CN115668725A

CN115668725A - Communication between a Valve Control Unit (VCU) and a Position Control Unit (PCU) in a High Voltage Direct Current (HVDC) power transmission system

Info

Publication number: CN115668725A
Application number: CN202080101116.8A
Authority: CN
Inventors: M·卢维索托; M·戴维德森; 庞智博; R·詹森; D·霍尔曼斯; J·厄曼; C·斯约贝格
Original assignee: Hitachi Energy Switzerland AG
Current assignee: Hitachi Energy Co ltd
Priority date: 2020-05-18
Filing date: 2020-05-18
Publication date: 2023-01-31
Also published as: WO2021233520A1

Abstract

A mechanism is provided for communicating with a PCU in an HVDC power transmission system. A method is performed by a VCU. The method comprises the following steps: the initiation pulse and the indication pulse are wirelessly communicated with the PCU over a first communication channel. The indication pulses are transmitted as unmodulated symbols over a first communication channel. The method comprises the following steps: keep-alive messages are wirelessly communicated with the PCU over a second communication channel. The second communication channel is separate from the first communication channel.

Description

Communication between a Valve Control Unit (VCU) and a Position Control Unit (PCU) in a High Voltage Direct Current (HVDC) power transmission system

Technical Field

Embodiments presented herein relate to a Valve Control Unit (VCU), a Position Control Unit (PCU), a method, a computer program and a computer program product for communication between a VCU and a PCU in a High Voltage Direct Current (HVDC) power transmission system.

Background

Generally, HVDC power transmission systems rely on inverters to convert power from Alternating Current (AC) to Direct Current (DC), and vice versa. In a conventional HVDC converter, a number of Power Electronic Components (PECs) are connected in series, forming a valve. Some examples of PECs are IGBTs, IGCTs, MOSFETs, thyristors, etc. In each valve, the VCU triggers the switching of all PECs by transmitting a startup impulse simultaneously to the PCUs connected to each PEC. Fiber optic links are typically used to connect the VCU to the PCU in a galvanically isolated manner, allowing the transmission of initiation pulses that trigger the switching of the PEC.

The number of PECs employed in HVDC systems grows with power rating, scaling up to several hundred per valve, with the consequent need to deploy thousands of fibres throughout the converter. Therefore, significant costs are involved in the installation and commissioning of all of these fibers. Furthermore, since optical fibers are subject to high electrical potentials, they may be damaged over time or even cause flammability problems, which require severe and costly restrictions on the climate in the valve. For these reasons, the possibility of replacing the optical fiber with a wireless link is attractive for both cost reduction and security reasons.

While the possibility of using a wireless link between the VCU and PCU to control the switching to the PEC is attractive, it is challenging to ensure the timing and reliability of communications over the wireless link due to the inherent limitations of the wireless channel. In fact, unlike a fiber optic network where each link is independent, the wireless channel is shared, and if more than one entity is transmitting on the same frequency band at the same time, the messages will collide at the receiver and may not be decoded correctly. Furthermore, the wireless channel is error prone and it is challenging to ensure that all messages are delivered, which causes problems in HVDC power transmission systems where all PECs in the valve have to be started. Finally, as the condition of the wireless channel varies over time, ensuring deterministic communication delays is also complex, which can cause problems in HVDC power transmission systems, as all PECs should be switched at the same time for correct power conversion.

There is therefore a need for improved communication between VCU and PCU in wireless HVDC power transmission systems.

Disclosure of Invention

It is an object of embodiments herein to provide efficient communication between a VCU and a PCU in a wireless HVDC power transmission system.

According to a first aspect, a method for communicating with a PCU in an HVDC power transmission system is presented. The method is performed by the VCU. The method comprises the following steps: the initiation pulse and the indication pulse are wirelessly communicated with the PCU over a first communication channel. The indication pulse is transmitted as an unmodulated symbol over the first communication channel. The method comprises the following steps: keep-alive messages are wirelessly communicated with the PCU over a second communication channel. The second communication channel is separate from the first communication channel.

