TRANSMISSION

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

In HVDC power transmission, alternating current (AC) electrical power is converted to high voltage direct current (DC) electrical power for transmission via overhead lines and/or undersea cables. This conversion reduces the cost per kilometre of the lines and/or cables, and thus becomes cost-effective when power needs to be transmitted over a long distance. Converters are required at each interface between AC and DC networks to effect the required conversion, and one such form of converter is a Voltage Source Converter (VSC) . The conventional voltage source converter is controlled by a global control unit which is connected to a plurality of modules of the converter via data transmission links such as fibre optic cables. Each module can include a single switch or a group of switches. It is necessary for each module to be operated independently of the other modules to enable voltage switching with fine resolution to generate a low distortion sinusoid AC waveform. To facilitate independent module operation, each module is therefore separately connected to the global control unit using unshared data transmission links. The large number of modules in a voltage source converter leads to a high number of data transmission links between the voltage source converter and the global control unit. Since the global control unit is located far from the vicinity of the high- voltage environment around the voltage source converter, the fibre optic cables tend to be relatively long in order to span the distance between the global control unit and the voltage source converter. The required length and number of fibre optic cables therefore results in increased size and cost of converter hardware due to the length of the fibre optic cables and the increased area of the station footprint. According to an aspect of the invention, there is provided an optical communications network for use in high voltage DC power transmission and reactive power compensation systems comprising at least one control unit, at least one passive communications hub and a plurality of power converter cells wherein the or each control unit is configured to communicate with the plurality of power converter cells via at least one passive communications hub; and each power converter cell is operably associated with a unique identification to define a unique communication path between the or each control unit and each power converter cell.

The provision of a passive communications hub, such as an optical power coupler, is advantageous because it eliminates the use of an active data distribution system such as a switching system to control the distribution of different data signals to a plurality of power converter cells forming part of a voltage source converter assembly. This is made possible by the assignment of a unique identification to each power converter cell which allows the or each control unit to communicate specifically with each power converter cell using the unique identification via the respective unique communication path. As a result, a combined data signal including different data signals with associated unique identifications can be transmitted between the or each control unit and the plurality of power converter cells via the passive communications hub to individually address each power converter cell without running the risk of miscommunication . There is therefore no need for an active data distribution system to address each power converter cell individually, which means a reduction in converter hardware costs and energy consumption. The use of the or each passive communications hub reduces the number of data transmission links between the voltage source converter and the or each control unit. Instead of linking the or each control unit directly to each individual power converter cell, the or each control unit is directly linked only to the or each passive communications hub which transmits signals between the or each control unit and the plurality of power converter cells. Using the passive communications hubs to establish communication between the or each control unit and the plurality of power converter cells therefore results in size, weight and cost savings of data transmission links such as fibre optic cables.

Fibre optic lines may be used in the optical communications network to provide data transmission with low loss and minimal crosstalk. The fibre optic lines also provides electrical isolation between the or each control unit and the plurality of power converter cells of the voltage source converter.

In embodiments of the invention, the optical communications network may include a plurality of control units, wherein each control unit is configured to communicate with the plurality of power converter cells via at least one passive communications hub.

The use of a plurality of control units allows duplication of the control of the plurality of power converter cells, so as to remove the risk of network downtime that is caused by failure of a single control unit, or a communication failure between a single control unit and a single passive communications hub . The reliability of the optical communications network may be improved by using a hierarchy of passive communications hubs between the or each control unit and each power converter cell, so that the failure of a single communications link in the network would not result in downtime for any of the power converter cells or would result in downtime for only a single power converter cell, instead of multiple power converter cells. This may be achieved, by for example, configuring at least one passive communications hub to communicate with at least one other passive communications hub. This allows each control unit to communicate with each power converter cell via multiple passive communications hubs.

It will be appreciated that the configuration of the hierarchy of passive communications hubs may vary depending on various user requirements such as, for example, cost, size, and weight.

In other embodiments the or each control unit may be controllable to communicate with the plurality of power converter cells using electronic level shifting circuits, magnetic isolation or electromagnetic waves. Preferably the or each passive communications hub includes at least one optical power coupler .

An optical power coupler can split the optical power of a optical signal in a single input fibre optic line between two or more output fibre optic lines. It can therefore be used to distribute an incoming signal from the or each control unit among multiple fibre optic lines leading to the plurality of power converter cells. In addition, the optical power coupler can also receive multiple data signals from the plurality of power converter cells and combine it into a single data signal before transmitting it to the or each control unit . In embodiments of the invention, the or each control unit may be controllable to assign a unique identification to each power converter cell.

The provision of a unique identification assignment process allows each power converter cell to be assigned a unique identification in a format recognizable by the or each control unit.

