WO2018193003A1

WO2018193003A1 - Short circuit detection in paralleled half-bridge modules

Info

Publication number: WO2018193003A1
Application number: PCT/EP2018/059963
Authority: WO
Inventors: Felix TRAUB
Original assignee: Abb Schweiz Ag
Priority date: 2017-04-20
Filing date: 2018-04-18
Publication date: 2018-10-25
Also published as: EP3610565A1

Abstract

A converter device (26) comprises at least two half-bridge modules (10), each half-bridge module (10) comprising two semiconductor switches (12, 14) connected in series between a DC+ output (16) and a DC- output (18) and providing an AC output (20) between them; wherein the half-bridge modules (10) are connected in parallel with each other, such that the DC+ outputs (16) are connected with each other, the DC- outputs (18) are connected with each other and the AC outputs (20) are connected with each other; and wherein at least one of the half-bridge modules (10) comprises a current sensor (30) adapted for detecting a differential current signal (32) of a differential current between the DC+ output (16) and the DC- output (18), the differential current being a difference between a current into the DC+ output (16) and a current out of the DC- output (18).

Description

DESCRIPTION

Short circuit detection in paralleled half-bridge modules

FIELD OF THE INVENTION

The invention relates to the field of short circuit detection in electrical converters. In particular, the invention relates to a converter device and a method for detecting a short circuit in such a device.

BACKGROUND OF THE INVENTION

In electrical converters, failure of chips or of the gate unit can result in a direct short circuit of the DC link. The resulting short circuit current leads usually to high stress on power modules and DC link capacitors, resulting in explosive release of energy if the short circuit current is not turned off within a very short time interval (such as less than 10 μβ). In future SiC power semiconductor based converters, the short circuit withstand time may be even shorter.

This situation may be even more severe in converters using several power modules in parallel. In this case, the short circuit current usually scales with the number of power modules in parallel. Furthermore, experiments have shown that oscillations between paralleled power modules may reduce the short circuit withstand time below the short circuit withstand time of a single power module.

In state-of-the-art IGBT converters, a DC link short circuit may be detected by de- saturation detection, i.e., the collector-emitter voltage may be measured to detect high voltages during on-state, indicating an excessive collector current leading to de-saturation of the IGBT. The main disadvantages of the de-saturation detection may be a complex implementation to measure a high-voltage signal, requiring large space due to isolation distances, and long detection times for short circuit events due to blanking times after IGBT turn-on.

Another solution to detect short circuit events is by means of sensing a time variation dl/dt of a current I through the power module. For example, a Rogowski coil may be used to detect a dl/dt in a power module terminal. Also, a parasitic emitter inductance of the IGBT may be used to detect the time variation dl/dt of the current.

DE 10 2013 108 078 Al describes a semiconductor switch circuit with paralleled half- bridges, which comprises an overcurrent detection unit.

DESCRIPTION OF THE INVENTION

It is an objective of the invention to detect short circuits in an electrical converter in a simple way, i.e. without complicated additional equipment and/or complicated evaluation of measurement signals.

This objective is achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

An aspect of the invention relates to a converter device. In general, the converter device may be a part of an electrical converter, for example an active rectifier and/or an inverter. The converter device may comprise a DC+ output, a DC- output and an AC output and may be used for converting a DC current into a phase of an AC current and vice versa.

It has to be noted that the converter device may be a power converter device adapted for converting currents of more than 10 A and/or more than 100 V.

The converter device comprises at least two half-bridge modules, each half-bridge module comprising two semiconductor switches connected in series between a DC+ output and a DC- output and providing an AC output between them. A half-bridge module may be a power module, which houses the two semiconductor switches and may provide terminals for electrically interconnecting the module with further devices, such as further power modules.

A semiconductor switch may comprise a transistor or thyristor connected in parallel with a free-wheeling diode. For example, the semiconductor switch may comprise an IGBT, IGCT, etc.

In the converter device, the half-bridge modules are connected in parallel with each other, such that the DC+ outputs are connected with each other, the DC- outputs are connected with each other and the AC outputs are connected with each other. In other words, the half- bridge modules may form a half-bridge of paralleled semiconductor switches that may be adapted for switching a higher current than one half-bridge module alone. Furthermore, at least one of the half-bridge modules may comprise a current sensor adapted for detecting a differential current signal of a differential current between the DC+ output and the DC- output, the differential current being a difference between a current into the DC+ output and a current out of the DC- output. The differential current signal may be indicative of the differential current.

