REACTIVE POWER COMPENSATION

This invention relates to a dual mode voltage source converter, and in particular a voltage source converter that is operable in a first mode of operation to exchange reactive power with an AC network, and operable in a second mode of operation to exchange real power with another network.

In high voltage direct current power transmission, alternating current (AC) electrical power is converted to high voltage direct current (DC) power for transmission via overhead lines and/or undersea cables. This conversion reduces the cost per kilometer of the lines and/or cables, and is therefore cost-effective when power needs to be transmitted over a long distance. Once the transmitted electrical power reaches its target destination, the high voltage DC electrical power is converted back to AC electrical power before being distributed to local networks.

Typically a pair of voltage source converters 10a, 10b is used to interconnect different AC networks 16a, 16b, as shown in Figure 1. The voltage source converters 10a, 10b are connected via their respective pair of DC terminals 12a, 12b, either back-to-back or via a DC cable or overhead line. Each voltage source converter 10a, 10b includes three AC terminals 14a, 14b, each of which is connected to a respective phase of a three-phase AC network 16a, 16b. Each voltage source converter also includes six chain-link converters 18 arranged in a six-pulse bridge configuration with each chain-link converter 18 defining a limb portion which connects a respective AC terminal 14a, 14b to a respective one of the DC terminals 12a, 12b. In this arrangement, each limb portion is required to produce only a unipolar voltage output (an AC voltage with a 100% DC voltage offset) and includes a plurality of half-bridge modules 20, each of which produces a zero or positive voltage.

Under different network conditions the transmission of electrical power through the AC transmission lines can experience fluctuations in voltage characteristics which may cause divergence from normal values. Such fluctuations can be minimised through the exchange of reactive power between a regulating device, typically a static synchronous compensator (STATCOM), and the AC transmission lines. The pair of voltage source converters 10a, 10b can be used as a static synchronous compensator by controlling the amplitude of the AC output voltage of each converter 10a, 10b to be above or below the AC system voltage at the respective AC terminal 14a, 14b. In this mode of operation, the real component of power, and the DC current, may be reduced or, if the equipment possesses sufficient rating, may be at their full values. According to a first aspect of the invention, there is provided a dual mode voltage source converter comprising: first and second terminals for connection in use to a first network; at least one converter limb extending between the first and second terminals and having first and second limb portions separated by a third terminal for connection in use to an AC network, each of the first and second limb portions including at least one module having at least one switching element connected to at least one energy storage device to selectively provide a voltage source; and a primary controller to switch the modules in the or each converter limb in a first mode to generate a zero voltage at each of the first and second terminals whereby the voltage source converter operates in a first switching mode of operation to exchange reactive power with the AC network, and to switch the modules in the or each converter limb in a further switching mode to generate a difference voltage across the first and second terminals whereby the voltage source converter operates in a second mode of operation to exchange real power with the first network. In the first mode of operation, the voltage source converter exchanges reactive power with the AC network in order to regulate the AC voltage characteristics of the AC network. The generation of a zero voltage at each of the first and second terminals eliminates the need for each limb portion to produce a voltage to offset the voltages at the first and second terminals during the first mode of operation. This in turn allows the voltage rating of each limb portion and thereby the number of energy storage devices in each limb portion to be reduced, which results in savings in terms of the size, weight and cost of the dual mode voltage source converter.

In the second mode of operation the first and second terminals of the voltage source converter are interfaced with the first network, e.g. external power equipment, and import or export real power to/from the first network. This enables the voltage source converter to be utilised also in other power applications such as voltage regulation of external power sources or sinks, and de-icing of transmission and distribution lines. The capability of the voltage source converter to import or export real power to a first network avoids the need for separate power equipment, and so reduces the overall amount of power equipment required in, for example, a power station. This leads to further savings in terms of the overall size, weight and cost of a given power transfer application. In addition the primary controller is able rapidly to control the difference voltage across the first and second terminals and to control it over a wide voltage range, whilst having a minimal effect on the external behaviour of the voltage source converter. This renders the voltage source converter compatible for use with power sources or sinks which operate across a wide voltage range.

Furthermore the dual mode operation of the voltage source converter is particularly beneficial in cases, such as the de-icing of AC transmission and distribution lines, where the need to import or export real power arises infrequently and where controllability of DC voltage over a wide range is required.

