WO2011160678A1

WO2011160678A1 - Cascaded multilevelconverter for hvdc application

Info

Publication number: WO2011160678A1
Application number: PCT/EP2010/058827
Authority: WO
Inventors: William Crookes; David Trainer; Colin Charnock Davidson
Original assignee: Areva T&D Uk Limited
Priority date: 2010-06-22
Filing date: 2010-06-22
Publication date: 2011-12-29
Also published as: EP2586112A1

Abstract

A power electronic converter for high voltage direct current power transmission and reactive power compensation comprises at least one primary module (54) operably connectable in use between at least two electrical networks, the or each primary module (54) including at least one set of series- connected primary switching elements (56) connected in parallel with an electronic block (58) in a H-bridge arrangement, the or each electronic block (58) including a plurality of secondary switching elements (62) connected to two or more energy storage devices (64) in a multi-level converter arrangement, the switching elements (56,62) of each primary module (54) being controllable in use to switch each energy storage device (64) into or out of circuit so the respective primary module (54) provides at least three discrete output voltages.

Description

CASCADED MULTILEVELCONVERTER FOR HVDC APPLICATION

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

In HVDC 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 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 voltage source converter (VSC) .

A known form of a voltage source converter 10 is shown in Figure 1. The voltage source converter comprises a plurality of half-link modules 12, each half-link module 12 including a pair of switching elements 14 connected in parallel with a capacitor 16 in a half-bridge arrangement. Each switching element 14 consists of an insulated gate bipolar transistor connected in parallel with an anti-parallel diode. The switching elements 14 of each half-link module 12 are operable so that the respective half-link module 12 provides zero or positive voltage and can conduct current in both directions. The magnitude of the positive voltage is dependent on the voltage level of the associated capacitor 16.

Instead of using half-link modules, the voltage source converter may utilise full-link modules 18 which include two pairs of switching elements 20 connected in parallel with a capacitor 22 in a full- bridge arrangement, as shown in Figure 2. The switching elements 20 of each full-link module 18 are operable so that the respective full-link module 18 provides positive, zero or negative voltage and can conduct current in both directions. The ability to provide a negative voltage and therefore an additional voltage output state increases the flexibility of the voltage source converter at the expense of increased converter size and costs.

According to an aspect of the invention, there is provided a power electronic converter for high voltage direct current power transmission and reactive power compensation comprising at least one primary module operably connectable in use between at least two electrical networks, the or each primary module including at least one set of series-connected primary switching elements connected in parallel with an electronic block in a H-bridge arrangement, the or each electronic block including a plurality of secondary switching elements connected to two or more energy storage devices in a multi-level converter arrangement, the switching elements of each primary module being controllable in use to switch each energy storage device into or out of circuit so the respective primary module provides at least three discrete output voltages .

The provision of the electronic block within each primary module results in an increased number of discrete output voltage states of each primary module when compared to conventional half-link and full-link modules. This is because the multilevel converter arrangement of the electronic block allows the secondary switching elements to switch each energy storage device into and out of circuit to define various configurations of the primary module and thereby generate the increased number of discrete output voltage states of each primary module, which results in a flexible power electronic converter arrangement .

In addition, the number of primary modules in such a power electronic converter may be reduced for a given voltage rating. This is because each primary module is capable of inserting multiple energy storage devices into circuit to generate a higher voltage than a conventional half-link or full-link module having only a single energy storage device. The reduction in the number of primary modules not only reduces the size, weight and cost of the voltage source converter, but also reduces overall conduction losses due to the reduction in overall number of switching elements. Furthermore, the reduction in the number of primary modules simplifies the control requirements for the power electronic converter by reducing the required number of transmission links and channels in the associated control unit.

Preferably the power electronic converter further includes at least one chain-link converter, the or each chain-link converter including a plurality of primary modules connected in series, the switching elements of each primary module of the or each chain- link converter being controllable in use so that the respective plurality of primary modules connected in series provides a stepped variable voltage source.