With the converter device as described in the above and the below, the current into the converter device and out of the converter device is substantially equally distributed between the half-bridge modules. The differential current in the DC terminals of every half-bridge module is substantially the current through the AC terminal.

In the case of an asymmetry caused by a short circuit event, the differential current I develops a rather high dl/dt, i.e. the time variations of the differential current become substantially higher than during normal operation. In other words, in the cause of a short circuit event, the high frequency components of the differential current becomes high.

In particular, in the case of N paralleled half-bridge modules, the half-bridge module with the short circuited semiconductor switch may experience the full short circuit current N*Isc in the DC output connected to the short circuited semiconductor switch and the per-module short circuit current Isc on the opposite DC terminal. The working half-bridge modules, on the other hand, see the per-module short circuit current Isc on one terminal and no current on the opposite DC terminal. Thus, during a short circuit event, all half-bridge modules experience a differential current in the DC terminals different from 0.

Furthermore, if one considers the time variations dl/dt of the differential current I during a short circuit event, these are even higher than during normal operation. To detect higher time variations in the differential current, a signal proportional to the differential current t may be high-pass filtered or a signal proportional to the time variations d/dt may be integrated with an integrator having a cut-off frequency below the switching frequency of the half-bridge modules.

When one or more of the half-bridge modules are provided with a current sensor that is adapted for sensing only the differential DC terminal current, the generated sensor signal may be used to detect a short circuit event in the converter device.

The current sensor may be adapted for detecting that there is a difference in the current flowing through the DC+ output and the current flowing through the DC- output. For example, the current sensor may comprise two individual sensors adapted for measuring a current in the DC+ output and the DC- output providing two current signals that are subtracted from each other. However, it also may be possible that the current sensor is adapted for directly providing the differential current signal as described below.

In general, measurements of the current sensor may be performed in the DC+ output and the DC- output. The current sensor may comprise detection means for detecting currents and/or a differential current in the DC+ output and the DC- output. The current sensor may generated the differential current signal based on the current through the DC+ output and the DC- output.

A differential current signal may be a voltage signal provided by the current sensor that may be in relation and/or proportional to the differential current.

According to an embodiment of the invention, only one half-bridge module comprises a current sensor adapted for detecting a differential current signal of a differential current between the DC+ output and the DC- output of the half-bridge module. As all paralleled half-bridge modules experience a non-zero differential current during a short circuit, only one differential current need to be measured.

According to an embodiment of the invention, at least two of the half-bridge modules comprise a current sensor adapted for detecting a differential current signal of a differential current between the DC+ output and the DC- output of the respective half-bridge module. In order to increase robustness, it is also possible to equip more than one or all of the half- bridge modules with a current sensor.

According to an embodiment of the invention, the differential current sensor is based and/or is adapted for detecting a varying magnetic field generated by the current in the DC+ output and the DC- output. In other words, the current sensor may be based on a dl/dt sensor, i.e. a sensor adapted for determining the time derivative of the differential current. This may be achieved by measuring time variations of a magnetic field generated by the currents through the DC+ output and the DC- output.

For example, the DC+ output and the DC- output may have conductors that are arranged antiparallel to each other with respect to a current flow through them. In such a way, when the currents through the conductors vary, the conductors generate a varying magnetic field around them, indicative of time derivate of the sum of the currents (wherein these currents have a sign with respect to a direction parallel to the conductors). Such a magnetic field may be transferred into a voltage signal induced in a coil.

According to an embodiment of the invention, the current sensor comprises a Rogowski coil surrounding at least one conductor of the DC+ output and at least conductor of the DC- output, in which Rogowski coil the differential current signal is induced. For example, the Rogowski coil may be wound around both DC terminals of the half-bridge power module. Compared to existing implementation of Rogowski coils wound around a single DC terminal, creepage and clearance constraints can easily be fulfilled because the Rogowski coil can be placed at arbitrary distance from the DC terminals conductors.

