TECHNICAL FIELD

The present disclosure relates to a device and method for protection of an auxiliary power supply from short-circuit currents in a converter cell in a

Modular Multilevel Converter (MMC).

BACKGROUND

An MMC, also known as Chain-Link Converter (CLC), comprises a plurality of converter cells, which may alternatively be called converter sub-modules, serially connected in converter branches, or phase legs, that in turn may be arranged in a star, delta, direct or indirect converter topology. Each converter cell comprises, in the form of a half-bridge or full-bridge circuit, a capacitor arrangement for storing energy and power semiconductor switches such as insulated gate bipolar transistor (IGBT) devices, gate-turn-off thyristor

(GTO) devices, integrated gate commutated thyristor (IGCT) devices, or

Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) devices for connecting the capacitor arrangement to the converter branch with one or two polarities. MMCs may be used in electric power transmission systems such as Static Synchronous Compensator (STATCOM), Frequency Converters in direct or indirect topology and High-Voltage Direct Current (HVDC)

transmission.

To power the semiconductor switches of a cell, the cell comprises an auxiliary power supply. In an MMC, the cells are floating in a wide voltage range, especially in high power, medium voltage and HVDC applications. To keep the insulation requirements of the auxiliary power supply simple and achieve an easy scalability, the power for the power supply to supply the gate drive and local control boards can be taken from the capacitor arrangement of the respective cell. By choosing this auxiliary power supply solution, the

protection of the power supply shifts from a low voltage protection scheme with rather low short-circuit current capability demand to the medium

voltage range with very high short-circuit capability demands. SUMMARY

The auxiliary power of a medium or high voltage MMC cell may be fed from its capacitor arrangement by connecting an auxiliary power supply, herein also called a Cell Power Supply (CPS), e.g. a DC/DC converter, which is reducing the medium direct current (DC) voltage to a low voltage level appropriate to supply the devices (e.g. gate drive and/or local control board) within the MMC cell. The capacitor arrangement of the cell may develop a very high short circuit current due to its low equivalent series resistance (ESR). Medium voltage DC fuses which offer the required short-circuit current capability are very bulky and expensive. As the CPS is fed from a medium DC voltage and the auxiliary power demand of the devices within the cell supplied by the CPS is rather low the current consumption of the CPS is typically small. Medium voltage fuses with a low rated current are small and inexpensive but offer only a very limited short-circuit current capability. In order to avoid the use of expensive and bulky fuses, the short-circuit current is in accordance with the present invention limited reliably by means of connecting a resistance in series with the CPS.

According to an aspect of the present invention, there is provided a cell of an MMC. The cell comprises an energy storing device, a CPS, a fuse, and a resistance (also called resistor chain). The CPS, fuse and resistance are connected in series with each other across the energy storing device.

According to another aspect of the present invention, there is provided an MMC comprising at leas one phase-leg having a plurality of series connected cells of the present disclosure. According to another aspect of the present invention, there is provided a method of protecting a CPS in a cell of an MMC. The method comprises reducing a current through a fuse by means of a resistance connected in series with the fuse and the CPS, across an energy storing device of the cell.

The present disclosure presents an inexpensive solution to protect an auxiliary power supply (CPS) within a medium or high voltage multilevel converter cell. The CPS is fed from the main capacitor arrangement of the MMC cell. The capacitor arrangement may develop very high short-circuit currents. In case of a failure within the CPS, a melting fuse is disconnecting the CPS from the capacitor arrangement. A medium voltage fuse with the corresponding high short-circuit capability is bulky and expensive in relation to the semiconductor switches and other devices it protects. The proposed circuit comprising a series connected resistance limits the short-circuit current to a level where inexpensive and small medium voltage fuses may be used. 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 l is a schematic illustration of an embodiment of an MMC, in accordance with the present invention.

