WO2015063108A1

WO2015063108A1 - Dc to dc converter assembly

Info

Publication number: WO2015063108A1
Application number: PCT/EP2014/073154
Authority: WO
Inventors: Francisco Jose Moreno Muñoz; Kevin James Dyke
Original assignee: Alstom Technology Ltd
Priority date: 2013-10-29
Filing date: 2014-10-28
Publication date: 2015-05-07
Also published as: GB2519762B; EP3063864A1; GB2519762A; GB201319057D0

Abstract

There is provided a DC to DC converter assembly (20) for interconnecting DC electrical networks (32,34). The DC to DC converter assembly (20) comprises: first and second converters (22,24), each converter (22,24) including first and second DC terminals (26,28) for connection to a respective one of the DC electrical networks (32,34), each converter (22,24) including a converter limb (30) extending between the corresponding first and second DC terminals (26,28), the converter limb (30) of the first converter (22) having a pair of first limb portions (36a,36b) separated by a first AC terminal (40), the converter limb (30) of the second converter (24) having a pair of second limb portions (38a,38b) separated by a second AC terminal (42), each limb portion (36a,36b,38a,38b) including at least one switching element, at least one first limb portion (36a,36b) including at least one first module (46a), at least one second limb portion (38a,38b) including at least one second module (46b), the or each module (46a,46b) including at least one switching element and at least one energy storage device, the or each switching element and the or each energy storage device in each module (46a,46b) combining to selectively provide a voltage source, the or each switching element in each limb portion (36a,36b,38a,38b) being switchable to switch the corresponding limb portion (36a,36b,38a,38b) into or out of circuit between the corresponding AC and DC terminals (26,28,40,42) to control the configuration of an AC voltage (66,68) at the corresponding AC terminal (26,28,40,42); an AC transmission link (56), the first AC terminal (40) being connected to the second AC terminal (42) via the AC transmission link (56), the first AC terminal (40) being connected at a first end of the AC transmission link (56), the second AC terminal (42) being connected at a second end of the AC transmission link (56); and a controller (65) configured to control switching of the or each switching element in each module (46a,46b) so as to transfer energy between at least one first module (46a) and at least one second module (46b), wherein the controller (65) is further configured to selectively control switching of the or each switching element in each limb portion (36a,36b,38a,38b) to control the configuration of the AC voltage (66,68) at the corresponding AC terminal (40,42) so as to minimise a net change in energy level of the DC to DC converter assembly (20) during a transfer of energy between at least one first module (46a) and at least one second module (46b).

Description

DC TO DC CONVERTER ASSEMBLY

This invention relates to a DC to DC converter assembly for interconnecting DC electrical networks, and a method of controlling a DC to DC converter assembly for interconnecting DC electrical networks.

In high voltage direct current (HVDC) power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or under-sea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the transmission line or cable, and thereby reduces the cost per 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. DC transmission and distribution networks are needed to support the emergence of HVDC power transmission. However, interconnecting the DC transmission and distribution networks to form a DC power grid is difficult because different DC networks may operate at different voltage levels. In order to interconnect two DC networks operating at different voltage levels, a DC to DC converter assembly is required.

According to a first aspect of the invention, there is provided a DC to DC converter assembly for interconnecting first and second DC electrical networks, the DC to DC converter assembly comprising:

first and second converters, the first converter including first and second DC terminals for connection to the first DC electrical network, the second converter including first and second DC terminals for connection to the second DC electrical network, each converter including a converter limb extending between the corresponding first and second DC terminals, the converter limb of the first converter having a pair of first limb portions separated by a first AC terminal, the converter limb of the second converter having a pair of second limb portions separated by a second AC terminal, each limb portion including at least one switching element, at least one first limb portion including at least one first module, at least one second limb portion including at least one second module, the or each module including at least one switching element and at least one energy storage device, the or each switching element and the or each energy storage device in each module combining to selectively provide a voltage source, the or each switching element in each limb portion being switchable to switch the corresponding limb portion into or out of circuit between the corresponding AC and DC terminals to control the configuration of an AC voltage at the corresponding AC terminal;

an AC transmission link, the first AC terminal being connected to the second AC terminal via the AC transmission link, the first AC terminal being connected at a first end of the AC transmission link, the second AC terminal being connected at a second end of the AC transmission link; and

a controller configured to control switching of the or each switching element in each module so as to transfer energy between at least one first module and at least one second module,

wherein the controller is further configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal so as to minimise a net change in energy level of the DC to DC converter assembly during a transfer of energy between at least one first module and at least one second module.

In use, the DC to DC converter assembly is operable to transfer power between the DC electrical networks. More particularly, to operate the DC to DC converter assembly to transfer power between the DC electrical networks, the switching elements of the limb portions of each converter are switched to selectively switch each limb portion into circuit between the corresponding AC and DC terminals over an operating cycle of the DC electrical networks.

Switching a limb portion into circuit between the corresponding AC and DC terminals enables the limb portion to be further operated to control the configuration of the AC voltage at the corresponding AC terminal when the limb portion includes at least one module. More particularly, the or each module of the limb portion can be controlled to selectively provide a voltage source to "push up" (add voltage steps to) and/or "pull down" (subtract voltage steps from) a DC voltage at the corresponding DC terminal to control the configuration of the AC voltage at the corresponding AC terminal. In this manner each converter is capable of generating a high quality AC voltage waveform at the corresponding AC terrninal and thereby enabling the transfer of high quality power between the DC electrical networks.

Meanwhile energy transferred from one DC electrical network to the other DC electrical network flows through the DC to DC converter assembly, and thereby flows through the or each energy storage device of the or each module that is controlled to selectively provide a voltage source, during the operation of the DC to DC converter assembly to transfer power between the DC electrical networks. Such a flow of energy through the or each energy storage device of the or each module could result in energy accumulation in (or energy loss from) at least one energy storage device, thus resulting in deviation of the energy level of at least one energy storage device from a reference value.

Such a deviation is undesirable because, if too little energy is stored within a given energy storage device then the voltage the corresponding module is able to generate is reduced, whereas if too much energy is stored in a given energy storage device then over-voltage problems may arise. The former would require the addition of a power supply to restore the energy level of the affected energy storage device to the reference value, while the latter would require an increase in voltage rating of one or more energy storage devices to prevent the over-voltage problems, thus adding to the overall size, weight and cost of the DC to DC converter assembly. The configuration of the controller of the DC to DC converter assembly according to the invention permits energy to be transferred between at least one first module and at least one second module (though selective storage and release of energy by at least one first module and at least one second module), and so permits regulation of the energy stored in a given energy storage device, thereby obviating the problems associated with a deviation of the energy level of at least one energy storage device from the reference value. In addition, configuration of the controller of the DC to DC converter assembly according to the invention permits minimisation of a net change in energy level of the DC to DC converter assembly (preferably over an operating cycle of the DC electrical networks) by way of generation of an optimal AC voltage at each AC terminal. This permits regulation of the energy level of the DC to DC converter assembly and thereby not only makes it easier to regulate the energy stored in a given energy storage device, thereby obviating the problems associated with a deviation of the energy level of at least one energy storage device from the reference value, but also enables energy balancing of the first and second converters.