For example, the Rogowski coil may be integrated in an existing PCB (such as the gate adapter board). However, it may also be realized as a separate component.

According to an embodiment of the invention, the current sensor comprises a pick-up coil arrangement partially surrounding a conductor of the DC+ output and a conductor of the DC- output. The pick-up coil arrangement may comprise one or more coils arranged besides the conductors. In the case of more than one coil, the coils may be connected in series with each other. In general, a pick-up coil may be designed like a Rogowski coil, which, however, does not form a closed loop but surrounds the two conductors only partially.

According to an embodiment of the invention, a number of conductor loops of the one or more coils are selected, such that the differential current signal is induced in the pick-up coil arrangement. The coils may be arranged at positions around the two DC terminals, at which differently strong magnetic fields from the different conductors are present. The sensitivity of the coils may be tuned with their number of turns, such that the effect of differently strong fields is mitigated.

The time variation or time derivative dl/dt of the differential current I may be detected by a pick-up coil arrangement, which is tuned in such a way that it detects non-zero net dl/dt in the pair of conductors, whereas it may be insensitive to a zero differential current such as during commutation events.

The pick-up coil arrangement may have two extension sections running along opposite sides of the conductors, the two extension sections having conductor loops wound in the same direction with respect to a direction around the conductors. The pick-up coil arrangement may have an intermediate extension section arranged between the two opposite extension sections.

According to an embodiment of the invention, the pick-up coil arrangement comprises at least two coils with conductor loops wound in opposite directions with respect to a direction around the conductors of the DC+ and the DC- output. In such a way, the pick-up coil arrangement may be placed much nearer to one of the conductors than to the other.

According to an embodiment of the invention, the one or more coils are provided by a printed circuit board (PCB). This printed circuit board may be a separate device and/or may carry further circuitry. For example, the pick-up coil arrangement may be integrated into a gate driver circuit board of the respective half-bridge module.

According to an embodiment of the invention, the current sensor comprises a ring of magnetic material surrounding a conductor of the DC+ output and a conductor of the DC- output. The current sensor furthermore may comprise a coil surrounding the ring, such that the differential current signal is induced in the coil. Such a current sensor may be seen as a differential current transformer on the conductors.

For example, the differential current transformer may be realized by placing magnetic material around both DC terminals and using a sensing wire to measure the magnetic field. In this case, the core of magnetic material only may pick up magnetic fields due to differential currents in the pair of DC terminals. As the magnetic field during normal operation only partly penetrates the core, the increase of stray inductance may be very small.

According to an embodiment of the invention, the at least one half-bridge module comprises a DC+ terminal and a DC- terminal interconnected with the DC+ output and the DC- output, wherein the DC+ terminal and the DC- terminal are together at least partially surrounded by the differential current sensor. The DC terminals may provide the conductors used for determining the differential current.

For example, the DC terminals may protrude side by side from the half-bridge module. In this case, the current sensor may be an additional device placed on the half-bridge module around the DC terminals. For example, the current sensor may be provided on a PCB (printed circuit board).

However, it is also possible that the current sensor is integrated into the half-bridge module, for example inside a housing of the half-bridge module.

According to an embodiment of the invention, the converter device comprises a controller adapted for receiving the at least one differential current signal and for determining from the differential current signal, whether the converter device has a short circuit.

The controller may be integrated into one half-bridge module or may be a separate device. For example, the controller also may provide the gate signals for the semiconductor switches.

A further aspect of the invention relates to a method for detecting a short circuit in a converter device, such as described in the above and in the following. For example, the method may be performed by the controller mentioned above. In general, the method may be implemented in software or at least partially in hardware. It has to be understood that features of the method as described in the above and in the following may be features of the controller or converter device as described in the above and in the following, and vice versa. The method is a fast and low-cost method to detect short circuit events in an electrical converter, which is composed of converter devices with paralleled half-bridge modules. As described above, the method uses the fact that a short circuit event is usually due to the failure of a single chip, leading to a current distribution which is strongly different from the well-balanced current distribution during normal operation.

