Patent Description:
In electrical converters, failures 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 <NUM>).

Current estimation and short circuit detection based on current sensing has been successfully demonstrated for <NUM>-level converters. However, in high current applications, multilevel converter topologies such as T-type or (A)NPC converters are very popular to reduce the required voltage rating of the semiconductor switches and to increase the effective switching frequency of the converter. In a multilevel converter, short circuit events may be detected by detecting a desaturation of IGBT semiconductors in the short circuit path. In order to identify the short circuit path, the information about the position of the desaturated IGBT has to be combined with information about the switching state.

A main challenge for short circuit detection in multilevel converters compared to <NUM>-level converters is the much higher number of commutation and short circuit paths. By consequence, a single current measurement signal is usually not sufficient to detect a short circuit event in a multi-level converter. As an example, there is only one DC-link short circuit path in a <NUM>-level converter. In contrast, in a <NUM>-level ANPC converter, there exist four different DC link short circuit paths and additionally two transfer commutation paths, which are not related to a short circuit path.

Furthermore, the number of switches in the short circuit path may be higher as in the case of a <NUM>-level converter, which also may mean that the short circuit path traverses more than one power module. Depending on the realization and the short circuit current, the short circuit current can traverse an entire power module (i.e., enter and leave the module through the pair of DC terminals), leading to a situation similar to a <NUM>-level converter. In some cases, however, the short circuit current can traverse only a part of a half-bridge power module (e.g., enter through a DC terminal and leave through the AC terminal). This situation usually does not appear in a <NUM>-level converter.

<NPL>, is a scientific article showing an ANPC type converter, which comprises current sensors in the form of Rogowski coils at special positions within the converter circuit. It is shown that all types of short-circuit paths may be detected with the current sensors. A table is provided, which maps sensor signals compared with thresholds to fault states.

<NPL>, also considers the detection of short-circuit paths.

<CIT> shows a T-type converter with a short-circuit detection and protection circuit, which is adapted for measuring currents through outputs of anti-series connected semiconductor switches.

<CIT> shows converter cells for a multi-level converter with a gate-driver unit adapted for detecting a short-circuit path through the converter cell.

<CIT> relates to a method for detecting short-circuit paths in an ANPC type converter.

<CIT> show Rogowski coils, which surround each of the DC terminals of a power module and which are connected in series. A sum signal of both Rogowski is integrated for detecting errors.

It is an objective to provide simple means for detecting short circuit events in an electrical multilevel converter, i.e. a converter with more than <NUM> DC voltage levels.

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 method for detecting a short circuit event in a multi-level converter. A further aspect of the invention relates to an electrical multi-level converter. The electrical converter may be seen as a multilevel converter. It has to be understood that features of the method as described in the above and in the following may be features of the electrical converter as described in the above and in the following, and vice versa.

According to an embodiment of the invention, the electrical converter comprises at least two power modules electrically interconnected with each other to generate a multilevel output Each power module comprises two semiconductor switches electrically connected with each other. The two semiconductor switches of each power module are electrically connected in series or in anti-series between two outputs. In other words, the series or anti-series connection of the two semiconductor switches provides the two (end) outputs at its ends, while an intermediate output may be provided between the two semiconductor switches. The end outputs may be DC outputs, while the intermediate output may be an AC output.

It has to be noted that a semiconductor switch may be one or more semiconductor devices, which are connected in parallel and which receive one collective gate signal, i.e. which are switched simultaneously.

Furthermore, each power modules comprises a current sensor adapted for determining a current along a flow path through the power module. It may be that each power module comprises solely one current sensor, which, however, is adapted for determining a collective current signal, which is indicative of the current through the two outputs. A power module may be a module adapted for converting currents of more than <NUM> A and/or more than <NUM> V. It has to be noted that the converter may be a power converter adapted for converting currents of more than <NUM> A and/or more than <NUM> V.

