EP2564496A1

EP2564496A1 - Converter

Info

Publication number: EP2564496A1
Application number: EP10715563A
Authority: EP
Inventors: Carl David Barker; David Trainer
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2010-04-29
Filing date: 2010-04-29
Publication date: 2013-03-06
Also published as: WO2011134521A1

Abstract

A power electronic converter (10) for use in high voltage direct current power transmission and reactive power compensation comprises at least one converter limb (12) including first and second DC terminals (14,16) connected in use to a DC network (22), and an AC terminal (18) connected in use to an AC network (26), the or each converter limb (12) defining first and second limb portions (32,34), each limb portion (32,34) including one or more primary switching elements (36) connected between a respective one of the first and second DC terminals (14,16) and the AC terminal (18), the or each primary switching element (36) being operable to switch the respective limb portion (32,34) in and out of circuit to facilitate the AC to DC conversion process; and at least one transformer (42) including mutually coupled first and second windings (38,40), the or each first winding (38) being operably associated with a respective converter limb (12) and the or each second winding (40) being operably associated with an auxiliary converter (44), wherein the or each auxiliary converter (44) is operable to modify the voltage and/or current waveforms at the AC and/or DC terminals (14,16,18) of the respective converter limb (12).

Description

CONVERTER

DESCRIPTION

TECHNICAL FIELD

This invention relates to a power electronic converter for use in high voltage direct current power transmission and reactive power compensation .

STATE OF THE PRIOR ART

In power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or undersea 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 kilometer 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 to DC power is also utilized in power transmission networks where it is necessary to interconnect the AC 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, and one such form of converter is a current source converter (CSC) . One form of known current source converter is based on the arrangement of large line commutated converter (LCC) thyristors configured in one or more bridges to form six, twelve (or higher) pulse structures that achieve the conversion between AC and DC power. These converters are capable of continuous operation at 3000 to 4000 Amperes and are suitable for plant installations capable of processing several gigawatts of electrical power.

Power plants based on these conventional converters absorb significant quantities of reactive power from the AC network to which they are connected. In addition, the commonly used twelve-pulse converter has unacceptably high levels of harmonic distortion in the converter current. Consequently both of these factors mean that the conventional power plants require the use of large passive inductors and capacitors to provide the required reactive power and filter the harmonic currents. This leads to an increase in size, weight and costs of converter hardware.

DESCRIPTION OF THE INVENTION

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 DC terminals for connection in use to a DC network, and an AC terminal for connection in use to an AC network, the or each converter limb defining first and second limb portions, each limb portion including one or more primary switching elements connected between a respective one of the first and second DC terminals and the AC terminal, the or each primary switching element being operable to switch the respective limb portion in and out of circuit to facilitate the AC to DC conversion process; and at least one transformer including mutually coupled first and second windings, the or each first winding being operably associated with a respective converter limb and the or each second winding being operably associated with an auxiliary converter, wherein the or each auxiliary converter is operable to modify the voltage and/or current waveforms at the AC and/or DC terminals of the respective converter limb.

The provision of at least one transformer allows a power electronic converter to incorporate components with different current ratings which would otherwise be incompatible with each other. This is because the voltage step-up/step-down capability of the transformer allows components with different current ratings to be installed and operated on different sides of the transformer. For example, self-commutated switches such as transistors provide excellent control and flexibility over the generation of a voltage waveform and therefore can be used to minimise harmonic distortion in converter current. The low current ratings of transistors means that conventional converters based on transistors tend to have a lower plant rating than conventional twelve-pulse LCC converters. The inclusion of the transformer however allows these transistors to be associated with a power electronic converter which includes components with high current ratings, such as thyristors, for power conversion. This allows the associated power plant to maintain a high power plant rating while minimising the issue of harmonic distortion associated with conventional twelve-pulse thyristor-based power electronic converters.