According to an embodiment of the invention, the method comprises: receiving at least one differential current signal from a differential current sensor of a half-bridge of the converter device; comparing the at least one differential current signal with a threshold; and determining that the converter device has a short circuit, when the differential current signal is higher than the threshold. For example, the threshold may be set higher than an average commutation current, since the short circuit current and also the differential current may be substantially higher than the commutation current. As described above, the differential current may be at least 1/N times the short circuit current, where N is the number of paralleled half-bridge modules.

It has to be noted that the detected short-circuit may be a short-circuit of the DC+ output and DC- output and/or of a DC link connected to the converter device.

Optionally, it is also possible that the differential current signal is not directly compared with the threshold, but is processed before it is compared with the threshold. Such processing may include integrating or high-pass filtering.

According to an embodiment of the invention, the method comprises: receiving at least one differential current signal from a differential current sensor of a half-bridge of the converter device; processing the differential current signal; comparing the processed differential current signal with a threshold; and determining that the converter device has a short circuit, when the processed differential current signal is higher than the threshold. According to an embodiment of the invention, the differential current signal is a signal proportional to the differential current; and the method further comprises: high-pass filtering the differential current signal; comparing the high-pass filtered differential current signal with the threshold. The integrator may be tuned that frequencies lower than a threshold frequency are filtered out. For example, the threshold frequency may be equal and/or higher than an output frequency of the converter and/or corresponding half-bridge module.

According to an embodiment of the invention, the differential current signal is a signal corresponding to a time derivative of the differential current. For example, the differential current signal may be generated with a current sensor as described in the above and the below, which measures a varying magnetic field generated by the currents through the DC terminals.

According to an embodiment of the invention, the method further comprises: comparing the differential current signal corresponding to a time derivative directly with the threshold. During normal commutation, a measurement of the time variations dl/dt of the differential current may not gather a significant differential current signal. Only during short circuit events, there may be a significant signal on the dl/dt measurement. A complex postprocessing of the differential current signal is not needed.

According to an embodiment of the invention, the method further comprises: integrating the differential current signal with an integrator having a lower cut-off frequency higher than a threshold frequency, before comparing the differential current signal with the threshold. For example, the cut-off frequency may be equal and/or higher than an output frequency of the converter and/or corresponding half-bridge module.

According to an embodiment of the invention, the method further comprises: receiving at least two differential current signals from two different half-bridge modules of the converter device and comparing them with a threshold; and determining that the converter device has a short circuit, when two of the received differential current signals are higher than the threshold. It also may be that not only one but two or more of the differential current signals are evaluated.

For example, by determining, which half-bridge module has the largest differential mode current, in the case of three or more half-bridge modules, the half-bridge module with the short circuit may be determined.

Furthermore, by determining directions of the differential currents in different half-bridge modules, even the short circuited semiconductor switch may be determined. A direction of a differential current may be determined based on a sign of the differential current signal.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings. Fig. 1A to ID schematically show different states of a half-bridge module used in a converter device according to an embodiment of the invention.

Fig. 2A and 2B show different states of a converter device according to an embodiment of the invention.

Fig. 3 schematically shows a perspective view of a half-bridge module for a converter device according to an embodiment of the invention.

Fig. 4 schematically shows a perspective view of a half-bridge module for a converter device according to an embodiment of the invention.

Fig. 5 schematically shows a perspective view of a half-bridge module for a converter device according to an embodiment of the invention.

Fig. 6 shows a flow diagram for a method for detecting a short circuit in a converter device according to an embodiment of the invention.

The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 1A to ID show a circuit diagram for a half-bridge module 10, which comprises two semiconductor switches 12, 14 connected in series. Each semiconductor switch 14 comprises a thyristor or transistor, i.e. a switchable semiconductor element, connected in parallel with a free-wheeling diode. The series connection of semiconductor switches 12, 14 provides a DC+ output 16 and a DC- output 18 at its ends. Between the semiconductor switches 12, 14, an AC output 20 is provided.

When the half-bridge module 10 is used as a part of an electrical converter, it may be interconnected with a DC link 22 as shown in Fig. 1A to ID.

Fig. 1A and Fig IB show a current path 24 through the half-bridge module 10 after and before a turn-on event of the upper semiconductor switch 12. The current in Fig. IB has a negative time derivative dl/dt, since the current through the free-wheeling diode is falling to zero. The current in Fig. 1 A has the same but opposite time derivative dl/dt. Since the current is taken over by the semiconductor switch 14, the transient current follows the current path as shown in Fig. 1C. Fig. ID shows the current path 24 during a short circuit event, which is qualitatively the same as for the transient current. However, in the case of a short circuit, the current is usually much higher.