A power module may be an assembly, which houses the two semiconductor switches and may provide terminals for electrically interconnecting the power module with further devices, such as further power modules. Also the current sensor may be provided in the power module. Alternatively, the current sensor may be attached to an outside of a housing or to conductors, to which the power module is electrically connected, such as a bus bar. It has to be noted that per power module, there may be only one current sensor but more than one semiconductor switch.

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..

The current measured by the current sensor may be measured inside or outside the power module. For example, the current may be a current through a semiconductor switch and/or through the transistor or thyristor. The current also may be a current through a conductor of the power module, such as a terminal. Furthermore, the current may be a current through a conductor outside of the power module, such as a bus bar. The current sensor may have to be arranged that a current along one single flow path through the module may be determined. There may be other current flow paths that are not detectable by the current sensor.

Furthermore, the converter may comprise a controller adapted for performing the method for detecting a short circuit event in a converter as described in the above and in the following. For example, the method may be implemented in software and the controller may comprise a processor on which the software is run. The method also may be implemented at least partially in hardware and/or the controller may comprise an FPGA and/or DSP.

According to an embodiment of the invention, the method comprises: determining at least two current signals with at least two current sensors of the power modules; determining a current signal pattern from the current signals by the comparing each current signal with a threshold; and detecting the short circuit event and identifying a short circuit flow path by searching the current signal pattern in a short circuit event table.

A current signal may be a signal provided by a current sensor. A current signal or current related signal may be a voltage signal that, for example, has been induced in the current sensor based on the current to be measured. It has to be noted that a current signal need not be proportional to the current, which is measured by the current sensor. For example, the current signal may be a signal proportional to a time derivative of the current. Also, a current signal may be indicative of the height of the current through the power module, i.e. as higher the current as higher the signal may be. In the method, at least two different current signals from different current sensors and in particular from different power modules are determined. Thus, currents associated with different power modules are evaluated with the method.

With the method, a current signal pattern is determined from the current signals. A current signal pattern may be seen as data with a data value for every current signal. This data value may be indicative of a state of the current, such as high/low present/not present, etc. A current signal pattern also may be a combined digital signal. By comparing the continuous and/or analogous current signal with one or more thresholds, the information content of the current signal is reduced, for example to only two or three different possible values.

The determined current signal pattern is then searched in a short circuit event table, which may be implemented as a lookup table. For every possible short circuit event and/or short circuit flow path, a pattern indicated of the short circuit event and/or short circuit flow path may be stored in the table. When the determined current signal pattern is found in the table, the corresponding short circuit event along a corresponding flow path may have taken place. A short circuit event may identify a short circuit flow path through the converter, which path may run through one or more power modules. It also may be possible that short circuit event identifies a short circuit in one semiconductor switch.

In general, different commutation events and short circuit events produce characteristic current signal patterns. In such a way, a commutation event, i.e. a regular switching event, may be discriminated from a short circuit event, i.e. an irregular event or fault. Even in a multilevel converter with more than one power module and more than one semiconductor switch per power module, a short circuit event may be detected with less current sensors as semiconductor switches are present. This reduces the number of needed current sensors and the complexity of the logic evaluating the current signals.

With the method, for each power module one collective current signal is determined with the respective current sensor, which collective current signal depends on two currents through the two outputs of the power module. For example, the collective current signal may be a differential current signal. In such a way, it is possible to only have one current signal per power module but to identify the position of a short-circuit event at least so exact that the affected power module can be identified. This may reduce the number of current sensors, which are necessary. Furthermore, the power modules may be standardized with two semiconductor switches and one current sensor, independently of their arrangement within the converter.

According to an embodiment of the invention, for determining a current signal pattern, each current signal is compared with a lower bound for a short circuit current as threshold. In such a way, commutation events may be discriminated from short circuit events. When no current is higher than the lower bound for a short circuit current, it may be assumed that no short circuit event is present.

According to an embodiment of the invention, for each current signal, the current signal pattern is indicative of the current signal higher than the threshold, the current signal lower than a negative value of the threshold and/or the current signal between the negative value of the threshold and the threshold. In other words, the data value from the current signal pattern for a current signal may be one of at least three values: no short circuit current, a positive short circuit current and a negative short circuit current.

