High voltage bypass device , voltage source converter and operating method

The present disclosure relates to a high voltage bypass device , its application in a voltage source converter and an operating method for a high voltage bypass device .

High voltage (HV) installations , such as voltage converters operating in excess of one or more kV, often comprise a number of switching devices , such as power semiconductor switching devices , that selectively connect or disconnect two di f ferent voltage potentials . For example , so-called modular multi-cell converters (MMC ) comprise a number of switching cells to generate a desired output voltage from a given input voltage . Such converters are useful , for example , in high voltage direct current (HVDC ) transmission systems . As such installations form part of the critical infrastructure , they should operate reliably, preferably even in cases where individual components of a converter fail .

It is known to bypass a failed switching component using a mechanical bypass switch (BPS ) or breaker . However, the provision of an additional electro-mechanical component adds complexity and signi ficant cost to the overall installation . Alternatively, speci fic semiconductor protection devices may be employed . However, due to the large voltages and currents involved in HV installation, semiconductor-based protection devices typically require active cooling, which also adds complexity and signi ficant cost to the HV installation . The added complexity and cost double , i f these components are provided redundantly, e . g . to comply with reliability requirements .

The present disclosure provides alternative devices , systems and methods for ensuring reliable operation of HV installations .

According to a first aspect , a high voltage bypass device is disclosed . The device comprises a cavity, a trigger element arranged in the cavity, a reservoir filled with a conductive flowable material , and a shutter separating the cavity and the reservoir . The trigger element is connected to a first terminal and a second terminal of the high voltage bypass device . The shutter is configured to open a passage between the cavity and the reservoir in case the trigger element is triggered by an overvoltage condition such that the conductive material at least partially fills the cavity, thereby forming a conductive path from the first terminal to the second terminal .

The device according to the first aspect can bypass another device or part of a HV installation . Contrary to known bypass devices , the disclosed bypass device does not require any form of active control or forced cooling . Accordingly, such a device is relatively easy to construct and integrate , even in existing installations .

In at least one embodiment , the trigger element comprises a first electrode connected to the first terminal , and a second electrode connected to the second terminal , the first electrode and the second electrode extending into the cavity and forming a spark gap therein . Spark gaps are relatively easy to form and generate a spark in case of an overvoltage condition, which can be utili zed to release the shutter almost instantly . Moreover, the electrodes forming the spark gap can form part of the conductive path after the bypass device is triggered .

In at least one embodiment , the trigger element comprises a varistor connected between the first terminal and the second terminal . Varistors have well known electrical characteristics . Among others , they can enter an electrically conductive state above a well-defined threshold voltage , known as clamping voltage . Due to the Ohmic losses in the conductive state , heat is generated by the varistor which can be utili zed to release the shutter .

In at least one embodiment , the conductive , flowable material comprises at least one of a liquid, a powder and/or a granular material . Such materials can freely flow, for example under the influence of gravity or another external force , from the reservoir to the cavity .

In at least one embodiment , the conductive , flowable material comprises at least one of a metal or metal alloy, in particular gallium ( Ga ) , Galinstan, mercury (Hg) , aluminum (Al ) or copper ( Cu) . Metal materials have a very high conductivity and are therefore suitable for carrying high currents typically occurring in HV installations .

In at least one embodiment , the trigger element is configured to generate heat in case of an overvoltage condition and the shutter comprises a heat sensitive material , in particular a polymer material , which is at least partially destroyed by one of melting, evaporation or combustion . Such materials can be engineered to reliably activate a shutter mechanism in case a spark or other predefined thermal event occurs within the cavity .

According to di f ferent embodiments , the shutter comprises either a heat sensitive membrane separating the cavity and the reservoir, or a heat sensitive trigger coupled to a spring loaded divider separating the cavity and the reservoir . Either solution provides a simple , mechanical shutter mechanism that can be activated by a heat created in case of an overvoltage condition .

In at least one embodiment , the reservoir is arranged above the cavity in a mounted position of the device , such that the conductive , flowable material is released by gravity when the trigger element is triggered and, as a consequence , the shutter is open . A gravity operated mechanism is particularly reliable and does not dependent on any external source of energy or control .