This may make it difficult, in practice, to distinguish a short circuit event from a normal switching event purely based on the time derivative signal dl/dt of the current. Instead, the dl/dt signal may need to be post-processed, for example using an integrator with a reset to eliminate a DC current component.

This situation is qualitatively different if the converter consists of multiple paralleled power modules 10. In the most likely case, a single chip of a semiconductor switch 12, 14 in a single half-bridge module 10 fails, producing the short circuit.

Fig. 2A and 2B show a converter device 26 composed of three half-bridge modules 10 in different states. In particular, Fig. 2A shows the current paths 24 for the commutation current flow during normal operation. Fig. 2B shows the current paths 24 for a short circuit current during a short circuit event 28.

The half-bridge modules 10 are connected in parallel, i.e. their DC+ outputs 16 are connected with each other providing a DC+ output of the converter device 26, their DC- outputs 18 are connected with each other providing a DC- output of the converter device 26 and their AC outputs 20 are connected with each other providing an AC output of the converter device 26.

As shown in Fig 2 A, during normal commutation, the current I or the time derivative dl/dt of the current is evenly distributed between the half-bridge modules 10. During normal commutation events, the current I commutates from the DC+ output 16 to the DC- output of every half-bridge module 10 or vice versa, making the sum of the currents or the time derivative dl/dt of the currents equal 0. Thus, the differential current, i.e. the difference of the current through the DC+ output and the current through the DC- output, and also the time derivative dl/dt of the differential current is substantially 0.

As shown in Fig. 2B, during a short circuit event 28, the current I or the time derivative dl/dt of the current is not evenly distributed between the half-bridge modules 10. On one side of the half-bridge modules 10 (here the upper side), the current I only flows through the short circuited semiconductor switch 12. The current may be concentrated in a single failed semiconductor switch 12, whereas it is shared by all working opposite semiconductor switches 12. This is due to the interconnection of the half-bridge modules 10 via their AC outputs 20. In this case, the differential current of every half-bridge module 10 is different from 0. The same applies to the time derivative dl/dt of the differential current.

In the example shown in Fig. 2B, the left-most half-bridge module 10 with the failed semiconductor switch 12 has a current of 3*Isc (Isc = short circuit current of single half- bridge module 10) in the DC+ output 16 and a current of Isc in the DC- output 18, resulting in a differential current of 2*Isc. The two half-bridge modules 10 on the right, i.e. the working half-bridge modules 10, have no current in the DC+ output 16 and a current of Isc in the DC- output 18, resulting in a differential current of -Isc.

It has to be noted that during normal operation, the output current I through the AC output 20 may also produce a differential current in the half-bridge modules 10. However, the corresponding current I may be lower by a factor of about 3 to 10 and/or time derivatives dl/dt may be much lower, for example by a factor of more than 100 or more than 1000.

As indicated in Fig. 2A and 2B, the differential current may be measured with a sensor 30 that may be provided in one, some or all of the half-bridge modules 10. It is possible that this sensor 30 is designed, such that it senses only the differential current and not the individual currents in the DC+ output and DC- output. It is also possible that the sensor 30 senses the time derivative of the differential current. Such sensors 30 will be described in the following.

The sensor 30 produces a differential current signal 32, which is received by a controller 34, which processes the differential current signal 32 to detect a short circuit event 28.

Fig. 3, 4 and 5 show a schematic perspective view of a half-bridge module 10 and in particular of its housing 36. The semiconductor switches 12, 14 are inside the housing 36. The DC+ output 16, DC- output 18 and AC output 20 provided by the half-bridge are connected with a DC+ terminal 38, a DC- terminal 40, and an AC terminal 42, respectively, which protrude from the housing 36 from one side.

The DC+ terminal 38 and the DC- terminal 40 are arranged side by side and provide two conductors, each of which generates a magnetic field based on the time derivate dl/dt of the current flowing through it. The two magnetic fields add up to an effective magnetic field indicative of the time derivative of the differential current. This magnetic field may be transformed into the differential current signal 32 in different ways.

