The present invention generally relates to hybrid circuit breakers . DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION

In order to overcome the most severe disadvantage of the solid state breakers a number of hybrid circuit breakers have been proposed. The hybrid breaker is a combination of a conventional mechanical circuit breaker and a solid state breaker. At nominal operation the current flows through the mechanical circuit breaker and the solid state breaker is only used during faults. As in the case of the solid state circuit breakers unidirectional and bidirectional switches can be used depending on the requirements of the application. In Figs, la-c a bidirectional hybrid circuit breaker including a conventional mechanical circuit breaker 11 and a solid state breaker 12 is displayed in schematic circuit diagrams during different operation stages.

During a pole-to-pole fault the current flows through the mechanical circuit breaker 11. At a time instant the mechanical circuit breaker 11 is opened and the solid state circuit breaker 12 is turned on. As the mechanical circuit breaker 11 opens an arc is initiated across the mechanical circuit breaker 11 and if the arc voltage is sufficient the hybrid circuit breaker will commutate the fault current to the solid state circuit breaker 12 as shown in Fig. Ia. After current commutation and arc extinction, the fault current flows through the solid state breaker 12 as shown in Fig. Ib. In order to avoid arc re- ignition the conduction time should be sufficiently long to allow the contact gap in the mechanical circuit breaker 11 to cool down to avoid reignition. When the solid state circuit breaker 12 is turned off the stored energy in the loop inductance is absorbed by the overvoltage protection element as shown in Fig. Ic. Figs. 2a-b are diagrams of the arc current and the arc voltage, respectively, as a function of time during operation of the bidirectional hybrid circuit breaker of Figs . la-c .

The hybrid circuit breaker requires the same amount of semiconductors as the fully solid state circuit breaker. However, for the hybrid circuit breaker, the conduction losses are not an issue since the current in nominal operation flows through the mechanical circuit breaker. Further, for the same reason, cooling is not a crucial issue for the hybrid circuit breaker. Yet, heat sinks are required because the fault current is commutated to the solid state circuit breaker during faults.

SUMMARY OF THE INVENTION

Nevertheless, the use of the conventional mechanical circuit breaker in combination with a solid state circuit breaker is challenging due to the limited current rating capabilities of the solid state circuit breaker. The mechanical breaker can interrupt a fault current of some tens of kiloamperes whereas controllable solid state devices can typically only interrupt currents of some kilo amperes.

Further, if the loop inductance is high, a long commutation time is required. Long commutation time results, however, in a further increase in magnitude of the fault current and therefore the solid state circuit breaker has to interrupt very high currents .

Still further, the conduction time of the solid state circuit breaker is critical due to that (i) long conduction time is required in order to completely commutate the current from the mechanical circuit breaker to the solid state circuit breaker, (ii) long conduction time is required when the loop inductance is high, and (iii) long conduction time is required in order to extinguish the arc voltage of the mechanical circuit breaker, i.e. to ensure that no current is flowing through the mechanical circuit breaker. However, long conduction times result in high conduction losses and as a result overheating of the device which can lead into device failures.

Accordingly, it is an object of the present invention to provide a hybrid circuit breaker which addresses the above issues.

It is a particular object of the invention to provide a hybrid circuit breaker, which has smaller peak current, shorter commutation time, and shorter conduction time.

It is a further object of the invention to provide such a hybrid circuit breaker, which has fast fault detection, and shorter dead time.

It is a further object of the invention to provide such a hybrid circuit breaker, which is compact, robust, and reliable.

These objects among others are, according to the present invention, attained by hybrid circuit breakers as claimed in the appended patent claims .

According to one aspect of the invention a hybrid circuit breaker for breaking fault currents is provided, wherein the circuit breaker comprises a mechanical switch through which a normal current is passed, the mechanical switch being arranged to open in case of being exposed to a fault current, and a solid state or semiconductor breaker device, to which the fault current is commutated, the semiconductor breaker device being arranged to break the fault current. The mechanical switch has a contact arm with two contacts abutting stationary contacts of a circuit when being closed, the contact arm being rotatable around an axis of rotation to allow the two contacts of the contact arm to be removed from the stationary contacts such that the mechanical switch opens. The contact arm is asymmetrically arranged, i.e. the two contacts of the contact arm are located at different distances from the axis of rotation such that the separations between the two contacts of the contact arm and the two stationary contacts are different when the mechanical switch opens. The semiconductor breaker device is connected in parallel with the contact of the contact arm and the stationary contact which separate most when the mechanical switch opens.

