DE1538286A1 - Short-circuit protection device - Google Patents

Description

49/66 _Tz / _schu

Aktiengesellschaft Brown ,, Boveri A Cie., Baden (Switzerland)

The invention relates to a short circuit protection device with im Current transformers saturated with overcurrent, with one AC half-wave via a consisting of two transistors and with the trigger connected to one pole of the current transformer, a pulse is formed which, depending on the polarity of the half-wave causes a trip or blocking and the current transformer is connected to a resistor in parallel with a current proportional Voltage is tapped. '*.

Such short-circuit protection devices, in which the currents converted into square-wave pulses via trigger circuits can be used for all types of protection that use the current as a measured variable. With overcurrent protection it is only important here that the impulse then the trigger actuated when a certain current level is exceeded. the

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Exceeding must last long enough within each half-wave so that actuation can take place. In this case, the actuation pulses can be shortened if the feeding converter or auxiliary converter connected to it are saturated. When such converters show signs of saturation, it is known that current peaks occur, but they quickly return to zero. The pulse width is only according to the then current peak exists "This may be too short to achieve a tripping" solution _β.

In the case of comparison protection systems, i.e. differential protection or busbar protection, in which at least two electrical variables are connected to one another are compared, it is important not only that the electrical size dee has a certain level, but also on the phase position between the two measured quantities. The impedance protection should also be seen as a comparison protection, insofar as it is it is fed, for example, with the help of rectifiers Bridge circuits exploits the overlap of the measured variables. A busbar protection that works on this principle is described in German Auslegeschrift 1 208 397, for example.

If now the sinusoidal shape of the current is converted into a rectangular shape wild, and the overlap of the Bechtedcimpulee is compared, actuation should take place when the overlap is small or is zero. A blocking or other actuation is to take place.

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if the overlap is large or complete. The width

the square pulse now corresponds to the positive or negative Half-wave of the primary current. It must therefore meet the condition ensure that the primary current is transferred true to shape to the secondary side will. This is the case as long as the current transformer cores used remain unsaturated.

Protective devices, however, should be used in the event of high overloads work properly. For this purpose, this condition is no longer always

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given the case. Especially with transient processes with high ^ The converter can easily reach the saturation range during equalization processes. But then the electric big ones are on the The secondary side of the converter no longer has the sinusoidal shape of the primary side identical. The current is distorted, there are peaks, which have high amplitudes, but narrower than the original SinuEform are. As a result, the zero crossings correspond on the primary side no longer the zero crossings on the secondary side. This makes the overlap of the electrical quantities also influenced. As the " Figure 1 shows, fish polarities can even occur here, which can cause false triggering of the protection. At the top Part of FIG. 1 is the primary current I "sinusoidal. He would Generate a square pulse, which is shown in the lower part as a dashed line.

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and denoted by i ^ The secondary e-romance I is now

distorted by the saturation of the magnetic core. You can see that the square pulse i is then considerably shorter, and even during BADORfGfNAL

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the positive half-wave of the primary current becomes negative. It 1st It is clear that the overlap tent is significantly influenced by this and can give rise to incorrect work of the protection.

If you now set the condition that the protection should trigger, if the overlap of the two measured quantities, for example lies between 60 and 180 °, the overlap would be limited to the range from 150 to 180 due to the distortion. In the range of 60 to 150 °, where the trigger should also be triggered, if the overlap time is too short to "enable tripping", however, false tripping can also occur conversely occur when a lock command is to be given. The beginning . of the pulse i is, as the lower part of FIG. 1 shows, in the direction to the left extended. It doesn't just start with Zero crossing of the sine curve of the primary current, but already before when the secondary stream passes through the gauze. This advancing the pulse extends the range for triggering, though this area should be blocked. This can therefore give rise to false triggers.

The invention is now intended to solve the problem of avoiding this disadvantage and, even if the current transformers are saturated, a flawless one To effect work of such comparison protection systems.

According to the invention it is therefore proposed that between the base

ο of the first transistor of the trigger circuit and its emitter with

^ the series connection of a capacitor and its emitter resistance

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cp of a resistor lies between and between the connection point

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^ this capacitor and this resistor and not to the si «-

σ associated trigger connected pole of the current transformer ε a further resistor is connected and that between the connection *

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point of the resistor capacitor circuit to the emitter resistor and the current transformer resistance a diode in the same tripping direction like that of the transistor.

