GB2537868A

GB2537868A - Powering voltage source converters

Info

Publication number: GB2537868A
Application number: GB1507284.6A
Authority: GB
Inventors: Javier Chivite Zabalza Francisco; Christopher Nicholls Jonathan; Reginald Trainer David; Jose Moreno Munoz Francisco
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2015-04-29
Filing date: 2015-04-29
Publication date: 2016-11-02
Anticipated expiration: 2035-04-29
Also published as: EP3289677A1; CN107567679A; GB201507284D0; WO2016173825A1; GB2537868B

Abstract

A voltage source converter (VSC) has one or more switching modules that may form a director switch of a converter arm of the VSC. A power distribution system 200 of the VSC has a power distribution loop 201 and one or more power distribution modules 205 that provide power from the power distribution loop to the switching module(s). A set of one or more floating power supply modules 202 generate an AC current in the power distribution loop. The floating power module(s) derive power from a voltage across one or more components of the VSC (e.g. an energy storage component or one or more semiconductor switches) and are not ground referenced power supplies, avoiding problems of suitable isolation. A ground referenced supply 207 may be removably connected to the power distribution loop to provide power when the VSC is not energised. A power controller may operate in: a single supply mode, to selectively activate a single floating power module with the other floating power modules inactive; or a multiple supply mode, where the power controller selectively activates more than one of the floating power modules. At least one of the floating power modules may be connected in series with, or inductively coupled to, the power distribution loop when active.

Description

POWERING VOLTAGE SOURCE CONVERTERS

This application relates to voltage source converters and to methods and apparatus for providing power to voltage source converters and in particular relates to providing power to one or more switching modules of a voltage source converter.

High Voltage Direct Current (HVDC) electric power systems can provide an economic and efficient alternative to High Voltage Alternating Current (HVAC) power systems, particularly when transporting power over long distances (>50km), where HVDC generally has lower losses and can provide increased transmission capacity.

For compatibility with AC systems, it is often necessary to convert the direct current (DC) of an HVDC system to alternating current (AC) and back again. Voltage source converters, (VSC) are one way of providing AC-DC and DC-AC conversion in HVDC transmission. VSCs use switching elements such as Insulated Gate Bipolar Transistors (IGBTs) that can be controllably turned on and turned off independently of any connected AC system.

Various designs of VSC are known. Typically each VSC will have a phase limb for each AC phase, with each phase limb having two converter arms connecting the relevant AC terminal to respective high and low DC terminals. Each arm comprises an apparatus which is termed a valve and which typically comprises a plurality of switching elements, with the switching elements of the valves being switched in a desired sequence by associated control circuitry.

In one form of known VSC, often referred to as a six pulse bridge or as a two-level converter, the valve of each converter arm comprises a set of series connected switching elements, typically IGBTs, each IGBT connected with an antiparallel diode. The IGBTs of the valve are switched together to connect or disconnect the relevant AC and DC terminals, with the valves of a given phase limb being switched in antiphase.

By using a pulse width modulated (PVVM) type switching scheme for each arm, conversion between AC and DC voltage can be achieved. In another form of VSC referred to as a modular multilevel converter (MMC) each converter arm comprises a plurality of series connected cells that each have an energy storage element such as a capacitor that can be selectively connected in series between the relevant AC and DC terminals or bypassed, using switches. By using a relatively large number of cells and timing the switching appropriately the valve can synthesise a stepped waveform that approximates to a sine wave, to convert from DC to AC or vice versa with low levels of harmonic distortion. In a further type of converter referred to as an Alternate-Arm-Converter (AAC) a plurality of series connected cells is connected in each converter arm for providing a stepped voltage waveform as described for the MMC type converter but each converter arm also comprises an arm switch, referred to as a director switch and each converter arm is turned off for at least part of the AC cycle. Other variants of VSC which may be based on similar principles are also known.

A feature common to all of the voltage source converter designs outlined above is the use of a plurality of switching elements which are selectively switched in sequence to produce an output waveform. Typically at least one switching element is arranged together with associated local control electronics for applying suitable gate control signals to the switching element and possibly some local monitoring circuitry, e.g. a gate board or the like.

