EP3063860A1

EP3063860A1 - Voltage source converter

Info

Publication number: EP3063860A1
Application number: EP14790564.0A
Authority: EP
Inventors: David Reginald Trainer; Omar Fadhel JASIM; Kevin James DYKE; Jonathan Charles CLARE
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2013-10-30
Filing date: 2014-10-23
Publication date: 2016-09-07
Also published as: WO2015062975A1; GB2519793B; CN105874691A; GB201319188D0; CN105874691B; GB2519793A

Abstract

A voltage source converter (10) comprising first and second DC terminals (12,14) for connection to a DC electrical network (42) and at least one limb (16) connected between the first and second terminals (12,14). The limb (16) includes a phase element (18) having a plurality of switching elements (54) and at least one AC terminal for connection to an AC electrical network (30), the plurality of switching elements (54) being configured to be controllable to facilitate power conversion between the AC and DC electrical networks (30,42). The voltage source converter (10) also includes a tertiary sub-converter (32) connected in series with the phase element (18) in an electrical block. The tertiary sub-converter (32) is configured to be controllable to act as a waveform synthesizer to modify a first DC voltage presented to a DC side of the phase element (18). A quarternary sub-converter (36) is connected in series with the tertiary sub-converter (32), the quarternary sub-converter (36) being configured to be controllable to act as a waveform synthesizer to modify a second DC voltage presented to the DC electrical network (42). An auxiliary sub-converter (38) is connected in parallel with the electrical block and connected to a common connection point between the tertiary and quarternary sub- converters (32,36) to form a "T" arrangement. The auxiliary sub-converter (38) is configured to be controllable to act as a waveform synthesizer to modify the first and second DC voltages.

Description

VOLTAGE SOURCE CONVERTER

The invention relates to a voltage source converter. In 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 transmission line or cable, and thereby reduces the cost per kilometre of the lines and/or cables. Conversion from AC to DC thus becomes cost-effective when power needs to be transmitted over a long distance.

The conversion of AC power to DC power is also utilised in power transmission networks where it is necessary to interconnect AC electrical networks operating at different frequencies. In any such power transmission network, converters are required at each interface between AC and DC power to effect the required conversion. One such form of converter is a voltage source converter (VSC).

According to an aspect of the invention there is provided a voltage source converter comprising first and second DC terminals for connection to a DC electrical network and at least one limb connected between the first and second terminals, the limb including: a phase element having a plurality of switching elements and at least one AC terminal for connection to an AC electrical network, the plurality of switching elements being configured to be controllable to facilitate power conversion between the AC and DC electrical networks;

a tertiary sub-converter connected in series with the phase element in an electrical block, the tertiary sub-converter being configured to be controllable to act as a waveform synthesizer to modify a first DC voltage presented to a DC side of the phase element;

a quarternary sub-converter connected in series with the tertiary sub-converter, the quarternary sub-converter being configured to be controllable to act as a waveform synthesizer to modify a second DC voltage presented to the DC electrical network; and an auxiliary sub-converter connected in parallel with the electrical block and connected to a common connection point between the tertiary and quarternary sub- converters to form a "T" arrangement, the auxiliary sub-converter being configured to be controllable to act as a waveform synthesizer to modify the first and second DC voltages. The invention provides advantages over arrangements that omit a quarternary sub- converter and in which a tertiary sub-converter is connected in series with a phase element in an electrical block, and an auxiliary sub-converter is connected in parallel with the electrical block.

Arranging the tertiary and quarternary sub-converters on opposite sides of the "T" arrangement that is formed with the auxiliary sub-converter allows the functionality of what might otherwise be provided solely by the tertiary sub-converter to be split between two separate sub-converters, the tertiary and quarternary sub-converters.

This allows each of the tertiary and quarternary sub-converters to be designed separately such that they are better able to satisfy the requirements of specific VSC applications than converter arrangements in which the functionalities of the tertiary and quarternary sub- converters are combined in a single sub-converter.

The two degrees of freedom thereby created also enable the energy in the tertiary and quarternary sub-converters to be managed separately without them interfering with each other. In embodiments of the invention the voltage source converter may include three limbs connected in series between the first and second DC terminals, the or each AC terminal of the phase element of each limb being for connection to a respective phase of a multiphase AC electrical network. In such embodiments the inclusion of a quarternary sub-converter in each limb allows the functionality of the quarternary sub-converter to be retained within each phase.

In other embodiments of the invention the phase element of the at least one limb may include at least one AC terminal for connection to a respective phase of a multiphase AC electrical network and the voltage source converter further includes at least two additional limbs connected in series with the at least one limb between the first and second DC terminals, each additional limb including:

an additional phase element having a plurality of switching elements and at least one AC terminal for connection to a respective phase of a multiphase AC electrical network, the plurality of switching elements being configured to be controllable to facilitate power conversion between the AC and DC electrical networks; an additional tertiary sub-converter connected in series with the additional phase element in an additional electrical block, the additional tertiary sub-converter being configured to be controllable to act as a waveform synthesizer to modify a first DC voltage presented to a DC side of the additional phase element; and

an additional auxiliary sub-converter connected in parallel with the additional electrical block, the additional auxiliary sub-converter being configured to be controllable to act as a waveform synthesizer to modify the first and second DC voltages,

wherein the limbs are arranged in series so that the quarternary converter of the at least one limb is connected directly to one of the first and second DC terminals.

In such embodiments the additional auxiliary sub-converter of each additional limb is not connected to a common connection point between the additional tertiary sub-converter and a quarternary sub-converter to form a "T" arrangement. In such embodiments the functionality of the quarternary sub-converter is consolidated at the DC network, thereby allowing for improvements in the layout of the voltage source converter and a reduction in the resultant footprint.

Preferably the voltage source converter further includes a controller configured to selectively control the or each quarternary sub-converter to synthesize at least one quarternary voltage component, the or each quarternary voltage component being a positive integer multiple of a 6^th harmonic component.

Configuring the controller to control the or each quarternary sub-converter in this manner permits active filtering of undesirable ripple harmonic components, each of which is a positive integer multiple of a 6^th harmonic component, that are present in the DC voltage across the or each limb. It thereby prevents the undesirable ripple harmonic components from appearing in the second DC voltage that is presented to the DC electrical network.

The synthesis of a voltage waveform that includes at least one quarternary harmonic component by each quarternary sub-converter obviates the need to control each auxiliary sub-converter to synthesize one or more zero-phase sequence triplen harmonic components. This in turn permits use of a star-connected plurality of transformer primary windings with a grounded neutral point, which could be desirable for high power applications.

Locating the harmonic filter provided by the quarternary sub-converter on the DC side of the "T" arrangement that is formed with the tertiary sub-converter and the auxiliary sub- converter means that the harmonic filter is located in a part of the converter structure where the current is DC, i.e. ripple free. The generation of harmonic voltages in the presence of a DC current means that in principle there is no energy exchange with the or each quarternary sub-converter because the voltage and current are at different frequencies.

In preferred embodiments however the or each quarternary sub-converter includes at least one energy storage device and the controller is configured to selectively control the or each quarternary sub-converter to synthesize at least one compensatory quarternary voltage component so as to transfer energy to or from that quarternary sub-converter and thereby minimise a net change in energy level of that quarternary sub-converter.

This configuration allows the injection of a relatively small DC voltage to interact with the DC current flowing through the quarternary sub-converter and create a power and energy exchange mechanism that may be used for dynamic stabilisation and compensation of losses.

The synthesis of a compensatory quarternary voltage component however affects both the auxiliary sub-converter and the tertiary sub-converter. The controller is therefore preferably configured to selectively control the or each auxiliary sub-converter to synthesize one or more auxiliary voltage components to minimise a net change in the DC voltage across the respective limb when the or each quarternary sub-converter synthesizes one or more compensatory quarternary voltage components.

The controller is also preferably configured to selectively control the or each tertiary sub- converter to synthesize one or more compensatory tertiary voltage components to minimise a net change in the DC voltage at the DC side of the respective phase element when the or each quarternary sub-converter synthesizes one or more compensatory quarternary voltage components. Configuring the controller in this manner allows the auxiliary sub-converter and the tertiary sub-converter to address the affects of the compensatory quarternary voltage components. The synthesis of a compensatory tertiary voltage component however causes power and energy drift as a result of the interaction between the compensatory tertiary voltage and the inherent DC current flow in that part of the circuit.