According to at least one embodiment , the trigger element comprises a first electrode connected to the first terminal , and a second electrode connected to the second terminal , the first electrode and the second electrode forming a spark gap . The first electrode is arranged at a bottom part of the cavity and protrudes upwards , and the second electrode is arranged at a top part of the cavity, in proximity to the shutter, and protrudes downwards . Alternatively or in addition, the first electrode is arranged at a first wall of the cavity and protrudes towards an opposite , second wall of the cavity, and the second electrode is arranged at the second wall of the cavity and protrudes towards the first wall of the cavity, wherein a direct path between the first and second electrode is in proximity to the shutter . These arrangements ensure that the energy of the spark will trigger the shutter mechanism reliably .

According to at least one embodiment , the trigger element comprises a varistor connected between the first terminal and the second terminal . The varistor is arranged in a central part of the cavity underneath the shutter . Alternatively or in addition, the varistor is arranged in direct thermal contact with the shutter . These arrangements ensure that heat generated by the varistor will trigger the shutter mechanism reliably .

According to at least one embodiment , the cavity is filled with one of vacuum or a protective gas before the trigger element is triggered . This may be advantageous in certain operating environments , where an unintentional discharge or chemical corrosion of the electrodes should be avoided .

In at least one embodiment , the conductive , flowable material is configured to form a permanent electrical path between the first and the second terminal after the trigger element is triggered . This may be achieved in di f ferent ways , for example by flooding the cavity with a conductive liquid, or by contacting a first and a second electrode extending into or bounding the cavity with a granular conducting medium . Formation of a permanent electrical pathway enables a HV installation to remain operational for a relatively long time , for example until a fault in another component has been identi fied and addressed .

According to a second aspect , a voltage source converter (VSC ) , in particular a modular multi-cell converter (MMC ) , is provided . The converter comprises at least one switching cell configured to switch a high voltage , and at least one high voltage bypass device according to the first aspect , which is configured to bypass the at least one switching cell in case of a failure .

A converter according to the second aspect remains partially or completely operational in case a switching cell turns out to be defective . In such a scenario , the switching cell can be bypassed by means of the high voltage bypass device , thereby providing a path for the current to bypass the cell .

In at least one embodiment , the switching cell comprises at least one power semiconductor switching device , in particular an IGBT , and the high voltage bypass device is connected in parallel to the at least one power semiconductor switching device . In this way, a core switching component of the switching cell can be bypassed .

According to at least one embodiment , the switching cell further comprises at least one mechanical bypass switch connected in parallel to the at least one power semiconductor switching device and the high voltage bypass device . Such a converter combines the advantages of a mechanical bypass switching device with the additional safety of the high voltage bypass device according to the first aspect . For example , a mechanical bypass switching device may be activated or deactivated by an external control signal and/or multiple times , or may have other advantages . The high voltage bypass device can be activated without external control in case the mechanical bypass fails to close . Moreover, it ful fils the requirement of providing redundant bypass components , thus avoiding the risks associated with a failure of a single safety component . In at least one embodiment , the at least one switching cell has a voltage rating in excess of 1 kV, preferably in excess of 3kV, and/or a current rating in excess of 100 A, preferably in excess of 1000 A.

In at least one embodiment , the at least one high voltage bypass device is triggered by an overvoltage exceeding a voltage rating of the switching cell by a factor of two or more .

In at least one embodiment , the at least one high voltage bypass device has an activation time of less than 10 ms for bypassing the at least one switching cell in case of an overvoltage condition .

In at least one embodiment , the at least one high voltage bypass device is configured to bypass the at least one switching cell for a period exceeding 24 hours , preferably exceeding 168 hours .

The above characteristics make the converter useful for typical voltages , currents , overvoltage conditions , switching times and operating times occurring in HV installations .

According to a third aspect , an operating method for a high voltage bypass device is disclosed . The method comprises : triggering a trigger element connected to two terminals of the bypass device in case of an overvoltage condition, in particular by creating heat through a spark or a ohmic heating; at least partially destroying, in particular by melting, evaporating or combusting, a shutter of the bypass device in response to triggering the trigger element , thereby releasing a conductive , flowable material ; and at least partially filling a cavity of the bypass device with the conductive , flowable material , thereby creating a permanent electrical path between the two terminals of the bypass device .

The steps according to the third aspect implement a simple mechanism that does not require any external control and therefore reliably bypasses parts of a HV installation .

The present disclosure comprises several aspects of HV installations . Every feature described with respect to one of the aspects is also disclosed herein with respect to the other aspect , even i f the respective feature is not explicitly mentioned in the context of the speci fic aspect .

The accompanying figures are included to provide a further understanding . In the figures , elements of the same structure and/or functionality may be referenced by the same reference signs . It is to be understood that the embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale .

Figures 1A to 1C show a schematic cross-section through a first HV bypass device before , during and after occurrence of an overvoltage condition . Figures 2A, 2B and 2C show a schematic cross-section through a second HV bypass device before , during and after occurrence of an overvoltage condition .

Figure 3 shows a schematic circuit diagram of part of a voltage source converter comprising a single switching cell .

Figure 4 shows a schematic circuit diagram of part of a modular multi-cell converter comprising multiple switching cells .

Figures 5A to 5C show a schematic cross-section through a third HV bypass device before , during and after occurrence of an overvoltage condition .

Figures 6A to 6C show di f ferent cross-sections through a fourth HV bypass device before and after occurrence of an overvoltage condition .

Figure 7 shows , in a schematic manner, a method for operating a HV bypass device .

Figure 1A shows , in a schematic manner, the functional parts of a first bypass device 10 . The bypass device 10 comprises a first terminal 11 and a second terminal 12 for connecting the bypass device 10 with corresponding electrical connections of a device to be bypassed . For example , the bypass device 10 may be connected in parallel with a switching device , such as an IGBT , and/or a mechanical bypass switch to be shorted in case of mal function of the respective device .

The two terminals 11 and 12 are electrically separated from one another by parts of an insulating housing or the like , thereby forming a cavity 13 . Parts of the first terminal 11 form a first electrode 14 protruding into the cavity 13 . Similarly, parts of the second terminal 12 form a second electrode 15 protruding into the cavity 13 . Together, the first electrode 14 and the second electrode 15 form a spark gap 16 there between . The spark gap 16 acts as a trigger element for the first bypass device 10 and is triggered i f an overvoltage condition occurs between the first terminal 11 and the second terminal 12 . The value of the overvoltage triggering the bypass device 10 is determined by the shape and the distance between the two electrodes 14 and 15 as well as a potential atmosphere filling the cavity 13 . According to one embodiment , a protective gas or vacuum may fill the cavity 13 . For example , a voltage of 5 kV may trigger arcing of the spark gap 16 .

In the bypass device 10 shown in Figure 1A, the second terminal 12 forms the outside walls of a reservoir 17 filled with a conductive , flowable material 18 . In the embodiment shown, the conductive , flowable material 18 is arranged in a central part of the second terminal 12 , i . e . directly over the first electrode 14 in a mounted position of the bypass device 10 . A passage between the reservoir 17 and the cavity 13 below it is sealed by a shutter 19 . The shutter 19 is arranged in proximity to the second electrode 15 . For example , a membrane formed from a thin polymer material with a low melting point may be used to seal the conductive , flowable material 18 within the reservoir 17 as shown .

In the situation shown in Figure IB, an overvoltage condition occurs between the first terminal 11 and the second terminal 12 . Accordingly, a spark 20 is formed between the first terminal 14 and the second electrode 15 . The heat associated with the spark 20, e.g. heated plasma, melts or otherwise destroys at least part of the shutter 19. In the embodiment shown in Figures 1A to 1C, essentially the entire membrane separating the reservoir 17 from the cavity 13 is destroyed. In another embodiment, only parts of the shutter 19 may be destroyed, thereby triggering a mechanical breakdown of the seal between the reservoir 17 and the cavity 13. For example, a fuse formed from a polymer material may be destroyed by the spark 20 and release a spring-loaded divider (not shown) . In consequence, the conductive, flowable material 18, flows, under the effect of gravity into the cavity 13.

In the situation depicted in Figure 1C, the lower part of the cavity 13 is filled with the conductive, flowable material 18 to such an extent that the conductive, flowable material 18 is in contact with both the first electrode 14 and the second electrode 15. Accordingly, a permanent, conductive path 21 is formed between the first terminal 11 and the second terminal 12. Contrary to high power semiconductor switching devices, no active cooling is required for the conductive path 21.

As an example, a metal material that is liquid at normal pressure and room temperature of 20°C may be used. For example, mercury and certain gallium-based metal alloys, such as a gallium, indium, and tin-alloy also known as Galinstan, have a melting point of approximately -39°C and -19°C, respectively, have a low electrical resistance. Depending on the operating conditions, also metals or metal alloys having a slightly higher melting point, such as gallium having a melting point of approximately 30°C, may be used in case the HV bypass device 10 is installed in a converter or other part of a HV electrical installation that constantly operates above a melting point of the respective metal. As another example , a powder or granular material may be used as conductive , flowable material 18 . For example , relatively small copper particles having particle si zes in the micrometer range , may be used to form the electrical pathway 21 . I f a relatively fine powder is used, a high current flowing through the bypass device may in turn melt parts of the powdered substance filling the gap between the electrodes , and form a conductive path 20 with an improved, lower electrical resistance compared to the original powder .