Since the AC transmission link is only connected between the first and second converters and is not directly connected to an AC grid, it is not necessary to comply with grid electrical regulations when controlling the configuration of the AC voltage at the corresponding AC terminal so as to minimise a net change in energy level of the DC to DC converter assembly during the transfer of energy between at least one first module and at least one second module, thus avoiding any problems associated with noncompliance with grid electrical regulations. In embodiments of the invention, minimising a net change in energy level of the DC to DC converter assembly may include maintaining a zero net change in energy level of the DC to DC converter assembly. This further improves regulation of the energy level of the DC to DC converter assembly, and any associated regulation of the energy stored in a given energy storage device.

In further embodiments of the invention, the controller may be configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal so as to concurrently. minimise a net change in energy level of the DC to DC converter assembly during a transfer of energy between at least one first module and at least one second module; and

transfer power between the DC electrical networks.

Configuration of the controller in this manner allows regulation of the energy level of the DC to DC converter assembly whilst power is being transferred between the DC electrical networks. This not only minimises or eliminates the need to bring the DC to DC converter assembly offline to minimise a net change in energy level of the DC to DC converter assembly, but also enables optimal operation of the DC to DC converter assembly through real-time regulation of the energy level of the DC to DC converter assembly.

The controller of the DC to DC converter assembly according to the invention may be configured in different ways in order to enable it to control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal so as to minimise a net change in energy level of the DC to DC converter assembly during a transfer of energy between at least one first module and at least one second module.

The controller may be configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal to:

equate a net change in energy level of the first converter with a minus of net change in energy levels of the first limb portions caused by a transfer of energy between at least one first module and at least one second module; and equate a net change in energy level of the second converter with a minus of net change in energy levels of the second limb portions caused by a transfer of energy between at least one first module and at least one second module,

so as to minimise a net change in energy level of the DC to DC converter assembly during a transfer of energy between at least one first module and at least one second module.

equate a sum of net changes in energy exchanged between the first and second converters and in energy levels of the first limb portions caused by a transfer of energy between at least one first module and at least one second module with a net change in energy imported by the first converter from the first DC electrical network; and

equate a sum of net changes in energy exchanged between the first and second converters and in energy levels of the second limb portions caused by a transfer of energy between at least one first module and at least one second module with a net change in energy exported by the second converter to the second DC electrical network, so as to minimise a net change in energy level of the DC to DC converter assembly during a transfer of energy between at least one first module and at least one second module.

A change in operating parameter of the DC to DC converter assembly may arise as a result of, for example, a fault or disturbance within the DC to DC converter assembly or within either or both of the first and second DC electrical networks.

The controller of the DC to DC converter assembly according to the invention may be configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal so as to minimise a net change in energy level of the DC to DC converter assembly during a transfer of energy between at least one first module and at least one second module in response to a change in operating parameter of the DC to DC converter assembly.

The change in operating parameter of the DC to DC converter assembly may be any one of:

• a change in DC voltage of either or each of the first and second DC electrical networks; • a change in energy storage capacity of at least one energy storage device of at least one module.

Configuration of the controller in this manner enhances the capability of the controller to carry out real-time regulation of the energy level of the DC to DC converter assembly in response to change in operating parameter of the DC to DC converter assembly, thus resulting in a more reliable DC to DC converter assembly.

It will be appreciated that the controller may be configured in different ways in order to enable it to control switching of the or each switching element in each module so as to transfer energy between at least one first module and at least one second module.

In embodiments of the invention, the controller may be configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal to:

form a current circulation path that includes the AC transmission link, at least one first limb portion and at least one second limb portion; and

inject a circulation current into the current circulation path to transfer energy between at least one first module and at least one second module in the circulation path.

The formation of the current circulation path allows energy to be transferred between at least one first module and at least one second module, and so permits regulation of the energy stored in a given energy storage device. In such embodiments of the invention, each converter may further include an auxiliary limb extending between the corresponding first and second DC terminals, the auxiliary limb being connected in parallel with the corresponding converter limb, the current circulation path further including at least part of the auxiliary limb. The inclusion of the auxiliary limb in each converter provides an additional route through which current can flow in the converter, and so provides greater flexibility in terms of the number of possible configurations of the current circulation path.

In embodiments of the invention employing the use of an auxiliary limb in each converter, the auxiliary limb of each converter may include an auxiliary terminal, and the auxiliary terminals of the first and second converters are electrically interconnected by a current return path, the current circulation path including the current return path. In such embodiments, the configuration of each auxiliary limb may vary. For example, each auxiliary limb may include a pair of DC link capacitors separated by the corresponding auxiliary terminal. The inclusion of the current return path in the current circulation path allows transfer of energy between at least one first module and at least one second module through control of the configuration of the AC voltage at each AC terminal to inject a common mode current into the current return path. Additionally, aside from providing a means of transferring energy between at least one first module and at least one second module, the provision of the current return path is particularly useful when it is required to limit the number of limb portions that are connected in the current circulation path.

In further embodiments of the invention, the DC to DC converter assembly may include a plurality of AC transmission links, wherein each converter includes a plurality of converter limbs, each first AC terminal being connected to a respective one of the second AC terminals via a respective one of the plurality of AC transmission links, each first AC terminal being connected at a first end of the corresponding AC transmission link, each second AC terminal being connected at a second end of the corresponding AC transmission link, the current circulation path including a first current circulation path portion and at least one further current circulation path portion,

wherein the first current circulation path portion includes: a first AC transmission link; at least one first limb portion connected at a first end of the first AC transmission link; and at least one second limb portion connected at a second end of the first AC transmission link, and

wherein the or each further current circulation path portion includes: a further AC transmission link; at least one first limb portion connected at a first end of the further AC transmission link; and at least one second limb portion connected at a second end of the further AC transmission link. The inclusion of the first current circulation path portion and the or each further current circulation path portion in the current circulation path portion permits the current circulation path to be formed within the converters and AC transmission links, thus obviating the need for inclusion of the above-described current return path in the current circulation path. In addition, the inclusion of the first current circulation path portion and the or each further current circulation path portion in the current circulation path enables multiple converter limbs of either or each converter to be connected in the current circulation path. In practice, the operation of the DC to DC converter assembly to transfer power between the DC electrical networks requires the AC voltage at the first AC terminal of the first converter to be phase-shifted relative to the AC voltage at the second AC terminal of the second converter. Accordingly the period in which each of the first and second limb portions are switched into conduction is longer than the common conduction period.

When each converter includes a plurality of converter limbs and a plurality of AC transmission links, the AC voltage at each AC terminal of a given converter is also phase-shifted relative to the AC voltage at each other AC terminal of the given converter during the operation of the DC to DC converter assembly to transfer power between the DC electrical networks. When the current circulation path includes the first current circulation path portion and the or each further current circulation path portion, the current circulation path can only be formed during an overlap between the common conduction periods of the first AC transmission link and the or each further AC transmission link, so as to enable concurrent formation of the current circulation path and transfer of power between the DC electrical networks. This limits the period in which the period in which energy can be transferred to or from a given energy storage device to regulate its energy level during the transfer of power between the DC electrical networks.

The controller may be configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal to form a first current circulation path including the first current circulation path portion and a further current circulation path portion, followed by a second current circulation path including the first current circulation path portion and another further current circulation path portion.