However, it also is possible that data value from the current signal pattern for a current signal may be one of two values: no short circuit current, short circuit current present.

According to an embodiment of the invention, a current signal is compared with more than one threshold. For example, there may be a threshold for a low current and a high current. A low current threshold in general may be lower than a high current threshold, which may be a short circuit current threshold. In general it may be that also a low current through one power module in combination with a high current through another module, such as a short circuit current, may help to discriminate different short circuit events.

In this case, the data value from the current signal pattern for the respective current signal may be selected from values discriminating the intervals between the thresholds and/or between the positive and negative values of the thresholds.

According to an embodiment of the invention, thresholds for different current signals are different. For example, there may be different threshold for different power modules. This may be the case, when the power modules are differently designed, for example have a different topology and/or a different current rating.

According to an embodiment of the invention, one current signal is determined for each power module. It may be that every power module is provided with one current sensor and the current signals from all power modules are evaluated.

Alternatively, a number of determined current signals is smaller than a number of power modules. It may be that some power modules do not have a current sensor. For example, it may be that two or more paralleled power modules only have one current sensor, which measures the current from both power modules.

In general, the number of current signals may be smaller or equal to the number of power modules.

According to an embodiment of the invention, the method further comprises: integrating the current signal, which current signal is indicative of a time derivative of the current through the power module; and comparing the integrated current signal with the threshold. For specific current sensors, in particular those that are based on sensing a magnetic field generated by the current, the current signal is indicative of a time derivative of the current. In this case, a signal indicative of the total current may be determined by integrating the current. In general, to detect a short circuit, the original current signal may be post-processed (such as integrated) in a way to get an indicative of the total current. It also may be possible that the current signal is low pass filtered.

It has to be noted that also in the case of a current signal is indicative of a time derivative of the current, the current signal may be directly compared with a threshold for forming the current signal pattern, since also the time derivative of a short circuit current is usually higher than the time derivative of a commutation current.

According to an embodiment of the invention, the method further comprises: blanking the current signals during specific time intervals, for example during time intervals, when the semiconductor switches are switched and the new switching state does not allow a short circuit event. In order to avoid false detections of short circuit events, the current signal from some or all of the current sensors and/or its post-processed form may be blanked during specific time intervals. As an example, transitions where thyristors or transistors are turned on and/or off may be blanked, especially if they result in a state where no short circuit can practically appear.

According to an embodiment of the invention, the method further comprises: optionally allowing the generation of short-circuit signals and overruling a detecting of short circuit events during specific time intervals, for example, when the new switching state does not allow a short circuit event. Blanking also may be implemented by overruling false short circuit detections during blanking periods.

According to an embodiment of the invention, the method further comprises: receiving at least one switching state signal indicative of a switching state of at least one of the semiconductor switches; and identifying the short circuit flow path by searching the current signal pattern and combining with information on the current switching state. For example, switching state may be stored in combination with the current signal patterns in the short circuit event table. A better resolution of the short circuit events may be achieved by additionally including information about the actual switching state of the semiconductor switches of the power modules. In this case, a specific short circuit current path and/or specific failed semiconductor switch may be identified.

The information on the switching state of the power converter also may be used to distinguish a short circuit event from a normal switching and/or commutation event, to identify a short circuit path, to identify the position of a failed chip, or to decide on a proper short circuit turn-off sequence.

In general, the method is applicable to electrical multilevel converters of different topologies, which may have equally designed power modules and/or differently designed power modules.

According to an embodiment of the invention, at least one of the power modules is a 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.

According to an embodiment of the invention, the current sensor of the half-bridge module is adapted for measuring a current through one of the DC outputs. For example, the current sensor may measure a current flowing directly through one of the semiconductor switches or through a conductor interconnected with one of the semiconductor switches.

