A unit comprising a power link with a variable inductance

Abstract

A cell (12) includes a first cell connection terminal (CC1), a second cell connection terminal (CC2), two switches (T1, T2) connected in series, a capacitor (C C ) and a first power link interconnecting the first series-connected two switches (T1, T2) and the energy storage branch (ESB), wherein the first power link has an inductance (LBB1) that varies depending on the current passing through it. A converter comprising such a unit is also provided.

Description

Technical Field

[0001] The present invention relates to the field of power converters, in particular to a voltage source converter (VSC). More particularly, the present invention relates to a unit that can be used in a modular multilevel converter (MMC). Background Art

[0002] A VSC (such as an MMC) includes switches. These switches can be arranged in so-called half-bridge or full-bridge units, where a unit can include two switches connected in series via a DC bus to a capacitor. Semiconductor components, particularly transistors such as IGBTs (insulated gate bipolar transistors), are used as switches. MMCs have become a popular choice for grid-connected converters due to their enhanced modularity, scalability, and excellent harmonic performance with reduced losses.

[0003] A faulty semiconductor module (e.g., an IGBT module) in an MMC unit can pose a high risk of large-scale damage. In many cases, if one IGBT is in a half-bridge or full-bridge circuit, the adjacent IGBT may be able to clear the resulting fault current from the DC bus capacitor. However, there are situations where this fault current may not be prevented.

[0004] Since the stray inductance between the IGBT and the DC bus can be intentionally very low (to achieve acceptable converter performance / low transient overvoltage at the IGBT collector-emitter terminals / low IGBT switching losses) and the energy stored in the cell DC capacitors can be high, the probability of fault currents occurring is also high, such as in the range of several hundred kA.

[0005] For MMC, large cell capacitors are often required to buffer the ripple energy generated by active / reactive power transfer. During a short circuit fault of a semiconductor component, the cell capacitor energy may discharge through the semiconductor component, resulting in explosion / significant destruction of the cell.

[0006] Without any countermeasures, the bond wires (wires connecting the semiconductor chip to the metal contacts) within the IGBT module will immediately fail, causing an arc. This arc will be fed by the high energy stored in the DC capacitors. This can cause the IGBT module to explode, resulting in massive damage. The difficulty of dealing with this problem increases as the energy stored in the DC capacitors increases.

[0007] There are some possibilities to overcome this problem, such as using presspack semiconductor components or using explosion boxes to protect the environment from the harmful effects of the explosion. These are only suitable for high power levels or add considerable cost without adding functional improvement for normal operation.

[0008] Another solution is to use a so-called DC crowbar to discharge most of the cell capacitor energy into the thyristor to limit the energy discharge into the semiconductor switch. However, in this case, the fault current may also become too large for the crowbar to handle.

[0009] In view of the above, there is a need to improve the way short circuit current is handled in cells.

[0010] An improved solution is therefore desired. Summary of the Invention

[0011] In a first aspect, the present invention may disclose a cell for a voltage source converter, wherein the cell comprises: a first cell connection terminal; a second cell connection terminal; two switches of a first series connection; an energy storage branch, the energy storage branch comprising a capacitor; and a first power link, the first power link interconnecting the two switches of the first series connection with the energy storage branch, wherein the first power link has an inductance that changes based on the current passing through it.

[0012] The inductance may have a static component and a transient component, and the first power link may have an inductance of the static component during steady-state operation of the unit and may be configured to introduce a transient component triggered by a short-circuit current through the link. After the transient component is introduced, the first power link may thus have an inductance that is the sum of the static component and the transient component.

[0013] The first power link may additionally include: a first busbar, which interconnects the first ends of the two switches connected in series with the first end of the energy storage branch; and a second busbar, which interconnects the second ends of the two switches connected in series with the second end of the energy storage branch.

[0014] To provide a static component of the first power link, the first busbar may be positioned adjacent to the second busbar. At least a portion of the first busbar may be movable from the second busbar due to a repulsive force caused by the short circuit current, thereby introducing a transient component of the power link.

[0015] During steady-state operation, the busbars may extend parallel to each other in a current carrying direction of the first busbar. The busbars may additionally be separated by insulators.