In such embodiments employing the use of unique identifications, the or each control unit may be controllable to concurrently assign a unique identification and transmit data to or receive data from each power converter cell. This prevents any unnecessary downtime of the voltage source converter.

In further embodiments of the invention, each power converter cell may include a unique serial number and is operable to transmit its serial number to the or each control unit in randomized time slots.

The unique serial number of each power converter cell can be used to generate a unique identification for the power converter cell. This is useful when the serial numbers given to the power converter cells during manufacture comprises too many data bits to be used as a unique identification in the optical communications network.

In such embodiments, each power converter cell is operable to repeatedly transmit its serial number to the or each control unit in randomized time slots until the power converter cell is operably associated with a unique identification. The transmission of the serial numbers is repeated for power converter cells without assigned unique identifications to ensure that each power converter cell is eventually assigned a unique identification .

In other embodiments, the bandwidth of the optical communications network may be larger than the bandwidth required by the transmission of the serial numbers .

This allows the unique identification assignment process to occur concurrently with other data transmission processes associated with the operation of the voltage source converter, and thereby prevent any unnecessary downtime of the voltage source converter.

In further embodiments the or each control unit may be manually controllable to operably associate each power converter cell with a unique identification. Each power converter cell's unique identification can be manually assigned and inputted into the or each control unit based on user requirements and/or preferences.

In embodiments of the invention the optical communications network may further include a plurality of identifiers, each identifier having a unique identification and being operably associated with a power converter cell.

In embodiments employing the use of identifiers, the identifier may be, but is not limited to, a plug or socket with hardwired coding, an optical bar code, a radio-frequency identification tag and/or a serial number.

The provision of an identifier means that the unique identification, and therefore each power converter cell, is now associated with a fixed physical location within the optical communications network. This therefore makes it straightforward to physically locate a power converter cell based on its unique identification. This also results in time and bandwidth savings because there is no need to perform a unique identification assignment process for each power converter cell.

In other embodiments each power converter cell may be operable to communicate with its neighbouring power converter cells. In such embodiments, each power converter cell may communicate with its neighbouring power converter cell via electrical, optical, electromagnetic or power link connections between the neighbouring power converter cells.

This provides the or each control unit with information to easily identify power converter cells which are physically adjacent to each other. This is useful when monitoring and operating groups of power converter cells.

In further embodiments each power converter cell is operably associated with a visual indicator.

A power converter cell can be physically located by using the or each control unit to identify a particular power converter cell and activating the associated visual indicator so that the operator can physically locate the specific power converter cell.

In embodiments of the invention, the optical communications networks may further include a plurality of visual indicators, wherein each visual indicator displays the unique identification of the respective power converter cell in a readable or scannable form.

The display of a unique identification on a power converter cell provides a straightforward way of positively identifying the physical location of the power converter cell associated with the unique identification .

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 an optical communications network according to a first embodiment of the invention; and

Figure 2 shows an optical communications networks according to a sixth embodiment of the invention . An optical communications network 10 according to a first embodiment of the invention is shown in Figure 1.

The communications network 10 for use in high voltage DC power transmission and reactive power compensation systems comprises a control unit 12, at least one passive communications hub 14 and a plurality of power converter cells 16 wherein the control unit 12 is configured to communicate with the plurality of power converter cells 16 via the or each passive communications hub 14; and each power converter cell 16 is operably associated with a unique identification.

The control unit 12, such as a valve base electronics unit, which may be a stand-alone unit or may receive instructions from another control unit 12. The purpose of the control unit 12 is to provide instructions in the form of control signals to the power converter cells 16 and monitor the status of the power converter cells 16 via data feedback signals. The plurality of power converter cells 16 may define part or all of a voltage source converter assembly.

The control unit 12 is connected to each passive communications hub 14 via data transmission links 18 such as fibre optic lines. When fibre optic cables are used as data transmission links 18, the passive communications hub 14 may be an optical power coupler 14. The optical power coupler 14 may include at least one input port and a plurality of output ports. In Figure 1, each input port is connected to the control unit 12 via fibre optic lines 18 and each output port is connected to a power converter cell 16 via fibre optic lines 18.

When the optical power coupler 14 receives a signal from the control unit 12, it splits the input signal into multiple output signals and transmits the output signals to each power converter cell 16. Conversely the optical power coupler 14 can also receive multiple data signals from the plurality of power converter cell 16 and combine it into a single data signal before transmitting it to the control unit 12. The data signals contain information on the status of each power converter cell 16. The use of the optical power coupler 14 reduces the number of data transmission links 18 because the control unit 12 is directly linked only to a small number of optical power couplers 14 which allows the control unit 12 to communicate with a larger number of power converter cells 16.