According to an embodiment of the invention, the converter is a <NUM>-level NPC converter comprising two half-bridge modules and a half-bridge diode module. A DC- output of an upper half-bridge module may be connected with the DC+ output of a lower power module. The AC outputs of the half-bridge modules may be interconnected with each other via diodes.

According to an embodiment of the invention, the converter is a <NUM>-Level NPC converter comprising one half-bridge module and two chopper modules, wherein a chopper module may be a series-connection of a diode and an semiconductor switch.

According to an embodiment of the invention, the converter is a <NUM>-level ANPC converter comprising three half-bridge modules, wherein an upper and a lower half-bridge module are interconnected with their AC outputs with the DC outputs of an intermediate half-bridge module. The DC- output of the upper half-bridge module may be connected with the DC+ output of the lower half-bridge module.

According to an embodiment of the invention, at least one of the power modules is a bidirectional switch module comprising two semiconductor switches connected in anti-series between two outputs.

According to an embodiment of the invention, the converter is a T-type converter with two parallel half-bridge modules interconnected with their AC outputs with an output of a bidirectional switch module.

According to an embodiment of the invention, the current sensor is adapted for measuring a current through one of the outputs. It may be that the bidirectional switch module has the same design with respect to its housing as a half-bridge module. The current sensor of the bidirectional switch module may be positioned like the one of a half-bridge module.

According to an embodiment of the invention, the short circuit events comprise current paths flowing through only one power module and/or the short circuit events comprise current paths flowing through more than one power module. In particular, the current paths may flow through differently designed modules, such as a half-bridge module and a bidirectional switch module.

There are different possibilities, how the current signal is measured, such as parasitic emitter inductance voltage sensing, current sensing with a Rogowski coil, and/or current sensing by a pick-up coil coupling to parasitic fields generated by the current.

According to an embodiment of the invention, the current sensor is adapted for measuring a current in one of the semiconductor switches. For example, this may be done by measuring the voltage drop on a bonding inductance of the semiconductor switch.

According to an embodiment of the invention, the current sensor is adapted for measuring a current in a power terminal of the power module and/or bus bar interconnected with the power terminal of the power module. For example, the time derivative of the current may be determined in the conductor with a Rogowski coil around the conductor.

According to another embodiment, one or more sensing coils may be positioned to couple to parasitic fields emitted by the power module and/or one or more of its interconnections. The transient magnetic fields produce a proportional voltage in the sensing coil as current signal.

For example, if a Rogowski coil is applied to the DC- terminal of a half-bridge module, the collector current of the lower semiconductor switch may be measured.

According to an embodiment of the invention, the current sensor is adapted for providing a collective current signal depending on two currents through two outputs of the power module. For example, the collective signal may depend on the currents through the DC outputs of a half-bridge module. In the case, only one current is present, the collective signal may be high and in the case, when both currents are substantially the same, the collective current signal may be low.

For example, the collective current signal may be indicative of a differential current of two outputs of the power module, the differential current being a difference between a current into the one output and a current out of the other output. Such a differential signal may be provided by a Rogowski coil around both conductors connected to the two outputs. For example, the Rogowski coil may be positioned around both corresponding terminals of the power module, which terminals may be positioned next to each other.

A collective current signal may be provided by a pick-up coil arrangement, which comprises several measurement coils positioned near the two conductors. The pick-up coil arrangement may couple to parasitic fields emitted by the conductors, such as wire bonds, power module terminals, or bus bars. The measurement coils may be positioned, such that a sum signal from the coils, for example provided by series and/or anti-series connecting the measurement coils, is high, when the difference between the currents is low and may be high, when the difference between the currents is high, which may be the case, when one of the currents is substantially zero and the other one is present.

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> shows a power module <NUM>, which comprises two semiconductor switches <NUM>, <NUM> connected in series. Each semiconductor switch <NUM> 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 <NUM>, <NUM> provides a DC+ output <NUM> and a DC-output <NUM> at its ends. Between the semiconductor switches <NUM>, <NUM>, an AC output <NUM> is provided.

