WO2017153553A1

WO2017153553A1 - Commutation pattern for director switches in hybrid modular multilevel converter

Info

Publication number: WO2017153553A1
Application number: PCT/EP2017/055618
Authority: WO
Inventors: Omar Fadhel Jasim; Raph FELDMAN
Original assignee: General Electric Technology GmbH
Current assignee: GE Vernova GmbH
Priority date: 2016-03-09
Filing date: 2017-03-09
Publication date: 2017-09-14
Anticipated expiration: 2018-09-09
Also published as: GB201604069D0; GB2548133A; GB2548133B

Abstract

In the field of high voltage direct current (HVDC) power transmission there is provided a voltage source converter (VSC) (10). The VSC (10) comprises a plurality of converter limbs (12A, 12B, 12C) which correspond to respective phases (A, B, C) of the VSC (10). Each converter limb (12A, 12B, 12C) extends between first and second DC terminals (14, 16) and includes first and second limb portions (12A+, 12A-, 12B+, 12B-, 12C+, 12C-) that are separated by an AC terminal (18A, 18B, 18C). Each limb portion (12A+, 12A-, 12B+, 12B-, 12C+, 12C-) includes a chain-link converter (24A+, 24A-, 24B+, 24B-, 24C+, 24C-) that is operable to provide a stepped variable voltage source and a primary switching element (Sw _i, Sw ₂, S _W3, Sw ₄, Sw ₅, S _W6) which is selectively operable in a conducting mode in which current is able to flow therethrough and a non-conducting mode in which current is unable to flow therethrough. Each converter limb (12A, 12B, 12C) is selectively operable in a fully-conducting mode in which the primary switching element (Swi, Sw ₂, S _W3, S _W4, S _W5, S _W6) in each limb portion (12A+, 12A-, 12B+, 12B-, 12C+, 12C-) thereof operates in its conducting mode and a partially-conducting mode in which the primary switching element (Sw _i, S _W2, S _W3, S _W4, Sw ₅, Sw ₆) in one limb portion (12A+, 12A-, 12B+, 12B-, 12C+, 12C-) thereof operates in its conducting mode and the primary switching element (Swi, S _W2, S _W3, S _W4, Sw ₅, S _W6) in the other limb portion (12A+, 12A-, 12B+, 12B-, 12C+, 12C-) thereof operates in its non-conducting mode. The VSC (10) also includes a controller (32) which is programmed to transition a given converter limb (12A, 12B, 12C) from its fully-conducting mode to its partially-conducting mode by (a) transitioning another converter limb (12A, 12B, 12C) from its partially-conducting mode to its fully-conducting mode while the given converter limb (12A, 12B, 12C) to be transitioned from its fully-conducting mode to its partially-conducting mode remains in its fully-conducting mode; and (b) circulating a commutation current (1 _c, _mA, I _comB, I _com c) around a commutation loop (40) defined by the converter limb (12A, 12B, 12C) transitioned into its fully-conducting mode and the converter limb (12A, 12B, 12C) remaining in its fully- conducting mode to facilitate the switching into its non-conducting mode of a primary switching element (Sw ₁, S _W2, S _W3, S _W4, Sw5, Sw ₆ ) in the given converter limb (12A, 12B, 12C) to be transitioned into its partially-conducting mode.

Description

COMMUTATION PATTERN FOR DIRECTOR SWITCHES IN HYBRID

MODULAR MULTILEVEL CONVERTER

This invention relates to a voltage source converter for use in high voltage direct current (HVDC) power transmission. In high voltage direct current (HVDC) power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or under-sea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the power transmission medium, i.e. the transmission line or cable, and reduces the cost per kilometre of the lines and/or cables, and thus becomes cost-effective when power needs to be transmitted over a long distance.

The conversion between DC power and AC power is utilized in power transmission networks where it is necessary to interconnect the DC and AC electrical networks. In any such power transmission network, converters are required at each interface between AC and DC power to effect the required conversion; AC to DC or DC to AC.

According to an aspect of the invention there is provided a voltage source converter, for use in high voltage direct current power transmission and distribution, comprising: a plurality of converter limbs corresponding to respective phases of the voltage source converter, each converter limb extending between first and second DC terminals and including first and second limb portions separated by an AC terminal, each limb portion including a chain-link converter operable to provide a stepped variable voltage source and a primary switching element selectively operable in a conducting mode in which current is able to flow therethrough and a non-conducting mode in which current is unable to flow therethrough, each converter limb being selectively operable in a fully-conducting mode in which the primary switching element in each limb portion thereof operates in its conducting mode and a partially-conducting mode in which the primary switching element in one limb portion thereof operates in its conducting mode and the primary switching element in the other limb portion thereof operates in its non-conducting mode; and a controller programmed to transition a given converter limb from its fully- conducting mode to its partially-conducting mode by: (a) transitioning another converter limb from its partially-conducting mode to its fully-conducting mode while the given converter limb to be transitioned from its fully-conducting mode to its partially-conducting mode remains in its fully-conducting mode; and (b) circulating a commutation current around a commutation loop defined by the converter limb transitioned into its fully-conducting mode and the converter limb remaining in its fully-conducting mode to facilitate the switching into its non-conducting mode of a primary switching element in the given converter limb to be transitioned into its partially-conducting mode. The selective creation of a commutation loop defined by a converter limb transitioned into its fully-conducting mode and a converter limb remaining in its fully-conducting mode controls the path taken by the commutation current. More particularly such a commutation loop restricts the flow of commutation current to solely within the voltage source converter of the invention, and so avoids causing unwanted current distortion and/or voltage distortion within the AC and DC networks that are, in use, interconnected by the voltage source converter.

Preferably each primary switching element includes a director switch selectively openable and closeable to respectively prevent and allow the flow of current therethrough and an anti-parallel diode electrically connected in anti-parallel with the director switch; and the controller is programmed to circulate a commutation current around the commutation loop to drive to zero the current flowing through the anti- parallel diode of the primary switching element to be switched into its non-conducting mode.

The circulation of a commutation current around the commutation loop to drive to zero the current flowing through the anti-parallel diode of the primary switching element to be switched into its non-conducting mode allows the anti-parallel diode to be held in a reverse-biased non-conducting condition, to help complete the switching of a given primary switching element into its non-conducting mode, even during unbalanced operating conditions of the voltage source converter when significant fluctuations in the current flowing through the said anti-parallel diode might otherwise arise.

