OF OPERATING-MECHANISM

FOR TRIPPING A VOLTAGE CIRCUIT-BREAKER DESCRIPTION

Technical field and prior art

The invention relates to an undervoltage release circuit of an operating mechanism for tripping a voltage circuit breaker and to an operating-mechanism for tripping a voltage circuit-breaker which comprises an undervoltage release circuit according to the invention .

The energy to trip a circuit-breaker is stored in a spring (spring-operated mechanism) or in a hydraulic tank (hydraulic-operated mechanism) . The spring is held in its charged position using a latching-system, while the hydraulic tank is kept under pressure by a closed valve. To trip the circuit- breaker, the latching-system or the valve is opened either manually or by an electromagnetic solenoid (coil) .

It is sometimes required a special equipment called "undervoltage-release circuit" to trip the circuit-breaker in case the control-voltage drops under a certain voltage level. As known by the man skilled in the art, the "control-voltage" is the power- supply with which the substation which contains the circuit-breaker is controlled (all mechanisms, protection relays, position indicators, etc. are activated by the control-voltage) . The tripping of the circuit-breaker is then specified as follows:

- The undervoltage-release circuit does not trip the circuit-breaker as long as the control-voltage is higher than 70% of rated voltage;

The undervoltage-release circuit must trip the circuit-breaker if the control-voltage is lower than 35% the rated voltage.

Furthermore, a reclosing must be possible as soon as the control-voltage is minirnum. 85-6 of rated voltage .

In some operating-mechanisms of the prior art, linear solenoids are used to realize the undervoltage-release function. The solenoid acts against a spring and is connected to a trip-latch. As long as the control-voltage is high enough, the solenoid holds the spring in charged position. When the control-voltage drops, the magnetic force of the solenoid gets weaker so it cannot hold the spring in its charged position. Thus, the spring discharges and trips the circuit-breeaker .

The main problem of such an electromagnetic system is to find the balance between the magnetic force induced by the solenoid and the force of the spring, especially when working under low or high temperatures. Due to the temperature-coefficient of the copper-windings of the solenoid, the magnetic force induced by the solenoid is depending on the temperature. Another problem is that the force induced by the solenoid is a very non linear force, while the force of the spring is a linear force. The Figure 1 represents the force induced by the solenoid and the force of the spring, in accordance with the stroke of both the solenoid and the spring, at room temperature (20°C), according to the prior art.

The curve CI represents the solenoid force at 70% rated voltage and the curve C3 represents the solenoid force at 35% rated voltage. The curve C2 represents the spring force. On the abscissa scale, the point "0" represents the solenoid pulled and the spring charged and the point "6" represents the solenoid released and the spring discharged.

The magnet force is higher than the spring force over the whole stroke as long as the control- voltage is 70% of rated voltage or higher. This means that the system stays at a position where the solenoid is pulled and the spring charged. If the control- voltage is at 35% of rated voltage or lower, the spring force is higher than the solenoid force over the complete stroke, which means that the sytem moves to the released, discharged position so that the circuit- breaker is tripped.

The Figure 2 represents the force induced by the solenoid and the force of the spring, in accordance with the stroke of both the solenoid and the spring, at minimum temperature, for example -40 °C, according to the prior art.

The curve C4 represents the solenoid force at 70% rated voltage and the curve C6 represents the solenoid force at 35% rated voltage. The curve C5 represents the spring force. The force induced by the solenoid at 35% rated voltage is only slightly lower than the spring force. After a certain stroke, both forces are equal. This means that the system starts to move and, most probably, stops at stroke 5 and the circuit-breaker is not tripped. Otherwise, as long as the control-voltage is 70% of rated voltage or higher, the magnet force is higher than the spring force over the whole stroke so that the system stays at a position where the solenoid is pulled and the spring charged.

The Figure 3 represents the force induced by the solenoid and the force of the spring, in accordance with the stroke of both the solenoid and the spring, at maximum temperature, for example +50 °C, according to the prior art.

The curve C7 represents the solenoid force at 70% of rated voltage and the curve C8 represents the solenoid force at 35% of rated voltage. The curve C9 represents the spring force. Even at 70% of rated voltage, the force induced by the solenoid is partially lower that the spring force. This means that the circuit-breaker could trip even at 70% of rated voltage .

