This invention relates to a power electronic converter for use in high voltage direct current power transmission and reactive power compensation. In particular the power electronic converter of the invention may be used in alternating current (AC) to AC power conversion to provide multilevel AC output.

Power converters are typically used to interconnect electrical networks having different power characteristics such as voltage amplitude and frequency. The design and structure of a power converter depends on the nature of the required power conversion and the power requirements of the connected electrical networks, which may vary greatly depending on the type and size of the intended application. One example of power conversion is in the field of power transmission. For example, in HVDC power transmission networks, AC electrical power is typically converted to direct current (DC) power for transmission via overhead lines and/or undersea cables. This conversion means that it is not necessary to compensate for AC capacitive load effects that are otherwise imposed by the transmission line or cable. This in turn reduces the cost per kilometre of the lines and/or cables, and thus conversion of AC power to DC power becomes cost-effective when power needs to be transmitted over a long distance. AC to DC and DC to AC power conversion is also commonly utilized in power transmission networks in circumstances where it is necessary to interconnect two AC networks operating at different frequencies. Converters are required at each interface between the AC and DC networks to effect the required conversion between the AC and DC networks.

Another example of power conversion is the use of a transformer to step up or step down the voltage so as to interconnect AC networks operating at different AC voltages. For example, transformers may be used in power transmission to increase the AC voltage before transmitting electrical energy over long distances or in electronic products to step down an AC supply voltage to a level that is compatible with low voltage circuits contained in the electronic products.

According to an aspect of the invention, there is provided a power electronic converter for use in high voltage direct current power transmission and reactive power compensation, the power electronic converter comprising at least one converter limb including first and second terminals for connection in use to a first electrical network and a third terminal for connection in use to a second electrical network; the or each converter limb defining first and second limb portions, each limb portion extending between a respective one of the first and second terminals and the third terminal and including at least two electronic blocks connected in parallel between a respective one of the first and second terminals and the third terminal, each electronic block including one or more series-connected modules, the or each module including at least one switching element connected to at least one energy storage device, the or each switching element of each module being controllable in use so that the respective module provides a voltage source . The provision of limb portions in the or each converter limb results in a flexible power electronic converter arrangement that is readily scalable to vary its voltage and current rating to match the requirements of the associated power application such as, for example, power transmission and electrical vehicles. Each limb portion may be modified to increase its current rating by increasing the number of parallel-connected electronic blocks and to increase its voltage rating by increasing the number of series-connected modules in each electronic block. The ability to increase the number of modules allows the use of switching elements having relatively low voltage and current ratings, which leads to a reduction in hardware costs.

Each module of the power electronic converter may be configured to have a standard circuit arrangement. This simplifies the installation and maintenance of the power electronic converter because an operator is only required to familiarize himself with the operation of the standard module. This also allows the application of a standard control scheme to each module, which simplifies the design and manufacture of the converter and thereby reduces manufacturing lead time and costs.

This modular approach therefore not only simplifies the design and manufacture of the power electronic converter so as to increase the cost- effectiveness of the converter, but also improves the reliability of the power electronic converter and its compatibility with a wide range of power applications having different power requirements.

Preferably each electronic block includes a plurality of series-connected modules.

In addition to improving the voltage rating of the converter and allowing the use of low-rated switching elements, the provision of the plurality of series-connected modules in each electronic block results in the or each converter limb defining a multilevel converter arrangement, which enables the generation of higher quality voltage waveforms. The multilevel converter arrangement of each electronic block also allows the power electronic converter to continue operating at a lower level in the event of one or more modules being offline and thereby improves the reliability of the power electronic converter. Preferably each electronic block includes a plurality of parallel-connected modules. The inclusion of parallel-connected modules improves the current rating of the converter.

Optionally the or each limb portion includes a plurality of pairs of parallel-connected electronic blocks connected in series between a respective one of the first and second terminals and the third terminal. Such an arrangement provides a multilevel converter with a high number of discrete output voltage states, and hence the ability to generate high quality voltage waveforms, while increasing the current rating of the converter.

In embodiments of the invention, the power electronic converter may include multiple converter limbs . The structure of the power electronic converter is compatible for connection to multi-phase AC networks. In AC to DC conversion, connection to a three-phase AC network minimises ripple in the generated DC voltage.

In other embodiments, the power electronic converter may include two converter limbs, wherein the converter limbs are symmetric to each other. The power electronic converter may include three converter limbs which are identical to one another . In such embodiments, each electronic block may have the same circuit arrangement.

The use of a standard circuit arrangement in the converter limbs and/or electronic blocks further simplifies the design, manufacture and control of the power electronic converter, which leads to reductions in converter hardware cost.

In a converter having two symmetric converter limbs and electronic blocks with the same circuit arrangement, the set of the first and second terminals of the converter limbs are interchangeable with the set of the third terminals of the converter limbs in that the first and second electrical networks may be connected to either set of terminals of the converter limbs without affecting the power conversion between the two electrical networks.