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 primary module, via the insertion of multiple primary 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. As such, the increased number of discrete output voltage states of each primary module increases the number of voltage steps in the generated voltage waveform and thereby enables the generation of higher quality voltage waveforms.

To define a 2-quadrant unipolar primary module that provides zero or positive voltage and can conduct current in two directions, at least one primary module may include one set of series-connected primary switching elements connected in parallel with the electronic block in a half-bridge arrangement.

To define a 4-quadrant bipolar primary module that provides negative, zero or positive voltage and can conduct current in two directions, at least one primary module may include two sets of series-connected primary switching elements connected in parallel with the electronic block in a full-bridge arrangement.

In embodiments of the invention, the or each electronic block includes a chain of secondary modules connected in series, the or each secondary module including one or more secondary switching elements connected to one or more energy storage devices .

In such embodiments, at least one secondary module may include at least one set of series-connected secondary switching elements connected in parallel with at least one energy storage device in an H-bridge arrangement .

In other such embodiments, at least one secondary module may include one set of series- connected secondary switching elements connected in parallel with an energy storage device in a half-bridge arrangement to define a 2-quadrant unipolar secondary module that provides zero or positive voltage and can conduct current in two directions.

In use, the secondary switching elements of the series-connected secondary modules are controllable to switch different combinations of energy storage devices into circuit and thereby produce a higher number of discrete output voltage states. As such, the series arrangement of the secondary modules allow each secondary module to utilise a simple two-level converter structure, which simplifies the design and manufacture of the power electronic converter.

In embodiments employing the use of series- connected secondary modules in combination with 4- quadrant bipolar primary modules, at least one electronic block may include a plurality of secondary modules connected in series and the switching elements of the or each primary module are controllable in use so that the respective primary module provides an asymmetric voltage source.

Such a provision results in a power electronic converter that is capable of producing an asymmetrical voltage output which allows the power electronic converter to interconnect AC and DC voltage of different magnitudes by increasing or decreasing the output AC voltage for a given DC voltage.

In addition, the asymmetric nature of the voltage output means that some of the primary switching elements of the respective primary module support a lower voltage level in their non-conducting state. This therefore enables the reduction of the voltage rating of these primary switching elements, which in turn leads to reductions in hardware size, weight and cost.

In other embodiments, each switching element includes a semiconductor device. In such embodiments, the semiconductor device is 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 semiconductor devices, each switching element 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 the or each chain-link converter to synthesize complex waveforms for injection into the AC side and/or DC side of 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 power electronic converters.

Furthermore the inclusion of such semiconductor devices allow the power electronic converter to respond quickly to the development of AC and DC side faults and/or other abnormal operating conditions, and thereby improve fault protection of the power electronic converter .

In further embodiments, the or each energy storage device 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 or each energy storage device of the or each primary module on an offshore wind farm may be provided in the form of an auxiliary AC generator connected to a wind turbine.

Preferably the power electronic converter further includes one or more converter limbs, the or each 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 at least one primary module connected in series between a respective one of the first and second DC terminals and the AC terminal, each primary module of each limb portion being operable to generate a voltage waveform at the respective AC terminal.

Such a power electronic converter arrangement may be utilised to facilitate power conversion between AC and DC networks.

In such embodiments, each limb portion may further include at least one tertiary switching element connected in series with the or each primary module between the respective DC terminal and the AC terminal, each tertiary switching element being controllable in use to switch the respective limb portion in and out of circuit to facilitate the conversion of power between the AC and DC networks .

The series combination of one or more secondary switching elements connected in series with the or each primary module in each limb portion is advantageous because it reduces the required voltage rating and therefore the number of components of the or each primary module in the respective limb portion required to carry out voltage conversion between the AC and DC networks .

In other such embodiments, the power electronic converter may include a plurality of converter limbs, the AC terminal of each converter limb being connected in use to a respective phase of a multi-phase AC network.

In such a power electronic converter, each 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 .

In embodiments of the invention, the secondary switching elements of the or each electronic block may be controllable in use so that the or each electronic block provides substantially zero voltage during the switching of at least one primary switching element of the respective primary module.