The inclusion of an auxiliary converter allows the injection of a controlled voltage waveform via the transformer into the AC side and/or DC side of the power electronic converter. The injected voltage waveform can be used to modify the shape of the AC and/or DC side voltage and current to control the flow of real power and reactive power and thereby improve the performance of the power electronic converter.

Preferably the or each first winding has a lower number of turns than the corresponding mutually coupled second winding.

The step-down in voltage from the second winding to the first winding means that any voltage waveform generated by the auxiliary converter will result in an injected voltage waveform with smaller voltage steps at the first winding of the transformer. The decrease in voltage step size of the injected waveform leads to a smoother waveform shape.

In embodiments of the invention each limb portion may include a first winding connected in series between the one or more primary switching elements and the respective DC terminal. In other embodiments a first winding may be connected in series between the AC network and the AC terminal of the respective converter lim .

The transformer may be inserted into the power electronic converter on the AC side and/or DC side of the power electronic converter. This flexibility in transformer location is advantageous in the design of layouts for complex power electronic converter arrangements.

In embodiments of the invention the or each auxiliary converter may be operable to modify the voltage waveform at the respective AC terminal to cancel harmonics in the voltage or current waveform.

The provision of the auxiliary converter allows for the generation of an AC waveform with less harmonic distortion. The current drawn from the AC voltage contains large low order harmonics which is undesirable at high power levels. The cancellation of harmonics in the voltage or current waveform however means that there is a reduced requirement for harmonic filters on the AC side of the power electronic converter to control power quality.

In other embodiments, the or each auxiliary converter may be operable to modify the magnitude of the fundamental frequency voltage waveform at the respective AC terminal.

In further embodiments, the or each auxiliary converter may be operable to modify the operating power factor of the power electronic converter .

Operation of the or each auxiliary manner in this manner provides additional control over the AC to DC power conversion process so as to improve the efficiency of the power electronic converter.

In embodiments of the invention, the or each auxiliary converter may be operable to compensate for inherent regulation effects caused, in use, by finite impedance in the AC network and the associated transformer .

The inclusion of the or each auxiliary converter also provides a means of compensating for the inherent regulation effects associated with the operation of converter equipment with finite impedance and can minimise or eliminate the need for transformer tap changing equipment normally required to maintain optimal operation. For example, to compensate for the inherent regulation effects, the auxiliary converter can be used to inject fundamental and harmonic waveforms and can provide both real and reactive power as required.

In other embodiments the or each auxiliary converter may be operable to generate a voltage in the respective first winding to oppose the flow of current created by a fault, in use, in the AC or DC networks.

The auxiliary converter may be used to inject a voltage so that the associated first winding provides 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 auxiliary converter components to carry out both voltage conversion and extinguishment of fault currents may eliminate the need for the installation of separate protective circuit equipment, such as a circuit breaker. This leads to savings in terms of hardware size, weight and costs.

In further embodiments the or each auxiliary converter may be operable to modify the voltage waveform at the respective AC terminal to minimise the voltage across the respective limb portion before the limb portion is switched into or out of circuit .

This feature is advantageous in that it allows the switching elements of the limb portion to switch at near zero voltage and thereby minimise switching losses and electromagnetic interference.

Since the use of near zero voltage switching also reduces voltage sharing errors and the rate of change of voltage seen by the switching elements, it becomes possible to simplify the design of converter hardware and associated snubber components.

Preferably each primary switching element includes a semiconductor device. In such embodiments, each semiconductor device may be a thyristor, a gate turn-off thyristor or a diode.

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 high power capability of thyristor- based and diode-based line commutated converters enables the associated power plant to achieve a high power rating. The or each auxiliary converter preferably includes one or more secondary switching elements. Preferably the or each secondary switching element includes a semiconductor device. In such embodiments, the or each semiconductor device may be an insulated gate bipolar transistor, a field effect transistor, a transistor, a gate-turn-off thyristor, an integrated gate-commutated transistor, an insulated gate commutated thyristor or an injection enhancement gate transistor.