In embodiments of the invention in the second mode of operation the primary controller may switch the modules in the or each converter limb in a second switching mode to generate a DC voltage at each of the first and second terminals and thereby generate a DC difference voltage across the first and second terminals. In such embodiments, the DC difference voltage across the first and second terminals may be symmetrical or asymmetrical. The generation of a DC difference voltage across the first and second terminals allows the voltage source converter to exchange real power with a DC source or sink to thereby regulate the voltage of the DC real power source or sink. This avoids the need for a separate DC voltage regulator in the associated power station plant. Such a DC real power source or sink may be or may include a battery, a fuel cell, a photovoltaic cell, a superconducting magnetic energy store or any other real power source or sink with a DC interface.

Alternatively the first and second terminals of such a voltage source converter may, for example, be interfaced with medium-voltage DC distribution lines or cables to import or export power to another power substation.

The first and second terminals of such a voltage source converter may also be connected to AC transmission and distribution lines to enable the export of power into these lines for de-icing purposes. This advantageously eliminates the need for separate equipment to perform the de-icing procedure, which would otherwise be a costly option for a procedure that is infrequently carried out. The wide voltage range of the voltage source converter in the second mode of operation allows such a voltage source converter to be used to de-ice AC transmission and distribution lines of varying length and conductor type.

In other embodiments in the second mode of operation the primary controller may switch the modules in the or each converter limb in a third switching mode to generate an AC voltage at each of the first and second terminals and thereby generate a single-phase AC difference voltage across the first and second terminals.

The generation of a single-phase AC difference voltage across the first and second terminals configures the voltage source converter as a multi-phase AC to single-phase AC converter, when the AC network connected to the third terminals is a multi-phase AC network, with built-in voltage regulation and able to handle differing input and output frequencies. Such a converter may be used for example, to supply AC power to isolated locations, induction machines, synchronous machines or a railway network operating at a different power frequency, and to connect a power generator to a power grid operating at a different frequency to that of the power generator.

The voltage source converter preferably further includes a transformer connected to the or each third terminal for connection in use to the AC network, the transformer including one or more tap changers, and wherein in the second mode of operation the primary controller further selectively switches the or each tap changer to reduce the voltage transmitted from the AC network to the or each third terminal.

The inclusion of a transformer having one or more tap changers increases the magnitude of voltage deliverable to the first and second terminals and so expands the usefulness of the second voltage source converter in the second mode of operation. This is particularly so in cases where the optimum voltage required in the first and second modes of operation differs greatly. Preferably each module includes two pairs of switching elements connected in parallel with the respective energy storage device to define a 4-quadrant bipolar module that can provide positive, zero or negative voltage or can conduct current in two directions.

Such a module can readily be switched, as required, to generate an AC voltage at the or each third terminal for connection to a corresponding phase of the AC network. In other embodiments, each limb portion may include a plurality of modules connected in series to define a chain-link converter operable to provide a stepped variable voltage source. The inclusion of a chain-link converter in each limb portion allows the build-up of a combined voltage to generate a voltage waveform of desired amplitude and phase angle.

In further embodiments, the voltage source converter may include multiple converter limbs, the third terminal of each converter limb being for connection in use to a respective AC phase of a multi-phase AC network.

Each converter limb adopts a modular arrangement within the voltage source converter, and so it is straightforward to increase or decrease the number of converter limbs to match the number of phases in the AC network.

Preferably the voltage source converter further includes a closed-loop controller to regulate the difference voltage across the first and second terminals.

The provision of a closed-loop control allows the voltage source converter to improve control over the voltages at the first and second terminals so as to enhance the operational reliability of the converter.

According to a second aspect of the invention, there is provided a voltage source converter assembly comprising a plurality of voltage source converters according to any preceding claim, the first and second terminals of each voltage source converter being connected in parallel or in series with the first and second terminals of at least one other voltage source converter.

The foregoing configurations of the voltage source converter assembly have increased output voltage or current characteristics and so are able to match to match the requirements of further power applications.

In particular, when the first and second terminals of each voltage source converter are connected in series with the first and second terminals of at least one other voltage source converter, the assembly provides an increase in output voltage, while when the first and second terminals of each voltage source converter are connected in parallel with the first and second terminals of at least one other voltage source converter, the assembly provides an increase in output current.