In order to address the resultant power and energy flow, the voltage source converter may further include a controller configured to selectively control the or each tertiary sub- converter to synthesize at least one tertiary voltage component so as to transfer energy to or from that tertiary sub-converter and thereby regulate an energy level of that tertiary sub- converter. Such a configuration allows the voltage source converter to address energy accumulation in (or energy loss from) at least one energy storage device included in the tertiary sub- converter, which results in deviation of the energy level of the at least one energy storage device from a reference value. Such a deviation is undesirable because, if too little energy is stored within a given energy storage device then the voltage the corresponding module is able to generate is reduced, whereas if too much energy is stored in a given in an energy storage device then over- voltage problems may arise. The former would require the addition of a power source to restore the energy level of the affected energy storage device to the reference value, while the latter would require any increase in voltage rating of one or more energy storage devices to prevent the over-voltage problems. This would add to the overall size, weight and cost of the voltage source converter. In addition if too little energy is stored within a given energy storage device then the voltage source converter might trip due to under- voltage protection.

The configuration outlined above thus allows energy to be transferred to and from the or each tertiary sub-converter to regulate the energy stored in one or more corresponding energy storage devices, thereby obviating the problems associated with a deviation of the energy level of at least one energy storage device from the reference value.

Optionally the magnitude of the or each tertiary voltage component may be altered in order to adjust the amount of energy transferred to or from the or each tertiary sub-converter.

When the voltage source converter includes a plurality of limbs connected between the first and second DC terminals, regulation of the energy level of each tertiary sub-converter may involve balancing of the energy levels of the plurality of tertiary sub-converters. This is useful when there is an imbalance in the energy levels of the plurality of tertiary sub- converters, which could be caused by, for example, an imbalance in the plurality of phase currents drawn from a multi-phase AC electrical network, or component failure in one or more modules of at least one tertiary sub-converter leading to a reduction in energy storage capacity. In embodiments of the invention the controller may be configured to selectively control the or each tertiary sub-converter to synthesize at least one tertiary voltage component so as to transfer energy to or from that tertiary sub-converter and thereby minimise a net change in energy level of that tertiary sub-converter. This further enhances the regulation of the energy level of the or each tertiary sub-converter and therefore any associated regulation of the energy stored in a given energy storage device.

Regulation of the energy level of the or each tertiary sub-converter to minimise a net change in energy level of that tertiary sub-converter preferably is carried out over a defined period of time, e.g. a single power frequency cycle.

In further embodiments of the invention the controller may be configured to selectively control the or each auxiliary sub-converter to synthesize an auxiliary voltage component that is in anti-phase with the respective tertiary voltage component. Control of the or each auxiliary sub-converter in this manner ensures that the DC voltage across the or each limb, and therefore the AC voltage at the AC side of the respective phase element, remains unmodified during the generation of the or each tertiary voltage component. This means that energy level regulation of the or each tertiary sub-converter may be carried out at any time during the operation of the voltage source converter without affecting the power transfer between the AC and DC electrical networks.

The or each tertiary voltage component may be a positive integer multiple of a 2^nd harmonic voltage component. Accordingly, in such embodiments, the controller is configured to selectively control the or each auxiliary sub-converter to synthesize an auxiliary voltage component that is the same positive integer multiple of a 2^nd harmonic voltage component and in anti-phase with the respective tertiary voltage component.

Synthesis of at least one auxiliary voltage component that is a positive integer multiple of a 2^nd harmonic voltage component enables summation of the DC voltages across the limbs, when the voltage source converter includes a plurality of limbs connected between the first and second DC terminals, so as to leave a combined, ripple-free DC voltage.

The or each tertiary voltage component is preferably a 2^nd harmonic voltage component, a 4^th harmonic component, an 8^th harmonic component or a 10^th harmonic component. It will be appreciated that the or each tertiary voltage component may be a (3(2n-1 ) ± 1 )^th harmonic voltage component, whereby n is a positive integer multiple. This prevents undesirable ripple harmonic components, each of which is a positive integer multiple of a 6^th harmonic component, from appearing in the DC voltage across the or each limb.

The or each tertiary voltage component may have the same frequency as a current component of a current flowing through that tertiary sub-converter. This provides a reliable means of producing real power when transferring energy to or from the or each tertiary sub-converter.

Optionally the controller may be configured to selectively control the or each tertiary sub- converter to modify a phase angle of the or each tertiary voltage component relative to a phase angle of a current flowing through that tertiary sub-converter. Control of the or each tertiary sub-converter in this manner permits adjustment of the amount of energy transferred to or from the or each tertiary sub-converter, and thereby provides an additional way of regulating the energy level of the or each tertiary sub-converter.

Further optionally the controller may be configured to selectively control the or each tertiary sub-converter to synthesize the or each tertiary voltage component to be in phase with a current component of a current flowing through that tertiary sub-converter. Control of the or each tertiary sub-converter in this manner maximises the amount of energy transferred to or from the or each tertiary sub-converter, and thereby optimises the operation of the voltage source converter to regulate the energy level of the or each tertiary sub-converter.

In preferred embodiments the voltage source converter further includes a controller configured to selectively control the or each tertiary sub-converter to generate a compensatory DC voltage component for presentation to the DC side of the respective phase element so as to compensate for a change in real power and/or reactive power generated or absorbed at an AC side of the respective phase element.

The configuration of the controller to control the or each tertiary sub-converter in this manner inhibits any effect a change in real power and/or reactive power generated or absorbed at an AC side of the or the respective phase element might have on the operation of the DC side of the voltage source converter. It thereby prevents any undesirable change in the DC side of the voltage source converter that would otherwise result from the change in real power and/or reactive power generated or absorbed at an AC side of the or the respective phase element. An alternative solution involves controlling the or each auxiliary sub-converter to synthesize one or more zero-phase sequence triplen harmonic components (e.g. 3^rd, 9^th and 15^th harmonic components) to compensate for any change in the DC side of the voltage source converter caused by a change in real power and/or reactive power generated or absorbed at an AC side of the respective phase element. Synthesis of one or more zero-phase sequence triplen harmonic components by the or each auxiliary sub- converter however means that a driving voltage is produced around a delta-connected plurality of transformer primary windings used to interconnect the AC electrical network and the AC side of the or each phase element. This in turn provides a path for a significant, continuous zero-phase sequence current to flow in the delta-connected plurality of transformer primary windings. Similarly use of a star-connected plurality of transformer primary windings, with a grounded neutral point, to interconnect the AC electrical network and the AC side of each phase element provides a path for a significant, continuous zero- phase sequence current to flow in the star-connected plurality of transformer primary windings.

The control of the or each tertiary sub-converter to generate a compensatory DC voltage component for presentation to the DC side of the or the respective phase element so as to compensate for a change in real power and/or reactive power generated or absorbed at an AC side of the or the respective phase element obviates the need to control the or each auxiliary sub-converter to synthesize one or more zero-phase sequence triplen harmonic components. This in turn permits use of a star-connected plurality of transformer primary windings with a grounded neutral point, which could be desirable for high power applications.

The controller may be configured to selectively control the or each tertiary sub-converter to generate a compensatory DC voltage component for presentation to the DC side of the respective phase element so as to compensate for a change in the respective first DC voltage caused by the change in real power and/or reactive power generated or absorbed at an AC side of the respective phase element. This prevents any undesirable change in the operation of the DC side of the voltage source converter that would otherwise result from the change in the respective first DC voltage.

The controller may be configured to selectively control the or each tertiary sub-converter to generate a compensatory DC voltage component for presentation to the DC side of the respective phase element so as to compensate for a change in the respective first DC voltage caused by the change in real power and/or reactive power generated or absorbed at an AC side of the respective phase element and thereby inhibit the change in the respective first DC voltage from modifying the DC voltage across the respective limb. This thereby prevents any undesirable change in the DC voltage across the respective limb that would otherwise result from the change in the respective first DC voltage.

Configuring the controller to generate a compensatory DC tertiary voltage component for presentation to the DC side of the respective phase element so as to compensate for a change in the respective first DC voltage caused by the change in real power and/or reactive power generated or absorbed at an AC side of the respective phase element therefore permits operation of the voltage source converter over a wide range of real power and reactive power with little to zero detrimental effect on the operation of the DC side of the voltage source converter.

In other preferred embodiments the voltage source converter may include a controller configured to selectively control the or each tertiary sub-converter to generate a compensatory DC tertiary voltage component for presentation to the DC side of the respective phase element so as to compensate for a reduction in the DC voltage across the first and second DC terminals. Configuring the controller to control the tertiary sub-converter in this manner inhibits any effect a reduction in the DC voltage of the DC electrical network might otherwise have at the AC side of the respective phase element.

