GB2537359A

GB2537359A - Converter

Info

Publication number: GB2537359A
Application number: GB1506131.0A
Authority: GB
Inventors: Briff Pablo; Reginald Trainer David; Jose Moreno Munoz Francisco
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2015-04-10
Filing date: 2015-04-10
Publication date: 2016-10-19
Anticipated expiration: 2035-04-10
Also published as: GB2537359B; GB201506131D0

Abstract

A multi-phase AC-DC voltage source converter comprises: first and second DC terminals 32, 34 for connection, in use, to a DC network; a plurality of AC terminals 36, each of the AC terminals 36 for connection, in use, to a respective phase of a multi-phase network; first, second, third and fourth converter limbs 1-4; and a plurality of switching blocks. The first and second converter limbs 1, 2 are operatively connected to the first DC terminal 32 and include a respective first waveform synthesiser (e.g. multi-level waveform synthesizer). The third and fourth converter limbs 3, 4 are operatively connected to the second DC terminal 34 and include a respective second waveform synthesiser. Each switching block 38 is arranged to interconnected each converter limb and a respective one of the AC terminals 36 and is arranged to be switchable to selectively switch each converter limb into, and out of, circuit between the corresponding DC terminal 32, 34 and the respective AC terminal 36, independently of the switching of each other converter limb into, and out of, circuit between the corresponding DC terminal 32, 34 and the respective AC terminal 36.

Description

CONVERTER

This invention relates to a converter.

In power transmission networks alternating current (AC) power is converted to direct current (DC) power for transmission via overhead lines, under-sea cables, underground cables, and so on. This conversion to DC power removes the need to compensate for the AC capacitive load effects imposed by the power transmission medium, i.e. the transmission line or cable, and reduces the cost per kilometre of the lines and/or cables, and thus becomes cost-effective when power needs to be transmitted over a long distance.

A converter provides the required conversion between AC power and DC power within the network.

According to an aspect of the invention, there is provided a converter comprising: first and second DC terminals for connection in use to a DC network; a plurality of AC terminals for connection in use to a respective phase of a multiphase AC network, which may have two, three or more phases; first and second converter limbs operatively connected to the first DC terminal, each of the first and second converter limbs including a respective first waveform synthesizer; third and fourth converter limbs operatively connected to the second DC terminal, each of the third and fourth converter limbs including a respective second waveform synthesizer; and a plurality of switching blocks, each switching block arranged to interconnect each converter limb and a respective one of the AC terminals, each switching block arranged to be switchable to selectively switch each converter limb into and out of circuit between the corresponding DC terminal and the respective AC terminal independently of the switching of each other converter limb into and out of circuit between the corresponding DC terminal and the respective AC terminal.

The arrangement of the switching blocks in the converter of the invention permits the switching blocks to function as a multiplexer to carry out the selective connection of each converter limb with each of the AC terminals This in turn allows each waveform synthesizer to, at least in part, control the configuration of the AC voltage waveforms at the AC terminals, i.e. the AC side of the converter. To this end, the converter may include a controller programmed to selectively operate each converter limb to control the configuration of an AC voltage waveform at the AC terminal with which that converter limb is connected into circuit. Preferably the AC voltage waveform is sinusoidal or near-sinusoidal in shape.

In contrast, a conventional converter with a plurality of AC terminals includes a plurality of phase legs connected in parallel between a pair of DC terminals, whereby the number of phase legs is the same as the number of AC terminals. Each phase leg of the conventional converter includes a pair of converter limbs separated by a respective one of the plurality of AC terminals, whereby each converter limb includes a respective waveform synthesizer.

On the other hand the configuration of the converter limbs and switching blocks of the converter of the invention allows the number of converter limbs and waveform synthesizers of the converter of the invention to be lower than the number of converter limbs and waveform synthesizers of the conventional converter, without reducing the number of AC terminals. The configuration of the converter limbs and switching blocks of the converter of the invention therefore allows a reduction in the number of components required of the converter when compared to the conventional converter, in order to provide a cost-efficient and space-saving converter due to the reductions in the size and weight of the converter and the associated commissioning, installation and transportation costs. In addition the provision of a lower number of components in the converter of the invention when compared to a.conventional converter reduces the number of operating variables to be monitored during the operation of the converter, thus simplifying the communications and data processing requirements of the converter.

Moreover the ability to selectively connect each converter limb to each of the AC terminals provides the converter of the invention with greater flexibility in controlling the configuration of the AC voltage waveforms at the AC terminals when compared to the conventional converter in which the AC voltage waveform at a given AC terminal is controlled through the operation of a single phase leg.

