Unit comprising a crowbar branch with a resistive element

Abstract

A cell (12) for a voltage source converter, comprising: a first cell connection terminal (CCI); a second cell connection terminal (CC2); a first series connection of two switches (T1, T2), a midpoint of the first series connection of the two switches forming the first cell connection terminal (CCI); an energy storage branch (ESB) comprising a capacitor (C C ); 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 detection of a short circuit fault, and a resistive element (R D ) connected in series with the first switching element and arranged to improve damping in the crowbar branch.

Description

Technical Field

[0001] This application relates to the field of power converters, particularly voltage source converters (VSCs). More specifically, this application relates to a unit that can be used in a modular multilevel converter (MMC). Background Technology

[0002] VSCs such as MMCs include switches. These switches can be arranged in so-called half-bridge or full-bridge units, where a unit may include two switches connected in series to a capacitor via a DC bus. Semiconductors are used as switches, particularly transistors such as IGBTs (Insulated Gate Bipolar Transistors). MMCs have become a popular choice for grid-connected converters due to their enhanced modularity, scalability, excellent harmonic performance, and reduced losses.

[0003] A faulty semiconductor module (e.g., an IGBT module) in an MMC cell can pose a high risk of large-scale damage. In many cases, if an IGBT in a half-bridge or full-bridge circuit fails, adjacent IGBTs may flush out fault currents generated by the DC capacitors (multiple). However, there are situations where such fault currents may not be preventable.

[0004] Because the stray inductance between the IGBT and the DC capacitor may be intentionally very low (in order to achieve acceptable converter performance / low transient overvoltage at the IGBT collector-emitter terminals / low IGBT switching losses), and the energy stored in the unit DC capacitor may be high, the expected fault current may be high, such as in the range of several hundred kA.

[0005] For MMCs, large cell capacitors are typically required to buffer the ripple energy generated by actual / reactive power transmission. During a short-circuit fault in the semiconductor, the energy in the cell capacitor can discharge through the semiconductor, potentially causing the cell to explode or be severely damaged.

[0006] Without any countermeasures, the bonding wires within the IGBT module (which connect the semiconductor chip to the metal contacts) will immediately fail, causing an electric arc. This arc will be fed by the high energy stored in the DC capacitor. This leads to an explosion of the IGBT module, resulting in massive destruction. The more energy stored in the DC capacitor, the more difficult it becomes to handle this problem.

[0007] One approach to this problem is to use a so-called DC crowbar to release most of the energy from the individual capacitors into crowbar components (such as thyristors), thus limiting the energy release to the semiconductor switches. However, in this case, the fault current may be too large for the crowbar to handle.

[0008] In view of the above, there is a need to improve the method of handling short-circuit current in units that include DC crowbars.

[0009] Therefore, an improved crowbar is needed. Summary of the Invention

[0010] In a first aspect, the present invention may disclose a unit for a voltage source converter, wherein the unit comprises: a first unit connection terminal; a second unit connection terminal; a first series connection of two switches, the midpoint of the first series connection of the two switches forming the first unit connection terminal; an energy storage branch including a capacitor; and a crowbar branch connected in parallel with the energy storage branch, wherein the crowbar branch includes a first switching element adapted to be activated by detection based on a short-circuit fault to short-circuit the capacitor of the energy storage branch, and a resistive element connected in series with the first switching element and configured to improve damping in the crowbar branch.

[0011] Furthermore, the resistive element is configured to dissipate the energy of the capacitor over a short period of time, such as within 10 to 200 μs from the activation of the first switching element. If the capacitance of the capacitor is C and the voltage across the capacitor is V, the resistive element may dissipate 0.5 * C * V within 10 to 200 μs from the activation of the first switching element. 2 Energy.

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

[0013] In one implementation, the element is a component with nonlinear resistance, such as a surge arrester. The surge arrester may additionally have a clamping voltage lower than the capacitor's rated voltage. The clamping voltage of the resistive element can be in the range of 10 to 20 times lower than the capacitor's rated voltage. In other words, the capacitor's rated voltage can be in the range of 10 to 20 times higher than the resistive element's clamping voltage.

[0014] Alternatively, the resistive element can be an element with a linear resistance R.