One of the challenges when building a voltage source converter for high voltage applications is how to power the local switching circuitry.

Embodiments of the present invention relate to methods and apparatus for powering switching modules of a voltage source converter.

Thus, according to the invention, there is provided a voltage source converter comprising: one or more switching modules; a first power distribution loop; one or more power distribution modules for providing, in use, power from the first power distribution loop to one or more of said switching modules; and a first set of one or more floating power supply modules for generating an AC current in the first power distribution loop; wherein the one or more floating power supply modules derive power from a voltage across one or more components of the voltage source converter.

Embodiments of the present invention thus have one or more floating power supply modules, i.e. power supply modules that are at a floating potential and not referenced to ground, to feed power to a power distribution loop. The floating power supply modules derive power from components of the VSC itself, i.e. components that form part of a phase limb of a VSC. This avoids the need for a ground referenced power supply in operation of the VSC, and thus avoids the problems with isolation associated with a ground referenced power supply in HVDC applications.

The first set of floating power supply modules may comprise a plurality of floating power supply modules. The VSC may further comprise a power controller configured to selectively control one or more of the floating power supply modules to provide power to the first power distribution loop. Thus there may be a plurality of sources of power available for the power distribution loop to ensure continuity of power supply.

In some embodiments the power controller is configured to be operable, in use, in a single supply mode wherein the power controller selectively controls one of the floating power supply modules to be active to provide power to the first power distribution loop and controls the other floating power supply modules to be inactive. In such a mode of operation the power controller may selectively vary which of the floating power supply modules is active based on power available to the floating power supply modules.

Additionally or alternatively the power controller may be configured to be operable, in use, in a multiple supply mode wherein the power controller selectively controls more than one of the floating power supply modules to be active to provide power to the first power distribution loop. In such a mode the power controller may be configured to send control signals to the one or more floating power supply modules to control the frequency and magnitude of the current generated by said floating power supply modules. In this way the power controller may manage the interaction between the various active floating power supply modules. In some embodiments however each floating power supply module may be configured, in use in the multiple supply mode, to measure the current in the first power distribution loop and synchronise the frequency and magnitude of the current generated by that floating power supply module based on the measured current.

In some embodiments the voltage source converter may further comprise: at least one additional power distribution loop; and an additional set of one or more floating power supply modules associated with each additional power distribution loop for generating an AC current in the respective additional power distribution loop. Each of the floating power supply modules may be configured to derive power from the voltage across one or more components of the voltage source converter. In some embodiments at least one power distribution modules may be electrically coupled to receive power from both the first power distribution loop and at least one additional power distribution loop.

In some embodiments there may additionally be at least one auxiliary power supply operable to generate an AC current in the first power distribution loop in a first mode of operation and to be isolated from the first power distribution loop in a second mode of operation. The auxiliary power supply may be a ground referenced power supply. The auxiliary power supply may be connected when the VSC is not energised, e.g. on start-up, and the floating power supply modules would not be able to deliver sufficient power. The auxiliary power supply may be configured to be removeably coupled to the first power distribution loop.

At least one of the floating power supply modules may be configured so as to be electrically connected in series to the power distribution loop when active. Additionally or alternatively at least one of the floating power supply modules may be inductively coupled to the power distribution loop.

As mentioned a floating power supply is configured to derive power from a voltage across one or more components of the VSC. Said one or more components of the voltage source converter may comprise at least one of: an energy storage component, a semiconductor switch, a group of semi-conductor switches and a switching-aid circuit.

Aspects also relate to an electrical transmission system, for instance a HVDC power transmission/distribution network comprising a voltage source converter as described in any of the variants above.

Aspects also relate to methods of operating a VSC. Thus in another aspect there is provided a method of operating a voltage source converter comprising: generating an AC current in a first power distribution loop using at least one of a first set of floating power supply modules; and using one or more power distribution modules to derive power from the first power distribution loop to power one or more switching modules of the voltage converter; wherein the one or more floating power supply modules derive power from a voltage across one or more components of the voltage source converter.

The method may be operated in any variants described above.