Figures 2A to 2C show an alternative implementation of a second high voltage bypass device 10 . Most components of the high voltage bypass device 10 according to Figures 2A to 2C correspond to the respective components of the high voltage bypass device 10 shown in Figures 1A to 1C . Accordingly, only the di f ferences are described here .

Unlike in the first embodiment , in the second high voltage bypass device 10 shown in Figure 2A, the first and second terminals 11 and 12 form opposite sidewalls of the bypass device 10 . Accordingly, the first electrode 14 and the second electrode 15 protrude from the respective sidewalls towards the centre of the cavity 13 . The bottom of the cavity 13 is formed by an insulating bottom part 22 . The flowable material 18 is arranged in a reservoir 17 formed in as an insulating top part 23 of the bypass device 10 . Depending on the flowable material 18 used, the reservoir 17 may be open on top as shown as sealed with a lid (not shown) .

As before , the shutter 19 is arranged over and in proximity to a direct line between the two electrodes 13 and 14 forming a spark gap 16 . As shown in Figure IB, a spark 20 is formed in case of an overvoltage condition between the first terminal 11 and the second terminal 12 . In consequence , the shutter 19 will be partially or completely destroyed, and the flowable material 18 will partially or completely fill the cavity 13 arranged underneath the reservoir 17 . As shown in the speci fic embodiment of Figure 2C, the conductive , flowable material 18 essentially fills the entire cavity 13 .

Figure 3 shows parts of a HV installation comprising a bypass device 10 according to the present disclosure . In particular, a single switching cell 30 , which may be used in a voltage source converter, such as a modular multi-cell converter (MMC ) as described later, is depicted .

The switching cell 30 shown in Figure 3 comprises a hal f bridge 31 consisting of a voltage source 32 represented by a capacitor in Figure 3 and two IGBTs 33a and 33b connected in series between the two terminals of the voltage source 32 . Each IGBT 33a and 33b also has a corresponding diode 34a and 34b, respectively, which conducts current in a reverse direction . This may take the form of an integrated body diode or an external diode .

Between the two IGBTs , an electrical contact 35 is formed, which may be used, for example , to output a voltage generated by the hal f bridge 31 to the HV installation (not shown) .

In case the lower IGBT 33a of the hal f bridge 31 shown in Figure 3 cannot be activated, i . e . closed, the entire switching cell 30 would permanently disconnect an electrical path between an electrical reference contact 36 and the electrical contact 35 . Accordingly, a converter or other part of electrical installation comprising said switching cell 30 would block any current to be transmitted .

To avoid this situation, according to Figure 3 , a bypass device 10 , such as the bypass device detailed above with regard to Figures 1A to 2C, is installed in parallel to the first IGBT 33a . As detailed above , in case the lower IGBT 33a cannot be activated, i . e . by providing a corresponding gate voltage to its control gate , an overvoltage condition will occur between the electrical contact 35 and the reference contact 36 . Accordingly, a spark 20 will be triggered within the spark gap 16 of the bypass device 10 , thereby releasing the conductive , flowable material 18 to form a permanent conductive path 21 through the bypass device 10 ( see also steps S I to S5 of Figure 7 ) . This in ef fect shortens the switching cell 30 and maintains an electrical pathway between the reference contact 36 and electrical contact 35 . Current can flow freely past cell 30 , thus enabling the whole converter to continue operation .

As detailed above , the shutter 19 may be formed from a polymer material that is essentially destroyed in case of an overvoltage condition . Accordingly, the electrical bypass device 10 may only be triggered a single time . It may therefore also be referred to as a sacri ficial component , in particular a sacri ficial spark-short device .

While the bypass device 10 is relatively simple in its setup, and therefore cheap to manufacture , it may be desirable to implement an additional way of bypassing the first IGBT 33a in a controlled manner . Accordingly, in the embodiment depicted in Figure 3 , an additional , mechanical bypass switch 37 may be connected in parallel to the first IGBT 33a and the bypass device 10 . The mechanical bypass switch 37 may be controlled by a corresponding control circuit (not shown) . In case the control circuit determines that the switching cell 30 should be deactivated or that the IGBT 33a cannot be activated success fully, the mechanical bypass switch 37 may be closed to establish an electrical pathway between the reference contact 36 and the electrical contact 35 .