In further embodiments, at least one module may include at least one set of series-connected switching elements connected in parallel with at least one energy storage device.

In such embodiments, each or at least one module may be provided in the form of a 2-quadrant module or a 4-quadrant module. To define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct current in two directions, at least one module may include a set of series-connected switching elements connected in parallel with the respective energy storage device in a half-bridge arrangement.

To define a 4-quadrant bipolar module that can provide zero, positive or negative voltage and can conduct current in two directions, at least one module may include two sets of series-connected switching elements connected in parallel with the respective energy storage device in a full-bridge arrangement. In certain circuit arrangements of each electronic block, some of the modules in each electronic block may only be required to generate a voltage of a single polarity. It is therefore advantageous to modify those modules to have a 2- quadrant unipolar module structure instead of a 4- quadrant bipolar module structure so as to reduce the size of the module while maintaining the required functionality of the electronic block in question. The ability of 4-quadrant bipolar modules to provide a positive or negative voltage facilitates AC to AC power conversion and AC to DC power conversion. The use of 4-quadrant bipolar modules not only removes the need to redesign the power electronic converter to fit both types of power conversion, but also allows a single AC to AC converter, instead of two AC to DC converters, to interconnect AC networks operating at different frequencies. Additionally this allows the power electronic converter to omit the use of a transformer or multiple AC to AC converters and thereby reduce hardware cost size and weight, particularly when multiple AC to AC power conversions are required.

When each electronic block of a limb portion includes a plurality of series-connected full- bridge modules, the full-bridge modules may be switched into and out of circuit to form different configurations of the electronic block without affecting the voltage output of the limb portion. As a result, there are redundant configurations of the electronic block for specific voltage output states of the limb portion. These redundant configurations provide the power electronic converter with added functionality. For example, switching between different configurations of the electronic block allows for voltage balancing between the energy storage devices of the modules. The switching of the full- bridge modules may be controlled so as to minimise switching losses, while maintaining a specific voltage output state of the respective limb portion.

Preferably the or at least one switching element of each module may include at least one semiconductor device. The or at least one semiconductor device may be an insulated gate bipolar transistor, a gate turn-off thyristor, a field effect transistor, an insulated gate commutated thyristor or an integrated gate commutated thyristor.

In such embodiments employing the use of at least one semiconductor device, the or at least one switching element of each module may further include an anti-parallel diode connected in parallel with the respective semiconductor device. The use of semiconductor devices is advantageous because such devices are small in size and weight and have relatively low power dissipation, which minimises the need for cooling equipment. It therefore leads to significant reductions in power converter cost, size and weight.

The fast switching capabilities of such semiconductor devices allow each limb portion to synthesize complex waveforms for injection into the electrical networks connected to the power electronic converter. The injection of such complex waveforms can be used, for example, to minimise the levels of harmonic distortion typically associated with thyristor-based voltage source converters. Furthermore the inclusion of such semiconductor devices allow the power electronic converter to respond quickly to the development of AC and/or DC side faults and/or other abnormal operating conditions, and thereby improve fault protection of the power electronic converter. In embodiments of the invention, the or at least one energy storage device of each module may be a capacitor, fuel cell, photovoltaic cell, battery or an auxiliary AC generator with an associated rectifier.

Such flexibility is useful in the design of converter stations in different locations where the availability of equipment may vary due to locality and transport difficulties. For example, the energy storage device of each module on an offshore wind farm may be provided in the form of an auxiliary AC generator connected to a wind turbine.

In other embodiments, the or each switching element of each module may be controllable in use to generate a voltage to oppose the flow of current created by a fault, in use, in the first or second electrical networks. Each module may be used to inject a voltage to provide the opposing voltage required to limit or extinguish the fault current and thereby prevent damage to the power electronic converter components. The use of the power electronic converter components to carry out both voltage conversion and extinguishment of fault currents may eliminate the need for separate protective circuit equipment, such as a circuit breaker or isolator. This leads to savings in terms of hardware size, weight and costs. In further embodiments, the or each switching element of each module may be controllable in use to regulate the voltage of the respective energy storage devices.

The regulation of voltage levels provides additional control over the voltage levels of the or each energy storage device of each module. This form of control may be used, for example, to balance the voltage levels of individual modules. This is advantageous because it means that the voltage of any particular module can be kept approximately equal to an average module voltage to simplify the control and improve the performance of a power electronic converter which uses the average module voltage as feedback to control switching of the modules.