Sequential formation of the first and second current circulation paths in this manner extends the period in which energy can be transferred to or from a given energy storage device to regulate its energy level during the transfer of power between the DC electrical networks.

According to a second aspect of the invention, there is provided a method of controlling a DC to DC converter assembly for interconnecting first and second DC electrical networks, the DC to DC converter assembly comprising:

first and second converters, the first converter including first and second DC terminals for connection to the first DC electrical network, the second converter including first and second DC terminals for connection to the second DC electrical network, each converter including a converter limb extending between the corresponding first and second DC terminals, the converter limb of the first converter having a pair of first limb portions separated by a first AC terminal, the converter limb of the second converter having a pair of second limb portions separated by a second AC terminal, each limb portion including at least one switching element, at least one first limb portion including at least one first module, at least one second limb portion including at least one second module, the or each module including at least one switching element and at least one energy storage device, the or each switching element and the or each energy storage device in each module combining to selectively provide a voltage source, the or each switching element in each limb portion being switchable to switch the corresponding limb portion into or out of circuit between the corresponding AC and DC terminals to control the configuration of an AC voltage at the corresponding AC terminal; and

an AC transmission link, the first AC terminal being connected to the second AC terminal via the AC transmission link, the first AC terminal being connected at a first end of the AC transmission link, the second AC terminal being connected at a second end of the AC transmission link,

wherein the method comprises the step of:

switching the or each switching element in each module so as to transfer energy between at least one first module and at least one second module; and

switching the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal so as to minimise a net change in energy level of the DC to DC converter assembly during a transfer of energy between at least one first module and at least one second module.

The method according to the invention shares the advantages associated with the corresponding features of the DC to DC converter assembly according to the invention.

A preferred embodiment of the invention will now be described, by way of a non-limiting example only, with reference to the accompanying drawings in which:

Figure 1 shows, in schematic form, a DC to DC converter assembly according to a first embodiment of the invention;

Figure 2 shows, in schematic form, the structure of a module forming part of the DC to DC converter assembly of Figure 1 ; and

Figures 3 illustrate, in graph form, the operation of the DC to DC converter assembly of Figure 1 to transfer power between DC electrical networks; Figures 4a to 4d illustrate the operation of the DC to DC converter assembly of Figure 1 to form a current circulation path during the transfer of power between DC electrical networks;

Figure 5 illustrates, in graph form, a comparison of the voltages, powers and energies for the first and second converters shown in Figure 1 during the transfer of energy between at least one first module and at least one second module through formation of a current circulation path;

Figure 6 illustrates, in graph form, variations in power in the first and second converters shown in Figure 1 during their operation to minimise a net change in energy level of the DC to DC converter assembly of Figure 1 ;

Figure 7 shows, in schematic form, a DC to DC converter assembly according to a second embodiment of the invention;

Figure 8 illustrates the operation of the DC to DC converter assembly of Figure 7 to transfer power between DC electrical networks; and

Figures 9 and 10 illustrate the operation of the DC to DC converter assembly of

Figure 7 to form a current circulation path during the transfer of power between DC electrical networks.

A first DC to DC converter assembly 20 according to a first embodiment of the invention is shown in Figure 1.

The first DC to DC converter assembly 20 comprises first and second converters 22,24. Each converter 22,24 includes first and second DC terminals 26,28, and a converter limb 30 extending between the corresponding first and second DC terminals 26,28.

In use, the first and second DC terminals 26,28 of the first converter 22 are respectively connected to first and second terminals of a first DC electrical network 32, while the first and second DC terminals 26,28 of the second converter 24 are respectively connected to first and second terminals of a second DC electrical network 34. The first terminal of each DC electrical network 32,34 carries a positive DC voltage, and the second terminal of each DC electrical network 32,34 carries a negative DC voltage.

The converter limb 30 of the first converter 22 has a pair of first limb portions 36a, 36b separated by a first AC terminal 40. More particularly, in the converter limb 30 of the first converter 22, an "upper" first limb portion 36a is connected between the first DC terminal 26 and the first AC terminal 40, and a "lower" first limb portion 36b is connected between the second DC terminal 28 and the first AC terminal 40. The converter limb 30 of the second converter 24 has a pair of second limb portions 38a,38b separated by a second AC terminal 42. More particularly, in the converter limb 30 of the second converter 24, an "upper" second limb portion 38a is connected between the first DC terminal 26 and the second AC terminal 42, and a "lower" second limb portion 38b is connected between the second DC terminal 28 and the second AC terminal 42.

Each first limb portion 36a, 36b includes a director switch 44 connected in series with a plurality of series-connected first modules 46a. Each second limb portion 38a,38b includes a director switch 44 connected in series with a plurality of series-connected second modules 46b.

Each director switch 44 is in the form of a single switching element. It is envisaged that, in other embodiments of the invention, each director switch may include a plurality of series-connected switching elements.

Each module 46a,46b includes two pairs of switching elements and an energy storage device 48 in the form of a capacitor. The pairs of switching elements are connected in parallel with the capacitor in a full-bridge arrangement, as shown in Figure 2.

Each switching element of the director switch 44 and modules 46a,46b in each limb portion 36a,36b,38a,38b includes a single switching device. Each switching element further includes a passive current check element that is connected in anti-parallel with each switching device.

Each switching device is in the form of an insulated gate bipolar transistor (IGBT) 50. It is envisaged that, in other embodiments of the invention, each IGBT may be replaced by a gate turn-off thyristor, a field effect transistor, an injection-enhanced gate transistor, an integrated gate commutated thyristor or any other self-commutated switching device. The number of switching devices in each switching element may vary depending on the required voltage rating of that switching element.

Each passive current check element includes a passive current check device in the form of a diode 52. It is envisaged that, in other embodiments, each diode may be replaced by any other device that is capable of limiting current flow in only one direction. The number of passive current check devices in each passive current check element may vary depending on the required voltage rating of that passive current check element.

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

The plurality of series-connected modules 46a,46b in each limb portion 36a,36b,38a,38b defines a chain-link converter 54.

The capacitor of each module 46a,46b is selectively bypassed or inserted into the corresponding chain-link converter 54 by changing the states of the switching elements. This selectively directs current through the capacitor or causes current to bypass the capacitor, so that the module 46a,46b provides a negative, zero or positive voltage.

The capacitor of the module 46a,46b is bypassed when the switching elements in the module 46a,46b are configured to form a short circuit in the module 46a,46b that bypasses the capacitor. This causes current in the chain-link converter 54 to pass through the short circuit and bypass the capacitor, and so the module 46a,46b provides a zero voltage, i.e. the module 46a, 46b is configured in a bypassed mode.

The capacitor of the module 46a, 46b is inserted into the chain-link converter 54 when the switching elements in the module 46a,46b are configured to allow the current in the chain-link converter 54 to flow into and out of the capacitor. The capacitor then charges or discharges its stored energy so as to provide a non-zero voltage, i.e. the module 46a,46b is configured in a non-bypassed mode. The full-bridge arrangement of the module 46a,46b permits configuration of the switching elements in the module 46a,46b to cause current to flow into and out of the capacitor in either direction, and so the module 46a,46b can be configured to provide a negative or positive voltage in the non- bypassed mode.