Preferred embodiments of the invention will now be described, by way of non- limiting examples, with reference to the accompanying drawings in which:

Figure 1 shows a pair of prior art voltage source converters for transmitting power between AC networks;

Figure 2 shows a voltage source converter according to a first embodiment of the invention;

Figures 3 and 4 illustrate control of the voltage source converter of Figure 2 in first and second modes of operation respectively;

Figure 5 illustrates the voltage source converter shown in Figure 2 connected to AC transmission and distribution lines for de-icing purposes;

Figures 6 to 8 illustrate the power characteristics of various voltage source converters according to the invention while in a second mode of operation; and

Figures 9 to 11 show a voltage source converter according to a fourth embodiment of the invention in differing modes of operation. A voltage source converter 30 according to a first embodiment of the invention is shown in Figure 2.

The voltage source converter 30 comprises first and second terminals 32,34, and three converter limbs 36. Each converter limb 36 extends between the first and second terminals 32,34, and has first and second limb portions 38,40 that are separated by a third terminal 42.

In use, the first and second terminals 32,34 are connected to a first network 44a, while the third terminal 42 of each converter limb 36 is connected to a respective phase of a second, three-phase AC network 44b.

In other embodiments (not shown), it is envisaged that the voltage source converter 30 may include a different number of converter limbs 36, the third terminal 42 of each converter limb 36 being for connection in use to a respective AC phase of a multi-phase AC network. In each converter limb 36, the first limb portion 38 includes a plurality of modules 48 connected in series between the first terminal 32 and the third terminal 42, while the second limb portion 40 includes a plurality of modules 48 connected in series between the second terminal 34 and the third terminal 42. The plurality of series-connected modules 48 in each limb portion 38,40 defines a chain-link converter 50. The number of modules 48 in each chain-link converter 50 is chosen according to the required voltage rating of the voltage source converter 30.

Each limb portion 38,40 of each converter limb 36 further includes an inductor 52 connected in series with the corresponding chain-link converter 50.

Each module 48 in each chain-link converter 50 includes two pairs of series-connected switching elements 54 connected in parallel with an energy storage device, in the form of a capacitor 56, to define a 4-quadrant bipolar module that can provide negative, zero or positive voltage and can conduct current in two directions. Each switching element 54 in each module 48 is a semiconductor device in the form of an insulated gate bipolar transistor (IGBT) which, in the embodiment shown, is connected in parallel with an anti- parallel diode 58. In other embodiments of the invention (not shown), one or more of the switching elements in each module may be a different semiconductor device, such as a field effect transistor, a gate-turn-off thyristor, an injection enhanced gate transistor, an integrated gate commutated thyristor or other forced commutated or self commutated semiconductor switches. In each instance, the semiconductor device is preferably connected in parallel with an anti-parallel diode.

It is envisaged that in other embodiments of the invention (not shown), the capacitor of each module may be replaced by a different energy storage device such as a fuel cell, a battery or any other energy storage device capable of storing and releasing its electrical energy to provide a voltage. This flexibility is useful in deploying the voltage source converter of the invention in different locations where the availability of equipment may vary due to locality or transport difficulties.

The capacitor 56 of each module 48 is selectively bypassed or inserted into the respective chain-link converter 50 by changing the state of the switching elements 54 in the corresponding module 48 so as to define a voltage source. In particular, the capacitor 56 of each module 48 is bypassed when the pairs of switching elements 54 in each module 48 are configured to form a short circuit in the module 48. This causes the current in the voltage source converter 30 to pass through the short circuit and thereby bypass the capacitor 56, and so the module 48 provides a zero voltage.

The capacitor 56 of each module 48 is inserted into the respective chain-link converter 50 when the pair of switching elements 54 in each module 48 is configured to allow the converter current to flow into and out of the capacitor. The capacitor 56 then charges or discharges its stored energy so as to provide a voltage. The bidirectional nature of the 4- quadrant bipolar module means that the capacitor 56 may be inserted into the module 48 in either forward or reverse directions so as to provide a positive or negative voltage.

It is therefore possible to build up a combined voltage across each chain-link converter 50 which is higher than the voltage available from each individual module 48 via the insertion of the capacitors 56 of multiple modules 48, each providing its own voltage, into the chain-link converter 50.