The invention will now be described by way of example only, with reference to the accompanying drawings, of which: Figure 1 illustrates an example voltage source converter; Figure 2 illustrates a power supply arrangement for a voltage source converter according to an aspect of the invention; Figure 3 shows two power supply modules inductively coupled to a power distribution loop; Figure 4 shows two power supply modules connected in series with a power distribution loop; Figures 5a and 5b illustrate further arrangements for coupling power supply modules and power distribution modules to a power distribution loop; Figure 6 illustrates an alternative arrangement of a voltage source converter according to the invention; and Figure 7 illustrates an implementation of the invention in an Alternate Arm Converter topology.

Figure 1 illustrates a known type of voltage source converter (VSC) 100. Figure 1 illustrates a VSC of the so-called Alternate Arm Converter (AAC) type. The example converter 100 has three phase limbs 101a-c, each phase limb having a high side converter arm connecting the relevant AC terminal 102a-c to the high side DC terminal DC+ and a low side converter arm connecting the relevant AC terminal 102a-c to the low side DC terminal DC-. Each converter arm comprises a circuit arrangement 103 of series connected cells, the arrangement 103 being in series with an arm switch 104.

The circuit arrangement 103 comprises a plurality of cells 105 connected in series. Each cell 105 has an energy storage element that can be selectively connected in series between the terminals of the cell or bypassed. In the example shown in figure 1 each cell 105 has terminals 106a, 106b for high-side and low-side connections respectively and comprises a capacitor 107 as an energy storage element. The capacitor 107 is connected with cell switching elements 108, e.g. IGBTs with antiparallel diodes, to allow the terminals 106a and 106b of the cell to be connected via a path that bypasses capacitor 107 or via a path that includes capacitor 107 connected in series. In the example illustrated in figure 1 each cell comprises four cell switching elements 108 in a full H-bridge arrangement. In some embodiments however at least some of the cells may comprise switching elements in a half bridge arrangement. The circuit arrangement 103 of such series connected cells can thus operate to provide a voltage level that can be varied over time to provide stepped voltage waveform for wave-shaping. The circuit arrangement 103 is sometimes referred to as a chain-link circuit or chain-link converter or simply as a chain-link.

In the AAC converter the chain-link 103 in each converter arm is connected in series with an arm switch 104, which will be referred to herein as a director switch, which may comprise a plurality of series connected arm switching elements 109. The director switch of an arm may for example comprise high voltage elements with turn-off capability such as IGBTs or the like with antiparallel diodes. The director switch 104 will typically comprise many such IGBTs 109. When a particular converter arm is conducting, the chain-link 103 is switched in sequence to provide a desired waveform.

However in the AAC converter each of the converter arms of a phase limb is switched off for part of the AC cycle and during such a period the director switch 104 is turned off.

A VSC controller 110 may be provided to control the director switch 104 and chain-link 103 of each converter arm as required. The VSC controller may therefore generate control signals CS for control of the switching elements 108 and 109 of the chain-link and director switch. It will be appreciated by one skilled in the art that the control system for a typical VSC may be distributed, with various components or groups of components having local controllers. Some control actions may be taken at a relatively local level, for instance within modules forming the chain-link or director switches, although there will be at least some top-level control of the overall VSC.

For operation of the switching elements there will be local gate control electronics 111 for generating an appropriate gate control signal to turn the switching element on and off as instructed by the VSC controller 110. There may also be other local circuitry 112 associated with the switching element which may comprise for instance monitoring circuitry for monitoring operation of the switching element and/or providing feedback signals for the VSC controller. In some instances the local circuitry 112 may also comprise switching aid circuits or the like as would be understood by one skilled in the art. At least one switching element 109 and its associated control electronics and other local circuitry may be identified as a switching module 113.

As used herein the term switching module shall be used to refer to at least one switching element and its associated local control/monitoring circuitry for the at least one switching element and any switching aid circuitry. In particular a switching module may comprise one or more switching elements forming at least part of a director switch of a voltage source converter.

In use the electronics, e.g. the gate control electronics 111 of the switching modules 113, need to be powered. Preferably however in operation the switching modules of a voltage source converter should be galvanically isolated from one another and the other component of the VSC. For instance to provide galvanic isolation between the switching modules 113 and the VSC controller 110 the control signals may be optical and provided by suitable fibre optic links.