In this manner the pairs of switching elements are connected in parallel with the capacitor in a full-bridge arrangement to define a 4-quadrant bipolar module 46a,46b that can provide negative, zero or positive voltage and can conduct current in two directions.

It is possible to build up a combined voltage across the chain-link converter 54, which is higher than the voltage available from each of its individual modules 46a,46b, via the insertion of the capacitors of multiple modules 46a,46b, each providing its own voltage, into the chain-link converter 54. In this manner switching of the switching elements in each module 46a,46b causes the chain-link converter 54 to provide a stepped variable voltage source, which permits the generation of a voltage waveform across the chain-link converter 54 using a step-wise approximation. As such the chain-link converter 54 is capable of providing a wide range of complex voltage waveforms.

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

The first DC to DC converter assembly 20 further includes an AC transmission link 56. The first AC terminal 40 is connected to the second AC terminal 42 via the AC transmission link 56. In particular, the first AC terminal 40 is connected at a first end of the AC transmission link 56, and the second AC terminal 42 is connected at a second end of the AC transmission link 56. The AC transmission link 56 includes a inductor 58.

Each converter 22,24 further includes an auxiliary limb 60 extending between the corresponding first and second DC terminals 26,28. In each converter 22,24, the auxiliary limb 60 is connected in parallel with the corresponding converter limb 30. Each auxiliary limb 60 includes a pair of "upper" and "lower" DC link capacitors 62a,62b separated by an auxiliary terminal 64, which can optionally be connected to ground. Accordingly the auxiliary terminals 64 of the first and second converters 22,24 are electrically interconnected by a current return path 67. The "upper" DC link capacitor 62a is connected between the first DC terminal 26 and the auxiliary terminal 64, while the "lower" DC link capacitor 62b is connected between the second DC terminal 28 and the auxiliary terminal 64. The first DC to DC converter assembly 20 further includes a controller 65 that is configured to control switching of the director switch 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b.

Operation of the first DC to DC converter assembly 20 of Figure 1 is described as follows, with reference to Figures 3 to 6. As described above, in use, the first and second DC terminals 26,28 of the first converter 22 are respectively connected to first and second terminals of a first DC electrical network 32, the first and second DC terminals 26,28 of the second converter 24 are respectively connected to first and second terminals of a second DC electrical network 34, the first terminal of each DC electrical network carries a positive DC voltage, and the second terminal of each DC electrical network carries a negative DC voltage.

In use, the director switches 44 in the "upper" and "lower" first and second limb portions 36a,36b,38a,38b are switchable to switch the corresponding limb portion 36a, 36b, 38a, 38b into or out of circuit between the corresponding AC and DC terminals 26,28,40,42. Accordingly the director switches 44 in the "upper" and "lower" first and second limb portions 36a,36b,38a,38b dictate which limb portion 36a,36b,38a,38b is in conduction and thereby is in use to control the configuration of the AC voltage 66,68 at the corresponding AC terminal 40,42. The configuration of the AC voltage 66,68 at each AC terminal 40,42 is controlled by combining first and second AC voltage components.

The operation of the first converter 22 to combine first and second AC voltage components to control the configuration of the AC voltage 66 at the first AC terminal 40 is described as follows.

To construct the first AC voltage component, the "upper" first limb portion 36a is in a conducting state by way of its director switch 44 being switched on, and the controller 65 controls the switching of the switching elements in each first module 46a of the "upper" first limb portion 36a to "push up" (add voltage steps to) and "pull down" (subtract voltage steps from) the positive DC voltage at the first DC terminal 26. The first AC voltage component is constructed to be in the form of a positive, half-sinusoidal voltage waveform. Meanwhile the "lower" first limb portion 36b is in a non-conducting state by way of its director switch 44 being switched off. To construct the second AC voltage component, the "lower" first limb portion 36b is in a conducting state by way of its director switch 44 being switched on, and the controller 65 controls the switching of the switching elements in each first module 46a of the "lower" first limb portion 36b to "push up" (add voltage steps to) and "pull down" (subtract voltage steps from) the negative DC voltage at the second DC terminal 28. The second AC voltage component is constructed to be in the form of a negative, half-sinusoidal voltage waveform. Meanwhile the "upper" first limb portion 36a is in a non-conducting state by way of its director switch 44 being switched off. It will be understood that the described operation of the first converter 22 to combine first and second AC voltage components to control the configuration of the AC voltage 66 at the first AC terminal 40 applies mutatis mutandis to the operation of the second converter 24 to combine first and second AC voltage components to control the configuration of the AC voltage 68 at the second AC terminal 42.

The AC voltage 66 at the first AC terminal 40 is phase-shifted relative to the AC voltage 68 at the second AC terminal 42 to generate a voltage 70 across the inductor 58 of the AC transmission link 56 and thereby cause a current 72 to flow from the first AC terminal 40 to the second AC terminal 42, as shown in Figure 3. In this manner the first DC to DC converter assembly 20 is operated to transfer power from the first DC electrical network 32 to the second DC electrical network 34. The energy transferred from the first DC electrical network 32 to the second DC electrical network 34 flows through the first DC to DC converter assembly 20, and therefore flows through the energy storage device 48 of each module 46a,46b that is controlled to selectively provide a voltage source, during the operation of the first DC to DC converter assembly 20 to transfer power from the first DC electrical network 32 to the second DC electrical network 34. Such a flow of energy through the energy storage device 48 of each module 46a,46b could result in energy accumulation in (or energy loss from) at least one energy storage device 48, thus resulting in deviation of the energy level of at least one energy storage device 48 from a reference value. Such a deviation is undesirable because, if too little energy is stored within a given energy storage device 48 then the voltage the corresponding module 46a,46b is able to generate is reduced, whereas if too much energy is stored in a given energy storage device 48 then over-voltage problems may arise. The former would require the addition of a power supply to restore the energy level of the affected energy storage device 48 to the reference value, while the latter would require an increase in voltage rating of one or more energy storage devices 48 to prevent the over-voltage problems, thus adding to the overall size, weight and cost of the first DC to DC converter assembly 20.

To enable regulation of the energy level of the first DC to DC converter assembly 20, the controller 65 is configured to selectively control switching of the director switch 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage 66,68 at the corresponding AC terminal 40,42 to form a current circulation path 74 and inject a circulation current 76 into the current circulation path 74.

To form the current circulation path 74, the controller 65 controls switching of the director switch 44 and switching elements in each module 46a,46b of the "upper" first and second limb portions 36a,38a to generate a voltage 78a,78b across each chain-link converter 54 of the "upper" first and second limb portions 36a, 38a, as shown in Figure 4a, so as to generate a differential AC voltage between the first and second AC terminals 40,42. The current circulation path 74 is formed during a common conduction period 80 of the AC transmission link 56. The common conduction period 80 of the AC transmission link 56 is the period in which the "upper" first and second limb portions 36a, 38a are both switched into conduction (i.e. switched into circuit between the corresponding AC and DC terminals 26,28,40,42) to transfer power between the DC electrical networks 32,34. Since the operation of the first DC to DC converter assembly 20 to transfer power between the DC electrical networks 32,34 requires the AC voltage 66 at the first AC terminal 40 to be phase-shifted relative to the AC voltage 68 at the second AC terminal 42, the period in which each of the "upper" first and second limb portions 36a, 38a are switched into conduction is longer than the common conduction period 80., as shown in Figure 4b. Meanwhile, during the formation of the current circulation path 74, the "lower" first and second limb portions 36b,38b are switched out of circuit by virtue of their director switches 44 being switched off.