The structure of the or each electronic block enables the secondary switching elements to switch the respective energy storage device out of circuit before the turn-on or turn-off of the primary switching elements. This allows soft switching of the primary switching elements of the respective primary module, which leads to lower switching losses in the power electronic converter and simplifies dynamic voltage sharing in series-connected primary modules. In other embodiments, the switching elements of the or each primary module may be controllable in use to generate a voltage to oppose the flow of current created by a fault, in use, in one of the electrical networks.

Each primary 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.

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 prior art power electronic converter based on half-link modules ;

Figure 2 shows, in schematic form, the structure of a prior art full-link module;

Figure 3 shows, in schematic form, a power electronic converter according to a first embodiment of the invention;

Figure 4 shows, in schematic form, the structure of a primary module in the form of a 2- quadrant unipolar module; Figure 5 shows, in schematic form, the structure of a primary module in the form of a 4- quadrant bipolar module;

Figure 6 shows, in schematic form, a primary module including an electronic block having six series-connected secondary modules;

Figure 7 shows, in schematic form, the operation of a 4-quadrant bipolar primary module to generate an asymmetric voltage output; and

Figure 8 shows, in schematic form, a power electronic converter according to a second embodiment of the invention.

A power electronic converter according to a first embodiment of the invention is shown in Figure 3.

The power electronic converter comprises a converter limb 30 including first and second DC terminals 32, 34 and an AC terminal 36.

In use, the first and second DC terminals 32, 34 of the converter limb 30 are respectively connected to positive and negative terminals of a DC network 38, the positive and negative terminals respectively carrying a voltage of +V_DC/2 and -V_DC/2, where V_DC is the voltage range of the DC network 38.

In use, the AC terminal 36 is connected to a single-phase AC network 40 via an inductor 42. In other embodiments, the AC terminal may be connected to the AC network via one or more additional inductors and/or one or more transformers.

The converter limb 30 defines first and second limb portions 44, 46, each limb portion 44, 46 including a tertiary switching element 48, 50, in the form of an insulated gate bipolar transistor with an reverse parallel-connected diode, connected in series with a chain-link converter 52 between a respective one of the first and second DC terminals 32, 34 and the AC terminal 36.

In embodiments of the invention, it is envisaged that each tertiary switching element may be omitted from the respective limb portion so that each limb portion includes a chain-link converter connected in series between the respective DC terminal and the AC terminal .

It is also envisaged that in other embodiments, each limb portion may include a string of tertiary switching elements connected in series with the chain-link converter between the respective DC terminal and the AC terminal .

The series connection between the tertiary switching element 48, 50 and the chain-link converter

52 of each of the first and second limb portions 44, 46 means that, in other embodiments, they may be connected in a reverse order between the AC terminal 36 and the respective DC terminal 32,34.

Each chain-link converter 52 includes a plurality of primary modules 54 connected in series.

Figures 4 and 5 respectively show a 2- quadrant unipolar primary module and a 4-quadrant bipolar primary module.

Each primary module 54 may include one set of series-connected primary switching elements 56 connected in parallel with an electronic block 58 in a half-bridge arrangement to define a 2-quadrant unipolar primary module that provides zero or positive voltage and can conduct current in two directions, as shown in Figure 4.

Alternatively each primary module 54 may include two sets of series-connected primary switching elements 56 connected in parallel with the electronic block 58 in a full-bridge arrangement to define a 4- quadrant bipolar primary module that provides negative, zero or positive voltage and can conduct current in two directions, as shown in Figure 5.

The number of series-connected primary switching elements 56 in each primary module 54 may vary depending on the voltage range of the respective electronic block 58.

In each of Figures 4 and 5, the electronic block 58 of the primary module 54 includes two secondary modules 60 connected in series. Each secondary module 60 includes one set of series- connected secondary switching elements 62 connected in parallel with a capacitor 64 in a half-bridge arrangement to define a 2-quadrant unipolar secondary module that provides zero or positive voltage and can conduct current in two directions.

It is envisaged that in other embodiments, the electronic block may have a plurality of secondary switching elements connected to one or more energy storage devices in a different multi-level converter arrangement .

