WO2021249657A1

WO2021249657A1 - A cell comprising a crowbar branch with a resistive element

Info

Publication number: WO2021249657A1
Application number: PCT/EP2020/066368
Authority: WO
Inventors: Aravind MOHANAVEERAMANI; Georgios STAMATIOU; Mohsen ASOODAR; Marta IBANEZ; Jürgen Steinke
Original assignee: Hitachi Energy Switzerland Ag
Priority date: 2020-06-12
Filing date: 2020-06-12
Publication date: 2021-12-16
Also published as: JP2023534120A; EP4165764A1; CN115699548A

Abstract

A cell (12) for a voltage source converter comprises a first cell connection terminal (CC1), a second cell connection terminal (CC2), a first series connection of two switches (T1, T2) the midpoint of which forms the first cell connection terminal (CC1), an energy storage branch (ESB) comprising a capacitor (Cc) and a crowbar branch connected in parallel with the energy storage branch (ESB), the crowbar branch comprising a first switching element (SCB1) adapted to be activated to short-circuit the capacitor (Cc) of the energy storage branch (ESB) based on the detection of a short-circuit fault and a resistive element (R_D) connected in series with the first switching element and set to improve damping in the crowbar branch.

Description

A CELL COMPRISING A CROWBAR BRANCH WITH A RESISTIVE ELEMENT

FIELD OF INVENTION

[0001] The present application is on the field of power converters, specifically voltage source converters (VSCs). It is more particularly concerned with a cell that can be used in a Modular Multilevel Converter (MMC).

BACKGROUND OF INVENTION

[0002] VSCs such as MMCs comprise switches. These switches may be arranged in so- called half bridge or full bridge cells, where a cell may comprise a series-connection of two switches connected to a capacitor via DC busbars. As switches, semiconductors are used, in particular transistors like IGBT (Insulated Gate Bipolar Transistor). MMCs have become a popular choice for the grid connected converters due to its enhanced modularity, scalability and excellent harmonic performance with reduced losses.

[0003] Faulty semiconductor modules (e.g. IGBT modules) in an MMC cell may pose a high risk of massive damage. In many cases, if one IGBT in a half-bridge or full-bridge circuit fails, the adjacent IGBT may be able to clear a resulting fault current from DC capacitor(s). However, there are cases where it may not be possible to prevent this fault current.

[0004] As a stray inductance between IGBT and a DC capacitor may be purposely very low (in order to achieve acceptable converter performance/low transient overvoltage at the IGBT collector-emitter terminals/low IGBT switching losses) and the stored energy in the cell DC capacitor may be high, the prospective fault current may be high, such as in the range of a few hundreds kA.

[0005] For MMCs, a large cell capacitor is often required to buffer ripple energy resulting from the real/reactive power transfer. During short-circuit failure of a semiconductor, the cell capacitor energy may be discharged via the semiconductor leading to explosion/significant destruction of the cell.

[0006] Without any countermeasure, bond wires (the wires which connect the semiconductor chip to metal contacts) within an IGBT module will immediately fail, resulting in an electric arc. This arc will be fed by the high energy stored in the DC capacitor. This results in an explosion of the IGBT module with the consequence of a massive destruction. The difficulty to handle this problem increases with increasing energy stored in the DC capacitor.

[0007] One way to address this problem is to use a so-called DC crowbar to discharge most of the cell capacitor energy into a crowbar element such as a thyristor in order to limit the energy discharge into the semiconductor switches. However in this case the fault current can become too large for the crowbar to handle.

[0008] There is in view of what has been mentioned above a need for improving the way that a short circuit current is handled in a cell comprising a DC crowbar.

[0009] An improved crowbar is therefore desirable.

SUMMARY OF INVENTION

[0010] In a first aspect the present invention may disclose a cell for a voltage source converter, where the cell comprises a first cell connection terminal, a second cell connection terminal, a first series connection of two switches the midpoint of which forms the first cell connection terminal, an energy storage branch comprising a capacitor and a crowbar branch connected in parallel with the energy storage branch, where the crowbar branch comprises a first switching element adapted to be activated to short-circuit the capacitor of the energy storage branch based on the detection of a short-circuit fault and a resistive element connected in series with the first switching element and set to improve damping in the crowbar branch.