In some converter systems it has been proposed to achieve such isolation by providing each switching module with a current transformer, such as a ferrite core with an appropriate secondary winding, to derive power from a power distribution loop that acts as a primary winding. A converter connected to the transformer converts the output of the transformer to a suitable voltage for the local circuitry. In such systems, the power distribution loop is fed by a ground-based power supply that injects a high frequency current into the loop.

Such an approach is acceptable at lower voltages; however at higher voltages, and particularly those associated with HVDC, the use of these types of systems become unfeasible as power supplies that are referenced to ground are potentially dangerous for HVDC applications due to the risk of electrical discharge. There is therefore a need for a power arrangement for switching modules of a voltage source converter that can safely be used at the high voltages associated with HVDC.

Figure 2 illustrates the principles of a power distribution system 200 for a voltage source converter, according to an embodiment of the invention. The power distribution system 200 comprises a power distribution loop 201. In use the power distribution loop is powered by one or more power supply modules 202 that are electrically coupled to the power distribution loop 201 to inject a high frequency current in the power distribution loop. The power supply modules arranged to supply power to a given power distribution loop shall be referred to herein as a set of power supply modules. Note the term 'set shall encompass a single such power supply module.

One or more power distribution modules 203 are arranged to derive power from the power distribution loop to distribute power to one or more switching modules 113. The power distribution modules 203 thus act as consumer points for the power distribution loop. The power distribution modules 203 may comprise a transformer 204 and a converter 205 for converting and regulating the power to the switching modules 113 as described above.

In embodiments of the present invention at least one power supply module 202 for generating current in the power distribution loop, i.e. to power the power distribution loop, is power supply module that derives power from a voltage across some of the components of the VSC itself, e.g. from the voltage across one more or semiconductor switching elements or from the DC voltage across an energy storage element of a cell of the chain-link. Power is thus derived from a voltage across components of a converter arm of a VSC, i.e. from the components forming a phase limb of the VSC. As such the power supply is not a ground referenced supply and instead is at a floating voltage potential. Such a power supply module shall be referred to herein as a floating power supply module.

Embodiments of the present invention thus provide a power source for the switching modules 113, that overcomes some of the limitations associated with conventional power supplies and in particular, ground based power supplies. As mentioned, it has been appreciated that when the voltage source converter is in operation, the floating power supply modules 202 can derive power from the components of the voltage source converter itself, e.g. by power harvesting. This provides the advantage that power can be delivered to the switching modules whilst maintaining the isolation requirements between each semi-conductor switching element in the switching modules and the ground potential.

As mentioned the floating power supply modules 202 in the power distribution system 200 may derive power from the voltage across one or more components of the voltage source converter. There are various ways in which the power supply modules may be powered by components of the voltage source converter, as illustrated in figure 2.

For example at least one floating power supply module 202 may derive power from one or more switching component in the voltage source converter. This power may be extracted from the voltage applied across the component and/or the rate of change of the voltage across the component. In some examples, the switching component may be one or more semiconductor switching elements 109 (and/or 108) such as insulated-gate bipolar transistors (IGBTs).

Additionally or alternatively, at least one floating power supply module 202 may derive power from an energy storage component in the voltage source converter. The energy storage component may be a capacitor, for example the power supply module may derive power from the voltage across the capacitor 107 of a chain-link cell. Alternatively still, the floating power supply module may derive power from the energy obtained from a switching-aid circuit 112 connected to the voltage source converter, or from any other source of power available in the circuit.

The power distribution system 200 may comprise a plurality of floating power supply modules 202, i.e. a first set of floating power supply modules for feeding power to a first power distribution loop may comprise more than one module. Each floating power supply module may derive power from the same type of component (i.e. they may all derive power from switching components), or at least some may derive power from different types of component (i.e. some may derive power from switching components, whilst others may derive power from energy storage components).