The power module <NUM> comprises a housing <NUM>, in which the semiconductor switches <NUM> are accommodated. The DC+ output <NUM>, DC- output <NUM> and AC output <NUM> are connected with a DC+ terminal <NUM>, a DC- terminal <NUM>, and an AC terminal <NUM>, respectively, which protrude from the housing <NUM> from one side.

As shown in <FIG>, the power module <NUM> furthermore comprises a current sensor <NUM>, which is adapted for measuring a current through the DC- terminal <NUM>. In <FIG>, a Rogowski coil <NUM> is shown, which surrounds the DC- terminal <NUM>. A magnetic field generated by the DC-terminal induces a voltage signal in the Rogowski coil <NUM>, which is indicative of a time derivative of the current through the DC- terminal <NUM> and which may be used as current signal <NUM>.

The current sensor <NUM> and in particular the Rogowski coil <NUM> also may surround the DC+ terminal <NUM> or any other conductor connected with the DC+ output <NUM> or the DC- output <NUM>.

The DC+ terminal <NUM> and the DC- terminal <NUM> are arranged side by side. Each of the DC terminals <NUM>, <NUM> generates a magnetic field based on the time derivate dI/dt of the current flowing through it. The two magnetic fields add up to an effective magnetic field indicative of the time derivative of a current, which is the difference of the current flowing into the DC+ terminal <NUM> and flowing out of the DC- terminal <NUM>. This magnetic field may be transformed into a collective current signal <NUM> by a Rogowski coil <NUM> surrounding both DC terminals <NUM>, <NUM>. The Rogowski coil <NUM> of <FIG> generates a voltage signal indicative of a time derivative of a difference between the currents through the DC outputs <NUM>, <NUM> and may be used as a further type of current signal <NUM>. The difference of the currents through the DC outputs <NUM>, <NUM> is the current flwoing through the AC terminal <NUM>.

The current sensor <NUM> and in particular the Rogowski coil <NUM> also may surround any other conductors connected with both the DC+ output <NUM> or the DC- output <NUM>.

<FIG> shows a further possibility, how a collective current signal <NUM> may be generated. A pick-up coil arrangement <NUM> is shown, which comprises several coils <NUM>, <NUM>', <NUM>" arranged around the DC terminals <NUM>, <NUM>, which coils <NUM>, <NUM>', <NUM>" may be integrated into a printed circuit board. As indicated by the arrows, the coils <NUM>, <NUM>' on opposite sides of the DC terminals <NUM>, <NUM> may be wound in the same direction with respect to a direction around the DC terminals <NUM>, <NUM>. A further coil <NUM>" at a further side of the DC terminals <NUM>, <NUM> may be wound in the opposite direction. The coils <NUM>, <NUM>', <NUM>" may be connected in series and/or the number of conductor loops per coil <NUM>, <NUM>', <NUM>" may be chosen such that the magnetic fields from the DC terminals <NUM>, <NUM> induce a sum voltage in the series connection of coils <NUM>, <NUM>', <NUM>", which is based on the time derivatives of the currents through the DC terminals <NUM>, <NUM>. The sum voltage of the coils <NUM>, <NUM>', <NUM>" then may be used as collective current signal <NUM>.

<FIG> show an electrical converter <NUM> that is composed of half-bridge power modules M1, M2, M3 that may be designed like the ones shown in <FIG>. In <FIG>, a further alternative for a current sensor <NUM> is shown, which may be a sensor adapted for bonding inductance measurement, i.e. the current sensor <NUM> may be adapted for measuring the current through one or more bonding wires, for example like in <FIG>. In particular, the current sensor <NUM> is adapted for measuring a current through the lower semiconductor switch <NUM>.

The current signals <NUM> from the current sensors are sent to a controller <NUM>, which is adapted for detecting a short circuit event <NUM> in the converter <NUM>.

The converter <NUM> is a <NUM>-level ANPC converter constructed from the half-bridge power modules M1, M2 and M3 in the following way: The middle power module M2 is connected with its DC+ output with the AC output of upper power module M1 and is connected with its DC- output with the AC output of the lower power module M3. The upper power module M1 is connected with its DC- output with the DC+ output of the lower power module M3.

