GB2197763A - Electronic protective relay - Google Patents

Description

1 Electronic Protective Relay 2197763 The present invention relates to an

electronic protective relay and more particularly to a relay with self-holding and storage of the release state produced by a release circuit.

Such a protective relay is known, for example, from US 4.509.088. This discloses an electronic motor protection relay which delivers a release signal in the event of overloading which might lead to endangering and overheating of the motor to be protected. With this release signal, a power circuit element, for example a contactor is actuated which switches off the motor. If switching off is effected as a result of overloading, switching on again is only possible when the motor has cooled down sufficiently. In order to prevent inadmissible switching on again after a supply voltage failure after the motor has been switched off as a result of overloading, it is proposed, in the US a capacitor with a voltage proportional to the heating of the motor. Connected in parallel with this capacitor is a discharging resistor. If the capacitor is sufficiently charged after a supply voltage failure, the switching on of the motor is possible; otherwise it is necessary to wait until the capacitor has been sufficiently charged by the resistor before the motor is started again.

Patent, to charge An electronic motor protection relay is known from GB Patent Applications 2 097 612 and 2 097 613 wherein the cooling times after the motor has been switched off as a result of overloading are determined by the discharging of an RC network. Here the times before a renewed switching on of the motor are a few minutes, preferably 1 1/4 minutes.

2 It is a disadvantage of this electronic protective relay that after a prolonged supply voltage failure, the cause which led to the release of the relay is no longer stored.

The present invention seeks to provide an electronic protective relay having a release-state storage circuit wherein, in the event of a supply voltage failure, the release state or condition is still stored even after hours, which is simple in construction and wherein the storage circuit causes the self-holding at the same time.

According to a f irst aspect of the present invent-ion there is provided an electronic protective self-holding relay with storage in a capacitor of the release state produced by a release circuit, wherein the noninverting input of a comparator is in electrical communication, through first resistor, with a release circuit and by means of the capacitor with: (a) the cathode of a diode which is connected, at the anode side, to a first reference voltage, and (b) a zener diode which is connected at the anode side to a second reference voltage, and a feedback loop formed by at least one feedback resistor to the non-inverting input of the comparator can be disconnected.

According to a second aspect of the present invention there is provided an electronic protective relay which is self-holding and has a capacitor which stores a value representative of a release state, the relay also comprising comparing means, one input of which is connected to: (i) resistive means arranged to be connected to a release circuit, (ii) one terminal of said capacitor, and (iii) a feedback loop from the output of the comparator and comprising a feedback 3 resistor, wherein the other terminal of said capacitor is connected via first and second rectifying means to respective reference voltages, and means are provided for disconnecting the feedback loop.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which:

Fig 1 shows diagrammatically possible functional units of electronic protective relays; Fig 2 shows the circuit diagram of an embodiment of the present invention; and Fig 3 shows a more detailed diagram of part of an electronic motor protection relay having a self-holding and release-state storage circuit according to the present invention.

In figure 1, the phase conductors of a supply system are designated by R, S, T. The load 1 can be connected to the power supply and disconnected from it again through a power circuit element 2f for example a contactor. Current transformers 3 measure the currents in the phase conductors R, S, T and deliver signals proportional to the current to an input circuit 4. Here the signals are rectified, standardized and distributed to the release circuits 5. The release circuits 5 examine the signals fot specific characteristics such as for short-circuit at circuit 6, overload at circuit 7, and asymmetry and phase failure at circuit 8. In addition, the temperature of the load 1 is measured with a thermister 9 of which the resistance value is dependent on temperature and is monitored by an appropriate release circuit 10. If a 4 f ixed cr i ter ion is exceeded in one of the release circuits 5, a release signal is delivered and conveyed to a self-holding and release- state storage circuit 11. Here the release signal is stored and at the same time the disconnection of the load 1 is effected by a disconnection instruction to the power circuit element 2.

The present invention relates to the self-holding and release-state storage circuit 11 and is illustrated in more detail in Figs 2 and 3 with reference to two examples.