The fast switching capabilities of semiconductor devices such as insulated gate bipolar transistors (IGBTs) allow the or each auxiliary converter to synthesize complex waveforms for injection into the AC side and/or DC side of the power electronic converter. Furthermore the inclusion of semiconductor devices allow the auxiliary converters to respond quickly to the development of AC and DC side faults or other abnormal operating conditions, and thereby improve fault protection of the power electronic converter .

In embodiments of the invention the or each auxiliary converter includes a chain-link converter including a chain of modules connected in series, each module including at least one pair of secondary switching elements connected in parallel with an energy storage device, the secondary switching elements being controllable in use so that the chain of modules defines a stepped variable voltage source. In such an embodiment, the or each second winding may be connected in series with a chain-link converter. The structure of the chain-link converter allows the build-up of a combined voltage, which is higher than the voltage provided by an individual module, via the insertion of multiple modules, each providing a voltage, into the chain-link converter. By varying the value of the combined voltage, the chain- link converter may be operated to generate a voltage waveform of variable amplitude and phase angle.

In such embodiments employing the use of a chain-link converter, the or each module may include two pairs of secondary switching elements connected in parallel with the respective energy storage device in a full-bridge arrangement to define a 4-quadrant bipolar module that can provide positive, zero or negative voltage and can conduct current in both directions.

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

The use of full-bridge 4-quadrant bipolar modules in any transformer connected on the AC side of the power electronic converter means that each of the modules is capable of conducting alternating current such that the chain-link converter is able to modify the voltage waveform at the AC terminal for presentation to the AC network. In further embodiments employing the use of a chain-link converter, the or each energy storage device may include a capacitor, fuel cell, a 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.

The power electronic converter preferably further comprises a plurality of converter limbs, each converter limb including an AC terminal for connection in use to a multiphase AC network.

In such power electronic converters, the switching elements of each converter limb and the transformer operably associated with the respective converter limb operates independently of that of the other converter limbs and therefore only directly affects the phase connected to the respective AC terminal, and has limited influence on the phases connected to the AC terminals of the other converter limbs .

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings in which: Figure 1 shows, in schematic form, a power electronic converter according to a first embodiment of the invention;

Figure 2 shows, in schematic form, a chain- link converter coupled to a second winding of a transformer;

Figure 3 shows a full-bridge chain-link module in the form of a 4-quadrant bipolar module;

Figure 4 shows a synthesis of a 50 Hz sinusoidal waveform by a chain-link converter; and

Figure 5 shows, in schematic form, a power electronic converter according to a second embodiment of the invention.

DETAILED EXPLANATION OF PARTICULAR EMBODIMENTS

A power electronic converter 10 according to a first embodiment of the invention is shown in Figure 1.

The power electronic converter 10 comprises three converter limbs 12, each converter limb 12 including first and second DC terminals 14,16, and an AC terminal 18.

In use, the first DC terminal 14 is connected to a positive terminal 20 of a DC network 22 which carries a voltage of +V_DC/2, where V_DC is the DC voltage range of the DC network 22. The second DC terminal 16 is connected to a negative terminal 24 of a DC network 22 which carries a voltage of -V_DC/2.

It is envisaged that in embodiments of the invention the first and second DC terminals 14,16 may be respectively connected in use to a positive terminal 20 carrying a voltage of +V_DC and a negative terminal 24 carrying a voltage of zero.

It is also envisaged that in other embodiments the voltages at the positive and negative terminals 20,24 of the DC network 22 may vary in magnitude and/or the first and second DC terminals 14,16 may be connected in use to a DC network 22 of reverse polarity such that the first and second DC terminals 14,16 are respectively connected to the negative and positive terminals 24,20 of the DC network 22..

