The present disclosure relates to a converter cell of a Modular Multilevel Chain-Link Converter (MMC). BACKGROUND

MMCs have become a popular choice for the grid connected converters due to enhanced modularity, scalability and harmonic performance with reduced losses. Development towards cell level protection concepts has been a focus lately. In a so called shoot-through failure of a half-bridge (HB) or full-bridge (FB) MMC cell, the cell capacitor is short-circuited via a leg of semiconductor switches, leading to discharge of cell capacitor energy into the switches with very large currents, e.g. in the range of 500 kA to 1 MA for Static

Synchronous Compensators (STATCOM) or in the range of 1-2 MA for High- Voltage Direct Current (HVDC) converters. Gate Drivers (GD) are typically equipped with protection circuits to sense large currents and safely turn OFF (i.e. non-conducting) the switches. From past experiences, it has been identified that safe turn OFF using GD does not always work and it is may be mandatory to have secondary protection circuits. Two problems that an MMC cell should overcome during a shoot- through failure of the switches are:

The large cell capacitor energy discharging into the switches may lead to either explosion, e.g. in case of industrial switch modules (for instance Insulated-Gate Bipolar Transistor (IGBT)) using bond wires, or need for more expensive Presspack switches (for instance Integrated Gate- Commutated Thyristor (IGCT), Bi-Mode Insulated Gate Transistor (BiGT), Thyristor, StackPak IGBT or Injection-Enhanced Gate Transistor (IEGT)) with hermetic sealing to handle such energies.

That it may be necessary to have a fast and stable bypass on the cell output before the switches explode or fail in high impedance mode during a shoot-through fault. It is to be noted that the current flowing through the cell output cannot be interrupted due to the presence of large arm reactors that would initiate an arc across the failed cell leading to significant damage of the valve structure of the converter. To address these problems, a direct current (DC) crowbar thyristor connected across the cell capacitor can be used. When a shoot-through fault is detected, the crowbar thyristor is triggered to turn ON (i.e. conducting). The large inrush current from the capacitor is then shared between the failed leg of switches and the thyristor, relaxing the energy handling requirements of the switch packaging. The thyristor may be designed such that most of the capacitor energy is discharged via the thyristor rather than via the switches. The thyristor can fail short and remains at low impedance for a long time duration (e.g. >i year), typically until the next service stop.

If the switches are e.g. IGCT, since it is a property of the IGCT to fail short while the diodes remain healthy, a stable cell output bypass through the diode bridge and failed thyristor may be obtained. Both of the problems mentioned above have been addressed by introducing the DC crowbar thyristor in the MMC cell.

Figure la illustrates a converter cell during normal operation, while figure lb illustrates when the DC crowbar thyristor is activated and goes into short circuit, whereby the cell is bypassed through the diodes.

SUMMARY

It is to be noted that even when using a crowbar, as discussed above, the diodes must remain healthy. For instance, industry IGBT modules use bond wires to connect the IGBT and diode chips to the power terminals. When a shoot-through failure occurs with a bond wire module, the bond wires may be damaged by the large inrush current, vaporizing the bond wires and the mechanical stress caused by the intense magnetic fields. Hence, this may lead to an open circuit failure, unlike in Presspack modules (e.g. IGCTs) which fail short circuit. An arc may be initiated within the IGBT module as the shoot- through currents are interrupted by the open circuit failure of the switches. This leads to increased temperature and eventually increased pressure inside the IGBT package and may ultimately lead to an explosion of the IGBT module. The gases expelled from the explosion along with the current interruption on the converter arm may then lead to arcing across the failed cell leading to further failure in the converter.

