SE540919C2 - Auxiliary power supply for control unit for switch in half-bridge Modular Multilevel Converter cell - Google Patents

Abstract

The present disclosure relates to a switch position 36 for a half-bridge Modular Multilevel Converter (MMC) cell 3. The switch position comprises a semiconductor switching unit 32 comprising a Position Control Unit (PCU) 34 for controlling the position of the switching unit and comprising an energy storage for powering the PCU. The switch position also comprises a onedirectional semiconductor device 33 connected anti-parallel to the switching unit. The switch position also comprises an energy transfer unit 35 arranged for charging the energy storage of the PCU by means of electromagnetic induction from a current flowing through the anti-parallel one-directional semiconductor device.

Description

Auxiliary power supply for control unit for switch in half-bridge Modular Multilevel Converter cell TECHNICAL FIELD The present disclosure relates to a switch for a half-bridge Modular Multilevel Converter (MMC).

BACKGROUND During a Direct Current (DC) fault in an MMC cell, the valve/switch is blocked and the Alternating Current (AC) side power is disconnected and the stored energy inside Position Control Units (PCU) of the switch starts to decay since the energy is taken from the position voltage (voltage difference between the different sides of the switch). This might create a critical situation to de-block the valve and retrieve the switching function. There is a general move towards increasing the time period within which the valve can be de-blocked/re-energized after loosing the position voltage. The extension of the time period could be achieved by using larger storage capacitances in the PCUs. A problem with this solution is the available space on the electronics boards, and another drawback is the limited life time of these capacitors which are usually of electrolytic type.

Due to the capacitive nature of the energy storage of PCUs and dependency on position voltage, the cell electronics energy decays fast during fault. This fast decay might result in lack of control if the blocking duration is more than the time period within which the PCU is sufficiently powered. As a consequence, the possibility to retrieve the switching function might be lost. During a fault, the fault current is flowing through the switches of the cell and all the series connected components within the converter switches, typically through the antiparallel diodes of the switches.

SUMMARY An objective of the present disclosure is to provide a way of prolonging the time period during a fault/short circuit of a cell switch within which the PCU is sufficiently powered to be able to de-block the switch.

In accordance with the present invention, energy is obtained from the magnetic flux around the switch components when the fault current is flowing to power the PCUs. In addition to position voltage as a source of energy for normal operation, the magnetic flux energy helps the PCU to stay energized during fault.

According to an aspect of the present invention, there is provided a switch position for a half-bridge Modular Multilevel Converter (MMC) cell. The switch position comprises a semiconductor switching unit 32 comprising a Position Control Unit (PCU) for controlling the position of the switching unit and comprising an energy storage for powering the PCU. The switch position also comprises a one-directional semiconductor device connected antiparallel to the switching unit. The switch position also comprises an energy transfer unit arranged for charging the energy storage of the PCU by means of electromagnetic induction from a current flowing through the anti-parallel one-directional semiconductor device.

According to another aspect of the present invention, there is provided an MMC comprising at least one phase leg comprising a plurality of cascaded converter cells. Each cell comprises a plurality of switches forming a halfbridge topology. Each of said switches comprises at least one switch position in accordance with an embodiment of the present disclosure.

During a fault, short circuit, of a converter cell, a fault current flows through the anti-parallel one-directional semiconductor devices, e.g. diodes, which are arranged in the switches to handle such fault currents and protect the semiconductor switching units, e.g. Insulated Gate Bipolar Transistors (IGBT) or Bi-Mode Insulated Gate Transistors (BIGT) in the switches. The fluctuations of such a fault current give rice to a flux in the magnetic field formed by the fault current. In accordance with the present invention, this magnetic flux is used to power the PCUs (typically comprised in the gates of the semiconductor switching units) of the semiconductor switching units during the fault, allowing the PCUs to be sufficiently powered to control the semiconductor switching units even after a prolonged fault period (e.g. of at least 1 s) without the need for larger energy storages in the PCUs.