[0015] The resistance R can be set as the desired damping ζ multiplied by a constant, which depends on the inductance L and capacitance C of the unit cell. More specifically, this constant can depend on the square root of an expression formed by dividing the unit capacitance by the unit inductance. More specifically, this constant can be set as 2 divided by the square root of the expression.

[0016] Linear resistance can be achieved using a valve valve resistor.

[0017] As an alternative, linear resistance can be achieved using conductor sheets of a crowbar branch extending along a longitudinal axis, wherein the conductor sheets include recesses alternately formed on opposite sides along the longitudinal axis to form a tortuous structure with bars interconnected at their edges and separated by grooves.

[0018] The conductor sheet may additionally include a first segment and a second segment, with one segment positioned on top of the other, and recesses provided on opposite sides of these segments. The two segments may additionally be separated by an insulator. The two segments may extend along a longitudinal axis. Current can enter the structure through one segment and exit the structure through the other. Current can enter and exit the structure at the first end, where the segments are insulated from each other, and the two segments may be electrically connected to each other at opposite ends to create a return path. The number of recesses can be selected to obtain the desired resistance.

[0019] The conductor sheet mentioned above can be formed from conventional conductor materials (such as aluminum). As an alternative, the conductor section can be formed from a high-resistivity material (such as a nickel-chromium alloy).

[0020] Additionally, the conductor sheet may be part of a clamping element for mechanically holding the first switching element. The first switching element may, for example, be mechanically clamped between two clamping elements. In this case, a tortuous structure may extend from the clamping body of the clamping element. Additionally, the other clamping element may also be made of a high-resistivity material.

[0021] The crowbar branch can be a separate crowbar branch. For this reason, the crowbar branch can also include a second switching element, wherein the first switching element is placed in the upper portion of the crowbar branch and the second switching element is placed in the lower portion of the crowbar branch. The junction between the upper portion and the adjacent portion having the switching element is connected to the midpoint of a first series connection of the two switches, and a resistive element can be connected in either the upper or lower portion of the crowbar branch.

[0022] The unit can be a half-bridge unit, wherein the midpoint of the first series connection of the two switches forms the first unit connection terminal, and the first end of the first series connection of the two switches or the second end of the first series connection of the switches forms the second unit connection terminal.

[0023] In the case of a split crowbar branch in a half-bridge unit, the adjacent portion having a switching element can be the lower portion having a second switching element. When a second unit connection terminal is formed at the second end of the first series connection of the switches, a resistive element is also connected in the upper portion of the crowbar branch; and when a second unit connection terminal is formed at the first end of the first series connection of the two switches, a resistive element is connected in the lower portion of the crowbar branch.

[0024] As an alternative, the unit can be a full-bridge unit, which further includes 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 the two switches forms a first unit connection terminal, and the midpoint of the second series connection of the two switches forms a second unit connection terminal.

[0025] In the case of a split crowbar branch in a full-bridge unit, the crowbar branch may include a third switching element connected in the intermediate section between the upper and lower sections. In this case, the adjacent section with the switching element is the intermediate section with the third switching element. In this case, the connection between the intermediate section and the lower section may also be connected to the midpoint of a second series connection of two switches.

[0026] Other aspects involve modular multilevel converters including units, wherein at least one unit is a component of such units. Attached Figure Description

[0027] The embodiments disclosed herein are presented in an exemplary sense, and their advantages will be described in more detail below with reference to the accompanying drawings, wherein:

[0028] Figure 1 A first variant of a modular multilevel converter including units is shown;

[0029] Figure 2 A half-bridge unit including a first variant with a crowbar branch having damping resistance is shown;

[0030] Figure 3 A full-bridge unit including a first variant with a crowbar branch having damping resistance is shown;

[0031] Figure 4 The diagram illustrates the fault that occurred in the unit;

[0032] Figure 5 The fault current in a cell without damping resistors during the positive half-cycle of resonance is shown.

[0033] Figure 6 The fault current in the cell during the negative half-cycle of resonance is shown;

[0034] Figure 7 A half-bridge unit with a first crowbar branch implementation is shown, wherein the damping resistor is implemented by a nonlinear resistive element;

[0035] Figure 8 The curves depicting the nonlinear resistance characteristics of the nonlinear resistive element are shown;

[0036] Figure 9A half-bridge unit with a first crowbar branch variant is shown, wherein the damping resistor is implemented as a linear resistor;

[0037] Figure 10 This illustrates a first implementation of a linear damping resistor;

[0038] Figure 11 A second implementation of the linear damping resistor is shown;

[0039] Figure 12 A second variant of the half-bridge unit implementation, including a crowbar branch with damping resistors, is shown.