In use each AC terminal 18 is connected to a respective phase of a three-phase AC network 26 via an inductor 28, which may be the AC system impedance. In other embodiments each AC terminal 18 may be connected to the AC network 26 via one or more transformers and/or one or more additional inductors 28.

A DC link capacitor 30 is connected in series between the first and second DC terminals 14,16.

Each converter limb 12 includes first and second limb portions 32,34, each limb portion 32,34 includes a plurality of thyristors 36 connected in series between a respective one of the first and second DC terminals 14,16 and the AC terminal 18. The thyristors 36 of the first and second limb portions 32,34 are operable to switch the respective limb portions 32,34 into and out of circuit to facilitate the AC to DC conversion process.

The high voltage and current ratings of thyristors 36 allows a power plant using thyristor- based converters to achieve a high power plant rating. It is envisaged that the thyristors 36 may be replaced by other semiconductor devices with high power capabilities such as diodes and gate turn-off thyristors.

The power electronic converter 10 also includes a first winding 38 connected in series between the AC network 26 and the AC terminal 18 of the respective converter limb 12. Each first winding 38 is mutually coupled with a second winding 40 to define a transformer 42. Each second winding 40 is operably associated with an auxiliary converter 44.

The provision of at least one transformer 42 allows a hybrid power electronic converter 10 to incorporate components with different current ratings which would otherwise be incompatible with each other. This is because the voltage step-up/step-down capability of the transformer 42 allows components with different current ratings to be installed and operated on different sides of the transformer 42. The auxiliary converter 44 may therefore include components with lower current ratings, such as transistors, without affecting the achievable power rating of the embodiment in Figure 1.

The thyristors 36 of each converter limb 12 and the auxiliary converter 44 operably associated with the respective converter limb 12 operates independently of that of the other converter limbs 12 and therefore only directly affects the phase connected to the respective AC terminal 18, and has limited influence on the phases connected to the AC terminals 18 of the other converter limbs 12.

Each auxiliary converter 44 is operable to modify voltage and/or current waveforms at the AC and/or DC terminals 14,16,18 of the respective converter limb 12. To modify the voltage and/or current waveforms, the auxiliary converter 44 preferably includes a chain-link converter 46, as shown in Figure 2.

In Figure 2, a chain-link converter 46 is connected in series with a second winding 40 of a transformer 42. The chain-link converter 46 includes a chain of modules 48 connected in series, each module 48 including two pairs of secondary switching elements 52 connected in parallel with a capacitor 50 in a full- bridge arrangement to form a 4-quadrant bipolar module 48, as shown in Figure 3, that can provide positive, zero or negative voltage, and can conduct current in both directions.

The ability of a 4-quadrant bipolar module

48 to provide positive or negative voltages means that the voltage across each chain-link converter 46 may be built up from a combination of modules 48 providing positive or negative voltages. The energy levels in the individual capacitors 50 may be maintained therefore at optimal levels by controlling the modules 48 to alternate between providing positive or negative voltage .

The secondary switching elements 52 are operable so that the chain of modules 48 provides a stepped variable voltage source, and are switched at near to the fundamental frequency of the AC network.

In the embodiment shown in Figure 2, each secondary switching element 52 includes an insulated gate bipolar transistor 54 accompanied by a reverse- parallel connected diode 56.

The fast switching capabilities of insulated gate bipolar transistors 54 allow the or each auxiliary converter 44 to synthesize complex waveforms for injection into the respective first winding 38, and thereby provide excellent control and flexibility over the generation of a voltage waveform.

It is envisaged that in other embodiments each secondary switching element 52 may include a different semiconductor device, such as a field effect transistor, a transistor, a gate-turn-off thyristor, an integrated gate-commutated transistor, an insulated gate commutated thyristor, an injection enhancement gate transistor or other forced commutated or self commutated semiconductor switches, accompanied by a reverse-parallel connected diode 56.