Each power converter cell 16 in the voltage source converter assembly may include a single semiconductor switch or a group of semiconductor switches. It is envisaged that each power converter cell 16 may further include a local control unit 12 which interconnects the semiconductor switches and the incoming fibre optic lines 18.

It is envisaged that instead of relying on fibre optic lines 18, data may be transmitted using electronic level shifting circuits, magnetic isolation or electromagnetic waves.

The passive nature of the optical power coupler 14 makes it necessary to introduce a data addressing scheme in order to direct control signals to specific power converter cells 16 and to uniquely reference power converter cell status data back from the power converter cells 16 to the control unit 12. This is achieved by assigning a unique identification to each power converter cell 16 to define a unique communication path between the control unit 12 and each power converter cell 16, which allows the control unit 12 to communicate specifically with each power converter cell 16 or with groups of power converter cells 16 using the unique identification of each power converter cell 16 via the respective unique communication path.

The assignment of unique identifications is performed either during start-up of the voltage source converter, or when new power converter cells 16 are added to the communications network 10.

Each power converter cell 16 includes a serial number which was given during the manufacture of the power converter cell 16. Often this serial number comprises too many bits to be used as a unique identification in the communications network 10. The control unit 12 may be controllable to request information from each power converter cell 16 to determine the state of each bit in their serial numbers. Alternatively the control unit 12 is controllable to command each power converter cell 16 to transmit their serial numbers to the control unit 12 in randomized time slots. After each power converter cell 16 has transmitted their serial number, all successfully identified serial numbers are recorded and assigned a corresponding unique identification. Commands are reissued to unidentified power converter cells 16 to transmit their serial numbers in randomized time slots again until each power converter cell 16 has been assigned a unique identification in a format usable by the control unit 12. The proximity of the power converter cells 16 within the voltage source converter assembly results in a quick assignment of unique identifications because the relatively small distances between power converter cells 16 result in small propagation delay times, and therefore small time slots.

The bandwidth of the communications network 10 is designed to be larger than the bandwidth required by the transmission of the serial numbers to enable the assignment process to occur concurrently with other data transmission processes associated with the operation of the voltage source converter, and thereby prevent any unnecessary downtime of the voltage source converter. As a result, the unique identification of the power converter cells 16 can be checked repeatedly, and updated if required, without disruption to the operation of the voltage source converter. The provision of a unique identification for each power converter cell 16 allows the transmission of a combined data signal including different data signals with associated identifications to be sent between the control unit 12 and the plurality of power converter cells 16 via the passive optical power coupler 14 to individually address each power converter cell 16 without the risk of miscommunication . There is therefore no need for an active data distribution system to address each power converter cell 16 individually, which means a reduction in hardware costs and energy consumption . There is however no correspondence between the assigned unique identifications and the physical positions of the power converter cells 16 within the voltage source converter assembly. This poses a problem when the control unit 12 receives status information indicating power converter cell failure which means that the failed power converter cell 16 must be located by the operator to carry out maintenance or repair. The unique identification of the power converter cell 16 does not provide any information in respect of the physical location of the failed power converter cell 16. Determining the location of a failed power converter cell 16 can be time-consuming due to the size of a HVDC power station and the potentially large number of power converter cells 16 in the voltage source converter assembly .

Since the control unit 12 records each serial number during assignment of unique identifications, the operator can rely on this record to positively identify the serial number of the failed power converter cell 16. The physical location of each power converter cell 16 can then be located by cross checking a manually prepared record of power converter cell serial numbers and their corresponding physical locations .

In a second embodiment of the invention the control unit 12 may be manually controllable to operably associate each power converter cell 16 with a unique identification. The operator manually assigns unique identifications to the power converter cell serial numbers and inputs the information into the control unit 12. A record of power converter cell serial numbers and their corresponding physical locations is again manually prepared to enable the operator to quickly determine the physical location of specific power converter cells 16.

In a third embodiment of the invention the communications network 10 may include a plurality of identifiers, each identifier having a unique identification and being operably associated with a power converter cell 16. Each identifier may be a connector, such as a plug or socket, which is connected to a power converter cell 16 and is hardwired with a unique identification which can be read by the control unit 12. Alternatively, the identifier may be encoded with the unique identification via an optical bar code, a radio-frequency identification tag and/or a serial number. The unique identification of each identifier can also be manually inputted into the control unit 12 by the operator.

The identifier provides a fixed unique identification for each physical location regardless of the connected power converter cell 16 and its serial number. This provides the operator with a fixed record of the unique identifications and their corresponding physical locations when it is necessary to locate specific power converter cells 16 based on their unique identifications. Consequently it is not necessary to manually prepare a new record of power converter cell serial numbers and their corresponding physical locations every time new power converter cells 16 are added or removed from the voltage source converter assembly.