[0011] The resistive element is furthermore configured to dissipate the energy of the capacitor in a short time duration, such as within 10 - 200 ps from the activation of the first switching element. If the capacitance of the capacitor is C and the voltage across the capacitor is V, then the resistive element may need to be able to dissipate the energy of 0.5*C*V² within 10 - 200 ps from the activation of the first switching element.

[0012] The cell may additionally comprise a bypass switch connected between the cell connection terminals, which bypass switch may be mechanical or electronic.

[0013] In one realization, the element is an element with non-linear resistance, such as a surge arrestor. The surge arrestor may additionally have a clamping voltage that is lower than the rated voltage of the capacitor. The clamping voltage of the resistive element may be in the range 10 - 20 times lower than the rated voltage of the capacitor. Put differently, the rated voltage of the capacitor may be in the range 10 - 20 times higher than the clamping voltage of the resistive element.

[0014] As an alternative the resistive element may be an element with linear resistance R.

[0015] The resistance R may be set to a value that is the desired damping z times a constant that depends on the inductance L of the cell and the cell capacitance C. The constant may more particularly depend on the square root of an expression formed as the cell capacitance divided by the cell inductance. The constant may more particularly be set as two divided by the square root of the expression.

[0016] The linear resistance may be realized using a disc resistor.

[0017] As an alternative the linear resistance may be realized using a piece of conductor of the crowbar branch stretching along a longitudinal axis, where the piece of conductor comprises indentations alternatingly formed on opposite sides along the longitudinal axis for forming a meandering structure with bars interconnected at their edges and separated by slots.

[0018] The piece of conductor may additionally comprise a first and a second section, where one section is placed on top of the other and with the indentations being provided on opposite sides of the sections. The two sections may additionally be separated by an insulator. The two sections may extend along the longitudinal axis. The current may enter the structure via one of the sections and leave the structure via the other. Current may enter and exit the structure at a first end where the sections are insulated from each other and the two sections may be electrically joined to each other at the opposite end in order to create a return path. The number of slots may be selected to achieve the desired resistance.

[0019] The above-mentioned piece of conductor may be formed of a conventional conductor material such as Aluminium. As an alternative the conductor sections may be formed of a high- resistive material such as Nichrome.

[0020] It is additionally possible that the piece of conductor is a part of a clamping element used to mechanically clamp the first switching element. The first switching element may for instance be mechanically clamped between two clamping elements. In this case the meandering structure may extend out from a clamping body of the clamping element. It is additionally possible that also the other clamping element is made of the high-resistance material. [0021] The crowbar branch may be a split crowbar branch. For this reason the crowbar branch may further comprise a second switching element, where the first switching element is placed in an upper part of the crowbar branch and the second switching element is placed in a lower part of the crowbar branch. A junction between the upper part and a neighbouring part with switching element is connected to a midpoint of the first series connections of two switches and the resistive element may be connected in the upper part or the lower part of the crowbar branch.

[0022] The cell may be a half bridge cell, where the midpoint of the first series connection of two switches forms the first cell connection terminal and either a first end of the first series connection of two switches or a second end of the first series connection of switches forms the second cell connection terminal.

[0023] In the case of split crowbar branch in a half-bridge cell, the neighbouring part with switching element may be the lower part with the second switching element. The resistive element is also connected in the upper part of the crowbar branch in case the second end of the first series connection of switches forms the second cell connection terminal and in the lower part of the crowbar branch if the first end of the first series connection of two switches forms the second cell connection terminal.

[0024] The cell may as an alternative be a full-bridge cell that further comprises a second series connection of two switches connected in parallel with the energy storage branch. In this case the midpoint of the first series connection of two switches forms the first cell connection terminal and the midpoint of the second series connection of two switches forms the second cell connection terminal.

[0025] In the case of split crowbar branch in a full-bridge cell, the crowbar branch may comprise a third switching element in an intermediate part connected between the upper and lower parts. In this case the neighbouring part with switching element is the intermediate part with the third switching element. A junction between the intermediate part and the lower part may in this case also be connected to the midpoint of the second series connection of two switches.