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

Figure 1 shows, in schematic form, a power electronic converter according to an embodiment of the invention;

Figure 2 shows a simplified version of the power electronic converter of Figure 1;

Figure 3 shows, in schematic form, the structure of a first limb portion of a first converter limb of the power electronic converter of Figure 1; Table 1 shows the different configurations of the first limb portion of the first converter limb of Figure 3;

Figures 4 and 5 show the charging and discharging of a module of a limb portion of the power electronic converter of Figure 1;

Table 2 shows the different voltage states of the limb portions of the power electronic converter during AC to AC power conversion;

Figures 6 and 7 show the results of a simulation of the operation of the power electronic converter of Figure 1;

Table 3 shows the different voltage states of the limb portions of the power electronic converter during DC to AC power conversion;

Figure 8(a) shows, in schematic form, a power electronic converter according to another embodiment of the invention;

Figure 8(b) shows, in schematic form, a unit cell forming part of the converter shown in Figure 8 (a) ;

Figure 9(a) shows a detailed view of the unit cell shown in Figure 8 (b) ;

Figure 9 (b) shows a simplified view of the unit cell shown in Figure 9(a); and

Figure 10 shows a schematic view of a converter limb forming part of the converter shown in Figure 8(a). A power electronic converter 10 according to an embodiment of the invention is shown in Figure 1. The power electronic converter 10 comprises first and second converter limbs 12,14, each converter limb including first, second and third terminals 16,18,20.

In use, the first and second terminals 16,18 of each converter limb 12,14 are connected to a first electrical network 22, which may be an AC or DC network, while the third terminal 20 of each converter limb 12,14 is connected to a second electrical network 24, which is an AC network.

Each converter limb 12,14 defines first and second limb portions 26,28. In each converter limb 12,14, the first limb portion 26 extends between the first terminal 16 and the third terminal 20 while the second limb portion 28 extends between the second terminal 18 and the third terminal 20. The first limb portion 26 includes first and second electronic blocks 30,32 connected in parallel between the third and first terminals 20,16 and the second limb portion 28 includes first and second electronic blocks 30,32 connected in parallel between the third and second terminals 20,18. Each of the first and second electronic blocks 30,32 includes two series-connected modules 34.

Each module 34 of the respective electronic block 30,32 includes two sets of series-connected switching elements 36 connected in parallel with a capacitor 38 in a full-bridge arrangement to define a 4-quadrant bipolar module. Each switching element 36 is an insulated gate bipolar transistor connected in parallel with an anti-parallel diode and each capacitor 38 has a voltage capacity of Vdc.

In use, each switching element 36 is controllable to turn on or off so that the respective module 34 defines a voltage source that can provide a positive, zero or negative voltage and conduct current in two directions.

In order for each module 34 to provide a voltage source, the capacitor 38 of each module 34 may be bypassed or inserted into circuit by changing the state of the switching elements 36.

The capacitor 38 of each module 34 is bypassed when the switching elements 36 are configured to form a short circuit in the module 34, causing the current in the power electronic converter 10 to pass through the short circuit and bypass the capacitor 38. This enables the module 34 to provide a zero voltage. The capacitor 38 of each module 34 is inserted into circuit when the switching elements 36 are configured to allow the converter current to flow into and out of the capacitor 38, which is then able to charge or discharge its stored energy and provide a voltage. The full-bridge arrangement of the module 34 allows the switching elements 36 to be configured to insert the capacitor 38 in circuit in either forward or reverse directions to allow either direction of current flow through the capacitor 38 so as to provide a positive voltage +Vdc or a negative voltage -Vdc .

In addition, each module 34 can conduct current in both directions when its capacitor 38 is either bypassed or inserted into circuit.

The above-described features of the power electronic converter 10 result in a symmetric arrangement, in which the set of the first and second terminals 16,18 of the converter limbs 12,14 are interchangeable with the set of the third terminals 20 of the converter limbs 12,14. This allows the first and second electrical networks 22,24 to be connected to either set of terminals 16,18,20 of the converter limbs 12,14 without affecting the power conversion between the first and second electrical networks 22,24.

This use of a standard circuit arrangement in each limb portion 26, 28 not only simplifies the installation and maintenance of the power electronic converter 10, but also simplifies the design, manufacture and control of the power electronic converter 10, which leads to reductions in converter hardware cost. It is envisaged that in other embodiments each switching element 36 may include a different semiconductor device, such as a field effect transistor, a gate-turn-off thyristor, a gate- commutated thyristor, an insulated gate-commutated thyristor, an integrated gate-commutated thyristor or other self commutated semiconductor switches, accompanied by a reverse-parallel connected diode. The fast switching capabilities of such semiconductor devices allow each limb portion 30,32 to synthesize complex waveforms for injection into the electrical networks 22,24 connected to the power electronic converter 10. The injection of such complex waveforms can be used, for example, to minimise the levels of harmonic distortion typically associated with thyristor-based voltage source converters. Furthermore the inclusion of such semiconductor devices allow the power electronic converter 10 to respond quickly to the development of AC and/or DC side faults and/or other abnormal operating conditions, and thereby improve fault protection of the power electronic converter 10.

It is also envisaged that in further embodiments, the capacitor 38 may be replaced by a fuel cell, photovoltaic cell, battery or an auxiliary AC generator with an associated rectifier.