The midpoint of the middle power module M2 provides the AC output of the converter <NUM>. The DC+ output of the upper power module M1 is connected with the positive point of a split DC link <NUM>. The DC- output of the upper power module M1 respectively the DC+ output of the lower power module M3 is connected with the midpoint point of the DC link <NUM>. The DC- output of the lower power module M3 is connected with the negative point of the DC link <NUM>.

The semiconductor switches <NUM>, <NUM> of the power modules M1, M2, M3 are numbered in the following way: Power module M1 contains semiconductor switches S1 and S5. Power module M2 contains semiconductor switches S2 and S3. Power module M3 contains semiconductor switches S6 and S4.

<FIG> show the converter <NUM> in different switching states, which are listed in a table below. For each figure, the current paths <NUM> during normal operation in this state, i.e. when none of the semiconductor switches is failed, and short circuit paths P1, P2, P3, P4 in the case of a failure of one of the semiconductor switches S1 to S6 are shown.

In the cases of <FIG> and <FIG>, there are two overlapping short circuit paths, namely the shorter and longer upper short circuit paths P1 and P2, and the shorter and longer lower short circuit paths P3 and P4.

In the following table, the figures, the corresponding short circuit paths and the switching states of the switches S1 to S6 are listed. In general, the current signals <NUM> from the current sensors <NUM> may be positive or negative, and they may have different magnitudes. However, the current signals <NUM> may be classified according to the following scheme as indicated in the table: "+" indicates a current that is higher than a lower bound for the short circuit current, "-" indicates a current that is lower than the negative value of the lower bound and "<NUM>" indicates a current in between.

It can be seen that every short circuit path P1 to P4 has a characteristic signature. In other words, every short circuit current path P1 to P4 is related to a corresponding current signal pattern, which can be determined by comparing the three different current signals of the power modules M1, M2, M3 with a threshold.

The following table shows the short circuit paths and the corresponding current signal patterns.

The controller <NUM> may compare the current signals <NUM> with the threshold and may determine a current signal pattern for the converter <NUM>. For example, this may be performed regularly, and/or after every switching event. When the determined current signal pattern is found in the above table, which may be stored in the controller <NUM>, the controller <NUM> is able to determine the short circuit paths P1 to P4, which are present in the short circuit event <NUM> and the semiconductor switch S1 to S6, which has failed.

In the case of <FIG>, the short circuit events <NUM> can be read from the current signal pattern in a unique way. In particular, no two short circuit paths produce the same current signal pattern. However, this does not necessarily need to be the case in general. In general, the current signal pattern may allow to identify the short circuit path, but does not in general may allow to identify the position of the failed semiconductor switch <NUM>, <NUM>. In order to detect a position of a failed semiconductor switch <NUM>, <NUM>, the available information about the short circuit path may have to be combined with information about the state of the semiconductor switches <NUM>, <NUM>.

It also has to be noted that the short circuit path P1 to P4 is not necessarily detected by the power module M1, M2, M3, <NUM>, which comprises the semiconductor switch <NUM>, <NUM> that has to be turned off in order to break the short circuit path P1 to P4. Thus, with the information about the short circuit event <NUM>, the controller <NUM> may break the short circuit path due to a failure in one power module <NUM> by switching a semiconductor switch <NUM>, <NUM> of another power module <NUM>.

<FIG> show an electrical converter <NUM>' that is composed of half-bridge power modules M1, M2, M3' that may be designed like the ones shown in <FIG>. Contrary to <FIG>, the power module M3' is a bidirectional switch module with the semiconductor switches <NUM>, <NUM> connected in anti-series.

In the T-type converter <NUM>, the half-bridge power modules M1 and M2 are connected in parallel each other via their DC+ outputs, DC- outputs and AC outputs. The DC+ outputs and the DC- outputs of the power modules M1 and M2 may be connected with the positive point and the negative point of a split DC link. The bidirectional switch module M3' is connected with one output to the neutral point of the DC link and with the other output to the AC outputs of the power modules M1 and M2 which also provide an AC output of the converter <NUM>'.

