DE3884204T2 - Overcurrent protection switch. - Google Patents

Description

The invention relates to a circuit breaker with a switching device for jump-over protection.

An example of such a circuit breaker is shown in Japanese Laid-Open Patent Application No. 32211/1985 and EP-A-0 133 968. It comprises a current transformer for detecting a fault current in a line. The output current of the current transformer is converted into a DC signal via a full-wave rectifier and then supplied to a shunt circuit. The waveform of the signal voltage induced in the shunt circuit is the well-known absolute value waveform. The output signal of the shunt circuit is converted into a signal corresponding to its effective value or average value in a signal conversion circuit.

The output signal of the signal conversion circuit is supplied to a fault processing circuit, and when the fault current exceeds a predetermined threshold, a level detection signal is supplied to a time delay circuit, which performs a certain time delay and triggers the gate of a thyristor to energize a release-type overcurrent switching coil and open a line contact and interrupt the current in the line.

In this case, a power supply circuit for the error processing circuit is connected in parallel to the shunt circuit.

In the arrangement described above, a part of the secondary current of the current transformer used to detect a current flows through the power supply circuit. As a result, the current flowing through the shunt circuit does not match that flowing through the AC line, and an error occurs in the detection level of the fault current. Since the current flowing through the power supply is not fixed, it is difficult to correct the error in the fault current level detection.

A measure for improving the accuracy of level detection of the leakage current is disclosed in Japanese Utility Model Publication No. 29931/1980. In the device, however, the capacitor-forming part of the time delay circuit is associated with a leakage current. In the case of a short-time delay circuit, the leakage current is very small compared with the leakage current and the leakage time is about 10 msec, so that the problem is not serious. In the case of a long-time delay circuit, even if the leakage current is as small as 0.5 µA, the leakage current and the charge current are equal when the current is 1 µA and the time delay operation is double that of the rated value. Thus, it deviates from the rated value.

An object of the invention is to solve the problems described above.

Another object of the invention is to provide a circuit breaker with improved accuracy in the level detection of the fault current and to provide an improved time delay operation.

EP-A-289 011 discloses a circuit breaking device having a line contact inserted into an AC line;

a current transformer for detecting the current flowing through the line contact;

a rectifier circuit connected to the secondary winding of the current transformer to convert the secondary alternating current into a direct current;

a DC constant voltage supply circuit connected between the output terminals of the rectifier circuit and having positive, negative and intermediate output terminals for generating a positive, negative and intermediate potential;

a current sensing resistor connected in series with the power supply circuit;

a differential amplifier circuit which is supplied by the output signal of the power supply circuit and converts the voltage drop across the current detection resistor, which is proportional to the rectified current, into an output signal which has a potential lying between the positive and the negative potential and taking the intermediate potential as a reference potential;

a time delay circuit supplied by the voltage supply circuit and receiving the output signal of the differential amplifier proportional to the rectified current and producing a certain time delay with respect to a predetermined magnitude of the rectified current;

a switching circuit which, in accordance with the output signal from the delay circuit, is switched by a from an open state to a closed state;

an electromagnetic switching coil connected in series with the switching circuit, the series connection between the electromagnetic switching coil and the switching circuit being supplied by the output signal of the rectifier circuit; and

a switching mechanism that is energized when the electromagnetic switching coil is energized, wherein the time delay circuit comprises a long time delay circuit.

US-A-3803455 corresponding to DE-A-2400084 discloses a multiphase circuit breaker with different timing circuits, each comprising conventional resistor-capacitor combinations.

In contrast to the prior art and in accordance with the present invention, the long-time delay circuit includes a capacitor charged by a current corresponding to a fault current and a current compensation circuit including a constant current source supplying to the capacitor a current equal to a maximum leakage current of the capacitor to compensate for the leakage current.

Short description of the drawing

Fig. 1 is a circuit diagram showing a circuit breaker device according to an embodiment of the present invention.

Fig. 2 is a circuit diagram showing an example of a long-time delay circuit.

Fig. 3 is a characteristic curve showing the operational characteristics of the circuit breaking device according to the present invention.

Fig. 4 is a circuit diagram of an example of the power supply circuit included in the circuit breaker device according to the present invention.

Fig. 5 is a circuit diagram of an example of a short-time delay circuit.

Detailed description of the implementation examples

The circuit breaker according to an embodiment of the invention shown in Fig. 1 is associated with a line 11 of an electrical power supply system or circuit to be protected. The electrical system may be of any type, such as a single-phase system or a multi-phase system. In Fig. 1, only a single line is shown for the purpose of simplification.