In Fig 2, an operational amplifier 20 is shown in broken lines; it symbolizes an output amplifier or output comparator of one of the release circuits 5. In the normal state, the output of this operational amplifier 20 is "high" and, on release, changes over to 11 1 o W". If the corresponding release circuit 5 finds that the fault is no longer present, the output signal of the operational amplifier 20 also changes back to "high" again.

The output signal of the release circuit 5 is conveyed through a dropping resistor 21 to the non-inverting input of an operational amplifier working as a comparator 22 and to a capacitor 23. A constant reference voltage, which is produced, for example by a voltage division between a first and second reference voltage (15V, OV), is applied to the inverting input of the comparator 22. The second connection of the capacitor 23 is connected to the cathode of a charging diode 24, the anode of which is again in communication with the first reference voltage (15V). At the same time, the second capacitor connection 23 is connected, by means of a zener diode 25 and a damping resistor 26 connected in series therewith, to the second reference voltage OV. A switching key 28 in series with a feedback resistor 27 forms the feedback to the non inverting input of the comparator 22.

The mode of operation of this circuit is as follows:

In the normal state, as described above, the output signal of the operational amplifier 20 and the output of the comparator 22 are "high", the capacitor 23 is discharged. In the output of the operational amplifier changes to "low", the capacitor 23 is charged by a current through the charging diode 24, the capacitor 23 and the dropping resistor 21. In the course of this, the potential at the non-inverting input of the comparator 22 drops below the voltage at the inverting input and the comparator 22 switches over. If the output of the operational amplifier 20 changes back to "high", the dropping resistor 21 and the feedback resistor 27 form an ohmic voltage divider, the potential of which, at the centre tap, is lower than the reference voltage at the inverting input of the comparator 22; the switching state of the comparator 22 is retained. Since the cathode of-the Zener diode 25 is connected to the capacitor 23, the capacitor cannot discharge or can only do so as far as the threshold voltage of the Zener diode 25. As a result of actuation of the switching key 28, the feedback and hence the voltage division is interrupted, the capacitor has to discharge through the dropping resistor 21, Zener diode 25 and damping resistor 26, while the comparator 22 switches over.

If, in the release state, the f irst and second reference voltages (15 V, OV) f a i 1, which are preferably the supply voltages of the protective relay, 6 the capacitor 23 remains charged for hours. A very slow discharge can only take place because of the nonideal diodes and capacitors. When the reference voltages return, the release state is maintained.

Components having the same action are designated by the same reference numerals in Fig 3 as in Fig 2, additionally preceded by the digit 1 or 2.

The output of the operational amplifier 120 of the release circuit 5 for asymmetry and phase failure 8, Fig 1, is connected to the first reference voltage 15V by means of a resistor 50. In the normal state, this resistor 50 defines the "high" state; in the event of a release, the operational amplifier 120 connects the output to the second reference voltage OV; this defines the "low" state. The output signal of the operational amplifier 120 is conveyed through the resistor 121 to the non-inverting input of the comparator 122. The voltage-divider circuit 51, 52 produces the reference voltage at the inverting input.

Instead of the switching key 28, Fig 2, here a diode 53 is connected into the feedback path of the comparator 122, the cathode of which diode is connected to the feedback resistor 127 and, by means of an oppositely poled diode 54, to the switching key 128.

On a release, the operational amplifier 120 switches to "low", the capacitor 123 is charged and the comparator 122 likewise switches to "low". A light-emitting diode 55, with a series-connected current-limiting resistor 56, indicates a release as a result of asymmetry or phase failure. A diode 57 becomes conducting and pulls the non-inverting input of an output comparator 58 to folow". At the same time, this input distinguishes the 7 r, reference voltage produced by the voltage division 59, 60 at the inverting input and the output comparator also switches to "low". The output of the output comparator 58 is connected, on the one hand through a resistor 62, which defines the "high" state, to the first reference voltage 15V and on the other hand to the base of a switching transistor 61, the emitter of which is connected to the second reference voltage OV and the collector of which is in electrical communication with the first reference voltage 15V through a series-connected relay 63.

In the normal state, output of the output comparator 58 "high", the switching transistor 61 is conducting and the relay 63 is attracted; otherwise the relay 64 drops and the power interrupter 2, Fig 1, disconnects.