According to a second aspect, a VCU for communicating with a PCU in an HVDC power transmission system is presented. The VCU includes processing circuitry. The processing circuitry is configured to cause the VCU to wirelessly communicate the initiation pulse and the indication pulse with the PCU over a first communication channel. The indication pulse is transmitted as an unmodulated symbol over the first communication channel. The processing circuitry is configured to cause the VCU to wirelessly communicate the keep-alive messages with the PCU over a second communication channel. The second communication channel is separate from the first communication channel.

According to a third aspect, a computer program for communicating with a PCU in an HVDC power transmission system is presented, the computer program comprising computer program code which, when run on processing circuitry of a VCU, causes the VCU to perform the method according to the first aspect.

According to a fourth aspect, a method for communicating with a VCU in an HVDC power transmission system is presented. The method is performed by the PCU. The method comprises the following steps: the start pulse and the indication pulse are wirelessly communicated with the VCU over a first communication channel. The indication pulses are transmitted as unmodulated symbols over a first communication channel. The method comprises the following steps: the keep-alive messages are wirelessly communicated with the VCU over a second communication channel. The second communication channel is separate from the first communication channel.

According to a fifth aspect, a PCU for communicating with a VCU in an HVDC power transmission system is presented. The PCU includes processing circuitry. The processing circuitry is configured to cause the PCU to wirelessly communicate the start pulse and the indication pulse with the VCU over a first communication channel. The indication pulses are transmitted as unmodulated symbols over a first communication channel. The processing circuitry is configured to cause the PCU to wirelessly transmit keep-alive messages with the VCU over a second communication channel. The second communication channel is separate from the first communication channel.

According to a sixth aspect, a computer program for communicating with a VCU in an HVDC power transmission system is presented, the computer program comprising computer program code which, when run on processing circuitry of a PCU, causes the PCU to perform the method according to the fourth aspect.

According to a seventh aspect, a computer program product is presented comprising a computer program according to at least one of the third and sixth aspects and a computer readable storage medium having the computer program stored thereon. The computer readable storage medium may be a non-transitory computer readable storage medium.

Advantageously, these aspects provide for efficient wireless communication between the VCU and the PCU in the HVDC power transmission system.

Advantageously, these aspects provide for efficient control of the PCU without the need to install cables for communication between the VCU and the PCU.

Advantageously, these aspects avoid potential hazards arising from the flammability of the optical fiber when subjected to high potential differences.

Advantageously, these aspects may extend the life cycle of control systems for power electronics, which is often shortened due to poor durability of optical fibers.

Advantageously, these aspects enable a wirelessly controlled HVDC power transmission system to meet the timing constraints required for initiating a PEC operatively connected to a PCU, including a minimum interval between initiation pulses, synchronization of switching instants and a minimum interval between keep alive messages.

Advantageously, these aspects may be applied in HVDC power transmission systems as well as in other applications (e.g. in flexible AC power transmission systems (FACTS)).

Other objects, features and advantages of the appended embodiments will be apparent from the following detailed disclosure, appended dependent claims and accompanying drawings.

In general, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, device, component, means, module, step, etc" are to be interpreted openly as referring to at least one instance of the element, device, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

Drawings

The inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:

fig. 1 is a schematic diagram illustrating an HVDC power transmission system in accordance with an embodiment;

fig. 2 and 3 are flow diagrams of methods according to embodiments;

fig. 4 is a schematic illustration of a communication protocol for communicating between a VCU and a PCU over a first communication channel, according to an embodiment;

fig. 5 is a schematic illustration of a communication protocol for communicating between a VCU and a PCU over a second communication channel, according to an embodiment;

FIG. 6 is a schematic diagram illustrating functional units of a VCU according to an embodiment;

fig. 7 is a schematic diagram showing a functional unit of the PCU according to the embodiment; and

fig. 8 shows an example of a computer program product comprising a computer readable means according to an embodiment.

Detailed Description

The inventive concepts will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concepts are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout. Any steps or features illustrated by dashed lines should be considered optional.

Fig. 1 schematically illustrates a system 100 that includes a VCU200 and N PCUs 300a, all operatively connected to respective Wireless Transceivers (WTRX) 110, 120n. Further, each PCU 300a is operatively connected to its own PEC 130a. Some examples of PECs are IGBTs, IGCTs, MOSFETs, thyristors, etc. In some aspects, the system 100 forms a portion of a valve. In some installations, there may be more than one VCU200 to provide a redundant system. In such systems, there may be a wireless link between each VCU and all PCUs.