Again as with respect to the converter <NUM> of <FIG>, the commutation paths or equivalently short circuit paths P1, P2, P3 are not necessarily contained in a single power module M1, M, M3'. For example, as shown in <FIG>, the short circuit path P1 from the positive point of the DC link to the neutral point only involves the path from the DC+ output to the AC output of the half-bridge power modules M1 and M2.

For example, the current signals <NUM> of the T-type converter <NUM>' may be generated by dI/dt sensing with a pick-up coil arrangement as shown in <FIG>. However, also other current measurement methods may be applicable.

Other as indicated in <FIG>, with a pick-up coil arrangement <NUM> as current sensor <NUM>, a collective current signal <NUM> is generated, which is based on the currents through both DC outputs of the respective module M1 and M2 or both outputs of the module M'. The pick-up coil arrangement <NUM> may have a stronger coupling coefficient to time derivatives of currents flowing from one DC output to the AC output compared to the ones between the DC outputs. The short circuit paths P1 and P2 shown in <FIG> and <FIG> lead to a higher collective current signal <NUM> as the short circuit path P3 of <FIG>, which, however, is based on the same magnitude of current.

The following table shows current signal patterns based on collective current signals <NUM> produced by the pick-up coil arrangements <NUM> of the modules M1, M2, M3'. In the table, "s" indicates a high absolute value of the collective current signals <NUM> and "w" indicates a weak collective current signal <NUM>. The values high and weak may be decimated with two different thresholds. The value "<NUM>" indicates a collective current signal <NUM> of substantially <NUM>.

Again, the controller <NUM> may distinguish the short circuit paths P1 to P3 based on the current signal patterns.

The coupling on the bidirectional switch module M3' may always have a relatively weak coupling coefficient, since the current is flowing through both of its terminals and/outputs. The current signal <NUM> detected with the current sensor <NUM> of the module M3 may be used to distinguish normal switching versus a short circuit event in the case of <FIG> and <FIG>. In this case, the current signals from M1 and M2 may be ignored, thereby avoiding false short circuit detections due to the different coupling coefficients.

Based on the sign of the current signal <NUM>, the two short circuit paths P1 and P2 from DC+ to NP and from NP to DC- can be distinguished. Additionally, using information about the switching state, the position of the failed semiconductor switch <NUM>, <NUM> may be uniquely identified.

If the current signal <NUM> from the bidirectional switch module M3' is <NUM>, the current signals <NUM> of the power modules M1 and M2 may be analyzed. This situation corresponds to <FIG>. The DC+ to DC- short circuit path P3 may be detected by integration of the current signals <NUM> from the modules M1, M2, which in the case of a pick-up coil arrangement correspond to a time derivate of a current, in order to estimate the current through the modules M1, M2.

<FIG> shows a flow diagram for a method for detecting a short circuit event <NUM> in a converter <NUM>, <NUM>', which may be performed by the controller <NUM>.

In step S10, at least two current signals <NUM> are determined with at least two current sensors <NUM> of the power modules M1, M2, M3, M3'. As described with respect to <FIG>, the current signals may be voltage signals provided by coils, in which the voltage signals are induced based on a current through the respective power module M1, M2, M3, M3'.

It also may be possible that the current signals are collective current signals that are indicative of the relationship of two or more currents through a power module <NUM>, such as the currents through DC outputs of the modules M1, M2, M3 or the currents through the outputs of the module M'.

It is furthermore possible that one current signal <NUM> is determined for each power module M1, M2, M3, M3' or, alternatively, that a number of determined current signals <NUM> is smaller than a number of power modules M1, M2, M3, M3'. For example, in the case of <FIG>, three current signals <NUM> are determined for three power modules M1, M2, M3. In the case of <FIG>, for the modules M1 and M2, only one current signal <NUM> may be necessary.