Typically the peak of the AC voltage on the AC side of the phase element is equal to the DC voltage of the DC electrical network. Accordingly any reduction in the DC voltage of the DC electrical network would lead to a reduction in the magnitude of the AC terminal voltage of the voltage source converter. This leads to undesirable lagging reactive power exchange with the AC network or, more preferably, the operation of a transformer tap changer to reduce the apparent AC network voltage to match the reduction in the DC voltage.

Arranging the tertiary and quarternary sub-converters on opposite sides of the "T" arrangement that is formed with the auxiliary sub-converter allows the voltage source converter to operate with a reduced DC network voltage and restore the nominal 1.0 per- unit voltage across the limbs of the voltage source converter and that of the unfolded AC terminal voltage. Configuring the controller to control the tertiary sub-converter in this manner results in energy drift, which must be managed by controlling the tertiary sub-converter to generate at least one 2^nd harmonic voltage component and controlling the auxiliary sub-converter to generate at least one 2^nd harmonic voltage component that is in anti-phase with the 2^nd harmonic voltage component generated by the tertiary sub-converter, as described above.

It is not practical to control the or each quarternary sub-converter to synthesize a compensatory quarternary voltage component because the size of the voltage component required would be much larger than the relatively small voltage component that is required to compensate for losses. The relatively larger compensatory quarternary voltage component that would be required would lead to a net energy exchange as a result of the DC voltage interacting with the DC current flowing through that quarternary sub-converter and it is not possible to compensate for such an energy drift using the interaction of harmonic voltage and current, as described above in connection with the tertiary sub- converter. This is because the or each quarternary sub-converter is located in a part of the voltage source converter that does not experience any harmonic currents.

So as to address faults in the DC network, the voltage source converter may include a controller configured to simultaneously control the or each tertiary sub-converter, the or each quarternary sub-converter and the or each auxiliary sub-converter so as to prevent or limit current flow from the AC network and prevent or limit current into the DC network in the event of a DC network low impedance fault.

The inclusion of such a controller in the voltage source converter permits control of the or each tertiary sub-converter, the or each quarternary sub-converter and the or each auxiliary sub-converter, which are normally used to facilitate transfer of power between the AC and DC electrical networks, to reliably minimise or block a fault current. This reduces or eliminates the need for additional fault current protection hardware (e.g. circuit breakers and surge arresters) to protect the voltage source converter from any detrimental effects of the fault current, thus resulting in an economical, space-saving voltage source converter that is capable of transferring power between the AC and DC electrical networks. It also minimises a fault current or blocks flow of a fault current through the voltage source converter resulting from a fault in the DC electrical network. The controller may be configured to simultaneously control the or each tertiary sub- converter, the or each quarternary sub-converter and the or each auxiliary sub-converter so that the DC voltage across the or each limb is zero. This results in a zero DC voltage across the first and second DC terminals and thus prevents a fault current from flowing between the first and second DC terminals via the or each auxiliary sub-converter.

Such a configuration allows the voltage source converter to be operated in a stable manner. It is also particularly advantageous in that control of the individual limbs offers very flexible operation and enables the voltage source converter to operate as a reactive power compensator (STATCOM) during a DC network fault.

In one particular embodiment the controller may be configured to simultaneously control the or each tertiary sub-converter, the or each quarternary sub-converter and the or each auxiliary sub-converter so as to block the or each auxiliary sub-converter and operate the or each tertiary sub-converter and the or each quarternary sub-converter so as to oppose a driving voltage applied by the AC network. Controlling the or each tertiary sub-converter and the or each quarternary sub-converter in the fault operating made to synthesize an opposing voltage permits distribution of the opposing voltage between the tertiary and quarternary sub-converters. This allows the individual voltage ratings of the or each tertiary sub-converter and the or each quarternary sub-converter to be reduced.

It will be appreciated that this configuration is particularly suitable in embodiments where the functionality of the quarternary sub-converter is consolidated at the DC network and the voltage rating of the quarternary sub-converter is therefore increased. The or each tertiary sub-converter and/or the or each quarternary sub-converter may be controlled in the fault operating mode to synthesize a variety of voltages in order to synthesize the opposing voltage.

It is also envisaged that in embodiments where the voltage source converter includes three limbs connected in series between the first and second DC terminals, the controller may be configured to simultaneously control each tertiary sub-converter, the or each quarternary sub-converter and each auxiliary sub-converter so that the voltage across each limb includes at least one harmonic component, the or each harmonic component being a positive integer multiple of a 2^nd harmonic component.

It will be appreciated that in such embodiments the or each harmonic component is a 2^nd harmonic voltage component, a 4^th harmonic voltage component, an 8^th harmonic component or a 10^th harmonic component, i.e. a (3(2n-1)±1)^lh harmonic component, whereby n is a positive integer multiple.

This configuration takes advantage of the 120 electrical degree displacements of the individual limbs and enables the summation of the DC voltages across the individual limbs to define a zero voltage for presentation to the faulty DC electrical network.

It will be appreciated that the or each limb and its components may be configured in different ways to vary the topology of the voltage source converter.

In embodiments of the invention at least one limb may include the auxiliary sub-converter being connected in parallel with an electrical block that includes the phase element.

In embodiments of the invention employing the use of at least one tertiary sub-converter, at least one limb may include an electrical block that includes a series connection of the tertiary sub-converter and phase element.

The configuration of the plurality of switching elements in each phase element may vary so long as the plurality of switching elements is capable of interconnecting a DC voltage and an AC voltage. For example, the plurality of switching elements in the or each phase element may include two parallel-connected pairs of series-connected switching elements, a junction between each pair of series-connected switching elements defining an AC terminal for connection to a respective phase of a multi-phase AC electrical network. The manner in which each limb is connected between the first and second DC terminals may vary. For example, a plurality of limbs may be connected in series between the first and second DC terminals.

In further embodiments of the invention, the or each sub-converter may be a multilevel converter.

In still further embodiments of the invention, the or each sub-converter may include at least one module, the or each module including at least one switching element and at least one energy storage device, the or each switching element and the or each energy storage device in each module combining to selectively provide a voltage source. The inclusion of the or each module in the or each sub-converter provides the or each sub- converter with a reliable means of acting as a waveform synthesizer.

The or each module in the or each sub-converter may vary in configuration.

In embodiments of the invention the or each switching element and the or each energy storage device in each module may combine to selectively provide a unidirectional voltage source. For example, the or each module in the auxiliary sub-converter may include a pair of switching elements connected in parallel with an energy storage device in a half-bridge arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct current in two directions.

In other embodiments of the invention, the or each switching element and the or each energy storage device in each module may combine to selectively provide a bidirectional voltage source. For example, the or each module in the tertiary sub-converter may include two pairs of switching elements connected in parallel with an energy storage device in a full-bridge arrangement to define a 4-quadrant bipolar module that can provide negative, zero or positive voltage and can conduct current in two directions. In embodiments of the invention where the voltage source converter may only have to generate leading reactive power, such that the or each tertiary sub-converter may only have to produce a positive DC voltage in order to generate an AC side voltage magnitude increase and a 2^nd harmonic component for energy management, the or each module of the or each tertiary sub-converter may include a pair of switching elements connected in parallel with an energy storage device in a half-bridge arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct in two directions.

In other embodiments where the leading and lagging reactive power requirements of the voltage source converter are unsymmetrical and biased heavily in one direction (e.g. leading) the or each tertiary sub-converter may include a plurality of modules, one or more of the modules including a pair of switching elements connected in parallel with an energy storage device in a half-bridge arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct in two directions, and one or more of the modules including two pairs of switching elements connected in parallel with an energy storage device in a full-bridge arrangement to define a 4-quadrant bipolar module that can provide negative, zero or positive voltage and can conduct current in two directions, so that the voltage available from the or each tertiary sub-converter in positive and negative directions is different and asymmetric.

The or each sub-converter may include a plurality of series-connected modules that defines a chain-link converter. The structure of the chain-link converter permits build up of a combined voltage across the chain-link converter, which is higher than the voltage available from each of its individual modules, via the insertion of the energy storage devices of multiple modules, each providing its own voltage, into the chain-link converter. In this manner switching of the or each switching element in each module causes the chain-link converter to provide a stepped variable voltage source, which permits the generation of a voltage waveform across the chain-link converter using a step-wise approximation. As such the chain-link converter is capable of providing a wide range of complex voltage waveforms for modifying the DC voltage at the DC side of the corresponding phase element.