[0026] Further aspects are directed towards a modular multilevel converter comprising cells, where at least one cell is such an assembly being a cell. BRIEF DESCRIPTION OF DRAWINGS

[0027] Embodiments of the present disclosure will be presented in the sense of examples and their advantages are explained in greater detail below, with reference to the accompanying drawings, wherein: [0028] Fig. 1 shows a first variation of a modular multilevel converter comprising cells;

[0029] Fig. 2 shows a half-bridge cell comprising a first variation of a crowbar branch with a damping resistance;

[0030] Fig. 3 shows a full-bridge cell comprising the first variation of a crowbar branch with a damping resistance; [0031] Fig. 4 schematically shows a fault occurring in a cell;

[0032] Fig. 5 shows a fault current in the cell without damping resistance during a positive half cycle of resonance;

[0033] Fig. 6 shows the fault current in the cell during a negative half cycle of resonance;

[0034] Fig. 7 shows the half-bridge cell with the first crowbar branch realization where the damping resistance is realized through a non-linear resistive element;

[0035] Fig. 8 shows a curve outlining the non-linear resistance characteristic of the non linear resistive element;

[0036] FIG. 9 shows the half-bridge cell with the first crowbar branch variation in which the damping resistance is realized as a linear resistance; [0037] FIG. 10 shows a first realization of the linear damping resistance;

[0038] FIG. 11 shows a second realization of the linear damping resistance;

[0039] Fig. 12 shows a half bridge cell realization comprising a second variation of a crowbar branch with a damping resistance;

[0040] Fig. 13 shows a full bridge cell realization comprising a third variation of a crowbar branch with a damping resistance; [0041] Fig. 14 schematically shows a second variation of a modular multilevel converter comprising cells; and

[0042] Fig. 15 schematically shows a third variation of a modular multilevel converter comprising cells.

DETAILED DESCRIPTION OF EMBODIMENTS

[0043] Hereinafter, the principle and spirit of the present disclosure will be described with reference to the illustrative embodiments. It should be understood that all these embodiments are given merely for the skilled in the art to better understand and further practice the present disclosure, but not for limiting the scope of the present disclosure. For example, features illustrated or described as part of one embodiment may be used with another embodiment to yield still a further embodiment. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions should be made to achieve the developers’ specific goals, such as compliance with system- related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time- consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

[0044] The disclosed subject matter will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the description with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the disclosed subject matter. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. [0045] The present subject matter provides a solution which intends to dampen the short- circuit current running through a crowbar branch of a cell, which can be used to prevent explosions when the crowbar branch is activated to discharge and bypass a cell capacitor.

[0046] Fig. 1 shows a converter 10 comprising cells where the above-mentioned peak short-circuit current limitation may be implemented.

[0047] The converter 10 is a voltage source converter (VSC) and may be realized as a modular multilevel converter (MMC). In the example shown in fig. 1, the converter 10 is an MMC. The MMC is in this case made up of a number of parallel phase legs, here three, where each phase leg comprises a number of cascaded cells 12. The midpoint of a phase leg may then form an AC output of the converter 10. As there are three phase legs, the converter may be connected to a three-phase AC system. A cell 12 may in turn be a full-bridge cell or a half bridge cell. A cell therefore comprises energy storage elements and switches configured to insert the energy storage elements with one out of a maximum of two different polarities in a phase leg or to bypass the energy storage element. The energy storage elements may with advantage be capacitors. Thereby each cell also has a cell voltage. This cell voltage is thus inserted into the phase leg or bypassed in order to form a waveshape.

[0048] The shown converter 10 is merely an example of an MMC where cells may be used. It is also possible that the phase legs are wye or delta-connected, where the junction between two phase legs is connected to a corresponding phase of a three-phase AC system.

[0049] In one variation of the converter 10, each phase leg is made up full-bridge cells. In another variation each phase leg is made up of half-bridge cells. In other variations each phase leg may be made up of a mixture of full-bridge and half-bridge cells.

[0050] Aspects of the invention are directed towards providing a crowbar branch comprising an element with a resistance used to dampen fault currents.

[0051] Fig. 2 schematically shows a cell 12 with a half-bridge realization. The cell may be used as a half-bridge cell of an MMC.

[0052] The cell 12 comprises an energy storage branch ESB comprising a capacitor Cc and having a first end and a second end. The cell 12 also comprises a first series-connection (series circuit) of two switches T1 and T2, where the first series connection of two switches is connected in parallel with the energy storage branch ESB. In the present example the energy storage branch ESB furthermore comprises an inductance Lc connected in series with the capacitor CC, which inductance is the stray inductance of the energy storage branch ESB

[0053] A first end of the first series circuit (series connection) of the two switches T1 and T2 is connected to the first end of the energy storage branch ESB via a busbar having a busbar inductance L_B.