In a preferred embodiment of the invention, the number of converter limbs operatively connected to the first DC terminal may be equal to N, the number of converter limbs operatively connected to the second DC terminal may be equal to N, and the number of AC terminals may be an integer that is equal to or less than 2N-1, wherein N may be an integer that is equal to or greater than two. In such a preferred embodiment of the invention, each converter limb represents a degree of freedom available for the control of the converter, and thus the provision of N converter limbs connected to each DC terminal permits the control of up to 2N variables, e.g. 2N-1 alternating current variables and 1 direct current variable when no alternating or direct current flows through a corresponding neutral terminal.

In embodiments of the invention the converter may include a controller programmed to control the switching of the switching blocks to form at least one switching state, the or each switching state defining a or a respective configuration of the converter limbs and AC terminals in which each converter limb is switched into or out of circuit with each of the AC terminals.

The or each switching state may vary depending on the required shapes of the AC voltage waveforms at the AC side of the converter.

In such embodiments of the invention, the or each switching state may define a or a respective configuration of the converter limbs and AC terminals in which one of the first and second converter limbs and one of the third and fourth converter limbs are switched into circuit with one of the AC terminals. Optionally the or each switching state may define a or a respective configuration of the converter limbs and AC terminals in which either the other one of the first and second converter limbs or the other one of the third and fourth converter limbs is switched into circuit with another of the AC terminals.

In further such embodiments of the invention, the or each switching state may define a or a respective configuration of the converter limbs and AC terminals in which: one of the first and second converter limbs and one of the third and fourth converter limbs are switched into circuit with one of the AC terminals; the other one of the first and second converter limbs is switched into circuit with another of the AC terminals; and the other one of the third and fourth converter limbs is switched into circuit with a further one of the AC terminals.

The provision of the or each switching state in which one of the first and second converter limbs and one of the third and fourth converter limbs is switched into circuit with one of the AC terminals permits a common current to flow through the converter limbs switched into circuit with the same AC terminal. The common current may be utilised to provide the converter with additional functionality.

For example, the common current may be in the form of a direct current flowing through the one of the first and second converter limbs and the one of the third and fourth converter limbs switched into circuit with the same AC terminal, thus permitting the converter limbs to exchange energy. The ability to exchange energy between converter limbs switched into circuit with the same AC terminal permits regulation of the energy level of a given waveform synthesizer that includes at least one energy storage device.

In addition the multiplexing functionality of the switching blocks permits the formation of a switching state in which a given converter limb is switched into circuit with the same AC terminal as a first other converter limb, and another switching state in which the given converter limb is switched into circuit with the same AC terminal as a second other converter limb, thus providing further options for utilising the common current to provide the converter with additional functionality.

Furthermore, for a given set of AC voltage waveforms at the AC side of the converter, the period in which each converter limb of the converter of the invention can be switched into circuit with the same AC terminal as one other converter limb is longer than the period in which each converter limb of the earlier-described conventional converter can be switched into circuit with the same AC terminal as one other converter limb. This in turn provides a longer period for the common current to flow through the converter limbs switched into circuit with the same AC terminal, thus providing greater flexibility in the utility of the common current to provide the converter with additional functionality.

In further such embodiments of the invention, the controller may be programmed to control the switching of the switching blocks to selectively form a sequence of a plurality of switching states. Each of the plurality of switching states may define a respective configuration of the converter limbs and AC terminals that is different from the configuration of the converter limbs and AC terminals defined by the or each other of the plurality of switching states. The ability to form the sequence of the plurality of switching states enables the operation of the converter limbs to generate a range of complex AC voltage waveforms at the AC side of the converter.

The period of each switching state and of the sequence of the plurality of switching states may vary depending on the shapes required of the AC voltage waveforms at the AC side of the converter. For example, the controller may be programmed to control the switching of the switching blocks to selectively form the sequence of the plurality of switching states over a period of 2Tr electrical degrees, and/or the controller may be programmed to control the switching of the switching blocks to maintain the or each switching state over a period of r/2 electrical degrees. In another example, the controller may be programmed to control the switching of the switching blocks to selectively form the sequence of the plurality of switching states over a period of 4-rr electrical degrees, and/or the controller may be programmed to control the switching of the switching blocks to maintain the or each switching state over a period of Tr/3 electrical degrees.

Each switching block may vary in structure so long as it is arranged to be switchable to selectively switch each converter limb into and out of circuit between the corresponding DC terminal and the respective AC terminal independently of the switching of each other converter limb into and out of circuit between the corresponding DC terminal and the respective AC terminal.

Each switching block is preferably distinct from each other switching block. In other words, each switching block preferably does not share any switching component with each other switching block.

In embodiments of the invention, each switching block may include a plurality of switching elements. Each switching element may be arranged to extend between each converter limb and the respective AC terminal. In such embodiments, each switching block may include four switching elements.