[0016] The first busbar may include a movable section having a first end engaged to a first stationary section via a first engagement structure, wherein the first stationary section is fixedly attached to the second busbar, and the first engagement structure is deformable due to the repulsive force to move the movable section away from the second busbar, thereby introducing a gap between the first busbar and the second busbar, causing the transient component of the power link inductance to be introduced.

[0017] In steady-state operation, the first stationary segment can be positioned adjacent to and aligned with the movable segment in the current transmission direction of the first busbar, and the first engagement structure can include at least one fold of excess material interconnecting the first stationary segment and the movable segment and configured to unfold due to a repulsive force caused by the short-circuit current. The number of folds used can correspond to the desired distance the movable segment is moved.

[0018] At least one fold, and advantageously all folds, may additionally comprise a loop of excess material. Said loop may have a radius that is at least one and a half times the thickness of said first generatrix.

[0019] The first busbar may additionally include a second stationary section joined to a second end of the movable section via a second engagement structure, wherein the second stationary section is fixedly attached to the second busbar, and the second engagement structure is deformable due to a repulsive force caused by the short-circuit current, thereby causing the movable section to move away from the second busbar.

[0020] During steady-state operation, the second stationary segment can be positioned adjacent to and aligned with the movable segment in the current-carrying direction of the first busbar, and the second engagement structure can include at least one fold of excess material, interconnecting the second stationary segment and the movable segment and configured to unfold due to the repulsive force induced by the short-circuit current. The number of folds used can correspond to the desired distance the movable segment is to be moved. This number can advantageously be the same as that in the first engagement structure. At least one, and advantageously all, folds can also comprise loops of excess material.

[0021] At least a portion of the second busbar may also be movable away from the first busbar due to the repulsive force caused by the short-circuit current. For this reason, the second busbar may include a stationary section and a movable section that engage with each other in the same manner as the corresponding stationary section and movable section of the first busbar. Therefore, the stationary section and movable section of the two busbars must be aligned with each other along the current transmission direction of the first busbar.

[0022] The cell may additionally comprise a bypass switch connected between the cell connection terminals, which bypass switch may be mechanical or electronic.

[0023] The cell may be a half-bridge cell, whereby the midpoint of the first series-connected two switches may form a first cell connection terminal, and the first ends of the first series-connected two switches or the second ends of the first series-connected switches may form a second cell connection terminal.

[0024] Alternatively, the cell may be a full-bridge cell. For this reason, the cell may also include a second series connection of two switches and a second power supply link interconnecting the first series connection of two switches with the energy storage branch. In this case, the midpoint of the first series connection of two switches may form a first cell connection terminal, and the midpoint of the second series connection of two switches may form a second cell connection terminal.

[0025] Furthermore, it is possible that the second power link has an inductance that varies depending on the current passing through it.To provide the inductance that varies depending on the current passing through the second power link, the second power link can be implemented in the same way as the first power link.

[0026] The cell may further include a crowbar branch connected in parallel with the energy storage branch, the crowbar branch may include a first switching element adapted to be activated to short-circuit the capacitor of the energy storage branch and dissipate energy stored therein upon detection of a short-circuit fault. The crowbar branch may additionally include a first impedance, such as a first inductor.

[0027] The crowbar branch may further include a second switching element and a midpoint connected to the midpoints of the two switches connected in series. In this case, one switching element is placed on one side of the midpoint and the other switching element is placed on the other side of the midpoint. In this case, a first impedance and a second impedance may also be used, where, in a similar manner, one impedance is placed on one side of the midpoint and the other impedance is placed on the other side of the midpoint.

[0028] Furthermore, it is possible that the energy storage branch also includes an inductor connected in series with the capacitor.