Fig 2 is a schematic circuit diagram of an embodiment of a bipolar converter cell comprising a CPS, in accordance with 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 l is a schematic circuit diagram of an embodiment of an MMC l, e.g. a high or medium voltage MMC, having cells with a DC voltage rating of at least 1500 Volts (V), e.g. between 1500 V and 10 kV. The MMC 1 of the figure may be used in electric power transmission systems such as STATCOM, but embodiments of the present invention are also relevant to other topologies such as for Frequency Converters in direct or indirect topology or HVDC transmission. One or more (high or medium voltage) phases, u, v and w having respective currents ii, 12 and 13 are input to the converter 1 via input lines, e.g. via bushings through a wall of the room or building in which the converter 1 is located. In this example the MMC is in delta configuration with three phase legs 2, but any other configuration, such as star (wye, Y) or double-star configuration, and number of phase legs, e.g. three-phase to two- phase MMC, is also possible with embodiments of the present invention. Each phase leg 2 comprises a plurality of cascaded (series connected) cells (also called sub-modules) 3. The currents in the converter 1 are referred to as "i", while the voltages are referred to as "U" in the figure, in combination with arrows indicating directions. The cells 3 may be of any suitable type, e.g. unipolar (also called half-bridge) or bipolar (also called full-bridge or H-bridge), comprising an energy storing device, e.g. a capacitor arrangement, and a plurality of semiconductor switches. Figure 2 illustrates an example of a bipolar cell 3. The cell comprises an energy storing device 5, here in the form of a DC-link capacitor. The energy storing device 5 may comprise a capacitor arrangement with any number of capacitors in series and/or parallel connection with each other. The cell also comprises four semiconductor switches S, forming the full-bridge (H-bridge) topology in the cell. The semiconductor switches of the bipolar cell are conventionally named in the figure as S11 switch, S12 switch, S13 switch and S14 switch. When the switches S11 and S14 are closed and S12 and S13 are open, a positive voltage will be applied. By opening S11 and S14 switches and closing S12 and S13 switches, this voltage is reversed. Each of the S switches may comprise e.g. an IGCT, an RC-IGCT or a BGCT, possibly in combination with an antiparallel one-direction conducting/blocking component such as a diode. In the example of figure 2, each S switch comprises an IGCT and antiparallel diode. If the power supply of the IGCT gate of each S switch will be taken from the DC-link capacitor 5, the IGCT gate unit is un -powered during start-up until the DC-link voltage reaches a certain voltage level. If instead the converter cell 3, and thus the S switches, is charged with an AC current, after zero crossing from >o to <o, the current will commutate from S11 to S13 and from S12 to S14, respectively, and vice versa.

However, in accordance with the present invention, the cell 3 is powered by means of a CPS 4 connected across the energy storing device 5. Preferably, each of the cells 3 of the converter 1 is provided with a CPS 4 connected in accordance with the present disclosure. The CPS 4 may e.g. be a transformer or DC/DC converter which outputs reduced current to semiconductors switches S (typically to gates thereof) and, possibly, to control board(s) of the cell 3. As mentioned above, the CPS 4 needs to be protected from any fault/short circuit current from the energy storing device 5 by means of a fuse 6, typically a melting fuse. In order to reduce the current rating of the fuse 6, to be able to use a smaller and less expensive fuse, a resistance 7 is connected in series with the fuse 6 and the CPS 4.

The resistance 7 may comprise any number of resistors connected in series, e.g. optimized for cost, and may thus be called a resistor chain 7. The resistor chain 7 may for example have a total resistance of at least 100 Ohm, such as at least 150 Ohm, at least 175 Ohm or at least 200 Ohm. One or a few resistors having a relatively high power rating each may be used, or a higher number of resistors each having a lower power rating may be used, depending on how much the possible fault/short circuit current from the energy storing device 5 is to be reduced and on the cost of each (typically commercially available) resistor of different power ratings.

How much the possible fault/short circuit current from the energy storing device 5 is to be reduced in order to be able to use a desired low cost fuse 6 may depend on the voltage of the cell 3. For instance, the possible fault/short circuit current may be above 1000 Amperes (A) if no resistance 7 is used. This fault/short circuit current may be reduced at least by a factor of fiver or of ten, e.g. to less than 250 A, such as less than 100 A, less than 50 A or less than 35 A by means of the resistance 7. How much resistance to introduce is a trade-off between how much to reduce the fault/short circuit current and the losses incurred during normal operations.

The resistance 7 is connected in series with the fuse 6 and the CPS 4, across (in parallel with) the energy storing device 5. In the figure, the resistance 7 is connected between the fuse 6 and the CPS 4 (on the plus-side of the CPS). This order of the series connected devices 4, 6 and 7 may be preferred in order for the resistance 7 to be without voltage if the fuse blows. However, the order is not critical and in alternative embodiments of the invention, the fuse 6 may be connected between the resistance 7 and the CPS 4, or the CPS 4 may be connected between the fuse 6 and the resistance 7 (e.g. with the resistance 7 connected on the minus side of the CPS 4). It is also possible to connect parts of the resistance 7, i.e. some of the series connected resistors of the resistance 7, in different parts of the loop across the energy storing device 5, e.g. with some resistors on either side of the fuse 6 and/or on either side of the CPS 4.

By introducing the resistor chain 7 of series connected (low voltage) resistors in series with the fuse 6, the worst case short-circuit current is limited to a level below the capability of the fuse 6 by choosing the appropriate total resistance of the resistor chain 7. The resistor chain's continuous power rating, pulse power rating and voltage rating may be chosen in accordance with the melting characteristic of the fuse 6. A resistor chain of cheap low voltage resistors may be more economical than one dedicated resistor rated for medium voltage. The resistor chain may be designed to sustain its function for substantially all pre-arcing and melting time intervals of the chosen fuse 6.

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.