Each switching block may include at least one AC switching element. The inclusion of the or each AC switching element in each switching block permits each switching block to handle any change in polarity of the voltage across each switching block during the switching of the switching blocks.

At least one of the waveform synthesizers may be a multi-level waveform synthesizer.

At least one of the waveform synthesizers may include at least one module. The or each module may include at least one switching element and at least one energy storage device. The or each switching element and the or each energy storage device in the or each module may be arranged to be combinable to selectively provide a voltage source.

The provision of the or each module provides a reliable means of operating the corresponding waveform synthesizer to facilitate the transfer of power between the corresponding AC and DC terminals.

The or each module may vary in configuration.

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

The or each switching element and the or each energy storage device in a module may be arranged to be combinable to selectively provide a bidirectional voltage source. In one example, the module may include two pairs of switching elements connected in parallel with an energy storage device in a full-bridge arrangement to define a 4-quadrant bipolar module that can provide negative, zero or positive voltage and can conduct current in two directions. In another example, the module may include first and second sets of series-connected current flow control elements connected in parallel with at least one energy storage device in a full-bridge arrangement to define a 2-quadrant bipolar rationalised module that can provide negative, zero or positive voltage and can conduct current in only one direction. In the rationalised module, each set of current flow control elements may include a series connection of an active switching element and a passive current check element arranged to selectively direct current through the or each energy storage device and to limit current flow through the rationalised module to a single direction.

At least one of the waveform synthesizers may include a plurality of modules (e.g. a plurality of series-connected modules) that defines a chain-link converter.

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

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

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

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

Each energy storage device may be any device that is capable of storing and releasing energy, e.g. a capacitor, fuel cell or battery.

A preferred embodiment of the invention will now be described, by way of a non-limiting example, with reference to the accompanying drawings in which: Figure 1 shows schematically a converter according to a first embodiment of the invention; Figures 2a to 2c show schematically the structures of a 4-quadrant bipolar module, a 2-quadrant unipolar module and a 2-quadrant bipolar rationalised module; Figures 3a to 3c illustrate a sequence of a plurality of switching states that define the configuration of the converter of Figure 1; Figure 4 illustrates one of the plurality of switching states of Figures 3a to 3c; Figures 5a to 5c illustrate, in graphical form, the on and off states of the AC switching elements during the sequence of Figures 3a to 3c; Figure 6 illustrates, in graphical form, the AC voltage experienced by a given AC switching element of the converter of Figure 1 during the sequence of Figures 3a to 3c; Figures 7a to 7d illustrate, in graphical form, voltage waveforms respectively generated by the waveform synthesizers of the converter of Figure 1 during the sequence of Figures 3a to 3c; Figure 8 illustrates, in graphical form, converter-side line-to-line AC voltage waveforms generated by the converter of Figure 1 during the sequence of Figures 3a to 3c; Figure 9 illustrates, in graphical form, transformer primary phase-to-ground AC voltage waveforms, transformer primary alternating currents, converter-side line-to-line AC voltage waveforms, and converter-side alternating current waveforms generated during the sequence of Figures 3a to 3c; Figure 10 shows schematically a converter according to a second embodiment of the invention; and Figure 11 illustrates a sequence of a plurality of switching states that define the configuration of the converter of Figure 10.

A converter according to a first embodiment of the invention is shown in Figure 1 and is designated generally by the reference numeral 30.

The converter 30 comprises: first and second DC terminals 32,34; first, second and third AC terminals 36; first, second, third and fourth converter limbs 1,2,3,4; and first, second and third switching blocks 38.

In use, the first and second DC terminals 32,34 are respectively connected to positive and negative poles of a DC network, the positive and negative poles carrying positive and negative DC voltages respectively, and each AC terminal 36 is connected via a transformer 20 to a respective phase of a three-phase AC network.

The first and second converter limbs 1,2 are operatively connected to the first DC terminal 32. More specifically each of the first and second converter limbs 1,2 includes first and second ends, and a first inductor connects the first DC terminal 32 to the first ends of the first and second converter limbs 1,2.

The third and fourth converter limbs 3,4 are operatively connected to the second DC terminal 34. More specifically each of the third and fourth converter limbs 3,4 includes first and second ends, and a second inductor connects the second DC terminal 34 to the first ends of the third and fourth converter limbs 3,4.

Each of the first and second converter limbs 1,2 includes a respective first waveform synthesizer connected in series with a respective optional limb inductor, and each of the third and fourth converter limbs 3,4 includes a respective second waveform synthesizer connected in series with a respective optional limb inductor.

Each waveform synthesizer is in the form of a chain-link converter. Each chain-link converter includes a plurality of series-connected modules 52. Each module 52 includes two pairs of switching elements 54 and an energy storage device in the form of a capacitor 56. In each module 52, the pairs of switching elements 54 are connected in parallel with the capacitor 56 in a full-bridge arrangement to define a 4-quadrant bipolar module 52 that can provide negative, zero or positive voltages and can conduct current in two directions. Figure 2a shows the structure of the 4-quadrant bipolar module 52.