Optionally prior to circulating a commutation current around the commutation loop the controller is programmed to establish whether current is flowing through the anti- parallel diode of the primary switching element to be switched into its non-conducting mode and, if current is not flowing through the said anti-parallel diode, the controller is programmed to: circulate an initiating commutation current around the commutation loop to cause the said anti-parallel diode to begin conducting; establish that the said anti-parallel diode is conducting current; and open the director switch in the said primary switching element to be switched into its non-conducting mod while the said anti-parallel diode is conducting current.

Opening the director switch of the primary switching element to be switched into its non-conducting mode while the corresponding anti-parallel diode is conducting current provides several benefits.

Firstly, by deliberately forcing the anti-parallel diode to conduct and establishing that this is the case, it is readily possible to determine that the director switch is no longer conducting the normal high load current required to transfer power between the DC and AC terminals. As a consequence reliable timing is provided for opening of the director switch, i.e. turning the director switch off, under desirable low current carrying conditions.

More particularly, and secondly, the magnitude of the initiating commutation current can be controlled, and so it is possible to establish a relatively low initiating commutation current, e.g. just 0.5A or so.

Therefore, thirdly, the net current flowing through the primary switching element including the director switch to be opened and the now-conducting corresponding anti- parallel diode, can be similarly controlled to be relatively low such that the actual current flowing through the director switch to be opened, i.e. the proportion of the net current shared between the director switch and the corresponding anti-parallel diode, is also very low. Consequently, when the said director switch is opened to prevent the flow of current therethrough the actual current flowing through it is very low, i.e. close to zero, and so soft switching of the said director switch is able to take place to thereby help avoid the occurrence of an undesirable and potentially harmful voltage spike across the director switch. Accordingly, the controller is programmed to help ensure soft switching of a director switch that is to be opened. In contrast conventional voltage source converters, which rely on the current flowing through a given director switch naturally being close to zero as a converter limb begins to transition from its fully-conducting mode to its partially- conducting mode, are susceptible to significant fluctuations in the current flowing through a director switch to be opened during unbalanced operating conditions of the converter, and so cannot guarantee that soft switching will occur.

The controller may be additionally programmed to control the initiating commutation current to drive to zero the current flowing through the said anti-parallel diode. Such a feature desirably helps to ensure that the said anti-parallel diode can be held in a reverse-biased non-conducting condition even during unbalanced operating conditions of the voltage source converter when significant fluctuations in the current flowing through the said anti-parallel diode might otherwise arise.

In another preferred embodiment of the invention prior to circulating a commutation current around the commutation loop the controller is programmed to establish whether current is flowing through the anti-parallel diode of the primary switching element to be switched into its non-conducting mode and, if current is flowing through the said anti- parallel diode, the controller is programmed to open the director switch in the said primary switching element to be switched into its non-conducting mode. Having a controller programmed to carry out the aforementioned steps helps to ensure that the said director switch is opened, so as to prevent the flow of current therethrough, when no current is flowing through it, and so soft switching of the said director switch takes place. As a result the occurrence of an undesirable and potentially harmful voltage spike across the director switch is avoided. In a still further preferred embodiment of the invention after opening the director switch of the primary switching element to be switched into its non-conducting mode the controller is programmed to circulate an extinguishing commutation current around the commutation loop to drive to zero the current flowing through the said anti-parallel diode. Such a feature desirably helps to ensure that the said anti-parallel diode can be held in a reverse-biased non-conducting condition even during unbalanced operating conditions of the voltage source converter when significant fluctuations in the current flowing through the said anti-parallel diode might otherwise arise.

Preferably the controller is further programmed to impose a reverse bias voltage across the said anti-parallel diode to maintain it in a reverse-biased non-conducting condition.

Such a controller assists the converter limb in question to operate normally in its partially-conducting mode, i.e. when current flows through only one limb portion thereof.

Optionally establishing whether current is flowing through the said anti-parallel diode includes observing the polarity of the voltage across the primary switching element to be switched into its non-conducting mode.

As the said anti-parallel diode within the primary switching element to be switched into its non-conducting mode begins to conduct current the voltage thereacross, and hence the voltage across the corresponding primary switching element, changes sense, e.g. changes from positive to negative. Consequently, observing the polarity of such a voltage reliably and repeatedly indicates whether the anti-parallel diode is conducting.

The controller may include a control sub-unit configured to determine from the observation of voltage polarity across the primary switching element to be switched into its non-conducting mode the direction of current flow through the said primary switching element. Such a determination can be helpful during, e.g. low power or hot start operation of the voltage source converter of the invention.

More particularly, in low power operation of the converter or hot start operation of the converter, i.e. operation of the converter when it is energised but only supplying losses, the current passing through the limb portions is relatively small. Therefore observing the polarity of the voltage across the primary switching element to be switched into its non-conducting mode is more reliable than, e.g. relying instead on the detection of a current polarity by a current transducer calibrated to measure large currents of typically 1.5kA which as a consequence has difficulty identifying the polarity of a relatively small current, particularly when noise is also present. Preferably the controller is still further programmed to: obtain a respective AC current demand phase waveform for each converter limb which the corresponding converter limb is required to track, and a DC current demand which each converter limb is also required to track; and carry out mathematical optimization to determine for each limb portion an optimal limb portion current which as necessary includes a commutation current component and which the limb portion must contribute to track the corresponding required AC current demand phase waveform and the required DC current demand.

Carrying out the aforementioned mathematical optimization, i.e. selecting the best individual limb portion current (with regard to chosen criteria) from a set of available alternatives, allows the AC and DC current demands to be controlled independently of one another, e.g. by a higher level controller.

It also permits individual limb portion currents to vary independently of one another to accommodate different current flow paths through the converter, e.g. as occasioned by the selective definition of fully-conducting and partially-conducting converter limbs throughout each operating cycle of the voltage source converter.

Moreover, the controller is able to carry out the aforementioned steps in real time so as to permit robust control of the voltage source converter of the invention.

The controller may be programmed to carry out mathematical optimization by considering an equivalent converter configuration which represents the flow of current through the voltage source converter.