During a shoot-through failure, when the converter cell energy storage is short circuited, current rushes through the one-directional crowbar switch. However, the DC voltage (i.e. the voltage over the energy storage) will not smoothly arrive at zero, but will resonate some time before eventually settling at zero, i.e. the DC voltage across the energy storage will go from positive to negative, and then back to positive again, for a number of cycles, each with a lower absolute peak voltage value. Thus, even when the crowbar thyristor is conducting, the diodes (antiparallel with the failed switches) may have to carry a large current during the negative half cycle(s) of resonance, which may reduce the lifetime and reliability of the diodes, e.g. of bond wires thereof, even if they survive the initial shoot-through event. Hence a healthy diode may not be guaranteed after such an event.

Having a DC crowbar thyristor can certainly prevent the explosion of a bond wire module. But the reliability of the diodes may be at question after such an event and a reliable, low impedance, long term cell output bypass may not be guaranteed.

It is an objective of the present invention to provide a converter cell with improved handling of DC capacitor discharge due to a shoot-through event. According to an aspect of the present invention, there is provided a converter cell for a power converter. The cell comprises a plurality of semiconductor devices forming a half-bridge or full-bridge topology in the cell with at least one switch leg comprising a plurality of the semiconductor devices connected in series, each semiconductor device comprising a one-directional switch and an anti-parallel diode. The cell also comprises an energy storage connected across the at least one switch leg. The cell also comprises a crowbar leg connected across the at least one switch leg. The crowbar leg comprises a one-directional first semiconductor crowbar switch arranged to short-circuit the energy storage when the crowbar switch is switched to conducting, and a one-directional reverse semiconductor crowbar device connected across the first crowbar switch and antiparallel to said first crowbar switch.

According to another aspect of the present invention, there is provided a power converter comprising a plurality of series connected converter cells of the present disclosure. By use of the one-directional reverse semiconductor crowbar device, e.g. a reverse-blocking semiconductor device such as a diode or an arrangement comprising at least one diode, the current during a negative half cycle of resonance during a shoot-through event in a converter cell can at least partly pass through said reverse device, relieving at least some of the strain on the anti-parallel diodes of the semiconductor devices in the cell. Thus, the antiparallel diodes remain healthy, allowing the faulty cell to be bypassed via the antiparallel diodes.

It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of “first”,“second” etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described, by way of example, with reference to the accompanying drawings, in which:

Fig ta and lb illustrate a converter cell according to prior art.

Fig 2 is a schematic circuit diagram of an MMC in double-star topology, in accordance with embodiments of the present invention.

Fig 3 is a schematic circuit diagram of a full-bridge converter cell illustrating the DC energy storage discharging path and the DC pole-to-pole fault path in the full-bridge converter cell when the crowbar leg is conducting in respective direction(s), in accordance with embodiments of the present invention. Fig 4a is a schematic circuit diagram of a half-bridge converter cell with a divided crowbar, in accordance with embodiments of the present invention.

Fig 4b is a schematic circuit diagram illustrating the AC bypass path in the half-bridge converter cell of figure 4a when the crowbar leg is conducting, in accordance with embodiments of the present invention. Fig 5 is a schematic circuit diagram of a full-bridge converter cell with a divided crowbar, illustrating the AC bypass path in the full-bridge converter cell when the crowbar leg is conducting, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown.

However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

Figure 2 illustrates power converter 1, here in the form of an MMC having converter arms 3 of series connected (also called chain-link or cascaded) converter cells 4, here an Alternating Current (AC) to Direct Current (DC) converter 1 in double-star topology comprising three phases 2, each having a first converter arm 3a connected to the positive DC terminal DC ⁺ and a second converter arm 3b connected to the negative DC terminal DC, each arm shown in series with an arm reactor.

MMC or double Wye (-star) converter topology are herein presented as an example. The developed cell protection method is applicable to any converter cell 4 for a power converter 1, e.g. that can be used to build any chain-link topology such as Delta, Wye, double Wye, Modular Matrix converters, etc. Also, the converter 1 is not limited to an AC-DC converter as shown in the figure, but may e.g. be an AC-AC converter or a Static Synchronous

Compensator (STATCOM).