In this manner switching of the switching elements 54 of each module 48 in each chain- link converter 50 causes the corresponding chain-link converter 50 to provide a stepped variable voltage source. The switching elements 54 of the modules 48 are preferably switched at near to the fundamental frequency of the AC network 44b.

Varying the timing of the switching operations for each module 48 to control the insertion and/or bypass of the capacitors 56 of individual modules 48 in the chain-link converter 50 results in the generation of a voltage waveform. For example, insertion of the capacitors 56 of the individual modules 48 may be staggered to generate a sinusoidal waveform. Other waveform shapes may be generated by adjusting the timing of switching operations for each module 48 in each chain-link converter 50.

In the embodiment shown, the modules 48 in the chain-link converters 50 are switched to generate a sinusoidal current waveform using a step-wise approximation.

The ability of the chain-link converters 50 to provide voltage steps, as set out above, allows them to increase or decrease the voltage at each of the first, second and third terminals 32,34,42. The voltage source converter 30 also includes a primary controller (not shown) which switches the modules 48 in each converter limb 36 in various switching modes to control the voltage source converter 30 in first and second modes of operation. In the first mode of operation, the primary controller switches the modules 48 in each of the converter limbs 36 in a first switching mode to generate a zero voltage at each of the first and second terminals 32,34, as shown in Figure 3.

In the first switching mode the primary controller further controls the switching of the modules 48 in the corresponding converter limbs 36 to generate an AC voltage waveform at each corresponding third terminal 42. This allows the voltage source converter 30 to exchange reactive power with the AC network 44b and thereby improve the stability and voltage control within the AC network 44b. The precision with which reactive power provided by the voltage source converter 30 matches the requirements of the AC network 44b may be increased by using a higher number of modules 48 with lower voltage levels. This increases the number of possible voltage steps and so improves the approximation of a given AC voltage waveform by providing each AC phase voltage waveform with a smoother profile.

In the first mode of operation of the voltage source converter 30, there is no real power exchange between the voltage source converter 30 and the AC network 44b, other than that needed to replenish the energy consumed by the losses of the converter, and transiently needed to increase or decrease the mean DC capacitor voltage of the chainlink modules .

Accordingly, with the primary controller switching the modules 48 of each converter limb 36 in a first switching mode, the voltage source converter 30 operates in a first, static synchronous compensator (STATCOM) mode.

Generating a zero voltage at each of the first and second terminals 32,34 means that none of the limb portions 38,40 needs to produce a voltage with a DC component to offset a respective voltage at the first and second terminals 32,34. Consequently each limb portion 38,40 can have a lower voltage rating, and so the number of energy storage devices, i.e. capacitors 56, needed in each limb portion 38,40 is reduced. Such capacitors 56 are large, heavy and expensive, and so reducing their number results in savings in terms of the size, weight and cost of the voltage source converter 30. Moreover, the ability of each chain-link converter 50 to generate different waveform shapes at the respective third terminals 42 allows the voltage source converter 30 to cope with different changes in AC voltage characteristics arising from a variety of conditions in the AC network 44b.

Additionally the provision of the chain-link converters 50 allows the voltage source converter 30 to continuously exchange reactive power with the AC network 44b because the switching operations of the chain-link converters 50 may be varied to match the changing needs of the AC network 44b without any need for disconnection.

In particular, the fast switching characteristics of the semiconductor switching elements 54 allows the voltage source converter 30 to respond quickly to changes in AC voltage characteristics of the AC network. The fast response of the voltage source converter 30 therefore helps to minimise the risk of any fluctuations in AC voltage characteristics causing damage to power transmission equipment during the first mode of operation.

In the second mode of operation the primary controller switches the modules 48 in the converter limbs 36 in a second switching mode to generate a different voltage at each of the first and second terminals 32,34 whereby a difference voltage appears across the first and second terminals 32,34, as shown in Figure 4. In the embodiment shown, the difference voltage is a symmetrical DC voltage, but in other embodiments where, for example, the primary controller switches the modules 48 in a third further switching mode it may be a single-phase AC voltage.

The generation of a difference voltage across the first and second terminals 32,34 allows the voltage source converter 30 to import and/or export real power to/from the first network 44a. Accordingly, with the primary controller switching the modules 48 of each converter limb 36 in a second switching mode, the voltage source converter 30 operates in a second, real power transfer mode of operation.