Preferably, the mechanical bypass switch 37 is activated by the control circuit before a spark 20 is formed in the bypass device 10 . In such a configuration, the sacri ficial bypass device 10 will only be activated i f both the first IGBT 33a as well as the mechanical bypass switch 37 fail together . In practice , the switching cell 30 or the IGBT 33a will have a speci fic voltage rating, for example 1 kV or 3 kV . In case of an IGBT failure , the mechanical bypass switch 37 will be triggered by the monitoring circuit . The gap between the first electrode 14 and the second electrode 15 of the bypass device 10 may be configured to be triggered only once the voltage potential between the electrical contact 35 and the reference contact 36 exceeds twice the rated voltage of the IGBT 33a . In operation, this situation may occur within 10 ms of the mechanical bypass switch 37 failing .

Figure 4 shows how multiple switching cells 30 may be combined to form a Modular Multi-Cell Converter 40 . In the depicted embodiment , the Modular Multi-Cell Converter 40 comprises a total of four, series-connected switching cells 30a to 30d . Therein, the reference contact of each switching cell is connected to the electrical contact of the previous switching cell . The reference contact 36 of the first switching cell 30a is connected to an electrical reference potential , such as an external voltage provided by a HV transmission line . The electrical contact 35 of the last switching cell 30d may in turn be connected to an output of the MMC 40 , e . g . a DC network into which energy from a HVAC network is to be provided .

Each of the switching cells 30a to 30d of the MMC 40 may comprise one of the bypass devices 10 as described above . For reasons of clarity, only a single bypass devices 10 is shown for the second switching cell 30b . I f one of the IGBTs 33 of any one of the switching cells 30 fails , an electrical transmission from the external reference potential to the output line is still possible , by means of the respective bypass device 10 . While the failure of , for example , the second switching cell 30b will negatively af fect the performance of the MMC 40 , it may still remain operational and thereby allow the control shutdown or repair of the failed part of the converter .

Figures 5A to 5C show an alternative implementation of a third high voltage bypass device 10 . Most components of the high voltage bypass device 10 according to Figures 2A to 2C correspond to the respective components of the high voltage bypass device 10 as shown in Figures 1A to 1C and 2A to 2C . Accordingly, only the di f ferences are described here .

Unlike in the first and second embodiment , in the third high voltage bypass device 10 , a varistor 50 is employed as a trigger element . The varistor 50 is place in a central part of the cavity 13 , underneath and physically separated from a shutter 19 . As can be seen in Figure 5A, the terminals of the varistor 50 are electrically connected with the ( external ) first and second terminals 11 and 12 of the bypass device 10 , respectively . In the depicted embodiment , the terminals of the varistor 50 are in direct contact with a first electrode 14 and a second electrodes 15 , respective , forming respective conductive parts of the sidewalls 51 of the cavity 13 . The electrodes 14 and 15 are in turn electrically connected to or integrally formed with the first and second terminals 11 and 12 .

As is well known, the resistance of varistors changes dependent on a voltage across them . Varistors are ceramic components with nonlinear electric properties . In general , a varistor, also known as a surge arrester, remains non- conductive or highly ohmic during normal operation when a voltage across it remains well below a threshold voltage , also known as its clamping voltage . At voltages above this threshold, also known as switching voltage , a varistor become conductive . In standard application, such as overvoltage protection, varistors are used to absorb a given maximal amount of energy . However, i f the energy is higher than this maximum amount , a strong temperature rise and even a thermal runaway occurs .

In the third embodiment , the varistor 50 is pushed on purpose to thermal runaway, so that a considerable amount of heat is generated by the varistor 50 . Hence , when an overvoltage condition occurs across the terminals 11 and 12 , the varistor 50 gets hot and triggers the shutter 19 , e . g . by causing it to melt or evaporate , etc .

As before , the shutter 19 is arranged over and in proximity to the trigger element formed by the varistor 50 . As shown in Figure 5B, heat 52 is generated and dissipated in case of an overvoltage condition between the first terminal 11 and the second terminal 12 . In consequence , the shutter 19 will be partially or completely destroyed, and the flowable material 18 will partially or completely fill the cavity 13 arranged underneath the reservoir 17 . As shown in the speci fic embodiment shown in Figure 5C, the conductive , flowable material 18 essentially fills the entire cavity 13 .