Considering an equivalent converter configuration in the aforementioned manner imposes constraints on the way in which the voltage source converter can be controlled and so assists in carrying out mathematical optimization to determine each optimal limb portion current. In a still further preferred embodiment of the invention the controller is programmed to consider an equivalent converter configuration which represents the flow of current through the voltage source converter by mapping possible current flow paths through the converter. Mapping the possible current flow paths through the converter helps the controller to tailor the mathematical optimization it provides to the topology, i.e. structure, of the voltage source converter of the invention.

In yet another preferred embodiment of the invention the controller is programmed to carry out mathematical optimization to determine one or more minimum individual limb portion currents that the corresponding limb portion must contribute to track the corresponding required AC current demand phase waveform and the required DC current demand.

Determining one or more minimum individual limb portion currents reduces the conduction and switching losses in each limb portion because ordinarily such losses are proportional to current and current squared, i.e. I and I².

There now follows a brief description of preferred embodiments of the invention, by way of non-limiting example, with reference being made to the following figures in which: Figure 1 shows a schematic view of a voltage source converter according to a first embodiment of the invention;

Figure 2 shows a preferred switching sequence of primary switching elements within the voltage source converter shown in Figure 1 during an operating cycle of the voltage source converter; Figures 3(a) to 3(c) show the voltage source converter of Figure 1 at various stages as a given converter limb transitions from its fully-conducting mode to its partially-conducting mode;

Figure 4 illustrates schematically the flow of current through a primary switching element within a limb portion of the given converter limb as the converter limb transitions in a first manner from a fully-conducting mode to a partially conducting mode;

Figure 5 shows the flow of current in various areas of the voltage source converter shown in Figure 1 during its operation as each converter limb therein transitions from a fully-conducting mode to a partially-conducting mode in the first manner illustrated in Figure 4; and Figure 6 shows a schematic representation of an equivalent converter configuration corresponding to the voltage source converter shown in Figure 1 .

A voltage source converter according to a first embodiment of the invention is designated generally by reference numeral 10, as shown in Figure 1 . The voltage source converter 1 0 includes three converter limbs 12A, 12B, 12C, each of which corresponds to a respective phase A, B, C of the converter 1 0. In other embodiments of the invention the voltage source converter 10 may include fewer than or more than three converter limbs.

Meanwhile, returning to the embodiment shown, each converter limb 12A, 12B, 12C extends between first and second direct current (DC) terminals 14, 1 6, and each converter limb 12A, 12B, 12C includes a first limb portion 12A+, 12B+, 12C+ and a second limb portion 12A-, 12B-, 12C- which are separated by a respective AC terminal 1 8A, 1 8B, 18C.

In use, the first and second DC terminals 14, 16 are connected to a DC network 20, with a voltage of VDC arising between the first and second DC terminals 14, 16, while each AC terminal 1 8A, 1 8B, 18C is connected to a corresponding phase A, B, C of a three-phase AC network 22 and carries a corresponding AC voltage phase waveform

V_A, V_B, V_C

Each limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C- has a chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- that includes a chain of modules 26 connected in series. The number of modules 26 in each chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- depends on the required voltage rating of the respective limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C-.

Each module 26 of each chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- includes two pairs of secondary switching elements (not shown) connected in parallel with an energy storage device, in the form of a capacitor (not shown), to define a 4- quadrant bipolar module 26 that can provide negative, zero or positive voltage and can conduct current in two directions.

In use, the secondary switching elements of the modules 26 of each chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- are operated to enable each chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- to provide a stepped variable voltage source.

It is therefore possible to build up a combined voltage across each chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- which is higher than the voltage available from each individual module 26 via the insertion of the capacitors of multiple modules 26, each providing its own voltage, into the chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C-.

The ability of a 4-quadrant bipolar module 26 to provide positive or negative voltages means that the voltage across each chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- may be built up from a combination of modules 26 providing positive or negative voltages. The energy levels in individual capacitors may be maintained therefore at optimal levels by controlling the modules 26 to alternate between providing positive or negative voltage.

It is possible to vary the timing of switching operations for each module 26 such that the insertion and/or bypass of the capacitors of individual modules 26 in the chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- results in the generation of a voltage waveform at a corresponding AC terminal 18A, 18B, 18C. For example, insertion of the capacitors of the individual modules 26 may be staggered to generate a sinusoidal waveform. Other waveform shapes may be generated by adjusting the timing of switching operations for each module 26 in the chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C-.

In this manner the chain-link converters 24A+, 24A-, 24B+, 24B-, 24C+, 24C- are able to facilitate power transfer between the AC and DC networks 22, 20.

In other embodiments of the invention (not shown) one or more modules 26 in at least one chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- may include a single pair of secondary switching elements that are connected in parallel with an energy storage device, in the form of a capacitor, to define a 2-quadrant unipolar module 26 that can provide zero or positive voltage and can conduct current in two directions.

In each instance each of the secondary switching elements in the respective chain-link converters 24A+, 24A-, 24B+, 24B-, 24C+, 24C- is an insulated gate bipolar transistor (IGBT) that is connected in parallel with an anti-parallel diode, although other semiconductor secondary switching elements may also be used.

Each limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C- also includes a primary switching element Sw₁, Sw₄, Sw₃, Sw₆, Sw₅, Sw₂ that is connected in series with the corresponding chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C-.

The series connection between each primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ and the corresponding chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- in each limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C- allows, in other embodiments of the invention, the respective primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ and corresponding chain-link converter 24A+, 24A-, 24B+, 24B-, 24C+, 24C- to be connected in a reverse order between the corresponding AC terminal 18A, 18B, 18C and the respective first or second DC terminal 14, 16.

Each primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ includes a director switch 28+, 28- which is electrically connected in anti-parallel with a corresponding anti-parallel diode 30+, 30-.

In the embodiment shown, each director switch 28+, 28- is an IGBT, although other forced commutated semiconductor switches, such as a field effect transistor, a gate- turn-off thyristor, an injection gate enhanced thyristor, or an integrated gate commutated transistor may also be used. Also, in other embodiments of the invention (not shown) one or more of the director switches 28+, 28- may include a plurality of, e.g. series-connected, switch elements depending on the required voltage rating of each limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C-.

In any event, each director switch 28+, 28- is selectively openable and closeable to respectively prevent and allow the flow of current therethrough. In other words, each director switch 28+, 28- can be turned off to prevent the flow of current therethrough and turned on to allow the flow of current therethrough.