The generation of a difference voltage across the first and second terminals 32,34, and the resulting ability to exchange real power with a first network 44a connected to the first and second terminals 32,34 allows the voltage source converter 30 to interface with a range of differently configured first networks 44a. For example, the first network 44a may comprise a real DC power source or sink such as a battery, a fuel cell, a photovoltaic cell, a superconducting magnetic energy store or any other real power source or sink with a DC interface. In such cases, the voltage source converter 30 imports or exports real power to regulate the voltage of the real power source or sink in the first network 44a.

The first network 44a may also consist of medium-voltage DC distribution lines or cables with the voltage source converter 30 importing or exporting real power to a power substation.

In a first network 44a including AC transmission and distribution lines 60 the voltage source converter 30 is able to export real power into these lines 60 for de-icing purposes, as shown schematically in Figure 5.

The fast switching characteristics of the semiconductor switching elements 54 means that the primary controller is able rapidly to control the difference voltage across the first and second terminals 32,34. As a result the voltage source converter 30 can be operated over a wide voltage range, which is particularly advantageous in relation to two of the first network configurations mentioned above.

For example, one or more batteries may have a peak voltage at the end of a charge cycle that is higher (typically in the order of 50%) than the minimum voltage at the end of a discharge cycle, and the voltage range of one or more photovoltaic cells can vary significantly depending on insolation conditions.

Also, AC transmission and distribution lines often vary in length and conductor type, and so a typical operating voltage range for de-icing AC transmission and distribution lines is of the order of 10 to 20 kV.

In the particular embodiment shown in Figure 4 the primary controller generates a DC difference voltage across the first and second terminals 32,34 by switching the modules 48 in the second switching mode to generate a positive DC voltage, +Vdc/2, at the first terminal 32 and a negative DC voltage, -Vdc/2, at the second terminal 34. The magnitude of the positive and negative DC voltages is equal, although in other embodiments the magnitudes may be unequal. The voltage and current operating characteristics of the voltage source converter 30 shown, while operating in the second, real power transfer mode, are set out below on the basis of an exemplary RMS line-to-line AC voltage Vacnom of 16kV at each third terminal 42.

Let:

• Pdc achievable DC power in the second real power transfer mode (with Q = 0)

• Vdc = achievable DC voltage between the first and second terminals 32,34 in the second mode of operation.

Idc achievable DC current in the second mode of operation

Vvrated peak rated voltage of each limb portion 38,40

Ivrated peak rated current of each limb portion 38,40

Vac(pu) actual working AC voltage at each third terminal 42, in p.u. Vacnom nominal rms line-to-line AC voltage at each third terminal

42

Ivpeak peak working current in each limb portion 38,40.

The peak voltage rating for each limb portion 38,40, Vvrated, is calculated using Equation 1.

Vvrated = ½ Vdc + (V2 / V3) ^■ Vac (rms, line-to-line)

= ½ Vdc + (V2 / V3) ^■ Vac(pu) ^■ Vacnom ^■■■ (1) Rearranging Equation 1 into Equation 2, the maximum achievable peak voltage Vdc before the first and second terminals 32,34 is given by:

^■ [Vvrated - (V2 / V3) ^■ Vac(pu) ^■ Vacnom] As an example, if Vvrated = 1.25 ^■ (V2 / V3) ^■ Vac(pu) ^■ Vacnom, i.e. each limb portion 38,40 has a voltage rating approximately 25% above the nominal AC voltage (which is typical for a voltage source converter operating in a STATCOM mode), we can simplify Equation 2 to: Vdc = 2 ^■ (V2 / V3) ^■ [1.25 - Vac(pu)] ^■ Vacnom (3) Such a voltage rating for each limb portion 38,40 is, for example, sufficiently large to permit the voltage source converter 30 to generate capacitive reactive power for regulation of the AC voltage characteristics of the AC network 44b while operating in the aforementioned first, STATCOM mode.

On the basis of Equation 2, if Vacnom = 16kVrms, the maximum achievable peak voltage Vdc is 6.5kV when Vac(pu) is set at 1.0 pu.