The Figure 4 represents the force induced by the solenoid at 85% of rated voltage (curve CIO) and the force of the spring (curve Cll), in accordance with the stroke of both the solenoid and the spring, at maximum temperature, for example +50 °C, according to the prior art.

This diagram shows the problem of recharging the spring at 85% of the rated voltage. At the circuit-breaker which means that the solenoid must be able to move the spring from its discharged to its charged position. The solenoid force is partially higher or equal to the spring force. The solenoid cannot charge the spring completely so that the closing of the circuit-breaker is not possible.

All the above Figures 1-4 show that it is not possible to find a solenoid and a spring that work correctly under all conditions. If the force induced by the solenoid is big enough (i.e. the solenoid is big enough) to fulfill the condition "charge the spring at 85% of the rated voltage", then the condition "trip the circuit-breaker (i.e. release the spring) at less than 35% of the rated voltage" cannot be fulfilled. If the solenoid has less force (smaller solenoid) so that the condition "trip the circuit-breaker at less than 35% of the rated voltage" can be fulfilled, then the condition "charge the spring at 85% of the rated voltage" cannot be fulfilled.

According to the prior art, several solutions are known to overcome all or part of the above mentioned drawbacks. These solutions are:

- using only the quasi-linear part of the stroke of the solenoid (this causes a bigger solenoid to have enough energy to charge the spring) ;

- using additional mechanical system that recharges the spring while tripping the circuit-breaker (doing so avoids the problem of recharging at 85%) ;

- using a special non linear spring or a cam-sytem to get a desired nonlinearity . All these systems use complicated additional mechanisms and/or very big solenoids, which increases both the size and the cost of the operating- mechanism.

The invention does not have such drawbacks.

Presentation of the invention

Indeed, the invention relates to an undervoltage release circuit comprising a solenoid and a spring, the solenoid acting against the spring and being connected to a trip-latch which trips a circuit- breaker as soon as a control voltage drops under a voltage level, characterized in that it comprises a voltage determining device and a switching element controlled by a command signal, the switching element being connected in series with the solenoid, the voltage determining device comprising means for measuring the control-voltage and means for generating a signal according to the control voltage measured, the signal generated according to the control voltage measured constituting the command signal so that the switching element is closed when the control voltage measured is equal or higher than the voltage level and opened as soon as the control voltage is less than the voltage level.

According to a another characteristic of the invention, the voltage determining device comprises a diode in series with a first resistor and a second resistor and the switching element is made of a transistor, the diode having an anode and a cathode, the first resistor having a first terminal and a second terminal, the second resistor having a first terminal and a second terminal and the transitor having a gate, a drain and a source, the first terminal of the first resistor being connected to the anode of the diode and to the gate of the transistor, the second terminal of the first resistor being connected to the second terminal of the undervoltage release circuit and to the drain of the transistor, the first terminal of the second resistor being connected to the first terminal of the undervoltage release circuit and to a first terminal of the solenoid, the second terminal of the second resistor being connected to the cathode of the diode, the source of the transistor being connected to a second terminal of the solenoid.

The invention also relates to an operating mechanism for tripping a voltage circuit-breaker, the operating mechanism comprising an undervoltage release circuit to trip the circuit-breaker if a control voltage drops under a voltage level, characterized in that the undervoltage release circuit is as circuit according to the invention.

Brief description of the drawings

Other characteristics and advantages of the invention will become clearer upon reading a preferred embodiment of the invention made in reference to the attached figures, wherein:

The Figures 1-4 - already described - represent a comparison between the force induced by a solenoid and the force of a spring, in different conditions, according to the prior art; - The Figure 5 represents a diagram of an undervoltage-release circuit of operating-mechanism for circuit-breaker according to the invention;

- The Figure 6 represents an example of the undervoltage-release circuit of operating-mechanism for circuit-breaker represented in Figure 5;

- The figure 7 represents an improvement of the undervoltage-release circuit represented in Figure 5;

The figure 8 represents an example of undervoltage-release circuit of operating-mechanism which uses a plurality of undervoltage-release circuits as the one shown in Figure 7.

In all the figures, the same references designate the same elements.

Detailed description of the invention

The Figure 5 represents a diagram of an undervoltage-release circuit of operating-mechanism for circuit-breaker according to the invention.