The inclusion in each primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆0f such a director switch 28+, 28- and anti-parallel diode 30+, 30- combination allows each primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ to selectively operate in a conducting mode in which current is able to flow therethrough and a non-conducting mode in which current is unable to flow therethrough.

Moreover, the inclusion in each limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C- of such primary switching elements Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ allows each converter limb 12A, 12B, 12C to selectively operate in:

(i) a fully-conducting mode in which the primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ in each limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C- thereof operates in its conducting mode; and

(ii) a partially-conducting mode in which the primary switching element Sw₁ , Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ in one limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C- thereof operates in its conducting mode and the primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ in the other limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C- thereof operates in its non-conducting mode.

In addition to the foregoing, the voltage source converter 10 includes a controller 32 that is arranged in operative communication with each primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆.

The controller 32 is a programmable device, such as a microcontroller, and more particularly is programmed to transition each converter limb 12A, 12B, 12C from its fully-conducting mode to its partially-conducting mode. By way of example, and for reasons of clarity, such programming of the controller 32 is illustrated and described herein with respect to the transition of a phase A converter limb 12A from its fully-conducting mode, as illustrated schematically in Figure 3(a) (i.e. with the primary switching element Sw₁, Sw₄ in each limb portion 12A+, 12A- thereof operating in its conducting mode to permit the flow of current therethrough) to its partially-conducting mode, as illustrated in Figure 3(c).

The controller 32 is programmed to accomplish the said transition by:

(a) transitioning another converter limb 12C from its partially-conducting mode to its fully-conducting mode while the given converter limb 12A to be transitioned from its full-conducting mode to its partially-conducting mode remains in its fully- conducting mode; and (b) circulating a commutation current -l_comA, I_comA, around a commutation loop 40 defined by the converter limb 12C transitioned into its fully-conducting mode and the converter limb 12A remaining in its fully-conducting mode to facilitate the switching into its non-conducting mode of a primary switching element Sw₁ in the given converter limb 12A to be transitioned into its partially-conducting mode.

In particular, the controller 32 is programmed to carry out step (a), i.e. transitioning another converter limb 12C from its partially-conducting mode to its fully-conducting mode while the phase A converter limb 12A remains in its fully-conducting mode, by selectively defining fully-conducting converter limbs 12A, 12B, 12C for regular fully- conducting periods 34₁, 34₂, 34₃, 34₄, 34₅, 34₆ of 62 electrical degrees during an operating cycle 36 of the voltage source converter 10.

The fully-conducting periods 34₁, 34₂, 34₃, 34₄, 34₅, 34₆ are arranged relative to one another such that a given period 34₁, 34₂, 34₃, 34₄, 34₅, 34₆ overlaps with each of the adjacent periods 34₁, 34₂, 34₃, 34₄, 34₅, 34₆ for a corresponding overlap period 38i , 382, 38₃, 38₄, 385, 38₆ of 2 electrical degrees, i.e. as shown in Figure 2.

During each respective overlap period 38i , 38₂, 38₃, 38₄, 38s, 38₆ two converter limbs 12A, 12B, 12C operate in their respective fully-conducting modes, i.e. the voltage source converter 10 is said to be operating with a double overlap.

For example, during a fourth fully-conducting period 34₄ the phase A converter limb 12A is in its fully-conducting mode. The controller 32 then transitions the phase C converter limb 12C from its partially-conducting mode, e.g. in which the primary switching element Sw₅ in the first limb portion 12C+ thereof is operating in its nonconducting mode to inhibit the flow of current therethrough and the primary switching element Sw₂ in the second limb portion 12C- is operating in its conducting mode to allow the flow of current therethrough (i.e. as shown in Figure 3(a)), to its fully- conducting mode in which the primary switching element Sw₅, Sw₂ in both the first and second limb portions 12C+, 12C- is operating in its conducting mode to permit the flow of current therethrough (i.e. as shown in Figure 3(b)).

Accordingly the phase C converter limb 12C operates in its fully-conducting mode during a fifth fully-conducting period 34₅ which overlaps with the fourth fully-conducting period 34₄ by a fourth said 2 electrical degree overlap period 38₄. In other embodiments of the invention each fully-conducting period may last for 64 electrical degrees such that adjacent fully-conducting periods overlap with one another for an overlap period of 4 electrical degrees. In still further embodiments of the invention the respective overlap periods may differ from the aforementioned 2 and 4 electrical degrees.

Returning to the example embodiment described above, once the controller 32 has transitioned the phase C converter limb 12C from its partially-conducting mode to its fully-conducting mode while the phase A converter limb 12A remains in its fully- conducting mode, i.e. during the fourth overlap period 38₄ and prior to circulating a commutation current -l_CcmA, I_comA, around the commutation loop 40, the controller 32 is programmed to establish whether current is flowing through the anti-parallel diode 30+ of the primary switching element Sw₁ to be switched into its non-conducting mode.

The controller 32 is programmed to establish whether current is flowing through the said anti-parallel diode 30+ by observing the polarity of the voltage across the primary switching element to be switched into its non-conducting mode, i.e. the primary switching element Sw₁ of the first limb portion 12A+. More particularly the controller 32 is programmed to establish that the anti-parallel diode 30+ is conducting when the polarity changes from positive (which corresponds to a flow of current from the first DC terminal 14 to the phase A AC terminal 18A) to negative (which corresponds to a flow of current from the phase A AC terminal 18A to the first DC terminal 14).

More particularly still, in the embodiment shown the controller 32 includes a plurality of respective control sub-units 4₂ (only one of which is shown in Figure 1 ), each of which is operatively associated with a respective primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ and each of which takes the form of a gate driver circuit to directly open and close the corresponding director switch 28+, 28-, i.e. to directly turn the corresponding director switch 28+, 28- off and on.

Each such control sub-unit 4₂ is additionally configured to determine from the observation of the voltage polarity across the corresponding primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ the direction of current flow through the said primary switching element Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆. In other embodiments of the invention the controller 32 may instead be programmed to establish that a given anti-parallel diode 30+, 30- is conducting by measuring the current flowing through the said diode 30+, 30-, e.g. using a current transducer.

In any event, depending on whether current is flowing through the anti-parallel diode 30+ of the primary switching element Sw₁ to be switched into its non-conducting mode, the controller 32 is programmed to continue with the transition of the given converter limb 12A from its fully-conducting mode to its partially-conducting mode in one of two different manners.

