HK1097961A - Method and circuit arrangement for the detection of ground faults on electronic trips for low-voltage circuit breakers comprising serially connected measuring amplifiers - Google Patents

Description

Method and circuit arrangement for detecting the earthing of an electrical release of a low-voltage power switch with a pre-measurement amplifier

Technical Field

The invention relates to a method and a circuit arrangement for detecting the grounding of an electrical discharge of a low-voltage power amplifier by means of a pre-measurement amplifier.

Background

The low-voltage power switch has other functions than its main functions, namely short-circuit monitoring and overcurrent monitoring. The most important function is ground monitoring. Grounding may produce intractable results in electrical devices by, for example, triggering sparks. The spark is thus eliminated or at least signaled as being generated with a delay by opening the power switch with a corresponding ground detection function. For this purpose, for a two-pole or bipolar power switch, the phase current and the vector sum of the currents, if an N-conductor converter is present, are formed in the N-conductor. If the residual current exceeds the actuation value, a display is made and the power switch is released if necessary. Alternatively, the grounding event can also be detected by only one additional current transformer in the N conductor or at the grounded star point.

The ground detection function can be implemented, for example, by special modules which can be plugged into the power switch, as is shown in DE69425916T 2.

The current, which is usually measured by a current transformer, is applied to the tripping unit of the low-voltage power switch via a measuring amplifier. The problem here is that the measuring amplifier has a large tolerance with respect to its amplification factor. The releaser may detect the grounding when the sum required for the grounding acquisition is made, although the grid is not grounded. The reverse is also conceivable, that is to say that grounding occurs, but the sum of the currents is close to 0.

To eliminate this problem, corrections are currently made in software in the processor of the releaser. The current signals are then summed in software. However, a large error occurs in that the small ground current that results in the discharge corresponds to only approximately two switching stages of the a/D converter of the electrical discharge unit, so that the phase current for detecting the ground cannot be formed with sufficient accuracy.

Disclosure of Invention

The object of the invention is to provide an electrical release for a low-voltage circuit breaker, which allows error-free detection of an earthing event.

According to the invention, this object is achieved by the features of claims 1 and 2. Suitable embodiments are given in the dependent claims.

According to the invention, the output signal of the measuring amplifier is applied to a summing amplifier which is pulse-width-modulated. 3 or 4 input signals are connected to the input of the summing amplifier via 3 or 4 analog switches. The correction is made by selecting the corresponding pulse duty factor of the analog switch, which amplifies the input of the grounding measurement to the same desired multiple.

The circuit of the invention has the advantage that the equalization in the form of hardware is dispensed with. The inaccuracy of the addition in the form of software is also eliminated. The amplification of the measuring amplifier can easily be determined in that the same input signal is applied to each current transformer in one test and the measurement result is then stored in the microprocessor of the releaser. These measurements can then be directly used in determining the pulse duty factor.

Drawings

The present invention is described in detail below by way of example with reference to the accompanying drawings. Wherein:

figure 1 shows an example of the circuit of the invention,

fig. 2 shows a second variant of the inventive circuit.

Detailed Description

Fig. 1 schematically shows an electrical discharge on a three-phase electrical network. The currents flowing through the current transformers SW1, SW2, SW3 are measured on the 3 conductors L1, L2 and L3 of the network and are led to the measuring amplifiers V1, V2, V3, the outputs of which are led to the a/D converters a/D1, a/D2, a/D3 of the microprocessor μ P, which is an important functional component of the releaser.

According to the invention, the output signals of the measuring amplifiers V1, V2, V3 are fed to switches S1, S2, S3 which are controlled by the microprocessor μ P in a pulse-width-modulated manner. The switches S1, S2, S3, which are only schematically illustrated here, can be realized, for example, by switching transistors. If one of the switches S1, S2, S3 is current-conducting, the other switches are open, wherein the pulse duty cycle corresponds to the amplification of the respective associated measuring amplifier V1, V2, V3. The outputs of the switches S1, S2, S3 are directed to a point where the input of the summing amplifier V4 is represented by the pre-resistor R. The summing amplifier V4 is implemented by an integrating circuit, that is to say by an operational amplifier OPV1, the output of which is fed back to the input thereof via a capacitor C1. Since the entire measurement moves in the direct current range, the second input of the operational amplifier OPV1 is biased with a reference voltage Uref, which corresponds to the average value of the voltage runs of the measurement amplifiers V1, V2, V3. The output signal of the operational amplifier OPV1 is then introduced into the a/D converter a/D4 of the microprocessor mup, and is the signal for ground monitoring,

the circuit of fig. 2 differs from the previously described variants only in that the output signals of the measuring amplifiers V1, V2, V3 are introduced via resistors R1, R2, R3 into switches S1, S2, S3, while the outputs of the switches S1, S2, S3 are applied directly to a capacitor C2, which capacitor C2 reflects the sum of the phase currents of conductors L1, L2, L3. This signal is input to the A/D converter A/D4 of the microprocessor μ P via an operational amplifier OPV 2.

Claims (4)

1. A method for detecting the grounding of an electrical discharge for a low-voltage circuit breaker with a pre-measuring amplifier, characterized in that the output signals of the measuring amplifiers are summed in a summing amplifier in phase-wise, pulse-modulated manner, wherein the duration of the pulses is controlled by the discharge as a function of the amplification of the respective measuring amplifier, and the output of the summing amplifier is the signal for monitoring the grounding.

2. A circuit arrangement for detecting ground at an electrical release for low-voltage circuit breakers, having a preceding measuring amplifier (V1, V2, V3), characterized by switches (S1, S2, S3) which are controllable by the release in a pulse-width-modulation manner as a function of the amplification of the measuring amplifier (V1, V2, V3), into which the outputs of the measuring amplifier (V1, V2, V3) are respectively introduced, wherein the outputs of the switches (S1, S2, S3) are together directed to the input of a summing amplifier whose output is a signal for ground monitoring.

3. A circuit arrangement as claimed in claim 2, characterized in that the summing amplifier is an integrating amplifier, consisting of an operational amplifier (OPV1) with capacitive feedback, the second input of which is biased with a reference voltage (Uref) which corresponds to the average of the voltage runs of the measuring amplifiers (V1, V2, V3).

4. A circuit arrangement as claimed in claim 2, characterized in that the summing amplifier is realized by means of a capacitor (C2) and a subsequent amplifier (OPV 2).