[0029] Other aspects are directed to a modular multilevel converter comprising cells, wherein at least one cell is a component, ie, a cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Embodiments of the present disclosure will be presented in an exemplary sense and their advantages will be explained in more detail below with reference to the accompanying drawings, in which:

[0031] Figure 1 A first variant of a modular multilevel converter comprising cells is shown;

[0032] Figure 2 A first variant of a cell with a half-bridge implementation is shown;

[0033] Figure 3 shows a perspective view of a first implementation of a first power chain in a cell during steady-state operation of the cell;

[0034] Figure 4 showing a perspective view of a first implementation of a first power link when there is a short circuit current flowing through the first power link;

[0035] Figure 5 Shown Figure 3 and Figure 4 two first engagement structures in the power link;

[0036] Figure 6 shows a perspective view of a second implementation of the first power chain during steady-state operation of the unit;

[0037] Figure 7 Shown Figure 6 A perspective view of two first engagement structures of a power link in FIG;

[0038] Figure 8 shows a side view of a second implementation of the first power chain during steady-state operation of the unit;

[0039] Figure 9 shows a side view of a second implementation of the first power link during an initial phase of a short circuit current flowing through the first power link;

[0040] Figure 10 shows a side view of a second implementation of the first power link during an intermediate phase of a short-circuit current flowing through the first power link;

[0041] Figure 11 a side view showing the final stage of the short-circuit current flowing through the first power link;

[0042] Figure 12 A cell with a full-bridge implementation is shown;

[0043] Figure 13 A second variant of the cell with a half-bridge implementation is schematically shown;

[0044] Figure 14 A third variant of the cell with a half-bridge implementation is schematically shown;

[0045] Figure 15A second variant of a modular multilevel converter comprising cells is schematically shown; and

[0046] Figure 16 A third variant of a modular multilevel converter comprising cells is schematically shown. DETAILED DESCRIPTION

[0047] Hereinafter, the principles and spirit of the present disclosure will be described with reference to exemplary embodiments. It should be understood that all of these embodiments are provided only for those skilled in the art to better understand and further practice the present disclosure, and are not intended to limit the scope of the present disclosure. For example, features illustrated or described as part of one embodiment can be used together with another embodiment to produce yet another embodiment. For the sake of clarity, not all features of the actual implementation are described in this specification. It should of course be understood that in the development of any such actual implementation, many implementation-specific decisions should be made to achieve the developer's specific goals, such as compliance with system-related and industry-related constraints, which will vary from one implementation to another. In addition, it should be understood that such development efforts may be complex and time-consuming, but are still routine tasks for those of ordinary skill in the art who benefit from the present disclosure.

[0048] 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 and so as not to obscure the description with details well known to those skilled in the art. Nevertheless, the 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 as having a meaning consistent with the understanding of these words and phrases by those skilled in the relevant art. Special definitions of terms or phrases, that is, definitions that are different from the ordinary and customary meanings understood by those skilled in the art, are intended to be implied by consistent use of the terms or phrases herein. To the extent that a term or phrase is intended to have a special meaning, that is, a meaning that is different from the meaning understood by those skilled in the art, such special definitions will be clearly set forth in the specification in a definition manner that directly and unambiguously provides the special definition of the term or phrase.

[0049] The present subject matter provides a solution intended to limit the peak short circuit current of a cell, which can be used to prevent explosions and / or simplify circuitry for preventing or handling explosions in the cell.

[0050] Figure 1 A converter 10 is shown, which comprises cells in which the above-mentioned peak short-circuit current limitation can be implemented.

[0051] The converter 10 is a voltage source converter (VSC) and can be implemented as a modular multilevel converter (MMC). Figure 1 In the example shown in FIG, converter 10 is an MMC. In this case, the MMC consists of a plurality of parallel phase legs, here three, each of which includes a plurality of cascaded cells 12. The midpoint of the phase legs can thus form the AC output of converter 10. Because there are three phase legs, the converter can be connected to a three-phase AC system. Cell 12 can, in turn, be a full-bridge cell or a half-bridge cell. The cell thus includes an energy storage element and a switch configured to insert an energy storage element having one of up to two different polarities into a phase leg or to bypass the energy storage element. The energy storage element can advantageously be a capacitor. Each cell thus also has a cell voltage. This cell voltage is thus inserted into a phase leg or bypassed to form the waveform.

[0052] The converter 10 shown is only one example of an MMC in which the cells may be used.The phase legs may also be star or delta connected, with the junction between two phase legs being connected to corresponding phases of a three-phase AC system.

[0053] In one variation of converter 10, each phase leg is composed of full-bridge cells. In another variation, each phase leg is composed of half-bridge cells. In other variations, each phase leg may be composed of a mixture of full-bridge cells and half-bridge cells.