[0054] A series-connection of two switches T1 and T2 may be provided, perhaps together with the energy storage branch ESB, in a semiconductor package or a module, such as a half bridge module, for instance in the form of a press pack or bond-wire based module like a so- called LinPak or Hipak module. In this case terminals of the series-circuit of two switches that are used for connection to the energy storage branch may be provided inside the semiconductor package or module.

[0055] The cell in fig. 2 is realized as a half bridge, comprising two switches T1 and T2 in a series connection. Generally, the switches may comprise a semiconductor like a transistor and an anti-parallel diode. The switches used in the series-circuits may thus comprise semiconductors. The semiconductors may be, for example, Silicone Carbide Metal Oxide Semiconductor Field Effect Transistors (SiC MOSFETs), Insulated Gate Bipolar Transistors (IGBTs) or Bi-Mode Insulated Gate Transistor (BIGT). The kind of semiconductor is not limiting and future semiconductors, suitable to cover the needs of a power converter or cell according to the present application, may be also included.

[0056] The diode is switched antiparallel to the conducting direction of the transistor. The diode may be an integral part of the transistor (power transistor).

[0057] A transistor only allows for a current flow in one direction, in this case from collector to emitter. The diode as a semiconductor allows for a current flow in a direction opposite to the direction which the transistors T1 and T2 allow. Preferably the diode (“free wheeling diode”) is adapted to bear the same power (or current) as the transistor.

[0058] The midpoint between the switches of the first series circuit of two switches T1 and T2 forms a first cell connection terminal CC1. In the half-bridge cell structure in fig. 2, the second end of the first series-circuit of two switches T1 and T2 forms a second cell connection terminal CC2. As an alternative, the first end of the first series-circuit of two switches T1 and T2 may instead form the second cell connection terminal CC2. [0059] The switches T1 and T2 are activated through corresponding gate drivers GD or gate control units that apply voltages capable of changing switch states.

[0060] The cell additionally comprises a crowbar branch CBB connected in parallel with the energy storage branch ESB. As can be seen in fig. 2, the crowbar branch comprises a first switching element SCB1 acting as a DC crowbar connected in series with an inductance LT, which is the stray inductance of the crowbar branch. The crowbar branch additionally comprises a resistive element RD connected in series with the first switching element SCI and set to improve damping in the crowbar branch. The shown crowbar branch is a solid crowbar branch only comprising two connection points for connection in parallel with the energy storage branch. As will be shown later other types of crowbar branches exist. The crowbar SCB1 is adapted to be externally actuated to short-circuit the energy storage branch ESB and then especially to short-circuit the capacitor Cc based on the detection of a short-circuit fault. The switching element SCB1 forming the crowbar may be a semiconductor, preferably a thyristor, which can be activated by a switching signal thereby short-circuiting the capacitor Cc.

[0061] As can be seen in fig. 2, the cell 14 also includes a bypass-switch BPS. The bypass switch BPS is connected between the two cell connection terminals CC1 and CC2. The bypass switch BPS may be realized as a mechanical switch. However, it too may be realized as an electronic switch, such as a thyristor. The bypass switch BPS may be controlled to bypass the whole cell after a crowbar has been used to discharge the capacitor. Through the addition of the bypass switch BPS, the converter in which the cell is provided may be possible to be continued to be used. It should here be realized that the bypass switch is optional and that it can be omitted in many cases.

[0062] Fig. 3 schematically shows a cell 12 with a full-bridge realization. The cell may be used as a full-bridge cell of an MMC. The difference from the previously shown half-bridge cell is that there is a second series connection of two switches T3 and T4 connected in parallel with the energy storage branch ESB. The first end of the first series-connection of two switches is in this case connected to the first end of the energy storage branch ESB via a first busbar having a busbar inductance LBA, while a first end of the second series circuit (series connection) of two switches T3 and T4 is connected to the first end of the energy storage branch ESB via a second busbar having a busbar inductance LBB. In this case the midpoint of the first series connection of two switches T1 and T2 forms the first cell connection terminal CC1 and the midpoint of the second series connection of two switches T3 and T4 forms the second cell connection terminal CC2.

[0063] The crowbar is provided for short-circuiting the cell capacitor in case of cell faults. The faults that may be problematic will now be described in some further detail, with reference being made to fig. 4, where the optional bypass switch as well as the crowbar branch have been omitted. In this case example, the second switch is failed and is in a continuous ON state (short-circuit mode) .