When the asymmetry or phase failure fault has disappeared, the operational amplifier 120 is switched back again to "high". The self-holding of the comparator 122 is now afforded by the voltage division, feedback resistor 127, conducting diode 53, dropping resistor 121 and resistor 50. In order to cancel the self-holding of the comparator 122, the switching key 128 must be actuated; this connects the positive reference voltage 15V to the feedback path through the oppositely poled diode 54 and cancels the voltage division given above. As shown in the explanation of Fig 2, the capacitor 123 discharges through the resistors 50, 121, 126 and the Zener diode 125 while the comparator 122 also switches over. On the other hand, if the asymmetry fault or phase failure is still present and the switching key 128 is nevertheless actuated, the comparator 122 cannot switch over; the release state is retained.

8 When the comparator 122 switches back to "high", the light-emitting diode 55 is extinguised, the diode 57 becomes blocking and the non-inverting input of the output comparator 58 is pulled to "high" by the seriesconnected resistors 64, 65.

The operation of the self-holding and release-state storage circuit for protection against overloading 7 is very similar.

1 ' The release circuit 5 for overloading 7 delivers a voltage proportional to the heating of the load 1, Fig to the inverting input of the operational amplifier 220. If the voltage exceeds a potential preset by the is voltage division by the resistors 66 and 67, the operational amplifier 220 switches to "low", the capacitor 223 is charged, the comparator 222 switches to "low", a light-emitting diode 68 indicates the overloading while a limiting resistor 69 limits the current through the light-emitting diode 68. Through the resistor 64, the release signal reaches the output comparator 58 which again leads to the disconnection of the power interrupter 2, Fig 1, as already outlined above. In this case, the diode 5'7 prevents a reaction on the circuit for protection against asymmetry and phase failure.

The self-holding is afforded by the voltage division:

feedback resistor 227, diode 70 and resistors 221, 71.

Here, too, the resetting of the comparator 222 is effected by cancelling the voltage division in that the cathode of the diode 70 is connected to the first reference voltage 15V through an oppositely poled diode 72 and the switching key 128.

A switch 73 renders possible a further possibility for 9 resetting the protective relay. With switch 73 closed, immediate connection in of the load 1 after sufficient cooling, preset by the release circuit 5 for overload 7, is again possible whereas in the event of phase failure or asymmetry, resetting can only be effected by actuation of the switching key 128.

Claims (7)

Claims

1. An electronic protective self-holding relay with storage in a capacitor of the release state produced by a release circuit, wherein the non-inverting input of a comparator is in electrical communication, through a first resistor, with a release circuit and by means of the capacitor with: (a) the cathode of a diode which is connected, at the anode side, to a first reference voltage, and (b) a zener diode which is connected at the anode side to a second reference voltage, and a feedback loop formed by at least one feedback resistor to the non-inverting input of the comparator can be disconnected.

2. A relay according to claim 1, wherein the connection between the first resistor and the release circuit is connected, through a further resistor to the first reference voltage.

3. A relay according to claims 1 or 2, wherein a damping resistor is connected in series with the zener diode.

4. A relay according to any one of claims 1 to 3, wherein a further diode is connected in series with the feedback resistor, the anode of this further diode being connected to the non-inverting input of the comparator and the connection between the further diode and the feedback resistor being able to be connected to the first reference voltage.

11

5. A relay according to claim 4, wherein the connection between the further diode and the feedback resistor, is electrically connected to the cathode of another charging diode, the anode of which can be connected to the first reference voltage.

6. An electric protective relay which is self-holding and has a capacitor which stores a value representative of a release state, the relay also comprising comparing means, one input of which connected to: (i) resistive means arranged to be connected to a release circuit, (ii) one terminal of said capacitor, and (iii) a feedback loop from the output of the comparator and comprising a feedback resistor, wherein the other terminal of said capacitor is connected via first and second rectifying means to respective reference voltages, and means are provided for disconnecting the feedback loop.

7. An electronic protective relay substantially as herein described with reference to Fig 2 or Fig 3 of the accompanying drawings.