[0040] Figure 13 A third variant of the full-bridge unit implementation, including a crowbar branch with damping resistors, is shown.

[0041] Figure 14 A second variant of the modular multilevel converter, including units, is schematically shown; and

[0042] Figure 15 A third variant of the modular multilevel converter, including units, is schematically shown. Detailed Implementation

[0043] In the following description, the concept and spirit of this disclosure will be described with reference to illustrative embodiments. It should be understood that all these embodiments are given merely to enable those skilled in the art to better understand and further practice this disclosure, and are not intended to limit the scope of this disclosure. For example, features shown or described as part of an embodiment may be used with another embodiment to produce yet another embodiment. For clarity, not all features of an actual implementation are described in this specification. Of course, it should be understood that in the development of any such actual embodiment, many implementation-specific decisions will be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from implementation to implementation. Furthermore, it should be understood that such development work can be complex and time-consuming, but it remains a routine task for those skilled in the art who have the benefits of this disclosure.

[0044] The disclosed subject matter will now be described with reference to the accompanying drawings. Various structures, systems, and devices are schematically depicted in the drawings for illustrative purposes only, so as not to obscure details well known to those skilled in the art. However, the drawings are included as illustrative examples used to describe and illustrate the disclosed subject matter. The words and phrases used herein should be understood and interpreted as having meanings consistent with those understood by one of skill in the art. Specific definitions of terms or phrases, i.e., definitions different from the common and customary meanings understood by those skilled in the art, are not intended to be implied by the consistent use of such terms or phrases herein. To a certain extent, terms or phrases are intended to have a specific meaning, i.e., a meaning different from that understood by one of skill in the art, which will be clearly stated in the specification in a manner that directly and explicitly provides a specific definition for the term or phrase.

[0045] This topic provides a solution designed to suppress short-circuit current through the crowbar branch of a cell, which can prevent explosion when the crowbar branch is activated to discharge and bypass the cell capacitor.

[0046] Figure 1 A converter 10 including a unit is shown, in which the aforementioned peak short-circuit current limiting can be implemented.

[0047] Converter 10 is a voltage source converter (VSC) and can be implemented as a modular multilevel converter (MMC). Figure 1 In the example shown, converter 10 is an MMC. In this case, the MMC consists of multiple parallel phase arms, here three, where each phase arm includes multiple cascaded units 12. The midpoint of the phase arm can form the AC output of converter 10. Due to the presence of three phase arms, the converter can be connected to a three-phase AC system. Unit 12 can also be a full-bridge unit or a half-bridge unit. Thus, the unit includes an energy storage element and a switch configured to insert or bypass the energy storage element having one of the two largest different polarities into the phase arm. The energy storage element can advantageously be a capacitor. Thus, each unit also has a unit voltage. This unit voltage is thus inserted into the phase arm or bypassed to form a waveform.

[0048] The converter 10 shown is merely an example of an MMC in which units can be used. The phase arms can also be Y-shaped or delta-connected, where the joint between two phase arms is connected to the corresponding phase of a three-phase AC system.

[0049] In one variant of converter 10, each phase arm is composed of a full-bridge unit. In another variant, each phase arm is composed of a half-bridge unit. In other variants, each phase arm may be composed of a mixture of full-bridge and half-bridge units.

[0050] Aspects of the present invention aim to provide a crowbar branch including an element having a resistor for suppressing fault current.

[0051] Figure 2 Unit 12, which has a half-bridge implementation, is schematically shown. This unit can be used as a half-bridge unit for MMC.

[0052] Unit 12 includes an energy storage branch ESB, which includes a capacitor C. C It also has a first terminal and a second terminal. Unit 12 further includes a first series connection (series circuit) of two switches T1 and T2, wherein the first series connection of the two switches is connected in parallel with the energy storage branch ESB. In this example, the energy storage branch ESB also includes an inductor LC connected in series with a capacitor CC, which is the stray inductance of the energy storage branch ESB.