[0064] If the second switch T2 is failed in a short circuit mode, i-e the collector and emitter are in constant electrical contact with each other, the turning ON of the first switch Tl, will lead to the cell capacitor Cc being short-circuited and discharging. A short circuit current will then run through the two switches Tl and T2. This in turn triggers the activation of the crowbar SCR1 (not shown). The short-circuit current is resonant and therefore has a first positive half period and a second negative half-period and keeps alternating until the cell capacitor is discharged.

[0065] The current in the first positive half-period for a cell without damping resistive element in the bypass branch and with the busbar inductance divided into two parts LBT and LBD is shown in fig. 5, where also the switching element of the crowbar branch is shown as a resistance RT and the two switches Tl and T2 are shown as a current source ITI+T2. The current in the second negative half period is shown in fig. 6, where now it passes the anti parallel diodes of the switches Tl and T2 and therefore the pair of switches is represented by a resistance R_D of these diodes.

[0066] During positive half-cycle of resonance, the current through the pair of switches Tl and T2 is clamped to a de-saturation current ITI+T2 of the transistors Tl and/or T2 until they may fail. The first switching element SCB1 of the crowbar branch may go into a Short-Circuit Failure Mode (SCFM) taking the peak capacitor discharge current. During the negative half cycle of resonance, the energy stored in the inductance LT of the crowbar branch and the part of the busbar inductance LBT connecting the crowbar branch to the energy storage branch will, due to the large peak current caused by the capacitor discharge, push a large reverse current via the diodes of the switches Tl and T2. This effect is pronounced since the inductance LT in the crowbar branch (due to the clamp circuit) is larger than the inductance Lc in the energy storage branch. If the current flowing via diodes in the negative half cycle of resonance is higher, arcing and explosion may occur. [0067] When the transistor T1 is turned-on, it thus goes into de-saturation, and does not fail immediately. The switching element of the crowbar branch is triggered shortly after transistor turn-on. The current in the switching element of the crowbar branch reaches a high peak value. The capacitor voltage is reversed during the negative half cycle of resonance. A negative voltage on capacitor leads to a high current through the diodes of the transistors. Such large current leads to arcing and explosion of the cell. In case the transistor is an IGBT, the IGBT gate oxide layer may also fail into short-circuit when the diode current becomes high, i.e., due to severe heating of the nearby IGBT chips.

[0068] The damping z of the resonance circuit depends on the capacitance Cc, the loop inductance L formed as the sum of L_T, L_BT and Lc and the resistance R of the crowbar branch according to:

This damping is typically too low and therefore needs to be improved. Aspects of the invention are concerned with increasing this damping through increasing the resistance in the crowbar branch.

[0069] Additionally, the introduced resistance needs to be able to handle the energy of the cell capacitor in a short time duration, such as within ten to two hundred microseconds after the activation of the crowbar.

[0070] Aspects of the invention are concerned with improving such damping.

[0071] A first aspect of the invention addresses the above-mentioned problem through using a first type of resistive element in the crowbar branch, which first element has a non linear resistance. This first type of resistive element R_DA can be seen in fig. 7, where the cell also comprises the bypass switch BPS. The element R_DA may be a surge arrester, which has the effect to introduce an extra non-linear resistance when the DC crowbar SCB1 is triggered. This non-linear resistance has the effect of clamping the voltage across itself to a clamping voltage. As the crowbar element SCB1 blocks the DC voltage during normal operation, the arrestor may have a lower voltage rating than the cell capacitor. It may also be able to handle high energy levels. The clamping voltage is thus lower than the voltage rating of the capacitor Cc and with advantage in the range ten to twenty times lower than the voltage rating of the capacitor Cc. Put differently, the voltage rating of the capacitor Cc is typically ten to twenty times higher than the clamping voltage of the surge arrestor RDA. The surge arrestor is also dimensioned for being able to dissipate the energy of the capacitor Cc in a short time duration. If the clamping voltage is Vclamp, the cell capacitance is C and the voltage across the capacitor Cc is V, then V = n* Vclamp, where 10 < n < 20 and the surge arrester may need to be able to dissipate the energy of 0.5*C*V² within 10 - 200 ps from the activation of the crowbar SCB1.

[0072] Fig. 8 shows a curve outlining the non-linear resistance characteristic of the non linear resistive element, often termed the voltage current characteristics. The voltage current characteristics may in essence follow the formula: 1,2, 3.

