This invention relates to a control circuit .

In DC power transmission schemes, DC transmission lines are used to interconnect a transmitting electrical network and a receiving electrical network to permit transfer of power between the two electrical networks. In the event of a fault preventing the receiving electrical network from receiving power from the DC transmission lines, the transmitting electrical network cannot interrupt the transmission of power into the DC transmission lines. This is because generators, such as wind turbines, cannot be switched off instantaneously and so will continue to feed energy into the DC transmission lines. Moreover, the transmitting electrical network is required by a Grid Code to ride through a supply dip, e.g. where the voltage is reduced to approximately 15% of its original value, and to resume the transmission of power upon the removal of the fault.

Continuing to transmit power into the DC transmission lines results in an accumulation of excess power in the DC transmission lines which adversely affects the balance between the transmission and receipt of power by the respective electrical networks. In order to prevent this imbalance the excess power is diverted away from the DC transmission lines using a load dump chopper circuit. Existing chopper circuits are slow to react to the need to direct power away from the DC transmission lines, and so there is a need for an improved means of removing excess power from the DC transmission lines which operates more quickly.

According to a first aspect of the invention there is provided a control circuit comprising first and second DC terminals for connection to respective DC transmission lines, the first and second DC terminals having a plurality of modules and at least one energy conversion element connected in series therebetween to define a current transmission path, each module including at least one energy storage device, the or each energy storage device being selectively removable from the current transmission path to cause current to flow from the DC transmission lines through the current transmission path and the or each energy conversion element and thereby remove energy from the DC transmission lines.

The ability to selectively remove the or each energy storage device of each module from the current transmission path allows the immediate transfer of energy, i.e. excess power, from the DC transmission lines to the control circuit and thereby enables rapid regulation of the energy levels in the DC transmission lines . In addition the ability to selectively remove the or each energy storage device of each module from the current transmission path also permits balancing of the voltage level of the or each energy storage device in one module with the or each energy storage device in other modules.

Balancing the voltage of respective energy storage devices simplifies the control of the control circuit by allowing, for example, the use of average voltage value as feedback to control selective removal of the energy storage devices from the current transmission path.

Each module may include a pair of primary switching elements connected in parallel with the or each energy storage device to selectively direct current through at least one energy storage device and cause current to bypass the or each energy storage device . Optionally the primary switching elements and the or each energy storage device are arranged 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.

Such modules provide a reliable means of selectively removing the or each energy storage device from the current transmission path. Preferably the primary switching elements of each module are controllable to delay the removal and/or return of the or each energy storage device of one module from the current transmission path relative to the corresponding removal and/or return of the or each energy storage device of the other modules.

The provision of such primary switching elements permits voltage balancing of the energy storage devices during normal operation of the control circuit. For example, the primary switching elements may be operated to charge an undercharged energy storage device in one module using current flowing through the current transmission path while the or each energy storage device of the other modules are removed from the current transmission path.

In a preferred embodiment of the invention at least one primary switching element is or includes a semiconductor device. The or each semiconductor device is preferably an insulated gate bipolar transistor, a gate turn-off thyristor, a field effect transistor, an injection enhanced gate transistor or an integrated gate commutated thyristor.

Preferably at least one primary switching element further includes an anti-parallel diode connected in parallel with the or each corresponding semiconductor device. The fast switching capabilities of such semiconductor devices helps the control circuit to respond quickly to changes in energy levels in the DC transmission lines, and also enables fine control over the selective removal of respective energy storage devices from the current transmission path. Moreover, the inclusion of such semiconductor devices permits the use of pulse width modulation, if desired. Optionally the control circuit includes an equal number of modules connected in series on either side of a single energy conversion element.

The control circuit may include a plurality of energy conversion elements connected in series with the plurality of modules.

Preferably the energy conversion elements and the modules are arranged to define an alternating sequence of energy conversion elements and modules.

Such arrangements help to minimise the amount of stray capacitive current flowing between the control circuit and ground, and so reduce the amount of electromagnetic interference generated by the control circuit .