If current is not flowing through the said anti-parallel diode 30+, the controller 32 is programmed to carry out the transition in a first manner, and more particularly is programmed to: circulate an initiating commutation current -l_comA around the commutation loop 40 to cause the said anti-parallel diode 30+ to begin conducting, as shown in Figure 3(b); establish that the said anti-parallel diode 30+ is conducting current; and open the director switch 28+ in the said primary switching element Sw₁ to be switched into its non-conducting mode while the said anti-parallel diode 30+ is conducting current.

By convention current flowing within the voltage source converter 10 is shown as flowing in a direction equivalent to the flow of current from the DC network 20 into the voltage source converter 10. The initiating commutation current -l_comA is, however, given a negative sign to show that its effect is to counteract any other opposing current flowing in the first limb portion 12A+ of the first converter limb 12A such that a net amount of current is urged to flow through the said anti-parallel diode 30+ such that it begins to conduct.

The controller 32 establishes that the anti-parallel diode 30+ is conducting current in the same manner as described herein above, i.e. by observing the polarity of the voltage across the primary switching element Sw₁ of the first limb portion 12A+.

Once the controller 32 has established that the anti-parallel diode 30+ in the primary switching element Sw₁ of the first limb portion 12A+ of the phase A converter limb 12A is conducting current it is programmed, while the said anti-parallel diode 30+ continues to conduct current, to open the corresponding director switch 28+, i.e. to open the director switch 28+ of the primary switching element Sw₁ of the first limb portion 12A+ of the phase A converter limb 12A which is to be switched into its non-conducting mode, to inhibit the flow of current through the said director switch 28+, i.e. as shown in Figure 3(c).

The controller 32 is also additionally programmed, once it has opened the said director switch 28+, to control the initiating commutation current -l_comA to drive to zero the current flowing through the corresponding anti-parallel diode 30+. More particularly, the controller 32 is programmed to control the said initiating commutation current -l_comA in a manner that reduces the rate of change of current experienced by the anti-parallel diode 30+ so that any reverse recovery loss is mitigated. One way in which the controller 32 might achieve this is to ramp the commutation current l_comA uniformly down to zero, although the controller 32 may achieve the same reduction to zero in a different manner.

Thereafter the controller 32 is programmed to impose a reverse bias voltage V_r across the anti-parallel diode 30+ to maintain it in a reverse-biased non-conducting condition, as also shown in Figure 3(c). In the embodiment shown the controller 32 establishes such a reverse bias voltage via control of the chain-link converters 24A+, 24A- in the first and second limb portions 12A+, 12A- of the phase A converter limb 12A.

It follows that, by way of example and with reference to Figure 4, in use a current I_A+ flows through the primary switching element Sw₁ in the first limb portion 12A+ of the phase A converter limb 12A in the following manner.

Initially during a first operating period 44₁, with the director switch 28+ in the primary switching element Sw₁ closed to permit the flow of current through the primary switching element Sw₁, as shown in Figure 3(a), current I_A+ flows through the director switch 28+ from the first DC terminal 14 to the phase A AC terminal 18A.

During a second operating period 44₂ which coincides with the phase A and phase C converter limbs 12A, 12C both operating in their respective fully-conducting modes, i.e. coincides with the fourth overlap period 38₄, the current I_A+ flowing through the primary switching element Sw₁ falls as power transfer is handed over from the phase A converter limb 12A to the phase C converter limb 12C. The initiating commutation current -l_comA is circulated around the commutation loop 40 created by both the phase A and phase C converter limbs 12A, 12C operating in their respective fully-conducting modes and the anti-parallel diode 30+ in the primary switching element Sw₁ begins to conduct as the current I_A+ flowing through the primary switching element Sw₁ changes direction to instead flow from the phase A AC terminal 18A to the first DC terminal 14.

It is established during a third operating period 44₃ that current is being conducted by the anti-parallel diode 30+, as illustrated schematically in Figure 3(b). After a short predetermined delay of, e.g. a few nano seconds, the director switch 28+ in the primary switching element Sw₁ is opened, e.g. by applying a 0 (zero) gate signal to the gate terminal of the corresponding IGBT, to inhibit the flow of current through the said director switch 28+.

Thereafter, during a fourth operating period 44₄ the initiating commutation current -l_comA flowing through the primary switching element Sw₁, which is now the only current flowing therethrough, is ramped uniformly down to zero, such that no current I_A+ flows through the primary switching element Sw₁ and hence no current I_A+ flows through the anti-parallel diode 30+ located therein.

During a fifth operating period 44₅ the reverse-bias voltage V_r is imposed on the anti- parallel diode 30+ to maintain it in a non-conducting condition and so, with both the anti-parallel diode 30+ and corresponding director switch 28+ preventing the flow of current therethrough, no current I_A+ flows through the primary switching element Sw₁.

In addition to the foregoing, the controller 32 is also programmed to carry out the transition of the first converter limb 12A from its fully-conducting mode to its partially- conducting mode in a second manner if current is flowing through the said anti-parallel diode 30+ when the controller 32 checks prior to circulating a commutation current around the commutation loop 40.

More particularly the controller 32 is programmed to immediately open the director switch 28+ in the primary switching element Sw₁ in the first limb portion 12A+ of the first converter limb 12A, i.e. to immediately open the director switch 28+ in the primary switching element Sw₁ to be switched into its non-conducting mode, as shown in Figure 3(b). After opening the said director switch 28+ the controller 32 is programmed to circulate an extinguishing commutation current +l_comA around the commutation loop 40 to drive to zero the current already flowing through the corresponding anti-parallel diode 30+, as also shown in Figure 3(b). The extinguishing commutation current +l_comA flows from the first DC terminal 14 to the phase A AC terminal 18A and so counteracts the current already flowing through the said anti-parallel diode 30+ such that the net amount of current flowing through the said anti-parallel diode 30+ falls to zero and the anti-parallel diode 30+ thereby ceases to conduct current, as shown in Figure 3(c). Thereafter the controller 32 is again programmed to impose a reverse bias voltage V_r across the anti-parallel diode 30+ to maintain it in a reverse-biased non-conducting condition, as also shown in Figure 3(c). The controller 32 again establishes such a reverse bias voltage via control of the chain-link converters 24A+, 24A- in the first and second limb portions 12A+, 12A- of the phase A converter limb 12A. Accordingly, with the reverse-bias voltage V_r imposed on the anti-parallel diode 30+ to maintain it in a non-conducting condition and the corresponding director switch 28+ opened to prevent the flow of current therethrough, no current is able to flow through the primary switching element Sw₁ in the first limb portion 12A+ of the first converter limb 12A, i.e. the said primary switching element Sw₁ is operating in its non-conducting mode, and so the first converter limb 12 is suitably transitioned into its partially- conducting mode.