It is also envisaged that in other embodiments, the capacitor 50 of each of the modules 48 may be replaced by a different energy storage device such as a fuel cell, a battery or an auxiliary AC generator with an associated rectifier.

The capacitor 50 of each module 48 may be bypassed or inserted into the chain-link converter 46 by changing the state of the secondary switching elements 52. A capacitor 50 of a module 48 is bypassed when a pair of secondary switching elements 52 is configured to form a short circuit in the module 48, causing the current in the power electronic converter 10 to pass through the short circuit and bypass the capacitor 50.

A capacitor 50 of a module 48 is inserted into the chain-link converter 46 when the pair of secondary switching elements 52 is configured to allow the converter current to flow into and out of the capacitor 50, which is then able to charge or to discharge its stored energy and provide a voltage.

It is therefore possible to build up a combined voltage across the chain-link converter 46 which is higher than the voltage available from each of the individual modules 48 via the insertion of the capacitors 50 of multiple modules 48, each providing its own voltage, into the chain-link converter 46.

It is also possible to vary the timing of switching operations for each module 48 such that the insertion and/or bypass of the capacitors 50 of individual modules 48 in the chain-link converter 46 results in the generation of a voltage waveform. An example of a voltage waveform generated using the chain-link converter is shown in Figure 4, in which the insertion of the capacitors of the individual modules is staggered to generate a 50Hz sinusoidal waveform. Other waveform shapes may be generated by adjusting the timing of switching operations for each module 48 in the chain-link converter 46. The operation of the chain-link converter 46 therefore results in a generation of a voltage waveform in the series-connected second winding 40. The change of voltage in the second winding 40 causes a change in magnetic flux which in turn induces a voltage waveform in the mutually coupled first winding 38. The voltage waveforms in the first and second windings 38,40 are similar in shape except for their respective amplitudes .

The amplitude of the induced voltage waveform relative to the generated voltage waveform is dependent on the ratio between the number of turns in each of the first and second mutually coupled windings 38, 40. For example, if the number of turns in the first winding 38 is equal to half of the number of turns in the second winding 40, the amplitude of the induced voltage waveform will be equal to half of the amplitude of the generated voltage waveform.

The provision of the first winding 38 having less turns than the second winding 40 is advantageous in that the induced voltage waveform will have smaller voltage steps 58 and therefore a smoother waveform shape. This enables the induced voltage waveform to form a nearer approximation of preferred waveforms such as sinusoidal waveforms.

The chain-link converter 46 is therefore controllable to inject a voltage waveform of variable amplitude and phase angle into the respective first winding 38. Each auxiliary converter 44 in Figure 1 including a chain-link converter may be controlled in this manner so as to modify the voltage waveform at the AC terminal 18 of the respective converter limb 12.

The injected waveform may be used to minimise harmonic distortion by cancelling harmonics present in the respective phase. The cancellation of harmonics in the voltage or current waveform means that there is a reduced requirement for large passive harmonic filters on the AC side of the power electronic converter 10 to control power quality. This leads to a reduction in converter size, weight and cost.

Additionally the auxiliary converter 44 may be used to inject a fundamental frequency voltage with variable magnitude and phase angle in order to control the real and reactive power flowing between the AC network 26 and the power electronic converter 10. The use of insulated gate bipolar transistors or other similar devices is advantageous in that the fast switching capabilities of such devices allow the generation of complex waveforms and therefore improve control over the or each voltage waveform presented to the AC and DC networks 26,22.

In other embodiments, the auxiliary converter 44 may be operable to modify the magnitude of the fundamental frequency voltage waveform at the respective AC terminal 18 and/or to modify the operating power factor of the power electronic converter 10, so as to provide additional control over the AC to DC power conversion process so as to improve the efficiency of the power electronic converter 10.