As an example, the reference current Iref = 1 kA, and the reference voltage Vref = 270 V, with

[0073] If the fault current is significantly larger than the reference current such as 100 - 200 times larger, which in the example would be 100 - 200 kA, then the arrestor voltage would be clamped to the clamping voltage Vclamp that is slightly higher than the reference voltage Vr_ef..

[0074] Through the introduction of the surge arrestor, the majority of the energy of the capacitor is dissipated therein and the destruction of the crowbar and cell is avoided. After the energy has been dissipated, it is then possible to activate the bypass switch BPS. The converter can thereby be continued to be used with the cell bypassed. Then the surge arrester has no influence on the losses in this continued use.

[0075] Through this type of operation the peak current through the crowbar branch may be significantly reduced and thereby it is possible to avoid cell explosion. It is thereby possible to realize the cell with simplified or even without Presspack modules and explosion boxes. It also opens up the possibility of using DC crowbar protection for SiC based Power Electronic Building Blocks (PEBBs). [0076] As an alternative to a non-linear resistive element, it is possible to use a linear resistive element in the crowbar branch. One such element RDB is schematically shown in fig. 9 in series with the crowbar switch SCBl.In this case the cell is also provided without a bypass branch.

[0077] Another way to increase the damping is thus through introducing an element having a linear resistance in the bypass branch. This is done in order obtain an over damped circuit i.e. in order to avoid a negative cycle of resonance. It is also here important that the element is able to dissipate energy from the cell capacitor in the same way as was discussed above. The damping may as an example be at least 4 times compared without the element. As an example it is possible that a resistance of up to 9 ihW is introduced which as an example may improve the damping z from a value of 0.136 to 0.6.

[0078] As the damping is decided by:

it can be seen that resistance R may be set to a value that is the desired damping z times a constant that depends on the inductance L of the cell and the cell capacitance C. The constant may more particularly depend on the square root of an expression formed as the cell capacitance divided by the cell inductance. The constant may more particularly be set as two divided by the square root of the expression.

[0079] In this way sufficient damping is achieved that avoids destruction of the cell, where the majority of the energy of the capacitor is dissipated in the linear resistance RDB and the destruction of the crowbar and cell is avoided. Furthermore, as the resistance is low, its contribution to the running losses when the cell is bypassed after cell capacitor discharge are continued to be low.

[0080] A first example of a suitable resistor RDBI is shown in fig. 10. The resistor RDBI is a so-called disc/washer resistor. This type has excellent capabilities to handle large peak energies for a short time duration without risk of explosion. The crowbar switch may additionally be provided in a mechanical clamping structure, where clamping elements are provided on both sides of a likewise disc-shaped switching element. Thereby the switching element is mechanically clamped between the two clamping elements. A disc resistor is easily placed in this clamping structure.

[0081] Another type of resistance RDB2 that forms a structure which makes up or is a part of an introduced linear resistive element is schematically shown in fig. 11. The structure is formed through providing a piece of conductor extending along a longitudinal axis AX.

[0082] The piece of conductor may be provided with indentations along the longitudinal axis AX in order to form slots. The indentations may be alternatingly formed or provided on opposite sides of the structure along the axis AX and thereby the conductor structure is meandering. The structure may thereby form bars that are perpendicular to the longitudinal axis AX and interconnected at their edges, where the bars additionally have a bar width BW. The bars are thus separated by slots and the slots are also oriented perpendicular to the longitudinal axis AX and have a slot with SW. An edge that interconnects two such bars separated by a slot may then have an edge width EW, where the edge is an edge that is likewise perpendicular to the axis AX. The slot will then have a depth that is the difference between the conductor plate width and the edge width EW.

[0083] The resistance may be formed in the above-mentioned way only using a first conductor section.

[0084] [0079] [0082] It is additionally possible that the piece of conductor also comprises a second conductor section, where the two conductor sections or conductive plates are placed above each other, i.e. one on top of another separated by an insulator. The two plates may then extend along the longitudinal axis AX. The current would then flow in the top section and return via the bottom section or vice versa. This means that the current would enter and exit the structure at a first end of the structure along the axis AX where the plates are insulated from each other. The two plates would then be electrically joined to each other at the opposite end of the structure along the axis AX in order to create the return path. The plates would thus be in galvanic contact with each other at this opposite end. Two sections of electrical conductor may thus be joined to each other with one placed above the other in order to form a forward current transporting path along the longitudinal axis of the structure and a return current path in the opposite direction.