The controller 32 is programmed, at an appropriate other operating point in the operating cycle of the converter 10, to similarly transition the phase A converter limb 12A from its fully-conducting mode to its partially-conducting mode by switching the primary switching element Sw₄ in the second limb portion 12A- thereof to operate in its non-conducting mode, and similarly, at still further other operating points, to transition each of the phase B and phase C converter limbs 12B, 12C from respective fully- conducting modes to partially-conducting modes by selectively switching the primary switching elements Sw₂, Sw₃, Sw₅, Sw₆ in each of the first and second limb portions 12B+, 12B-, 12C+, 12C- thereof into their respective non-conducting modes.

The controller 32 is so programmed to carry out the further transitions in either the first or second manner according to whether or not current is flowing through the anti- parallel diode 30+, 30- of the primary switching element Sw₁ , Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ when the controller 32 checks prior to circulating a commutation current around the commutation loop 40.

In other embodiments of the invention the controller may, however, be programmed to transition a given converter limb from its fully-conducting mode to its partially- conducting mode in only one of the first and second manners described herein.

Accordingly, operation of the controller 32 to transition the converter limbs 12A, 12B, 12C from their respective fully-conducting modes to their respective partially- conducting modes gives rise to various current flows throughout the voltage source converter 10 during a given operating cycle 36. When each such transition is carried out by the controller 32 in the first manner described hereinabove, the various current flows are as shown in Figure 5, i.e.:

- current flows I_A+, I_B+, I_C+ in each first limb portion 12A+, 12B+, 12C+ which sum to give a steady DC current demand e in the DC network 20; - current flows I_A-, I_B-, I_C- in each second limb portion 12A-, 12B-, 12C- which similarly sum to give a steady DC current demand be in the DC network 20;

- current flows I_A, b, Ic at each AC terminal 18A, 18B, 18C; and

- respective initiating commutation current flows -l_comA, -I_comB, -l_comC in each corresponding converter limb 12A, 12B, 12C. It follows, as also shown in Figure 5, that the introduction of respective commutation currents l_comA, I_comB, l_comC to facilitate the switching of a corresponding primary switching element Sw₁ , Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ from its conducting mode to its non-conducting mode, and more particularly the introduction of respective initiating commutation currents -I_comA,- I_comB, -l_comC to allow soft switching of a corresponding director switch 28+, 28- with the attendant benefits such switching provides, is achieved without adversely impacting, i.e. distorting, either the sinusoidal currents I_A, I_B, I_C that are generated at the AC terminals 18A, 18B, 18C or the DC current demand be in the DC network 20, all of which remain smooth throughout the operating cycle 36 of the voltage source converter 10. In addition to the foregoing, the controller 32 is also programmed to: obtain a respective AC current demand phase waveform I_A, I_B, I_C for each converter limb 12A, 12B, 12C which the corresponding converter limb 12A, 12B, 12C is required to track, and a DC current demand e which each converter limb 12A, 12B, 12C is also required to track; and carry out mathematical optimization to determine for each limb portion 12A+,

12A-, 12B+, 12B-, 12C+, 12C- an op ^rtimal limb p ^rortion current

which as necessary includes a commutation current component l_comA, I_comB, l_comC and which the limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C- must contribute to track the corresponding required AC current demand phase waveform I_A, I_B, I_C and the required DC current demand e

The determined optimal limb portion currents act

as individual current references that can be compared with the corresponding actual measured limb portion currents to determine a subsequent control action, e.g. to reduce the error between the reference and measured current values. The various AC current demand phase waveforms I_A, I_B, I_C and the DC current demand be may be obtained directly from a higher-level controller (not shown) within the voltage source converter 10 or from some other external entity. Alternatively the voltage source converter 10 may obtain them directly by carrying out its own calculations. Meanwhile the controller 32 is programmed to carry out the aforementioned mathematical optimization by considering an equivalent converter configuration 100, as shown in Figure 6, which represents the flow of current through the corresponding voltage source converter 10 of the invention.

The equivalent converter configuration 100 includes similar features to the voltage source converter 10 of the invention and these like features share the same reference numerals. To that end the equivalent converter configuration 100 includes three converter limbs 12A, 12B, 12C, each of which corresponds to a respective phase A, B, C of the converter 10 of the invention.

In the equivalent converter configuration 100 each converter limb 12A, 12B, 12C similarly extends between first and second DC terminals 14, 16, and each converter limb 12A, 12B, 12C includes a first limb portion 12A+, 12B+, 12C+ and a second limb portion 1 2A-, 12B-, 12C-. Each pair of first and second limb portions 1 2A+, 12A-, 1 2B+, 1 2B-, 1 2C+, 12C- in each converter limb 1 2A, 1 2B, 1 2C is separated by a corresponding AC terminal 1 8A, 1 8B, 1 8C.

The equivalent converter configuration 1 00 also represents the respective AC current demand phase waveforms I_A, I_B, I_C that each converter limb 1 2A, 12B, 1 2C is required to track, e.g. match as closely as possible, and the DC current demand e that the converter limbs 12A, 12B, 12C are also required to track.

In practice each converter limb 1 2A, 1 2B, 1 2C also operates within the constraints of the corresponding AC voltage phase waveforms VA, VB, VC of the AC network 22, as well as a DC voltage Voc of the DC electrical network 20, to which the converter 1 0 is, in use, connected, and so the equivalent converter configuration 1 00 may also represent these elements.

The controller 32 is programmed to consider an equivalent converter configuration 1 00 which represents the flow of current through the voltage source converter 1 0 by mapping possible current flow paths through the converter 1 0.