The proposed DC crowbar arrangement for a FB MMC cell 4 is shown in figure 3. However, the crowbar 7 may in the same way be provided in a HB cell 4. Each cell comprises an energy storage 5, e.g. comprising a capacitor, supercapacitor or battery arrangement, and at least one switch leg 6 connected across (i.e. in parallel with) the energy storage 5 and comprising at least two semiconductor devices T. Each semiconductor device T comprises a one-directional switch S and an antiparallel diode D. A HB cell 4 typically comprises a single switch leg 6 having a first semiconductor device Ti and a second semiconductor device T2 in series. A FB cell 4 as in figures 3 typically comprises both a first switch leg 6a and a second switch leg 6b, where the additional second switch leg 6b is connected in parallel with the first switch leg 6a and comprises a first semiconductor device T3 and a second

semiconductor device T4 in series. At least a first one-directional crowbar switch (e.g. thyristor) Cl, capable of withstanding the full DC voltage, is connected in a crowbar leg 7 connected in parallel to the cell capacitor 5 (and thus also in parallel with the switch leg 6) with minimal inductance. In the event of a shoot-through failure where the GD fails to turn OFF the healthy switch S successfully (or open circuit failure of a switch), the DC crowbar thyristor Cl is triggered to turn ON. In some embodiments, the at least a first one-directional crowbar switch may comprise a plurality of one-directional crowbar switches, e.g. connected in series. Herein, the first crowbar switch is exemplified as a thyristor, but any other suitable one-directional semiconductor switch may be used.

As seen from past experiences, the thyristor Cl would conduct most of the cell capacitor energy, relaxing the energy handling requirements of the switch S modules. Thus, an explosion of the bond wire modules may be prevented. Also, the thyristor fails (conduct) into low impedance, long time duration, short circuit mode after conducting such large amounts of energies.

Thus, if e.g. the first and second switches Si and S2 or S3 and S4 of a switch leg 6 has failed, the respective antiparallel diodes D conduct in order to bypass the cell at the AC side thereof.

The diodes D in switch legs 6 may carry large currents with the activation of the DC crowbar switch Cl during a cell shoot-through fault, reducing the reliability/lifetime of the diodes D after such an event. In order to not damage the antiparallel diodes D, and thus ensure the cell can be reliably bypassed, a one-directional reverse semiconductor crowbar device CR (e.g. comprising a diode) which is antiparallel to the first crowbar switch Cl is included in the cell. That the reverse crowbar device CR is antiparallel to the first crowbar switch implies that it is connected in parallel with the first crowbar switch, and thus connected across the energy storage 5 as well as the switch leg(s) 6, but is arranged to, when conducting, conduct in the opposite direction of the direction in which the first crowbar switch is able to conduct (when switched to conducting). Thus, the reverse crowbar device CR is able to handle at least some of the current during a negative half cycle of resonance after a shoot- through fault when the energy storage is short circuited, taking some of the current load from the antiparallel diodes D.

The reverse crowbar device CR is herein exemplified as a diode, e.g. a presspack diode, but any other suitable reverse-blocking semiconductor device may alternatively be used. A presspack diode may be added in parallel to the energy storage 5 to reduce the surge current rating of the diodes D in the semiconductor devices T. During the negative half cycle of resonance, the current is split between the diodes in the switch legs 6 and the reverse crowbar device CR placed across the energy storage 5. The switch legs 6 each has two diodes D in series whereas only one reverse crowbar device, e.g. diode, CR may be needed across the energy storage, which may result in that most of the current flows through the added reverse crowbar device, which helps to increase the reliability /lifetime of the diodes D after such an event, especially if the semiconductor devices T are bond wire modules. The proposed crowbar arrangement may be combined with a split crowbar comprising at least first and second series connected semiconductor crowbar switches Cl and C2 as shown in figures 4a, 4b and 5 for HB and FB cells 4, respectively.