Having a plurality of floating power supply modules operable to provide power to the power distribution loop can ensure that suitable power is always available for the power distribution loop. For instance if the voltage across a particular component of the VSC drops, such that a given floating power supply module can no longer supply sufficient power, another floating power supply module may be used to ensure continuity of power supply. A power controller 206 may therefore control the various power supply modules accordingly, as will be described in more detail later.

Embodiments of the invention may, in particular, be arranged to supply power to switching modules forming the director switch of a converter arm. The director switches of a VSC typically have no, or very little, energy storage capacity (say typically of the order of 4J peak or so) whereas a module of the chain-link with a large capacitor may have an energy storage capacity of the order of 8-9 KJ on average. Thus the embodiments herein are particularly useful for supplying power to the switching modules of the director switches which otherwise have no suitable source of power. However the principles can be applied to providing power to any switching module or indeed other components of the VSC.

The exact arrangement of the VSC will determine the suitable arrangement of the power distribution loop(s). For example in one embodiment of an AAC type converter the components of the converter arm may be distributed with the elements forming the director switch of a converter arm being arranged in stacks, each stack comprising a number of switching elements, for example between say 6 to 10 IGBTs. Such director switch stacks may be arranged between modules of the chain-link 103.

The power distribution system may be arranged to be relatively local such that the floating power supply modules 103 derive energy from components in a converter arm and the power distribution modules 203 supply power to components, e.g. switching modules 113, that are relatively near neighbours, i.e. from the same section of the converter arm. Thus in the example mentioned above of a distributed AAC type converter the power supply modules 202 may derive power from one or more modules of the chain-link 103 and the power distribution modules 203 may provide power to the neighbouring director switch stacks.

Such a relatively local distribution of power avoids the need for long power distribution loops 201. However if desired one or more power distribution loops could be more global so as to derive power from and/or distribute power to the majority or all of a converter arm or between converter arms. A power distribution loop could be global to receive power from, and distribute power to, components distributed across the full VSC in any given loop.

The floating power supply modules 202 may comprise an inverter to convert the DC voltage derived from the component of the VSC into an AC voltage of a relatively high frequency. The output of the inverter may be transferred from the power supply modules to the power distribution loop 201 by means of an electrical coupling.

In some examples, a floating power supply module 202 may be electromagnetically coupled, i.e. inductively coupled, to the power distribution loop, for example, by a transformer. In such an example, the power supply modules 202 further comprise a primary winding or primary coil for generating the current in the power distribution loop.

This is illustrated in Figure 3 which shows a pair of floating power supply modules 202a and 202b, with respective inverters 301a and 301b that are inductively coupled to a power distribution loop 201 via respective transformer coils or windings 302a and 302b. Such coils 302a and 302b are effectively intermediate windings between the inverter and the main winding of the power distribution loop 201.

Additionally or alternatively at least some floating power supply modules 202 may be connected in series with the power distribution loop 201. This is illustrated in Figure 4 which shows a pair of floating power supply modules, 202a and 202b, with respective inverters 301a and 301b connected in series with a power distribution loop 201, in a daisy chain manner.

Turning now to the power distribution modules 203, as mentioned above each power distribution module acts as a consumer point and transfers power from the power distribution loop to one or more switching modules 113. As mentioned above to aid in galvanic isolation each power distribution module 203 may be electrically connected to the power distribution loop 201, via a transformer 204, e.g. via an inductive coupling. The transformer may, for example, contain a ferrite core with appropriate secondary windings that is arranged such that power distribution loop acts as a primary winding. Each power distribution module 203 may further comprise a converter 205 that regulates the power output and converts the current into a suitable voltage to be connected to the switching modules 113.

Further alternatives for connecting the power supply elements 301 and/or the power distribution modules to a power distribution loop are illustrated with respect to figures 5a and 5b. Figure 5a illustrates an inverter 301 that is part of a power supply module 202 coupled to a power distribution loop 201 by a double wound transformer. The windings of the transformer may be connected in series to form the power distribution loop 201 Figure 5b illustrates that power distribution modules 205 may also be connected to the power distribution system using double wound transformers with the primary windings connected in series. Figure 7b illustrates that the inverters may be connected via any of the methods discussed previously.