Such flexibility is useful in the design of converter stations in different locations where the availability of equipment may vary due to locality and transport difficulties.

Figure 2 shows a simplified diagram of the power electronic converter of Figure 1.

For the purposes of this specification, the voltage across the first and second terminals 16,18 of the converter limbs 12,14 is referred to as Vin and the voltage across the third terminals 20 of the converter limbs 12,14 is referred to as Vout .

In addition the voltage across the first and second limb portions 26,28 of the first converter limb 12 are respectively referred to as Va and Vb while the voltage across the first and second limb portions 26, 28 of the second converter limb 14 are respectively referred to as Vc and Vd. The relationships between the first and second electrical networks 22,24 are identified in Equations 1 to 3.

Vin = Va + Vout + Vd ( 1 )

Vin = Vc - Vout + Vb ( 2 )

Vin = Va + Vb = Vc + Vd (3) The currents flowing throughout the circuit are calculated in Equations 4 and 5. Iin lout

la = Id = h (4)

2 2

Iin lout

Ib = Ic =— (5)

2 2

Where la and lb are the currents 40,42 respectively flowing through the first and second limb portions 26, 28 of the first converter limb 12; Ic and Id are the currents 44,46 respectively flowing through the first and second limb portions 26,28 of the second converter limb 14; and Iin and lout are the currents 48,50 flowing respectively through the set of the first and second terminals 16,18 and the set of the third terminals 20 of the converter limbs 12,14. Figure 3 shows, in schematic form, the structure of the first limb portion 26 of the first converter limb.

In Figure 3, the voltages across the two series-connected modules 34 of the first electronic block 30 of each limb portion 26, 28 are respectively referred to as VI and V3 while the voltages across the two series-connected modules 34 of the second electronic block 30 of each limb portion 26,28 are respectively referred to as V2 and V4.

The voltage Va across the first limb portion 26 of the first converter limb is built up from the individual voltages VI, V2, V3, V4 generated by the modules 34. use, the switching elements 36 of each module 34 are configured so that the voltages across the first and second electronic blocks 30,32 are equal. The voltage Va across the first limb portion 26 of the first convertor limb is therefore calculated using Equation 6.

Va = V3 - Vl = V2 - V4 (6) outlined earlier, the switching elements

36 of each module 34 are controllable in use so that the respective module 34 is capable of providing a voltage of +Vdc, 0 or -Vdc . As such, using Equation 6, the modules 34 of the first limb portion 26 of the first converter limb may be configured to generate different voltage output states of Va, which are -2Vdc, -Vdc, 0, +Vdc and +2Vdc, as shown in Table 1.

When Va is equal to -Vdc, 0 or +Vdc, it is possible to form different configurations of the first limb portion 26 of the first converter limb so as to generate the same voltage Va . For example, the switching elements 36 of the modules 34 of the first limb portion 26 of the first converter limb may be configured using Equation 1 to form a first configuration in which VI = 0; V2 = 0; V3 = -Vdc; and V4 = +Vdc, or a second configuration in which VI = 0 ; V2 = +Vdc; V3 = +Vdc; and V4 = 0. Both configurations result in the voltage Va being equal to +Vdc . Since only one configuration is necessary to generate the desired voltage Va, the other configuration may be regarded as redundant .

There are multiple redundant configurations of the first limb portion 26 of the first converter limb when Va is equal to -Vdc, 0 or +Vdc, as shown in Table 1. The operation of the power electronic converter may include the control of the switching elements 36 of the modules 34 to form these redundant configurations so as to provide the power electronic converter with added functionality.

One such added function is the switching between different configurations of the limb portion 26 to carry out voltage balancing between the capacitors 38 of the modules 34, which means that the voltage of any particular module 34 can be kept approximately equal to an average module voltage to allow the use of an average module voltage as feedback to control switching of the modules 34 and thereby simplify the control and improve the performance of the power electronic converter.

Figures 4 and 5 show the charging and discharging of the capacitor 38 of a module 34 when inserted into circuit in forward and reverse directions. In the event of an abnormal condition leading to voltage unbalance in the limb portion, the capacitor 38 of each module 34 may be inserted in either forward or reverse directions without affecting the output voltage of the limb portion, which enables the continuous operation of the power electronic converter. Otherwise it would be necessary to place the power electronic converter in offline mode to recharge or discharge the capacitors 38 of the modules 34 to the desired voltage level required for optimal performance.

Another such added function is the switching of the full-bridge modules 34 to form different configurations of the limb portion so as to minimise switching losses, while maintaining a specific voltage output state of the respective limb portion.