The capacitor 64 of each secondary module 60 in Figures 4 and 5 can be bypassed or inserted into the electronic block 58 by changing the state of the respective secondary switching elements 62.

The capacitor 64 of each secondary module 60 is bypassed when the secondary switching elements 62 are configured to form a short circuit in each secondary module 60, causing the current in the power electronic converter to pass through each short circuit and bypass the respective capacitor 64. This enables the electronic block 58 to provide a zero voltage.

The capacitor 64 of each secondary module

60 is inserted into the electronic block 58 when the secondary switching elements 62 of each secondary module 60 are configured to allow the converter current to flow into and out of the respective capacitor 64. This allows each secondary module 60 to provide a voltage equivalent to the voltage across the respective capacitor 64.

The output voltage 66 of the electronic block 58 is the sum of the voltages across the respective series-connected secondary modules 60 and is therefore dependent on the number of inserted capacitors 64 in the electronic block 58. In Figures 4 and 5, the bypass of both capacitors 64 results in a zero voltage step of the electronic block 58; the insertion of one capacitor 64 into the electronic block 58 result in a voltage step of +V; and the insertion of two capacitors 64 into the electronic block 58 results in a voltage step of +2V, where V is the voltage across each capacitor 64.

The provision of the secondary modules 60 in the electronic block 58 therefore results in an increased number of discrete voltage output states of the electronic block 58 when compared to the single voltage state of the capacitor in conventional half- link and full-link modules.

The electronic block 58 of each primary module 54 can be bypassed or inserted into the chain- link converter by changing the state of the respective primary switching elements 56.

The electronic block 58 of each primary module 54 is bypassed when the respective primary switching elements 56 are configured to form a short circuit in the primary module 58, causing the current in the power electronic converter to pass through the short circuit and bypass the electronic block 58. This enables each primary module 54 to provide a zero voltage, regardless of the voltage across the electronic block 58.

The electronic block 58 is inserted into the chain-link converter when the primary switching elements 56 are configured to allow the converter current to pass through the electronic block 58.

Insertion of the electronic block 58 into the 2-quadrant unipolar primary module of Figure 4 results in an output voltage 68 of the 2-quadrant unipolar primary module that is equal in magnitude and polarity to the output voltage of the electronic block 58. This results in three discrete output voltage states of the 2-quadrant unipolar primary module, which is higher than the two discrete voltage output states of the conventional half-link module. The full-bridge arrangement of the 4- quadrant bipolar primary module of Figure 5 allows the primary switching elements 56 to be configured to insert the electronic block 58 in the chain-link converter in either forward and reverse positions to allow either direction of current flow through the electronic block 58 so as to provide a positive or negative voltage. This is seen in Figure 5, in which the electronic block 58 has an output voltage 70 with voltage steps of +V and +2V when inserted in a forward direction, and with voltage steps of -V and -2V when inserted in a reverse direction. This results in five discrete output voltage states of the 4-quadrant bipolar primary module, which is higher than the three discrete voltage output states of the conventional full-link module.

The provision of the electronic block 58 within each primary module 54 therefore increases the number of discrete output voltage states of each primary module 54 when compared to conventional half- link and full-link modules.

In addition, each of the primary modules 54 in Figures 4 and 5 can conduct current in both directions when the electronic block 58 is either bypassed or inserted into the chain-link converter.

In use, the secondary switching elements 62 of the electronic block 58 of each primary module 54 are controllable to bypass the capacitors 64 so that the respective electronic block 58 provides substantially zero voltage during the switching of one or more primary switching elements 56 and simplifies dynamic voltage sharing in the series-connected primary modules 54.

Operation of the secondary switching elements 62 in this manner allows soft switching of the primary switching elements 56 of each primary module 54, which leads to lower switching losses in the power electronic converter.

In a chain-link converter based on full- bridge primary modules, the ability of each full-bridge primary module to provide positive or negative voltages means that the voltage across each chain-link converter may be built up from a combination of full-bridge primary modules providing positive or negative voltages. The energy levels in the individual capacitors may be maintained therefore at optimal levels by controlling the full-bridge primary modules to alternate between providing positive or negative voltage .