The power distribution loop may be formed from a suitable conductive material such as a power cable. In some embodiments however the power distribution loop 201 may be a magnetic core made of insulated magnetic material.

It will be appreciated that, as the floating power supply modules derive power from voltages across various components of the VSC, the VSC has to be energised in order for the floating power supply modules to feed the power distribution loop. During initial start-up of the VSC such sources of power may not be available. In some embodiments therefore, the voltage source converter may further comprise at least one auxiliary power supply 207 that can feed power to the power distribution loop and which can operate to provide power independently of the state of operation of the VSC. The auxiliary power supply may, for instance, comprise a ground-based power supply that can be electrically coupled to the power distribution loop when required, e.g. during start-up to provide the initial power needed to operate the necessary components of the switching modules during the starting sequence. At the point of such start-up the VSC will not be operational at the high-voltage experienced in use and thus a ground reference power supply may be used at this point. This ground referenced power supply may then be isolated from the system before the DC terminals are fully energised and the AC terminals are connected to the grid. In some embodiments therefore the auxiliary power supply may be selectively connected to or isolated from the power distribution loop by a switch 208, which may for instance be a mechanical switch.

In some examples, the auxiliary power supply may form at least part of a start-up module that may be removeably connected to the system in the form of a portable power source device. This is particularly advantageous to energise particular sections of the apparatus during maintenance. The portable start-up module can then be isolated or fully detached from the circuit prior to energising the main HVDC converter.

As mentioned above a power controller 206 may control the operation of the floating power supply modules 202 (and the auxiliary power supply if present). The controller 206 may be configured to selectively enable or disable the operation of different power supply modules 202 to ensure suitable power continuity for the power distribution loop.

It will be appreciated that where there are a plurality of floating power supply modules 202 all able to feed power to a single power distribution loop 201 it will be necessary to manage the interactions between the various power supply modules.

In one example the power controller 206 may be operable in a single supply mode such that there is a single floating power supply module active to feed power to the power distribution loop at any time. The controller may therefore be configured to selectively connect a single active floating power supply module to the power distribution loop 201 at any given time whilst ensuring that the remaining floating power supply module(s) are in an inactive or idle state. VVhilst in an inactive or idle state, a power supply modules does not feed power into the power distribution loop. However if the presently active floating power supply modules fails, or is unable to supply the required amount of energy to the power distribution loop, the control system can activate another power supply module as a replacement to the original power supply module. This adds redundancy to the system and adds to its overall integrity.

When a floating power supply module 202 is disabled by the power controller (i.e. when it is in idle mode or inactive), its output terminals may be shorted. For instance, referring back to Figure 3 which illustrates the floating power supplies 202 being inductively coupled to the power distribution loop, floating power supply module 202a may be active and thus inverter 301a is driving power to intermediate winding 302a.

Floating power supply module 202b may be inactive and thus the terminals may be shorted by a switch 303 to provide a circulating path for the current in winding 302b. Likewise referring back to Figure 4 which shows the floated power supply modules 202 being connected in series in a daisy chain-arrangement, the inverter 301a of an active power supply module 202a may be connected to the power distribution loop whilst the terminals of an inactive module are shorted by switch 303 such that current flows freely around the power distribution loop 302.

The power controller may therefore control which of the floating power supply modules able to feed power to the power distribution loop is active based on the power available to each power supply module and generate controls signals to activate and deactivate power supply modules as required.

In some embodiments the power controller 206 may additionally or alternatively be operable in a multiple supply mode such that a plurality of floating power supply modules may be active at the same time as one another to feed power to the same power distribution loop 201.

When two or more floating power supply modules are active to supply power to the power distribution loop simultaneously, the current supplied to the power distribution loop from the different sources must be synchronised.

In some examples, the synchronisation is co-ordinated by the controller. For example, the control system may send out control signals, e.g. such as synchronisation pulses or the like, to control the frequency and magnitude of operation of each floating power supply module.