In the situation shown in Figure 5C, the varistor 50 is ef fectively bypassed by the conductive , flowable material 18 , which directly contacts the electrodes 14 and 15 , which form respective conductive parts of the sidewalls 51 of the cavity 13 . The varistor 50 itsel f may be partially or completely destroyed by the thermal runaway condition . This , however, is immaterial to the formation of the electrical bypath by the conductive , flowable material 18 .

Figures 6A to 6C show an alternative implementation of a fourth high voltage bypass device 10 . Most components of the high voltage bypass device 10 according to Figures 6A to 6C correspond to the respective components of the high voltage bypass device 10 shown in Figures 5A to 5C . Accordingly, only the di f ferences are described here .

As describe before , a varistor 50 is used as trigger element in the fourth high voltage bypass device 10 . Contrary to the previous embodiment , the varistor 50 is arranged in direct thermal contact with the shutter 19 . Thus , heat generated by the varistor 50 in response to an overvoltage condition acts directly on the shutter material . This will generally lead to a fast response time . Moreover, such a configuration may be useful in scenarios where the cavity is filled with vacuum or a protective gas with poor thermal properties . As shown in the cross section of Fig . 6B, the varistor 50 does not extend over the entire depth of the cavity 13 , leaving open areas 53 between the varistor 50 and an insulating front wall 54 and/or an insulating back wall 55 before the shutter 19 is released .

As shown in the cross section of Fig . 6C, upon triggering of the varistor 50 , the shutter 19 is at least partially destroyed, and the flowable material 18 runs through the open areas 53 to partly or completely fill the cavity 13 .

Figure 7 once more shows , in a schematic manner, the steps occurring during activation of the bypass device 10 .

In a first step S I , an overvoltage occurs across the bypass device 10 . Attention is drawn to the fact that the creation of the overvoltage may occur automatically as a consequence of the failure of another component , such as the failed activation of a mechanical bypass switch .

In step S2 , a trigger element connected to two terminals 11 and 12 carrying the overvoltage is triggered . For example , heat may be generated by the trigger element in response to the overvoltage condition . For example , i f a voltage potential between the first terminal 11 and the second terminal 12 exceeds a predefined voltage , a spark 20 will be generated between the two electrodes 14 and 15 . Alternatively, a thermal runaway condition in a varistor 50 or similar electrical component may be used to generate heat , e . g . by ohmic heating .

In a step S3 , a shutter 19 separating a flowable material 18 from a corresponding cavity 13 will be released in response to the trigger event of step S2. For example, a shutter 19 comprising a heat sensitive material may be released by the heat generated through arcing, i.e. a spark 20, or a thermal runaway conduction of the trigger element. For example, creation of a spark 20 may cause the local creation of a hot plasma, which will in turn melt, evaporate or combust the polymer material, thereby releasing the shutter 19. Similarly, a thermal runaway situation of the varistor 50, also release an amount of heat sufficient to melt, evaporate or combust the polymer material.

In absence of the shutter 19, in a step S4, the conductive, flowable material 18 will fill the cavity 13 arranged on the other side of the destroyed shutter 19. This may occur naturally, due to the effect of gravity acting on the flowable material 18, or may be aided or performed by an external force, such as spring-loaded mechanism.

Once the conductive, flowable material 18 has filled the space between the first terminal 11 and the second terminal 12 to a sufficient degree, for example by touching the first electrode 14 and the second electrode 15, a solid or liquid conductive path 21 is formed.

While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the figures and will be described in detail. It should be understood, however, that the intention is not to limit the disclosure to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure defined by the appended claims. For example, while the invention has been described with respect to a half bridge 31 and a MMC converter 40, it may equally be applied in a full bridge and/or another type of power converter.

Reference Signs

10 bypass device

11 first terminal

12 second terminal

13 cavity

14 first electrode

15 second electrode

16 spark gap

17 reservoir

18 flowable material

19 shutter

20 spark

21 conductive path

22 insulating bottom part

23 insulating top part

30 switching cell

31 hal f bridge

32 voltage source

33a/b IGBT

34a/b body diode

35 electrical contact

36 reference contact

37 mechanical bypass switch

40 modular multi-cell converter

50 varistor

51 conductive part of the sidewall

52 heat

53 open area

54 insulating front wall

55 insulating back wall