[0085] With this type of structure, it is as an example possible that the resistance may be increased by a factor of 30 and the inductance may be increased by a factor of 8. For the chosen slot, bar and edge width, the resistance may increase four-fold for one-fold increase in inductance. This ratio can be further increased by choosing a smaller slot width (without exceeding the insulation limits). Further, it is possible to vary the resistance linearly by increasing or decreasing the number of slots. The slots may then be changed in pairs, where such a pair is a pair of slots formed in different sides of the structure. For every additional repeatable slot pair, the resistance may as an example be increased by 1.5 ihW. This feature enhances the scalability of the damping element without a significant increase in cost.

[0086] The conductor structure may be made of Aluminium. However, it is as an alternative possible to realize the conductor structure using a high resistance material such as Nichrome, which is an alloy made of Nickel and Chromium. It is additionally possible that the structure is a part of one of the previously mentioned clamping elements used to mechanically clamp the switching element of the crowbar branch. In this case the structure may extend out from a clamping body of the clamping element. It is additionally possible that also the other clamping element is made of the high-resistance material. This use of Nichrome allows the number of slots in the conductor structure to be reduced for achieving the same resistance as in the Aluminium example. Thereby the increase in inductance can be minimal. The mechanical forces exerted on the meandric slots may be very high during a short-circuit leading to significant deformation. In order to ensure structural integrity, support structures of non-conductive material may be provided on either side of the conductor structure to hold the bars in place.

[0087] As can be seen in fig. 12, which shows a half bridge cell realization comprising a second variation of a crowbar branch, the resistive element R_D element can also be implemented in a split-DC crowbar protection realization. In this realization the crowbar branch comprises a first switching element SCB1 and a first inductance L-n in an upper part of the crowbar branch as well as a second switching element SCB2 and a second inductance L_T2 in a lower part of the crowbar branch. A junction between the upper and lower parts of the crowbar branch is in this case also connected to the midpoint of the first series connection of two switches Tl, T2. It can thereby be seen that a junction between the upper part with the first switching element and a neighbouring part with another switching element is connected to the midpoint of the first series connections of two switches, where the neighbouring part with another switching element in this case is the lower part with the second switching element. A Split-DC crowbar is an improved version of DC crowbar protection concept especially for bond-wire based semiconductor modules. This protection concept does not rely on the functioning of the diode in the switches for the cell bypass. In this case the resistive element R_D is with advantage placed in the upper part of the bypass branch. This has the advantage of achieving low running losses during cell bypass following cell capacitor discharge as the element RD is outside of the bypass path. As can be seen it is in this case therefore also possible to omit the bypass switch. It should be realized that in case the first end of the first series-connection of two switches forms the second cell connection terminal, then the resistive element would be placed in the lower part of the bypass branch instead.

[0088] Another example of a split DC crowbar branch is shown in fig. 13, which shows a full bridge cell realization comprising a third variation of a crowbar branch. As compared with the split DC crowbar branch of the half-bridge cell, the crowbar branch in this case comprises a third switching element SCB3 connected in the branch between the first and second switches SCB1 and SCB2. A junction between the upper part of the branch comprising the first switching element SCB1 and an intermediate part of the branch comprising the third switching element SCB3 is in this case connected to the midpoint of the first series connection of two switches T1 and T2, while a junction between the intermediate part of the branch comprising the third switching element SCB3 and the lower part of the branch comprising the second switching element SCB2 is connected to a midpoint between the second series connection of switches T3 and T4. It can thereby be seen that also here a junction between the upper part with the first switching element and a neighbouring part with another switching element is connected to the midpoint of the first series connections of two switches, where the neighbouring part with another switching element in this case is the intermediate part with the third switching element. The third/second switching element SCB3, SCB2 ensures a reliable cell bypass in the event of diode failure. Just as the cell in fig. 12, the resistive element is placed in the upper part of the crowbar branch. It is thereby also placed outside any bypass branch used after cell capacitor discharge. It is also possible to place the resistive element in the lower branch.

[0089] The resistance introduced in the crowbar branch thus reduces the negative cycle of the fault current i.e., discharges the cell capacitor in the positive half-cycle and thereby the crowbar as well as the cell is protected. Various solutions have additionally been presented in order to limit the running losses during cell bypass after cell discharge.