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 1 is a schematic block diagram of an embodiment of a phase leg of an MMC, in accordance with the present invention.

Fig 2 is a schematic circuit diagram of an embodiment of a converter cell of an MMC, in accordance with the present invention.

Fig 3 is a schematic circuit diagram of an embodiment of switching positions within a converter cell, 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 1 illustrates an embodiment of a phase leg (typically one of three phase legs) 2 of an MMC 1. The phase leg 2 comprises a plurality of series connected (also called cascaded) converter cells 3. In the embodiment of figure 1, the MMC is in a double-wye (double-Y) configuration, which is preferred in some applications, in which a first arm 2a of the phase leg is connected between the AC and DC<->terminals and a second arm 2b of the phase leg is connected between the AC and DC<+>terminals of the converter 1. However, in other embodiments of the present invention, the converter may have any configuration, such as a single-wye or delta configuration. The converter may have any number of phase legs 2, but a three-phase AC system corresponding to three phase legs 2 may be typical.

Typically, the MMC 1 is a Voltage Source Converter (VSC). However, embodiments of the present invention may also be used with current source converters.

In some embodiments of the present invention, the MMC 1 is configured for being connected to a High Voltage Direct Current (HVDC) system, e.g. between a (high voltage, HV) three-phase AC system/grid and an HVDC system/grid. Embodiments of the present invention may be convenient for HVDC systems which may demand more retrieval time because of application of reclosing function in the overhead DC line or keeping the valve in blocked mode for longer period of time.

In some embodiments of the present invention, the MMC 1 is a rectifier or an inverter, e.g. between a (high voltage, HV) three-phase AC system/grid and an HVDC system/grid.

Figure 2 illustrates an embodiment of a converter cell 3 for an MMC 1. The cell is a half-bridge (also called mono-polar/directional) cell comprising two switches S (S1 and S2, also called valves) forming the half-bridge topology. The cell 3 also comprises an energy storage 31, typically formed by one or a plurality of capacitors. Each switch S comprises at least one switch position 36. Each switch position comprises a semiconductor switching unit 32 comprising a gate G, and a one-directional semiconductor device 33 connected anti-parallel to the switching unit 32. The semiconductor switching unit 32 may e.g. be or comprise an IGBT or a BIGT, preferably an IGBT. The one-directional semiconductor device 33 may e.g. be or comprise a diode or a thyristor, preferably a diode.

Figure 3 illustrates two series connected switch positions 36, which may be part of the same switch S, or be part of different adjacent switches S1 and S2 in a converter cell 3. Each switch may comprise a single switch position or a plurality of switch positions 36, e.g. eight switch positions 36 per switch/valve S. As mentioned in relation to figure 2, each switch position comprises a semiconductor switching unit 32 comprising a gate G, and a onedirectional semiconductor device 33 connected anti-parallel to the switching unit 32. The semiconductor switching unit 32 may e.g. be or comprise an IGBT or a BIGT, preferably an IGBT. The one-directional semiconductor device 33 may e.g. be or comprise a diode or a thyristor, preferably a diode. The gate G of each semiconductor switching unit 32 comprises a PCU 34 for controlling (opening/conducting and closing/blocking) the semiconductor switching unit 32. This PCU 34 may be powered conventionally by the position voltage over the semiconductor switching unit 32, i.e. the position voltage is used to charge a power storage in the PCU, e.g. in the form of a capacitor arrangement such as comprising electrolytic capacitor (s).