[0054] Aspects of the present invention are directed to providing a power link in a cell that limits short-circuit current in the event of a fault in the cell.

[0055] Figure 2 A cell 12 with a half-bridge implementation is schematically shown. This cell can be used as a half-bridge cell of an MMC.

[0056] The unit 12 comprises an energy storage branch ESB comprising a capacitor C C The unit 12 further comprises two switches T1 and T2 in a first series connection (series circuit), wherein the two switches in the first series connection are connected in parallel with the energy storage branch ESB. In this example, the energy storage branch ESB further comprises a capacitor C C An optional inductor LC is connected in series.

[0057] The first ends of the two switches T1 and T2 of a first series circuit (series connection) are connected to a first end of the energy storage branch ESB via a first busbar BB1, and the second end of the first series circuit of switches T1 and T2 is connected to a second end of the energy storage branch ESB via a second busbar BB2, wherein the two busbars together form a power link for transferring power to or from the energy storage branch ESB. The first power link thus includes the first busbar BB1 interconnecting the first ends of the two switches T1 and T2 of the first series connection and the first end of the energy storage branch ESB, and the second busbar BB2 interconnecting the second ends of the two switches T1 and T2 of the first series connection and the second end of the energy storage branch ESB.

[0058] The two switches T1 and T2 connected in series can be arranged—possibly together with the energy storage branch ESB—in a semiconductor package or module (such as a half-bridge module), for example in the form of a press pack or wire bond-based module (e.g., a so-called LinPak or Hipak module). In this case, the terminals of the series circuit of the two switches for connection to the energy storage branch can be arranged inside the semiconductor package or module.

[0059] Figure 2 The unit in is implemented as a half-bridge, which includes two switches T1 and T2 connected in series. Generally, the switches may include semiconductor components, such as transistors and diodes. The switches used in the series circuit may therefore include semiconductor components. The semiconductor components may be, for example, silicon carbide metal oxide semiconductor field effect transistors (SiC MOSFETs), insulated gate bipolar transistors (IGBTs), or dual-mode insulated gate transistors (BIGTs). The type of semiconductor component is not restrictive and may also include future semiconductor components suitable for covering the needs of the power converter or unit according to the present application.

[0060] The diode is switched antiparallel to the conduction direction of the transistor. The diode may be an essential part of the transistor (power transistor).

[0061] Transistors only allow current to flow in one direction, in this case from collector to emitter. Diodes, as semiconductor components, allow current to flow in the opposite direction to that allowed by transistors T1 and T2. Preferably, the diodes ("freewheeling diodes") are suitable for carrying the same power (or current) as the transistors.

[0062] The midpoint between the switches of the first series circuit of two switches T1 and T2 forms the first cell connection terminal CC1. Figure 2In the half-bridge cell structure, the second end of the first series circuit of the two switches T1 and T2 forms the second cell connection terminal CC2. As an alternative, the first end of the first series circuit of the two switches T1 and T2 may instead form the second cell connection terminal CC2.

[0063] The unit comprises an optional crowbar branch connected in parallel with the energy storage branch ESB. Figure 2 As can be seen in FIG, the crowbar branch further comprises a first switching element SCB1 acting as a DC crowbar connected in series with an optional inductor LCB1. The inductor is an example of a type of impedance that can be used in the crowbar branch. Other types that can be used are capacitors and resistors or any combination of inductors, capacitors and resistors to achieve a specific goal. The crowbar SCB1 is adapted to be externally actuated to short-circuit the energy storage branch ESB and then in particular the capacitor C C The switching element forming the crowbar may be a semiconductor component, preferably a thyristor, which may be activated by a switching signal, thereby causing the capacitor C C Short circuit.

[0064] If you can Figure 2 As can be seen in FIG, 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 can be implemented as a mechanical switch. However, it can also be implemented as an electronic switch, such as a thyristor. After the crowbar has been used to discharge the capacitor, the bypass switch BPS can be controlled to bypass the entire cell. By adding the bypass switch BPS, the converter in which the cell is installed can continue to be used.