[0053] The first terminal of the first series circuit (connected in series) of two switches T1 and T2 is connected via a bus inductance L B The busbar is connected to the first end of the energy storage branch ESB.

[0054] Two switches T1 and T2 may be connected in series within a semiconductor package or module, possibly together with the energy storage branch ESB, such as a half-bridge module, for example in the form of a press-fit or bonding-wire module, such as a so-called LinPak or Hipak module. In this case, the terminals of the series circuit for connecting the two switches to the energy storage branch can be located inside the semiconductor package or module.

[0055] Figure 2 The unit in the circuit is implemented as a half-bridge, comprising two switches T1 and T2 connected in series. Typically, the switches can include semiconductors, such as transistors and anti-parallel diodes. Therefore, the switches used in the series circuit can include semiconductors. Semiconductors can be, for example, silicon carbide metal-oxide-semiconductor field-effect transistors (SiC MOSFETs), insulated-gate bipolar transistors (IGBTs), or two-mode insulated-gate transistors (BIGTs). The type of semiconductor is not limited, and future semiconductors suitable for meeting the needs of the power converter or unit according to this application may also be included.

[0056] The diode is switched to be connected in antiparallel to the conduction direction of the transistor. The diode can be an integral part of the transistor (power transistor).

[0057] A transistor allows current to flow in only one direction, in this case from the collector to the emitter. A diode, being a semiconductor, allows current to flow in the opposite direction to that allowed by transistors T1 and T2. Preferably, the diode (freewheeling diode) is adapted to carry the same power (or current) as the transistor.

[0058] The midpoint between the switches in the first series circuit of two switches T1 and T2 forms the first unit connection terminal CC1. Figure 2 In the half-bridge unit structure, the second end of the first series circuit of the two switches T1 and T2 forms the second unit connection terminal CC2. As an alternative, the first end of the first series circuit of the two switches T1 and T2 can alternatively form the second unit connection terminal CC2.

[0059] Switches T1 and T2 are activated by applying a voltage to a corresponding gate driver GD or gate control unit that can change the switching state.

[0060] The unit also additionally includes a crowbar branch CBB connected in parallel with the energy storage branch ESB. (As from...) Figure 2 As can be seen from the diagram, the crowbar branch includes a first switching element SCB1 that acts as a DC crowbar, which is connected to the inductor L. T The inductor is a stray inductance of the crowbar branch, connected in series. The crowbar branch additionally includes a resistive element RD connected in series with the first switching element SC1 and configured to improve damping in the crowbar branch. The crowbar branch shown is a solid crowbar branch, which includes only two connection points for parallel connection with the energy storage branch. Other types of crowbar branches exist, as will be shown later. The crowbar SCB1 is adapted to be externally energized to detect a short-circuit fault in the energy storage branch ESB, and then specifically short-circuit the capacitor C. C The switching element SCB1 forming the crowbar can be a semiconductor (preferably a thyristor), which can be activated by a switching signal, thereby short-circuiting capacitor C. C .

[0061] like Figure 2 As can be seen, unit 14 also includes a bypass switch BPS. The bypass switch BPS is connected between the two unit connection terminals CC1 and CC2. The bypass switch BPS can be implemented as a mechanical switch. However, it can also be implemented as an electronic switch, such as a thyristor. The bypass switch BPS can be controlled to bypass the entire unit after the capacitor has been discharged using a crowbar. By adding the bypass switch BPS, the converter in which the unit is located can continue to be used. It should be understood here that the bypass switch is optional and can be omitted in many cases.

[0062] Figure 3 Unit 12, implemented as a full-bridge, is schematically shown. This unit can be used as a full-bridge unit for MMC. Unlike the previously shown half-bridge unit, it features a second series connection of two switches T3 and T4 connected in parallel with the energy storage branch ESB. In this case, the first terminal of the first series connection of the two switches is connected via a bus inductance L.BA The first busbar is connected to the first terminal of the energy storage branch ESB, while the first terminal of the second series circuit (connected in series) of the two switches T3 and T4 is connected via a busbar inductance L. BB The second busbar is connected to the first end of the energy storage branch ESB. In this case, the midpoint of the first series connection of the two switches T1 and T2 forms the first unit connection terminal CC1, and the midpoint of the second series connection of the two switches T3 and T4 forms the second unit connection terminal CC2.

