DE1075724B - Long differential protection with saturated converters, especially for lines - Google Patents

Description

GERMAN

The invention relates to a longitudinal differential protection with voltage comparison, which works with transformers that are saturated even at low overcurrents, to avoid overstressing the pilot wires required for the comparison. She wants the job solve, to render harmless the current peaks caused by the saturated converter.

A line differential protection compares the currents at both ends of an object to be protected, for example a line. The currents are not immediate, especially in the case of longer lines compared, but one creates tensions out of them. This is done by the current transformer mostly feed impedances via intermediate transformers, the voltage drops of which are compared by relays will.

Ohmic resistances have hitherto been used as such impedances. The one so formed at each end Voltage is compared by relays which are in the connecting lines between the two Ends are switched. If the two voltages are the same, the relays remain idle; if they are different, that's how they address.

These connecting lines must not be overstrained. But there especially in the event of malfunctions If the currents in the network rise very sharply, there is a risk that the resulting high voltages the lines are overstressed and destroyed precisely when they are for the How the protection works are most necessary. One has therefore made the converter so that they already are saturated with low overloads of the highest possible operating current, thus the secondary current grows only slowly even if the primary current continues to increase. Mostly one uses for this Purpose intermediate transformers, which are connected to the main transformers. These transducers produce but now with high primary currents due to saturation high current peaks, which are also caused by the Go through comparison impedance. The consequence of this is that with continuous fault currents, that is, with External short circuits, in which the protection should not work, due to the saturation never being exactly the same voltage differences can occur in the converter, which cause the protection to respond. Complete security against overloading of the auxiliary lines is also achieved when using saturation converters not achieved, since the voltage with the current level is still slow even with high currents grows.

It has already been proposed to keep these current peaks away from the relay, the relay winding Connect capacitors in parallel so that the peaks are essentially absorbed by the capacitors

with saturated transducers,

especially for cables

Applicant:

Public limited company Brown, Boveri & Cie., Baden (Switzerland)

Representative: Dr.-Ing. E. Sommerfeld, patent attorney,
Munich 6, Dunantstr. 23

Claimed priority:
Switzerland from April 21, 1959

Dipl.-Ing. Hendrik Johann Mezger,

Rotterdam (Netherlands),
has been named as the inventor

men and only the operating-frequency currents go through the relay. This arrangement has the Disadvantage that the tips are only kept away from the relay, not from the line. Also are at these arrangements have resistors for obtaining the measurement voltage and capacitors for the derivation the tips required separately.

The aim of the invention is to remedy the disadvantages mentioned and to save switching means. According to the invention, it is proposed to compare the impedances that arise at the comparison impedances To accomplish voltages in that capacitors are used as impedances.

Fig. 1 explains the embodiment. 1 is the line to be protected, which is shown as single-pole is. Current transformers 2 and 12 are installed at both ends. They dine over even at low levels Do saturated transducers 4 and 14 and resistors 5 and 15 overcurrent to differential relays 6 and 16 with capacitors 7 and 17. Each relay has two opposing systems, one of which one, 9 and 19, seeks to press a contact not shown, the other, 8 and 18, as a so-called Holding system counteracts the response system and increases the response current.

The relays are thus switched as percent relays in a known manner. The two sides of the protection are with the exception of main converters 2 and 12 symmetrical

909 730/372

trically switched. The secondary lines of the main converter 12 are crossed, so with the same current direction on the primary side opposite voltages arise at the impedances 7 and 17, so that the relays 6 and 16 can not respond.

The mode of operation of the arrangement is now as follows; Let us first consider the left side. The current flowing in the converter 2 is fed to the intermediate converter 4; Its secondary current flows through the resistor 5 and the holding system 8 of the relay 6 into the capacitor 7. This creates a voltage U _{7 on the capacitor 7, and the voltage M 17} on the right side of the line in the same way on the capacitor 17. When current flows through the line, for example from left to right, the voltages M ₇ and M _{17 are} equal, so that no current can flow through the relay coils 9 and 19 connected to one another on the auxiliary line 10. The relays remain idle. If there is a short circuit on line 1, current flows into the line from both sides or, in the case of stub lines, only from one side. The current in the converter 12 thus reverses its direction and generates a _{voltage M 17} which is shifted by 180 ° _{with respect to M 7} , or there is no current at all, so that a comparison voltage is only available on the feeding side. This creates a voltage between the relay coils 9 and 19, and the relays trip. The converters 4 and 14 are already saturated with relatively small overcurrents, so the secondary current and the secondary voltage arising at the terminals of the converter do not increase in proportion to the ratio with the primary current, but more slowly.

In FIG. 2, curve A shows the increase in the secondary voltage M ₄ at the intermediate converter as a function of the primary current i . As a result of the saturation, this voltage now has high peaks which must be kept away from the auxiliary lines. Therefore, the secondary current of the converters 4 and 14 is conducted via the capacitors 7 and 17. Since the capacitors have a frequency-dependent impedance, the higher frequencies generate a lower voltage on the capacitor than the fundamental wave, which means that the voltage on the capacitor becomes more sinusoidal. As the short-circuit current i increases, the rise in voltage M ₇ becomes even less than secondary _{voltage M 4} . Furthermore, the holding effect of the systems 8 and 18 is thereby increased, since the current flowing through them still contains the peaks. The effect of the capacitor is therefore a twofold stabilization. It removes the tips from the response system and the auxiliary lines, but increases the effect of the fundamental wave on the holding system through the tips.

One can improve the effect of the capacitors 7 and more by changing the resistance 5 and 15 respectively. This increases the time constant for the discharge. This has the Advantage that with large currents, where the curve shape of the secondary current consists of peaks and one in between The low-current part consists of the voltage on the capacitor during this low-current Part is still retained and thus enough voltage to trigger the relay when it is inside Errors is present.

If you use the intermediate transformers 4 and 14, you can determine the resistance of the secondary winding as follows interpret that it has the same effect as the resistors 5 and 15, which can then be omitted.

The advantage of the arrangement described is that the auxiliary lines 10 are not overloaded can and that stabilization through the capacitors beyond the effect of the holding winding the protection is further improved so that no false tripping occurs as a result of the voltage peaks can.

Claims (4)

Patent claims. 1. Line differential protection, especially for lines, by comparing the impedances resulting voltages when using saturated with low overcurrents Converters, characterized in that capacitors are used as impedances. 2. Line differential protection according to claim 1, characterized in that means are present are to influence the discharge of the capacitors. 3. Longitudinal differential protection according to claim 1, characterized in that in each converter circuit Resistors (5, 15) are connected, which are dimensioned so that the discharge in the low-current Pause between two current peaks caused by the saturated transducers. 4. Line differential protection according to claim 3, characterized in that the resistance of the Intermediate converter is dimensioned so that the discharge of the capacitor in the low-current There is a break between two current peaks. Considered publications:
German Auslegeschrift No. 1 023 127;
German patent specification No. 762 923. 1 sheet of drawings © 909 730/372 2.