Figure 6 illustrates the maximum achievable DC voltage Vdc, i.e. the maximum difference voltage, across the first and second terminals 32,34 as a function of per-unit AC voltage Vac(pu), with a nominal AC voltage of 16kV, and for two cases: a first case 62 where Vvrated = 1.25 ^■ (V2 / V3) ^■ Vac(pu) ^■ Vacnom (as in Equation 1) and a second case 64 where the Vvrated has been increased by a further 10% so that Vvrated = 1.375 ^■ (V2 / V3) ^■ Vac(pu) ^■ Vacnom.

It is shown in Figure 6 that a modest increase in the voltage rating of each limb portion 38,40 results in an increase in the value of the maximum achievable difference voltage appearing across the first and second terminals 32,34. The maximum achievable DC current Idc at the first and second terminals 32,34 also varies with the AC voltage at each respective third terminal 38, and is limited by the peak rated current Ivrated in each limb portion 38,40, which in a typical example is 1200A in steady state. In the voltage source converter 30 the peak working current Ivpeak flowing through a given limb portion 38,40 is equal to one third of the DC current plus one half of the peak AC current.

For "ideal" HVDC transmission, the peak working current Ivpeak in each limb portion 38,40 is equal to the DC current while the peak voltage in each limb portion 38,40 is equal to the DC voltage. Under these conditions, the converter is operating at a modulation index, λ of 1. λ is defined as: λ = Vac ^■ (V2 / V3)/ (Vdc/2) = 2 ^■ V2 Vac / ( V3 ^■ Vdc) ^■■■ (4)

The peak working current Ivpeak in each limb portion 38,40 can therefore be expressed as: Ivpeak = ldc/3 ^■ (1 + 2 / λ) (5)

To obtain the maximum achievable DC current Idc, Ivpeak is equated to Ivrated. Hence, Equation 4 is substituted into Equation 5 to obtain Equation 6 as follows:

Idc = 3 ^■ Ivrated / (1 + ( V3 ^■ Vdc / V2 ^■ Vac)) ^■■■ (6) with Ivrated being approximately 1200A as set out above.

Figure 7 illustrates the maximum achievable DC current, as a function of per-unit AC voltage, for the same two cases as illustrated in Figure 6, i.e. the first case 66 where Vvrated = 1.25 ^■ (V2 / V3) ^■ Vac(pu) ^■ Vacnom and the second case 68 where Vvrated = 1.375 ^■ (V2 / V3) ^■ Vac(pu) ^■ Vacnom.

It is shown in Figure 7 that the maximum achievable DC current Idc produced by the static synchronous converter 30 during the second mode of operation is greater than the peak valve current rating Ivrated, i.e. 1200A, of each limb portion 38,40. This is possible because the DC current Idc at the first and second terminals 32,34 is divided between the three converter limbs 36.

The maximum amount of real power which can be transferred, i.e. exported or imported, by the voltage source converter 30 is the product of the maximum achievable difference voltage Vdc and the maximum achievable DC current Idc. This is illustrated in Figure 8 for the same first 70 and second 72 cases as illustrated in Figures 6 and 7. It is shown that increasing the voltage rating of each limb portion 38,40 gives a proportional increase in the available real power and shifts the maximum of the power-voltage curve towards the right. A voltage source converter according to a second embodiment of the invention (not shown) includes a primary controller that switches the modules in the respective limb portion thereof in a third switching mode to generate an AC voltage waveform at each of the first and second terminals, and thereby produce a single phase AC difference voltage across the first and second terminals.

The generation of a single-phase AC difference voltage across the first and second terminals configures the second voltage source converter as a step-down three-phase AC to single-phase AC converter with built-in voltage regulation and different input and output frequencies. Having built-in voltage regulation eliminates the need for on-load tap changers, and so saves space and costs. The second voltage source converter is again able to import or export real power via the first and second terminals to, for example: supply AC power to isolated rural communities, induction machines or synchronous machines;

. supply AC power to railway networks operating at a different power frequency (typically 16.67 Hz in many European countries); or

connect a 50 Hz power generator to a 60 Hz power grid, or vice versa.

A voltage source converter according to a third embodiment of the invention (not shown) includes a primary controller that is able to simultaneously switch the modules 48 in each limb portion 38,40 in the first switching mode and the second or third switching modes so that the third voltage source converter operates simultaneously in both the first STATCOM mode and the second real power transfer mode, i.e. regulates the AC voltage characteristics of an AC network connected to the or each AC terminal whilst simultaneously providing intermittent or continuous real power to a first network including, for example, external power equipment connected to the first and second terminals.