[0063] A crowbar is provided to short-circuit the cell capacitor in the event of a cell failure. Reference will now be made to... Figure 4 The potential fault is described in more detail, omitting the optional bypass switch and crowbar branch. In this case, the second switch fails and is in a continuously conducting state (short-circuit mode).

[0064] If the second switch T2 fails in short-circuit mode, i.e., the collector and emitter remain in electrical contact with each other, the conduction of the first switch T1 will cause the unit capacitor C to fail. C Short-circuit and discharge. The short-circuit current will pass through two switches T1 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-cycle and a second negative half-cycle, and continues to alternate until the unit capacitor discharges.

[0065] Figure 5 The diagram shows that there is no damping resistor in the bypass branch and the bus inductance is divided into two parts L. BT and L BD The current in the first positive half-cycle of the unit, where the switching element of the crowbar branch is also shown as a resistor R. T And the two switches T1 and T2 are shown as current sources I. T1+T2 . Figure 6 The diagram shows the current during the second negative half-cycle, where it now flows through the anti-parallel diodes of switches T1 and T2, and thus this pair of switches is controlled by the resistance R of these diodes. D express.

[0066] During the positive half-cycle of resonance, the current through the switch pair T1 and T2 is clamped to the desaturation current I of transistors T1 and / or T2. T1+T2 Until they may fail. The first switching element SCB1 in the crowbar branch may enter a short-circuit fault mode (SCFM) employing a peak capacitor discharge current. During the negative half-cycle of resonance, due to the large peak current caused by capacitor discharge, the inductance L stored in the crowbar branch... T and the bus inductance L that connects the crowbar branch to the energy storage branch BTThe energy in this section will drive a large reverse current through the diodes of switches T1 and T2. This effect is significant because of the inductance L in the crowbar branch. T (Due to the clamping circuit) the inductance L in the energy storage branch is greater. C If the current flowing through the diode is high during the negative half-cycle of resonance, an electric arc and explosion may occur.

[0067] When transistor T1 is turned on, it enters a desaturation state and does not immediately fail. Shortly after the transistor turns on, the switching element in the crowbar branch is triggered. The current in the switching element in the crowbar branch reaches its peak value. The capacitor voltage is reversed during the negative half-cycle of resonance. The negative voltage on the capacitor causes a high current through the diode of the transistor. Such a large current causes arcing and explosion of the cell. In the case of an IGBT, when the diode current becomes high, the IGBT gate oxide layer may also fail and short-circuit, i.e., due to severe heating of the nearby IGBT chip.

[0068] In the following equation, the damping ζ of the resonant circuit depends on the capacitance C. C Forming as L T L BT and L C The sum of the loop inductance L and the resistance R of the crowbar branch:

[0069]

[0070] This damping is typically too low and therefore requires improvement. Aspects of the present invention relate to increasing this damping by increasing the resistance in the crowbar branch.

[0071] Additionally, the introduced resistor needs to be able to handle the energy of the unit capacitor for a short period of time, such as within 10 to 200 microseconds after the crowbar is activated.

[0072] Various aspects of the present invention relate to improving this damping.

[0073] A first aspect of the invention solves the above-mentioned problem by using a first type of resistive element in the crowbar branch, the first element having a non-linear resistance. The first type of resistive element R... DA It is possible Figure 7 As seen in the image, this unit also includes a bypass switch (BPS). Component R DA It can be a surge arrester that introduces an additional non-linear resistance when the DC crowbar SCB1 is triggered. This non-linear resistance clamps the voltage across itself to a clamping voltage. Because the crowbar element SCB1 blocks the DC voltage during normal operation, the arrester can have a lower rated voltage than the unit capacitor. It can also handle high energy levels. Therefore, the clamping voltage is lower than that of the capacitor C.C The rated voltage, and advantageously in the capacitor C C The rated voltage is ten to twenty times lower. In other words, capacitor C C The rated voltage of the surge arrester R is usually DA The clamping voltage is ten to twenty times that of the surge discharger. The size of the surge discharger is determined to dissipate the capacitor C for a short duration. C The energy. If the clamping voltage is Vclamp, the unit capacitance is C, and the capacitor C C If the voltage on the circuit is V, then V = n * Vclamp, where 10 ≤ n ≤ 20, and the surge discharger may need to be able to dissipate 0.5 * C * V within 10 to 200 μs from the activation of the crowbar SCB1. 2 Energy.