[0065] The power link has a first inductance LBB1 that varies based on the current passing through it. The first inductance of the power link can more specifically include a first steady-state or static component LBBS1 and a first transient component LBBT1, wherein the power link has an inductance of the static component LBBS1 during steady-state operation of the unit and is configured to insert or introduce a transient component LBBT1 in the event of, or triggered by, a short-circuit current flowing through the power link. During a short circuit, the first power link thus has a first inductance that is the sum of the first static component LBBS1 and the first transient component LBBT1.

[0066] During a cell short circuit fault, the peak current through the power link can be given by

[0067]

[0068] Ignoring the resistance in the commutation loop, the expression can be simplified to:

[0069]

[0070] Where C is the cell capacitance, L is the inductance in the commutation loop, and U C0 is the cell capacitor voltage

[0071] The peak short-circuit current is inversely proportional to the square root of the loop inductance. Therefore, adding inductance across the short-circuit path helps reduce the peak current (and the mechanical force Fα). However, during normal switching operation, increasing the loop inductance will result in increased switching losses and higher overvoltages. This, in turn, will result in a reduction in the nominal cell voltage. Therefore, it is recommended that the power link have a steady-state or minimum inductance during normal steady-state operation, and that transient inductance be added or inserted during a cell short-circuit fault. The added or transient inductance can be large, such as up to 10 times the nominal value of the cell inductance.

[0072] Reference will be made below Figure 3 、 Figure 4 and Figure 5 How this is achieved is described in the first implementation of the power link PL shown in FIG. Figure 3 shows a perspective view of the first power link PL during steady-state operation, Figure 4 shows a perspective view of the first power link PL when there is a short-circuit current flowing through the first power link PL, and Figure 5 Two first engagement structures of the power train in steady-state operation are schematically shown.

[0073] The power link PL is composed of a first busbar BB1 and a second busbar BB2, wherein the first busbar BB1 is positioned adjacent to the second busbar BB2 and at least a portion of the first busbar BB1 is movable from the second busbar BB2 due to a repulsive force caused by a short-circuit current, thereby introducing a transient component of the power link. In order to achieve this effect, the first busbar BB1 includes a movable section 20A having a first end engaged to a first stationary section 16A via a first engagement structure 18A, wherein the first stationary section 16A is fixedly attached to the second busbar BB2, and the first engagement structure 18A is configured to be deformed by a magnetic repulsive force so as to move the movable section 20A away from the second busbar BB2. Thereby, a gap is introduced between the first busbar BB1 and the second busbar BB2, which results in the introduction of a transient component LBBT of the power link inductance. In Figure 3-Figure 5In the power train implementation shown in FIG, it can be seen that, during steady-state operation, first stationary segment 16A is positioned adjacent to and aligned with movable segment 20A in the longitudinal current transmission direction of first busbar BB1 during steady-state operation, and first engaging structure 18A includes at least one fold of excess material interconnecting first stationary segment 16A and movable segment 20A, wherein the at least one fold is configured to unfold or move away from second busbar BB2 due to repulsive forces caused by a short-circuit current. It can also be seen that first busbar BB1 includes a second stationary segment 24A joined to a second end of movable segment 20A via a second engaging structure 22A, wherein second stationary segment 24A is fixedly attached to second busbar BB2, and second engaging structure 22A is deformable due to repulsive forces caused by a short-circuit current to cause movable segment 20A to move away from second busbar BB2. In this case, second engaging structure 22A also has the same implementation as the first engaging structure.

[0074] exist Figure 3-Figure 5 In the power link variant shown in FIG, the second busbar BB2 also has the same implementation as the first busbar BB1. At least a portion of the second busbar BB2 is thus movable away from the first busbar BB1 due to the repulsive force caused by the short-circuit current, and includes stationary sections 16B, 24B and a movable section 20B, which are mutually engaged via engagement structures 18B and 22B in the same manner as the corresponding stationary sections 16A, 24A and movable section 20A of the first busbar BB1. The stationary sections 16A, 16B, 24A, 24B and movable sections 20A, 20B of the two busbars BB1 and BB2 are aligned with each other along the longitudinal current transmission direction of the first busbar BB1, wherein the longitudinal current transmission direction of the first busbar BB1 during steady-state operation is opposite to the longitudinal current transmission direction of the second busbar during steady-state operation.