Referring back to Figure 3, the switching elements of each primary module 54 are operable so that the chain of primary modules 54 provides a stepped variable voltage source, and are switched at near to the fundamental frequency of the AC network 40.

The structure of each chain-link converter 52 allows the build-up of a combined voltage, which is higher than the voltage provided by an individual primary module 54, via the insertion of multiple primary modules 54, each providing a voltage, into the chain-link converter 52. By varying the value of the combined voltage, the chain-link converter 52 may be operated to generate a voltage waveform of variable amplitude and phase angle. As such, the increased number of discrete output voltage states of each primary module 54 increases the number of voltage steps in the generated voltage waveform and thereby enables the generation of higher quality voltage waveforms

The number of primary modules 54 in such a chain-link converter 52 may be reduced for a given voltage rating because the chain-link converter 52 includes primary modules 54 that are capable of inserting multiple capacitors into circuit to generate a higher voltage than a conventional half-link or full- link module having a single capacitor. For example, in Figure 6, the electronic block 58 of the primary module 54 includes six series-connected 2-quadrant unipolar secondary modules 60. To achieve the same voltage rating as a chain-link converter based on the primary module of Figure 6, a conventional chain-link converter based on conventional half-link modules would require twelve series-connected conventional half-link modules.

The reduction of the number of primary modules 54 in the chain-link converter 52 conveys several advantages. One advantage is that the converter arrangement leads to a reduction in the overall number of primary switching elements per power electronic converter and therefore a decrease in conduction losses and hardware size, weight and costs. Another advantage of the converter arrangement is that it is possible to reduce the number of bypass switches and crowbar thyristors in the power electronic converter, whereby each primary module 54 is associated with a bypass switch and a crowbar thyristor to prevent over-voltage of the primary module 54. In addition, the reduction in the number of required primary modules 54 simplifies the control requirements for the power electronic converter by reducing the required number of transmission links and channels in the associated control unit.

It is possible to vary the timing of switching operations for each primary module 54 such that the insertion and/or bypass of the electronic block of individual primary modules in the chain-link converter results in the generation of a voltage waveform. For example, the insertion of the electronic blocks of the individual primary modules 54 may be staggered to generate a sinusoidal waveform. Other waveform shapes may be generated by adjusting the timing of switching operations for each primary module 54 in the chain-link converter 52.

In other embodiments, it is envisaged that each chain-link converter may include at least one primary module connected in series with at least one conventional half-link module and/or at least one conventional full-link module.

Preferably each switching element includes an insulated gate bipolar transistor accompanied by a reverse-parallel connected diode.

It is envisaged that in other embodiments each of the primary and/or secondary switching elements 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 the chain-link converter to synthesize complex waveforms for injection into the power electronic converter, and thereby provide excellent control and flexibility over the generation of a voltage waveform. The synthesis and injection of complex waveforms can be used to minimise harmonic distortion which are typically present in thyristor- based power conversion.

It is also envisaged that, in other embodiments, each capacitor of each secondary module 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 power electronic converter may further include a pair of DC link capacitors 72 connected between the first and second DC terminals 32, 34, a mid-point between the pair of DC link capacitors 72 being connected in use to ground 74, as shown in Figure 3.

It is envisaged that in embodiments of the invention the power electronic converter may include a plurality of converter limbs, each converter limb including an AC terminal for connection in use to a respective phase of a multi-phase AC network.

In use, the tertiary switching elements 48,50 of the first and second limb portions 44,46 are operable to switch each of the chain-link converters 52 in and out of circuit between the respective DC terminal 32, 34 and the AC terminal 36. Each chain-link converter 52 is operable to generate a voltage waveform at the AC terminal 36 via bypass or insertion of either or both of the electronic blocks of its primary modules 54.

Each chain-link converter 52 is preferably operable to generate a sinusoidal voltage waveform using a step-wise approximation. The chain-link converters 52 are suitable for use in step-wise waveform generation due to their ability to provide voltage steps to increase or decrease the output voltage at the AC terminal 36.