The capacitor 56 of each module 52 is selectively bypassed and inserted into the chain-link converter by changing the states of the corresponding switching elements 54. This selectively directs current through the capacitor 56 or causes current to bypass the capacitor 56 so that the module 52 provides a negative, zero or positive voltage.

The capacitor 56 of the module 52 is bypassed when the switching elements 54 are configured to form a current path that causes current in the chain-link converter to bypass the capacitor 56, and so the module 52 provides a zero voltage, i.e. the module 52 is configured in a bypassed mode.

The capacitor 56 of the module 52 is inserted into the chain-link converter when the switching elements 54 are configured to allow the current in the chain-link converter to flow into and out of the capacitor 56. The capacitor 56 then charges or discharges its stored energy so as to provide a non-zero voltage, i.e. the module 52 is configured in a non-bypassed mode. The full-bridge arrangement of the switching elements 54 permits configuration of the switching elements to cause current to flow into and out of the capacitor 56 in either direction, and so each module 52 can be configured to provide a negative or positive voltage in the non-bypassed mode.

The structure of the chain-link converter permits build-up of a combined voltage across the chain-link converter, which is higher than the voltage available from each of its individual modules 52, via the insertion of the capacitors 56 of multiple modules 52, each providing its own voltage, into the chain-link converter. In this manner the chain-link converter is capable of providing a stepped variable voltage source, which permits the synthesis of a voltage waveform across the chain-link converter using a step-wise approximation. As such each chain-link converter is capable of providing complex voltage waveforms.

It is envisaged that, in other embodiments of the invention, each module 52 may include a pair of switching elements 54 and an energy storage device in the form of a capacitor 56.

In each such module 52, the pair of switching elements 54 is connected in parallel with the capacitor 56 in a half-bridge arrangement to define a 2-quadrant unipolar module 52 that can provide zero or positive voltages and can conduct current in two directions. Figure 2b shows the structure of the 2-quadrant unipolar module 52.

It is envisaged that, in still other embodiments of the invention, each module 52 may include first and second sets of series-connected current flow control elements 54 connected in parallel with at least one capacitor 56 in a full-bridge arrangement to define a 2-quadrant bipolar rationalised module 52 that can provide negative, zero or positive voltage and can conduct current in only one direction. In the rationalised module 52, each set of current flow control elements 54 may include a series connection of an IGBT-antiparallel diode pair and a diode arranged to selectively direct current through the or each capacitor 56 and to limit current flow through the rationalised module 52 to a single direction. Figure 2b shows the structure of the 2-quadrant bipolar rationalised module 52.

Each switching element constitutes an IGBT that is connected in anti-parallel with a diode.

It is envisaged that, in other embodiments of the invention, each IGBT may be replaced by a gate turn-off thyristor, a field effect transistor, an injection-enhanced gate transistor, an integrated gate commutated thyristor or any other self-commutated switching device.

It is envisaged that, in other embodiments of the invention, each diode may be replaced by another type of passive current check element that includes at least one passive current check device, which may be any device that is capable of limiting current flow in only one direction.

It is envisaged that, in other embodiments of the invention, each capacitor 56 may be replaced by another type of energy storage device that is capable of storing and releasing energy, e.g. a battery or fuel cell.

The first switching block 38 includes four AC switching elements SA1, SA2, SA3, SA4, each of which is arranged to extend between the second end of a respective one of the converter limbs 1,2,3,4 and the first AC terminal 36. The second switching block 38 includes four AC switching elements SB1, SB2, SB3, S64, each of which is arranged to extend between the second end of a respective one of the converter limbs 1,2,3,4 and the second AC terminal 36. The third switching block 38 includes four AC switching elements SC1, SC2, SC3, SC4, each of which is arranged to extend between the second end of a respective one of the converter limbs 1,2,3,4and the third AC terminal 36. In use, each AC switching element is switchable to switch the corresponding converter limb 1,2,3,4 into and out of circuit with the corresponding AC terminal 36.

Each AC switching element is in the form of a pair of inverse-series connected IGBT-anti-parallel diode pairs, but may take a different form in other embodiments.

In this manner each switching block 38 is arranged to interconnect each converter limb 1,2,3,4 and the respective AC terminal 36, and is arranged to be switchable to selectively switch each converter limb 1,2,3,4 into and out of circuit between the corresponding DC terminal and the respective AC terminal 36 independently of the switching of each other converter limb 1,2,3,4 into and out of circuit between the corresponding DC terminal and the respective AC terminal 36.

