DE1245485B - Device for current limitation in alternating current circuits - Google Patents

Description

GERMAN WTTW ^ PATENT OFFICE

EDITORIAL

German class: 21 d2 - 42/01

Number: 1 245 485

File number: F 33713 VIII b / 21 d2

1245485 filing date: April 20, 1961

Opened on: July 27, 1967

Faults in electrical networks or with electricity consumers are usually associated with overcurrents, which cause undesirable effects. It is therefore quite a number of bodies have been created, the power supply unit or consumer concerned when an overcurrent occurs turn it off, so shut it down. In many cases, however, the repercussions on the disturbed circuit or on the feeding network as a result of the inevitable delay in the shutdown process so big that you have to use current-limiting means.

Generally known for this is the interposition of a choke coil at the beginning of the protected area Circuit. With such a choke coil, however, only a limited current limitation can be achieved because if the inductance is too high, the voltage fluctuations between no-load and full load will lag behind the inductor would become unacceptably large. For example, if you leave a voltage change of 5%, in the event of a short circuit the short circuit current is approximately 20 times the nominal current, so it is still very big. Similar relationships occur when two networks are coupled; here, interference in one network should be kept away from the other network as far as possible.

It is known (German patent specification 641 335) to provide a choke coil with taps for connecting a consumer to two different networks, to which the consumer can be connected. The consumer current, which is divided between the two networks, causes opposite induction currents in the choke coil, so that the resulting inductance remains relatively small, while the full inductance is effective for the equalizing current between the two networks. This arrangement can therefore only be used where no value is placed on an exchange of power between the two networks. Finally, there are also cases in which a shutdown is not actually desired, because, for example, the disturbance can be eliminated in a way other than switching off, but in which the effects on the undisturbed power supply are too great even if a choke coil is interposed that they could be accepted, so a shutdown must take place. In the cases under consideration, the choke coil must be designed as an air choke coil, since it is known that an iron choke coil would saturate at high currents and thus lose its protective effect.
A current-limiting device that is used to limit the current in AC circuits

Applicant:

Paint factories Bayer Aktiengesellschaft, Leverkusen

Named as inventor:
Dipl.-Ing. Bernhard Drescher, Leverkusen

to avoid the disadvantages described, must therefore meet the following conditions: During normal operation, d. H. between idle and full load, the voltage drop across the device should be negligible be small, but when the full load current is exceeded, the current-limiting effect should be immediate occur and in the event of a short circuit, the overcurrent that occurs is reduced to the lowest possible value, for example, about 2 to 3 times the rated current can be limited. In special cases it would be • even desirable to make the short-circuit current smaller than the rated current. After all, supposed to no disruptive equalization processes occur when the current limitation becomes effective.

In order to achieve these objects, it is proposed according to the invention to include a saturable inductor in the circuit to be connected in series with a capacitor and the inductance of the choke with an unsaturated iron core with the capacitance of the Adjust the capacitor so that a small total capacitive resistance is close to the voltage resonance point is available.

If the iron saturates when the rated current is exceeded, the inductive resistance drops the choke coil; but since the capacitive resistance of the capacitor remains constant, so the volume resistance of the series connection increases, which is essentially due to the difference of the two resistance values is given, thereby limiting the current. Will the short circuit or the overload is eliminated, the saturation of the iron inductor, its resistance, disappears increases and the initial state is restored. Compensatory processes practically do not occur because

709 618/201

the whole process adjusts itself to the new value with each half-wave.

The process is explained in more detail with reference to Figs. Ib and la.

Fig. Ib shows a simple consumer circuit: The generator G is to be protected against overload and short-circuit by the current limiting device, consisting of an iron choke coil and a capacitor, so that its voltage U is maintained and the effects of the overcurrent and especially the short-circuit current are sufficiently limited . In the case of the current /, the voltage Ul occurs at the choke coil and the voltage U _c occurs at the capacitor. Such a series resonant circuit has been examined in detail in an article by Walter Koch in Siemens-Zeitschrift, 33 (1959), 6, pp. 385 to 395, "Tilting phenomena in alternating current - high-voltage systems", with the tilting phenomena being dealt with in particular Use of saturable reactors can occur. Also in Fr. Sammer's book, "Schwingungskreise mit Eisenkernspulen" (Schwingungskreise mit Eisenkernspulen), pages 66 to 68 describe the occurrence of breakover oscillations in series resonant circuits with saturable reactors, with the behavior at different frequencies being examined in particular. In both cases the magnetization curve is expressed by a transcendent function. Since, in the arrangement described in the patent application, the choice of the sizes of inductance and capacitance prevents breakover oscillations with certainty, it is sufficient to get an insight into the current-voltage ratios to approximate the characteristics of the inductor coil with two straight lines and to approximate the effective resistances to neglect. Below the saturation current J _s , to which the voltage U _s corresponds, the choke coil has the constant inductance L ₁ , and above it the inductance L ₂ . The relationship applies:

40

U _l = U _s + (JJ _s ) CoL ₂ = U _s (l— j ^ J + J-oL ₂ .

With the current J, the voltage is applied to the capacitance

"- ^j C

ω C

or because of the vote U _c = J · co L ₁ .

The resulting voltage on the series resonance circuit is U = U _c - Ul and after some transformations

and Un = 2.126. If you wanted to

J _s U _s

1 -

55

L ₁

In the event of a short circuit behind the series connection - in the case under consideration, on the general side Network - becomes the maximum short-circuit current

A ₌ 1 4th E * L L_
Λ U _s

L ₁

65

(with Um = mains voltage).

For example, if you make L ₂ = 0.2 L ₁
= 0.9 U _s , then -A

to achieve the same current limitation with a simple choke coil, this would have to have a short-circuit voltage of Yiy ₅ = 47%; normal operation would not be possible with it.

To illustrate the mode of operation, voting was taken with full response; how As already indicated above, it is advisable to make the capacitive resistance a little larger than the inductive one, so that tilting vibrations can be avoided with certainty.

The use of resonance circuits consisting of a series connection of inductance and capacitance exist for control purposes is in itself not unknown, with either the change in frequency the alternating current or the inductance causes the detuning of the resonance circuit. In such But circling is the phenomenon that in the case of resonance due to the vanishing reactance very high partial voltages and through currents can occur, special attention should be paid to give. Thereupon z. B. in the book by Fr. Bauer, "The Capacitor in Heavy Current Technology" (1934), pp. 21/22, although this phenomenon is felt to be extremely disadvantageous. In the subject matter of the invention, however, it is precisely this disappearance of the volume resistance that is used to switch between Idle and full load no or only a very small voltage drop at the current limiting device to have, whereby the level of the partial voltages can be set as desired by appropriate dimensioning. As when exceeding of the full load current the through current is limited, the partial voltages are thereby also automatically kept within permissible limits.

Series resonance circuits have - as indicated above - found various applications, z. B. for protection against overvoltages according to German patent specification 401972, the circle is matched to a network harmonic. Of course, no saturation reactors may be used here be used. A more well-known field of application is the magnetic voltage and current stabilizer, the influence through the capacitance connected in the equalizing circuit the power factor of the connected load (German patent specification 671 310) or the load fluctuations itself (German patents 753 322 and 759 853) should be compensated. With another Execution is the phase jump that occurs when the resonance point is exceeded for the control a downstream discharge vessel is used (German patent specification 722 243).

As the name suggests, voltage and current stabilizers have the task of maintaining the output voltage or to keep the through current constant, with series circuits only as controlling or elements that improve stability are used.

But it is by no means thought of or in the way in which the resonant circuit is part of the general circuit is inserted, justifies that the output voltage behind the device or the through current should be smaller if a certain limit value,

Claims (2)

1, for example, the rated current is exceeded. The use of the series resonant circuit with a saturable choke coil for this purpose is new and cannot be readily seen from the characteristics of the series circuit inductance-capacitance. In the case of the conditions that a current limiting device should meet, it was also stated at the beginning that in the event of a short circuit, the current flowing through the short circuit should, under certain circumstances, be less than the rated current. This can be achieved in the simplest way with the device as it has been described in the invention so far by connecting an inductance in parallel with the series resonant circuit. As explained above, the saturation of the iron inductor decreases its inductance, so that a capacitive residual resistance remains in the series connection of the inductor-capacitor. If you connect an inductive resistor in parallel, the result is a parallel resonance circuit, the volume resistance of which is known to be very high, i.e. H. in the event of a short circuit, the short circuit current drops below the nominal current. The circuit is shown in Fig. 3. The saturation choke 1 and the capacitor 2 are in series and the choke 3 in parallel . B. in the event of a short circuit on the consumer side - the resistance is practically infinite, so no current flows into the short circuit point. The full voltage with the corresponding partial currents is applied to the rail parallel connection. Another application example is the coupling of two networks via the device described. In other words, the greater the difference between the two mains voltages, the less the current flowing through the coupling point increases in order to strive for a final value - which can be kept low - in the event of a short circuit in one of the networks. Finally, as a further example, the case is shown in Fig. 2 in which a "secured" network is to be fed in from two different networks, whereby the voltage in the secured network is to be maintained without lowering. In this case, the secured network will be cordoned off on both sides by two current limiting devices. If, for example, a short circuit occurs in the general network or if it fails, which in most cases has the same effect on the secured network as a short circuit in an outlet, an energy current flows from network 2 to network 1, the level of which is determined by the series connection of the saturation reactor - Capacitor in the outlet is automatically limited to a harmless value as soon as it arises. By limiting the current to a low value, the voltage in the "secured" network is also maintained. Patent claims: 1. Device for current limitation in AC circuits, consisting of a saturation choke coil, which is located between the voltage source and the power supply in which the current is to be limited, characterized in that the saturation choke coil is connected in series with a capacitor and the inductance of the choke coil unsaturated iron core is matched with the capacitance of the capacitor so that a small total capacitive resistance is present near the voltage resonance point. 2. Device according to claim 1, characterized in that a second choke coil is arranged parallel to the series connection of the saturation choke coil — capacitor. Considered publications:
German Patent Nos. 401972, 641 335, 671310, 722 243, 753 322, 759 853; Books by Friedrich Bauer, "The Capacitor in Heavy Current Technology," Berlin, 1934, p. 21; Book by Friedlich Sammer, "Schwingungskreise mit Eisenkernspulen", Leipzig, 1950, pp. 66 to 68; Siemens-Zeitschrift, 33 (1959), pp. 385 to 395. 1 sheet of drawings 709 618/201 7.67 © Bundesdruckerei Berlin