The proposed measure does two things: One becomes a generated square pulse through the establishment of long sustained, thus the range eweiterto is also achieved that when the polarity of the primary current changes, the square-wave pulse must stop ο

Figure 2 shows an embodiment of the invention. The current in line 1 is _{labeled I p} , it is the primary current in current transformer 2. This current is fed to the downstream protective device. It is then compared to another stream. This second device is not shown because the pulses generated from the current are obtained in the same way and the overlapping devices are known. The invention relates only to the shaping of the pulses themselves. The current I _g flows in the secondary winding of the current transformer 2. The Zener diodes 3 are used to reduce the overvoltage in a known manner. A voltage proportional to the current is obtained from the current separately for each half-wave »This is done with the diodes 4 and 5, as well as the resistors 6 and 7 Diode 5 · In this case it is assumed that there is a positive voltage across resistor 6. In the other half-wave the current flows through the resistor 7 and the diode 4. The so

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A ntstanoene positive voltage is now fed to the transistor 8, which belongs to a trigger circuit. It is switched ao so that whenever a positive voltage occurs at the base of transistor 8, a positive signal in techteckform i arises at the output of transistor 9. The other resistors 10 to 1'4 αηα the diode 15 are components of the trigger circuit known per se, which does not need to be explained in more detail here. The positive voltage P is applied to the circuit via the resistors. The trigger circuit responds when the half-wave of the current I _{p is} positive which corresponds to the rectangular shape Ι ″ shown in FIG. 1. If, however, the secondary current I ″ is distorted due to the saturation of the magnetic core of the transducer 2, the Bechteck pulse is also incorrectly reproduced as a result. The pulse length is shortened To prevent this, the resistor-diode combination is provided, which consists of the resistors 16 and 17, the capacitor 18 and the diode 19.

This combination, which represents the actual feature of the invention, now acts as follows: The capacitor 18 stores energy during the saturation peak of the current I ₀ . When the current goes to zero, it emits this energy again via transistor 8, so that it continues to receive positive voltage and keeps the trigger open. This acts as a dropout delay of the trigger. So that the capacitor does not over-calibrate

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can discharge the resistor 6, the diode 19 is provided.

But if the negative saturation peak arises, it must Transistor 8 will be blocked immediately. This is achieved in that the voltage occurring at the resistor 7 after the zero crossing through the resistor 16, which must be high-resistance to a Prevent coupling between the positive and negative circle, is given to the capacitor 18. Then the same potentials are on both sides of the capacitor 18 and the discharge is prevented

In the same way, he becomes through the negative half-wave of the Pritnärstronies resulting pulse i is extended by appropriate circuits. This is shown in the lower part of FIG. The single ones Switching elements are similar to what is shown in the left part the same and therefore unspecified .. It should also be mentioned that P and N the plus and minus poles of the required for the transistors Mean auxiliary voltage.

These measures ensure that the pulse duration despite Saturation of the iron core of converter 2 or of an auxiliary converter connected to it can be extended to a value. which does not cause false or false tripping of the protection. The extension does not necessarily need to extend over the entire half-wave to extend. It only has to be the part of the period during which no signal should be so small that no false triggering of the trigger can occur. You need a certain period of time to trigger it of the impulse «If this is not achieved, there is none

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False or nuisance tripping. So it is enough, for example, the impulse i in FIG. 1, which is about 80 ° wide, to about 150 2u extend. Then the lack of the pulse or an incorrectly occurring negative pulse would only be 30 available to have. This is then no longer sufficient for triggering.

This device is used for overcurrent protection, Differential protection systems and the collective protection, as well as k also the distance protection in question.

The technical advantage is that there are no saturation phenomena, which can occur in particular during transient processes Have an influence on the effectiveness of the protection.

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Claims (2)

- 9 - ..osiiJHcBrf ^ rSj- * 9/66 '? a t_e_n_ t_a_n_s ρ r ü c h e 1) Short-circuit protection device with saturated in the overcurrent area Current transformers, with an alternating current half-wave via one consisting of two transistors and with one pole of the current transformer connected trigger a pulse is formed which, depending on the polarity of the half-wave, triggers or blocks, and the current transformer has a resistor in parallel, at which a current-proportional voltage is tapped, thereby marked that between dfe base of erasn The transistor and its emitter with its emitter resistor is the series connection of a capacitor and a resistor and between the connection point between this capacitor and this resistor and the one not connected to the associated trigger Pole of the current transformer is connected to another resistor, and that between the connection point of the resistor capacitor circuit to the emitter resistor and the current transformer resistor a diode in the same forward direction as that of the Transistor lies. 2) short circuit protection device according to claim 1, characterized in that that the device is connected separately to both poles of the current transformer for each half-wave and that the resistor generating the proportional voltage is double and the center point is connected to the diodes. 009808/0570 1538280 - 10 - 49/66 Short-circuit protection device according to claim 1, characterized in that the resistors and the capacitor are dimensioned such that the pulses are lengthened by approximately 90%. Public limited company BROWN, BOVERI & CIE. 009808/0570 "'■■ *"? ---, ■■■ Empty page