The converter 30 further includes a controller 100 programmed to operate the waveform synthesizers and control the switching blocks 38. More particularly, the controller 100 is programmed to selectively operate the waveform synthesizer of each converter limb 1,2,3,4 to control the configuration of an AC voltage waveform at the AC terminal 36 with which that converter limb 1,2,3,4 is connected into circuit, and to control the switching of the switching blocks 38 to form a plurality of switching states, each switching state defining a respective configuration of the converter limbs 1,2,3,4 and AC terminals 36 in which each converter limb 1,2,3,4 is switched into or out of circuit with each of the AC terminals 36.

In use, the converter 30 is operated to facilitate the transfer of power between its AC and DC terminals, i.e. between the AC and DC sides of the converter 30. This is achieved through the operation of the waveform synthesizers of the converter limbs 1,2,3,4 to control the configuration of the AC voltage waveforms at the AC terminals 36.

To permit the waveform synthesizers of the converter limbs 1,2,3,4 to control of the configuration of the AC voltage waveforms at the AC terminals 36, the controller 100 is programmed to control the switching of the switching blocks 38 to form twelve switching states, whereby each of the twelve switching states defines a respective configuration of the converter limbs 1,2,3,4 and AC terminals 36 in which: one of the first and second converter limbs 1,2 and one of the third and fourth converter limbs 3,4 are switched into circuit with one of the AC terminals 36; the other one of the first and second converter limbs 1,2 is switched into circuit with another of the AC terminals 36; and the other one of the third and fourth converter limbs 3,4 is switched into circuit with a further one of the AC terminals 36.

As shown in Figure 3a, the controller 100 is further programmed to control the switching of the switching blocks 38 to selectively form a sequence of a plurality of switching states over a period of 47 electrical degrees, and to control the switching of the switching blocks 38 to maintain each switching state over a period of r/3 electrical degrees. The sequence of the plurality of switching states is described as follows, with reference to Figures 3b and 3c.

In a first switching state over the period of 0 to r/3 electrical degrees, the AC switching elements SA1, SA3, SB4 and SC2 are turned on so that the first and third converter limbs 1,3 are switched into circuit with the first AC terminal 36, the fourth converter limb 4 is connected into circuit with the second AC terminal 36, and the second converter limb 2 is switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In a second switching state over the period of r/3 to 27/3 electrical degrees, the AC switching elements SA1, S64, SC2, SC3 are turned on so that the first converter limb 1 is switched into circuit with the first AC terminal 36, the fourth converter limb 4 is connected into circuit with the second AC terminal 36, and the second and third converter limbs 2,3 are switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In a third switching state over the period of 2r/3 to if electrical degrees, the AC switching elements SA1, SB2, SB4 and SC3 are turned on so that the first converter limb 1 is switched into circuit with the first AC terminal 36, the second and fourth converter limbs 2,4 are connected into circuit with the second AC terminal 36, and the third converter limb 3 is switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In a fourth switching state over the period of rr to 47/3 electrical degrees, the AC switching elements SA1, SA4, SB2, SC3 are turned on so that the first and fourth converter limbs 1,4 are switched into circuit with the first AC terminal 36, the second converter limb 2 is connected into circuit with the second AC terminal 36, and the third converter limb 3 is switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In a fifth switching state over the period of 4u/3 to 5r13 electrical degrees, the AC switching elements SA4, S82, SC1, SC3 are turned on so that the fourth converter limb 4 is switched into circuit with the first AC terminal 36, the second converter limb 2 is connected into circuit with the second AC terminal 36, and the first and third converter limbs 1,3 are switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In a sixth switching state over the period of 5u/3 to 2u electrical degrees, the AC switching elements SA4, 882, 883, SC1 are turned on so that the fourth converter limb 4 is switched into circuit with the first AC terminal 36, the second and third converter limbs 2,3 are connected into circuit with the second AC terminal 36, and the first converter limb 1 is switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In a seventh switching state over the period of 27r to 77/3 electrical degrees, the AC switching elements SA2, SA4, SB3, SC1 are turned on so that the second and fourth converter limbs 2,4 are switched into circuit with the first AC terminal 36, the third converter limb 3 is connected into circuit with the second AC terminal 36, and the first converter limb 1 is switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In an eighth switching state over the period of 71i/3 to 8u/3 electrical degrees, the AC switching elements SA2, 883, SC1, SC4 are turned on so that the second converter limb 2 is switched into circuit with the first AC terminal 36, the third converter limb 3 is connected into circuit with the second AC terminal 36, and the first and fourth converter limbs 1,4 are switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In a ninth switching state over the period of 8n/3 to 3rr electrical degrees, the AC switching elements SA2, 561, SB3, SC4 are turned on so that the second converter limb 2 is switched into circuit with the first AC terminal 36, the first and third converter limbs 1,3 are connected into circuit with the second AC terminal 36, and the fourth converter limb 4 is switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In a tenth switching state over the period of 3-rr to 10-rr/3 electrical degrees, the AC switching elements SA2, SA3, SB1, SC4 are turned on so that the second and third converter limbs 2,3 are switched into circuit with the first AC terminal 36, the first converter limb 119 connected into circuit with the second AC terminal 36, and the fourth converter limb 4 is switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In a eleventh switching state over the period of 10-rr/3 to 117/3 electrical degrees, the AC switching elements SA3, SB1, SC2, SC4 are turned on so that the third converter limb 3 is switched into circuit with the first AC terminal 36, the first converter limb 1 is connected into circuit with the second AC terminal 36, and the second and fourth converter limbs 2,4 are switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