At least one switching element may include at least one self-commutated switching device. The or each self-commutated switching device may be an insulated gate bipolar transistor, a gate turn-off thyristor, a field effect transistor, an injection-enhanced gate transistor, an integrated gate commutated thyristor or any other self-commutated switching device. The number of switching devices in each switching element may vary depending on the required voltage and current ratings of that switching element.

The or each switching element may further include a passive current check element that is connected in anti-parallel with the or each switching device.

The or each passive current check element may include at least one passive current check device. The or each passive current check device may be any device that is capable of limiting current flow in only one direction, e.g. a diode. The number of passive current check devices in each passive current check element may vary depending on the required voltage and current ratings of that passive current check element.

Each energy storage device may be any device that is capable of storing and releasing energy, e.g. a capacitor, fuel cell or battery. It will be appreciated that each embodiment of the invention may be optionally combined with one or more other embodiments of the invention. It will also be appreciated that use of the terms "tertiary", "auxiliary" and "quarternary" in the patent specification is merely intended to help distinguish between similar features (e.g. the auxiliary, tertiary and quarternary sub-converters), and is not intended to indicate the relative importance of one feature over another feature.

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

Figure 1 shows a voltage source converter according to an embodiment of the invention;

Figures 2a and 2b respectively show, in schematic form, the structure of a 2- quadrant unipolar module and a 4-quadrant bipolar module;

Figures 3a and 3b illustrate the voltage and current components of the voltage source converter shown in Figure 1 ;

Figures 4a-4h illustrate a strategy for operating the voltage source converters shown in Figure 1 to manage energy flow in the or each quarternary sub-converter;

Figures 5a-5e illustrate a strategy for operating the voltage source converters shown in Figure 1 to manage energy flow in each tertiary sub-converter;

Figures 6a-6f illustrate a strategy for operating the voltage source converter shown in Figure 1 in the event of a depressed DC network voltage;

Figure 7 shows a voltage source converter according to another embodiment of the invention;

Figures 8a-8c illustrate potential current loops through the auxiliary, tertiary and quarternary sub-converters of the voltage source converters shown in Figures 1 and 3 in the event of a DC network fault;

Figures 9a and 9b illustrate a method of addressing a DC network fault in the voltage source converter shown in Figure 7; and

Figure 10 illustrates a method of addressing a DC network fault in the voltage source converter shown in Figure 1.

A voltage source converter 10 according to an embodiment of the invention is shown in Figure 1.

The voltage source converter 10 includes first and second DC terminals 12,14 for connection to a DC electrical network and three limbs 16 connected in series between the first and second DC terminals 12,14. Each of the limbs 16 includes a phase element 18 having two parallel-connected pairs of series-connected switching elements 20. A junction between each pair of series- connected switching elements 20 defines an AC terminal. The AC terminals of each phase element 18 define the AC side 22 of that phase element 18.

In use, the AC terminals of each phase element 18 are interconnected by a respective one of a plurality of open secondary transformer windings 24. Each secondary transformer winding 24 is mutually coupled with a respective one of a plurality of primary transformer windings 26. The plurality of primary transformer windings 26 are connected in a star configuration in which a first end of each primary transformer winding 26 is connected to a common junction 28 and a second end of each primary transformer winding 26 is connected to a respective phase of a three-phase AC electrical network 30. In this manner, in use, the AC side 22 of each phase element 18 is connected to a respective phase of the three-phase AC electrical network 30.

The common junction 28 defines a neutral point of the plurality of primary transformer windings 26, and is grounded.

The phase element 18 of each limb 16 is connected in series with a tertiary converter 32 to define an electrical block, and the tertiary converter 32 is further connected in series with a quarternary converter 36.

Each limb 16 further includes an auxiliary sub-converter 38 that is connected in parallel with the electrical block of the limb 16 and is connected to a common connection point between the tertiary sub-converter 32 and the quarternary sub-converter 36 to form a "T" arrangement.

The sub-converters 32,36,38 each include a plurality of modules 40. Each module 40 of each auxiliary sub-converter 38 includes a pair of switching elements 54 and an energy storage device 56 in the form of a capacitor. In each auxiliary sub- converter 38, the pair of switching elements 54 is connected in parallel with the capacitor 56 in a half-bridge arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct current in two directions, as shown in Figure 2a.

Each module 40 of each tertiary sub-converter 32 and each quarternary sub-converter 36 includes two pairs of switching elements 54 and an energy storage device 56 in the form of a capacitor. In each tertiary sub-converter 32 and each quarternary sub-converter 36, the pairs of switching elements 54 are connected in parallel with the capacitor 56 in a full- bridge arrangement to define a 4-quadrant bipolar module that can provide negative, zero or positive voltage and can conduct current in two directions, as shown in Figure 2b.

As outlined above, the limbs 16 are connected in series between the first and second DC terminals 12, 14. In use, the first and second DC terminals 12, 14 are respectively connected to first and second terminals of a DC electrical network 42, the first terminal of the DC electrical network 42 carrying a negative DC voltage and the second terminal of the DC electrical network 42 carrying a positive DC voltage.

The configuration of each limb 16 as set out above means that, in use, a DC voltage appears across the parallel-connected pairs of series-connected switching elements 20 of each phase element 18.

As such, in use, each phase element 18 interconnects a DC voltage and an AC voltage.

In other embodiments it is envisaged that each phase element 18 may include a plurality of switching elements with a different configuration to interconnect a DC voltage and an AC voltage.

Each switching element 20,54 includes a single switching device. Each switching element 20,54 further includes a passive current check element that is connected in anti-parallel with each switching device.

Each switching device is in the form of an insulated gate bipolar transistor (IGBT). It is envisaged that, in other embodiments of the invention, each IGBT may be replaced by a gate turn-off thyristor, a field effect transistor, an injection-enhanced gate transistor, an integrated gate commutated thyristor or any other self-commutated switching device. The number of switching devices in each switching element may vary depending on the required voltage rating of that switch.

Each passive current check element includes a passive current check device in the form of a diode. It is envisaged that, in other embodiments, each diode may be replaced by any other device that is capable of limiting current flow to only one direction. The number of passive current check devices in each passive current check element may vary depending on the required voltage rating of that passive current check element. It is further envisaged that, in other embodiments of the invention, each capacitor may be replaced by another type of energy storage device that is capable of storing and releasing energy, e.g. a fuel cell or battery.

The plurality of series-connected modules 40 in each sub-converter 32,36,38 define a chain-link converter.

The capacitor 56 of each module 40 is selectively bypassed or inserted into the chain-link converter by changing the states of the switching elements 54. This selectively directs current through the capacitor 56 or causes current to bypass the capacitor 56 so that the module 40 provides a zero or positive voltage in the case of each auxiliary sub-converter 38, the module 40 provides a negative, zero or positive voltage in the case of each tertiary sub-converter 32 and each quarternary sub-converter 36.

The capacitor 56 of the module 40 is bypassed when the switching elements 54 in the module 40 are configured to form a short-circuit in the module 40. This causes current in the chain-link converter to pass through the short-circuit and bypass the capacitor 56, and so the module 40 provides a zero voltage, i.e. the module 40 is configured in a bypassed mode.

The capacitor 56 of the module 40 is inserted into the chain-link converter when the switching elements 54 in the module 40 are configured to allow the current in the chain- link converter to flow into and out of the capacitor 56. The capacitor 56 then charges or discharges its stored energy so as to provide a non-zero voltage, i.e. the module 40 is configured in a non-bypassed mode.

It is envisaged that, in other embodiments of the invention, each module 40 may be replaced by another type of module that includes at least one switching element and at least one energy storage device, the or each switching element and the or each energy storage device in the or each module combining to selectively provide a voltage source.

The structure of the chain-link converter permits a build-up of a combined voltage across the chain-link converter, which is higher than the voltage available from each of its individual modules 40, via insertion of the energy storage devices 56 of multiple modules 40, each providing its own voltage, into the chain-link converter. In this manner switching of each switching element 54 in each module 40 causes the chain-link converter to provide a stepped variable voltage source, which permits the generation of a voltage waveform across the chain-link converter using a step-wise approximation. As such each chain-link converter is capable of providing a wide range of complex voltage waveforms. The series connection of the tertiary sub-converter 32 and phase element 18 in each limb 16 permits the tertiary sub-converter 32 to selectively act as a waveform synthesizer to modify a first DC voltage at a DC side of the corresponding phase element 18. Such modification of the DC voltage at the DC side of the corresponding phase element 18 results in a corresponding modification of the AC voltage at the AC side 22 of the corresponding phase element 18.