By such hybrid circuit breaker faster interruption and comparably lower turn-off current are obtained.

More in detail, the mechanical switch will be latched in open position at lower current and therefore the dead time will be shorter implying a comparably lower turn-off current. Lower turn-off current in turn results in considerably lower energy dissipation in the semiconductor breaker device and therefore the footprint of the device can be reduced.

Further, a control strategy is proposed in combination with the asymmetric hybrid circuit breaker.

In one embodiment the inventive semiconductor breaker device comprises accordingly a control unit arranged to control the fault current commutated to the semiconductor breaker device as a function of time. The current commutated is allowed to increase during the fault handling in a controlled manner.

Preferably, the control unit is arranged to fully commutate the fault current to the semiconductor breaker device for a certain time period when the mechanical switch has fully opened and been latched in the open position to make sure that the contact gap cools down sufficiently. When the certain time period has lapsed, the current through the semiconductor breaker device is interrupted. Hereby, it can be made sure that the current is interrupted without risk of reigniting the contact gap.

In a particular implementation, the semiconductor breaker device comprises an insulated gate bipolar transistor (IGBT) and the control unit, which may be a gate drive unit, is arranged to control the fault current commutated to the semiconductor breaker device by means of controlling the gate voltage of the insulated gate bipolar transistor.

The gate drive unit can measure the voltage across the semiconductor breaker device and when the voltage across the device is increased and reaches a certain threshold level, this implies that a fault is occurred, and the control of the fault current commutated to the semiconductor breaker device is initiated. The gate voltage is thereafter increased with time, preferably stepwise, in a controlled manner.

The hybrid circuit breaker of the present invention can be a unidirectional device or a bidirectional device capable of breaking a DC fault current in any direction or an AC fault current. The hybrid circuit breaker has preferably a voltage rating of up to 1 kV.

Further details of the invention are set out in the dependent claims .

Advantages of the present invention may include:

• a fast fault detection, • turn-on at low fault current,

• comparably short dead time,

• comparably short commutation time,

• short conduction time of the semiconductor breaker device, • low turn-off current,

• only low energy has to be handled (dissipated) by the semiconductor breaker device,

• compact solution, • low temperature rise in the semiconductor breaker device due to lower peak current,

• fast interruption and current limiting ability, and

• use in both AC and DC current interruptions .

Further characteristics of the invention, and advantages thereof, will be evident from the following detailed description of preferred embodiments of the present invention given hereinafter and the accompanying Figs. 1-5, which are given by way of illustration only, and are thus not limitative of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

Figs, la-c display in schematic circuit diagrams a bidirectional hybrid circuit breaker during different operation stages according to prior art.

Figs. 2a-b are diagrams of the arc current and the arc voltage, respectively, as a function of time during operation of the bidirectional hybrid circuit breaker of Figs. la-c.

Fig. 3 displays schematically, partly in a side view, partly in a circuit diagram, a hybrid circuit breaker according to an embodiment of the present invention. Fig. 4 is a diagram of the current as a function of time during operation of the hybrid circuit breaker of Fig. 3. Figs. 5a-b illustrate how an insulated gate bipolar transistor of a semiconductor breaker device of the hybrid circuit breaker of Fig. 3. controls the current through the semiconductor breaker device by means of introducing a variable resistance controlled by a gate voltage of the insulated gate bipolar transistor. Fig. 5a illustrates the variable resistance whereas Fig. 5b is a diagram of the gate voltage and the variable resistance as functions of time during operation of the hybrid circuit breaker. DETAILED DESCRIPTION OF EMBODIMENTS

In Fig. 3 is illustrated a hybrid circuit breaker for interrupting fault currents according to an embodiment of the invention. The hybrid circuit breaker comprises a mechanical switch 31 and has a voltage rating of up to 1 kV. During normal operation conditions the current is passed through the mechanical switch 31, but when being exposed to a fault current, the mechanical switch 31 is arranged to open, thereby commuting current to the semiconductor breaker device 32. The semiconductor breaker device 32 is eventually arranged to interrupt the fault current.

The mechanical switch 31 has a contact arm 33 with two contacts 33a-b abutting stationary contacts 34a-b of a circuit when being closed. The contact arm 33 is rotatable around an axis of rotation 35 to allow the two contacts 33a-b of the contact arm 33 to be removed from the stationary contacts 34a-b such that the mechanical switch 31 opens. Fig. 3 illustrates the mechanical switch 31 in an open state.

