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. Presspack 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.

It is to be noted that for this method to work, 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.

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 of the resonance period which may reduce the lifetime and reliability of the bond wires even if they survive the shoot-through event itself. Hence a healthy diode cannot be guaranteed after such an event.

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

It is an objective of the present invention to provide a converter cell with improved handling of both DC capacitor discharge due to a shoot-through event and fast and reliable bypass of the failed cell.

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 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, said crowbar leg comprising a plurality of series connected semiconductor crowbar switches arranged to short-circuit the energy storage when the crowbar switches are switched to conducting. The cell also comprises first and second AC terminals, wherein at least one of said AC terminals is connected to one of the at least one switch leg, between two of the plurality of series connected semiconductor devices of said switch leg, and connected to the crowbar leg, between two of the plurality of series connected crowbar switches of said crowbar leg, whereby the first and second AC terminals are arranged to bypass the semiconductor devices via one of said crowbar switches when said one crowbar switch is switched to

conducting.

According to another aspect of the present invention, there is provided a power converter, e.g. an MMC, comprising a plurality of series connected converter cells of the present disclosure.

By the use of a plurality of series connected crowbar switches (e.g. thyristors), the cell bypass path between the output terminals may be formed via at least one of said crowbar switches instead of via the antiparallel diodes of the failed switches, improving the fault handling and reliability of the cell and thus the whole converter.

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 a power converter, here in the form of an MMC in double-star topology, in accordance with embodiments of the present invention. Fig 3a is a schematic circuit diagram of a half-bridge converter cell, in accordance with embodiments of the present invention.

Fig 3b is a schematic circuit diagram illustrating the DC energy storage discharging path and the AC bypass path in the half-bridge converter cell of figure 3a when the crowbar leg is conducting, in accordance with

embodiments of the present invention.

Fig 4a is a schematic circuit diagram of a full-bridge converter cell, in accordance with embodiments of the present invention.

Fig 4b is a schematic circuit diagram illustrating the DC energy storage discharging path and the AC bypass path in the full-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 half-bridge converter cell with a reverse crowbar switch, in accordance with embodiments of the present invention. Fig 6 is a schematic circuit diagram of a full-bridge converter cell with a reverse crowbar switch, 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.

The invention proposes a split DC crowbar arrangement, having a plurality of one-directional crowbar switches in series, to provide a fast and reliable cell output bypass in addition to the handling of a capacitor energy discharge.

The crowbar switches are herein exemplified with thyristors, which is preferred in some embodiments of the invention, but it should be noted that other semiconductor switches may alternatively be used in other

embodiments. Advantageous properties of thyristors as crowbar switches include that they can be fast switched to conducting and then remain conducting at low impedance for a long time after failure. Additionally they are available in hermetically sealed packages capable of handling very high discharge energies making them suitable for Crowbar applications.. Figure 2 illustrates a 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 (also called double-wye or double-Y) 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 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 split DC crowbar arrangement for HB and FB cells 4 is shown in figures 3 and 4, respectively. 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. An HB cell 4 as in figures 3 typically comprises a single switch leg 6 having a first semiconductor device Ti and a second semiconductor device T2 in series. An FB cell 4 as in figures 4 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. 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 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 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 3b, 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. 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. As seen in figure 4b, 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 3 and 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 figures 4, 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 3.

In some embodiments, one, some or all of the crowbar switches C may be complemented with an antiparallel reverse crowbar switch CR, e.g.

comprising a thyristor. Some examples of this are illustrated in figures 5 and 5 for a HF and a FB cell 4, respectively. As can be seen, it may be especially useful with a reverse crowbar switch which is antiparallel to the second crowbar switch C2, as shown in the figures 5 and 6 in order to be able to handle e.g. a DC pole-to-pole fault, where there is a current running in an opposite direction, from the negative DC terminal DC to the positive DC terminal DC ⁺ . The reverse crowbar switch may then form a bi-directional switching device together with the crowbar switch C that it is antiparallel to, e.g. a Bi-Directional Controlled Thyristor (BCT) which may also be called a Phase Control Thyristor (PCT).