[0074] Figure 8 The diagram shows curves depicting the nonlinear resistive characteristics (commonly referred to as voltage-current characteristics) of a nonlinear resistive element. The voltage-current characteristics can essentially follow the formula:

[0075]

[0076] As an example, the reference current Iref = 1kA and the reference voltage Vref = 270V, where

[0077] Section 1 Section 2 Section 3 k 1.036 1.131 0.775 α 10.02 28.3688 6.1614

[0078] If the fault current is significantly greater than the reference current, such as 100 to 200 times, which in this example is 100 to 200 kA, then the discharger voltage will be clamped to slightly above the reference voltage Vr. ef The clamping voltage Vclamp.

[0079] By introducing a surge arrester, most of the capacitor's energy is dissipated, preventing damage to the crowbar and the unit. After the energy has dissipated, the bypass switch (BPS) can be activated. Therefore, the converter can continue to operate with the unit bypassed. The surge arrester has no impact on losses during continued operation.

[0080] This type of operation significantly reduces the peak current through the crowbar branch, thus preventing cell explosion. Therefore, simplified cells, or even cells without press-fit modules and explosion chambers, can be achieved. It also provides the possibility of using DC crowbar protection for SiC-based power electronic building blocks (PEBBs).

[0081] As an alternative to nonlinear resistive elements, linear resistive elements can be used in the crowbar branch. One such element R... DB exist Figure 9The diagram schematically shows the unit connected in series with the crowbar switch SCB1. In this configuration, the unit is set to have no bypass branch.

[0082] Therefore, another way to increase damping is by introducing a component with a linear resistance in the bypass branch. This is done to obtain an overdamped circuit to avoid the negative cycle of resonance. Here, it is equally important that the component is able to dissipate energy from the unit capacitor in the same way as described above. As an example, the damping can be at least four times greater than without the component. As an example, a resistor up to 9 mΩ can be introduced, which can increase the value of the damping ζ from 0.136 to 0.6.

[0083] Since damping is determined by the following formula:

[0084]

[0085] Therefore, it can be seen that the resistance R can be set as the desired damping ζ multiplied by a constant value, which depends on the inductance L and capacitance C of the unit cell. More specifically, this constant can depend on the square root of an expression formed by dividing the unit capacitance by the unit inductance. More specifically, this constant can be set as 2 divided by the square root of the expression.

[0086] In this way, sufficient damping to prevent damage to the cell is achieved, where most of the capacitor's energy is dissipated in the linear resistance R. DB This avoids damage to the crowbar and the unit. Furthermore, due to its low resistance, the unit's contribution to operating losses remains low when it is bypassed after the unit capacitor has discharged.

[0087] Suitable resistor R DB1 The first example in Figure 10 As shown in the figure. Resistor R DB1 This is a so-called disc / washer resistor. This type has an excellent ability to handle large peak energies over short periods without the risk of explosion. The crowbar switch can be additionally mounted in a mechanical clamping structure, where clamping elements are positioned on both sides of a switch element that is also disc-shaped. Thus, the switch element is mechanically clamped between the two clamping elements. Disc resistors are easily placed in this clamping structure.

[0088] Figure 11 Another type of resistor R is illustrated schematically. DB2 The resistor R DB2 A structure is formed that constitutes or is part of the introduced linear resistive element. This structure is formed by providing a conductor sheet extending along the longitudinal axis AX.

[0089] The conductor sheet can be provided with recesses along the longitudinal axis AX to form grooves. These recesses can be alternately formed or located on opposite sides of the structure along the axis AX, thus making the conductor structure tortuous. Consequently, the structure can form rods perpendicular to the longitudinal axis AX and interconnected at their edges, with each rod additionally having a rod width BW. The rods are thus separated by grooves, and the grooves are also oriented perpendicular to the longitudinal axis AX and have grooves with a SW. The edges of two such rods interconnected by the grooves can have an edge width EW, where this edge is perpendicular to the axis AX. The depth of the groove is the difference between the conductor sheet width and the edge width EW.

[0090] A resistor can be formed using only the first conductor segment in the manner described above.