The above-described operation of the first limb portion 26 of the first converter limb 12 is also applicable to the operation of the remaining limb portions 26,28 of the power electronic converter 10 of Figure 1. As such, the voltages Vb,Vc,Vd across the other three limb portions 26,28 are also built up from the individual voltages VI, V2, V3, V4 generated by the modules 34 in the respective limb portion 26,28. Since the four limb portions 26, 28 share the same circuit arrangement, each limb portion 26,28 also shares the same voltage output states of -2Vdc, -Vdc, 0, +Vdc and +2Vdc.

To facilitate AC to AC power conversion between the first and second electrical networks 22,24, the power electronic converter 10 of Figure 1 is operated as follows: In use, the first and second terminals 16,18 of each converter limb 12,14 are connected to a first AC network 22 while the third terminal of each converter limb is connected to a second AC network 24.

Each of the first, second and third terminals 16,18,20 of each converter limb 12,14 may be connected in series with one or more inductors. In use, the first AC network 22 generates an AC voltage waveform having peak positive and negative values of +Vdc and -Vdc respectively. As such, Vin has three voltage states of +Vdc, 0 and -Vdc. Inserting the possible voltage states of

Vin, Va, Vb, Vc and Vd into Equations 1 to 3 leads to the different configurations of the limb portions 26,28 of the converter limbs 12,14 so as to generate different voltage states of Vout, which are -4Vdc, 3Vdc, -2Vdc, -Vdc, 0, +Vdc, +2Vdc, +3Vdc and +4Vdc.

Table 2 shows some of the possible configurations of the limb portions of the converter limbs to generate these voltage states of Vout.

In use, the switching elements 36 of the modules 34 are controllable to generate an AC voltage waveform at the third terminals 20 of the converter limbs 12,14, the AC voltage waveform having peak positive and negative values of +4Vdc and -4Vdc respectively and voltage steps of +Vdc or -Vdc. In other embodiments, the first and second AC networks 22,24 may have different voltage amplitudes and/or operating frequencies. Figures 6 and 7 show the results of a simulation of the power conversion between a first AC network having a Vin 52 with a peak voltage of +/- 1.1 kV and a frequency of 1 kHz and a second AC network having a Vout 54 with peak voltage of +/- 1.1 kV and a frequency of 50 Hz. In this simulation, the output current lout 56 acts as a reference signal to carry out pulse-width modulation so as to obtain a Vout having a desired operating frequency of 50 Hz. The input current Iin 58 may have a ripple of 50Hz, which may be eliminated by using an appropriate filter.

In the simulation shown in Figures 6 and 7, the operation of the power electronic converter to facilitate power conversion between the first and second AC networks leads to voltage balance between the capacitors of the modules.

To facilitate DC to AC power conversion between the first and second electrical networks 22,24, the power electronic converter 10 of Figure 1 is operated as follows:

In use, the first and second terminals

16,18 of each converter limb 12,14 are connected to positive and negative terminals of a DC network 22 carrying a voltage Vin of +Vdc while the third terminals 20 of each converter limb 12,14 are connected to an AC network 24.

Inserting Vin and the possible voltage states of Va, Vb, Vc and Vd into Equations 1 to 3 leads to the different configurations of the limb portions 26, 28 of the converter limbs 12,14 so as to generate different voltage states of Vout, which are -3Vdc, 2Vdc, -Vdc, 0, +Vdc, +2Vdc and +3Vdc, as shown in Table 3.

In use, the switching elements 36 of the modules 34 are controllable to generate an AC voltage waveform at the third terminals 20 of the converter limbs 12,14, the AC voltage waveform having peak positive and negative values of +3Vdc and -3Vdc respectively and voltage steps of +Vdc or -Vdc.

During the operation of the power electronic converter 10 to carry out either AC to AC or DC to AC power conversion, it is envisaged that in other embodiments the switching elements 36 of the modules 34 may be controllable in use to generate different shapes of the AC voltage waveform at the third terminals 20 of the converter limbs 12,14, each AC voltage waveform including all or some of the possible voltage states of Vout, so as to enable connection of the power electronic converter 10 to different AC networks 24 having different power requirements such as, for example, amplitude and frequency . In similar fashion to the operation of the individual limb portion 26,28 in Figure 3, it is possible to form different configurations of the limb portions 26,28 to generate a desired Vout for a specific Vin using Equations 1 to 3. Since only one configuration is necessary to generate the desired Vout for the specific Vin, the other configurations may be regarded as redundant. As outlined earlier, the operation of the power electronic converter 10 may include the control of the switching elements 36 of the modules 34 to form these redundant configurations so as to provide the power electronic converter 10 with added functionality, such as, for example, the earlier described added functions.

In other embodiments, it is envisaged that the switching elements 36 of the modules 34 may be controllable in use to form different configurations of the limb portions 26,28 of the converter limbs 34 so as to generate the same value of Vout for different values of Vin. For example, the switching elements 36 of the modules 34 may be configured using Equations 1 to 3 to form a first configuration in which Va = +2Vdc; Vb = - Vdc; Vc = -Vdc; and Vd = +2Vdc when Vin = +Vdc, or a second configuration in which Va = +2Vdc; Vb = -2Vdc; Vc = -Vdc; and Vd = +Vdc when Vin = 0. Both configurations results in Vout being equal to -3Vdc.