In a preferred embodiment of the invention each module includes a local discharge circuit connected in parallel with a first energy storage device, the local discharge circuit including at least one secondary resistor connected in series with at least one secondary switching element.

The provision of a local discharge circuit in each module permits local discharge of the corresponding first energy storage device, and so allows the continuous flow of current through each module. As a result the first energy storage device of each module can be selectively removed from the current transmission path individually, i.e. the first energy storage devices do not have to be all removed together.

It therefore becomes possible to vary the current flowing through the current transmission path using a step-wise approximation so as to accurately match a desired excess instantaneous current.

The inclusion of a local discharge circuit in each module therefore helps to minimise switching losses in the control circuit, and removes the need for a separate DC link capacitor between the DC transmission lines. Avoiding the need for such a capacitor reduces the size and cost of the associated power transmission system.

In another preferred embodiment of the invention each module includes an energy storage circuit connected in parallel with a first energy storage device. The inclusion of such an energy storage circuit allows the control circuit to store energy, instead of dissipating energy, when regulating the energy levels of the DC transmission lines. This stored energy can be returned to the DC transmission lines at a later stage.

Preferably the or each energy conversion element is a resistor.

The resistance value may be adjusted to match the requirements of the control circuit, such as, for example, the rate of dissipation of excess energy flowing into the control circuit from the DC transmission lines.

In other embodiments of the invention the or each energy conversion element is an inductor. The inclusion of one or more inductors facilitates the storage of energy removed from the DC transmission lines by permitting the diversion of current drawn from the DC transmission lines into an energy storage device.

The or each energy storage device may include a capacitor, a battery, or a fuel cell.

A respective energy storage device may be any device that is capable of storing and releasing electrical energy to provide a voltage. This flexibility is useful in designing control circuits in different locations where the availability of equipment may be limited due to locality or transport difficulties .

Preferably the control circuit further includes a bypass arrangement operably associated with at least one module, the or each bypass arrangement being operable to isolate the corresponding module from the current transmission path.

The inclusion of such a bypass arrangement allows the ready isolation of, e.g. a damaged module, while permitting the control circuit to continue normal operation without interruption.

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

Figure 1 shows a control circuit according to a first embodiment of the invention;

Figure 2 shows the connection of the first control circuit shown in Figure 1 to a DC power transmission scheme;

Figure 3 illustrates a current waveform shape during pulse width modulation;

Figures 4 illustrates a current transmission path in the first control circuit when delaying the removal of the energy storage device in one module from a current transmission path relative to other modules;

Figure 5 illustrates a current transmission path in the first control circuit when delaying the return of the energy storage device in one module to a current transmission path relative to other modules;

Figure 6 illustrates schematically the circuit inductance present in the first control circuit ;

Figure 7 shows a control circuit according to a second embodiment of the invention;

Figure 8 shows a control circuit according to a third embodiment of the invention;

Figure 9 shows a control circuit according to a fourth embodiment of the invention;

Figure 10 shows an alternative module including a local discharge circuit;

Figure 11 illustrates respective current transmission paths in the fourth control circuit when differing numbers of energy storage devices are removed from a current transmission path;

Figure 12 shows a control circuit according to a fifth embodiment of the invention; and

Figure 13 shows a control circuit according to a sixth embodiment of the invention.

A control circuit 40 according to a first embodiment of the invention is shown in Figure 1. The control circuit 40 comprises first and second DC terminals 42,44. In use, the first DC terminal 42 is connected to a first DC transmission line that is at a positive voltage, +Vdc/2, while the second DC terminal 44 is connected to a second DC transmission line that is at a negative voltage, -Vdc/2.

The control circuit 40 further includes a plurality of modules 46 that are connected in series with a primary resistor 48 between the first and second DC terminals 42, 44 to define a current transmission path. The primary resistor 48 is connected between the plurality of series-connected modules 46 and the first DC terminal 42, and is a fixed resistor with a resistance of Rrheo.