As discussed above, in case of a short circuit, where a fault current typically flows through the one-directional semiconductor devices 33 instead of being regulated by the semiconductor switching units 32, the PCUs 34 may no longer be powered by the position voltages (e.g. if the position voltage falls below a threshold, such as 300 V). In accordance with the present invention, an energy transfer unit 35 is used, in addition to or as an alternative to the position voltage (e.g. as determined by a control unit of the converter 1), to power the PCU 34 of each switch position 36. Preferably, each switch position 36 has a respective energy transfer unit 35, but in some embodiments a single energy transfer unit 35 may power the PCUs 34 of a plurality of semiconductor switching units 32, e.g. of adjacent switch positions, in which case an energy transfer unit is shared by a plurality of switch positions. The energy transfer unit 35 of each switch position 36 is arranged for charging the energy storage of the PCU 34 by means of electromagnetic induction from a current flowing through the anti-parallel one-directional semiconductor device 33, i.e. a fault current. This fault current is typically fluctuating, allowing the thus formed magnetic flux around the conductor path through which the fault current flows to generate voltage in the energy transfer unit 35 which charges the energy storage of the PCU 34.

The energy transfer unit 35 of each switch position 36 may comprise e.g. a silicon steel ring core wound with turns of copper wire, or a current transformer (CT) or a Rogowski coil, arranged around a part of the conductor path (including the one-directional semiconductor device 33) through which the fault current flows. The energy transfer unit 35 may e.g. be arranged around the one-directional semiconductor device 33, around a wire conductor connecting to the one-directional semiconductor device, or around a heat sink or other component connected in series with the one-directional semiconductor device 33 in the switch position. Due to the time variations in the flowing fault current (rise and fall periods as well as needle spikes in the current wave form), a variant current is induced in a secondary winding of the energy transfer unit 35. This induced current can be conducted to the PCU 34 and converted to an appropriate wave form which can power the PCU during the fault time period.

The energy transfer unit 35 may be integrated with the one-directional semiconductor device 33 and/or with the switching unit 32. For instance, the energy transfer unit 35 may be integrated in a frame within which both the one-directional semiconductor device 33 and the switching unit 32 are arranged. In some embodiments of the present invention, the switch position 36 is formed as an exchangeable module. The module, comprising the semiconductor switching unit 32, the one-directional semiconductor device 33 and the energy transfer unit 35 (as well as any other suitable components), may then be removed and serviced or replaced without the need to affect other switch positions 36 within a switch S.

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.

Claims (11)

1. A switch position (36) for a half-bridge Modular Multilevel Converter, MMC, cell (3), the switch position comprising: a semiconductor switching unit (32) comprising a Position Control Unit, PCU, (34) for controlling the position of the switching unit and comprising an energy storage for powering the PCU; a one-directional semiconductor device (33) connected anti-parallel to the switching unit (32); and an energy transfer unit (35) arranged for charging the energy storage of the PCU (34) by means of electromagnetic induction from a current flowing through the anti-parallel one-directional semiconductor device (33).

2. The switch position (36) of claim 1, wherein the energy transfer unit (35) comprises a current transformer or a Rogowski coil arranged around a conductor in series with the anti-parallel one-directional semiconductor device (33).

3. The switch position (36) of any preceding claim, wherein the energy transfer unit (35) is integrated with the one-directional semiconductor device (33) and/or with the switching unit (32).

4. The switch position (36) of any preceding claim, wherein the switch position (36) is formed as an exchangeable module.

5. The switch position (36) of any preceding claim, wherein the switching unit (32) is an Insulated Gate Bipolar Transistor, IGBT, or a Bi-Mode Insulated Gate Transistor, BIGT, preferably an IGBT.

6. The switch position (36) of any preceding claim, wherein the antiparallel one-directional semiconductor device (33) is a diode.

7. An MMC (1) comprising at least one phase leg (2) comprising a plurality of cascaded cells (3), each cell comprising a plurality of switches (S) forming a half-bridge topology, each switch (S) comprising at least one switch position (36) in accordance with any preceding claim.

8. The MMC of claim 7, wherein the MMC (1) is a Voltage Source Converter, VSC.

9. The MMC of any claim 7 or 8, wherein the MMC (1) is configured for being connected to a High Voltage Direct Current, HVDC, system.

10. The MMC of any claim 7-9, wherein the MMC (1) is a rectifier or an inverter.

11. The MMC of any claim 7-10, wherein the MMC (1) is in a double-wye configuration.