The inclusion of each auxiliary converter

44 also provides a means of compensating for the inherent regulation effects associated with the operation of converter equipment with finite impedance and can minimise or eliminate the need for transformer tap changing equipment normally required to maintain optimal operation. For example, to compensate for the inherent regulation effects, the auxiliary converter 44 can be used to inject fundamental and harmonic waveforms and can provide both real and reactive power as required.

Preferably the or each auxiliary converter

44 is operable to generate a voltage in the respective first winding 38 to oppose the flow of current created by a fault, in use, in the AC or DC networks 22, 26. When the or each auxiliary converter 44 includes a chain-link converter, the modules of each chain-link converter may be switched into circuit to inject the opposing voltage into the respective first winding 38 to limit or extinguish the fault current and thereby prevent damage to the power electronic converter components.

In the event of a fault in the DC network 22 resulting in high fault current in the power electronic converter 10, the secondary switching elements of each module of one or more chain-link converters may be operated to insert the full-bridge modules to inject a voltage in the respective first winding 38 which opposes the driving voltage of the non-faulty AC network 26 and thereby reduces the fault current in the power electronic converter 10.

For example, a short circuit occurring across the DC link capacitor 30 connected to the DC network 22 results in both voltages at the positive and negative terminals 20,24 dropping to zero volts. When this happens, a high fault current can flow from the AC network 26 through the first limb portion 32 of a converter limb 12, and return to the AC network 26 through the short circuit and the second limb portion 34 of another converter limb 12.

The low impedance of the short circuit means that the fault current flowing in the power electronic converter 10 may exceed the current rating of the power electronic converter 10.

The fault current may be minimized by opposing the driving voltage from the AC network 26. This is carried out by configuring the secondary switching elements of each chain-link module such that the modules are inserted into the respective AC side chain-link converter to provide a voltage which opposes and thereby reduces the driving voltage.

The use of the auxiliary converter components to carry out both voltage conversion and extinguishment of fault currents may eliminate the need for the installation of separate protective circuit equipment, such as a circuit breaker. This leads to savings in terms of hardware size, weight and costs. In addition, the fast switching capabilities of insulator gate bipolar transistors allow the chain-link converter to respond quickly to the development of faults or other abnormal operating conditions in the AC or DC networks 22,26 and provide the opposing voltage to limit or extinguish the fault current. The voltage waveform at the respective AC terminal 18 may be modified using the injected voltage waveform to minimise the voltage at the respective AC terminal 18 when the respective limb portion 32,34 is switched into or out of circuit. This feature is advantageous in that it allows the thyristors 36 of the limb portion to switch at near zero voltage and thereby minimise switching losses and electromagnetic interference. Since the use of near zero voltage switching also reduces voltage sharing errors and the rate of change of voltage seen by the switching elements, it becomes possible to simplify the design of converter hardware and associated snubber components.

A power electronic converter 60 according to a second embodiment of the invention is shown in Figure 5. The structure of this embodiment of the invention and its operation is similar to the embodiment in Figure 1 except for the locations of the first windings 38 in the respective power electronic converters 10,60.

In Figure 5, each limb portion 32,34 includes a first winding 38 connected in series between the one or more thyristors 36 and the respective DC terminal 20,24. Operation of each auxiliary converter 44 results in the injection of a voltage waveform into the respective first winding 38, and therefore into the associated DC voltage, so as to modify the voltage waveform at the AC terminal 18.

Each auxiliary converter 44 of each first winding 38 operably associated with the respective converter limb 12 operates independently of that of the other converter limbs 12 and therefore only affects the phase connected to the respective AC terminal 18, and has no influence on the phases connected to the AC terminals 18 of the other converter limbs 12.

In embodiments of the invention it is envisaged that the power electronic converter may further comprise a plurality of converter limbs 12, each converter limb 12 including an AC terminal 18 for connection in use to a multiphase AC network 26.

In other embodiments it is envisaged that the power electronic converter may include first windings 38 at both AC and DC sides of the power electronic converter.

