AU669927B2

AU669927B2 - Power circuit breaker

Info

Publication number: AU669927B2
Application number: AU47579/93A
Authority: AU
Inventors: Hermann Zierhut
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1992-09-24
Filing date: 1993-09-23
Publication date: 1996-06-27
Anticipated expiration: 2013-09-23
Also published as: JPH06196986A; EP0590167B1; AU4757993A; EP0590167A1; DE59208693D1; ATE155297T1

Abstract

If a dead short circuit with a high rate of current rise occurs in a circuit, in which an electronic power switch or circuit breaker is switched on, the short-circuit current can assume very high values until regulation starts. If the construction of the circuit with the short circuit is very disadvantageous, the power switch can be destroyed if it is not correspondingly overrated. This disadvantage can be avoided by providing inductances (8), which limit the short-circuit current undelayed due to the voltages dropped across them proportionally to the rate of current rise, in the current path of the power switch (1) carrying the load current. <IMAGE>

Description

LL I d L_~ 92 P 3539 E Siemens Aktiengesellschaft Power circuit breaker The invention relates to a power circuit breaker, as well as a line circuit breaker, which operates with a semiconductor element whose internal impedance has a low value when there is a specific control voltage on a control electrode and an operating voltage on operating electrodes connected in the forward direction in a cable run, and whose internal impedance rises suddenly when the voltage on the operating electrodes rises.

Such a power circuit breaker, on which the invention is based, is described in European Patent Application (EP 91 120 690.2) which, at the moment, is S 15 still unpublished. In the version as a line protection 15 circuit breaker, this power circuit breaker has the advantage that it manages using simple means and a semiconductor element which requires no cost-increasing derating. For this purpose, the tripping element of a component which acts as a relay and is in normally-closed effective connection with an interrupter contact in the cable run is arranged electrically in parallel with the semiconductor element. Very high rates of current increase di/dt can occur when a true short-circuit occurs in a circuit. Since the reaction times of control loops for controlling the short-circuit current are in the range of several ten's of nanoseconds to 100 ns, the *short-circuit current can assume undesirably high values before the controller operates. In the case of a highly unfavourable construction of the circuit in which the short-circuit occurs, an electronic power or line protection circuit breaker which is provided therein and is in the form of a semiconductor switch can be destroyed if it is not correspondingly derated.

Ithe invention is thus based on -the e-bjct- -f constructing a power circuit breaker, as well as a line protection circuit breaker, in a simple and cost-effec- -o tive manner so that it withstands a true short-circuit -2with a high rate of current increase without derating.

In accordance with aspect of the present invention, there is provided a circuit breaker comprising: a semiconductor device whose internal impedance has a low value when there is a specific control voltage on a control electrode of the semiconductor device and an operating voltage on operating electrodes of the semiconductor device, and whose internal impedance rises suddenly when the voltage on the operating electrodes rises; and a small inductance, connected in a current path which carries a load current, said inductance representing a substantial short-circuit during normal operation, said inductance connected to a second control electrode for connection of the control voltage in such a manner that, in the event of a large increase in the load current caused by a i short-circuit, only a correspondingly small portion of the control voltage acts on the semiconductor device.

In the case of said power circuit breaker, a small inductance, which represents a true short-circuit in normal operation, is connected in the current path, which carries the load current, of the power circuit breaker. High rates of current increase di/dt as a result of short-circuits lead to a large voltage drop in the inductance, which voltage drop reduces the control voltage of the semiconductor switch without any time delay and thus chops off the dangerous current surge until the controller operates. This small inductance represents a true short-circuit for the normal 50 to 60 Hz current. It has no influence on normal operation of the power circuit breaker.