As shown in Fig. 3, both DC terminals 38, 40 may be surrounded by a Rogowski coil 44. The magnetic field of both DC terminals 38, 40 induces a voltage in the Rogowski coil 44, which is substantially proportional to the time derivative of the differential current and may be used as differential current signal 32.

Fig. 4 shows a sensor 30 comprising a transformer arrangement 46 with a ring of magnetic material 48, which surrounds the DC terminals 38, 40, which may be seen as primary side of the transformer arrangement 46. A coil 50 is wound around the ring of magnetic material 48, which may be seen as secondary side. The differential current generates a magnetic field in the ring 48, which induces a voltage in the coil 50, which is substantially proportional to the time derivative of the differential current and may be used as differential current signal 32.

In Fig. 5, a pick-up coil arrangement 52 is shown, which comprises several coils 54, 54', 54" arranged around the DC terminals 38, 40. The pick-up coil arrangement may be a printed circuit board 52, in which the coils 54, 54', 54" are realized with metallization layers of the printed circuit board 52.

As indicated by the arrows, the coils 54, 54' on opposite sides of the DC terminals 38, 40 may be wound in the same direction with respect to a direction around the DC terminals 38, 40. A further coil 54" at a further side of the DC terminals 38, 40 may be wound in the opposite direction. The coils 54, 54', 54" may be connected in series and the number of conductor loops per coil 54, 54', 54" may be chosen such that the magnetic fields from the DC terminal 38, 40 induces a sum voltage in the series connection of coils 54, 54', 54", which is indicative of the time derivative of the differential current. The sum voltage of the coils 54, 54', 54" then may be used as differential current signal 32.

Fig. 6 shows a flow diagram for a method for detecting a short circuit 28 in a converter device 26, which may be performed by the controller 34.

In step S10, the controller 34 receives one or more differential current signals 32 from one or more current sensors 30 of the half-bridge modules 10. As a short circuit event 28 produces a differential current in every half-bridge module 10 as described with respect to Fig. 2B, it is enough that only one differential current signal 32 is evaluated. However, more than one differential current signal 32 may be used for redundancy reasons and/or for determining the failed half-bridge module 10.

In step S12, the controller 34 optionally processes the one or more differential current signals 32. Furthermore, after the optional processing, the controller 34 compares each of the one or more differential current signals 32 with a threshold and determines that the converter device 26 has a short circuit 28, when the one or all differential current signals 32 are higher than the threshold.

The processing of the one or more differential current signals 32 may performed in the following way:

When a differential current signals 32 is proportional to the differential current, it may be high-pass filtered, such that only frequencies higher than the usual frequencies generated during normal operation of the corresponding half-bridge module are filtered out. In such a way, only differential current components with frequencies generated by a short-circuit event may be compared with the threshold.

It also may be that the differential current signal 32 is a signal corresponding to a time derivative of the differential current, such as provided by the current sensors 30 as described with respect to Fig. 3, 4 and 5. In this case, the differential current signal 32 may be integrated with an integrator having a lower cut-off frequency higher the usual frequencies generated during normal operation.

However, it may not be necessary that such a differential current signal 32 is integrated for comparing it with the threshold, since also the time derivate of the differential current signal may be much higher than the one of an ordinary commutation event.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. LIST OF REFERENCE SYMBOLS

10 half-bridge module

12 semiconductor switch

14 semiconductor switch

16 DC+ output

18 DC- output

20 AC output

22 DC link

24 current path

26 converter device

28 short circuit event

30 current sensor

32 differential current signal

34 controller

36 housing

38 DC+ terminal

40 DC- terminal

42 AC terminal

44 Rogowski coil

46 transformer arrangement

48 ring of magnetic material

50 coil

52 pick-up coil arrangement, printed circuit board

54 coil

54' coil

54" coil

Claims

1. A converter device (26),

wherein the converter device (26) comprises at least two half-bridge modules (10), each half-bridge module (10) comprising two semiconductor switches (12, 14) connected in series between a DC+ output (16) and a DC- output (18) and providing an AC output (20) between them;

wherein the half-bridge modules (10) are connected in parallel with each other, such that the DC+ outputs (16) are connected with each other, the DC- outputs (18) are connected with each other and the AC outputs (20) are connected with each other;

wherein at least one of the half-bridge modules (10) comprises a current sensor (30) adapted for detecting a differential current signal (32) of a differential current between the DC+ output (16) and the DC- output (18), the differential current being a difference between a current into the DC+ output (16) and a current out of the DC- output (18).