[0091] Additionally, the conductor sheet may also include a second conductor segment, wherein two conductor segments or conductor plates are placed on top of each other, i.e., one on top of the other and separated by an insulator. The two plates can then extend along the longitudinal axis AX. Current will then flow in the top segment and return through the bottom segment, and vice versa. This means that current will enter and exit the structure at a first end along the axis AX, where the plates are insulated from each other. The two plates will then be electrically joined to each other at opposite ends along the axis AX to create a return path. Thus, the plates will be in electrical contact with each other at these opposite ends. The two segments of the electrical conductor can thus be joined to each other, with one placed on top of the other, to form a forward current delivery path along the longitudinal axis of the structure and a return current path in the opposite direction.

[0092] Using this type of structure, for example, the resistance can be increased by 30 times and the inductance by 8 times. For the chosen groove, rod, and edge widths, the resistance can increase fourfold with a doubling of the inductance. This ratio can be further increased by choosing a smaller groove width (within the insulation limit). Furthermore, the resistance can be linearly changed by increasing or decreasing the number of grooves. The grooves can then be changed in pairs, where such a pair is a pair of grooves formed on different sides of the structure. For each additional repeatable groove pair, for example, the resistance can increase by 1.5 mΩ. This feature enhances the scalability of the damping element without significantly increasing cost.

[0093] The conductor structure can be made of aluminum. However, as an alternative, a high-resistivity material, such as nichrome, an alloy made of nickel and chromium, can be used to realize the conductor structure. Additionally, the structure can be part of one of the previously mentioned clamping elements for the switching element used to mechanically clamp the crowbar branch. In this case, the structure can extend from the clamping body of the clamping element. Additionally, another clamping element can also be made of a high-resistivity material. This use of nichrome allows for a reduction in the number of grooves in the conductor structure to achieve the same resistance as in the aluminum example. The increase in inductance can thus be minimal. During a short circuit, the mechanical force applied to the central groove can be very high, resulting in significant deformation. To ensure structural integrity, a non-conductive support structure can be provided on either side of the conductor structure to hold the bar in place.

[0094] like Figure 12 As shown, it illustrates a second variant of the half-bridge unit implementation including a crowbar branch, with resistive element R. D The component can also be implemented as a separate DC crowbar protection implementation. In this implementation, the crowbar branch includes a first switching element SCB1 and a first inductor L in the upper part of the crowbar branch. T1 And the second switching element SCB2 and the second inductor L in the lower part of the crowbar branch. T2 In this case, the junction between the upper and lower portions of the crowbar branch is also connected to the midpoint of the first series connection of the two switches T1 and T2. It can be seen that the junction between the upper portion with the first switching element and the adjacent portion with the other switching element is connected to the midpoint of the first series connection of the two switches, where the adjacent portion with the other switching element is the lower portion with the second switching element. The split DC crowbar is an improved version of the DC crowbar protection concept, especially for semiconductor modules based on junction lines. This protection concept does not rely on the function of the diode in the unit bypass switch. In this case, the resistive element R... D Advantageously, it is placed in the upper part of the bypass branch. This has the advantage of achieving low operating losses during the unit bypass period after the unit capacitor discharges, because the element RD is outside the bypass path. As can be seen, the bypass switch can also be omitted in this case. It should be understood that when the first end of the first series connection of the two switches forms the second unit connection terminal, the resistive element will be placed in the lower part of the bypass branch.

[0095] Figure 13Another example of a split DC crowbar branch is shown, illustrating a full-bridge unit implementation including a third variant of the crowbar branch. Compared to the split DC crowbar branch of a half-bridge unit, this crowbar branch includes a third switching element SCB3 in the branch connecting the first and second switches SCB1 and SCB2. In this case, the junction between the upper portion of the branch including the first switching element SCB1 and the middle portion of the branch including the third switching element SCB3 is connected to the midpoint of the first series connection of the two switches T1 and T2, while the junction between the middle portion of the branch including the third switching element SCB3 and the lower portion of the branch including the second switching element SCB2 is connected to the midpoint of the second series connection of switches T3 and T4. It can be seen that the junction between the upper portion with the first switching element and the adjacent portion with the other switching element is connected to the midpoint of the first series connection of the two switches, where the adjacent portion with the other switching element is the middle portion with the third switching element. The third / second switching elements SCB3 and SCB2 ensure reliable unit bypass in the event of diode failure. As... Figure 12 Similar to the unit in the circuit, the resistive element is placed in the upper part of the crowbar branch. Therefore, it is also placed outside any bypass branch used after the unit capacitor has discharged. Alternatively, the resistive element can be placed in the lower branch.

