DE3344208A1 - Method for detecting an earth or short-circuit current - Google Patents

Abstract

To detect the actual value of an earth or short-circuit current in earthed alternating-current systems, a measuring method is proposed in which the value of the current is measured and stored in successive periods. The stored value is replaced by the next-higher measured value when the measured values exceed the stored value in two successive measuring periods. The measurement is not taken synchronously with the zero crossings of the current. The second measuring period can be delayed by the length of at least one period.

Description

Procedure for the detection of a ground or short-circuit current

The invention relates to a method for detecting an earth or short-circuit current in an earthed or semi-rigidly earthed AC network.

In earthed and semi-rigidly earthed AC systems, in particular Medium voltage networks, is the current flowing in different earth faults from Interest. Since the earth fault current often contains harmonics, it is important to determine of the current to pay attention to the correct evaluation of the harmonics.

In addition, fluctuations in the event of a change in errors as well as signs of saturation the current transformer timed the zero crossings of the current, so that during an evaluation Incorrect measurements occur within the current zero crossings.

The object of the invention is to provide a method for detecting the maximum Specify the amount of a ground or short-circuit current that allows the influences of transient processes when a fault occurs or when a fault is switched on (rush) to eliminate during the detection of the fault current and the occurring Correctly evaluate harmonics.

The task is given in the characterizing part of claim 1 Features solved. A suitable evaluation variable for the earth or short-circuit current is the arithmetic mean, the peak value or the effective value. Through the Harmonics frequently present in earth or short-circuit currents would be caused by a Peak value measurement taking into account the peak factor of sinusoidal currents the measured value is falsified by the harmonics.

The peak value measurement is therefore preferred for currents with low harmonics suitable. In the case of an arithmetic mean value measurement, the Harmonic components, however, in the case of earth or short-circuit currents containing strong harmonics is fraught with a major flaw.

When measuring the rms value, all harmonic components are correctly rated. In the following, the inventive method is based on the in the time diagram shown in Figure 1, in which the effective value measurement is used as the basis for evaluation.

FIG. 1 shows the amplitude profile of various types of earth fault currents shown over time t. These are periodic alternating currents with a Period of 20 milliseconds. The effective value of a ground fault current is determined during one period of the alternating current. The measurement cycle n, n + 1, ... to determine the effective value is repeated periodically and lasts 20 Milliseconds. It is noteworthy that each measuring cycle is not synchronous with the Zero crossing of the current amplitude to be measured must begin and within one period can be anywhere.

In the method for recording the maximum effective value of a Earth fault current is continuously a new one Reading 11 with the largest previously measured and displayed measured value I compared. If the new measured value I1 greater than the displayed value I, and also the measured value I2 of the next measuring cycle greater than the value I, the displayed value I is deleted and the value I2 is saved and brought to the display. Otherwise the memory and display status remains I obtain.

To switch off the evaluation of transient processes, according to an advantageous development of the method provided, each measuring cycle to delay one or more periods of the fault current to be measured.

The introduction of a delay time tv defines a waiting time within which the alternating current network to be monitored after a transient process can adjust to a steady state again.

The Pigur 1 illustrates the effect of a measurement method in which a delay time tv of three periods is introduced. Assuming that a current period is again 20 milliseconds, there is a delay time tv of 60 milliseconds.

A preferred circuit arrangement for implementing the described The measuring method is shown as a block diagram in FIG.

The fault current to be examined is fed into the primary winding depending on the current strength one of four current transformers 5 fed in. The current transformers are on the secondary side 5 connected via a connection to a common reference point (measurement ground), while the other connection can be connected to a measuring circuit 6 via a closing contact is. This measuring circuit 6 contains an evaluator 1, a memory 2, and a display 3 and is fed by an auxiliary voltage source UH. The evaluator 1 determines telt each over one period of the supplied fault current in a measuring cycle, the does not have to begin synchronously with the current zero crossing, the effective value of the current and compares this new value with the largest last measured value. Results the new measured value reaches a higher value than the previously stored value the next measured value in memory 2, if this is greater than the one previously stored. The last measured value stored in memory 2 is taken over by display 3. With The measuring circuit can be reset to zero with the aid of a button 4.

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Claims (5)

Claims 1. A method for detecting a ground or short-circuit current (Fault current) in an earthed or semi-rigidly earthed alternating current network by the following features: a) the fault current is over the period of time a Period (= one measuring cycle) measured; b) that measured during a first measuring cycle The value is compared with the largest value recorded and displayed last; c) results the comparison that the measured value is greater than the displayed value follows a second measurement cycle with a new comparison; d) is the one during the second Measurement cycle is again greater than the displayed value, the new value saved and displayed. 2. The method according to claim 1, characterized in that the effective value the earth or short-circuit current is measured 3. The method according to claim 1, characterized characterized in that the second measuring cycle is delayed by at least one period is. 4. The method according to claim 1, characterized in that a measuring cycle does not have to begin synchronously with the zero crossing of the fault current. 5. A circuit for performing the method according to claim 1, characterized by an evaluator (1) for measuring the fault current, a memory (2) for Storage of the maximum value, a display (3) to signal the current one Maximum value and a button (4) for manual resetting of the measuring circuit (6).