The series connection of the quarternary sub-converter 36 with the tertiary sub-converter 32 in each limb 16 permits the quarternary sub-converter 36 to selectively act as a waveform synthesizer to modify a second DC voltage that is presented to the DC electrical network.

The parallel connection of the auxiliary sub-converter 38 and electrical block in each limb 16, and connection to a common connection point between the respective tertiary and quarternary sub-converters 32,36, permits the auxiliary sub-converter 38 to selectively act as a waveform synthesizer to modify the first and second DC voltages.

It is envisaged that, in other embodiments of the invention, the configuration of each tertiary sub-converter 32, each quarternary sub-converter 36 and each auxiliary sub- converter 38 may vary as long as the sub-converter is capable of selectively acting as a waveform synthesizer to modify the first and/or second DC voltages, as required. For example, each auxiliary sub-converter may be a multilevel converter.

The voltage source converter 10 further includes a controller 44 configured to control the tertiary, quarternary and auxiliary sub-converters 32,36,38.

More particularly the controller 44 is configured to perform a first control function, which is selective control of each quarternary sub-converter 36 to synthesize a voltage waveform that includes at least one quarternary harmonic component, the or each quarternary harmonic component being a positive integer multiple of a 6^th harmonic component.

Controlling the quarternary sub-converter 36 in this manner actively filters undesirable ripple harmonic components, each of which is a positive integer multiple of a 6^th harmonic component, that are present in the first DC voltage and thereby prevents undesirable ripple harmonic components from appearing in the second DC voltage presented to the DC electrical network 42. An alternative solution would involve controlling each auxiliary sub-converter 38 to synthesize one or more zero-phase triplen harmonic components (e.g. 3^rd, 9^th and 15^th harmonic components) to cancel the undesirable ripple harmonic components, each of which is a positive integer multiple of a 6^th harmonic component, that are present in the second DC voltage. Synthesis of one or more zero-phase sequence triplen harmonic components by each auxiliary sub-converters 38 however means that use of a delta- connected plurality of transformer primary windings to interconnect the AC electrical network 30 and the AC side 22 of each phase element 18 would result in a driving voltage around the delta-connected plurality of transformer windings, thus providing a path for a significant, continuous zero-phase sequence current to flow in the delta-connected plurality of transformer windings. Similarly use of a star-connected plurality of transformer primary windings, with a grounded neutral point, to interconnect the AC electrical network 30 and the AC side 22 of each phase element 18 would provide a path for a significant, continuous zero-phase sequence current to flow in the star-connected plurality of transformer primary windings.

Controlling each quarternary sub-converter 36 to synthesize a voltage waveform including at least one quarternary harmonic component obviates the need to control each auxiliary sub-converter 38 to synthesize one or more zero-phase triplen harmonic components. This in turn permits use of the star-connected plurality of transformer primary windings 26 with a grounded neutral point 28, which could be desirable for high power applications.

Locating the harmonic filter provided by the quarternary sub-converter 36 of each limb 16 on the DC side of the "T" arrangement (Figures 3a and 3b) that is formed with the respective tertiary and auxiliary sub-converters 32,38 means that the harmonic filter is located in a part of the converter structure where the current is DC, i.e. ripple free, as illustrated in Figure 3b.

It is not therefore necessary to control each of the auxiliary sub-converters 38 to synthesize at least two 2^nd harmonic components at the same time as controlling the quarternary sub- converter to synthesize one or more 6^th harmonic components, which would be the case if, for example, the tertiary sub-converter 32 located on the other side of the "T" arrangement was used as a harmonic filter. The generation of harmonic voltages in the presence of a DC current means that in principle there is no energy exchange with each quarternary sub-converter 36 because the voltage and current are at different frequencies.

The controller 44 is however further configured to selectively control each quarternary sub- converter 36 to synthesize at least one compensatory quarternary voltage component 46 so as to transfer energy to or from that quarternary sub-converter 36 and thereby minimise a net change in energy level of that quarternary sub-converter 36.

Controlling each quarternary sub-converter in this manner allows the injection of a relatively small DC voltage to interact with the DC current be flowing through the quarternary sub-converter 36 and create a power and energy exchange mechanism that may be used for dynamic stabilisation and compensation of losses, as shown in Figure 4a.

So as to compensate for the effect of the compensatory quarternary voltage component 46 on the auxiliary sub-converter 38 in each limb 16, the controller 44 is configured to selectively control each auxiliary sub-converter 38 to synthesize one or more auxiliary voltage components 48 to minimise a net change in the DC voltage across the respective limb 16, as shown in Figure 4b.

In addition, the controller 44 is configured to selectively control each tertiary sub-converter 32 to synthesize one or more compensatory tertiary voltage components 50 to minimise a net change in the DC voltage at the DC side of the respective phase element 18, as shown in Figure 4c.

The compensatory tertiary voltage components 50 generated by each tertiary sub- converter 32 however interacts with the DC current e flowing through the tertiary sub- converter 32, which causes power and energy drift in that part of the circuit (Figure 4d).

To regulate the energy levels of the tertiary sub-converters 32, the controller 44 is configured to selectively control each tertiary sub-converter 32 to synthesize at least one tertiary voltage component 52 so as to transfer energy to or from that tertiary sub-converter 32 and thereby regulate an energy level of that tertiary sub-converter 32, as shown in Figure 4e. The or each tertiary voltage component 52 is synthesized to have the same frequency as a current component of a current flowing through that tertiary sub-converter 32 and to be a positive integer multiple of a 2^nd harmonic voltage component. In addition, the or each tertiary voltage component 52 is synthesized to be in phase with a current component of the current fe flowing through that tertiary sub-converter 32. Control of each tertiary sub-converter 32 in this manner maximises the amount of energy transferred to or from each tertiary sub-converter 32, and thereby optimises the operation of the voltage source converter 10 to regulate the energy level of each tertiary sub- converter 32.

So as to compensate for the effect of the tertiary voltage component 52 on the auxiliary sub-converter 38 in each limb 16, the controller 44 is configured to selectively control each auxiliary sub-converter 38 to synthesize an auxiliary voltage component 58 that is in anti- phase with the respective tertiary voltage component 52, as shown in Figure 4f.

The displacement of 120 electrical degrees between the three limbs 16 of the voltage source converter means that summation of the auxiliary voltage component 58 synthesized by each auxiliary sub-converter 38 sums to zero across the limbs 16 and does not affect the second DC voltage that is presented to the DC network 42, as shown in Figure 4g.

The strategy of energy management described above with reference to Figures 4a-4g, and summarised in Figure 4h, is more complex than would be required in a voltage source converter in which the control is configured to selectively control each tertiary sub- converter 32 to synthesize a 6^th harmonic component for harmonic filtering purposes.

This is a result of the location of each quarternary sub-converter 36 on the DC side of the "T" arrangement that is formed in each limb 16 with the respective tertiary and auxiliary sub-converters 32,38.

The benefits however of providing sub-converters on opposite sides of the "T" arrangement, which leads to two degrees of freedom in sub-converter energy management and far greater design freedom, have however been found to outweigh the need for the more complex strategy for energy management. In the embodiment shown in Figure 1 the controller 44 is also configured to perform a second control function, which is selective control of each tertiary sub-converter 32 to generate a compensatory DC tertiary voltage component 50 for presentation to the DC side of the respective phase element 18 so as to compensate for a change in the respective first DC voltage caused by a change in real power and/or reactive power generated or absorbed at an AC side of the respective phase element 18 and thereby inhibit the change in the respective first DC voltage from modifying the respective second DC voltage. During operation of the voltage source converter 10, the first and second DC voltages in each limb 16 may be set to be equal so that it is not necessary to control the respective tertiary converter 32 to synthesize a DC voltage waveform. Since the voltage source converter 10 includes three limbs 16 connected in series between the first and second DC terminals 12, 14, each of the first and second DC voltages is equal during normal operation to one-third of the voltage across the first and second DC terminals 12,14 (i.e. the voltage of the DC network 42).