Accordingly, as shown in Figure 4c, the current circulation path 74 passes through the "upper" first limb portion 36a, the AC transmission link 56, the "upper" second limb portion 38a, the "upper" DC link capacitor 62a of the auxiliary limb 60 of the second converter 24, the current return path 67 and the "upper" DC link capacitor 62a of the auxiliary limb 60 of the first converter 22. The differential AC voltage between the first and second AC terminals 40,42 results in injection of a common mode current 76 into the current return path 67 (and therefore injection of a circulation current 76 into the current circulation path 74).

The circulation current 76 (as shown in Figure 4d) combines with the voltage 82a, 82b across the chain-link converter 54 of each "upper" limb portion 36a,38a during the common conduction period 80 to modify the power 84a, 84b generated in each "upper" limb portion 36a, 38a and, in turn, affect the energy 86a, 86b stored in the chain-link converter 54 of each "upper" limb portion 36a,38a. During the common conduction period 80, the polarity of the voltage 82a across the chain-link converter 54 of the "upper" first limb portion 36a is opposite to the polarity of the voltage 82b across the chain-link converter 54 of the "upper" second limb portion 38a. Consequently the circulation current 76 flowing through the current circulation path 74 has an opposite effect on each of the powers 84a,84b generated in each of the "upper" limb portions 36a, 38a (as shown in Figure 4d).

In addition, as shown clearly in Figure 4d, the flow of the circulation current 76 through the current circulation path 74 results in an increase in energy 86a stored in the chain- link converter 54 of the "upper" first limb portion 36a at the end of the common conduction period 80, and in a decrease in energy 86b stored in the chain-link converter 54 of the "upper" second limb portion 38a at the end of the common conduction period 80. In the foregoing manner formation of the current circulation path 74 and injection of the circulation current 76 into the current circulation path 74 during the common conduction period 80 causes a transfer of energy from the chain-link converter 54 of the "upper" second limb portion 38a to the chain-link converter 54 of the "upper" first limb portion 36a. Similarly the current circulation path 74 can be formed by the controller 65 controlling switching of the director switch 44 and switching elements in each module 46a, 46b of the "lower" first and second limb portions 38 to generate a voltage across each chain-link converter 54 of the "lower" first and second limb portions 38, and thereby generate a differential AC voltage between the first and second AC terminals 40,42. In this case, the current circulation path 74 is formed during a common conduction period 88 of the AC transmission link 56 in which the "lower" first and second limb portions 36b,38b are both switched into conduction to transfer power between the DC electrical networks 32,34. Meanwhile, during the formation of the current circulation path 74, the "upper" first and second limb portions 36a,38a are switched out of circuit by virtue of their director switches 44 being switched off.

Accordingly the current circulation path 74 passes through the "lower" first limb portion 36b, the AC transmission link 56, the "lower" second limb portion 38b, the "lower" DC link capacitor 62b of the auxiliary limb 60 of the second converter 24, the current return path 67 and the "lower" DC link capacitor 62b of the auxiliary limb 60 of the first converter 22. Injection of a circulation current 76 into the current circulation path 74 causes transfer of energy from the chain-link converter 54 of the "lower" second limb portion 38b to the chain-link converter 54 of the "lower" first limb portion 36b in the same manner as the above-described transfer of energy from the chain-link converter 54 of the "upper" second limb portion 38a to the chain-link converter 54 of the "upper" first limb portion 36a. The formation of the current circulation path 74 therefore allows energy to be transferred between at least one first module 46a and at least one second module 46b, and so permits regulation of the energy stored in a given energy storage device 48, thereby obviating the problems associated with a deviation of the energy level of at least one energy storage device 48 from the reference value.

Figure 5 illustrates, in graph form, a comparison of the voltages 82a,82b, powers 8a,84b and energies 86a,86b for the first and second converters 22,24 during the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74. It can be seen from Figure 5 that the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74 alone is insufficient to balance the energies 86a, 86b of the first and second converters 22,24. This is due to a difference in DC voltages of the first and second DC electrical networks 32,34. This in turn would result in the first DC to DC converter assembly 20 experiencing an energy drift (i.e. the net change in energy level of the first DC to DC converter assembly 20 is not equal to zero), which adversely affects balancing of the energies 86a,86b of the first and second converters 22,24.

In order to prevent the aforementioned energy drift of the first DC to DC converter assembly 20, the controller 65 is further configured to selectively control switching of the director switch 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage at the corresponding AC terminal 40,42 so as to minimise a net change in energy level of the first DC to DC converter assembly 20 during a transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74.

More particularly, in order to minimise a net change in energy level of the first DC to DC converter assembly 20 during a transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74, the controller 65 is further configured to selectively control switching of the director switch 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage at the corresponding AC terminal 40,42 to:

equate a net change in energy level of the first converter 22 with a minus of net change in energy levels of the first limb portions 36a,36b caused by the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74 (shown below as Equation 1); and equate a net change in energy level of the second converter 24 with a minus of net change in energy levels of the second limb portions 38a,38b caused by the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74 (shown below as Equation 2).

AE_convl = -AE_cl (1) AE_conv2 = -AE_C2 (2) where ΔΕ is the net change in energy level of the first converter 22,

AEci is the net change in energy levels of the first limb portions 36a,36b caused by the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74,

ΔΕ is the net change in energy level of the second converter 24,

ΔΕο2 is the net change in energy levels of the second limb portions 38a, 38b caused by the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74. Switching of the director switch 44 and switching elements in each module 46a, 46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage at the corresponding AC terminal 40,42 to comply with Equations 1 and 2 results in generation of an optimal AC voltage at each AC terminal 40,42 that minimises a net change in energy level of the first DC to DC converter assembly 20 during the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74.

The configuration of the controller 65 of the first DC to DC converter assembly 20 therefore permits minimisation of a net change in energy level of the first DC to DC converter assembly 20 (preferably over an operating cycle of the DC electrical networks 32,34) by way of generation of an optimal AC voltage at each AC terminal 40,42. This permits regulation of the energy level of the first DC to DC converter assembly 20 and thereby not only makes it easier to regulate the energy stored in a given energy storage device 48, thereby obviating the problems associated with a deviation of the energy level of at least one energy storage device 48 from the reference value, but also enables energy balancing of the first and second converters 22,24.

Optionally minimising a net change in energy level of the first DC to DC converter assembly 20 may include maintaining a zero net change in energy level of the first DC to DC converter assembly 20. This further improves regulation of the energy level of the first DC to DC converter assembly 20, and any associated regulation of the energy stored in a given energy storage device 48.

Figure 6 illustrates, in schematic form, variations in DC and AC power 180,182 for the first converter 22, a variation in power 184 for the first limb portions 36a, 36b resulting from a net change in energy levels of the first limb portions 36a,36b caused by the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74, variations in DC and AC power 186,188 for the second converter 24, and a variation in power 190 for the second limb portions 36a,36b resulting from a net change in energy levels of the first limb portions 36a,36b caused by the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74, during the switching of the director switch 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage at the corresponding AC terminal 40,42 to comply with Equations 1 and 2.