For a HB cell 4, two crowbar switches (thyristors) C, a first thyristor Cl and a second thyristor C2, each capable of withstanding the full DC voltage, are connected in series in the crowbar leg 7 connected antiparallel to the reverse semiconductor crowbar device CR (and thus also in parallel with the switch leg 6 and cell capacitor 5) with minimal inductance. In the event of a shoot- through failure where the GD fails to turn OFF the healthy switch S successfully (or open circuit failure of a switch), the DC crowbar thyristors C are triggered to turn ON.

As seen from past experiences, the thyristors C would conduct most of the cell capacitor energy, relaxing the energy handling requirements of the switch S modules. Thus, an explosion of the bond wire modules may be prevented. Also, the thyristors fail (conduct) into low impedance, long time duration, short circuit mode after conducting such large amounts of energies. Hence, as seen from the illustration of figure 4b, the cell output is bypassed by the conducting second thyristor C2. This does not depend on the diodes D for the cell output bypass. A single gate pulse trigger can be paralleled to all the thyristors since it is not of concern how the thyristors are triggered. It is to be noted that though two series connected thyristors of cell DC voltage rating is required, the energy handling capabilities of each thyristor is reduced to half compared to a single thyristor.

The same is in applicable parts true for a FB cell 4, as seen in figure 5. Three thyristors C, a first thyristor Cl, a second thyristor C2 and a third thyristor C3, connected in series, each capable of withstanding the full DC voltage but typically with only 1/3 energy handling capability, are connected in parallel to the cell capacitor 5 with minimal inductance. In the event of a shoot-through failure, all three thyristors are triggered to turn ON. Again, the cell output is, as for the HF cell, bypassed by the conducting second thyristor C2.

For both HF and FB cells 4, the semiconductor devices T, especially the diodes D thereof, can be bypassed with regard to the AC conduction path through the cell between the first AC terminal A and the second AC terminal B. As shown in figures 4, the first AC terminal A is connected (typically by direct galvanic connection) to both the a switch leg (here the only switch leg 6 for a HF cell and to the first switch leg 6a for a FB cell), between the first and second semiconductor devices Ti and T2 thereof, and to the crowbar leg 7, between the first and second crowbar switches Cl and C2 thereof.

Additionally, for a FB cell and as shown in figure 5, the second AC terminal B is connected (typically by direct galvanic connection) to both the second switch leg 6b, between the first and second semiconductor devices T3 and T4 thereof, and to the crowbar leg 7, between the second and third crowbar switches C2 and C3. For a HB cell, the second AC terminal B may be connected in a conventional manner as shown in figures 4. In some embodiments of the present invention, the reverse crowbar device CR comprises a diode.

In some embodiments of the present invention, the first crowbar switch Cl comprises a thyristor. In some embodiments of the present invention, each of the switches S of the semiconductor devices T comprises an Insulated-Gate Bipolar Transistor (IGBT), Bi-Mode Insulated Gate Transistor (BiGT), or an Integrated Gate- Commutated Thyristor (IGCT), preferably an IGBT which is suitable for some applications. In some embodiments of the present invention, each of the diodes (D) and switches S of the semiconductor devices T is connected via bond wires. As discussed above, the present invention may be especially useful when bond wires are used, but embodiments of the invention may also be used when bond wires are not used. In some embodiments of the present invention, the reverse crowbar device CR has a presspack configuration.

In some embodiments of the present invention, the power converter 1 is an MMC comprising a plurality of series-connected (cascaded or chain-linked) converter cells of the present disclosure. The power converter 1 may have any suitable topology. MMC or double-Star (-Wye) converter topology are herein presented as an example. The developed cell protection method is applicable to any converter cell 4 for a power converter 1, e.g. that can be used to build any chain-link topology such as Delta, Wye, double Wye, Modular Matrix converters, etc. Also, the converter 1 is not limited to an AC-DC converter as shown in the figure, but may e.g. be an AC-AC converter or a Static

Synchronous Compensator (STATCOM). In some embodiments, as is currently preferred for some applications, the MMC is an AC-to-DC converter, e.g. in a double-star topology. The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.