Furthermore, the current signals <NUM> may be indicative of a time derivate of the current(s) to be measured. In such a case, all or some of the current signals <NUM> may be integrated. For example, in the case of <FIG>, only the current signals <NUM> from the modules M1 and M2 may be integrated.

It also may be that the current signals <NUM> are blanked, i.e. set to <NUM>, for example in time intervals, during which the semiconductor switches <NUM>, <NUM> of the modules M1, M2, M3, M3' are switched. In such a way, false detections may be suppressed.

In step S12, a current signal pattern is determined from the current signals <NUM>. This may be done by comparing each current signal <NUM> with a threshold. For example, in the embodiment of <FIG>, each current signal <NUM> is compared with a lower bound for a short circuit current as threshold.

There may be more than one threshold for a specific current signal <NUM> from a specific power module. This is the case for the embodiment shown in <FIG>, where thresholds for weak and strong current signals are present. It also may be that thresholds for current signals <NUM> from different power modules are different.

With the thresholds, the current signals <NUM> may be associated to different categories, such as the "-", "<NUM>" and "+" of the embodiment of <FIG>. From these categories, the current signal pattern may be composed.

In step S14, the short circuit event <NUM> is determined by searching the current signal pattern in a short circuit event table. Such tables are shown in the above. The short circuit event table may be stored as a table in the controller. However, it also may be possible that only parts of the table are stored and that another part (or the complete table) is implemented as program logic. For example, in the embodiment of <FIG>, the current signals of power modules M1 and M2 only may be evaluated, when the current signal of M3' is in the category "<NUM>".

It is also possible that the controller receives and/or uses at least one switching state signal indicative of a switching state of at least one of the semiconductor switches <NUM>, <NUM> of the power modules M1, M2, M3, M3'. Then, the short circuit event <NUM> may be identified additionally based on the switching state. Also, specific switching states may be part of the short circuit event table.

Based on the short circuit event <NUM> that was identified, the controller <NUM> may select a semiconductor switch <NUM>, <NUM> that has to be shut off for breaking a short circuit path P1 to P4.

Claim 1:
A method for detecting a short circuit event (<NUM>) in a multi-level converter (<NUM>, <NUM>'),
wherein the converter (<NUM>) is composed of at least two power modules (M1, M2, M3, M3') electrically interconnected with each other to generate a multilevel output, and each of the power modules comprises a current sensor (<NUM>) adapted for determining a current along a flow path through the power module (M1, M2, M3, M3');
wherein each power module comprises two semiconductor switches (<NUM>, <NUM>) electrically connected in series or anti-series between two outputs (<NUM>, <NUM>) with each other, wherein the two outputs are connected with two terminals (<NUM>, <NUM>), which are positioned next to each other;
the method comprising:
determining for each power module (M1, M2, M3, M3') one collective current signal (<NUM>) with a current sensor (<NUM>, <NUM>, <NUM>) of the respective power module, which collective current signal (<NUM>) depends on two currents through the two outputs (<NUM>, <NUM>) of the power module (M1, M2, M3, M3');
determining a current signal pattern from the collective current signals (<NUM>) by comparing each collective current signal with a threshold;
detecting the short circuit event (<NUM>) and identifying a short circuit flow path (P1 to P4) by searching the current signal pattern in a short circuit event table;
characterized in that
the collective current signal is indicative of a differential current of the two outputs of the power module, the differential current being a difference between a current into the one output and a current out of the other output;
a) the current sensor comprises a Rogowski coil that surrounds the two terminals; or
b) the current sensor comprises a pick-up coil arrangement, which comprises several measurement coils (<NUM>, <NUM>', <NUM>") arranged on opposite sides and a further side of the two terminals (<NUM>, <NUM>), wherein the measurement coils (<NUM>, <NUM>') on opposite sides of the terminals (<NUM>, <NUM>) are wound in the same direction with respect to a direction around the terminals (<NUM>, <NUM>) and the measurement coil (<NUM>") at a further side of the terminals (<NUM>, <NUM>) is wound in the opposite direction.