As such, the power electronic converter 10 may be configured to facilitate AC to DC power conversion, where the first electrical network 22 is an AC network and the second electrical network 24 is a DC network .

The power electronic converter 10 is therefore capable of carrying out power conversion between different AC networks and between AC and DC networks .

The structure of the limb portions of the converter limbs results in a flexible power electronic converter arrangement that is readily scalable to vary its voltage and current rating to match the requirements of the associated power application. In embodiments of the invention, each limb portion may include more than two electronic blocks connected in parallel between the third terminal and a respective one of the first and second terminals of the converter limbs and/or each electronic block may include one module or a plurality of series-connected modules .

Such modification of the limb portions leads to a change in current and/or voltage rating of the power electronic converter.

Increasing the number of modules in each limb portion reduces the required voltage rating of the individual module and thereby allows the use of switching elements having relatively low voltage and current ratings, which leads to a reduction in hardware costs .

In addition to improving the voltage rating of the converter and allowing the use of low-rated switching elements, increasing the number of series- connected modules in each electronic block results in an increased number of discrete output voltage states of the respective limb portion and converter limb. This is because having a plurality of series-connected modules in each electronic block results in the respective limb portion defining a multilevel converter arrangement, which allows the switching elements to switch each capacitor into and out of circuit to define various configurations of the limb portion and thereby generate the increased number of discrete output voltage states of the respective limb portion. As such, the increased number of discrete output voltage states of each limb portion increases the number of voltage steps in the generated voltage waveform and thereby enables the generation of higher quality voltage waveforms.

The multi-level arrangement of the limb portions also improves the reliability of the power electronic converter. In the event of one or more modules being offline, the remaining online modules may be used to carry out the required power conversion at a lower level whilst the offline modules are being repaired. Figure 8 shows such a multi-level arrangement in a power electronic converter 70 according to a second embodiment of the invention. The second converter 70 shares common features with the first converter 10 and these are designated by the same reference numeral.

The second converter 70 includes first, second and third converter limbs 12, 14, 72 which allows connection to a three phase AC network.

In other embodiments of the invention, the power electronic converter may include further converter limbs so as to enable connection to a multi- phase AC network with more than three phases.

Each converter limb 12, 14, 72 includes first, second and third terminals 16, 18, 20. As in the first converter 10, the first and second terminals 16, 18 are connectable to a first electrical network 22 which may be an AC or DC network. The third terminal 20 is connectable to a second electrical network 24 which is an AC network. The connection to the second electrical network 24 may include an inductor.

Each converter limb 12, 14, 72 includes first and second limb portions 26, 28. In each converter limb 12, 14, 72, the first limb portions 26 extends between the first terminal 16 and the third terminal 20 to define a top arm 74, and the second limb portion 28 extends between the second terminal 18 and the third terminal 20 to define a bottom arm 76.

Each limb portion 26, 28 includes a plurality of pairs of parallel-connected first and second electronic blocks 30, 32. Each pair of parallel- connected first and second electronic blocks 30, 32 together define a unit cell 78. The converter 70 shown includes four unit cells 78 in each limb portion 26, 28 although greater or fewer numbers of unit cell 78 in each limb portion 26, 28 are also possible. In any event, the unit cells 70 are connected in series between a respective one of the first and second terminals 16, 18 and the third terminal 20.

Each of the first and second electronic blocks 30, 32 in each unit cell 78 includes a pair of module assemblies 80 that are connected in series with one another .

Each module assembly 80 includes four modules 34 which are series-connected in pairs to define first and second module arms 82, 84 that are connected in parallel with one another.

As in the first converter 10, each module 34 includes two sets of series-connected switching elements 36 connected in parallel with a capacitor 38 in a full-bridge arrangement to define a 4-quadrant bipolar module. Each switching element 36 is an insulated gate bipolar transistor connected in parallel with an anti-parallel diode and each capacitor 38 has a voltage capacity of Vdc.

Each switching element 36 is again controllable to turn on or off so that the respective module 34 defines a voltage source that can provide a positive, zero or negative voltage and conduct current in two directions. In particular, in use each module can produce three voltage levels: +Vdc, 0, and -Vdc (where Vdc is the voltage across the capacitor 38 in the given module 34). Given that each module 34 is able to produce three voltage levels (+Vdc, 0, and -Vdc), the possible voltages across each module assembly 80 are +2 Vdc, + Vdc, 0, - Vdc, and -2 Vdc. As a consequence the possible voltages across each unit cell 78, i.e. Vcell, are +4 Vdc, +3 Vdc, +2 Vdc, + Vdc, 0, - Vdc, -2 Vdc, -3 Vdc, and -4 Vdc. Accordingly, while the parallel paths defined by the first and second module arms 82, 84 do not contribute to an increase in the number of discrete output voltage states of each limb portion 26, 28, they do contribute to an increase in the current rating of the converter 70, as set out below. Each converter limb 12, 14, 72 defines a single phase power converter circuit, as illustrated schematically in Figure 10. Vtop and Vbottom are the respective total voltages across the top and bottom arms 74, 76 of the converter 70.