A wireless link between the VCU200 and the PCU 300a is used to transmit the startup pulse from the VCU200 to the PCU 300a. The start pulse triggers the switching of PEC 300a. Each PCU 300a. The VCU200 may be at a fixed interval from the first start pulse (denoted as the start pulse interval (T)) _pulse ) A new start pulse is sent afterwards, which fixed interval can be as short as a few microseconds.

In addition, the VCU200 should also know whether all PCUs 300a are operating correctly (also referred to as surviving). Such information passingKeep-alive (KA) messages sent by each PCU 300a _alive Wherein T is _alive Which may be on the order of a few milliseconds. The keep-alive messages are triggered by an open keep-alive (SKA) message transmitted from the VCU200 to the PCU 300a.

All PEC 300a. In practice, this requirement is interpreted as the maximum allowable deviation Δ between the times at which two PCUs 300a receive a start pulse _pulse . The deviation may be as low as 1 microsecond or even lower.

Therefore, communication between the VCU200 and the PCU 300a over a wireless link should be designed to support such requirements, and in particular to ensure that it is possible to support every T _pulse Send a start pulse to each PCU 300a _pulse Receiving the start pulse, transmitting an instruction pulse by the PCU 300a in response to each start pulse, and every T _alive Keep-alive messages are sent by each PCU 300a.

A conventional HVDC converter based on valves comprises a large number of PECs 300a. If a wireless network is used to replace the currently employed optical fiber for communication between the VCU200 and the PCU 300a, it is challenging to ensure switching accuracy and fast startup. In accordance with at least some of the embodiments disclosed herein, there is thus provided a communication protocol employed by the VCU200 and the PCU 300a, according to which there are two separate communication channels; namely a first communication channel for the start pulse and the indication pulse and a second communication channel for the keep-alive messages. Such communication protocols are disclosed in further detail below.

Embodiments disclosed herein relate particularly to mechanisms for communication between the VCU200 and the PCU 300a in an HVDC power transmission system. To obtain such a mechanism, a VCU200, a method performed by the VCU200, a computer program product comprising code (e.g. in the form of a computer program) which, when run on processing circuitry of the VCU200, causes the VCU200 to perform the method are provided. To achieve such mechanisms, there is further provided a PCU 300a.

Referring now to fig. 2, a method for communicating with a PCU 300a in an HVDC power transmission system, as performed by the VCU200, is illustrated, in accordance with an embodiment.

S102: the VCU200 wirelessly communicates the start pulse and the instruction pulse with the PCU 300a via a first communication channel. The indication pulse is transmitted as an unmodulated symbol over the first communication channel.

S104: the VCU200 wirelessly communicates the keep-alive messages with the PCU 300a over a second communication channel. The second communication channel is separate from the first communication channel.

One reason for utilizing two different communication channels is: the two types of traffic (start pulse/indication pulse on the one hand and keep alive messages on the other hand) have different properties and requirements.

Embodiments will now be disclosed that relate to further details of communicating with the PCU 300a in the HVDC power transmission system as performed by the VCU 200.

The unmodulated symbols may be transmitted as an uncoded symbol sequence on a first communication channel for the indication pulse. The modulation symbols can be transmitted over a first communication channel for the start pulse and over a second communication channel for the keep-alive messages.

Aspects of the delivery of the initiation pulse will now be disclosed. The startup pulse is transmitted from the VCU200 to the PCU 300a. The startup pulse may be transmitted by broadcasting to the PCU 300a in the form of a packet. The startup pulse may define a trigger for switching a PEC 300a. Further aspects of the transmission of the start pulse will be disclosed below with reference to fig. 4.

Aspects of the transmission of the indication pulse will now be disclosed. The instruction pulse is transmitted from the PCU 300a. The indication pulse is delivered in response to the PCU 300a having received the initiation pulse. Further aspects of the transmission of the indication pulse will be disclosed below with reference to fig. 4.