2. The converter device (26) of claim 1,

wherein at least two of the half-bridge modules (10) comprise a current sensor (30) adapted for detecting a differential current signal (32) of a differential current between the DC+ output (16) and the DC- output (18) of the respective half-bridge module (10).

3. The converter device (26) of claim 1 or 2,

wherein the current sensor (30) is based on detecting a varying magnetic field generated by the current in the DC+ output (16) and the DC- output (18).

4. The converter device (26) of one of the preceding claims,

wherein the current sensor (30) comprises a Rogowski coil (44) surrounding at least one conductor (38) of the DC+ output (16) and at least one conductor (40) of the DC- output (18), in which Rogowski coil (44) the differential current signal (32) is induced.

5. The converter device (26) of one of the preceding claims, wherein the current sensor (30) comprises a pick-up coil arrangement (52) partially surrounding a conductor (38) of the DC+ output (16) and a conductor (40) of the DC- output (18), the pick-up coil arrangement (52) comprising one or more coils (50, 50', 50") arranged besides the conductors (38, 40), wherein a number of conductor loops of the one or more coils (50, 50', 50") are selected, such that the differential current signal (32) is induced in the pick-up coil arrangement (52).

6. The converter device (26) of claim 5,

wherein the pick-up coil arrangement (52) comprises at least two coils (50, 50") with conductor loops wound in opposite directions with respect to a direction around the conductors (38, 40) of the DC+ output (16) and the DC- output (18); and/or

wherein the one or more coils (50, 50', 50") are provided by a printed circuit board

(52).

7. The converter device (26) of one of the preceding claims,

wherein the current sensor (30) comprises a ring of magnetic material (48) surrounding a conductor (38) of the DC+ output (16) and a conductor (40) of the DC- output (18) and a coil (50) surrounding the ring (48), such that the differential current signal (32) is induced in the coil (50).

8. The converter device (26) of one of the preceding claims,

wherein the at least one half-bridge module (10) comprises a DC+ terminal (38) and a DC-terminal (40) interconnected with the DC+ output (16) and the DC- output (18); wherein the DC+ terminal (38) and the DC- terminal (40) are together at least partially surrounded by the current sensor (30).

9. The converter device (26) of one of the previous claims,

wherein the converter device (2) comprises a controller (34) adapted for receiving the at least one differential current signal (32) and for determining from the differential current signal (32), whether the converter device (26) has a short circuit (28).

10. A method for detecting a short circuit (28) in a converter device (26) according to one of the previous claims, the method comprising:

receiving at least one differential current signal (32) of a differential current between a DC+ output (16) and a DC- output (18) from a differential current sensor (30) of a half- bridge module (10) of the converter device (26), the differential current being a difference between a current into the DC+ output (16) and a current out of the DC- output (18);

comparing the differential current signal (32) or the processed differential current signal (32) with a threshold;

determining that the converter device (26) has a short circuit (28), when the differential current signal (32) or the processed differential current signal (32) is higher than the threshold.

11. The method of claim 10,

wherein the differential current signal (32) is a signal proportional to the differential current;

wherein the method further comprises:

high-pass filtering the differential current signal (32);

comparing the high-pass filtered differential current signal (32) with the threshold.

12. The method of claim 10,

wherein the differential current signal (32) is a signal corresponding to a time derivative of the differential current;

wherein the method further comprises:

comparing the differential current signal (32) corresponding to a time derivative directly with the threshold.

13. The method of claim 10,

wherein the method further comprises: integrating the differential current signal (32) with an integrator having a lower cutoff frequency higher than a threshold frequency, before comparing the differential current signal (32) with the threshold.

14. The method of one of the preceding claims, further comprising:

receiving at least two differential current signals (32) from two different half-bridge modules (10) of the converter device (26) and comparing them with a threshold;

determining that the converter device (26) has a short circuit (28), when two of the received differential current signals (32) are higher than the threshold.