It can be seen from Figure 6 that the variation in AC power 182 for the first converter 22 is identical to the variation in AC power 188 for the second converter 24.

It can also be seen from Figure 6 that a sum of net changes in AC energy exported by the first converter 22 (as defined by the area under the curve for the AC power 182) and in energy levels of the first limb portions 36a,36b (as defined by the area under the curve for the power 184 for the first limb portions 36a, 36b resulting from a net change in energy levels of the first limb portions 36a,36b caused by the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74) is equal to a net change in DC energy imported by the first converter 22 from the first DC electrical network 32 (as defined by the area under the curve for the DC power 180. It can further be seen from Figure 6 that a sum of net changes in AC energy imported by the second converter 24 (as defined by the area under the curve for the AC power 188) and in energy levels of the second limb portions 38a, 38b (as defined by the area under the curve for the power 190 for the second limb portions 38a, 38b resulting from a net change in energy levels of the first limb portions 36a,36b caused by the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74) is equal to a net change in DC energy exported by the second converter 24 to the second DC electrical network 34 (as defined by the area under the curve for the DC power 188).

Selection of the aforementioned optimal AC voltage at each AC terminal 40,42 therefore allows control of the switching of the director switch 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage at the corresponding AC terminal 40,42 to:

equate a sum of net changes in energy exchanged between the first and second converters 22,24 and in energy levels of the first limb portions 36a,36b caused by the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74 with a net change in energy imported by the first converter 22 from the first DC electrical network 32 (shown below as Equation 3); and

equate a sum of net changes in energy exchanged between the first and second converters 22,24 and in energy levels of the second limb portions 38a,38b caused by the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74 with a net change in energy exported by the second converter 24 to the second DC electrical network 34 (shown below as Equation 4).

AE_AC + AE_cl = AE_DC{ (3)

M_AC + AE_a = ΔΕ^ (4) where AEAC is the net change in energy exchanged between the first and second converters 22,24,

AEDCI is the net change in energy imported by the first converter 22 from the first

DC electrical network 32, AEDCZ is the net change in energy exported by the second converter 24 to the second DC electrical network 34.

A change in operating parameter of the first DC to DC converter assembly 20 may arise as a result of, for example, a fault or disturbance within the first DC to DC converter assembly 20 or within either or both of the first and second DC electrical networks 32,34. The change in operating parameter of the first DC to DC converter assembly 20 may be any of:

• a change in DC voltage of either or each of the first and second DC electrical networks 32,34;

• a change in energy storage capacity in at least one capacitor 48 of at least one module 46a,46b.

The change in operating parameter of the first DC to DC converter assembly 20, if left unchecked, may result in a considerable net change in energy level of the first DC to DC converter assembly 20.

Optionally the controller 65 may be configured such that, in response to a change in operating parameter of the first DC to DC converter assembly 20, the controller 65 selectively controls switching of the director switch 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b to generate a new optimal AC voltage at the corresponding AC terminal 40,42 that complies with Equations 3 and 4. As such the controller 65 is able to minimise a net change in energy level of the first DC to DC converter assembly 20 during a transfer of energy between at least one first module 46a and at least one second module 46b in response to a change in operating parameter of the first DC to DC converter assembly 20.

Configuration of the controller 65 in this manner enhances the capability of the controller 65 to carry out real-time regulation of the energy level of the first DC to DC converter assembly 20 in response to a change in operating parameter of the first DC to DC converter assembly 20, thus resulting in a more reliable first DC to DC converter assembly 20.

In the foregoing manner the controller 65 of the first DC to DC converter assembly 20 is configured to selectively control switching of the director switch 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage at the corresponding AC terminal 40,42 to transfer energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74 and also to minimise a net change in energy level of the first DC to DC converter assembly 20 during the transfer of energy between at least one first module 46a and at least one second module 46b.

Since the AC transmission link 56 is only connected between the first and second converters 22,24 and is not directly connected to an AC grid, it is not necessary to comply with grid electrical regulations when controlling the configuration of the AC voltage at the corresponding AC terminal to minimise a net change in energy level of the first DC to DC converter assembly 20 during the transfer of energy between at least one first module 46a and at least one second module 46b, thus avoiding any problems associated with non-compliance with grid electrical regulations.

Regulating the energy level of the first DC to DC converter assembly 20 whilst power is being transferred between the DC electrical networks 32,34 not only minimises or eliminates the need to bring the first DC to DC converter assembly 20 offline to minimise a net change in energy level of the first DC to DC converter assembly 20, but also enables optimal operation of the first DC to DC converter assembly 20 through real-time regulation of the energy level of the DC to DC converter assembly.

A second DC to DC converter assembly 120 according to a second embodiment of the invention is shown in Figure 7. The second DC to DC converter assembly 120 of Figure 7 is similar in structure and operation to the first DC to DC converter assembly 20 of Figure 1 , and like features share the same reference numerals.

The second DC to DC converter assembly 120 differs from the first DC to DC converter assembly 20 in that the second DC to DC converter assembly 120 includes a plurality of AC transmission links 56a, 56b, 56c and each converter 22,24 includes three converter limbs 30.

Each first AC terminal 40 is connected to a respective one of the second AC terminals 42 via a respective one of the plurality of AC transmission links 56a,56b,56c. Each first AC terminal 40 is connected at a first end of the corresponding AC transmission link 56a, 56b, 56c. Each second AC terminal 42 is connected at a second end of the corresponding AC transmission link 56a,56b,56c. The operation of the second DC to DC converter assembly 120 to transfer power between the DC electrical networks 32,34 is similar to the operation of the first DC to DC converter assembly 20 to transfer power between the DC electrical networks 32,34 in that:

· the earlier-described operation of the limb portions 36a, 36b in the single converter limb 30 of the first converter 22 in the first DC to DC converter assembly 20 applies mutatis mutandis to the operation of the limb portions 36a, 36b in each converter 22,24 limb of the first converter 22 in the second DC to DC converter assembly 120; and

· the earlier-described operation of the limb portions 38a, 38b in the single converter limb 30 of the second converter 24 in the first DC to DC converter assembly 20 applies mutatis mutandis to the operation of the limb portions 38a, 38b in each converter 22,24 limb of the second converter 24 in the second DC to DC converter assembly 120.

Furthermore, as shown in Figure 8, the AC voltage 66a,66b,66c,68a,68b,68c at each AC terminal 40,42 of a given converter 22,24 is also phase-shifted by 120 electrical degrees relative to the AC voltage 66a,66b,66c,68a,68b,68c at each other AC terminal 40,42 of the given converter 22,24 during the operation of the DC to DC converter assembly to transfer power between the DC electrical networks 32,34.

To enable regulation of the energy level of each energy storage device 48 in each module 46a,46b of the first and second converters 22,24 of the second DC to DC converter assembly 120, a current circulation path 74 may be formed in the second DC to DC converter assembly 120 in respect of each AC transmission link 56 and the corresponding limb portions in the same manner as the current circulation path 74 formed in the first DC to DC converter assembly 20.

Formation of a current circulation path 74 in the second DC to DC converter assembly 120 in this manner not only provides a means of transferring energy between at least one first module 46a and at least one second module 46b, but also is particularly useful when it is required to limit the number of limb portions 36a, 36b, 38a, 38b that are connected in the current circulation path 74. Another way of enabling regulation of the energy level of each energy storage device 48 in each module 46a,46b of the first and second converters 22,24 of the second DC to DC converter assembly 120 is by forming an alternative current circulation path 90 with a different configuration.