When the voltage source converter 10 is controlled to generate or absorb reactive power at the AC sides 22 of the phase elements 36 (e.g. through switching of the switching elements 20 of the phase elements 18), the AC voltage at the AC side 22 of each phase element 18 must increase or decrease in magnitude. An increase or decrease in the AC voltage at the AC side 22 of each phase element 18 in turn results in an increase or decrease in the first DC voltage presented to the DC side of each phase element 18, which then causes an increase or decrease in each second DC voltage.

Consequently each second DC voltage will no longer be equal to one-third of the voltage across the first and second DC terminals 12,14 (i.e. the voltage of the DC electrical network 42). To inhibit the change in the respective first DC voltage from modifying the second DC voltage, the controller 44 controls each tertiary sub-converter 32 to generate a compensatory DC voltage component 50 for presentation to the DC side of the respective phase element 18 so as to compensate for a change in the respective first DC voltage caused by the change in reactive power generated or absorbed at an AC side 22 of the respective phase element 18. This is shown schematically in Figure 5a. Control of each tertiary sub-converter 32 in this manner inhibits any effect a change in reactive power generated or absorbed at an AC side 22 of the respective phase element 18 might have on the respective second DC voltage. It thereby prevents any undesirable change in the respective second DC voltage that would otherwise have resulted from the change in reactive power generated or absorbed at an AC side of the respective phase element 18.

It is therefore possible to operate the voltage source converter 10 over a wide range of real power and reactive power with little to zero detrimental effect on the operation of the DC side of the voltage source converter 10.

An alternative solution would involve controlling each auxiliary sub-converter 38 to synthesize one or more zero-phase sequence triplen harmonic components (e.g. 3^rd, 9^th and 15^th harmonic components) to compensate for any change in the DC side of the voltage source converter 10 caused by a change in real power and/or reactive power generated or absorbed at an AC side 22 of the respective phase element 18. As mentioned above, controlling each auxiliary sub-converter 38 to synthesize one or more zero-phase sequence triplen harmonic components however means that use of a delta-connected plurality of transformer primary windings to interconnect the AC electrical network 30 and the AC side 22 of each phase element 18 would result in a driving voltage around the delta-connected plurality of transformer primary windings, thus providing a path for a significant, continuous zero-phase sequence current to flow in the delta-connected plurality of transformer primary windings. Similarly use of a star-connected plurality of transformer primary windings, with grounded neutral point, to interconnect the AC electrical network 30 and the AC side 22 of each phase element 18 would provide a path for a significant continuous zero-phase sequence current to flow in the star-connected plurality of transformer primary windings.

Controlling each tertiary sub-converter 32 to generate compensatory DC tertiary voltage component 50 for presentation to the DC side of the respective phase element 18 obviates the need to control each auxiliary sub-converter 38 to synthesize one or more zero-phase sequence triplen harmonic components. This in turn permits use of a star-connected plurality of transformer primary windings with a grounded neutral point, which could be desirable for high power applications.

As described above with reference to Figure 4d, the compensatory tertiary voltage component 50 generated by each tertiary sub-converter 32 will interact with the DC current IDC flowing through the tertiary sub-converter 32, which causes power and energy drift in that part of the circuit.

To regulate the energy levels of the tertiary sub-converters 32, the controller 44 is configured to selectively control each tertiary sub-converter 32 to synthesize at least one tertiary voltage component 52 so as to transfer energy to or from that tertiary sub-converter 32 and thereby regulate an energy level of that tertiary sub-converter 32, as shown in Figure 5b. Again the or each tertiary voltage component 52 is synthesized to have the same frequency as a current component of a current l₂ flowing through that tertiary sub-converter 32 and to be a positive integer multiple of a 2^nd harmonic voltage component.

In addition, the or each tertiary voltage component 52 is synthesized to be in phase with a current component of the current flowing through that tertiary sub-converter 32. Control of each tertiary sub-converter 32 in this manner maximises the amount of energy transferred to or from each tertiary sub-converter 32, and thereby optimises the operation of the voltage source converter 10 to regulate the energy level of each tertiary sub- converter 32.

So as to compensate for the effect of the tertiary voltage component 52 on the auxiliary sub-converter 38 in each limb 16, the controller 44 is configured to selectively control each auxiliary sub-converter 38 to synthesize an auxiliary voltage component 58 that is in antiphase with the respective tertiary voltage component 52, as shown in Figure 5c.

The displacement of 120 electrical degrees between the three limbs 16 of the voltage source converter means that summation of the auxiliary voltage component 58 synthesized by each auxiliary sub-converter 38 sums to zero across the limbs 16 and does not affect the second DC voltage that is presented to the DC network 42, as shown in Figure 5d.

The strategy for energy management described above with reference to Figures 5a-5d is summarised in Figure 5e. In each of the energy management strategies described above, and summarised in Figures 4h and 5e, each tertiary sub-converter 32 is selectively controlled to synthesize at least one tertiary voltage component 52 so as to transfer energy to or from that tertiary sub-converter 32 and thereby regulate an energy level of that tertiary sub-converter 32.

In each case the or each tertiary voltage component 52 is synthesized to have the same frequency as a current component of a current l₂ flowing through the corresponding tertiary sub-converter 32, and to be a positive integer multiple of a 2^nd harmonic voltage component.

In addition, the or each tertiary voltage component 52 is synthesized to be in phase with a current component of the current flowing through that tertiary sub-converter 32. Controlling each tertiary sub-converter 32 in this manner maximises the amount of energy transferred to or from each tertiary sub-converter 32, and thereby optimises the operation of the voltage source converter 10 to regulate the energy level of each tertiary sub- converter 32.

The controller 44 is further configured to selectively control each auxiliary sub-converter 38 to synthesize an auxiliary voltage component 58 that is in anti-phase with the respective tertiary voltage component 52. Controlling each auxiliary sub-converter 38 in this manner ensures that the respective first DC voltage, and therefore the AC voltage at the AC side of the respective phase element 18, remains unmodified during the generation of the or each tertiary voltage component 52.

This means that it is possible to regulate the energy level of each tertiary sub-converter 32 at any time during operation of the voltage source converter 10 without affecting the transfer of power between the AC and DC electrical networks 30,42.

The product of the voltage and current of each tertiary sub-converter defines its power profile which, when integrated over time, provides an energy profile. Operation of the voltage source converter 10 to transfer power between the AC and DC electrical networks 30,42 could result in energy accumulation in (or energy loss from) at least one capacitor 56, thus resulting in deviation of the energy level of at least one capacitor 56 from a reference value.

Such a deviation is undesirable because, if too little energy is stored within a given capacitor 56 then the voltage the corresponding module 40 is able to generate is reduced, whereas if too much energy is stored in a given capacitor 56 then over-voltage problems may arise. The former would require the addition of a power source to restore the energy level of the affected capacitor 56 to the reference value, while the latter would require an increase in voltage rating of one or more capacitors 56 to prevent the over-voltage problems, thus adding to the overall size, weight and cost of the voltage source converter 10. In addition if too little energy is stored within a given capacitor 56 then the voltage source converter 10 might trip due to under-voltage protection.

The configuration of the voltage source converter 10 allows energy to be transferred to and from each tertiary sub-converter 32 to regulate the energy stored in one or more corresponding capacitors 56, thereby obviating the problems associated with a deviation of the energy level of at least one capacitor 56 from the reference value.

Regulation of the energy level of each tertiary sub-converter 32 may involve balancing of the energy levels of the plurality of tertiary sub-converters 32. This is useful when there is an imbalance in the energy levels of the plurality of tertiary sub-converters 32, which could be caused by, for example, an imbalance of the plurality of phase currents drawn from the AC electrical network 30, or component failure in one or more modules 40 of at least one tertiary sub-converter 32 leading to a reduction in energy storage capacity.

Optionally the controller 44 may be configured to selectively control each tertiary sub- converter 32 to modify a phase angle of the or each tertiary voltage component relative to a phase angle of a current flowing through each tertiary sub-converter 32. Control of each tertiary sub-converter 32 in this manner permits adjustment of the amount of energy transferred to or from each tertiary sub-converter 32, and thereby provides an additional way of regulating the energy level of each tertiary sub-converter 32.

In the embodiment shown in Figure 1 the controller 44 is further configured to perform a third control function, which is to selectively control each tertiary sub-converter 32 to generate a compensatory DC tertiary voltage component 50 for presentation to the DC side of the respective phase element 18 so as to compensate for a reduction in the DC voltage 60 across the first and second DC terminals 12,14 (Figures 6a and 6b).