Claims

1. 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 DC terminals for connection in use to a DC network, and an AC terminal for connection in use to an AC network, the or each converter limb defining first and second limb portions, each limb portion including one or more primary switching elements connected between a respective one of the first and second DC terminals and the AC terminal, the or each primary switching element being operable to switch the respective limb portion in and out of circuit to facilitate the AC to DC conversion process; and at least one transformer including mutually coupled first and second windings, the or each first winding being operably associated with a respective converter limb and the or each second winding being operably associated with an auxiliary converter, wherein the or each auxiliary converter is operable to modify the voltage and/or current waveforms at the AC and/or DC terminals of the respective converter limb.

2. A power electronic converter according to Claim 1 wherein the or each first winding has a lower number of turns than the corresponding mutually coupled second winding.

3. A power electronic converter according to Claim 1 or Claim 2 wherein each limb portion includes a first winding connected in series between the one or more primary switching elements and the respective DC terminal.

4. A power electronic converter according to any of the preceding claims wherein a first winding is connected in series between the AC network and the AC terminal of the respective converter limb.

5. A power electronic converter according to any of the preceding claims wherein the or each auxiliary converter is operable to modify the voltage waveform at the respective AC terminal to cancel harmonics in the voltage or current waveform.

6. A power electronic converter according to any of the preceding claims wherein the or each auxiliary converter is operable to modify the magnitude of the fundamental frequency voltage waveform at the respective AC terminal.

7. A power electronic converter according to any of the preceding claims wherein the or each auxiliary converter is operable to modify the operating power factor of the power electronic converter.

8. A power electronic converter according to any of the preceding claims wherein the or each auxiliary converter is operable to compensate for inherent regulation effects caused, in use, by finite impedance in the AC network and the associated transformer .

9. A power electronic converter according to any of the preceding claims wherein the or each auxiliary converter is operable to generate a voltage in the respective first winding to oppose the flow of current created by a fault, in use, in the AC or DC networks.

10. A power electronic converter according to any of the preceding claims wherein the or each auxiliary converter is operable to modify the voltage waveform at the respective AC terminal to minimise the voltage across the respective limb portion before the limb portion is switched into or out of circuit.

11. A power electronic converter according to any of the preceding claims wherein the or each primary switching element includes a semiconductor device .

12. A power electronic converter according to Claim 11 wherein each semiconductor device is a thyristor, a gate turn-off thyristor or a diode.

13. A power electronic converter according to any of the preceding claims wherein the or each auxiliary converter includes one or more secondary switching elements.

14. A power electronic converter according to Claim 13 wherein the or each secondary switching element includes a semiconductor device.

15. A power electronic converter according to Claim 13 wherein the or each semiconductor device is an insulated gate bipolar transistor, a field effect transistor, a transistor, a gate-turn-off thyristor, an integrated gate-commutated transistor, an insulated gate commutated thyristor or an injection enhancement gate transistor.

16. A power electronic converter according to any of Claims 13 to 15 wherein the or each auxiliary converter includes a chain-link converter including a chain of modules connected in series, each module including at least one pair of secondary switching elements connected in parallel with an energy storage device, the secondary switching elements being controllable in use so that the chain of modules defines a stepped variable voltage source.

17. A power electronic converter according to Claim 16 wherein the or each second winding is connected in series with a chain-link converter.

18. A power electronic converter according to Claim 16 or Claim 17 wherein the or each module includes two pairs of secondary switching elements connected in parallel with the respective energy storage device in a full-bridge arrangement to define a 4-quadrant bipolar module that can provide positive, zero or negative voltage and can conduct current in both directions.

19. A power electronic converter according to any of Claims 16 to 18 wherein the or each energy storage device includes a capacitor, fuel cell, a battery or an auxiliary AC generator with an associated rectifier .

20. A power electronic converter according to any of the preceding claims further comprising a plurality of converter limbs, each converter limb including an AC terminal for connection in use to a multiphase AC network.