WO2007022744A1

WO2007022744A1 - Current-limiting switch

Info

Publication number: WO2007022744A1
Application number: PCT/DE2006/001306
Authority: WO
Inventors: Christoph Meyer; Rik W.A.A. De Doncker; Maurice Kowal
Original assignee: Rwth Aachen
Priority date: 2005-08-25
Filing date: 2006-07-26
Publication date: 2007-03-01
Also published as: DE102005040432A1

Abstract

The invention relates to a current-limiting switch having a main current path, which contains a mechanical switching unit, and having at least one commutation path, which is in parallel with the main current path and in which a power-electronic switching unit is arranged, a capacitive short-circuit limitation unit being arranged in series with the power-electronic switching unit.

Description

Current limiting switch

The invention relates to a current-limiting switch having a main circuit path containing a mechanical switching unit and having at least one commutation path running parallel to the main circuit path, in which a power-electronic switching unit is arranged.

From DE 199 53 551 Cl a current-limiting switch is known, which has as a so-called hybrid switch a main current path and two Kommutierungspfade, wherein the paths each have different switching units. The main current path, through which normally a mains current is conducted, has a mechanical switching unit, which is opened in the event of a short circuit (switching case), whereby an arc is formed. To limit the short-circuit current, the current is commutated in the first and second Kommutierungspfad. For this purpose, the first commutation path has a mechanical switching unit and a power-electronic switching unit arranged in series therewith, so that the first commutation path in the event of a short-circuit occurs briefly and impulsively can be traversed by the koπtmutierten stream. The second commutation path includes a current limiting resistor whose time delay is sufficient to complete the opening operation in the main current path and in the first commutation path.

A disadvantage of the known current-limiting switch is that the short-circuit current during the off period can continue to increase unhindered. The short-circuit current is only diverted from the main current path to the first commutation path to relieve the main current path, at the mechanical switching unit of which an arc is formed. A current limitation takes place only after further commutation on the second commutation path. A further disadvantage is that a turn-off semiconductor is necessary.

The object of the present invention is to further develop a current-limiting switch in order to effectively limit currents with low component expenditure, in particular in the event of a short circuit in high and medium-voltage networks.

In order to achieve this object, the invention is characterized in conjunction with the preamble of claim 1 characterized in that a capacitive Kurzschlussbegrenzungsein- unit is arranged in series with the power electronic switching unit.

The particular advantage of the switch according to the invention is that by providing a capacitive short-circuit Final limitation unit in a Kommutierungspfad a counter to the mains voltage counter voltage is established during the switching operation, which limits the increase of the short-circuit current. Because the resistance of the commutation path is substantially smaller than the resistance of the mechanical switching unit in the main current path, the mains current can commutate relatively quickly to the commutation path. The associated relatively short arc time in the main current path allows mechanical contacts of the mechanical switching unit of the main current path to experience relatively little wear during the turn-off process. The fact that the capacitive short-circuit limiting unit reduces the maximum short-circuit current allows the short-circuit current to be reduced more quickly to a permissible level, which reduces the switch-off time of the switch. It forms a relatively steep, but short short-circuit current increase, so that there is the possibility of switching off short circuits in DC networks of high power.

According to a preferred embodiment of the invention, the capacitive short-circuit limiting unit has a capacity which is indirectly dependent on the inductance of the network and / or the level of the mains current at the beginning of the switch-off process. After the power electronic switching unit of the commutation path has been set to a conducting state, the inductance of the network or the arc voltage of the main current path causes the charging of the capacitive short-circuit limiting unit. According to a development of the invention, the capacitive short-circuit limiting unit is designed to be discharged in an initial state. Consequently, an opposite charging of the capacitive short-circuit limiting unit in the initial state is not required.

According to a development of the invention, the capacitive short-circuit limiting unit is formed only by a capacitor and the power electronic switching unit only by at least one thyristor, so that the component cost is relatively low. Advantageously, a thyristor and no turn-off semiconductor is used.