[0075] Thus it can be seen that the second busbar BB2 comprises a movable section 20B having a first end joined to a first stationary section 16B via a first joining structure 18B, wherein the first stationary section 16B is fixedly attached to the first busbar BB1, and in this case to the first stationary section 16A of the first busbar BB1. The first joining structure 18B is dimensioned to deform due to the magnetic repulsive force caused by the short circuit current so as to move the movable section 20B away from the first busbar BB1, thereby facilitating the introduction of a gap between the first busbar BB1 and the second busbar BB2 that results in the introduction of a transient component LBBT of the power link inductance. Figure 3-Figure 5As can be seen in the variation shown in FIG, in steady-state operation, the first stationary segment 16B is positioned adjacent to and aligned with the movable segment 20B in the longitudinal current transmission direction of the second busbar BB2, and the first engagement structure 18B includes at least one fold of excess material (annular engagement structure) interconnecting the first stationary segment 16B and the movable segment 20B, and is configured to deform or unfold due to repulsive forces caused by a short-circuit current, thereby moving the movable segment 20B away from the first busbar BB1. It can also be seen that the second busbar BB2 includes a second stationary segment 24B that is engaged with the second end of the movable segment 20B via a second engagement structure 22B, wherein the second stationary segment 24B is fixedly attached to the first busbar BB1, in this case, to the second stationary segment 24A of the first busbar BB1. The second engagement structure 22B deforms or unfolds due to repulsive forces caused by the short-circuit current, thereby moving the movable segment 20B away from the first busbar BB1. By this implementation, the two first stationary segments 16A, 16B, the two first engagement structures 18A and 18B, the two movable segments 20A and 20B, the two second engagement structures 22A and 22B, and the two second stationary segments 24A and 24B are aligned with each other along the two current transmission directions.

[0076] The first and second busbars BB1 and BB2, which can be considered the positive and negative busbars, are separated by a thin insulator sheet to minimize inductance during normal operation. During a cell short-circuit fault, very high currents (i.e., exceeding 100 kA) may flow through the power link PL. Each engagement structure 18A, 18B, 22A, and 22B in this case includes at least one fold of excess material and is implemented as multiple folds of excess conductor stacked perpendicular to the direction of current flow. The number of folds used can correspond to the desired distance the movable segment is moved. Thus, the number of folds used in the engagement structure can correspond to the desired distance the end of the movable segment engaged with it is moved. The magnetic force generated in the power link repels the positive and negative busbars outward, causing the folds of the first and second engagement structures 18A, 18B, 22A, 22B in the first and second busbars BB1 and BB2 to unfold, thereby moving the movable segments 20A and 20B and forming an elliptical structure. This creates a large air gap, significantly increasing the busbar inductance.

[0077] This type of operation significantly reduces peak switching currents, preventing the cell from exploding. This allows for cells with simplified or even no press-pack modules and explosion boxes. It also opens the possibility of using DC crowbar protection for SiC-based power electronic building blocks (PEBBs).

[0078] A DC crowbar is used to short-circuit the corresponding capacitor within a few microseconds, thereby diverting any fault current on the DC bus away from the semiconductor (such as IGBT) module through the crowbar component. By introducing a transient inductive component into the power link, the short-circuit current can be reduced to a level that the crowbar can handle.

[0079] The design of a second implementation forming the first power link PL is schematically shown in Figure 6 、 Figure 7 、 Figure 8 、 Figure 9 、 Figure 10 and Figure 11 ,in Figure 6 shows a perspective view of the first power link PL during steady-state operation of the unit, Figure 7 shows the two first engagement structures of the power chain, Figure 8 shows a side view of the first power link PL during steady-state operation of the unit, Figure 9 shows a side view of the first power link PL during the initial phase of the short-circuit current, Figure 10 shows a side view of the first power link PL during an intermediate phase of a short-circuit current, and Figure 11 A side view showing the final stage of the flowing short-circuit current.