As can be seen in figures 5 and 6, in case of an opposite-direction current, a current can flow from the second AC terminal B to the first AC terminal A via the reverse crowbar switch CR.

One of the critical faults for e.g. MMC HB is the DC pole-to-pole fault. With such a fault, the diodes D (D2 in case of HB, D2 and D3 in case of FB) carry large currents (e.g. in the range of 10-20 kA) limited only by the arm reactor impedance, typically for a half to one fundamental cycle until the AC breaker trips to break the current. This puts a requirement on the diode surge current rating. With bond wire modules, the diode surge current rating (i.e., ratio of surge current to nominal current) may be relatively limited compared to a Presspack device. If the second crowbar switch C2 with the proposed split DC crowbar protection is replaced with e.g. a BCT, then the reverse crowbar switch can be triggered to turn ON for such a fault case. The reverse crowbar switch CR would then share the fault current with the diode, increasing the surge current handling capability of the diode significantly. It also helps to increase the lifetime of the bond wire diodes for such fault cases.

Further, it has been seen that STATCOM may require handling of very high Transient Overvoltage (TOV) of up to 2.1 p.u. (per unit). In such cases, the converter arms 3 are usually over-rated (up to 50% with increased reactor impedance) to handle such high TOV. During normal operation i.e., with 1.0 p.u. voltage at the Point of Common Coupling (PCC), many cells (around 30%) are bypassed normally, which cells are mainly required only during the TOV condition. To bypass a FB cell either both first switches Si and S3 or both second switches S2 and S4 are turned ON. With the proposed split DC crowbar, having a reverse crowbar switch CR antiparallel with the second crowbar switch C2 (see figure 6), the reverse crowbar switch CR can be turned ON to bypass the cell to reduce the conduction losses since only one switch is used to bypass the cell rather than two switches. During TOV, the converter l is operated in inductive mode which naturally aids the thyristor commutation bringing back the cell 4 from bypass mode to cell insert mode. 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.

In some embodiments of the present invention, the at least one switch leg 6 consists of one switch leg, comprising a first Ti and a second T2

semiconductor device, forming a half-bridge topology. Then, in some embodiments, the plurality of series connected semiconductor crowbar switches C consists of a first Cl and a second C2 crowbar switch. Then, in some embodiments, the first AC terminal A is connected to the switch leg 6 between the first and second semiconductor devices Ti and T2, and connected to the crowbar leg 7 between the first and second crowbar switches Ci and C2.

In some other embodiments of the present invention, the at least one switch leg consists of a first switch leg 6a and a second switch leg 6b, each switch leg comprising a first semiconductor device Ti or T3 and a second

semiconductor device T2 or T4, forming a full-bridge topology. Then, in some embodiments, the plurality of series connected semiconductor crowbar switches consists of a first Ci, a second C2 and a third C3 crowbar switch. Then, in some embodiments, the first AC terminal A is connected to the first switch leg 6a between the first and second semiconductor devices Ti and T2 thereof, and connected to the crowbar leg 7 between the first and second crowbar switches Cl and C2, and the second AC terminal B is connected to the second switch leg 6b between the first and second semiconductor devices T3 and T4 thereof, and connected to the crowbar leg 7 between the second and third crowbar switches C2 and C3.

In some embodiments of the present invention, a reverse crowbar switch CR is connected across the second crowbar switch C2, antiparallel to the second crowbar switch, whereby the reverse crowbar switch CR is arranged to allow current to flow from the second terminal B to the first terminal A via said reverse crowbar switch when said reverse crowbar switch is switched to conducting. In some embodiments, the second crowbar switch C2 and the reverse crowbar switch CR are comprised in a Bidirectional Control Thyristor (BCT).

In some embodiments of the present invention, each of the crowbar switches C comprises a thyristor. Using a thyristor may be convenient but other suitable semiconductor switches may alternatively be used.

In some embodiments of the present invention, each of the switches S of the semiconductor devices T comprises an Insulated-Gate Bipolar Transistor (IGBT), a 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 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.

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.