For a circuit breaker which is designed for a rated current of 10 to 16 A, it is particularly advantageous if the inductance has a value of 10 to 100 ni-I. A particularly simple and cost-effective development of the invention arises if the semiconductor element comprises two FETs connected in opposite directions in series with their gates connected to the common control voltage. It has furthermore been found to be advantageous if the inductance is 1-'.igned as a series circuit comprising at least two IN:\LIBoojOO573:MXL 2a inductance elements and if one inductance element is connected to each source connection of the FETs, and the common junction point of the inductance elements forms the negative connection point for the control voltage. If in each case one diode is connected in parallel with the inductance elements, the cathodes being in each case connected to the source connection of the two FETs, this thus ensures simple protection of the gate-source junctions. This prevents control voltage increases occurring in the case of the opposite current direction, since the diodes are connected such that the opposing control voltage, produced by the current increase, of that FET which carries inverse current is bridged by a diode and thus becomes ineffective. A further preferred embodiment arises if current i IN:\LIBoo100573:MXL 92 P 3539 E 3 transformers are provided in the circuit instead of the inductance elements, the primary circuit inductances of which current transformers are dimensioned to be of the same magnitude as the inductance elements and whose secondary circuits are connected to a control unit. Only one inductance is required if it is connected between the source connections of the two FETs and separate control voltages are provided for the FETs. The use of only one inductance and only oze control voltage is sufficient if means are provided using which the negative connection point of the control voltage can be switched over depending on the current direction. It has been found to be expedient for this purpose if an H-shaped network having diodes in the two parallel branches and having a 15 resistor in the transverse connection are provided as means, and if the network is connected in parallel with the inductance and the negative connection point of the control voltage is supplied to the resistor. A further *l Qd v o^'ec3^ simple and at the same time expedient development of the invention results if the inductance is designed as the primary-circuit inductance of a current/voltage converter.

The invention is explained in more detail in the following text, with reference to various exemplary embodiments.

FIG. 1 shows a power circuit breaker having a seriesconnected inductance, FIG. 2 shows a power circuit breaker having MOSFETs which are connected together in series, in opposite directions, to form a bipolar switch, and having current-increase limiting which is produced by an inductance, FIG. 3 shows a power circuit breaker "cording to FIG. 2 with protection of the inductance elements, FIG. 4 shows a power circuit breaker according to FIG.

3, in which the inductance elements are replaced by current transformers, FIG. 5 shows a power circuit breaker having only one inductance and two control voltages, 92 P 3539 E 4 FIG.. 6 shows a power circuit breaker having only one inductance and one control voltage.

FIG. 7 shows a power circuit breaker having a current transformer in a similar circuit to that according to FIG. 6.

FIG. 1 shows a power circuit breaker having an FET 6 as the semiconductor element 1, to which a control voltage 2 is applied via a control electrode 3. The internal impedance of the FET 6 assumes a low value in the case of a specific control voltage 2 and an operating voltage 4 on operating electrodes 5 which are connected in the forward direction in a cable run. A small inductance 7 is connected in series with that electrode, which is common to the load circuit and to the control circuit, 15 of the semiconductor switch 1, which is designed in particular as an MOSFET 6. In the event of a current "increase as a consequence of a short-circuit or of any other undesirable overcurrent pulse, a voltage drop u L di/dt, which follows the induction law, is produced in the inductance. In consequence, the control voltage of the semiconductor switch 1 is reduced without any time delay and, in this way, the dangerous current surge is chopped off until the current controller operates.

C

*e FIG. 2 shows a power circuit breaker having two MOSFETs 6 which are connected together in series, in opposite directions, to form a bipolar switch. The current-limiting inductance in this circuit comprises two inductance elements 8 which are connected in series a:nd are in each case electrically connected to a source 0 30 connection 10 of the two FETs 6. The common junction point of the two inductance elements 8 forms the negative connection point for the control voltage 2. In order to protect the gate-source junctions of the two FETs 6, in each case one further diode 11 can additionally be connected across each inductance element 8 (see FIG. 3).

The polarity in which the diodes 11 are connected is in this case defired such that their cathodes are connected in each case to one source connection 10 of the FETs 6 in order to prevent the control voltage increase which 92 P 3539 E 5 occurs in the opposite current direction. The diodes are connected such that the opposing control vcltage, produced by the current increase, of that FET 6 which carries inverse current is bridged by a diode 11 and thus becomes ineffective. The power circuit breaker according to FIG. 4 differs from that shown, in FIG. 3 essentially in that current transformers 12 are used in the circuit instead of the inductance elements 8, the inductances being formed by the primary circuits of the current transformers 12. The secondary circuits are in this case connected to a control unit 13 which supplies the necessary control voltage.

The design of the power circuit breaker as a bipolar switch having two MOSFETs 6 is, however, also 15 possible in conjunction with only one inductance 7 (see FIG. The inductance 7 is in this case connected between the two source connections 10 of the two FETs 6.