Aspects of the transmission of keep-alive messages and turn-on keep-alive messages will now be disclosed. Keep-alive messages are transmitted from the PCU 300a to the VCU 200. Each keep-alive message may include different information, such as identity information, status information, analysis of past events, etc. of that PCU 300a that has transmitted the keep-alive message. The open-keep-alive messages may be wirelessly communicated from the VCU200 to the PCU 300a over a second communication channel. Further aspects of the transmission of keep-alive messages and turn-on keep-alive messages will be disclosed below with reference to fig. 5.

Referring now to fig. 3, a method for communicating with a VCU200 in an HVDC power transmission system as performed by the PCU 300a.

S202: the PCU 300a. The indication pulse is transmitted as an unmodulated symbol over the first communication channel.

S204: the PCU 300a. The second communication channel is separate from the first communication channel.

Embodiments will now be disclosed that relate to further details of communicating with the VCU200 in an HVDC power transmission system as performed by the PCU 300a.

The unmodulated symbols may be transmitted as an uncoded symbol sequence on a first communication channel for the indication pulse. The modulation symbols may be transmitted over a first communication channel for the initiation pulse and over a second communication channel for the message.

Aspects of the delivery of the initiation pulse will now be disclosed. As disclosed above, the initiation pulse is transmitted from the VCU200 to the PCU 300a. One of the startup pulses defines a trigger for switching the PEC 300a. Further aspects of the transmission of the start pulse will be disclosed below with reference to fig. 4.

Aspects of the transmission of the indication pulse will now be disclosed. As disclosed above, the instruction pulse is transmitted from the PCU 300a. An indication pulse is transmitted to the VCU200 in response to the PCU 300a. Further aspects of the transmission of the indication pulse will be disclosed below with reference to fig. 4.

Aspects of the transmission of keep-alive messages and turn-on keep-alive messages will now be disclosed. As disclosed above, keep-alive messages are transmitted from the PCU 300a to the VCU 200. As disclosed above, the keep-alive messages may include identity information of the PCU 300a. The keep-alive messages may be transmitted by the PCU 300a in unicast mode. As disclosed above, the open keep-alive messages may be wirelessly communicated from the VCU200 to the PCU 300a over a second communication channel. The keep-alive messages may be transmitted only in response to the PCU 300a having received an open keep-alive message from the VCU 200. Further aspects of keep-alive messages and turning on the transmission of keep-alive messages will be disclosed below with reference to fig. 5.

Embodiments will now be disclosed that relate to further details of communication between the VCU200 and the PCU 300a in an HVDC power transmission system.

Generally, the first communication channel is at a first carrier frequency F _pulse Is centered and has a first bandwidth B _pulse And the second communication channel is at a second carrier frequency F _alive Is centered and has a second bandwidth B _alive . Two separate communication channels are thus used to exchange messages between the VCU200 and the PCU 300a; i.e. a first communication channel, at a carrier frequency F _pulse Is central and has a bandwidth B _pulse Which is used to exchange the start pulse and the indication pulse; and a second communication channel at a carrier frequency F _alive Is central and has a bandwidth B _alive Which is used to transmit keep-alive messages. The first carrier frequency may be the same as or different from the second carrier frequency. The first bandwidth may be the same as or different from the second bandwidth.

In some examples, the second communication channel is logically separate from the first communication channel. Thus, the two communication channels may differ only logically, but are physically deployed on the same frequency band (i.e., F) _pulse ＝F _alive And B _pulse ＝B _alive ) Or also in two different physical channels.

In other examples, there may be more than two channels. For example, the impulse channel may be duplicated to allow for redundant communication between the PCU and two or more redundant VCUs 200. In another example, multiple parallel keep-alive channels can be used to allow keep-alive messages to be transmitted in parallel from different PCUs to a VCU.

Details of communication over the first communication channel as applicable to both the VCU200 and the PCU 300a will now be disclosed with reference to fig. 4. Fig. 4 is a schematic illustration of a communication protocol 400 for communicating between the VCU200 and the PCU 300a over a first communication channel.

At a given time (determined by a higher level of control), the VCU200 begins preparing for a start pulse. A certain amount of time is required to process the packet to be transmitted ("TX processing FP" block) and then a certain amount of time is required to transmit the packet ("start pulse" block).