In alternative examples, the synchronisation can be made by each power supply module monitoring the current in the power distribution loop, or at least an indication of the current in the power distribution loop The power controller 206 may be a central controller for controlling the power distribution system and may be integrated with some other controller of the VSC. Alternatively however at least some aspects of the control of the power distribution system may be implemented by distributed local control, close to or forming part of the relevant power supply module or power distribution module. For instance, some control decisions may be taken at the local level of the power supply module, for example if a power supply module detects it is faulty, it may short its terminals out and otherwise will lock to the main current in the power distribution loop.

Embodiments of the invention thus use at least one floating power supply to feed power into a power distribution loop to provide power for the switching modules of a VSC. It was noted above that a plurality of floating power supply modules 202 may be able to feed the same power distribution loop so as to ensure continuity of power in the power distribution loop.

Additionally or alternatively redundancy in power supply for a particular switching module of a VSC may be achieved by having a plurality of separate power distribution loops all provide power to an individual power distribution module. In other words a transformer and converter associated with a given switching module, or set of switching modules, may be arranged to receive power simultaneously from more than one power distribution loop. Thus there may be a first power distribution loop and at least one additional power distribution loop, each with an associated set of floating power supply modules.

Figure 6 shows an alternative configuration of a power supply arrangement of a voltage source converter 600, wherein each of three power supply modules 602a, 602b and 602c is electrically coupled to a power distribution loop 601a, 601b, and 601c respectively. The three power distribution loops are each electrically coupled to the same power distribution module 603 that supplies power to a switching module 113. In this arrangement therefore, multiple power distribution loops, each connected to separate power supply modules, provide power to a single power distribution module.

As described above, according to an aspect of the invention, the power distribution loops may be powered by floating power supply modules that derive power from the voltage across one or more components of the voltage source converter, in an analogous way to the floating power supply modules described previously.

The power distribution module 603 may be magnetically coupled to each of the three power distribution loops, 601a, 602b and 603c by means of a single transformer 604 in the power distribution module. Thus for example the various power distribution loops may be provided as separate windings through the same ferrite core of a transformer 604 of the power distribution module 603.

The power supply modules, 602a, 602b and 602c, may each derive power from different sources within the VSC. As with the embodiments described above providing multiple possible sources of power helps ensure power continuity and redundancy.

For instance consider that a source of power is the voltage across a capacitor of a cell of the chain-link. If the relevant cell fails the capacitor may discharge but there may still be a need to provide power for the switching modules. Having multiple power sources available such as illustrated in figure 6 means that even in the event of a cell failure, the gate drive for the switching modules remains active.

As mentioned previously to maintain a steady voltage for the switching module, e.g. the gate drive of a switching module, the power distribution module 603 may comprise a converter 603 that includes a regulator such as an on board shunt regulator. This has the advantage of ensuring that the power distribution module can operate equally well with multiple active primary windings or just one.

To ensure flux is maintained in the core of the transformer of the power distribution module, the current in the power distribution loops 601a, 601b and 601c may be synchronised, e.g. by controller 606 by a control unit that sends control signals such as synchronisation pulses to each power supply module to align the high frequency waveforms of the three power distribution loops.

The core of the transformer of the power distribution modules 202 may be, for example, a ferrite material, iron, a powder core or a nanocrystaline material.

In some embodiments, at least one of the power distribution loops illustrated in figure 6 may provide power to a plurality of power distribution modules. According to this arrangement, multiple power distribution loops may each be arranged to power multiple power distribution modules. It may be possible for each power distribution loop to provide power for four or more power distribution modules before isolation requirements of the power distribution loop become significant. The advantage of such an arrangement is that it ensures a good level of redundancy so that sufficient power is maintained in the gate drivers in the event of a localised failure.

Additionally or alternatively in some embodiments each of the power distribution loops illustrated in figure 6 could be arranged with multiple power supply modules able to feed power into that loop. In other words each of the power distribution loops 601a, 601b and 601c could have the form of loop 201 illustrated in figure 2.

Figure 7 illustrates an embodiment of how a power supply arrangement as described above may be applied to a phase limb of an Alternate Arm Converter type VSC 700. In this embodiment power is supplied to the power distribution loop, 201 by a series of power supply modules 202 that derive current from one or more switching cells of the chain-link 103. Director switches 104 are positioned between the switching modules and these derive power from the power distribution loop 201. In this arrangement, the power supply modules may derive power from the cells of the chain-link which are next to the director switches 104, thereby getting power to the director switches over a reasonable insulation barrier that can be traversed with the power distribution loops.