As previously described, the switching operations in the primary modules 54 may be configured so that the insertion and bypass of the electronic blocks are staggered to form a step-wise approximation of a sinusoidal waveform. The step-wise approximation of the voltage waveform may be improved by using a higher number of primary modules 54 with lower voltage levels to increase the number of voltage steps, and/or by controlling one or more primary modules 54 of the chain-link converter 52 to each present an intermediate voltage so as to provide a smoother transition between voltage steps. Initially the tertiary switching element 48 of the first limb portion 44 is closed while the tertiary switching element 50 of the second limb portion 46 is open. The chain-link converter 52 in the first limb portion 44 is controlled to provide a voltage of +V_DC/2 so that it opposes the voltage at the positive terminal of the DC network 38. The output voltage at the AC terminal 40 is therefore zero volts i.e. halfway between the positive DC voltage at the positive terminal, +V_DC/2, and the negative DC voltage at the negative terminal, -V_DC/2. Any unused primary modules 54 are left in bypass mode.

In order to generate the positive voltage component of the sinusoidal voltage waveform, the output voltage is slowly increased by reducing the number of inserted electronic blocks of primary modules 54 in the chain-link converter 52 and thereby reducing the chain-link converter voltage. The change in the chain-link converter voltage can be observed in the step-wise increments of the output voltage at the AC terminal 36. At the peak of the positive voltage component, the chain-link converter 52 may be bypassed to produce a peak value equal to the positive DC voltage, +V_DC/2, or it may produce a voltage that adds to the positive DC voltage of the DC network 38. The positive voltage component produced may therefore have a peak that is higher than the positive DC voltage of the DC network 38, if desired.

During the generation of the positive voltage component of the sinusoidal voltage waveform, the voltage across the second limb portion 46 is equal to the difference between the output voltage and the negative DC voltage at the negative terminal, -V_DC/2 of the DC network 38.

The chain-link converter 52 of the first limb portion 44 is then controlled to reduce the output voltage in step-wise decrements by controlling the combined voltage across the chain-link converter 52 until the output voltage returns to zero.

When the output voltage returns to zero, the tertiary switching element 48 in the first limb portion 44 can remain closed when the tertiary switching element 48 of the second limb portion 46 is closed and before the tertiary switching element 50 in the first limb portion 44 is opened.

The full voltage range of the DC network

38, V_DC, is opposed by the voltage provided by the chain-link converters 52 in both limb portions 44,46 during the switching operations of both tertiary switching elements 48, 50 from one state to the other.

The chain-link converter 52 in the first limb portion 44 is controlled to provide a voltage of +V_DC/2 while the chain-link converter 52 in the second limb portion 46 is controlled to provide a voltage of -V_DC/2. As a result, there is zero or minimal voltage across the tertiary switching elements 48, 50 of the first and second limb portions 44, 46 when the tertiary switching elements 48, 50 switch from one state to the other. The low voltage across the tertiary switching element 48, 50 of each of the limb portions 44, 46 leads to low switching losses. The generation of the negative voltage component of the sinusoidal waveform is similar to the generation of the positive voltage component except that the tertiary switching element 48 of the first limb portion 44 remains open and the tertiary switching element 50 of the second limb portion 46 remains closed, and the generation of the voltage waveform is caused by the insertion and bypass of the electronic blocks of the primary modules 54 in the chain-link converter 52 of the second limb portion 46.

During generation of the negative voltage component of the sinusoidal voltage waveform, the voltage across the first limb portion 44 is equal to the difference between the output voltage and the positive DC voltage at the positive terminal, +V_DC/2 of the DC network 38.

The voltage capability of each limb portion 44, 46 is a combination of the voltage capability of the respective chain-link converter 52 and the voltage rating of the respective tertiary switching element 48, 50 and can be distributed in a non-symmetrical manner if desired. It is possible to reduce the number of primary modules 54 in each chain-link converter 52 by increasing the number of tertiary switching elements 48,50 in each limb portion 44, 46.