To form the alternative current circulation path 90, the controller 65 controls:

• switching of the director switch 44 and switching elements in each module 46a,46b of "upper" first and second limb portions 36a, 38b, that are connected at both ends of a first AC transmission link 56a, to generate a voltage across each chain-link converter 54 of the "upper" first and second limb portions 36a,38a so as to generate a differential AC voltage between the first and second AC terminals 40,42 connected at both ends of the first AC transmission link 56a; and

• switching of the director switch 44 and switching elements in each module 46a,46b of "lower" first and second limb portions 36a, 38b, that are connected at both ends of a second AC transmission link 56b, to generate a voltage across each chain-link converter 54 of the "lower" first and second limb portions 38 so as to generate a differential AC voltage between the first and second AC terminals 40,42 connected at both ends of the second AC transmission link 56.

The alternative current circulation path 90 is formed during an overlap 92 between the common conduction periods 80a, 80b of the first and second AC transmission links 56a, 56b, so as to enable concurrent formation of the alternative current circulation path 90 and transfer of power between the DC electrical networks 32,34.

Accordingly, as shown in Figure 9, the alternative current circulation path 90 passes through a first current circulation path portion, a second current circulation path portion and the auxiliary limbs 60 of the first and second converters 22,24. The first current circulation path portion includes the first AC transmission link 56a, the "upper" first limb portion 36a connected at a first end of the first AC transmission link 56a, and the "upper" second limb portion 38a connected at a second end of the first AC transmission link 56a. The second current circulation path portion includes the second AC transmission link 56b, the "lower" first limb portion 36b connected at a first end of the second AC transmission link 56b, and the "lower" second limb portion 38b connected at a second end of the second AC transmission link 56b. The respective differential AC voltage between the first and second AC terminals 40,42 connected at both ends of a respective one of the first and second AC transmission links 56a, 56b results in injection of a circulation current 76 into the alternative current circulation path 90. The circulation current 76 (as shown in Figure 10) combines with the voltage 94a,94b,94c,94d across the chain-link converter 54 of each limb portion 36a,36b,38a,38b in the alternative current circulation path 90 during an overlap 92 between the common conduction periods 80a, 80b of the first and second AC transmission links 56a,56b to modify the power 96a,96b,96c,96d generated in each limb portion 36a,36b,38a,38b in the alternative current circulation path 90 and, in turn, affect the energy 98a,98b,98c,98d stored in the chain-link converter 54 of each limb portion 36a, 36b, 38a, 38b in the alternative current circulation path 90. During the overlap 92 between the common conduction periods 80a,80b of the first and second AC transmission links 56a, 56b, the polarities of the voltages 94a,94c across the chain-link converters 54 of the first limb portions 36a, 36b of the first converter 22 that are connected in the alternative current circulation path 90 are opposite to the polarities of the voltages 94b,94d across the chain-link converters 54 of the second limb portions 38a,38b of the second converter 24 that are connected in the alternative current circulation path 90. Consequently the circulation current 76 flowing through the alternative current circulation path 90 not only has an opposite effect on each of the powers 96a,96b generated in each of the "upper" limb portions 36a, 38a connected at both ends of the first AC transmission link 56a, but also has an opposite effect on each of the powers 96c,96d generated in each of the "lower" limb portions 36b,38b connected at both ends of the second AC transmission link 56b.

As shown clearly in Figure 10, the flow of the circulating current 76 through the alternative current circulation path 90 results in an increase in energy 98a stored in the chain-link converter 54 of the "upper" first limb portion 36a connected to the first AC transmission link 56a at the end of the overlap 92 between the common conduction periods 80a,80b of the first and second AC transmission links 56a, 56b, and in a decrease in energy 98b stored in the chain-link converter 54 of the "upper" second limb portion 38a connected to the first AC transmission link 56a at the end of the overlap 92 between the common conduction periods 80a, 80b of the first and second AC transmission links 56a,56b. Also, as shown clearly in Figure 10, the flow of the circulating current 76 through the alternative current circulation path 90 results in an increase in energy 98c stored in the chain-link converter 54 of the "lower" first limb portion 36b connected to the second AC transmission link 56b at the end of the overlap 92 between the common conduction periods 80a, 80b of the first and second AC transmission links 56a, 56b, and in a decrease in energy 98d stored in the chain-link converter 54 of the "lower" second limb portion 38b connected to the second AC transmission link 56b at the end of the overlap 92 between the common conduction periods 80a, 80b of the first and second AC transmission links 56a,56b.

In the foregoing manner formation of the alternative current circulation path 90 and injection of the circulation current 76 into the alternative current circulation path 90 during the overlap 92 between the common conduction periods 80a, 80b of the first and second AC transmission links 56a, 56b enables regulation of the energy level of each energy storage device 48 in the limb portions 36a,36b,38a,38b of multiple converter limbs 30 of the first and second converters 22,24 of the second DC to DC converter assembly 120.

The inclusion of the first and second current circulation path portions in the alternative current circulation path 90 permits the alternative current circulation path 90 to be formed within the converters 22,24 and first and second AC transmission links 56a, 56b, thus obviating the need for inclusion of the above-described current return path 67 in the alternative current circulation path 90. Therefore, the second DC to DC converter assembly 120 may omit the current return path 67.

It is envisaged that the alternative current circulation path 90 may be formed to further include a third current circulation path portion in addition to the first and second current circulation path portion, wherein the third current circulation path portion includes a third AC transmission link 56c, at least one first limb portion 36a, 36b connected at a first end of the third AC transmission link 56c, and at least one second limb portion 38a,38b connected at a second end of the third AC transmission link 56c. In this case, the alternative current circulation path 90 is formed during an overlap between the common conduction periods 80a,80b,80c of the first, second and third AC transmission links 56a,56b,56c.

It is further envisaged that the combination of the first and second current circulation path portions in the alternative current circulation path 90, as set out above, may be replaced by a combination of the second and third current circulation path portions, or a combination of the first and third current circulation path portions.

Forming the alternative current circulation path 90 during an overlap 92 between the common conduction periods 80a, 80b of the first and second AC transmission links 56a, 56b limits the period in which energy can be transferred to or from a given energy storage device 48 to regulate its energy level during the transfer of power between the DC electrical networks 32,34. To extend the period in which energy can be transferred to or from a given energy storage device 48 to regulate its energy level during the transfer of power between the DC electrical networks 32,34, the controller 65 is configured to selectively control switching of the director switches 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage at the corresponding AC terminal 40,42 to first form a first current circulation path, the first current circulation path being identical to the aforementioned alternative current circulation path 90, followed by a second current circulation path.

The second current circulation path passes through a first current circulation path portion, a third current circulation path portion and the auxiliary limbs 60 of the first and second converters 22,24. The first current circulation path portion includes the first AC transmission link 56a, the "upper" first limb portion 36a connected at a first end of the first AC transmission link 56a, and the "upper" second limb portion 38a connected at a second end of the first AC transmission link 56a. The third current circulation path portion includes the third AC transmission link 56c, the "lower" first limb portion 36b connected at a first end of the third AC transmission link 56c, and the "lower" second limb portion 38b connected at a second end of the third AC transmission link 56c.