The enable pulse packet is broadcast to all PCUs 300a. Depending on the propagation time between the VCU200 and each PCU 300a 300n, the PCU 300a. From T _prop，max Represents the maximum propagation time between the PCU 300a _prop，diff To represent the maximum difference between the propagation times experienced by the two PCUs 300a.

As soon as a PCU 300a has completed receiving the start pulse packet, the PCU 300a starts reception processing ("RX processing FP" block). Once the process is complete, the PCU 300a will wait a fixed amount of time ("idle time" block) during which to perform internal measurements.

After the idle time, the PCU 300a sends an instruction pulse to the VCU 200. A certain amount of time is required to process the packets to be transmitted ("TX processing IP" block). A certain amount of time is required to actually transmit the packet (the "indicator pulse" block).

The VCU200 receives the indication pulses (even if they collide with each other) and processes them. A new start pulse may be triggered after a time interval from a previous start pulse, where T _pulse Is thatMinimum value of time interval.

To respect timing requirements, all these operations should be at T _pulse The method is implemented internally. Specifically, the following conditions should be satisfied:

T _{proc，tx，FP} +T _propmax +T _FP +T _{proc，rx，FP} +T _idle +T _{proc，tx，IP} +T _prop，max +T _IP +T _{proc，rx，IP} ≤T _pulse (1)

T _{proc，tx，FP} and T _{proc，tx，IP} Is the time required for the transmission processing of the pulse packet. Since the content of these packets is fixed and will not change, these packets can be pre-processed by the VCU200, and thus T _{proc，tx，FP} ＝T _{proc，tx，IP} ＝0。

T _{proc，rx，FP} And T _{proc，rx，IP} Is the time required for the reception processing of the pulse packet in the PCU 300a. Unlike the transmission processing time, these times cannot be skipped.

T _FP And T _IP Is the time required for over the air (over the air) wireless transmission of the start pulse packet and the indication pulse packet.

T _prop，max Is the maximum propagation time, which is equal to the maximum distance between the VCU200 and the PCU 300a divided by the speed of light. In common HVDC power transmission systems, such distances can be up to several tens of meters, hence T _prop，max Will be below 1 microsecond (. Mu.s).

T _idle It is the idle time for the PCU 300a to wait for internal measurements. T is _idle The values of (a) are hardware dependent.

The signal transmitted over the first communication channel for the indication burst is not structured like a regular packet, i.e. comprises an uncoded preamble (for transmitter-receiver synchronization) and a coded payload. Instead, each indicator pulse is sent as an uncoded sequence. This sequence may be provided as a carrier at a particular carrier frequency, or it may be provided as a wider bandwidth resulting from a sequence of uncoded symbols (similar to the preamble of a regular packet)Of (2) is performed. This allows a significant reduction in the receive processing time (since complex decoding/demodulation operations are avoided), as well as the transmission time. In fact, the quantity T _{proc，rx，IP} And T _IP Can be very short (e.g., if channel bandwidth B) _pulse About a few MHz, then about one microsecond).

A further advantage of using an uncoded sequence for the indicator pulses is that it is not a problem for several indicator pulses to collide with each other at the VCU200, as shown in fig. 4. Indeed, the WRTX of VCU200 need only detect that a sequence, rather than noise, is received to determine that at least one indicator pulse has been received. It is independent of the WRTX decoding sequence of the VCU200 and is aware of its contents. In particular, after transmitting the start pulse, WRTX of VCU200 may begin to correlate the unmodulated symbols with the known symbols corresponding to the indicator pulse. If no correlation peak is detected within a certain amount of time (i.e., within the detection window), the VCU200 concludes that no indication pulse has been sent. The duration DW of the detection window may be defined as:

DW＝2T _prop，max +T _{proc，rx，FP} +T _idle +T _{proc，tx，IP+} T _IP (2)

a further requirement may be that the maximum deviation Δ be _pulse The start pulse is received by all PCUs 300a. As can be seen from FIG. 4, such time intervals are equal to T _prop，diff This, in turn, can be calculated as:

here, d _max Is the maximum distance, d, between any of the PCUs 300a _min Is the minimum distance between any of the PCUs 300a. Under the assumption that the distance between the VCU200 and the PCU 300a 300n may vary between a few meters and one hundred meters, T _prop，diff About half a microsecond.