[0080] The busbars forming the first and second busbars BB1 and BB2 can, for example, be made of aluminum, each having a thickness of 2 mm, a length from edge to edge of 389 mm, and a width of 100 mm. It can be seen that in the second embodiment of the power train, each fold comprises a loop, so that each joint structure 18A, 18B, 22A, and 22B includes two folds, each provided with a loop. A rule of thumb is that the loops in the folds of sheet metal should not have a radius less than 1.5 times the thickness of the corresponding sheet, otherwise cracks may form in the metal along the folded area. For the presented design, a 4 mm radius (i.e., 2 times) was considered for the loops in the folds of the joint structure. The folds are then unfolded due to magnetic repulsion to form a large volume with the movable sections 20A and 20B. For example, when the busbar is fully unfolded, the inductance can increase by 20 to 25 times in this case. It is assumed that the force required to fully unfold the busbar (2 mm thickness) can be achieved between 50 kA and 70 kA. Reducing the busbar thickness helps achieve expansion at lower peak currents. However, at this thickness, using very thin busbars runs the risk of busbar open circuits.

[0081] The power links described above may also be used in other types of units.

[0082] Figure 12 Shown is the corresponding Figure 2The full-bridge unit of the half-bridge unit in FIG. 1 includes, in addition to the first series-connected switches T1 and T2, the energy storage branch ESB, and the crowbar branch, a second series-connected switch T3 and T4. The first ends of the two switches T3 and T4 of the second series circuit (series connection) are connected to the first end of the energy storage branch ESB, and the second ends of the switches T3 and T4 of the second series circuit are connected to the second end of the energy storage branch ESB.

[0083] In the cell, the midpoint between switches T1 and T2 of the first series circuit again forms the first cell connection terminal CC1. However, in this case, the midpoints of the second series circuit of two switches T3 and T4 together form the second cell connection terminal CC2. Again, the bypass switch BPS is connected between the first and second cell connection terminals CC1 and CC2, i.e., in parallel with the cell. In this case, a second power link exists between the second series-connected switches T3 and T4 and the energy storage branch ESB, wherein the second power link comprises a third busbar BB3 and a fourth busbar BB4. In this case, the inductance of the first power link comprises a first steady-state or static component LBBS1 and a first transient component LBBT1, while the inductance of the second power link comprises a second steady-state or static component LBBS2 and a second transient component LBBT2. It should be appreciated that, alternatively, one of the power links can have a conventional implementation.

[0084] The power chain implementation with static and transient components can be used in other types of cell structures. Figure 13 As can be seen in the half-bridge cell implementation in

[15] , this power link implementation can also be used in a split DC crowbar protection scheme, where the crowbar branch includes a first switching element CBC1 and a second switching element CBC2 and a first impedance LCB1 and a second impedance LCB2, where the midpoint of the branch is connected to the midpoint of the first series connection of two switches T1 and T2, and each branch half includes a switching element and an impedance. As can be seen, the bypass switch can also be omitted in this case.

[0085] Another half-bridge cell implementation is shown in Figure 14 In this case, there is no crowbar branch, but only a bypass branch. It should be appreciated that in some variants, the bypass branch may also be omitted.

[0086] The transient inductance introduced by the extended busbar during a cell short-circuit fault reduces the peak current through the faulty switch, thereby preventing the explosion of bond wire modules or reducing the cost and design complexity of compact package modules. This is also achieved without the use of sensors and active control elements, which simplifies the cell and makes it more robust. Therefore, the speed at which the transient component of the power link inductance is introduced can also be rapid.

[0087] The converter may be a power converter as used, for example, in HVDC power transmission, a FACTS system or a static frequency converter system.

[0088] As mentioned above, Figure 1 The MMC converter in is only one converter type variant. Figure 15 A second variant with star-connected phase legs is shown, and Figure 16 A third variant with a delta-connected phase leg is shown.

[0089] Generally, the present invention discloses providing a power link between two switches connected in series and an energy storage branch, wherein the power link has two busbars that together provide an inductance, wherein a static component and a transient component are introduced triggered by the occurrence of a short circuit fault.

[0090] Furthermore, 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.This description is intended to include such modifications and variations.

[0091] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope of the disclosure is determined by the claims that follow.