One way in which the possible current flow paths through the converter 1 0 may be mapped is by conducting a Kirchhoff analysis of the equivalent converter configuration 1 00 to obtain the following equations:

where the binary variables indicate whether a given limb portion 12A+, 12A-, 12B+, 1 2B-, 12C+, 12C- is switched into circuit or out of circuit i.e.

represent the state of the corresponding primary switching element Sw₁ , Sw₂, Sw₃, Sw₄, Sw₅, Sw₆ (i.e. operating in a conducting or nonconducting mode) in each limb portion 12A+, 12A-, 1 2B+, 12B-, 1 2C+, 1 2C- of the converter 1 0 (details of which may be provided to the controller 32 by each respective control sub-module 4₂ );

I_DC+ is the sum of currents I_A+, I_B+, + iIn_C the first limb portions 12A+, 12B+,

12C+; I_DC_ is the sum of currents I_A-, I_B-, I_C- in the second limb portions 12A-, 12B-,

12C-; and

In addition, further equality constraints can be considered so as to include a commutation current l_ComA, I_comB, l_comC component within the determined optimal limb portion currents while continuing to correctly

route the AC current demand phase waveforms I_A, I_B, I_C and the DC current demand I DC through the voltage source converter 1 0, i.e. according to the following:

Cmod C where the signed binary variable β_k indicates the nature of the conducting mode in which the corresponding converter limb 12A, 12B, 12C is operating (details of which are provided to the controller 32, e.g. by a switch control algorithm), i.e. with k = 1 indicating that the corresponding commutation current is circulating through the converter limb which is fully-conducting and includes the director switch that is to be opened (i.e. the phase A converter limb 12A in the above-described example which temporarily remains in its fully-conducting mode), β_k = -1 indicating that the corresponding commutation current is circulating through the converter limb which has been transitioned from its partially-conducting mode to its fully-conducting mode during a corresponding overlap period (i.e. the phase C converter limb 12C in the above-described example), and β_k = 0 indicating that the corresponding converter limb is in its partially- conducting mode such that no commutation current is flowing therethrough; and are the respective commutation currents which in the

embodiment described are each initially set as a single, equal fixed value, e.g. 0.5A (although this may be different), followed by an equal ramp signal to zero, such that

The preceding equations are then combined, simplified and expressed in a matrix form, i.e.:

A.x = b where,

A is a matrix which maps the possible current flow paths provided by the lim portions 12A+, 12A-, 12B+, 12B-, 12C+, 12C-, i.e.

and

Other equivalent converter configurations and corresponding analysis techniques are, however, also possible.

Optionally, the controller 32 may also be programmed to carry out mathematical optimization by applying a current weighting to the relative current contribution provided by each limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C-. The respective current weighting for each limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C- may be determined according to measured operating parameters of the converter 10 during its operation. The various current weightings can be determined throughout operation of the said converter 10 so as to permit an updating of the current weightings, e.g. in response to changing environmental conditions. As a result the various current weightings can vary as the converter 10 operates. Preferably during normal operation of the converter 10 an identical current weighting is applied to each limb portion current

However, when the converter 10 is operating under certain conditions, e.g. an abnormal operating condition, a different current weighting may be applied to the current contribution, i.e. the limb portion current

provided by at least one limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C-. For example, a larger current weighting may be applied to the optimal limb portion current that a particular limb portion 12A+, 12A-, 12B+,

12B-, 12C+, 12C must contribute, so as to reduce an actual limb portion current that the said limb portion contributes relative to an actual current contribution of each of the other limb portions, which are otherwise all the same as one another.

Returning to the embodiment described hereinabove, the controller 32 is programmed to carry out mathematical optimization to determine a minimum individual limb portion current that each of the limb portion 12A+, 12A-,

12B+, 12B-, 12C+, 12C- must contribute so as to track the corresponding required AC current demand phase waveform _C and the required DC current demand be, i.e.

minimum individual limb portion currents that act

as individual current references with which the corresponding actual measured limb portion currents can be compared in order to, via some subsequent control action, reduce the error between the reference and measured current values. One way in which minimum individual limb portion currents

the unknown elements of x in the equation set out above, may

be determined is by solving a nonlinear optimization of the general form:

subject to the equality constrained equation of the form:

A. x = b where is the current objective function to be minimized;

Ψ is the current weighting at time ti ; f is the current cost function which may optionally include a current weighting matrix

x is the transpose of

reflected in a column vector;

t₀ is the time at which a particular period of control of the converter 10 starts; and t is the time at which a particular period of control of the converter 10 ends. As mentioned above, the current weighting matrix Q_l may be determined according to measured operating parameters of the converter 10, and may be so determined throughout the operation of the converter 10, such that it can vary as the said converter 10 is controlled in response to changes in the operation of the converter 10. Meanwhile one example of a current objective function to be minimized is

where the current objective function, i.e. J(x), is minimised subject to the equality constraints expressed as linear equations above, with

being the transpose of the vector x set out above.

When subject only to an equality constrained equation, as mentioned above, the Lagrangian (or the method of Lagrange multipliers) is a technique for solving the above-identified nonlinear optimization in order to find local minima of the current objective function It may also be solved using other optimization algorithms,

including iterative and programming algorithms.

As a general optimal control problem, the aforementioned nonlinear optimization could additionally include one or more inequality constraints in which case it could be solved by using the further method of Hamiltonian (Pontryagin's minimum principle).

One example of such an inequality constraint is:

where

C is a matrix which maps possible maximum current flow paths provided by the limb portions 12A+, 12A-, 12B+, 12B-, 12C+, 12C-; and d is a vector representing of the maximum desired current in each limb portion 12A+, 12A-, 12B+, 12B-, 12C+, 12C-.

In either case the minimum individual limb portion currents

may also be determined by solving a nonlinear optimization of the form

Thereafter the controller 32 is still further programmed to apply a feedback control algorithm to directly establish limb portion voltage sources

necessary to provide the determined minimum individual limb portion current

that each limb portion 12A+, 12A-, 12B+, 12B-, 12C+,

12C- must contribute.