An output DC voltage V _DC can be generated between the first and second terminals 16, 18, i.e. in the first electrical network 22, by combining the voltages of the first and second limb portions 26, 28 in each converter limb 12, 14, 72. V _DC is the average output DC voltage generated in the first electrical network 22 with a tolerance of approximately +/- 20% associated with it.

The relationship between the top arm voltage Vtop and the output DC voltage V _DC is:

Vtop > V _DC ( 7 )

The same relationship applies to the arm voltages Vtop, Vbottom in each converter limb 12, 14, 72.

The number N of modules 34 required in series in each limb portion 26, 28 is given by: Vdc _ min

Where Varm is Vtop or Vbottom, and Vdc_min is the minimum value of the voltage Vdc across the capacitor 38 in each module 34.

Having a greater number of modules N than the minimum required simply to generate a required output DC voltage provides for redundant modules 34 in the first and second limb portion 26, 28, i.e. the top and bottom arms 74, 76. Such redundant modules 34 permit voltage balancing between the capacitors 38 of the modules 34, as set below. The inclusion of redundant modules 34 also improves the reliability of the converter 70 since, in the event of one or more modules being offline, the remaining online modules may be used to carry out the required power conversion while the offline modules are being repaired.

Each electronic block 30, 32 includes four modules 34 in series, i.e. two modules 34 in series in each of the module assemblies 80, and hence the series arrangement of modules in each unit cell 78 is equivalent to four series-connected modules 34 (noting that the first and second blocks 30, 32 are connected in parallel with one another) . Hence the number of unit cells 78, i.e. pairs of parallel-connected electronic blocks 30, 32, in each of the top and bottom arms 74, 76 is given by:

N

Ncells =— ( 9 )

4

The voltage across each of the top and bottom arms 74, 76, i.e. Varm, can therefore be defined by :

N / 4

Varm = Vcelli (10)

i=l

Where Vcell is the voltage across each unit cell 78.

The output DC voltage V _DC can be defined in terms of the voltage across the top and bottom arms Vtop, Vbottom, as follows: V _DC = Vtop + Vbottom (11)

Accordingly, the maximum possible voltage across an arm 74, 76 is

N

+— x Vcell (12)

4 and the minimum possible voltage across an arm 74, 76 is x Vcell (13)

4

However when the number of unit cells in the top arm 74 is equal to N/4, the minimum possible voltage across each arm 74, 76 is zero.

This is because, as set out in equation 11, the total voltage across the top and bottom arms 74, 76 should be equal to the output DC voltage V _DC .

It follows that the maximum voltage across one arm 74, 76 is the output DC voltage V _DC , and so the minimum voltage of the other arm 74, 76 should be zero so as to maintain the required output DC voltage V _DC across the corresponding converter limb 12, 14, 72.

In arrangements that include a redundant unit cell 78 in the top arm 74 it is possible to generate the aforementioned minimum possible voltage across an arm 74, 76.

If the maximum current through each unit cell 78 is I then, as illustrated schematically in Figure 9(b), the maximum current through each module 34 (and hence through each switching element 38 in a given module 34) is 1/4. As such the parallel arrangement of the modules 34 in each unit cell 78 increases the current rating of the second converter 70. The second converter 70 has a number of redundant modules 34 in the bottom arm 76 of each converter limb 12, 14, 72 which permits voltage balancing between the capacitors 38 of the modules 34.

As shown in Figures 4 and 5, the capacitor 38 of a module 34 can be charged and discharged by inserting it into circuit in forward and reverse directions. In the event of an abnormal condition leading to voltage unbalance in a limb portion 26, 28, i.e. a top or bottom arm 74, 76, the capacitor 38 of each module 34 may be inserted in either forward or reverse directions without affecting the output voltage of the limb portion 26, 28, which enables the continuous operation of the power electronic converter.

Control of such voltage balancing may be achieved by including a control loop (not shown) to monitor the voltage across each capacitor 38 and the current I through each unit cell 78 in the top arm 74 of each converter limb 12, 14, 72 and in the bottom arm 76 of each converter limb 12, 14, 72.

It is envisaged that in other embodiments, at least one module may include a set of series- connected switching elements connected in parallel with the respective capacitor 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 certain circuit arrangements of each electronic block, some of the modules in each electronic block may only be required to generate a zero voltage and a voltage of a single polarity. It is therefore advantageous to modify those modules to have a 2-quadrant unipolar module structure instead of a 4- quadrant bipolar module structure so as to reduce the size of the module while maintaining the required functionality of the electronic block in question.