The undervoltage-release circuit comprises a voltage determining device 1, a switch 2, a solenoid 3 and a spring. The spring is not represented in Figure 5 because the representation of the spring is not useful for the understanding of the invention.

The switch 2 is in series with the solenoid 3 and the voltage determining device 1 is in parallel with the set constituted by the switch 2 in series with the solenoid 3. The control-voltage VC is applied between the terminals of the voltage determining device 1. The voltage determining device 1 measures the control voltage VC and outputs a signal K which a function of the control voltage measured. The signal K constitutes the command signal of the switch 2.

When the control voltage VC is equal to the rated voltage, the signal K commands the switch 2 in its closed position and a current flows through the solenoid 3. The spring (not represented) stays in the charged position.

As soon as the control voltage VC drops to a voltage level less than 70% of the rated voltage, for example a value equal to 50% of the rated voltage, the signal K commands the switch 2 so that the switch 2 is put in its opened position. There is no more current in the solenoid 3. Therefore, the solenoid 3 cannot keep the spring in the charged position and the circuit- breaker is tripped.

An advantage of the invention - in comparison with the prior art - is that there is no more need to find a balance between the magnetic force induced by the solenoid and the force of the spring so that the conditions (1) and (2) below are obtained:

(1) the magnetic force induced by the solenoid is big enough to fulfill the condition "charge the spring at 85% of the rated voltage" or "keep the spring charged at 70% of the rated voltage when temperature is maximum", and

(2) the magnetic force induced by the solenoid is small enough to fulfill the condition "trip the circuit breaker (i.e. release the spring) at less than 35% of the rated voltage". With the device of the invention, the only condition to realise is to choose a solenoid big enough so that its magnetic force is big enough too. Advantegeously, this condition is very easy to realise with standard solenoids (solenoids bought in the market) .

The Figure 6 represents an example of the undervoltage-release circuit of operating-mechanism for circuit-breaker represented in Figure 5.

The voltage determining device 1 is made of a Zener diode Dl and two resistors Rl and R2 in series with the Zener diode. The switch 2 is made of a field effect transistor T, for example a power MOSFET. The resistor Rl has a first terminal connected to the anode of the Zener diode Dl and to the gate of the field effect transistor T, the second terminal of the resistor Rl being connected to the source of the field effect transistor T. The second resistor R2 has a first terminal connected to a first terminal of the solenoid 3, the second terminal of which is connected to the drain of the field effect transistor T. The second terminal of the resistor R2 is connected to the cathode of the Zener diode Dl . The control-voltage VC is applied between the first terminal of the resistor R2 and the second terminal of the resistor Rl .

As long as the control-voltage VC is higher than the Z-voltage of the diode Dl, the gate of the transistor T is charged and a current flows through the solenoid 3. As soon as the control-voltage gets lower than the Z-voltage of the diode Dl, the gate of the transistor T gets discharged through the resistor Rl and the transistor T interrupts the current through the solenoid .

The figure 7 represents an improvement of the undervoltage-release circuit represented in Figure 5.

In addition to the elements represented in Figure 6, the circuit represented in Figure 7 comprises a rectifier B placed at the input of the undervoltage release circuit. This rectifier makes advantageously the circuit independent of polarity and suitable for active current AC and direct current DC.

The figure 8 represents an example of undervoltage-release circuit of operating-mechanism which uses a plurality of undervoltage-release circuits as the one shown in Figure 7.

In addition to the elements represented in Figure 7, the voltage determining device of Figure 8 comprises a 6 way switching controller 4 having one input and six outputs and six branches comprising each at least one Zener diode in series with at least one resistor. Each branch is palced between a different output of the 6 way switching controller and the first terminal of the solenoid. As an example, each branch among five branches comprise a diode Di (i=l, 2, 3, 4, 5) and a resistor Ri (i=l, 2, 3, 4, 5) and the sixth branch comprises two Zener diodes D6a, D6b and two resistors R6a, R6b. The 6 way controller 4 allows to connect the gate of the transistor T to anyone of the six branches. The resistors Ri (i=l, 2, 3, 4, 5) have different values. This allows to get different values of voltage levels to which the control-voltage may be compared .

With the example of Figure 8, the voltage determining device comprises a 6 way switching controller and six branches. More generally, the voltage determining device of the undervoltage-release circuit of the invention comprises a N way switching controller and N branches.