The reduction in the DC voltage 60 could be caused, for example, by a high impedance fault in the DC network or a deliberate operating strategy to lower insulation voltage stress and prevent flash-over, as might occur during operation of an over-head DC line in the presence of a salt fog. Configuring the controller 44 to control each tertiary sub-converter 32 in this manner inhibits any effect a reduction in the DC voltage 60 of the DC electrical network might otherwise have at the AC side 22 of the respective phase element 18. As described above with reference to Figures 4d and 5a, the compensatory tertiary voltage component 50 generated by each tertiary sub-converter 32 will interact with the DC current be flowing through the tertiary sub-converter 32, which causes power and energy drift in that part of the circuit. To regulate the energy levels of the tertiary sub-converters 32, the controller 44 is configured to selectively control each tertiary sub-converter 32 to synthesize at least one tertiary voltage component 52 so as to transfer energy to or from that tertiary sub-converter 32 and thereby regulate an energy level of that tertiary sub-converter 32, as shown in Figure 6c.

Again the or each tertiary voltage component 52 is synthesized to have the same frequency as a current component of a current flowing through that tertiary sub-converter 32 and to be a positive integer multiple of a 2^nd harmonic voltage component. In addition, the or each tertiary voltage component 52 is synthesized to be in phase with a current component of the current I2 flowing through that tertiary sub-converter 32. Control of each tertiary sub-converter 32 in this manner maximises the amount of energy transferred to or from each tertiary sub-converter 32, and thereby optimises the operation of the voltage source converter 10 to regulate the energy level of each tertiary sub- converter 32.

So as to compensate for the effect of the tertiary voltage component 52 on the auxiliary sub-converter 38 in each limb 16, the controller 44 is configured to selectively control each auxiliary sub-converter 38 to synthesize an auxiliary voltage component 58 that is in anti- phase with the respective tertiary voltage component 52, as shown in Figure 6d.

The displacement of 120 electrical degrees between the three limbs 16 of the voltage source converter means that summation of the auxiliary voltage component 58 synthesized by each auxiliary sub-converter 38 sums to zero across the limbs 16 and does not affect the second DC voltage that is presented to the DC network 42, as shown in Figure 6e. The strategy for energy management described above with reference to Figures 6a-6e is summarised in Figure 6f.

A voltage source converter 70 according to a second embodiment of the invention is shown in Figure 7. The voltage source converter 70 is similar in structure and operation to the voltage source converter 10 shown in Figure 1 , and like features share the same reference numerals.

The voltage source converter 70 shown in Figure 7 differs from the voltage source converter 10 shown in Figure 1 by virtue of the fact that the functionality of the quarternary sub-converter 36 of each phase is consolidated at the DC network.

In the embodiment shown in Figure 7 the consolidated quarternary sub-converter 36 is connected in series between the tertiary sub-converter 32 of a first limb 16a and the first DC terminal.

It will be appreciated that the rating of the consolidated quarternary sub-converter 36 of the voltage source converter 70 shown in Figure 7 is three times that of each of the individual quarternary sub-converters 36 of the voltage source converter 10 shown in Figure 1.

Operation of the voltage source converter 70 shown in Figure 7 is identical to that shown of the voltage source converter 10 shown in Figure 1. Accordingly the energy management strategies described above with reference to Figures 4a-4h, 5a-5e and 6a-6f apply mutatis mutandis to the voltage source converter 70 shown in Figure 7.

It will be appreciated that the controllers of voltage source converters according to other embodiments of the invention may be configured so as to omit the capability to perform the first and/or second and/or third control function.

The controller 44 of each of the voltage source converters 10,70 shown in Figures 1 and 7 is preferably configured to simultaneously control each of the tertiary, quarternary and auxiliary sub-converters 32,36,38 so as to prevent or limit current flow from the AC network 30 and prevent or limit current into the DC network 42 in the event of a DC network low impedance fault. The inclusion of such a controller 44 in the voltage source converters 10,70 permits control of the tertiary, quarternary and auxiliary sub-converters 32,36,38, which are normally used to facilitate the transfer of power between the AC and DC electrical networks 30,42 to reliably minimise or block a fault current.

Figures 8a, 8b and 8c illustrate three possible current loops that might occur in each limb 16 of the voltage source converters 10,70 in the event of zero DC voltage across the first and second DC terminals 12,14 as a result of a DC network low impedance fault. The controller 44 must simultaneously control each of the tertiary, quarternary and auxiliary sub-converters 32,36,38 to produce opposing voltages to control or extinguish these current loops.

Controlling the individual limbs 16 and hence the individual phases in this manner enables flexible operation of the voltage source converters 10,70 and allows the voltage source converters 10,70 to be operated as a reactive power compensator (STATCOM) during a DC network fault.

In order to operate each of the voltage source converters 10,70 as a reactive power compensator; the tertiary, quarternary and auxiliary sub-converters 32,36,38 would be simultaneously controlled so that a current loop circulating through the phase element 18, tertiary sub-converter 32 and auxiliary sub-converter 38 of each limb 16 (shown in Figure 8a) would be leading or lagging, as required to generate or absorb reactive power in response to the requirements of the AC network 30. The controller 44 would simultaneously control the tertiary, quarternary and auxiliary sub- converters 32,36,38 in such circumstances to extinguish a current loop flowing through the auxiliary sub-converter 38, quarternary sub-converter 36 of each limb 16 and across the first and second DC terminals 12,14 (shown in Figure 8b) and a current loop flowing through the phase element 18, the tertiary sub-converter 32 and the quarternary sub- converter 36 of each limb 16 and across the first and second DC terminals 12,14 (shown in Figure 8c).

In other circumstances, where reactive power compensation is not required, the controller 44 would simultaneously control the tertiary, quarternary and auxiliary sub-converters 32,36,38 to extinguish all three current loops described above and shown in Figures 8a, 8b and 8c. The current loops passing through the auxiliary sub-converter 38 of each limb 16, and shown in Figures 8a and 8b, may be extinguished by blocking the auxiliary sub- converters 38, as shown in Figures 9a and 9b.

The third current loop, omitting the auxiliary sub-converter 38 in each limb 16 and shown in Figure 8c would be extinguished by simultaneously controlling the tertiary and quarternary sub-converters 32,36 to produce voltages that oppose the AC driving voltage. It is also envisaged that the current loops shown in Figures 8a and 8b may be extinguished by simultaneously controlling the tertiary, quarternary and auxiliary sub-converters 32,36,38 of each limb 16 so that the voltage across each limb 16 includes at least one harmonic component, the or each harmonic component being a positive integer multiple of a 2^nd harmonic component.

This configuration takes advantage of the 120 electrical degree displacements of the individual limbs 16 and enables the summation of the voltages across the individual limbs to define a zero DC voltage for presentation to the faulty DC network, as shown in Figure 10.

Whilst each of the voltage source converters 10,70 shown in Figures 1 and 7 are constructed to include full-bridge modules 40 in the tertiary and quarternary sub-converters 32,36, it will be appreciated that the structure of each tertiary sub-converter 32 may be varied to allow for different voltage source converter requirements.

For example, in embodiments where the voltage source converter will only be required to produce leading reactive power, such that each tertiary sub-converter will only have to produce a positive DC voltage in order to generate an AC side voltage magnitude increase and a 2^nd harmonic component for energy management, each tertiary sub-converter 32 may include a plurality of half-bridge modules 40.

In other embodiments where, for example, the voltage source converter will the leading and lagging power requirements will be unsymmetrical and biased heavily in one direction (e.g. leading) each tertiary sub-converter 32 may include a mixture of full and half-bridge modules 40 so that the voltage available from each tertiary sub-converter 32 is different and asymmetric. This flexibility is rendered possible by the use of tertiary and quarternary sub-converters 32,36 located on opposite sides of the "T" arrangement that the tertiary and quarternary sub-converters 32,36 form with the auxiliary sub-converter 38.

Claims

1. A voltage source converter comprising first and second DC terminals for connection to a DC electrical network and at least one limb connected between the first and second terminals, the limb including:

a phase element having a plurality of switching elements and at least one AC terminal for connection to an AC electrical network, the plurality of switching elements being configured to be controllable to facilitate power conversion between the AC and DC electrical networks;

a quarternary sub-converter connected in series with the tertiary sub-converter, the quarternary sub-converter being configured to be controllable to act as a waveform synthesizer to modify a second DC voltage presented to the DC electrical network; and an auxiliary sub-converter connected in parallel with the electrical block and connected to a common connection point between the tertiary and quarternary sub- converters to form a "T" arrangement, the auxiliary sub-converter being configured to be controllable to act as a waveform synthesizer to modify the first and second DC voltages.