In a fourth embodiment of the invention each power converter cell 16 may be operable to communicate with its neighbouring power converter cells 16. The provision of communication between neighbouring power converter cells enables each power converter cell 16 to establish their physical position relative to their neighbouring power converter cells 16 and to provide the control unit 12 with information to easily identify which power converter cells 16 are physically located near each other without relying on a manually prepared record of power converter cells and their corresponding physical locations. This is useful during the operation and monitoring of multiple power converter cells 16 of the voltage source converter.

It is envisaged that each power converter cell may be operable to communicate with its neighbouring power converter cell via electrical, optical, electromagnetic or power link connections between the neighbouring power converter cells. Communication via power link connection may be achieved by modulating a carrier onto the power link between neighbouring power converter cells. In a fifth embodiment of the invention each power converter cell 16 may be operably associated with a visual indicator. The visual indicator may be a display of a power converter cell's unique identification in a readable or scannable form. For example, the visual indicator may be a liquid crystal display which shows the unique identification in readable form. The unique identification may also be displayed in a scannable form such as a bar code. Displaying the unique identification on each power converter cell 16 enables the operator to positively identify the physical location of a specific power converter cell 16 when there is no previous record of power converter cell serial numbers and their corresponding physical locations .

An optical communications network 110 according to a sixth embodiment of the invention is shown in Figure 2. The sixth embodiment of the optical communications network 110 shown in Figure 2 is similar in structure and operation to the first embodiment of the optical communications network 10 shown in Figure 1 and like features share the same reference numerals.

The sixth embodiment of the optical communications network 110 differs from the first embodiment of the optical communications network 10 in that the sixth embodiment of the optical communications network 110 includes a plurality of control units 12 and a plurality of passive communications hubs 14,15.

Having a plurality of control units 12 allows the optical communications network 110 to duplicate the control of the plurality of power converter cells 16. Each control unit 12 is connected to a respective one of a pair of primary passive communications hubs 14 via data transmission links 18.

It is envisaged that, in other embodiments of the invention, each control unit 12 may be connected to a plurality of primary passive communications hubs 14, instead of a single primary passive communications hub 14, via data transmission links 18.

In such embodiments, each primary passive communications hub 14 is connected to each of a plurality of secondary passive communications hubs 15 via a data transmission link 18. Each secondary passive communications hub 15 is then connected to a plurality of power converter cells 16 via data transmission links 18. This results in the duplication of a data transmission link path between each control unit 12 and each power converter cell 16. As such, a hierarchy of passive communications hubs 14,15 is defined between each control unit 12 and each power converter cell 16 that prevents the failure of a single data transmission link 18 from inhibiting communication with more than one power converter cell 16. It is envisaged that, in yet further embodiments of the invention, the plurality of secondary passive communications hubs 15 connected to each primary passive communications hub 14 may vary in number. For example, the plurality of secondary passive communications hubs 15 connected to each primary passive communications hub 14 may vary in number to accommodate a specific number of power converter cells 16. Duplication of control of the power converter cells 16 removes the risk of network downtime that is caused by failure of a single control unit 12, or by failure of a data transmission link 18 between a single control unit 12 and a single primary passive communications hub 14. In addition, the use of the hierarchy of passive communications hubs 14,15 means that failure of a single data transmission link 18 in the network 110 would not result in downtime for any of the power converter cells 16 or would result in downtime for only a single power converter cell 16. The latter may be mitigated through inclusion of one or more additional power converter cells 16 over the number of power converter cells 16 required for a particular power application. As such, in the event of downtime for one or more power converter cells 16, the function of the or each power converter cell 16 that is experiencing downtime may be carried out by a corresponding number of additional power converter cells 16 to ensure continuity of the power application. For example, when a voltage source converter assembly requires a specific number of power converter cells 16 to be connected in series to match a specific voltage, additional power converter cells 16 over the specific number of power converter cells 16 may be added to the voltage source converter assembly. In this manner, the operation of the voltage source converter assembly is unaffected by downtime for a number of power converter cells 16 that is less or equal to the number of additional power converter cells 16.

The configuration of the control units 12 and the passive communications hubs 14,15 therefore improves the reliability of the optical communications network 110.

It will be appreciated that the plurality of passive communications hubs 14,15 may be arranged to define different configurations of the hierarchy of passive communications hubs 14,15 to connect each control unit 12 to each power converter cell 16.

It is envisaged that, in other embodiments of the invention, the number of control units 12 and passive communications hubs 14,15 in the optical communications network 110 may vary depending on user requirements such as cost, size and weight.