Each module 46 includes a pair 50 of first and second primary switching elements 50a, 50b connected in parallel with a first energy storage device in the form of capacitor 52. The primary switching elements 50a, 50b and the capacitor 52 are connected in a half-bridge arrangement which defines a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct current in two directions .

The primary capacitor 52 of each module 46 may be selectively removed from the current transmission path, i.e. switched in or out of circuit with the primary resistor 48, by changing the state of the primary switching elements 50a, 50b. This allows the current in the control circuit 40 to selectively flow through or bypass each capacitor 52.

The capacitor 52 of each module 46 is removed from the current transmission path, i.e. switched out of circuit with the primary resistor 48, when the pair 50 of primary switching elements 50a, 50b is configured to form a short circuit in the module 46, i.e. when the first primary switch element 50a is switched on and the second primary switching element 50b remains off. This causes the current in the control circuit 40 to pass through the short circuit and bypass the capacitor 52. Such a configuration enables the module 46 to provide a zero voltage.

The capacitor 52 of each module 46 is returned to the current transmission path, i.e. switched back into circuit with the primary resistor 48, when the pair 50 of primary switching elements 50a, 50b is configured to allow the current in the control circuit 40 to flow into and out of the capacitor 52, i.e. when the second primary switching element 50b is switched on and the first primary switching element 50a is switched off. The capacitor 52 is then able to charge or discharge its stored energy and provide a voltage .

It is envisaged that the pair 50 of first and second primary switching elements 50a, 50b may be replaced by other configurations that are capable of selectively removing a corresponding energy storage device, e.g. a capacitor 52, from the current transmission path in the aforementioned manner.

Each primary switching element 50a, 50b includes an insulated gate bipolar transistor connected in parallel with a diode. In other embodiments each primary switching element may include a gate turn-off thyristor, a field effect transistor, an injection enhanced gate transistor or an integrated gate commutated thyristor, or other force-commutated or self-commutated semiconductor switches.

In still further embodiments each capacitor may be replaced by another energy storage device such as a battery, or a fuel cell, or any device that is capable of storing and releasing electrical energy to provide a voltage.

The operation of the control circuit 40 shown in Figure 1 within a DC power transmission scheme is described below with additional reference to Figure 2.

First and second DC transmission lines interconnect first and second power converters 54a, 54b that are themselves connected to respective phases of corresponding first and second AC networks 56a, 56b. Power is transmitted from the first AC network 56a to the second AC network 56b via the corresponding power converters 54a, 54b and the first and second DC transmission lines. During normal operation the control circuit 40 adopts a standby configuration in which the capacitor 52 of each module 46 is connected in the current transmission path, i.e. switched into circuit with the primary resistor 48. Such a configuration is achieved by switching on each second primary switching element 50b.

The total voltage across the modules 46 is approximately equal to Vdc, which is the voltage across the DC transmission lines. In this configuration there is zero or minimal current flowing through the current transmission path, i.e. through the primary resistor 48 and the modules 46.

In the event that the second power converter 54b is unable to receive the transmitted power as a result of, for example, a fault in the second AC network 56b, the first AC network 56a must continue transmitting power into the DC transmission lines according to the established Grid Code.

In order to allow the first AC network 56a to continue transmitting power into the DC transmission lines via the first power converter 54a, the primary switching elements 50a, 50b of each module 46 are controlled to remove each capacitor 52 from the current transmission path, i.e. to switch each primary capacitor 52 out of circuit with the primary resistor 48. This is achieved by having each second primary switching element 50b turned off and each first primary switching element 50a switched on. This configuration imposes the voltage across the DC transmission lines, Vdc, across the primary resistor 48 and so a current flows through the primary resistor 48. The value of the current is equal to Vdc divided by Rrheo.

The flow of current through the primary resistor 48 enables excess energy in the DC transmission lines to be transferred to the control circuit 40 and dissipated via the primary resistor 48. The energy levels in the DC transmission lines are therefore regulated which helps to ensure power balance between each of the first and second AC networks 56a, 56b and the control circuit 40.

