GB2049323A - Short-circuit protection in low- voltage systems - Google Patents

Abstract

In short-circuit protection of induction motors utilizing phase-sensitive rating of short-circuit currents, in certain conditions currents are rated in an ideally rectified mode.

Description

SPECIFICATION Short-circuit protection in low-voltage systems The invention relates to a short-circuit protection in low-voltage systems in which large induction motors are operated, with phase-sensitive rating of the short-circuit currents.

In such low-voltage systems, medium and large two and three-pole short circuits must be detected within a very short time. To this effect, short-circuit currents which are smaller than the starting current of large induction motors are detected in a few half-waves. In this manner, it is possible, by using appropriate high-speed circuit breakers, to prevent arcing out of wires and cables in the case of a line short circuit. Moreover, in premises with explosion hazards, especially in underground coal mines, safety can be improved by the short-circuit protection according to the invention.

It is known that in modern circuit breakers triggering against overloads is effected thermally, whilst triggering against medium and large short-circuit currents is effected by electromagnetic means. In special cases, for example in the case of 15-limiters, the trigger criterion for detecting medium and high short-circuit currents is constituted by the rate of rise of the current (ETZ-A, Vol. 81 (1960), pages 741 -744).

It is further known that weak short-circuit currents and short-circuits can be detected in their formative phase with a short-circuit protection having phasesensitive rating means (Mining Technology, April 1978, pages 133-136). However, in the case of two-pole short-circuits the phase position of the current in the short-circuit bearing phases shifts, relative to the three-pole short-circuit current by amounts up to + 30 or - 30 , respectively. This partly incapacitates the phase-sensitive rating. In addition, bouncing protective contacts, which generate a two-pole starting current when large induction motors are switched on, lead to a phase shift of the starting current by about + 30 in one phase and 30 in the other.For this reason, a phase-sensitive short-circuit detector can lead to a spurious triggering of the circuit breaker.

It is an object of the invention to provide improved short-circuit protection in the case of weak shortcircuit currents and in the case of short-circuits during the formative phase of the latter.

According to the invention, there is provided short-circuit protection in low-voltage systems in which large induction motors are operated, with phase-sensitive rating of the short-circuit currents, wherein for the short-circuit detection there is executed a more than proportional phase-sensitive rating with a disproportionality factor of 1 to 10 excess in which leading currents preceding the voltage, are rated not in phase-sensitive but in ideally rectified mode.

Preferably, the disproportionality factor is 3 to 6.

By means of a more than proportional short-circuit protection, two-pole short-circuit currents are rated in a surprisingly advantageous manner, and accidental tripping due to bouncing contacts is prevented. The more than proportional, phasesensitive short-circuit protection is designated for systems in which the three-pole short-circuit current does not exceed a maximum phase angle. By contrast, three-pole starting currents must bring about a phase shift, which is greater than the phase shift caused by the short-circuit currents.

In the more than proportional, phase-sensitive short-circuit protection according to the invention the currents and voltages of the three phases are fed via transformers to the short-circuit detection unit. In the case of equal polarity of current and voltage, the current is given a positive rating. By contrast, if current and voltage are of different polarity, the current is given negative rating and is multiplied by an excess factor of 1 to 10, preferably 3 to 6. In contrast to the known phase-sensitive rating, the mean value of this more than proportional phasesensitively rated current is reduced substantially on motor start-up and only slightly in case of short ci rcu it.

This function is smoothed in a following delay circuit and fed to a maximum-value module. The output signal of the maximum-value module is then evaluated in a comparator with respect to an adjustable limit value.

In case of a two-pole short-circuit, the current leads the voltage by preceding it with a phase.

However, since the leading current is always ideally rectified, it is thereby displaced on a larger scale than in the known phase-sensitive rating.

If, on the other hand, the current follows the voltage with a phase shift greater than a specific preset angle, then the current in the two other phases is not taken into consideration. Therefore, the two-pole protective contact cannot have a negative effect on the more than proportional phasesensitive short-circuit protection when the motor is started up (bouncing contacts).

The special advantage of the plus-proportional phase-sensitive rating of the current according to the invention consists in that weak short-circuit currents are more clearly differentiated from the starting currents of large induction motors than hitherto possible with the known phase-sensitive shortcircuit protection.

An example of short-circuit protection according to the invention is illustrated in the drawings, in electronic hybrid techology mode by way of example, and described in more detail in the following. In the drawings: Figure 'shows the mode of functioning a shortcircuit indicator module in a block circuit diagram, Figure 2 illustrates the function of block 7 of Figure 1, and Figure 3 illustrates the plus-proportional phasesensitive rating of a phase in blocks 1 to 3 of Figure 2.

In Figure 1, the input signals are differentiated in the blocks 1 to 3. In blocks 4 and 5, the sum of the input signals and subsequently the maximum value of the three sums is formed. The maximum values are then evaluated in comparators in Block 6 relative to set values and are digitally processed to produce an output signal, which directly acts on a tripping device of an appropriate circuit breaker. The plus proportional phase-sensitive rating is carried out in block 7.

Referring to Figure 2, the plus-proportional phasesensitive rating of the individual phases is effected in blocks 1 to 3. From their three output signals the maximum value is formed in block 4, to be evaluated in block 5 relative to a reference value in a comparator circuit.

Referring to Figure 3, the voltage U is fed through a first-order delay circuit 1, which causes a phase shift of 65" at 50 Hz, respectively to the zero-level switch 2 or to the zero-level switch 4 directly. The current I is digitized in the zero-level switch 6. Atthe output of phase discriminator 3, in case of a current lead relative to the voltage U phase-shifted by 65 , there appears a logical 1, so that in the other phases the analog switches 8 and 9 remain in the position shown.

If the current I leads the voltage U, then at the output of the phase discriminator 5 there appears the digitized current signal and, in the case of following current I, the digitized voltage signal.

The analog switch 8 passes the current signal in the case of logical 1 and, in the case of logical 0, the signal inverted in amplifier7. The phase-sensitive rated analog current signal is multiplied by 1 in amplifier 10 when the values are positive, and, when the values are negative, by 4.7 in the present case, for example. This plus-proportional, phase-sensitive rate signal is finally smoothed in the first-order delay circuit 11.

Claims (3)

1. Short-circuit protection in low-voltage systems in which large induction motors are operated, with phase-sensitive rating of the short-circuit currents, wherein for the short-circuit detection there is executed a more than proportional phase-sensitive rating with a disproportionalityfactorofl to 10 excess in which leading currents preceding the voltage, are rated not in phase-sensitive but in ideally rectified mode.

2. Short-circuit protection as claimed in claim 1, wherein the disproportionality factor is 3 to 6.

3. Short-circuit protection substantially as hereinbefore described with reference to and as shown in the accompanying drawings.