A circuit breaker includes a circuit breaker mechanism 100 and a line contact 201 for isolating or separating the different areas of the electrical system under certain abnormal or faulty conditions, such as an overcurrent condition. The circuit breaker mechanism 100 may include an electromagnetic Switching coil 80 which, when energized while the line contact 201 is closed, causes switching or opening of the line contact 201.

In order to obtain an output current which is substantially proportional to the line current in the line 11, a current transformer 21 is provided as shown in Fig. 1. The current transformer 21 is energized in accordance with the line current of the line 11.

The current transformer 21 has a secondary output winding connected to the AC terminals of a bridge full-wave rectifier circuit 30 which rectifies the secondary output current of the current transformer 21. The rectifier circuit 30 comprises a series connection of diodes 31 and 32 and another series connection of diodes 33, 34.

A power supply circuit 500 is provided to receive the output signal of the rectifier circuit 30. More specifically, a first input terminal 5b of the power supply circuit 500 is connected to the positive DC output terminal of the rectifier circuit 30. The power supply circuit 500 has a positive output terminal 5a, a negative output terminal 5d and an intermediate terminal 5c. The intermediate terminal 5c is grounded. The potential at the positive output terminal 5a is positive with respect to and 12 V higher than the intermediate terminal 5c. The potential at the negative terminal 5d is negative with respect to and approximately 3 V lower (more negative) than the intermediate terminal. 5d. The negative output terminal 5d is connected to a negative DC output terminal of the rectifier circuit 30 through a current sensing resistor 400. Thus, a full-wave rectified current corresponding to the line current in the line 11 flows through the sensing resistor 40.

An example of the power supply circuit 500 is shown in Fig. 4. As shown, it comprises a series connection of a resistor 500a and a Zener diode 500b connected between the terminals 5b and 5c. The Zener diode 500b has a Zener voltage of approximately 12 V. The voltage supply circuit 500 further comprises a series connection of three diodes 500c, across which a forward voltage drop occurs which is approximately 2 V and which are connected between the terminals 5d and 5c, a diode 500e, the anode of which is connected to the connection point between the resistor 500a and the Zener diode 500b and the cathode of which is connected to the terminal 5a, and a capacitor 500d which is connected between the terminals 5a and 5c, and another capacitor 500f which is connected between the terminals 5c and 5d. The capacitors 500d and 500f stabilize the voltage across the terminals 5a, 5c and 5d. The diode 500e prevents the capacitors 500d and 500f from discharging.

A differential amplifier circuit 60 amplifies the voltage drop across the current sensing resistor 40 and at the same time shifts the level of the potential which is below the ground potential into a signal with the intermediate potential V₀ of the power supply circuit 500 as a reference. In other words, the differential amplifier circuit 60 produces an output signal Vout through an output signal line and a ground line, which corresponds in magnitude to the voltage drop across the current sensing resistor. The differential amplifier circuit 60 comprises an operational amplifier 63 and four resistors 64, 65, 66 and 67. The supply for the differential amplifier circuit 60 is provided by the power supply circuit 500. The input terminals of the differential amplifier circuit 60 are connected to both ends of the current sensing resistor 40.

The gain A of the differential amplifier circuit 60 is:

A = Vout/Vin = Rout/Rin,

where Rin = R64 = R66

and Rout = R65 = R67

Here, R64, R65, R66 and R67 are resistance values of resistors 64, 65, 66 and 67.

For the differential amplifier circuit 60 to operate properly, the input potentials V₁ and V₂ should be between the positive and negative potentials applied to its positive and negative power supply terminals.

A time delay circuit 70 includes an instantaneous circuit 230, a short-term delay circuit 220, and a long-term delay circuit 170. The output terminals of the long-term delay circuit 170, the short-term delay circuit 220, and the instantaneous circuit 230 are connected in parallel to each other and have an output terminal 70a.

The instantaneous circuit 230 is connected to the input terminal of the differential amplifier circuit 60. In parallel with the instantaneous circuit 230 are a series connection of a peak converter circuit 210 and the short time delay circuit 220 and another series connection of a peak converter circuit 211 and the instantaneous circuit 170. The respective peak converter circuits 210 and 211 convert the input signal of the full-wave rectified waveform into a continuous DC signal having a magnitude proportional to the peak value of the input signal.

The electromagnetic switching coil 80 is connected to the positive and negative DC terminals of the rectifier circuit 30 via a switch 120. An armature (not shown) driven by the electromagnetic switching coil 80 is mechanically coupled to the line contact 201 via the circuit breaker mechanism 100. When the switch 120 changes from its open state to its closed state, the line contact 201 is opened.