Details of communication over the second communication channel as applicable to both the VCU200 and the PCU 300a will now be disclosed with reference to fig. 5. Fig. 5 is a schematic illustration of a communication protocol 500 for communicating between the VCU200 and the PCU 300a over a second communication channel.

The signals transmitted over the second communication channel may be transmitted in the form of standard modulated packets. Specifically, the keep-alive messages may be transmitted as packets by each PCU 300a in a contention free mode (i.e., no collisions at the WRTX of the VCU 200) in a unicast mode. To ensure that communication channel access is collision-free, each PCU 300a.

The VCU200 begins communication by broadcasting an open keep alive (SKA) message in the form of packets. Such packets need to be first processed by the VCU200 and then transmitted over the second communication channel.

SKA packets are received and processed by each PCU 300a. Then, each PCU 300a.

Once the wait time is over, the PCU 300a transmits a Keep Alive (KA) message in packets to the VCU200 using unicast.

The latency is inserted such that KA packets do not collide at the VCU 200. The latency for PCU N (where 1. Ltoreq. N.ltoreq.N) can be estimated as:

T _wait，n ＝(n-1)·T _KA +T _margin (4)

here, T _KA Is the time required to transmit KA messages over the air, and T _margin Is a margin to account for the difference in propagation time (which can be estimated as T) _prop，diff )。

To respect timing requirements, all these operations should be at T _alive And (4) carrying out internal implementation. Specifically, the following conditions should be satisfied:

T _{proc，tx，SKA} +T _prop，max +T _SKA +T _{proc，rx，SKA} +T _wait，N +T _{proc，tx，KA} +T _prop，max +T _KA +T _{proc，rx，KA} ≤T _alive (5)

T _{proc，tx，SKA} and T _{proc，tx，KA} The time required for transmission processing of SKA packets and KA packets, respectively. These times are negligible because SKA packets can be prepared in advance (the contents of which do not change over time), and KA packets can be prepared during the latency of each PCU 300a. T is _{proc，rx，SKA} And T _{proc，rx，KA} The time required for receive processing of SKA packets and KA packets, respectively. Unlike the transmission processing time, these times cannot be skipped and can be as long as several tens of microseconds. T is _SKA And T _KA Is the time required to transmit SKA packets and KA packets over the air. The length of these packets is about a few bytes and the channel bandwidth B _active On the assumption of about several MHz, T _SKA And T _KA Is about several tens of microseconds. T is _wait，N Given by equation (4), and is also about several tens of microseconds. Under these assumptions, it is feasible if T _alive At about 10 milliseconds, equation (5) holds.

The communication between the two communication channels does not require synchronization. However, since operation is initiated by the VCU200 in both communication channels, the communication protocol may be designed such that SKA messages are only sent after all start pulses have been sent.

Fig. 6 schematically illustrates the components of a VCU200 in terms of several functional units, according to an embodiment. The processing circuit 210 is provided using any combination of one or more of suitable Central Processing Units (CPUs), multiprocessors, microcontrollers, digital Signal Processors (DSPs), etc., and is capable of executing software instructions stored in a computer program product 810a (as in fig. 8), for example in the form of storage medium 230. The processing circuit 210 may further be provided as at least one Application Specific Integrated Circuit (ASIC) or Field Programmable Gate Array (FPGA).

In particular, the processing circuit 210 is configured to cause the VCU200 to perform a set of operations or steps as disclosed above. For example, the storage medium 230 may store the set of operations, and the processing circuit 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the VCU200 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuit 210 is thereby arranged to perform the method as disclosed herein.

The storage medium 230 may also include persistent storage, which may be, for example, any single memory or combination of magnetic memory, optical memory, solid state memory, or even remotely mounted memory.

The VCU200 may further include a communication interface 220 for communicating with the PCU 300a. Thus, communication interface 220 may include one or more transmitters and receivers, including analog and digital components.

The processing circuit 210 controls the general operation of the VCU200, for example, by sending data and control signals to the communication interface 220 and the storage medium 230, by receiving data and reports from the communication interface 220, and by retrieving data and instructions from the storage medium 230. Other components of VCU200 and related functions have been omitted so as not to obscure the concepts presented herein.