However, a supply having two separate control voltages 2 is necessary in the case of this design. FIG. 6 shows a modification of this circuit where, in addition to one inductance 7, there is also only one control voltage 2.

A network having four diodes 14 is connected in parallel with the inductance 7, in each case two of which diodes 14 are connected in series, in opposite directions, in 25 each branch and the diodes which are connected in parallel with one another in the branches are connected in parallel, in opposite directions. Connected in the transverc connection of the two parallel branches of this network is a resistor 15, which is connected to the 30 negative connection point for the control voltage 2. FIG.

7 shows a modification of this power circuit breaker having a current transformer 12 instead of the indictance 7.

The reaction time of the short-circuit current limiting by electronic power or line circuit breakers can thus be shortened by the use of an inductance.

Claims

1. A circuit breaker comprising: a semiconductor device whose internal impedance has a low value when there is a specific control voltage on a control electrode of the semiconductor device and an operating voltage on operating electrodes of the semiconductor device, and whose internal impedance rises suddenly when the voltage on the operating electrodes rises; and a small inductance, connected in a current path which carries a load current, said inductance representing a substantial short-circuit during normal operation, said inductance connected to a second control electrode for connection of the control voltage in such a manner that, in the event of a large increase in the load current cai'ed by a short-circuit, only a correspondingly small portion of the control voltage acts on the semiconductor device.

2. A circuit breaker according to claim 1, wherein the inductance has a value from 10 to 100 nlH.

3. A circuit breaker according to claim 1 or 2, wherein the semiconductor device is a FET. 0

4. A circuit breaker according to claim 1 or 2, wherein the semiconductor :,,device comprises two FETs which are connected in series, in opposite directions, and 0 •whose gates are connected to the control voltage.

A circuit breaker according to claim 4, wherein the inductance is formed as a series circuit comprising at least two inductance elements, one of said inductance elements being connected to each source terminal of the FETs, and a common junction point of the inductance elements forming a negative connection point for the control voltage.

6. A circuit breaker according to claim 5, wi rein a diode is connected in parallel with each of said inductance elements, a cathode of each said diode being connected to the source terminal of the respective one of said FETs.

7. A circuit breaker according to claim 6, wherein each said inductance element comprises a primary winding of a current transformer, a secondary winding of Seach said current transformer being connected to a control unit. IN:\LIBoo00573:MXL 'C

8. A circuit breaker according to claim 3, wherein the inductance is connected between the source connections of the two FETs, and in that separate control voltages are provided for the FETs.

9. A circuit breaker according to claim 3, further comprising switching means by which the negative connection point of the control voltage can be switched over depending on the direction of said load current.

A circuit breaker according to claim 9, further comprising a H-shaped 1 o network, having at least four diodes in the two parallel branches and a resistur in the transverse connection, said network being connected in parallel with the inductance, wherein the negative connection point of the control voltage is supplied to said resistor.

11. A circuit breaker according to claim 8, 9 or 10, wherein said inductance is formed to a primary circuit inductance of a current/voltage transformer. i

12. A circuit breaker as claimed in any one of the preceding claims, wherein said circuit breaker is a power circuit breaker.

13. A circuit breaker as claimed in any one of the preceding claims, wherein said circuit breaker is a line protection circuit breaker.

14. A circuit breaker substantially as herein described, with reference to Fig. 1, or Fig. 2, rr Fig. 3, or Fig. 4, or Fig. 5, or Fig. 6. DATED this Thirtieth Day of April 1996 Siemens Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON IN:\LIBoolOO573:MXL 92 P 3539 E Abstract Power circuit breaker If a true short-circuit with a high rate of current increase occurs in a circuit in which an electronic power circuit breaker or line protection circuit breaker is connected, then the shcrt-circuit current can assume very high values until the controller operates. In the case of a very unfavourable construction of the circuit in which the short-circuit occurs, the power circuit breaker can be destroyed if it is not correspondingly derated. This disadvantage can be avoided by providing inductances in the current path, carry- ing the load current, of the power circuit breaker which inductances limit the short-circuit current without being delayed by the voltages dropped on them, which are proportional to the rate of current increase. FIG. 1 0e 6 e* J S 0 01