Following the removal of excess energy from the DC transmission lines, the primary switching elements 50a, 50b of the modules 46 are controlled to switch each capacitor 52 back into circuit with the primary resistor 48 i.e. each first primary switching element 50a is switched off and each second primary switching element 50b is switched on. Such a configuration turns off the current flowing in the control circuit 40.

Each DC transmission line is normally held at a constant DC voltage, within a tolerance band, which means that the instantaneous value of the current flowing into the control circuit 40 is always constant. However, it is desirable to allow the excess energy to be dissipated via the primary resistor 48 in a controlled manner. Such controlled energy dissipation can be achieved by varying the time averaged value of the current flowing through the primary resistor 48. In the first control circuit 40 this is done using Pulse Width Modulation (PWM) . The primary switching elements 50a, 50b are switched at high frequency (>lkHz) with different on-off time ratios 58a (first primary switching element 50a) , 58b (second primary switching element 50b) , as shown in Figure 3. This varies the energy transferred per switching cycle, and thereby varies the time averaged value of the current flowing through the primary resistor 48.

In practice, the voltage capacities of the capacitors 52 are similar but not identical, which means that respective capacitors 52 will assume a different level of charge.

Voltage balance between the capacitors 52 can be achieved by providing a time delay between the switching of the primary switching elements 50a, 50b of one module 46 and the switching of the primary switching elements 50a, 50b of one or more of the other modules 46.

For example, if a first capacitor 52' has a lower voltage than the other capacitors 52, this can be corrected by delaying the turn on of the first primary switching element 50a in the module 46 containing the depleted primary capacitor 52, so as to delay the removal of the depleted capacitor 52' from the current transmission path. During the delay current flowing through the current transmission path 60 flows into the depleted capacitor 52' while bypassing the other capacitors 52, as shown in Figure 4. This results in the depleted capacitor 52' being charged to restore voltage balance with the other capacitors 52.

If the first capacitor 52' has a higher voltage than the other capacitors 52 then this can be corrected by delaying the turn off of the first primary switching element 50a in the module 46 containing the overcharged capacitor 52', so as to delay the return of the overcharged capacitor 52' to the current transmission path. During the delay, the current flowing through the current transmission path 60 flows through the other capacitors 52 and bypasses the overcharged capacitor 52' as shown in Figure 5. This results in the other capacitors 52 being charged to restore the voltage balance with the overcharged capacitor 52 ' . A typical value for each capacitor 52 is

4mF and a typical current is lkA. This means that the rate of change of voltage in each capacitor 52 in the control circuit 40 is equal to 1000A/4mF, i.e. 0.25ν/μ3. Such a low rate of change of voltage in each capacitor 52 means that voltage balancing of the control circuit 40 is straightforward to perform. As a result the voltage capacity, and thereby the size and cost, of each capacitor 52 can be reduced.

Voltage balancing the control circuit 40 means that the voltage of any particular capacitor 52 can be kept approximately equal to an average voltage value. This simplifies the control and improves the performance of the control circuit 40 because it allows the use of average voltage value as feedback to control switching of the primary switching elements 50a, 50b.

Voltage balance between the capacitors 52 can also be achieved when the control circuit 40 adopts the standby configuration, i.e. during normal operation of the DC power transmission scheme.

While in the standby configuration any capacitors 52 having a higher voltage than other capacitors 52 are removed from the current transmission path. This causes current to flow through the primary resistor 48 and the lower charged capacitors 52. When acceptable balance is achieved between the capacitors 52, the first primary switching element 50a of each module 46 is turned off so that the previously overcharged capacitors 52 are returned into circuit with the primary resistor 48. This stops the current flowing in the control circuit 40.

Voltage balancing of the capacitors 52 while the control circuit 40 is in the standby configuration has minimal effect on the DC transmission lines because, although the instantaneous value of the current is high, the duration of the instantaneous current is short, which means that the amount of energy dissipated in the primary resistor 48 is low.

