DE2917398A1 - SHORT CIRCUIT PROTECTION IN LOW VOLTAGE NETWORKS - Google Patents

Description

BERGWERKSVERBAND GMBH

EXPERIMENTAL COMPANIES OF MINING RESEARCH

43 Essen-Kray, April 24, 1979

Frillendorfer Strasse 351 Telephone (0201) 105-1

A 8 / Str-Be

Short-circuit protection in low-voltage networks

The invention relates to short-circuit protection in low-voltage networks in which large asynchronous motors are operated with phase-sensitive Evaluation of the course closing currents.

In such low-voltage networks, medium and large 2- and 3-pole short circuits should be detected in a very short time. Here Short-circuit currents that are smaller than the starting current are greater Asynchronous motors are / recognized in a few half waves. In this way it is possible to use suitable fast switching Circuit breaker to prevent the arc from escaping from lines and cables in the event of a line short circuit. aside from that can be used in plants at risk of explosion, especially in underground coal mining, by means of the short-circuit protection according to the invention security can be increased.

It is known that modern circuit breakers trigger a trip takes place thermally against overcurrents and electromagnetically against medium and large short-circuit currents. In special cases, for example in the case of I-limits, is used as a trigger criterion for the detection

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Solution of medium and high short-circuit currents uses the current gradient (ETZ-A, vol. 81 (1960), pages 741-744).

It is also known that small short-circuit currents and short-circuits can be detected in their development phase with a short-circuit protection with phase-sensitive evaluation (mining Technology, April 1978, pages 133-136). With 2-pole short circuits However, the phase position of the current shifts in the short-circuit-carrying phases compared to the 3-pole short-circuit current up to + 30 or - 30 °. This partially eliminates the phase-sensitive evaluation. Also lead Bouncing contactor contacts when switching on large asynchronous motors cause a 2-pole starting current to a phase shift of the starting current by about + 30 in one phase and - 30 in the other phase. Because of this, it can be a phase sensitive Short-circuit evaluation lead to a false tripping of the circuit breaker.

The invention is based on the object with small short-circuit currents as well as to offer better short-circuit protection in the case of short-circuits in their development phase.

According to the invention, this object is achieved in that a disproportionately phase-sensitive sensor is used for short-circuit detection Evaluation is carried out with a disproportionality factor of greater than 1 to 10, preferably from 3 to 6, in which currents, which lead the voltage, are not rated as phase-sensitive, but ideally rectified.

Through a disproportionate phase-sensitive short-circuit protection Surprisingly, 2-pole short-circuit currents become special valued cheaply and false triggers due to bouncing

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Contacts prevented. The disproportionately phase sensitive Short-circuit protection is designed for networks in which the 3-pole short-circuit current does not exceed a maximum phase angle. On the other hand, 3-pole starting currents must cause a phase shift that is greater than that caused by short-circuit currents maximum phase shift.

With disproportionate phase-sensitive short-circuit protection according to According to the invention, the currents and voltages of the three phases are fed to the short-circuit detection element via converters. At a If the polarity of current and voltage is the same, the current is rated positively. With different polarity of current and Voltage, on the other hand, the current is assessed negatively and multiplied by a factor greater than 1 to 10, preferably 3 to 6. In contrast to the known phase-sensitive evaluation, the mean value of this is evaluated disproportionately phase-sensitive The current is considerably reduced when the motor starts and slightly reduced in the event of a short circuit.

This function is smoothed in a subsequent delay and fed to a maximum value module. The output signal of the maximum value module is then given an adjustable limit value evaluated in a comparator.

In the case of a 2-pole short circuit, the current of the voltage rushes in one Phase ahead. However, since leading current is always ideally rectified, it is displayed larger than with the known one phase sensitive evaluation.

If, on the other hand, the current of the voltage rushes by more than one phase in one phase after a certain predetermined angle, the current in the other two phases is not taken into account. The 2-pole contactor

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contact can therefore occur when a motor is switched on (bouncing Contacts) do not have a negative effect on the disproportionately phase-sensitive short-circuit protection.

The particular advantage of the disproportionate phase-sensitive evaluation of the current according to the invention is that Distinguish small short-circuit currents more clearly from the starting currents of large asynchronous machines than the previously known phase-sensitive ones Short circuit protection enabled.

An embodiment of the invention is electronic in the drawing in hybrid technology, for example, and is described in more detail below. Show it

Fig. 1 The mode of operation of a short-circuit reporting element in a block diagram,

Fig. 2 shows the function of block 7 in Fig. 1 and

3 shows the disproportionate phase-sensitive evaluation of a phase in blocks 1 to 3 of FIG. 2.

In Fig. 1, the input signals are differentiated in blocks 1 to 3. In blocks 4 and 5, the amount of the input signals and then the maximum value of the three amounts are formed. The maximum values are then evaluated in comparators with setting values in block 6 and digitally processed to an output signal that acts directly on the tripping device of a suitable circuit breaker. The disproportionate phase-sensitive evaluation is carried out in block 7.

According to FIG. 2, the disproportionate phase-sensitive evaluation of the individual phases in blocks 1 to 3.

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The maximum value in block 4 is formed from its three output signals, and that in block 5 in a comparator circuit with a Comparison value is evaluated.

According to FIG. 3, the voltage U becomes over a delay of the 1st order 1, which causes a phase shift of 65 at 50 Hz, to the zero point switch 2 or directly to the zero point switch 4 given. The current I is digitized in the zero point switch 6. At the output of the phase discriminator 3 is at Current lead compared to the voltage phase-shifted by 65 ° U a logical 1, so that the analog switches 8 and 9 remain in the position shown in the other phases.

If the current I leads the voltage U, then appears at the output of the phase discriminator 5, the digitized current signal and, if the current I is lagging, the digitized voltage signal.

The analog switch 8 switches the current signal through with logic 1 and with logic 0 the signal inverted in amplifier 7. That The analog current signal evaluated in a phase-sensitive manner is indicated by 1 in the amplifier 10 for positive values and with 1 for negative values for example in the present case 4.7 multiplied. This disproportionately phase-sensitive signal is used in the 1st order delay 11 then finally smoothed.

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Claims (1)

BERGWERKSVERBAND GMBH EXPERIMENTAL COMPANIES OF MINING RESEARCH 43 Essen-Kray, April 24, 1979 Frillendorfer Straße 351 Telephone (0201) 105-1 A 8 / Str-Be Short-circuit protection in low voltage networks Claim Short-circuit protection in Nxeder voltage networks in which large asynchronous motors are operated, with phase-sensitive evaluation of the short-circuit currents, characterized in that a disproportionate phase-sensitive evaluation with a disproportionate factor of greater than 1 to 10, preferably 3 to 6, is carried out for the short-circuit detection, for the currents that lead the voltage, are not rated as phase-sensitive but ideally rectified. 0300AA / 0512