[0096] Therefore, the resistor introduced in the crowbar branch reduces the negative period of the fault current, that is, it discharges the unit capacitor during the positive half-cycle, thereby protecting the crowbar and the unit. Additionally, various solutions are proposed to limit operating losses during unit bypass after unit discharge.

[0097] The converter can be, for example, a power converter used in HVDC power transmission, FACTS systems, or static frequency converter systems.

[0098] As mentioned above, Figure 1 The MMC converter in the text is just a variant of the converter type. Figure 14 A second variant with a Y-shaped connection is shown. Figure 15 A third variant with a triangularly connected phase arm is shown.

[0099] In summary, the present invention discloses providing a resistive element in the crowbar branch of a unit.

[0100] Furthermore, features shown or described as part of one embodiment can be used in or in combination with other embodiments to produce yet another embodiment. The description is intended to include such modifications and variations.

[0101] Although the foregoing describes embodiments of the present disclosure, other and additional embodiments of the present disclosure may be devised without departing from the basic scope of the present disclosure, the scope of which is defined by the appended claims.

Claims

1. A unit (12) for a voltage source converter (10), the unit (12) comprising: First unit connection terminal (CC1). Second unit connection terminal (CC2). The first series connection of the two switches (T1, T2) forms the first unit connection terminal (CC1) at the midpoint of the first series connection of the two switches. An energy storage branch (ESB) includes a capacitor (Cc). as well as A crowbar branch, connected in parallel with the energy storage branch (ESB), includes a first switching element (SCB1) of a capacitor (Cc) adapted to be activated to short-circuit the energy storage branch (ESB) based on short-circuit fault detection, and a surge discharger (R) with nonlinear resistance connected in series with the first switching element to improve damping in the crowbar branch. DA ), wherein the surge discharger (R) DA It has a clamping voltage (V) that is 10 to 20 times lower than the rated power of the capacitor (Cc). clamp ), The crowbar branch further includes a second switching element (SCB2), wherein the first switching element (SCB1) is placed in the upper portion of the crowbar branch, and the second switching element (SCB2) is placed in the lower portion of the crowbar branch. The junction between the upper portion and the adjacent portion containing the switching elements (SCB2; SCB3) is connected to the midpoint of the first series connection of the two switches (T1, T2). The second end of the series connection of the two switches (T1, T2) forms the second unit connection terminal (CC2), and the surge arrester (R) DA It is connected to the upper part of the crowbar branch.

2. The unit (12) according to claim 1, wherein, The surge discharger is configured to dissipate the energy of the capacitor over a short period of time.

3. The unit (12) according to claim 2, wherein, The surge discharger is configured to dissipate the energy of the capacitor within 10 to 200 μs from the activation of the first switching element.

4. The unit (12) according to any one of the preceding claims further includes a bypass switch (BPS) connected between the unit connection terminals (CC1, CC2).

5. The unit (12) according to any one of claims 1 to 3, wherein the crowbar branch further comprises a second switching element (SCB2), wherein, The first switching element (SCB1) is placed in the upper portion of the crowbar branch, and the second switching element (SCB2) is placed in the lower portion of the crowbar branch. The junction between the upper portion and the adjacent portion containing the switching elements (SCB2; SCB3) is connected to the midpoint of the first series connection of the two switches (T1, T2). The first end of the series connection of the two switches (T1, T2) forms the second unit connection terminal (CC2), and the surge arrester (R...) DA It is connected to the lower part of the crowbar branch.

6. The unit (12) according to any one of claims 1 to 3 further comprises a second series connection of two switches (T3, T4), the second series connection of the two switches (T3, T4) being connected in parallel with the energy storage branch (ESB), wherein, The midpoint of the second series connection of the two switches (T3, T4) forms the second unit connection terminal (CC2).

7. A modular multilevel converter (10) including units, wherein, At least one unit is the unit (12) according to any one of the preceding claims.

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