An undervoltage operation blocking circuit 50 is connected between the positive and negative output terminals 5a and 5b of the power supply circuit 500. A switch 55 is controlled by the undervoltage operation blocking circuit 50.

Fig. 2 shows an example of the long-term delay circuit 170. As shown, it comprises a comparator 35 which receives an output signal e of the peak converter circuit 211 at an input terminal. A long-term delay switching current reference voltage setting circuit 37 is connected to the other input terminal of the comparator 35 to provide a rated current value as a reference. A switch 36 is controlled by the comparator 35 which is normally closed but is opened in the event of an overcurrent.

A long delay switch capacitor 38 and a discharge resistor 39 are connected in parallel with the switch 36. The discharge resistor 39 is designed to discharge the capacitor 38 with a predetermined time constant. The time constant of the resistor 39 and the capacitor 38 is selected to correspond to the time constant of the heat distribution of the line. The junction point a connecting the capacitor 38 and the resistor 39 is connected to the output of a voltage/current converter circuit 44. The input of the voltage/current converter circuit 44 is connected to receive the output signal e of the peak converter circuit 211. The output signal of the voltage/current converter circuit 44 is proportional to the square of the input signal.

The output current I1 of the voltage/current converter circuit 44 flows into the capacitor 38 for charging the same.

The connection point a is also connected to the input terminal of a comparator 41. A long-time reference voltage setting circuit 42 is connected to the other input terminal of the comparator 41 to provide a potential corresponding to the long-time delay operation, which becomes the reference for the comparator 41.

A current compensation circuit 43 is connected between the connection point a and the positive DC output terminal 5a. The current compensation circuit 43 serves to compensate the leakage current of the capacitor 38. The leakage current of the capacitor 38 is considered to be not greater than 0.5 µA, and the current compensation circuit 43 includes a constant current source which supplies 0.5 µA to compensate the maximum leakage current.

A short-time delay circuit 220 is shown in Fig. 5. It is similar to the long-time delay circuit 170. As shown, it includes a comparator 221, a switch 222, a short-time switching current setting circuit 223, a voltage/current converter circuit 224, a capacitor 225, a resistor 226, a comparator 227 and a short-time delay time reference voltage setting circuit 228, which are similar to the comparator 35, the switch 36, the voltage/current converter circuit 44, the long-time delay switching current reference voltage setting circuit 37, the capacitor 38, the resistor 39, the comparator 41 and the long-time delay time reference voltage setting circuit 42. However, the output signal of the voltage/current converter 224 is proportional to its input signal.

The operation of the arrangement described above is described below.

When a current flows through the AC line 11, a secondary current flows through the secondary winding of the current transformer 21. The magnitude of the secondary current is determined by the transformer ratio. The secondary current is converted into a rectified current by the rectifier circuit 30. The output current from the rectifier circuit 30, which is a full-wave rectified current corresponding to the AC line 11, passes through the power supply 500 and the current detection resistor 40.

When a full-wave rectified current flows through the power supply circuit 500, the output terminals 5a and 5d of the power supply circuit 500 will produce potentials +V (= approximately +12 V) and -V (= approximately -2 V), respectively, when the potential at the intermediate terminal 5c is selected as a reference Vo (= 0 V).

The differential amplifier circuit 60 is supplied by the power supply circuit 500 and the input signal of the differential amplifier circuit 60 is supplied via the current sensing resistor 40. The gain A of the differential amplifier circuit 60 is given by

A = Vout/Vin = Rout/Rin,

as previously stated.

When the line current as represented by the output signal of the differential amplifier circuit 60 exceeds the instantaneous switching current range, the short-time delay switching range and the long-time delay switching range as shown in Fig. 3, that is, when the magnitude of the line current as represented by the magnitude of the current of the differential amplifier circuit 60 exceeds or continuously exceeds an overcurrent value for a time along the characteristic curve, a Output signal generated according to the respective long time delay circuit 170, short time delay circuit 220 or instantaneous circuit 230.

The output of the peak conversion circuit 211 is input to the comparator 35 as shown in Fig. 2, and when this output exceeds the output of the long-time delay switching current reference setting circuit 37, the switch 36 is controlled by the comparator 35 from the closed state to the open state, thereby allowing the charging of the capacitor 38. The time delay operation is thereby started.

The voltage/current converter circuit 44 converts the input voltage into a current signal. Its output current I1 is used for charging the capacitor 38. When the voltage across the capacitor 38 is increased due to the charging and exceeds the output voltage of the long-time delay time reference voltage setting circuit 42, an output signal is provided from the long-time delay circuit 170 as an indication of the elapse of a long delay time. With higher input voltage e, the operation time is shorter.