In a twelfth switching state over the period of 11-rr/3 to 4-rr electrical degrees, the AC switching elements SA3, SB1, S134, SC2 are turned on so that the third converter limb 3 is switched into circuit with the first AC terminal 36, the first and fourth converter limbs 1,4 is connected into circuit with the second AC terminal 36, and the second converter limb 2 are switched into circuit with the third AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off.

The controller 100 is further programmed to control the switching of the switching blocks 38 to cyclically form the sequence of the plurality of switching states such that the twelfth switching state of an electrical cycle of the converter 30 transitions to the first switching state of the next electrical cycle of the converter 30.

The arrangement of the switching blocks 38 in the converter 30 of Figure 1 therefore permits the switching blocks 38 to function as a multiplexer to carry out the selective connection of each converter limb 1,2,3,4 with each of the AC terminals 36.

It will be appreciated that the above first switching state that defines the starting point of the AC voltage waveform is merely selected to illustrate the working of the invention, and a different switching state may be selected to define the starting point of the AC voltage waveform.

During the formation of each switching state, the controller 100 operates the waveform synthesizer of each converter limb 1,2,3,4 to control the configuration of an AC voltage waveform at the AC terminal 36 with which that converter limb 1,2,3,4 is connected into circuit. In the embodiment shown, each AC voltage waveform at each AC terminal 36 is controlled to be sinusoidal in shape.

Moreover, in each switching state, one of the first and second converter limbs 1,2 and one of the third and fourth converter limbs 3,4 are switched into circuit with one of the AC terminals 36. An example is shown in Figure 4 which corresponds to the first switching state over the period of 0 to Tr/3 electrical degrees. This permits a common current in the form of a direct current to flow through the converter limbs 1,2,3,4 switched into circuit with the same AC terminal 36 to enable the converter limbs 1,2,3,4 to exchange energy. The ability to exchange energy between converter limbs 1,2,3,4 switched into circuit with the same AC terminal 36 permits regulation of the energy level of the capacitors 56 of each waveform synthesizer.

In addition the sequence of the plurality of switching states provides greater flexibility in regulating of the energy level of the capacitors 56 of each waveform synthesizer, not only because each converter limb 1,2,3,4 is switched into circuit with different converter limbs 1,2,3,4 at different times during the sequence of the plurality of switching states, which permits each converter limb 1,2,3,4 to exchange energy with different converter limbs 1,2,3,4 using the common current, but also because each converter limb 1,2,3,4 is connected to one of the AC terminals 36 at all times during the sequence.

Figures 5a to Sc illustrate the on and off states of the AC switching elements in a period of time from t = 1 second to t = 1.02 second during the sequence of the plurality of switching states, Figure 6 illustrates, in graphical form, the AC voltage experienced by a given AC switching element in the same period of time. Figures 7a to 7d illustrate the voltage waveforms respectively generated by the waveform synthesizers in the same period of time.

Figure 8 illustrates the converter-side line-to-line AC voltage waveforms generated by the converter 30 in the same period of time, while Figure 9 illustrates, from top to bottom, the transformer primary phase-to-ground alternating voltage waveforms Vabc0, transformer primary alternating currents labc0, converter-side line-to-fine AC voltage waveforms Vconverter, and the converter-side alternating current waveforms labc generated in the same period of time.

It can be seen from Figures 5a to 9 that the converter 30 of Figure 1 can be readily operated in the above described manner in order to generate high quality AC side voltage and current waveforms. The reduction in the number of converter limbs 1,2,3,4 when compared to the earlier-mentioned conventional converter therefore has no impact on the ability of the converter 30 of Figure 1 to transfer high quality power.

The configuration of the converter limbs 1,2,3,4 and switching blocks 38 of the converter 30 of Figure 1 therefore reduces the number of components required of the converter 30 when compared to the conventional converter and thereby results in a cost-efficient and space-saving converter 30 due to the reductions in the size and weight of the converter 30 and the associated commissioning, installation and transportation costs. In addition the lower number of components in the converter 30 of Figure 1 when compared to the conventional converter reduces the number of operating variables to be monitored during the operation of the converter 30, thus simplifying the communications and data processing requirements of the converter 30.