When current flows in the control circuit 40 energy is stored in stray inductances 62 that occur naturally in the control circuit 40 as a result of its construction, as illustrated schematically in Figure 6. An unwanted inductive current flows in the control circuit 40 when the energy stored in the stray inductances is released.

In the control circuit 40 of the invention the inductive current can be dealt with by directing it into the capacitor 52 of each module 46 via the diode of each corresponding primary switching element 50a, 50b. The flow of inductive current leads to a temporary charging of the capacitors 52 which are able subsequently to discharge, via the primary switching elements 50a, 50b and the primary resistor 48, into the DC transmission lines. Alternatively, if the additional charge on each capacitor 52 is sufficiently small, the additional charge can be stored on the capacitor 52 without significantly affecting the operation of the control circuit 40.

A control circuit 140 according to a second embodiment of the invention is shown in Figure 7. The second control circuit 140 is similar in structure and operation to the first control circuit 40 shown in Figure 1 and similar features share the same reference numerals. The second control circuit 140 differs, however, in that it includes an equal number of modules 46 connected in series on either side of a single primary resistor 48. Half of the modules 46 are connected in series between the first DC terminal 42 and a first end of the primary resistor 48 to define a set of top modules 46a, while the other half of the modules 146 are connected in series between the second DC terminal 44 and a second end of the primary resistor 148 to define a set of bottom modules 46b.

During the selective removal of the primary capacitor 52 in each module 46 from the current transmission path a change in voltage across any stray capacitance 64 from each of the top modules 46a is with respect to the voltage of the first DC transmission line 42, and a change in voltage across any stray capacitance 64 from each of the bottom modules 46b is with respect to the voltage of the second DC transmission line 44.

The resulting capacitive currents from the set of top modules 46a are therefore essentially equal and opposite to those of the set of bottom modules 46b, which results in a substantially net zero capacitive current flowing from the second control circuit 140 to earth 166. Having a zero or very low capacitive current flowing in the second control circuit 140 reduces the amount of electrical interference generated by the circuit, and so improves the operation of the second control circuit 140.

A control circuit 240 according to a third embodiment of the invention is shown in Figure 8. The third control circuit 240 is similar in structure and operation to the first control circuit 40 and similar features share the same reference numerals. However, the third control circuit 240 includes a plurality of second primary resistors 248, which together with a plurality of modules 46 are arranged to define an alternating sequence of second primary resistors 248 and modules 46. The alternating sequence is connected in series between the first and second DC terminals 42,44 to define a current transmission path. The total resistance of the plurality of second primary resistors 248 in the third control circuit 240 is chosen to equal to Rrheo, which is the resistance value of the single first primary resistor 48 in each of the first and second control circuits 40; 140.

During the selective removal of the capacitor 52 in each module 46 of the third control circuit 240, the change in voltage across each stray capacitance 64 with respect to the voltages of the first and second DC transmission lines is essentially equal to the voltage across the corresponding module 46. As a result the capacitive current flowing from each module 46 to earth 66 is identical for all the modules 46. The total capacitive current flowing from the third control circuit 240 to earth 66 is calculated as follows:

Total capacitive current = n-Ic

Where Ic is the capacitive current for the module 46 closest to the second DC transmission line; and n is the number of series-connected modules 46 in the control circuit 240.

The total capacitive current flowing between the third control circuit 240 and earth 66 is lower than, e.g. the total capacitive current flowing between the first control circuit 40 and earth 66, and so the third control current 240 experiences less electromagnetic interference.