According to a development of the invention, the electronic power switching unit is formed by two antiparallel connected thyristors, which allow actuation of the switch according to the invention in two opposite directions of current.

According to a development of the invention, a further commutation path is provided which has an upper voltage protection unit. The overvoltage protection unit is preferably formed by a varistor whose threshold voltage is greater than the mains voltage. The thyristor of the first commutation path is used for switching off the same and thus for current commutation on the second Kommutierungspfad in which the varistor demagnetizes the inductance of the network. Through the thyristor of the first Incidentally, commutation paths avoid oscillations.

Embodiments of the invention are explained in more detail below with reference to the drawings.

Show it:

FIG. 1 shows a block diagram of a current-limiting switch which is connected to a single-phase medium-voltage network or to a direct voltage network,

FIG. 2 shows a representation of the currents during the switch-off process and

Figure 3 is an illustration of the voltage during the shutdown.

A current-limiting switch 1 can be used in all networks and current forms of the electrical power supply for switching operations and in particular as a short-circuit breaker. When used in alternating voltage networks, the advantages of the switch 1 are preferably in the area of the reduction of short-circuit currents and the reduced wear of a mechanical switching unit. In DC voltage networks, the advantage is preferably to switch off high currents. The switch 1 described below can be used in the form of a parallel and / or series connection for different voltage or power classes. Particularly when used in DC networks, the switch 1 according to the invention makes it possible to avoid relatively high losses of additional inductances for current limitation. The field of application may preferably be the protection of DC network structures for offshore wind farms.

According to the embodiment according to FIG. 1, the current-limiting switch 1 comprises on the one hand a mechanical switching unit 2 which is arranged in a main current path H. The mechanical switching unit 2 may be configured as a fast mechanical switch without the provision of an arc extinguishing device. Normally, the mechanical switching unit 2 is in a closed state, so that a mains current I _G (rated current) of a generator, not shown, can flow to a consumer, not shown, exclusively via the main current path H.

In the present case, the switch 1 is connected to a 2OkV DC network.

The switch 1 further comprises a first commutation path Ki and a second commutation path K ₂ , which are arranged parallel to the main current path H. __ _" η

The first commutation path Ki has a power electronic switching unit 3 and a capacitive short circuit limiting unit 4 connected in series with it.

The second coinmutation path K ₂ has an overvoltage protection unit 5, which preferably comprises a voltage-dependent resistor (varistor). According to an embodiment of the invention, not shown, the second commutation K ₂ omitted.

The power electronic switching unit 3 comprises two antiparallel-connected thyristors T, which initiate the current commutation from the main current path H to the first commutation path Ki in the event of a short circuit by ignition. According to an embodiment of the invention, not shown, the power electronic switching unit 3 may also have only a single thyristor T, so that the switch 1 is only used unidirectionally.

In the following, the turn-off behavior of the switch 1 will be explained with reference to FIGS. 2 and 3. The switch 1 is connected to a 20kV DC power supply. The rated current IG is 2 kA; the rated power is 40 MW. Upon reaching a current of 3 kA, the mechanical switching unit 2 detects an overcurrent and initiates by opening the switching unit 2, the shutdown. At a first commutation time t _k , at which the current has increased to 3.4 kA, a first current commutation takes place from the main current path H to the first commutation time. Ki. The delay of the current commutation between the current value 3 kA and 3.4 kA results from the detection time and the tripping delay of a measuring device of the measuring device integrated in the mechanical switching unit 2 and the actual switch of the switching unit 2.

After the current commutation from the main current path H to the first commutation path K ₁ , a further increase in the current I ₅ takes place, the capacitive short-circuit limiting unit 4 consisting of a capacitor C being gradually charged. The thereby increasing voltage Uc on the capacitor C corresponds to the voltage at the mechanical switch U _M. The capacitor C is charged in the direction of the mains voltage U _M and thereby provides for a reverse voltage, which leads to a reduced voltage across the network inductance and thus limits the current increase.