Furthermore, for a given set of AC voltage waveforms at the AC side of the converter 30, the period in which each converter limb 1,2,3,4 of the converter 30 of the invention can be switched into circuit with the same AC terminal 36 as one other converter limb 1,2,3,4 is longer than the period in which each converter limb of the earlier-described conventional converter can be switched into circuit with the same AC terminal 36 as one other converter limb. This in turn provides a longer period for the common current to flow through the converter limbs 1,2,3,4 switched into circuit with the same AC terminal 36, thus providing greater flexibility in the utility of the common current to provide the converter 30 with additional functionality.

It will be appreciated that the limb inductors of the converter 30 may be omitted by increasing the size of each inductor connected between each DC terminal and the corresponding converter limbs 1,2,3,4.

A converter according to a second embodiment of the invention is shown in Figure 10 and is designated generally by the reference numeral 130. The converter 130 of Figure 10 is similar in structure and operation to the converter 30 of Figure 1, and like features share the same reference numerals.

The converter 130 of Figure 10 differs from the converter 30 of Figure 1 in that the converter 130 of Figure 10 omits the third AC terminal 36 and the third switching block 38.

Accordingly, in use, each AC terminal 36 is connected via a transformer (not shown) to a respective phase of a two-phase AC network.

To permit the waveform synthesizers of the converter limbs 1,2,3,4 to control of the configuration of the AC voltage waveforms at the AC terminals 36, the controller 100 is programmed to control the switching of the switching blocks 38 to form four switching states, whereby each of the four switching states defines a respective configuration of the converter limbs 1,2,3,4 and AC terminals 36 in which: one of the first and second converter limbs 1,2 and one of the third and fourth converter limbs 3,4 are switched into circuit with one of the AC terminals 36; and either the other one of the first and second converter limbs 1,2 or the other one of the third and fourth converter limbs 3,4 is switched into circuit with the other of the AC terrninals 36.

The controller 100 is further programmed to control the switching of the switching blocks 38 to selectively form a sequence of a plurality of switching states over a period of 2n-electrical degrees, and to control the switching of the switching blocks 38 to maintain each switching state over a period of 'rr/2 electrical degrees. The sequence of the plurality of switching states is described as follows, with reference to Figure 11.

In a first switching state over the period of 0 to Tr/2 electrical degrees, the AC switching elements SA1, SA3 and SB2 are turned on so that the first and third converter limbs 1,3 are switched into circuit with the first AC terminal 36, and the second converter limb 2 is switched into circuit with the second AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off, and the fourth converter limb 4 is switched out of circuit with each of the AC terminals 36.

In a second switching state over the period of rr/2 to -rr electrical degrees, the AC switching elements SA1, 5A4 and SB3 are turned on so that the first and fourth converter limbs 1,4 are switched into circuit with the first AC terminal 36, and the third converter limb 3 is connected into circuit with the second AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off, and the second converter limb 2 is switched out of circuit with each of the AC terminals 36.

In a third switching state over the period of -rr to 3-rr/2 electrical degrees, the AC switching elements SA1, SB2 and SB4 are turned on so that the first converter limb 1 is switched into circuit with the first AC terminal 36, and the second and fourth converter limbs 2,4 are connected into circuit with the second AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off, and the third converter limb 3 is switched out of circuit with each of the AC terminals 36.

In a fourth switching state over the period of 3r12 to 2-rr electrical degrees, the AC switching elements SA4, SB2 and SB3 are turned on so that the fourth converter limb 4 is switched into circuit with the first AC terminal 36, and the second and third converter limbs 2,3 are connected into circuit with the second AC terminal 36. Meanwhile the remaining AC switching elements of the switching blocks 38 are turned off, and the first converter limb 1 is switched out of circuit with each of the AC terminals 36.

The controller 100 is further programmed to control the switching of the switching blocks 38 to cyclically form the sequence of the plurality of switching states such that the fourth switching state of an electrical cycle of the converter 130 transitions to the first switching state of the next electrical cycle of the converter 130.

The arrangement of the switching blocks 38 in the converter 130 of Figure 10 therefore permits the switching blocks 38 to function as a multiplexer to carry out the selective connection of each converter limb 1,2,3,4 with each of the AC terminals 36.

In each of the above embodiments, the ability to selectively connect each converter limb 1,2,3,4 to each of the AC terminals 36 provides the converter 30,130 with greater flexibility in controlling the configuration of the AC voltage waveforms at the AC terminals 36 when compared to the conventional converter in which the AC voltage waveform at a given AC terminal is controlled through the operation of a single phase leg.