The power electronic converter may be operated to produce a peak AC voltage that exceeds the DC voltage across the positive and negative terminals of the DC network 38. In order to produce such a peak AC voltage, the switching elements of each primary module 54 may be configured to provide a voltage such that the combined voltage across the chain-link converter 52 is added to the positive or negative DC voltage of the AC network 40 to increase the magnitude of the output voltage at the AC terminal 36. This leads to a greater power output for a given current rating of the power electronic converter.

Each chain-link converter 52 may also be operated to generate an asymmetric voltage waveform by controlling the primary and secondary switching elements of each primary module 54 to generate an asymmetric voltage output. For example, the secondary switching elements 62 of the 4-quadrant bipolar primary module in Figure 7 are controlled in use to insert the capacitors 64 into the electronic block 58 to generate an output voltage 66 with voltage steps of +V and +2V, and the primary switching elements 56 are controlled in use to insert the electronic block 58 into the primary module 54 so as to generate an output voltage 76 with a voltage step of +V during the positive voltage component of the voltage waveform and with voltage steps of -V and -2V during the negative voltage component of the voltage waveform.

A power electronic converter according to a second embodiment of the invention is shown in Figure 8. The power electronic converter comprises three converter limbs 30. Each converter limb is similar to the converter limb 30 of the embodiment in Figure 3 except that each of the first and second limb portions 44,46 includes two tertiary switching elements 48, 50 connected in series with a single primary module 54.

In other embodiments, it is envisaged that the power electronic converter may have a different number of converter limbs, each limb having an AC terminal for connection in use to a single-phase or multi-phase AC network.

The electronic block 58 of each primary module 54 includes three series-connected secondary modules 60. Each secondary module 60 includes one set of series-connected secondary switching elements 62 connected in parallel with a capacitor 64 in a half- bridge arrangement to define a 2-quadrant unipolar secondary module that provides zero or positive voltage and can conduct current in two directions.

As outlined earlier, the arrangement based on the use of a single primary module 54 in each limb portion 44, 46 is advantageous when it comes to reducing converter size and simplifying control requirements.

It is envisaged that in other embodiments, the number of series-connected secondary modules in each primary module may vary in number. This enables the modification of the voltage characteristics of each primary module 54 to suit the associated power application. For example, an increase in the number of series-connected secondary modules 60 in each primary module 54 not only leads to an increase in voltage rating of the primary module 54, which improves the compatibility of the power electronic converter with a wide range of operating voltages, but also leads to an increase in the number of discrete voltage output states of the respective primary module 54, which allows the generation of higher quality voltage waveforms at the respective AC terminal 36.

Preferably each primary module 54 of the power electronic converter in Figures 3 and 8 is operable to generate a voltage to oppose the flow of current created by a fault, in use, in the AC network 40 or the DC network 38. Each primary module 54 may be switched into circuit to inject the opposing voltage into the power electronic converter to extinguish the fault current and thereby prevent damage to the power electronic converter components. The fault may be 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 40 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.

The fault current may be minimized by opposing the driving voltage from the AC network. This is carried out by configuring the primary and secondary switching elements of each primary module 54 to respectively insert the electronic block and one or more capacitors into circuit so as to inject a voltage which opposes the driving voltage of the non-faulty AC network 40 and thereby reduces the fault current in the power electronic converter.

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. In addition, the fast switching capabilities of the switching elements allow the respective primary module to respond quickly to the development of faults in the AC or DC networks and provide the opposing voltage to extinguish the fault current.

Claims

1. A power electronic converter for high voltage direct current power transmission and reactive power compensation comprising at least one primary module (54) operably connectable in use between at least two electrical networks (38, 40), the or each primary module including at least one set of series- connected primary switching elements (56) connected in parallel with an electronic block (58) in a H-bridge arrangement, the or each electronic block including a plurality of secondary switching elements (62) connected to two or more energy storage devices (64) in a multi-level converter arrangement, the switching elements (56, 62) of each primary module being controllable in use to switch each energy storage device into or out of circuit so the respective primary module (54) provides at least three discrete output voltages .