The embodiments described above provide a number of advantages such as providing a local power supply to switching modules, and especially the switching modules of the director switches, which need local power to operate their gate driver systems. During operation, power is taken from nearby components that have sufficient energy storage and are at a floating potential. The embodiments described galvanically isolate floating potential gating electronics from low voltage control and ground potential for safety. Furthermore, when the HVDC converter is off, a start-up module can be incorporated to the feeder distribution system to provide power for the starting sequence. This can subsequently be disconnected which helps black-start up requirements.

The embodiments have principally been described with respect to AAC type VSCs, however it will be appreciated that the power arrangements described herein may be implemented in any type of VSC for HVDC applications.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.

Claims

Claims 1. A voltage source converter comprising: one or more switching modules; a first power distribution loop; one or more power distribution modules for providing, in use, power from the first power distribution loop to one or more of said switching modules; and a first set of one or more floating power supply modules for generating an AC current in the first power distribution loop; wherein the one or more floating power supply modules derive power from a voltage across one or more components of the voltage source converter.
2. The voltage source converter of claim 1 wherein the first set comprises a plurality of floating power supply modules and the voltage source converter comprises a power controller configured to selectively control one or more of the floating power supply modules to provide power to the first power distribution loop.
3. The voltage source converter of claim 2 wherein the power controller is configured to be operable, in use, in a single supply mode wherein the power controller selectively controls one of the floating power supply modules to be active to provide power to the first power distribution loop and controls the other floating power supply modules to be inactive.
4. The voltage source converter of claim 3 wherein the power controller is configured to, in use in the single supply mode, selectively vary which of the floating power supply modules is active based on power available to the floating power supply modules.
5. The voltage source converter of any of claims 2 to 4 wherein the power controller is configured to be operable, in use, in a multiple supply mode wherein the power controller selectively controls more than one of the floating power supply modules to be active to provide power to the first power distribution loop.
6. The voltage source converter of claim 5 wherein the power controller is configured to, in use in the multiple supply mode, to send control signals to the one or more floating power supply modules to control the frequency and magnitude of the current generated by said floating power supply modules.
7. The voltage source converter of claim 5 wherein each floating power supply module is configured, in use in the multiple supply mode, to measure the current in the first power distribution loop and synchronise the frequency and magnitude of the current generated by that floating power supply module based on the measured current.
8. The voltage source converter of any preceding claim wherein the voltage source converter further comprises: at least one additional power distribution loop; and an additional set of one or more floating power supply modules associated with each additional power distribution loop for generating an AC current in the respective additional power distribution loop wherein each floating power supply module is configured to derive power from the voltage across one or more components of the voltage source converter; wherein at least one of said one or more power distribution modules is electrically coupled to receive power from both the first power distribution loop and at least one additional power distribution loop
9. The voltage source converter of any preceding claim wherein the voltage source converter further comprises at least one auxiliary power supply operable to generate an AC current in the first power distribution loop in a first mode of operation and to be isolated from the first power distribution loop in a second mode of operation.
10. The voltage source converter of claim 9 wherein the auxiliary power supply is configured to be removeably coupled to the first power distribution loop.11 The voltage source converter of any preceding claim wherein at least one of the floating power supply modules is configured to be electrically connected in series with the power distribution loop when active.12. The voltage source converter of any preceding claim wherein at least one of the floating power supply modules is inductively coupled to the power distribution loop when active.13. The voltage source converter of any preceding claim wherein said one or more components of voltage source converter comprises at least one of: an energy storage component, a semiconductor switch, a group of semi-conductor switches and a switching-aid circuit.14. An electrical transmission system comprising a voltage source converter as in any preceding claim.15. A method of operating a voltage source converter comprising: generating an AC current in a first power distribution loop using at least one of a first set of floating power supply modules; and using one or more power distribution modules to derive power from the first power distribution loop to power one or more switching modules of the voltage converter; wherein the one or more floating power supply modules derive power from a voltage across one or more components of the voltage source converter.