Preferably the switching elements of each module are controllable in use to generate a voltage to oppose the flow of current created by a fault, in use, in the first or second electrical networks.

For example, high fault current may flow in a power electronic converter interconnecting AC and DC networks when a fault is caused by commutation failure of one or more thyristor valves in another converter station, which results in conducting thyristors being connected directly across the DC network to form a short circuit path. The low impedance of the short circuit means that the fault current flowing in the power electronic converter may exceed the current rating of the power electronic converter.

To reduce the fault current in either the first or second power electronic converter 10; 70, the switching elements 36 of each module 34 may be operated to insert the full-bridge modules 34 into the respective limb portion 26,28 to inject a voltage which opposes the driving voltage of the non-faulty AC network so as to extinguish the fault current and thereby prevent damage to the power electronic converter components.

As well as functioning in the foregoing manner, the second power electronic converter 70 can also simultaneously operate as a breaker during a fault condition. In particular, during a DC fault the top and bottom arms 74, 76 can be configured to generate a total of zero voltage, e.g. the top arm 74 generates C _e ii x +4Vdc (where N _ce n is the number of unit cells 78 in each of the top and bottom arms 74, 76 and +4Vdc is the maximum possible voltage across each unit cell) and the bottom arm 76 generates N _ce n x -4Vdc (where -4Vdc is the minimum possible voltage across each unit cell 78) .

The use of the power electronic converter components to carry out both voltage conversion and extinguishment of fault currents simplifies or eliminates the need for separate protective circuit equipment, such as a circuit breaker or isolator. This leads to savings in terms of hardware size, weight and costs.

V1 V2 V3 V4 Va

+Vdc -Vdc -Vdc +Vdc -2Vdc

+Vdc 0 0 +Vdc

+Vdc -Vdc 0 0 -Vdc

0 0 -Vdc +Vdc

0 -Vdc -Vdc 0

+Vdc +Vdc +Vdc +Vdc

+Vdc -Vdc +Vdc -Vdc

+Vdc 0 +Vdc 0

-Vdc 0 -Vdc 0 0

0 -Vdc 0 -Vdc

0 +Vdc 0 +Vdc

-Vdc +Vdc -Vdc +Vdc

-Vdc -Vdc -Vdc -Vdc

-Vdc 0 0 -Vdc

-Vdc +Vdc 0 0

+Vdc

0 0 -Vdc +Vdc

0 +Vdc +Vdc 0

-Vdc +Vdc +Vdc -Vdc +2Vdc

Table 1

Va Vb Vc Vd Vout

-2Vdc +2Vdc +2Vdc -2Vdc +4Vdc

-Vdc +2Vdc +2Vdc -Vdc +3Vdc

-Vdc +Vdc +Vdc -Vdc +2Vdc

0 +Vdc +Vdc 0 +Vdc

-Vdc +Vdc -Vdc +Vdc

0

+Vdc -Vdc +Vdc -Vdc

0 -Vdc -Vdc 0 -Vdc

+Vdc -Vdc -Vdc +Vdc -2Vdc

+Vdc -2Vdc -2Vdc +Vdc -3Vdc

+2Vdc -2Vdc -2Vdc +2Vdc -4Vdc

Table 2

Vin Va Vb Vc Vd Vout

+2Vdc -Vdc -Vdc +2Vdc -3Vdc

+2Vdc -Vdc 0 +Vdc

-2Vdc

+Vdc 0 -Vdc +2Vdc

+2Vdc -Vdc +Vdc 0

-Vdc

0 +Vdc -Vdc +2Vdc

+Vdc 0 0 +Vdc

0 +Vdc 0 +Vdc

+Vdc 0 +Vdc 0 .

+Vdc 0

-Vdc +2Vdc -Vdc +2Vdc

+2Vdc -Vdc +2Vdc -Vdc

+Vdc 0 +2Vdc -Vdc

-Vdc +2Vdc 0 +Vdc +Vdc

0 +Vdc +Vdc 0

0 +Vdc +2Vdc -Vdc +2Vdc

-Vdc +2Vdc +Vdc 0

-Vdc +2Vdc +2Vdc -Vdc +3Vdc

Table 3

14. A power electronic converter according to Claim 12 or Claim 13 wherein the or at least one switching element of each module further includes an anti-parallel diode connected in parallel with the respective semiconductor device.

15. A power electronic converter according to any preceding claim wherein the or at least one energy storage device is a capacitor, fuel cell, photovoltaic cell, battery or an auxiliary AC generator with an associated rectifier.

16. A power electronic converter according to any preceding claim wherein the or each switching element of each module is controllable in use to generate a voltage to oppose the flow of current created by a fault, in use, in the first or second electrical networks. 17. A power electronic converter according to any preceding claim wherein the or each switching element of each module are controllable in use to regulate the voltage of the respective energy storage devices.