A control circuit 340 according to a fourth embodiment of the invention is shown in Figure 9. The fourth control circuit 340 is similar in structure and operation to the first control circuit 40 and similar features share the same reference numerals. However, the fourth control circuit 340 includes a plurality of modified modules 346 each of which includes a local discharge circuit 368 connected in parallel with the capacitor 52. The local discharge circuit 368 includes a secondary resistor 370, which is a variable resistor, connected in series with a secondary switching element 372 Figure 10 shows another module 346' in which a second primary switching element 350b is in the form of a diode only. The voltage ratings of the secondary resistor 370 and the secondary switching element 372 must be sufficient to limit the voltage across the corresponding capacitor 52 during the flow of current through the capacitor 52. If the modified modules 346 are identical, the resistance of the secondary resistor 370 must be less than or equal to ≤ (4 x Rrheo/n) to handle a situation when half the modules 346 are required . If any capacitor 52 becomes overcharged, the secondary switching element 372 can be turned on to discharge the capacitor 52 via the corresponding secondary resistor 370 until the capacitor 52 reaches a desired voltage level.

The provision of the local discharge circuit 368 in each modified 346 permits current to flow continuously through the module 346 and thereby permits individual selective remove of the capacitors 52 from the current transmission path (rather than them all having to be selectively removed together) during regulation of the energy levels in the DC transmission lines. It is therefore possible to vary the current flowing through the fourth control circuit 340 using a step-wise approximation, instead of pulse width modulation, to accurately match the required instantaneous current.

In particular, when one capacitor 52 is removed from the current transmission path and the remaining capacitors 52 remain in circuit, the voltage across the primary resistor 48 is determined by the voltage across the removed capacitor 52. Assuming the voltages across each capacitor 52 are identical, the resulting current through the primary resistor 48 is given by Vdc/ (n-Rrheo) .

Such an arrangement is illustrated by a first current transmission path 374 in Figure 11.

The current through the primary resistor 48 can be increased by removing additional primary capacitors 52 from the current transmission path. For example, an additional capacitor 52 can be removed, as illustrated by a second current transmission path 376 in Figure 11.

The step-wise approximation of the required current may be improved by using a higher number of modules 346 with a lower voltage level across each one so as to provide a greater number of finer voltage steps .

The inclusion of a local discharge circuit 368, therefore removes the need to use pulse width modulation and so helps to reduce switching losses in the fourth control circuit 340.

Moreover, the ability to use step-wise approximation to match the required instantaneous current helps to ensure that the DC transmission lines are not discharged and overcharged by taking large currents for short intervals. The modular arrangement of the fourth control circuit 340 makes it possible to use any combination of capacitors 52 to provide a partial voltage and achieve a given resulting current. For example, if two capacitors 52 are required to be removed from the current transmission path, the capacitor 52 of any two series-connected modules 346 may be removed.

In addition, when two capacitors 52 are removed these same capacitors 52 can be returned into the current transmission path, and another two capacitors 32 can be removed without affecting the partial voltage and the resulting current flowing in the fourth control circuit 340. This allows the required power rating of the secondary resistor 370 and the secondary switching element 372 to be minimised because the switching of the capacitors 52, as outlined above, evens out the energy absorbed by each resistor 368. The modified modules 346, 346' can be used to replace each module 46 in other embodiments of the invention having one or more primary resistors 48, 248, and can be operated in the above described manner to perform similar functions.

For example, the third control circuit 240 shown in Figure 8 may be modified to include respective modified modules 346, 346', in place of the normal modules 46.

Such a modified control circuit 440 defines a fifth embodiment of the invention and is illustrated in Figure 12.

The total resistance of the plurality of primary resistors 248 in the fifth control circuit 440 is chosen to equal to Rrheo, and so the resistance of the secondary resistor 370 of each module 346 need only be less than or equal to 4 x Rrheo/n.

A control circuit 540 according to a sixth embodiment of the invention is shown in Figure 13. The sixth control circuit 540 is similar in structure and operation to the fifth control circuit 440, and similar features share the same reference numerals.

The sixth control circuit 540 differs, however, in that it includes respective inductors 580 in place of each primary resistor 248, and each module 546 includes an energy storage circuit 582 connected in parallel with the capacitor 52. Each energy storage circuit 582 includes a second energy storage device in the form of a battery 584, connected in series with a storage controller 586. The storage controller 586 may be omitted if desired.

