing through the circuit breaker. Such measuring means can, for example, detect short-circuit currents by means of a resistor or a voltage drop across the internal resistance of a circuit breaker.

The circuit breaker is suitably a power semiconductor. An advantageous embodiment of the power semiconductor is a field effect transistor (FET) or an insulated gate bipolar transistor IGBT (Insulated Gate Bipolar Transistor.

According to a preferred solution, the resistor is connected in series with the thyristor or the triac, so that the resistor is directly conductively connected to the thyristor or the triac. Therein, further components can be provided, which are connected to the terminals of the resistor or the thyristor or the triac, in particular also at the connection between the resistor and the thyristor or the triac .

According to one embodiment of the invention, the component has a varistor, which is connected to the gate of the thyristor or the gate of the triac. This voltage-dependent varistor is a high-resistance device until it reaches a threshold voltage, and when the voltage above it exceeds the threshold voltage, its resistance drops significantly. Wherein, the threshold voltage is specially set for the short circuit condition, therefore, the thyristor or the triac will not be triggered under the normal working voltage condition. But when the circuit breaker is suddenly opened under short-circuit conditions, the induced voltage caused by the inductance will cause a voltage that is greater than both the operating voltage and the threshold voltage. In this case, an effective current flows through the above-mentioned varistor, which in turn flows into the gate of the thyristor or the gate of the triac, thereby triggering the thyristor or triac.

Another advantageous embodiment consists in replacing the varistor with a triac, which is connected to the gate of the thyristor or the gate of the triac. The triac has similar electrical characteristics to the varistor, and thus also has the function of triggering the gate of the thyristor or the gate of the triac.

According to a preferred construction solution, at least two (for example) anti-series or anti-parallel power semiconductors are arranged, in particular two Metal Oxid Semiconductor Field Effect Transistors MOSFET (MetalOxid Semiconductor Field Effect Transistor) or two insulated gate Bipolar transistor IGBT (Insulated Gate Bipolar Transistor). Preferably, a freewheeling diode (Freilaufdiode) is connected between the source terminal (Source) and the drain terminal (Drain) of each power semiconductor. This freewheeling diode can be an external diode, or it can be formed by a p-n junction (such as a drain-body junction, Drain-bulk) of a power semiconductor.

According to one embodiment of the invention, two diodes are connected to the circuit breaker and/or the thyristor in such a way that current flows through at least one diode and at least partially through the thyristor when the thyristor is in the triggered state. These diodes have a rectifying effect in case of alternating current, so the short-circuit current will flow through the thyristor regardless of the direction of the current flow.

According to a further preferred embodiment, the resistor is dimensioned such that the voltage drop across the resistor at maximum current is smaller than the reverse voltage of the at least one circuit breaker. In this case it can be considered that the voltage drop across the thyristor or triac (and thus the diode) is less than the voltage drop across the resistor. Alternatively, the sum of the voltage drops is decisive so that the maximum reverse voltage of the circuit breaker cannot be exceeded.

Another aspect of the invention relates to the use of the switching circuit described above in a switching device of a low voltage installation. Such low voltage equipment may have, for example, three switching circuits of this type in order to control the three phase currents.

This object is achieved by the features of claim 16 according to the method of the invention.

The method is used to control at least one circuit breaker for switching an inductive load. This method is: firstly detect the short-circuit current flowing through the circuit breaker; then open the at least one circuit breaker, thereby triggering the thyristor or triac through the voltage drop on the circuit breaker, and the next short-circuit current The circuit breaker is bridged by a triggered thyristor or a triggered triac before the zero crossing.

Description of drawings

Embodiments of the present invention are described in detail below with the aid of the accompanying drawings, wherein:

Figure 1 is a first partial circuit diagram showing a first embodiment with thyristors; and

Figure 2 is a second partial circuit diagram showing a second embodiment with a triac.

Detailed ways

The same components are denoted by the same reference symbols in the various figures.

An inductive load X _L is exemplarily shown in the embodiment shown in FIG. 1 . Wherein, the current flowing through the inductive load X _L can be switched on and off through two reversely connected circuit breakers in the form of MOS field effect transistors (MOSFET) M1 and M2 shown in the drawing. The inductive load X _L is connected to the ground wire GND, and the MOSFET M1 is connected to the phase wire OC. If there are multiple (for example three) phase lines OC, three switch circuits described below can be used to respectively connect each current from the phase line OC to the ground line GND through the inductive load _XL . In this embodiment, the current flowing through the inductive load X _L is alternating current (AC power supply), so by connecting the two MOSFETs M1 and M2 in reverse series, the current can be controlled or cut off in two current directions of the alternating current. In order to control the MOSFETs M1 and M2 , the gate input ends of the MOSFETs M1 and M2 are connected to the control logic circuit 1 .

For switching devices in, for example, low-voltage installations, the two MOSFETs M1 and M2 must also be able to interrupt short circuits in addition to the normal switching function. Figure 1 shows a short circuit with a lightning bolt. Since a short circuit may occur at any point in a cycle, it must be limited and/or disconnected as soon as possible. Therefore, if no pre-limiting measures are taken for the short-circuit current, the next time the current crosses zero, it will not be able to Effectively break the short circuit.

Only when the energy stored in the inductor can be converted can the current be cut off immediately through MOSFETs M1 and M2. Wherein, the inductive load X _L can be understood as any kind of inductance that can function under short-circuit conditions, such as the inductance of the load coil or the inductance of the power supply (grid), which are collectively expressed as inductive load X in this embodiment _L. When the current is cut off, there is still a large amount of energy stored in the inductor _XL . If the short-circuit current is cut off quickly, it will cause a high overvoltage, resulting in damage to MOSFETM1 and/or M2.

With the help of the implementation shown in FIG. 1 , effective current limiting can be performed first in the electronic switch circuit, so that the circuit between the phase line OC and the ground line GND can be cut off when the current crosses zero next time. Current limiting is realized by actively connecting an equivalent load RL, so as to at least partially compensate for the "absence or stop" of the load that can function under normal operating conditions under short-circuit conditions. The equivalent load RL must be able to carry the current flowing through it for a maximum of half a grid cycle, i.e. a maximum of 10 ms at a grid frequency of 50 Hz, so even at potentially very high current intensities , the equivalent load RL can only absorb less energy.

The MOSFETs M1 and M2 shown in Fig. 1 have a built-in or an external freewheeling diode D _M1 and D _M2 respectively. If both MOSFETs M1 and M2 are turned on, current can flow from the phase line OC to the neutral line GND or the second phase line (not shown) through the MOSFETs M1 and M2 and the inductive load _XL . And when MOSFETs M1 and M2 are blocked, the circuit is disconnected.

The circuit has two diodes D1 and D2, a thyristor Th, an equivalent load RL and a trigger for the thyristor, which has elements V3 and R1. The circuit as a whole can be produced as a single device or integrated on a semiconductor chip with so-called "smart power" solutions.

In the event of a short circuit, the current rises to a high value within a short time, so the short circuit detection in the control logic 1 switches off the two MOSFETs M1 and M2 as quickly as possible. For example, short-circuit detection can be carried out by means of current limit value detection or based on a voltage increase at MOSFET M1 and/or M2 which is caused by the saturation current of MOSFET M1, M2 being exceeded.

Due to the rapid current cut-off and the presence of inductance X _L in the circuit, a high voltage significantly greater than the normal operating voltage appears across at least one of the MOSFETs M1 or M2. Once this high voltage exceeds the threshold voltage of the trigger varistor V3, the trigger varistor V3 will become a low-ohm resistor, and the flowing current will trigger the thyristor Th. In this case, the current can further flow through the resistor RL and one of the diodes D1 or D2, thereby limiting the overvoltage. When the current crosses zero next time, the thyristor Th will automatically cut off the current.

Regardless of the polarity of the instantaneous power supply, this function is reliably performed by two diodes D1 and D2. When the positive half-wave (that is, the phase line OC is connected to the positive voltage), the current flows through D1, RL, Th and freewheeling diode D _M2 when the protection circuit is activated; when the negative half-wave, the current flows through D2, RL, Th and freewheeling diode D _M1 .

The thyristor Th (same as the equivalent load RL) only needs to be energized for a short time. Therefore, these two elements can be implemented with relatively small devices that only need to withstand overloads for half the length of the grid cycle. The size of the resistor RL is determined so that the voltage generated by the resistor RL under the condition of maximum current (here, the saturation current of MOSFETM1 and M2) is smaller than the reverse voltage of MOSFET M1 or M2.

The circuit in the embodiment shown in Fig. 1 can be used without a fuse (fuses), but can break the short circuit, so there is no need to trigger a superimposed circuit breaker. A varistor designed for short-circuit currents or an RC element designed for short-circuit currents is provided for this purpose. Varistors are capable of absorbing inrush currents of relatively high magnitude, but this is often limited. The RC element requires a larger capacitor to absorb energy from the inductor _XL . The "active clamping" achieved by power transistors, that is, the control of power transistors to a state where the voltage is still below the maximum limit, requires these semiconductors to be oversized. On the contrary, if the equivalent load RL is actively connected, only MOSFETs M1 and M2 designed for normal working conditions need to be used.

The embodiment shown in Fig. 2 uses a triac TR to replace the thyristor, and the triac TR is connected in series with the equivalent load RL, and the two MOSFETs M1 and M2 are bridged in the triggered state. Forming the triggering circuit for the triac TR are a diac Di and a (if necessary) resistor R1. Wherein, the trigger current of two poles can be used to trigger the triac TR. Therefore, regardless of the polarity of the instantaneous power supply when a short circuit occurs, the triac TR can be triggered in principle.

Claims (16)

1. Switching circuit comprising: at least one circuit breaker (M1, M2) for switching the load (X _L ); a resistor (RL); a thyristor (Th) or a triac (TR ); and a plurality of members (R1, Di, V3) for switching said thyristor (Th) or said triac (TR) according to the voltage drop across said circuit breaker (M1, M2) triggering; wherein said resistor (RL) and said thyristor (Th) or said triac (TR) are connected to each other across said at least one circuit breaker (M1, M2) connected. 2. Switching circuit according to claim 1, wherein the circuit breaker (M1, M2) is connected to the control logic circuit (1). 3. The switching circuit according to claim 2, wherein the control logic circuit (1) is configured to open the circuit breaker (M1, M2) in case of a short circuit. 4. The switching circuit as claimed in claim 3, wherein the control logic (1) has measuring means for determining a short-circuit current flowing through the circuit breaker (M1, M2). 5. Switching circuit according to claim 1, wherein said circuit breaker is a power semiconductor (M1, M2). 6. Switching circuit according to claim 5, wherein said power semiconductors are field effect transistors (M1, M2). 7. The switching circuit of claim 5, wherein the power semiconductor is an insulated gate bipolar transistor (IGBT). 8. Switching circuit according to claim 1, wherein said resistor (RL) is connected in series with said thyristor (Th) or said triac (TR). 9. The switching circuit according to claim 1, wherein said member comprises a varistor (V3) connected to the gate of the thyristor (Th) or to the gate of the triac (TR) connected to the grid. 10. Switching circuit according to claim 1, wherein said member comprises a diac (Di) connected to the gate of said thyristor (Th) or said triac The gate of the switch (TR) is connected. 11. The switching circuit according to claim 5, wherein at least two power semiconductors (M1, M2), in particular two Metal Oxid Semiconductor Field Effect Transistors (Metal Oxid Semiconductor Field Effect Transistor, MOSFET) or two Insulated Gate Bipolar Transistor (IGBT). 12. The switching circuit according to claim 11, wherein a freewheeling diode (D) is connected between the source terminal (Source) and the drain terminal (Drain) of each of the power semiconductors (M1, M2). _M1 , D _M2 ). 13. Switching circuit according to claim 1, wherein two diodes (D1, D2) are connected to said circuit breaker (M1, M2) and/or said thyristor (Th) in such a way that when said When said thyristor (Th) is in an activated state, a current flows through one of the diodes (D1, D2) and at least partly through said thyristor (Th). 14. Switching circuit according to claim 1, wherein said resistor (RL) is dimensioned such that the voltage drop across said resistor (RL) at maximum current is smaller than at least one of said open circuits device (M1, M2) reverse voltage. 15. Use of the switching circuit according to claim 1 in a switching device of a low-voltage apparatus. 16. A control method for controlling at least one circuit breaker (M1, M2) for switching a load (X _L ), especially a power semiconductor, wherein, detecting a short circuit current through said at least one circuit breaker (M1, M2), and opening said at least one circuit breaker (M1, M2), thereby triggering said thyristor (Th) or triac (TR) by a voltage drop across said circuit breaker (M1, M2), such that A triggered thyristor (Th) or a triggered triac (TR) bridges the circuit breaker (M1, M2) before the next zero crossing of the short circuit current.

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