Note that Fig. 3 only illustrates the mechanical switch 31 schematically. The exact shapes of the contacts may be different than illustrated and further, the mechanical switch

31 is provided with a device for holding the mechanical switch 31 in an open state, and for subsequent closing of the mechanical switch 31 (when the fault has been handled) .

The two contacts 33a-b of the contact arm 33 are located at different distances xl, x2 from the axis of rotation 35 such that the separations zl, z2 between the two contacts 33a-b of the contact arm 33 and the two stationary contacts 34a-b are different when the mechanical switch 31 opens or is open. Such mechanical switch 31 is referred to as an asymmetric mechanical switch. Preferably, the contacts 33a-b of the contact arm 33 are located on opposite sides of the axis of rotation 35.

The semiconductor breaker device 32 is connected in parallel with the contact 33b of the contact arm 33 and the stationary contact 34b which separate most when the mechanical switch 31 opens . Hereby, the mechanical switch 31 can be latched in open position at lower current and therefore the dead time will be shorter implying a comparably lower turn-off current.

The semiconductor breaker device 32 comprises an insulated gate bipolar transistor (IGBT) 36, a diode bridge 37, and a gate drive unit 38. The diode bridge is commonly used in hybrid circuit breakers in order to be capable of interrupting a DC fault current in any direction or an AC fault current, see e.g.

US 2005/146814 and US 6,760,202, the contents of which being hereby incorporated by reference. The gate drive unit 38 is arranged to measure the voltage across the semiconductor breaker device 32. When the voltage increases and reaches a certain threshold level this implies that a fault is occurred. As a result fast fault detection can be achieved.

Then, the gate driver unit 38 is arranged to control the fault current commutated to the semiconductor breaker device 32 by means of controlling the gate voltage of the insulated gate bipolar transistor 36 and operating the insulated gate bipolar transistor 36 in its linear region. Hereby, the current can be clamped at different levels.

The gate drive unit 38 controls the fault current commutated to the semiconductor breaker device as a function of time and the current commutated is allowed to increase in a controlled manner, by means of a gradual or stepwise change of the gate voltage of the insulated gate bipolar transistor 36.

Fig. 4 is a diagram of the fault current and the current through the semiconductor breaker device, respectively, as a function of time during operation of the inventive hybrid circuit breaker.

The fault current is denoted by 41 and the current through the semiconductor breaker device 32 is denoted by 42. It can be seen that the insulated gate bipolar transistor 36 is operated in the linear region at e.g. 43. The latching of the mechanical switch

31 is indicated at 44.

The following parameters are indicated in Fig. 4: i _on is the fault current level where current is started to commutate to the semiconductor breaker device 32 and t _on the time instant when the semiconductor breaker device 32 is switched on, i _O ff is the peak fault current and t _Off the time instant when the semiconductor breaker device 32 is switched off, and Ri, R ₂ , R3, and R _ce are resistances of the insulated gate bipolar transistor 36. Figs. 5a-b illustrate how the insulated gate bipolar transistor 36 controls the current through the semiconductor breaker device

32 by means of introducing the variable resistance controlled by a gate voltage of the insulated gate bipolar transistor 36. Fig. 5a illustrates the variable resistance, whereas Fig. 5b is a diagram of the gate voltage V _g and the variable resistance R _va r as functions of time during operation of the hybrid circuit breaker. The values V _g i, V _g2 , V _g3 , and Vg _ma* of the gate voltage give the respective resistances Ri, R ₂ , R3, and Rce-

As can be seen in Figs. 4 and 5, at a certain time instant after the fault has been detected the insulated gate bipolar transistor 36 of the semiconductor breaker device 32 is turned on applying the lowest gate voltage V _g i at the gate thereof. This implies that the semiconductor breaker device 32 is at high resistance and a small amount of the fault current is flowing through the semiconductor breaker device 32. Then the gate voltage is increased allowing a higher current to flow through the semiconductor breaker device. This is repeated, i.e. the gate voltage is increased stepwise, until the mechanical switch 31 is latched at a secure position in the open state. Thus, the semiconductor breaker device 32 is operated as a linear regulator.

When the mechanical switch 31 is locked or latched in open state, re-strike of the contacts is not possible. Consequently, the gate voltage is increased so the semiconductor breaker device 32 fully commutates the current from the mechanical switch 31. When the current is commutated to the semiconductor breaker device 32, the semiconductor breaker device 32 will remain turned on for a certain time instance to make sure that the arc is extinguished. Finally, the semiconductor breaker device is turned off at considerably lower peak current compared with the symmetrical hybrid breaker.