Fig. 7 schematically illustrates the components of the PCU 300a according to the embodiment with respect to several functional units. The processing circuit 310 is provided using any combination of one or more of suitable Central Processing Units (CPUs), multiprocessors, microcontrollers, digital Signal Processors (DSPs), etc., capable of executing software instructions stored in a computer program product 810b (as in fig. 8), for example in the form of storage medium 330. The processing circuit 310 may further be provided as at least one Application Specific Integrated Circuit (ASIC) or Field Programmable Gate Array (FPGA).

In particular, the processing circuit 310 is configured to cause the PCU 300a to perform a set of operations or steps as disclosed above. For example, the storage medium 330 may store the set of operations, and the processing circuit 310 may be configured to retrieve the set of operations from the storage medium 330 to cause the PCU 300a to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuit 310 is thereby arranged to perform a method as disclosed herein.

The storage medium 330 may also include persistent storage, which may be, for example, any single memory or combination of magnetic memory, optical memory, solid state memory, or even remotely mounted memory.

The PCU 300a. Thus, communication interface 320 may include one or more transmitters and receivers, including analog and digital components.

The processing circuit 310 controls the general operation of the PCU 300a, for example, by sending data and control signals to the communication interface 320 and the storage medium 330, by receiving data and reports from the communication interface 320, and by retrieving data and instructions from the storage medium 330. Other components of the PCU 300a.

Fig. 8 shows an example of a

computer program product

810a, 810b comprising a computer readable device 830. On the computer-readable device 830, a computer program 820a may be stored, which computer program 820a may cause the processing circuit 210 and the entities and devices operatively coupled thereto, such as the communication interface 220 and the storage medium 230, to perform a method according to embodiments described herein. Thus, the computer program 820a and/or the computer program product 810a may provide means for performing any of the steps of the VCU200 as disclosed herein. On the computer-readable device 830, a computer program 820b may be stored, which computer program 820b may cause the processing circuit 310 and entities and devices operatively coupled thereto, such as the communication interface 320 and the storage medium 330, to perform a method according to embodiments described herein. Accordingly, the computer program 820b and/or the computer program product 810b may provide means for performing any steps of the PCU 300a.

In the example of fig. 8, the

computer program products

810a, 810b are illustrated as optical discs, such as CDs (compact discs) or DVDs (digital versatile discs) or blu-ray discs. The

computer program product

810a, 810b may also be embodied as a memory such as a Random Access Memory (RAM), a Read Only Memory (ROM), an Erasable Programmable Read Only Memory (EPROM) or an Electrically Erasable Programmable Read Only Memory (EEPROM), and more particularly as a non-volatile storage medium of the device in external memory, such as a USB (universal serial bus) memory or a flash memory, such as a compact flash memory. Thus, although the

computer programs

820a, 820b are here schematically shown as tracks on the depicted optical disc, the

computer programs

820a, 820b may be stored in any way suitable for the

computer program products

810a, 810 b.

The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily apparent to a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept as defined by the appended patent claims.

Claims

1. A method for communicating with a position control unit, PCU, (300a):

wirelessly transmitting (S102) a start pulse and an indication pulse with the PCU (300a) over a first communication channel, wherein the indication pulse is transmitted as an unmodulated symbol over the first communication channel; and

wirelessly communicating (S104) a keep-alive message with the PCU (300a) over a second communication channel, wherein the second communication channel is separate from the first communication channel.

2. The method of claim 1, wherein the unmodulated symbols are transmitted as a sequence of uncoded symbols.

3. The method of claim 1, wherein a modulation symbol is communicated over the first communication channel for the initiation pulse and over the second communication channel for the keep-alive messages.

4. The method according to claim 1, wherein the initiation pulse is transmitted from the VCU (200) to the PCU (300a.

5. The method according to claim 4, wherein the initiation pulse is transmitted by broadcasting to the PCU (300a).

6. The method of claim 1, wherein the start pulse defines a trigger for switching a PEC (130a.

7. The method according to claim 1, wherein the indication pulse is transmitted from the PCU (300a.

8. The method according to claim 1, wherein the indication pulse is transmitted in response to the PCU (300a).

9. The method according to claim 1, wherein the keep-alive messages are transmitted from the PCU (300a.

10. The method according to claim 1, wherein each keep-alive message comprises identity information of the PCU (300a.

11. The method of claim 1, wherein an open keep-alive message is wirelessly communicated from the VCU (200) to the PCU (300a.