The sum of the voltages across each module 546, i.e. the sum of the voltages across the corresponding primary capacitor 52 and battery 584, of each module 546 is configured to be greater than the voltage across the DC transmission lines, Vdc .

In other embodiments, the plurality of inductors 580 may be replaced by a single inductor, and/or each battery 584 may be replaced by a second capacitor, a fuel cell, a photovoltaic cell, an AC generator with associated rectifier or any device that is capable of storing and releasing electrical energy to provide a voltage. The capacitor 52 in each module 546 may also be omitted.

This provision of an energy storage circuit 582 allows the sixth control circuit 540 to store any excess energy removed from the DC transmission lines (as opposed to dissipating the excess energy) , and to return the stored energy to the DC transmission lines as desired.

To remove excess energy from the DC transmission lines, the first primary switching element 50a of one or more modules 546 is switched on to remove the corresponding capacitor 52 and associated energy storage circuit 582 from the current transmission path. This imposes a voltage across the primary inductors 580 and thereby establishes a current through the primary inductors 580. The or each first primary switching element 50a is then switched off to return each capacitor 52 and associated energy storage current 582 to the current transmission path. Current then flows into the capacitor 52 and the energy storage circuit 582, i.e. battery 584 of the corresponding module 546. Energy is therefore stored in the or each module 546 first removed from the current transmission path.

The foregoing steps may be repeated multiple times, depending on the amount of excess energy to be removed. The rate of change of the current is determined by the number of modules 546 removed initially. In this manner the sixth control circuit 540 acts as a boost chopper.

To return stored energy to the DC transmission lines, the second primary switching element 50b of one or more modules 546 is switched on to direct current to flow from the energy storage circuit 582 and capacitor 52 of each selected module 546 into the DC transmission lines. This is followed by switching the second primary switching element 50b off so that the current is diverted through the diode of the corresponding first primary switching element 50a. These steps result in the return of stored energy, from the energy storage circuit 582 of each module 546 initially returned to the current transmission path, to the DC transmission lines.

The steps may be repeated multiple times, depending on the amount of energy to be returned. The rate of change of the current is again determined by the number of modules 546 initially returned. In this manner the sixth control circuit 540 acts as a buck chopper .

The operation of the sixth control circuit 540 as described above provides additional control over the regulation of the energy levels in the DC transmission lines. In addition, the sixth control circuit 540 can be operated for long periods of time because the operational duration is limited only by the storage capability of the respective energy storage circuits, i.e. batteries 584.

In other embodiments of the invention (not shown) the control circuit 40; 140; 240; 340; 440; 540 may include a plurality of additional series-connected redundant modules.

During normal operation the capacitor 52 in each such redundant module is removed from the current transmission path, i.e. switched out of circuit, and so does not affect the regulation of energy levels in the DC transmission lines. In the event of damage to one or more operational modules, a redundant module can be returned to the current transmission path to perform the function of the damaged module. The reliability of the associated power transmission scheme is therefore further improved.

The modular arrangement of each of the control circuits mentioned hereinabove also means that it is straightforward to increase or decrease the number of modules in a given control circuit. As such, each of the control circuits mentioned hereinabove can be readily modified to match the voltage requirements of the associated power transmission scheme. Moreover, the use of such a modular arrangement decreases the complexity of the design, manufacture and assembly of each control circuit, which reduces the associated development costs and duration.

In other embodiments of the invention (not shown) the control circuit 40; 140; 240; 340; 440; 540 may further include a bypass arrangement operably associated with one or more modules 46; 346; 346'; 546. Each bypass arrangement is controllable to completely isolate the corresponding module from the current transmission path.

In the event of damage to a module, the corresponding bypass arrangement removes the damaged module from the current transmission path and so enables the control circuit to continue normal operation without interruption. The operation of the or each bypass arrangement may be coordinated with the switching of one or more redundant modules, as mentioned above, to allow the or each redundant module to take over the role of the damaged module in the control circuit.