2. A voltage source converter according to Claim 1 wherein the voltage source converter includes three limbs connected in series between the first and second DC terminals, the or each AC terminal of the phase element of each limb being for connection to a respective phase of a multiphase AC electrical network.

3. A voltage source converter according to Claim 1 wherein the phase element of the at least one limb includes at least one AC terminal for connection to a respective phase of a multiphase AC electrical network and the voltage source converter further includes at least two additional limbs connected in series with the at least one limb between the first and second DC terminals, each additional limb including:

an additional phase element having a plurality of switching elements and at least one AC terminal for connection to a respective phase of a multiphase AC electrical network, the plurality of switching elements being configured to be controllable to facilitate power conversion between the AC and DC electrical networks;

an additional tertiary sub-converter connected in series with the additional phase element in an additional electrical block, the additional tertiary sub-converter being configured to be controllable to act as a waveform synthesizer to modify a first DC voltage presented to a DC side of the additional phase element; and

4. A voltage source converter according to any one of the preceding claims further including a controller configured to selectively control the or each quarternary sub- converter to synthesize at least one quarternary voltage component, the or each quarternary voltage component being a positive integer multiple of a 6^th harmonic component.

5. A voltage source converter according to Claim 4 wherein the or each quarternary sub-converter includes at least one energy storage device and the controller is configured to selectively control the or each quarternary sub-converter to synthesize at least one compensatory quarternary voltage component so as to transfer energy to or from that quarternary sub-converter and thereby minimise a net change in energy level of that quarternary sub-converter.

6. A voltage source converter according to Claim 5 wherein the controller is configured to selectively control the or each auxiliary sub-converter to synthesize one or more auxiliary voltage components to minimise a net change in the DC voltage across the respective limb when the or each quarternary sub-converter synthesizes one or more compensatory quarternary voltage components.

7. A voltage source converter according to Claim 5 or Claim 6 wherein the controller is configured to selectively control the or each tertiary sub-converter to synthesize one or more compensatory tertiary voltage components to minimise a net change in the DC voltage at the DC side of the respective phase element when the or each quarternary sub- converter synthesizes one or more compensatory quarternary voltage components.

8. A voltage source converter according to any one of the preceding claims wherein the or each tertiary sub-converter includes at least one energy storage device and the voltage source converter further includes a controller configured to selectively control the or each tertiary sub-converter to synthesize at least one tertiary voltage component so as to transfer energy to or from that tertiary sub-converter and thereby regulate an energy level of that tertiary sub-converter.

9. A voltage source converter according to Claim 8 wherein the controller is configured to selectively control the or each tertiary sub-converter to synthesize at least one tertiary voltage component so as to transfer energy to or from that tertiary sub- converter and thereby minimise a net change in energy level of that tertiary sub-converter.

10. A voltage source converter according to Claim 8 or Claim 9 wherein the controller is configured to selectively control the or each auxiliary sub-converter to synthesize an auxiliary voltage component that is in anti-phase with the respective tertiary voltage component.

11 . A voltage source converter according to any one of Claims 8 to 10 wherein the or each tertiary voltage component is a 2^nd harmonic voltage component, a 4^th harmonic voltage component, an 8^th harmonic component or a 10^th harmonic component.

12. A voltage source converter according to any one of Claims 8 to 10 wherein the or each tertiary voltage component is a (3(2n-1)±1 )^th harmonic voltage component, wherein n is a positive integer multiple.

13. A voltage source converter according to any one of Claims 8 to 12 wherein the or each tertiary voltage component has the same frequency as a current component of a current flowing through that tertiary sub-converter.

14. A voltage source converter according to any one of Claims 8 to 13 wherein the controller is configured to selectively control the or each tertiary sub-converter to modify a phase angle of the respective tertiary voltage component relative to a phase angle of a current flowing through that tertiary sub-converter.

15. A voltage source converter according to any one of the preceding claims further including a controller configured to selectively control the or each tertiary sub-converter to generate a compensatory DC tertiary voltage component for presentation to the DC side of the respective phase element so as to compensate for a change in real power and/or reactive power generated or absorbed at an AC side of the respective phase element.

16. A voltage source converter according to Claim 15 wherein the controller is configured to selectively control the or each tertiary sub-converter to generate a compensatory DC tertiary voltage component for presentation to the DC side of the respective phase element so as to compensate for a change in the respective first DC voltage caused by the change in real power and/or reactive power generated or absorbed at an AC side of the respective phase element.

17. A voltage source converter according to Claim 16 wherein the controller is configured to selectively control the or each tertiary sub-converter to generate a compensatory DC tertiary voltage component for presentation to the DC side of the respective phase element so as to compensate for a change in the respective first DC voltage caused by the change in real power and/or reactive power generated or absorbed at an AC side of the respective phase element and thereby inhibit the change in the respective first DC voltage from modifying the DC voltage across the respective limb.

18. A voltage source converter according to any one of the preceding claims further including a controller configured to selectively control the or each tertiary sub-converter to generate a compensatory DC tertiary voltage component for presentation to the DC side of the respective phase element so as to compensate for a change in the DC voltage across the first and second DC terminals.

19. A voltage source converter according to any one of the preceding claims further including a controller configured to simultaneously control the or each tertiary sub- converter, the or each quarternary sub-converter and the or each auxiliary sub-converter so as to prevent or limit current flow from the AC network and prevent or limit current into the DC network in the event of a DC network low impedance fault.

20. A voltage source converter according to Claim 19 wherein the controller is configured to simultaneously control the or each tertiary sub-converter, the or each quarternary sub-converter and the or each auxiliary sub-converter so that the DC voltage across the or each limb is zero.

21. A voltage source converter according to Claim 20 wherein the controller is configured to simultaneously control the or each tertiary sub-converter, the or each quarternary sub-converter and the or each auxiliary sub-converter so as to block the or each auxiliary sub-converter and operate the or each tertiary sub-converter and the or each quarternary sub-converter so as to oppose a driving voltage applied by the AC network.

22. A voltage source converter according to Claim 19 wherein the voltage source converter includes three limbs connected in series between the first and second DC terminals and the controller is configured to simultaneously control each tertiary sub- converter, the or each quarternary sub-converter and each auxiliary sub-converter so that the voltage across each limb includes at least one harmonic component, the or each harmonic component being a positive integer multiple of a 2^nd harmonic component.

23. A voltage source converter according to Claim 22 wherein the or each harmonic component is a 2^nd harmonic voltage component, a 4^th harmonic voltage component, an 8^th harmonic component or a 10^th harmonic component.

24. A voltage source converter according to any one of the preceding claims wherein the or each tertiary sub-converter, the or each quarternary sub-converter and the or each auxiliary sub-converter is a multilevel converter including at least one module having at least one switching element and at least one energy storage device, the or each switching element and the or each energy storage device in each module combining to selectively provide a voltage source.

25. A voltage source converter according to Claim 24 wherein the or each module of the or each auxiliary sub-converter includes a pair of switching elements connected in parallel with an energy storage device in a half-bridge arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct in two directions.

26. A voltage source converter according to Claim 25 wherein the or each module of the or each quarternary sub-converter includes two pairs of switching elements connected in parallel with an energy storage device in a full-bridge arrangement to define a 4- quadrant bipolar module that can provide negative, zero or positive voltage and can conduct current in two directions.

27. A voltage source converter according to Claim 26 wherein the or each module of the or each tertiary sub-converter includes two pairs of switching elements connected in parallel with an energy storage device in a full-bridge arrangement to define a 4-quadrant bipolar module that can provide negative, zero or positive voltage and can conduct current in two directions.

28. A voltage source converter according to Claim 26 wherein the or each module of the or each tertiary sub-converter includes a pair of switching elements connected in parallel with an energy storage device in a half-bridge arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct in two directions.

29. A voltage source converter according to Claim 26 wherein the or each tertiary sub-converter includes a plurality of modules, one or more of the modules including a pair of switching elements connected in parallel with an energy storage device in a half-bridge arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct in two directions, and one or more of the modules including two pairs of switching elements connected in parallel with an energy storage device in a full- bridge arrangement to define a 4-quadrant bipolar module that can provide negative, zero or positive voltage and can conduct current in two directions, so that the voltage available from the or each tertiary sub-converter in positive and negative directions is different and asymmetric.