Claims

CLAIMS:

1 . A voltage source converter (10), for use in high voltage direct current power transmission and distribution, comprising: a plurality of converter limbs (12A, 12B, 12C) corresponding to respective phases of the voltage source converter (10), each converter limb (12A, 12B, 12C) extending between first and second DC terminals (14, 16) and including first and second limb portions (12A+, 12A-, 12B+, 12B-, 12C+, 12C-) separated by an AC terminal, each limb portion including a chain-link converter (24A+, 24A-, 24B+, 24B-, 24C+, 24C-) operable to provide a stepped variable voltage source and a primary switching element (Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆) selectively operable in a conducting mode in which current is able to flow therethrough and a non-conducting mode in which current is unable to flow therethrough, each converter limb (12A, 12B, 12C) being selectively operable in a fully-conducting mode in which the primary switching element (Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆) in each limb portion (12A+, 12A-, 12B+, 12B-, 12C+, 12C-) thereof operates in its conducting mode and a partially-conducting mode in which the primary switching element (Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆) in one limb portion thereof operates in its conducting mode and the primary switching element (Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆) in the other limb portion thereof operates in its non-conducting mode; and a controller (32) programmed to transition a given converter limb (12A, 12B, 12C) from its fully-conducting mode to its partially-conducting mode by:

(a) transitioning another converter limb (12A, 12B, 12C) from its partially- conducting mode to its fully-conducting mode while the given converter limb (12A, 12B, 12C) to be transitioned from its fully-conducting mode to its partially-conducting mode remains in its fully-conducting mode; and

(b) circulating a commutation current around a commutation loop (40) defined by the converter limb (12A, 12B, 12C) transitioned into its fully-conducting mode and the converter limb remaining in its fully-conducting mode to facilitate the switching into its non-conducting mode of a primary switching element (Sw₁, Sw₂, Sw₃, Sw₄, Sw₅, Sw₆) in the given converter limb (12A, 12B, 12C) to be transitioned into its partially-conducting mode.

2. A voltage source converter according to Claim 1 , wherein: each primary switching element (Sw1 , Sw2 , Sw3 , Sw4, Sw5, Sw6 ) includes a director switch (28+, 28-) selectively openable and closeable to respectively prevent and allow the flow of current therethrough and an anti-parallel diode electrically connected in anti-parallel with the director switch (28+, 28-); and the controller (32) is programmed to circulate a commutation current around the commutation loop (40) to drive to zero the current flowing through an anti-parallel diode (30+, 30-) of the primary switching element (Sw1 , Sw2 , Sw3 , Sw4, Sw5, Sw6 ) to be switched into its non-conducting mode.

3. A voltage source converter according to Claim 2, wherein prior to circulating a commutation current around the commutation loop (40) the controller (32) is programmed to establish whether current is flowing through the anti-parallel diode (30+, 30-) of the primary switching element (Sw1 , Sw2 , Sw3 , Sw4, Sw5, Sw6) to be switched into its non-conducting mode and, if current is not flowing through the said anti-parallel diode (30+, 30-), the controller (32) is programmed to: circulate an initiating commutation current around the commutation loop (40) to cause the said anti-parallel diode (30+, 30-) to begin conducting; establish that the said anti-parallel diode (30+, 30-) is conducting current; and open the director switch (28+, 28-) in the said primary switching element (Sw1 , Sw2 , Sw3 , Sw4, Sw5, Sw6) to be switched into its non-conducting mode while the said anti-parallel diode (30+, 30-) is conducting current.

4. A voltage source converter according to Claim 3, wherein the controller (32) is additionally programmed to control the initiating commutation current to drive to zero the current flowing through the said anti-parallel diode (30+, 30-).

5. A voltage source converter according to any of Claims 2 to 4, wherein prior to circulating a commutation current around the commutation loop (40) the controller (32) is programmed to establish whether current is flowing through the anti-parallel diode (30+, 30-) of the primary switching element (Sw1 , Sw2 , Sw3 , Sw4, Sw5, Sw6 ) to be switched into its non-conducting mode and, if current is flowing through the said anti- parallel diode (30+, 30-), the controller (32) is programmed to open the director switch (28+, 28-) in the said primary switching element (Sw1 , Sw₂ , Sw₃ , Sw₄, Sw₅, Sw₆ ) to be switched into its non-conducting mode.

6. A voltage source converter according to Claim 5, wherein after opening the director switch (28+, 28-) of the primary switching element (Sw1 , Sw₂ , Sw₃ , Sw₄, Sw₅, Sw₆ ) to be switched into its non-conducting mode the controller (32) is programmed to circulate an extinguishing commutation current around the commutation loop (40) to drive to zero the current flowing through the said anti-parallel diode (30+, 30-).

7. A voltage source converter according to Claim 4 or Claim 6, wherein the controller (32) is further programmed to impose a reverse bias voltage across the said anti-parallel diode (30+, 30-) to maintain it in a reverse-biased non-conducting condition.

8. A voltage source converter according to any of Claims 3 to 7, wherein establishing whether current is flowing through the said anti-parallel diode (30+, 30-) includes observing the polarity of the voltage across the primary switching element (Sw1 , Sw₂ , Sw₃ , Sw₄, Sw₅, Sw₆ ) to be switched into its non-conducting mode.

9. A voltage source converter according to Claim 8, wherein the controller (32) includes a control sub-unit configured to determine from the observation of voltage polarity across the primary switching element (Sw1 , Sw₂ , Sw₃ , Sw₄, Sw₅, Sw₆ ) to be switched into its non-conducting mode the direction of current flow through the said primary switching element (Sw1 , Sw₂ , Sw₃ , Sw₄, Sw₅, Sw₆ ).

10. A voltage source converter according to any preceding claim, wherein the controller (32) is still further programmed to: obtain a respective AC current demand phase waveform for each converter limb which the corresponding converter limb (12A, 12B, 12C) is required to track, and a DC current demand which each converter limb (12A, 12B, 12C) is also required to track; and carry out mathematical optimization to determine for each limb portion (12A+, 12A-, 12B+, 12B-, 12C+, 12C-) an optimal limb portion current which as necessary includes a commutation current component and which the limb portion (12A+, 12A-, 12B+, 12B-, 12C+, 12C-) must contribute to track the corresponding required AC current demand phase waveform and the required DC current demand.

1 1 . A voltage source converter according to Claim 10, wherein the controller (32) is programmed to carry out mathematical optimization by considering an equivalent converter configuration (100) which represents the flow of current through the voltage source converter (10).

12. A voltage source converter according to Claim 1 1 , wherein the controller (32) is programmed to consider an equivalent converter configuration (100) which represents the flow of current through the voltage source converter (10) by mapping possible current flow paths through the converter (10).

13. A voltage source converter according to any of Claims 10 to 12, wherein the controller (32) is programmed to carry out mathematical optimization to determine one or more minimum individual limb portion currents that the corresponding limb portion (12A+, 12A-, 12B+, 12B-, 12C+, 12C-) must contribute to track the corresponding required AC current demand phase waveform and the required DC current demand.