In still further embodiments of the invention (not shown) the primary controller is able to selectively switch the modules in each limb portion to separately generate both a DC difference voltage and an AC difference voltage across the first and second terminals, as required.

Any of the aforementioned voltage source converters may further include a closed-loop controller (not shown) to regulate the difference voltage across the first and second terminals to improve further the degree of control over the voltage at each of the first and second terminals, and thereby enhance the operational reliability of the associated converter. A voltage source converter 80 according to a fourth embodiment of the invention is shown in Figures 9 to 1 1. The fourth voltage source converter 80 is similar to the first voltage source converter 30 shown in Figures 2 to 4 and like features share the same reference numerals.

The fourth voltage source converter 80 differs from the first voltage source converter 30 in that it further includes a transformer 46 connected to each third terminal 42 so as to lie between the third terminals 42 and an AC network 44 to which, in use, it is connected.

The transformer 46 includes a tap changer (not shown) to selectively reduce the peak AC voltage transmitted from the AC network 44b to each third terminal 42. The primary controller of the fourth voltage source converter 80 operates the tap changer as required to produce a desirable, lower, peak AC voltage Vac(pu) at each third terminal 42. The tap changer may be an on-load or off-load tap changer.

The ability to decrease the peak AC voltage Vac(pu) at each third terminal 42 provides a significant increase in the maximum achievable difference voltage Vdc across the first and second terminals 32,34 while the voltage source converter 80 is operating in the second, real power transfer mode.

For example, referring to Equation 2 above, with a nominal rms line-to-line AC voltage Vacnom of 16kVrms, reducing the peak AC voltage to 60% of the nominal voltage Vacnom, i.e. setting Vac(pu) at 0.6, the maximum achievable difference voltage Vdc is 17.0 kV (compared to 6.5 kV in the first voltage source converter where 100% of the nominal voltage Vacnom is seen at each third terminal, i.e. Vac(pu) is 1.0). Such an increase is illustrated in Figure 6 as the maximum achievable difference voltage Vdc increases towards the left of the graph as the peak AC voltage Vac(pu) reduces (as a ratio of the nominal AC voltage Vacnom).

Figure 8 illustrates that the peak of the power-voltage curve, for the example voltage source converter embodiments described herein, arises when Vac(pu) is approximately 0.75, i.e. when the tap changer reduces the nominal AC voltage of 16kVrms to approximately 75% of this level at each third terminal 42.

The primary controller may, therefore, switch the tap changer to provide a reduced, e.g. 0.75, peak AC voltage at each third terminal 42 while operating the fourth voltage source converter 80 in the second real power transfer mode, to maximise the real power transfer with the first network 44a connected to the first and second terminals 32,34. Operating the fourth voltage source converter 80 in such a manner would render it particularly useful for de-icing AC transmission and distribution lines 60 (as illustrated schematically in Figure 5).

In other embodiments of the invention which are similar to the fourth voltage source converter 80 and which are intended, for example, to act as a Battery Energy Storage System (BESS) or to interface with a photovoltaic cell array, the primary controller switches the tap changer to provide a maximum voltage, i.e. no voltage reduction thereacross and Vac(pu) = 1.0, while the voltage source converter is in the first STATCOM mode (to ensure maximum reactive power transfer is available), and switches the tap changer to provide a reduced voltage, i.e. voltage reduction thereacross and Vac(pu) < 1.0, while the voltage source converter is in the second, real power transfer mode (to ensure a suitable level of real power transfer is available).

A voltage source converter assembly according to a further embodiment of the invention comprises a plurality of voltage source converters. Each voltage source converter of the assembly has the same structure and modes of operation as the first voltage source converter shown in Figure 2.

The first and second terminals of each voltage source converter are connected in series with the first and second terminals of at least one other voltage source converter to provide an increase in output voltage of the assembly. In other embodiments of the invention, the first and second terminals of each voltage source converter may be connected in parallel with the first and second terminals of at least one other voltage source converter to provide an increase in output current of the assembly, such as may be required, for example, when de-icing AC transmission and distribution lines.