As can be seen from FIG. 3, the rise in the voltage drop _{U.sub.M of} the high-speed mechanical switching unit 2 never exceeds a value of 80 V / .mu.s, so that the reconsolidation of the air gap is always faster than the voltage rise. A re-ignition of the arc in the main current path H can be avoided. The rise in the voltage drop Ü _{M of} the mechanical switching unit 2 can be influenced by the capacitance of the capacitor C. By increasing the capacitance of the capacitor C, an arbitrary value can be set, in particular also for slow mechanical switches. It will accepted that the maximum value of the current is increased.

The capacitor C allows a current limiting, wherein at the time tjjax the maximum IMAX of the current I _s, the voltage drop ü _c at the capacitor C exactly the mains voltage U _N , ie 20 kV corresponds. With further charging of the capacitor C increasingly negative voltage U _c falls over the line inductance, so that there is already at this time to reduce the short-circuit current.

In a further current commutation of the first Kommutierungspfad Ki on the second commutation K ₂ , the voltage drop across the mechanical switching unit 2 is 30 kV. A falling on the anti-parallel thyristors T voltage U _τ is approximately during the current flow through the first and second commutation path Ki commutation K _{2 is} zero.

As can be seen from FIG. 2, the current commutation takes place from the first commutation path Ki to the second commutation _path K2 at a time t _ö2r in which the short-circuit current I _{3 has} again _{fallen below} its maximum value. The current commutation on the Kommutierungspfad K2 occurs as soon as the capacitor voltage U _{c is} above the threshold voltage of the varistor 5. The voltage Ü _M has _reached the voltage value 30 kV at the time t _k2 , which corresponds to the threshold value of the varistor 5. The further falling short-circuit current I _v now flows solely through the second commutation K ₂ until it drops to zero. The firing signal for the thyristor T is deactivated in order to prevent the oscillations between the mains inductance and the capacitor C which occur after the mains current has become zero. The switch-off process is completed when the current value zero is reached.

Claims

Claims:

1. Current-limiting switch with a main circuit path containing a mechanical switching unit and with at least one parallel to the main current path Kommutierungspfad in which a power electronic switching unit is arranged, characterized in that in series with the power electronic switching unit (3) a capacitive Kurzschlussbegrenzungsein- unit ( 4) is arranged.

2. Switch according to claim 1, characterized in that the capacitive short-circuit limiting unit (4) can be charged by the electric current (I ₃ ) which is commutated by the main current path (H) into the commutation path (Ki) as a result of the power electronic switch unit ( ₃ ) being actuated.

3. A switch according to claim 1 or 2, characterized marked, that the capacitance of the capacitive short-circuit limiting unit (4) is dependent on the inductance of the network and / or the amount of the useful current at the beginning of a shutdown.

4. Switch according to one of claims 1 to 3, characterized in that the capacitive short-circuit limiting unit (4) is designed to discharge in an initial state.

5. Switch according to one of claims 1 to 4, characterized in that the capacitive Kurzschlussbegrrenzung unit (4) only one capacitor (C) comprises.

6. Switch according to one of claims 1 to 5, characterized in that the power electronic switching unit (3) comprises at least one thyristor (T).

7. Switch according to one of claims 1 to 6, characterized in that the power electronic switching unit (3) comprises two anti-parallel connected thyristors (T).

8. Switch according to one of claims 1 to 7, characterized in that a parallel to the first Koπunu- tierungspfad (Ki) arranged second commutation path (K ₂ ) is provided which has an overvoltage protection unit (5).

9. Switch according to claim 8, characterized in that the overvoltage protection unit (5) has a varistor whose threshold voltage is greater than the mains voltage.

10. Switch according to one of claims 1 to 9, characterized in that the power electronic switching unit (3) is in the blocking state as soon as the current from the first Kommutierungspfad (Ki) on the second commutation path (K ₂ ) commutes.