12. A method for communicating with a valve control unit VCU (200) in a high voltage direct current, HVDC, power transmission system, the method being performed by a position control unit, PCU, (300a:

wirelessly transmitting (S202) a start pulse and an indication pulse with the VCU (200) over a first communication channel, wherein the indication pulse is transmitted as an unmodulated symbol over the first communication channel; and

wirelessly communicating (S204) a keep-alive message with the VCU (200) over a second communication channel, wherein the second communication channel is separate from the first communication channel.

13. The method of claim 12, wherein the unmodulated symbols are transmitted as a sequence of uncoded symbols.

14. The method of claim 12, wherein a modulation symbol is communicated over the first communication channel for the activation pulse and over the second communication channel for the keep-alive messages.

15. The method according to claim 12, wherein the initiation pulse is transmitted from the VCU (200) to the PCU (300a.

16. The method of claim 12, wherein one of the start pulses defines a trigger for switching a PEC (130a.

17. The method according to claim 12, wherein the indication pulse is transmitted from the PCU (300a.

18. The method according to claim 17, wherein an indication pulse is transmitted in response to the PCU (300a).

19. The method according to claim 12, wherein the keep-alive messages are communicated from the PCU (300a).

20. The method according to claim 12, wherein the keep-alive message comprises identity information of the PCU (300a.

21. The method of claim 12, wherein the keep-alive messages are transmitted in a unicast mode.

22. The method of claim 12, wherein an open keep-alive message is wirelessly communicated from the VCU (200) to the PCU (300a.

23. The method of claim 17 and claim 22 in combination, wherein keep-alive messages are transmitted only in response to the PCU (300a).

24. The method of any preceding claim, wherein the second communication channel is logically separate from the first communication channel.

25. The method of any preceding claim, wherein the first communication channel is centred on a first carrier frequency and has a first bandwidth.

26. The method of any preceding claim, wherein the second communication channel is centred on a second carrier frequency and has a second bandwidth.

27. The method of claim 25 and claim 26 in combination, wherein the first carrier frequency is the same as or different from the second carrier frequency.

28. The method of claim 25 and claim 26 in combination, wherein the first bandwidth is the same as or different from the second bandwidth.

29. A valve control unit, VCU, (200) for communicating with a position control unit, PCU, (300a):

wirelessly communicating with the PCU (300a) over a first communication channel an initiation pulse and an indication pulse, wherein the indication pulse is communicated over the first communication channel as an unmodulated symbol; and

wirelessly communicating keep-alive messages with the PCU (300a) over a second communication channel, wherein the second communication channel is separate from the first communication channel.

30. The VCU (200) of claim 28, further configured to perform the method of any one of claims 2-11.

31. A position control unit, PCU, (300a, 300n) for communicating with a valve control unit, VCU, (200) in a high voltage direct current, HVDC, power transmission system, the PCU (300a:

wirelessly communicating a start pulse and an indication pulse with the VCU (200) over a first communication channel, wherein the indication pulse is communicated over the first communication channel as an unmodulated symbol; and

wirelessly communicating keep-alive messages with the VCU (200) over a second communication channel, wherein the second communication channel is separate from the first communication channel.

32. The PCU (300a 300n) according to claim 30, further configured to perform the method according to any one of claims 13 to 28.

33. A computer program (820 a) for communicating with a position control unit, PCU, (300a 300n) in a high voltage direct current, HVDC, power transmission system, the computer program comprising computer code which, when run on processing circuitry (210) of a valve control unit, VCU, (200), causes the VCU (200) to:

34. A computer program (820 b) for communicating with a valve control unit, VCU, (200) in a high voltage direct current, HVDC, power transmission system, the computer program comprising computer code which, when run on a processing circuit (310) of a position control unit, PCU, (300a:

35. A computer program product (810 a, 810 b) comprising a computer program (820 a, 820 b) according to any of claims 33 and 34 and a computer readable storage medium (830) on which the computer program is stored.