It will be appreciated that the above first and second embodiments of the invention are non-limiting examples of the invention, and are merely chosen to illustrate the working of the invention. It will be further appreciated that the working principles of the invention may be extended to other embodiments of the invention with a plurality of AC terminals.

Claims

CLAIMS1. A converter comprising: first and second DC terminals for connection in use to a DC network; a plurality of AC terminals, each of the AC terminals for connection in use to a respective phase of a multi-phase AC network; first and second converter limbs operatively connected to the first DC terminal, each of the first and second converter limbs including a respective first waveform synthesizer; third and fourth converter limbs operatively connected to the second DC terminal, each of the third and fourth converter limbs including a respective second waveform synthesizer; and a plurality of switching blocks, each switching block arranged to interconnect each converter limb and a respective one of the AC terminals, each switching block arranged to be switchable to selectively switch each converter limb into and out of circuit between the corresponding DC terminal and the respective AC terminal independently of the switching of each other converter limb into and out of circuit between the corresponding DC terminal and the respective AC terminal.
2. A converter according to Claim 1 wherein the number of converter limbs operatively connected to the first DC terminal is equal to N, the number of converter limbs operatively connected to the second DC terminal is equal to N, and the number of AC terminals is an integer that is equal to or less than 2N-1, wherein N is an integer that is equal to or greater than two.
3. A converter according to Claim 1 or Claim 2 including a controller programmed to selectively operate each converter limb to control the configuration of an AC voltage waveform at the AC terminal with which that converter limb is connected into circuit.
4. A converter according to Claim 3 wherein the AC voltage waveform is sinusoidal or near-sinusoidal in shape.
5. A converter according to any one of the preceding claims including a controller programmed to control the switching of the switching blocks to form at least one switching state, the or each switching state defining a or a respective configuration of the converter limbs and AC terminals in which each converter limb is switched into or out of circuit with each of the AC terminals.
6. A converter according to Claim 5 wherein the or each switching state defines a or a respective configuration of the converter limbs and AC terminals in which one of the first and second converter limbs and one of the third and fourth converter limbs are switched into circuit with one of the AC terminals.
7. A converter according to Claim 6 wherein the or each switching state defines a or a respective configuration of the converter limbs and AC terminals in which either the other one of the first and second converter limbs or the other one of the third and fourth converter limbs is switched into circuit with another of the AC terminals.
8. A converter according to any one of Claims 5 to 7 wherein the or each switching state defines a or a respective configuration of the converter limbs and AC terminals in which: one of the first and second converter limbs and one of the third and fourth converter limbs are switched into circuit with one of the AC terminals; the other one of the first and second converter limbs is switched into circuit with another of the AC terminals; and the other one of the third and fourth converter limbs is switched into circuit with a further one of the AC terminals.
9. A converter according to any one of Claims 5 to 8 wherein the controller is programmed to control the switching of the switching blocks to selectively form a sequence of a plurality of switching states, each of the plurality of switching states defining a respective configuration of the converter limbs and AC terminals that is different from the configuration of the converter limbs and AC terminals defined by the or each other of the plurality of switching states.
10. A converter according to Claim 9 when dependent from any one of Claims 5 to 7 wherein the controller is programmed to control the switching of the switching blocks to selectively form the sequence of the plurality of switching states over a period of 2tr electrical degrees.
11. A converter according to any one of Claims 5, 6, 7, 9 and 10 wherein the controller is programmed to control the switching of the switching blocks to maintain the or each switching state over a period of tr/2 electrical degrees.
12. A converter according to Claim 9 when dependent from any one of Claims 5 to 8 wherein the controller is programmed to control the switching of the switching blocks to selectively form the sequence of the plurality of switching states over a period of 4-rr electrical degrees.
13. A converter according to any one of Claims 5, 6, 7, 8, 9 and 12 wherein the controller is programmed to control the switching of the switching blocks to maintain the or each switching state over a period of 1r/3 electrical degrees.
14. A converter according to any one of the preceding claims wherein each switching block is distinct from each other switching block.
15. A converter according to any one of the preceding claims wherein each switching block includes a plurality of switching elements, each switching element arranged to extend between each converter limb and the respective AC terminal, and optionally wherein each switching block includes four switching elements.
16. A converter according to any one of the preceding claims wherein each switching block includes at least one AC switching element.
17. A converter according to any one of the preceding claims wherein at least one of the waveform synthesizers is a multi-level waveform synthesizer.
18. A converter according to any one of the preceding claims wherein at least one of the waveform synthesizers includes at least one module, the or each module including at least one switching element and at least one energy storage device, the or each switching element and the or each energy storage device in the or each module arranged to be combinable to selectively provide a voltage source.