2. A power electronic converter according to Claim 1 further including at least one chain-link converter (52), the or each chain-link converter including a plurality of primary modules (54) connected in series, the switching elements (56, 62) of each primary module (54) of the or each chain-link converter being controllable in use so that the respective plurality of primary modules connected in series provides a stepped variable voltage source.

3. A power electronic converter according to Claim 1 or Claim 2 wherein at least one primary module (54) includes one set of series-connected primary switching elements (56) connected in parallel with the electronic block (58) in a half-bridge arrangement to define a 2-quadrant unipolar primary module that provides zero or positive voltage and can conduct current in two directions.

4. A power electronic converter according to any preceding claim wherein at least one primary module (54) includes two sets of series-connected primary switching elements (56) connected in parallel with the electronic block (58) in a full-bridge arrangement to define a 4-quadrant bipolar primary module that provides negative, zero or positive voltage and can conduct current in two directions.

5. A power electronic converter according to any preceding claim wherein the or each electronic block (58) includes a chain of secondary modules (60) connected in series, the or each secondary module including one or more secondary switching elements (62) connected to one or more energy storage devices (64) .

6. A power electronic converter according to Claim 5 wherein at least one secondary module (60) includes at least one set of series-connected secondary switching elements (62) connected in parallel with at least one energy storage device (64) in a H-bridge arrangement .

7. A power electronic converter according to Claim 5 or Claim 6 wherein at least one secondary module (60) includes one set of series-connected (62) secondary switching elements connected in parallel with an energy storage (64) device in a half-bridge arrangement to define a 2-quadrant unipolar secondary module that provides zero or positive voltage and can conduct current in two directions.

8. A power electronic converter according to any of Claims 5 to 7 when dependent from Claim 4 wherein at least one electronic block (58) includes a plurality of secondary modules (60) connected in series and the switching elements (56) of the or each primary module (54) are controllable in use so that the respective primary module (54) provides an asymmetric voltage source.

9. A power electronic converter according to any preceding claim wherein each switching element (56, 62) includes a semiconductor device.

10. A power electronic converter according to Claim 9 wherein the semiconductor device (56, 62) is 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 .

11. A power electronic converter according to Claim 9 or Claim 10 wherein each switching element (56, 62) further includes an anti-parallel diode connected in parallel with the respective semiconductor device.

12. A power electronic converter according to any preceding claim wherein the or each energy storage (64) device is a capacitor, fuel cell, photovoltaic cell, battery or an auxiliary AC generator with an associated rectifier.

13. A power electronic converter according to any preceding claim further including one or more converter limbs (30), the or each converter limb including first and second DC terminals (32, 34) for connection in use to a DC network (38) and an AC terminal (36) for connection in use to an AC network (40), the or each converter limb defining first and second limb portions (44, 46), each limb portion (44, 46) including at least one primary module (54) connected in series between a respective one of the first and second DC terminals (32, 34) and the AC terminal (36), the or each primary module (54) of each limb portion (44, 46) being operable to generate a voltage waveform at the respective AC terminal (36) .

14. A power electronic converter according to Claim 13 wherein each limb portion (44, 46) further includes at least one tertiary switching element (48, 50) connected in series with the or each primary module (54) between the respective DC terminal (32, 34) and the AC terminal (36) , each tertiary switching element

(48, 50) being controllable in use to switch the respective limb portion (44, 46) in and out of circuit to facilitate the conversion of power between the AC and DC networks (38, 40) .

15. A power electronic converter according to Claim 13 or Claim 14 including a plurality of converter limbs (30), the AC terminal (36) of each converter limb being connected in use to a respective phase of a multi-phase AC network (40) .

16. A power electronic converter according to any preceding claim wherein the secondary switching elements (62) of the or each electronic block (58) is controllable in use so that the or each electronic block (58) provides substantially zero voltage during the switching of at least one primary switching element (56) of the respective primary module (54) .

17. A power electronic source converter according to any preceding claim wherein the switching elements (56, 62) of the or each primary module (54) are controllable in use to generate a voltage to oppose the flow of current created by a fault, in use, in one of the electrical networks.