In general, the long delay circuit 170 should operate when the current flowing through the line contact 201 is within 125% of the rated current.

When the current I1 corresponding to the fault current flows into the capacitor 38, a leakage current of the capacitor 38 flows. A compensation current I3 corresponding to the leakage current is supplied by the current compensation circuit 43 is added. As a result, the capacitor 38 is charged in accordance with the magnitude of the leakage current I1, with the effect that the leakage current is removed. An accurate time delay operation is thereby achieved.

The output of the long delay circuit 170 passes through the now closed switch 55 of the undervoltage operation blocking circuit 50 and triggers the switch 120, thereby moving the switch 120 from the open state to the closed state, which in turn energizes the electromagnetic switching coil 80. The electromagnetic switching coil 80 then actuates the line contact 201 from the closed state to the open state, thereby interrupting the fault current.

When the current flowing through the line contact 201 is less than 10 to 20% of the rated current, the output voltage of the power supply circuit 500 is insufficient for the operation of the time delay circuit 70. In such a state, the time delay circuit 70 may produce an erroneous output signal. To prevent such an erroneous output signal, the undervoltage operation blocking circuit 50 is provided. This blocking circuit 50 opens the switch 55 when the current across the line contact 201 is small and the voltage across the terminals 5a and 5b is low.

In the described embodiment, the electrical system is a single-phase alternating current system. The invention is also applicable to multi-phase systems.

As described according to the invention, a total current corresponding to the current through the AC conductor flows through the power supply circuit and the current detection resistor, so that the accuracy of current detection is improved.

In addition, a current compensation circuit is provided in the long-time delay circuit to compensate for the leakage current through the discharge resistor. A current exactly equal to the leakage current therefore flows into the capacitor for its charging, so that the time delay operation is accurate.

Claims (4)

1. Circuit breaker device with: a line contact (201) inserted into an AC line (11); a current transformer (21) for detecting the current flowing through the line contact (201); a rectifier circuit (30) connected to the secondary winding of the current transformer (21) in order to convert the alternating secondary current into a direct current of the same direction; a DC constant voltage supply circuit (500) connected between the output terminals of the rectifier circuit (30) and having positive, negative and intermediate output terminals (5a, 5d, 5c) for generating a positive, negative and an intermediate potential; a current sensing resistor (40) connected in series with the voltage supply circuit (500); a differential amplifier circuit (60) which is supplied by the output signal of the voltage supply circuit (500) and converts the voltage drop across the current detection resistor (40), which is proportional to the rectified current, into an output signal (Vout) which has a potential lying between the positive and the negative potential and taking the intermediate potential as a reference potential; a time delay circuit (70) which is supplied by the voltage supply circuit (500) and which is proportional to the rectified current receives the output signal (Vout) of the differential amplifier circuit (60) and produces a certain time delay with respect to a predetermined magnitude of the rectified current; a switch circuit (120) which is switched from an open state to a closed state in accordance with the output signal from the delay circuit; an electromagnetic switching coil (80) which is connected in series with the switching circuit, the series connection between the electromagnetic switching coil (80) and the switching circuit being supplied by the output signal of the rectifier circuit; and a switching mechanism (100) which is energized when the electromagnetic switching coil is energized, the time delay circuit (70) comprising a long-time delay circuit (170), characterized in that the long-time delay circuit has a capacitor (38) which is charged by a current corresponding to a fault current and a current compensation circuit (43) which comprises a constant current source which supplies the capacitor (38) with a current equal to a maximum leakage current of the capacitor in order to compensate for the leakage current. 2. Device according to claim 1, in which the series connection of electromagnetic switching coil (80) and switching circuit (120) is located between the input terminals (5b, 5d) of the voltage supply circuit (500). 3. The device of claim 1 or 2, wherein the time delay circuit (70) further comprises a peak value converter circuit (211) which converts the peak value of the output signal of the differential amplifier circuit (60) into a DC signal having a magnitude corresponding to the peak value of the output signal of the differential amplifier circuit (60). 4. The apparatus of claim 3, wherein the long-term delay circuit (170) further comprises: a first comparator (35) for comparing the output signal (e) of the peak converter circuit (211) with a predetermined rated current reference voltage and for generating a signal when the former crosses the latter; a switch (36) connected in parallel with the capacitor (38), the switch being normally closed and being opened when the comparator (35) generates the signal; a voltage/current converter (44) which converts the output signal of the peak converter circuit (211) into a current proportional to the square of the output signal of the peak converter circuit (211) and a second comparator (41) for comparing the voltage across the capacitor (38) with a predetermined reference voltage for the time delay operation, forming the output signal of the long delay circuit (170).