[0090] The converter may be a power converter as e.g. used in HVDC power transmission, FACTS systems or static frequency converter systems. [0091] As was mentioned above the MMC converter in fig. 1 is only one converter type variation. Fig. 14 shows a second variation with wye-connected phase legs and fig 15 shows a third variation with delta-connected phase legs.

[0092] Generally, the present invention discloses the provision of a resistive element in a crowbar branch of a cell.

[0093] Further, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It may be intended that the description includes such modifications and variations.

[0094] While the foregoing is directed to embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

1. A cell (12) for a voltage source converter (10), the cell (12) comprising: a first cell connection terminal (CC1), a second cell connection terminal (CC2), a first series connection of two switches (Tl, T2) the midpoint of which forms the first cell connection terminal (CC1), an energy storage branch (ESB) comprising a capacitor (Cc); and a crowbar branch connected in parallel with the energy storage branch (ESB), the crowbar branch comprising a first switching element (SCBi) adapted to be activated to short- circuit the capacitor (Cc) of the energy storage branch (ESB) based on the detection of a short- circuit fault and a resistive element (R_D) connected in series with the first switching element and set to improve damping in the crowbar branch.

2. The cell (12) according to claim 1, wherein the resistive element is configured to dissipate the energy of the capacitor in a short time duration, such as within 10 - 200 ps from the activation of the first switching element.

3. The cell (12) according to any previous claim, further comprising a bypass switch (BPS) connected between the cell connection terminals (CC1, CC2).

4. The cell (12) according to any previous claim, wherein the element is an element with non-linear resistance.

5. The cell (12) according to claim 4, wherein the element with non-linear resistance is a surge arrestor (R_DA).

6. The cell (12) according to claim 5, wherein the surge arrestor (R_DA) has a clamping voltage (Vclamp) that is lower than the rated voltage of the capacitor (Cc), where the rated voltage of the capacitor (Cc) with advantage is in the range 10 - 20 times higher than the clamping voltage of the resistive element. -

7. The cell (12) according to any of claims 1 - 3, wherein the resistive element is an element with linear resistance (R_DB).

8. The cell (12) according to claim 7, wherein the resistive element is a disc resistor (RDBI).

9. The cell (12) according to claim 7, wherein the resistive element (RDB2) comprises a piece of conductor of the crowbar branch stretching along a longitudinal axis (AX), said piece of conductor comprising indentations alternatingly formed on opposite sides along the longitudinal axis (AX) for forming a meandering structure with bars interconnected at their edges and separated by slots.

10. The cell according to claim 9, wherein the piece of conductor comprises a first and second section, where one section is placed on top of the other, said sections comprising said indentations.

11. The cell (12) according to claim 9 or 10, wherein said piece of conductor is formed of a high-resistive material such as Nichrome.

12. The cell (12) according to any of claims 9 - 11, wherein said piece of conductor is part of a clamping element used to mechanically clamp the first switching element.

13. The cell (12) according to any previous claim, the crowbar branch further comprising a second switching element (SCB2), where the first switching element (SCB1) is placed in an upper part of the crowbar branch and the second switching element (SCB2) is placed in a lower part of the crowbar branch, where a junction between the upper part and a neighbouring part with switching element (SCB2; SCB3) is connected to a midpoint of the first series connections of two switches (Tl, T2), with the first end of the series connection of two switches (Tl, T2) forming the second cell connection terminal (CC2) and the resistive element (RD) is connected in the lower part of the crowbar branch.

14. The cell (12) according to any of claims 1 - 12, the crowbar branch further comprising a second switching element (SCB2), where the first switching element (SCB1) is placed in an upper part of the crowbar branch and the second switching element (SCB2) is placed in a lower part of the crowbar branch, where a junction between the upper part and a neighbouring part with switching element (SCB2; SCB3) is connected to a midpoint of the first series connections of two switches (Tl, T2), with the second end of the series connection of two switches (Tl, T2) forming the second cell connection terminal (CC2) and the resistive element (R_D) is connected in the upper part of the crowbar branch.

15. The cell (12) according to any of claims 1 - 14, further comprising a second series connection of two switches (T3, T4) connected in parallel with the energy storage branch (ESB), wherein the midpoint of the second series connection of two switches (T3, T4) forms the second cell connection terminal (CC2).

16. A modular multilevel converter (10) comprising cells, wherein at least one cell is a cell (12) according to any previous claim.