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), First, two switches (T1, T2) are connected in series. Energy storage branch (ESB), which includes capacitors (C C ); as well as a first power link (PL) interconnecting the first series-connected two switches (T1, T2) and the energy storage branch (ESB), The first power link (PL) has an inductance (LBB1) that changes based on a current passing therethrough, wherein the first power link (PL) includes: a first busbar (BB1) interconnecting a first end of the first series-connected two switches (T1, T2) with a first end of the energy storage branch (ESB); and a second busbar (BB2) interconnecting a second end of the first series-connected two switches (T1, T2) with a second end of the energy storage branch (ESB), wherein the first busbar (BB1) and the second busbar (BB2) are positioned adjacent to each other, and at least a portion of the first busbar (BB1) is movable from the second busbar (BB2) due to a repulsive force caused by a short-circuit current, thereby introducing a transient component to the power link (PL).

2. A unit (12) according to claim 1, wherein the inductance (LBB1) has a static component (LBBS1) and a transient component (LBBT), wherein the first power link (PL) has an inductance of the static component (LBBS1) during steady-state operation of the unit and is configured to introduce a transient component (LBBT1) triggered by a short-circuit current through the link.

3. The unit (12) of claim 1, wherein the first busbar (BB1) comprises: A movable section (20A) having a first end engaged to a first stationary section (16A) via a first engagement structure (18A), wherein the first stationary section is fixedly attached to the second busbar (BB2), and the first engagement structure (18A) is deformable due to the repulsive force to move the movable section (20A) away from the second busbar (BB2), thereby introducing a gap between the first and second busbars (BB1, BB2) that causes the transient component (LBBT) of the power link inductance to be introduced.

4. A unit (12) according to claim 3, wherein in steady-state operation, the first stationary segment (16A) is placed adjacent to and aligned with the movable segment (20A) in the longitudinal current transmission direction of the first busbar (BB1), and the first engaging structure (18A) includes at least one fold of excess material, the at least one fold interconnecting the first stationary segment (16A) and the movable segment (20A) and being configured to unfold due to the repulsive force caused by the short-circuit current.

5. The unit (12) of claim 4, wherein at least one fold comprises a loop of excess material.

6. A unit (12) according to any one of claims 3 to 5, wherein the first busbar (BB1) includes a second stationary segment (24A) joined to the second end of the movable segment (20A) via a second joining structure (22A), wherein the second stationary segment (24A) is fixedly attached to the second busbar (BB2), and the second joining structure (22A) is deformable due to a repulsive force caused by the short-circuit current to move the movable segment (20A) away from the second busbar (BB2).

7. A unit (12) according to any one of claims 3 to 5, wherein at least a portion of the second busbar (BB2) is movable away from the first busbar (BB1) due to a repulsive force caused by the short-circuit current, and comprises stationary and movable sections (16B, 20B, 24B) that engage with each other in the same manner as the corresponding stationary and movable sections (16A, 20A, 24A) of the first busbar (BB1), wherein the stationary and movable sections (16A, 16B, 20A, 20B, 24A, 24B) of the two busbars (BB1, BB2) are aligned with each other along the longitudinal current transmission direction of the first busbar.

8. The cell (12) according to any one of claims 1 to 5, further comprising a bypass switch (BPS) connected between the cell connection terminals (CC1, CC2).

9. A cell (12) according to any one of claims 1 to 5, wherein a midpoint of the first series-connected two switches (T1, T2) forms a first cell connection terminal (AC1), and a first end of the first series-connected two switches (T1, T2) or a second end of the first series-connected switches (T1, T2) forms a second cell connection terminal (AC2).

10. The cell (12) according to any one of claims 1 to 5, further comprising a second series connection of two switches (T3, T4) and a second power link interconnecting the second series connection of the two switches with the energy storage branch (ESB), wherein the midpoint of the first series connection of the two switches (T1, T2) forms a first cell connection terminal (AC1) and the midpoint of the second series connection of the two switches (T3, T4) forms a second cell connection terminal (AC2).

11. The unit (12) of claim 10, wherein the second power link has an inductance (LBB2) that varies based on a current passing therethrough.

12. The unit (12) according to any one of claims 1 to 5 and 11, further comprising 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 cause the capacitor (C C ) is short-circuited, and the energy stored therein is dissipated based on the detection of the short-circuit fault.

13. The unit (12) of claim 12, the crowbar branch further comprising a second switching element (SCB2) and a midpoint connected to the midpoints of the two switches (T1, T2) of the first series connection.

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

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