Sequential formation of the first and second current circulation paths in this manner extends the period in which energy can be transferred to or from a given energy storage device 48 of the limb portions 36a, 38a connected at both ends of the first AC transmission link 56a to regulate its energy level during the transfer of power between the DC electrical networks 32,34. It will be appreciated that, whilst the sequential formation of the first and second current circulation paths is described above with respect to the first AC transmission link 56a, the described formation of the first and second current circulation paths applies mutatis mutandis to each of the second and third AC transmission links 56b, 56c to extend the period in which energy can be transferred to or from a given energy storage device 48 of the limb portions 36a,36b,38a,38b connected at both ends of each of the second and third AC transmission links 56b,56c.

In a similar fashion to the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the current circulation path 74 in the first DC to DC converter assembly 20 of Figure 1 , the transfer of energy between at least one first module 46a and at least one second module 46b through formation of the alternative current circulation path 90 alone is insufficient to balance the energies 98a,98b,98c,98d of the first and second converters 22,24. This is due to a difference in DC voltages of the first and second DC electrical networks 32,34. This in turn would result in the second DC to DC converter assembly 120 experiencing an energy drift (i.e. the net change in energy level of the second DC to DC converter assembly 120 is not equal to zero), which adversely affects balancing of the energies 98a,98b,98c,98d of the first and second converters 22,24.

In order to prevent the aforementioned energy drift of the second DC to DC converter assembly 120, the controller 65 of the second DC to DC converter assembly 120 is further configured in a similar manner to the controller 65 of the first DC to DC converter assembly 20 to enable selective control of the switching of the director switch 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage at the corresponding AC terminal 40,42 so as to minimise a net change in energy level of the first DC to DC converter assembly 20 during a transfer of energy between at least one first module 46a and at least one second module 46b through formation of the alternative current circulation path 90.

For each of the embodiments shown, the controller controls switching of the director switch 44 and switching elements in each module 46a,46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage at the corresponding AC terminal 40,42 so as to minimise a net change in energy level of the DC to DC converter assembly 20,120 during a transfer of energy between at least one first module 46a and at least one second module 46b and during the transfer of power between the DC electrical networks 32,34 for optimal performance of the DC to DC converter assembly 20,120. It will be appreciated however that the controller may control switching of the director switch 44 and switching elements in each module 46a, 46b of each limb portion 36a,36b,38a,38b to control the configuration of the AC voltage at the corresponding AC terminal 40,42 so as to minimise a net change in energy level of the DC to DC converter assembly 20,120 during a transfer of energy between at least one first module 46a and at least one second module 46b when the DC to DC converter assembly 120 is not being operated to transfer power between the DC electrical networks 32,34.

In other embodiments of the invention, it is envisaged that the number of converter limbs and AC transmission links in the DC to DC converter assembly may vary in number. The number of converter limbs and AC transmission links would depend on the requirements of the DC to DC converter assembly (such as converter voltage rating, active filtering capability). In still other embodiments of the invention, it is envisaged that each limb portion may omit the director switch. Omission of the director switch from each limb portion would require the plurality of series-connected modules to provide a voltage to offset the DC voltage at the corresponding DC terminal so as to configure the limb portion in a nonconducting state.

It will be appreciated that, whilst each of the embodiments of Figures 1 and 7 is primarily described with reference to the formation of a current circulation path 74,90 to transfer energy between at least one first module 46a and at least one second module 46b, the operation of the embodiments of Figures 1 and 7 to minimise a net change in energy level of the DC to DC converter assembly 20,120 during a transfer of energy between at least one first module 46a and at least one second module 46b may be carried out in respect of other ways of controlling switching of the switching elements in each module 46a,46b so as to transfer energy between at least one first module 46a and at least one second module 46b.

Claims

1. A DC to DC converter assembly for interconnecting first and second DC electrical networks, the DC to DC converter assembly comprising:

2. A DC to DC converter assembly according to any preceding claim wherein minimising a net change in energy level of the DC to DC converter assembly includes maintaining a zero net change in energy level of the DC to DC converter assembly.

3. A DC to DC converter assembly according to any preceding claim wherein the controller is configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal so as to concurrently:

minimise a net change in energy level of the DC to DC converter assembly during a transfer of energy between at least one first module and at least one second module; and

transfer power between the DC electrical networks.

4. A DC to DC converter assembly according to any preceding claim wherein the controller is configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal to:

equate a net change in energy level of the first converter with a net change in energy levels of the first limb portions caused by a transfer of energy between at least one first module and at least one second module; and

equate a net change in energy level of the second converter with a net change in energy levels of the second limb portions caused by a transfer of energy between at least one first module and at least one second module,

5. A DC to DC converter assembly according to any preceding claim wherein the controller is configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal to:

6. A DC to DC converter assembly according to any preceding claim wherein the controller is configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal so as to minimise a net change in energy level of the DC to DC converter assembly during a transfer of energy between at least one first module and at least one second module in response to a change in operating parameter of the DC to DC converter assembly.

7. A DC to DC converter assembly according to Claim 6 wherein the change in operating parameter of the DC to DC converter assembly may be any one of:

· a change in DC voltage of either or each of the first and second DC electrical networks;

• a change in energy storage capacity of at least one energy storage device of at least one module.

8. A DC to DC converter assembly according to any preceding claim wherein the controller is configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal to:

9. A DC to DC converter assembly according to Claim 8 wherein each converter further includes an auxiliary limb extending between the corresponding first and second DC terminals, the auxiliary limb being connected in parallel with the corresponding converter limb, the current circulation path further including at least part of the auxiliary limb.

10. A DC to DC converter assembly according to Claim 9 wherein the auxiliary limb of each converter includes an auxiliary terminal, and the auxiliary terminals of the first and second converters are electrically interconnected by a current return path, the current circulation path including the current return path.

11. A DC to DC converter assembly according to Claim 10 wherein each auxiliary limb includes a pair of DC link capacitors separated by the corresponding auxiliary terminal.

12. A DC to DC converter assembly according to any of Claims 8 to 11 including a plurality of AC transmission links, wherein each converter includes a plurality of converter limbs, each first AC terminal being connected to a respective one of the second AC terminals via a respective one of the plurality of AC transmission links, each first AC terminal being connected at a first end of the corresponding AC transmission link, each second AC terminal being connected at a second end of the corresponding AC transmission link, the current circulation path including a first current circulation path portion and at least one further current circulation path portion,

wherein the or each further current circulation path portion includes: a further AC transmission link; at least one first limb portion connected at a first end of the further AC transmission link; and at least one second limb portion connected at a second end of the further AC transmission link.

13. A DC to DC converter assembly according to Claim 12 when dependent from Claim 11 wherein the controller is configured to selectively control switching of the or each switching element in each limb portion to control the configuration of the AC voltage at the corresponding AC terminal to form a first current circulation path including the first current circulation path portion and a further current circulation path portion, followed by a second current circulation path including the first current circulation path portion and another further current circulation path portion.

14. A method of controlling a DC to DC converter assembly for interconnecting first and second DC electrical networks, the DC to DC converter assembly comprising:

wherein the method comprises the steps of: