Communication between valve control units (VCUs) and position control units (PCUs) in high voltage direct current (HVDC) transmission systems

Abstract

Mechanisms for communicating with a PCU in a HVDC power transmission system are provided. A method is performed by a VCU. The method includes wirelessly transmitting, over a first communication channel, a start-up pulse and an indication pulse to the PCU. The indication pulse is transmitted as an unmodulated symbol over the first communication channel. The method includes wirelessly transmitting, over a second communication channel, keep-alive messages to the PCU. The second communication channel is separate from the first communication channel.

Description

TECHNICAL FIELD

[0001] The embodiments presented herein relate to a Valve Control Unit (VCU), a Position Control Unit (PCU), a method for communication between a VCU and a PCU in a High Voltage Direct Current (HVDC) power transmission system, a computer program and a computer program product. BACKGROUND

[0002] Generally, HVDC power transmission systems rely on converters to convert power from Alternating Current (AC) to Direct Current (DC) and vice versa. In a traditional HVDC converter, many Power Electronic Components (PECs) are connected in series, forming a valve. Some examples of PECs are IGBTs, IGCTs, MOSFETs, thyristors, etc. In each valve, a VCU triggers the switching of all PECs by transmitting a start-up pulse simultaneously to the PCUs connected to each PEC. Optical fiber links are usually used to connect the VCU to the PCUs in a galvanically isolated way, allowing the transmission of the start-up pulse triggering the switching of the PECs.

[0003] The number of PECs employed in a HVDC system grows with the rated power, increasing proportionally to several hundreds in each valve, with the consequent need to deploy several thousands of optical fibers throughout the converter. Therefore, significant costs are involved in the installation and commissioning of all these optical fibers. Moreover, since optical fibers are subject to high potentials, they can be damaged over time or even cause flammability issues, requiring strict and costly restrictions on the climate in the valve. For these reasons, the possibility of replacing optical fibers with wireless links is attractive both for cost reduction and safety reasons.

[0004] While the possibility of using wireless links between the VCU and the PCUs to control the switching of the PECs is attractive, due to the intrinsic limitations of the wireless channel, it is challenging to guarantee the timing and reliability of the communication made through the wireless links. Indeed, unlike an optical fiber network where each link is independent, the wireless channel is shared and if more than one entity transmits at the same time on the same frequency band, the messages will collide at the receiver and can not be decoded correctly. Moreover, the wireless channel is error prone and it is challenging to guarantee that all messages are delivered, which raises issues in a HVDC power transmission system where all PECs in a valve must be started. Finally, since the condition of the wireless channel varies over time, it is also complex to guarantee a deterministic communication delay, which raises issues in a HVDC power transmission system since all PECs should be switched at the same time for the power conversion to be done correctly.

[0005] Therefore, there is a need to improve the communication between a VCU and a PCU in a wireless HVDC power transmission system. SUMMARY

[0006] It is an object of embodiments herein to provide efficient communication between a VCU and a PCU in a wireless HVDC power transmission system.

[0007] According to a first aspect, a method for communicating with a PCU in a HVDC power transmission system is presented. The method is performed by a VCU. The method comprises wirelessly transmitting, over a first communication channel, a start pulse and an indication pulse to the PCU. The indication pulse is transmitted as an unmodulated symbol over the first communication channel. The method comprises wirelessly transmitting, over a second communication channel, keep-alive messages to the PCU. The second communication channel is separate from the first communication channel.

[0008] According to a second aspect, a VCU for communicating with a PCU in a HVDC power transmission system is presented. The VCU comprises processing circuitry. The processing circuitry is configured to cause the VCU to wirelessly transmit, over a first communication channel, a start pulse and an indication pulse to the PCU. 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 transmit, over a second communication channel, keep-alive messages to the PCU. The second communication channel is separate from the first communication channel.

[0009] According to a third aspect, a computer program for communicating with a PCU in a 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.

[0010] According to a fourth aspect, a method for communicating with a VCU in a HVDC power transmission system is presented. The method is performed by a PCU. The method comprises wirelessly transmitting, over a first communication channel, a start pulse and an indication pulse to the VCU. The indication pulse is transmitted as an unmodulated symbol over the first communication channel. The method comprises wirelessly transmitting, over a second communication channel, keep-alive messages to the VCU. The second communication channel is separate from the first communication channel.

[0011] According to a fifth aspect, a PCU for communicating with a VCU in a HVDC power transmission system is presented. The PCU comprises processing circuitry. The processing circuitry is configured to cause the PCU to wirelessly transmit, over a first communication channel, a start pulse and an indication pulse to the VCU. The indication pulse is transmitted as an unmodulated symbol over the first communication channel. The processing circuitry is configured to cause the PCU to wirelessly transmit, over a second communication channel, keep-alive messages to the VCU. The second communication channel is separate from the first communication channel.

[0012] According to a sixth aspect, a computer program for communicating with a VCU in a 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.

[0013] According to the 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 on which the computer program is stored. The computer-readable storage medium may be a non-transitory computer-readable storage medium.

[0014] Advantageously, these aspects provide efficient wireless communication between the VCU and PCU in HVDC transmission systems.

[0015] Advantageously, these aspects provide efficient control of the PCU without the need for cables to be installed for communication between the VCU and the PCU.

[0016] Advantageously, these aspects avoid the potential dangers caused by the flammability of optical fibers when subjected to high potential differences.

[0017] Advantageously, these aspects can extend the lifespan of control systems used in power electronics, which is often shortened due to the poor durability of optical fibers.

[0018] Advantageously, these aspects enable wirelessly controlled HVDC transmission systems to meet the timing constraints required for starting PECs operationally connected to the PCU, including minimum intervals between start pulses, synchronization of switching times, and minimum intervals between keep-alive messages.

[0019] Advantageously, these aspects can be applied to HVDC transmission systems as well as other applications (e.g., Flexible AC Transmission Systems (FACTS)).

[0020] Other objects, features, and advantages of the accompanying embodiments will become apparent from the following detailed disclosure, the appended dependent claims, and the accompanying drawings.

[0021] Generally, unless otherwise expressly defined herein, all terms used in the claims shall be interpreted according to their ordinary meaning in the art. Unless otherwise expressly stated, all references to “a / an / the element, device, component, apparatus, module, step, etc.” shall be openly interpreted as referring to at least one instance of an element, device, component, apparatus, module, step, etc. Unless expressly stated otherwise, the steps of any method disclosed herein need not be performed in the exact order disclosed. Attached Figure Description

[0022] The inventive concept will now be described by way of example with reference to the accompanying drawings, in which:

[0023] Figure 1 This is a schematic diagram of an HVDC power transmission system according to an embodiment;

[0024] Figure 2 and Figure 3 is a flowchart of a method according to an embodiment;

[0025] Figure 4 is a schematic illustration of a communication protocol for communication between a VCU and a PCU over a first communication channel according to an embodiment;

[0026] Figure 5 is a schematic illustration of a communication protocol for communication between a VCU and a PCU over a second communication channel according to an embodiment;

[0027] Figure 6 is a schematic diagram illustrating functional units of a VCU according to an embodiment;

[0028] Figure 7 is a schematic diagram illustrating functional units of a PCU according to an embodiment; and

[0029] Figure 8 One example of a computer program product comprising computer readable means according to an embodiment is shown. DETAILED DESCRIPTION

[0030] The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventive concept are illustrated. 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 the description. Any step or feature illustrated by dashed lines should be regarded as optional.

[0031] Figure 1 A system 100 is schematically illustrated, comprising one VCU 200 and N PCUs 300a:300N, all of which are operatively connected to respective wireless transceivers (WTRX) 110, 120a:120N. Further, each PCU 300a:300N is operatively connected to its own PEC 130a:130N. Some examples of PECs are IGBT, IGCT, MOSFET, thyristor, etc. In some aspects, the system 100 forms part of a valve. In some installations, there can be more than one VCU 200 to provide a redundant system. In such systems, there can be a wireless link between each VCU and all PCUs.

[0032] The wireless link between the VCU 200 and the PCUs 300a:300N is used to send start pulses from the VCU 200 to the PCUs 300a:300N. The start pulses trigger switching of the PECs 300a:300N. Each PCU 300a:300N performs local measurements after each start pulse and replies to the VCU 200 with a message, called an indication pulse. The VCU 200 can send a new start pulse after a fixed interval, denoted as start pulse interval (T pulse ), from the first start pulse, which can be as short as a few microseconds.

[0033] In addition, the VCU 200 should also know whether all PCUs 300a:300N are operating correctly (also referred to as alive). Such information is conveyed by keep-alive (KA) messages sent by each PCU 300a:300N, with a maximum interval between two messages from the same PCU 300a:300N of T alive , where T alive may be on the order of a few milliseconds. The keep-alive messages are triggered by a start-keep-alive (SKA) message transmitted from the VCU 200 to the PCUs 300a:300N.

[0034] All PECs 300a:300N should switch at the same time, with the consequence that all PCUs 300a:300N should receive a start pulse at the same time. In practice, this requirement is interpreted as a maximum allowed deviation between the times at which two PCUs 300a:300N receive a start pulse of Δ pulse . This deviation can be as low as 1 microsecond or even lower.

[0035] Therefore, the communication between the VCU 200 and the PCUs 300a:300N over the wireless link should be designed to support such requirements and, in particular, to ensure that a start pulse can be sent to each PCU 300a:300N every T pulse , received with a maximum time deviation of Δ pulse , an indication pulse is sent by the PCUs 300a:300N in response to each start pulse, and a keep-alive message is sent by each PCU 300a:300N every T alive .

[0036] A valve-based traditional HVDC converter comprises a large number of PECs 300a:300N that shall be started simultaneously and often. It is challenging to guarantee switching accuracy and fast start-up if a wireless network is used instead of the optical fiber currently employed for communication between the VCU 200 and the PCUs 300a:300N. According to at least some of the embodiments disclosed herein, there is thus provided a communication protocol employed by the VCU 200 and the PCUs 300a:300N according to which there are two separate communication channels; i.e. a first communication channel for start-up pulses and indication pulses, and a second communication channel for keep-alive messages. Such a communication protocol will be disclosed in further detail below.

[0037] The embodiments disclosed herein relate in particular to mechanisms for communication between a VCU 200 and a PCU 300a:300N in an HVDC power transmission system. In order to obtain such mechanisms, there is provided a VCU 200, a method performed by the VCU 200, a computer program product comprising code, e.g. in the form of a computer program, which when run on processing circuitry of the VCU 200 causes the VCU 200 to perform the method. In order to obtain such mechanisms, there is further provided a PCU 300a:300N, a method performed by the PCU 300a:300N, and a computer program product comprising code, e.g. in the form of a computer program, which when run on processing circuitry of the PCU 300a:300N causes the PCU 300a:300N to perform the method.

[0038] Reference is now made to Figure 2 which illustrates a method for communication with a PCU 300a:300N in an HVDC power transmission system as performed by a VCU 200 according to an embodiment.

[0039] S102: The VCU 200 wirelessly communicates start-up pulses and indication pulses with the PCU 300a:300N over a first communication channel. The indication pulses are communicated over the first communication channel as unmodulated symbols.

[0040] S104: The VCU 200 wirelessly communicates keep-alive messages with the PCU 300a:300N over a second communication channel. The second communication channel is separate from the first communication channel.

[0041] One reason for utilizing two different communication channels is that the two types of traffic, on the one hand start-up pulses / indication pulses and on the other hand keep-alive messages, have different properties and requirements.

[0042] Embodiments relating to further details of the communication with the PCU 300a:300N in the HVDC power transmission system as performed by the VCU 200 will now be disclosed.

[0043] The unmodulated symbol can be transmitted for the indication pulse as an unencoded symbol sequence on the first communication channel. The modulated symbol can be transmitted for the start-up pulse over the first communication channel and for the keep-alive messages over the second communication channel.

[0044] Aspects of the transmission of the start-up pulse will now be disclosed. The start-up pulse is transmitted from the VCU 200 to the PCU 300a:300N. The start-up pulse can be transmitted by being broadcasted in the form of a packet to the PCU 300a:300N. The start-up pulse can define a trigger for switching the PEC 300a:300N operatively connected to the PCU 300a:300N. Further aspects of the transmission of the start-up pulse will be disclosed below with reference to Figure 4

[0045] Aspects of the transmission of the indication pulse will now be disclosed. The indication pulse is transmitted from the PCU 300a:300N to the VCU 200. The indication pulse is transmitted in response to the PCU 300a:300N having received the start-up pulse. Further aspects of the transmission of the indication pulse will be disclosed below with reference to Figure 4

[0046] Aspects of the transmission of the keep-alive messages and the open keep-alive message will now be disclosed. The keep-alive messages are transmitted from the PCU 300a:300N to the VCU 200. Each keep-alive message can comprise different information, such as identity information of that PCU 300a:300N which has transmitted the keep-alive message, status information, analysis of past events, etc. The open keep-alive message can be wirelessly transmitted from the VCU 200 to the PCU 300a:300N over the second communication channel. Further aspects of the transmission of the keep-alive messages and the open keep-alive message will be disclosed below with reference to Figure 5

[0047] Reference will now be made to Figure 3 which illustrates a method for communicating with the VCU 200 in the HVDC power transmission system as performed by the PCU 300a:300N according to an embodiment.

[0048] S202: The PCU 300a:300N wirelessly transmits a start-up pulse and an indication pulse with the VCU 200 over a first communication channel. The indication pulse is transmitted as an unmodulated symbol over the first communication channel.

[0049] S204: The PCU 300a:300N wirelessly transmits a keep-alive message with the VCU 200 over a second communication channel. The second communication channel is separate from the first communication channel.​​​

[0050] Embodiments relating to further details of the communication with the VCU 200 in the HVDC power transmission system as performed by the PCU 300a:300N will now be disclosed.

[0051] The unmodulated symbols can be transmitted on the first communication channel as an unencoded sequence of symbols indicating the pulses. The modulated symbols can be transmitted through the first communication channel for the start-up pulses and through the second communication channel for the messages.

[0052] Aspects of the transmission of the start-up pulses will now be disclosed. As disclosed above, the start-up pulses are transmitted from the VCU 200 to the PCU 300a:300N. One of the start-up pulses defines a trigger for switching the PEC 300a:300N operatively connected to the PCU 300a:300N. Further aspects of the transmission of the start-up pulses will be disclosed below with reference to Figure 4

[0053] Aspects of the transmission of the indication pulses will now be disclosed. As disclosed above, the indication pulses are transmitted from the PCU 300a:300N to the VCU 200. One indication pulse is transmitted to the VCU 200 in response to the PCU 300a:300N having received one start-up pulse. Further aspects of the transmission of the indication pulses will be disclosed below with reference to Figure 4

[0054] Aspects of the transmission of the keep-alive messages and the open keep-alive messages will now be disclosed. As disclosed above, the keep-alive messages are transmitted from the PCU 300a:300N to the VCU 200. As disclosed above, the keep-alive messages can comprise identity information of the PCU 300a:300N. The keep-alive messages can be transmitted by the PCU 300a:300N in a unicast mode. As disclosed above, the open keep-alive messages can be wirelessly transmitted from the VCU 200 to the PCU 300a:300N through the second communication channel. The keep-alive messages can only be transmitted in response to the PCU 300a:300N having received the open keep-alive message from the VCU 200. Further aspects of the transmission of the keep-alive messages and the open keep-alive messages will be disclosed below with reference to Figure 5

[0055] Embodiments relating to further details of the communication between the VCU 200 and the PCU 300a:300N in the HVDC power transmission system will now be disclosed.

[0056] Generally, the first communication channel is centered at a first carrier frequency F pulse and has a first bandwidth B pulse , and the second communication channel is centered at a second carrier frequency F alive and has a second bandwidth B​​​alive Two separate communication channels are thus used to exchange messages between the VCU 200 and the PCUs 300a:300N; namely, a first communication channel, centered on a carrier frequency F pulse and having a bandwidth B pulse which is used to exchange start-up pulses and indication pulses; and a second communication channel, centered on a carrier frequency F alive and having a bandwidth B alive which is used to transmit keep-alive messages. The first carrier frequency is the same or different from the second carrier frequency. The first bandwidth is the same or different from the second bandwidth.

[0057] In some examples, the second communication channel is logically separate from the first communication channel. Thus, the two communication channels can only differ 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.

[0058] In other examples, there can be more than two channels. For example, the pulse channel can be replicated to account 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 the VCU.

[0059] Details of the communication over the first communication channel, as applicable to both the VCU 200 and the PCUs 300a:300N, will now be disclosed with reference to Figure 4 . Figure 4 is a schematic illustration of a communication protocol 400 for communicating over the first communication channel between the VCU 200 and the PCUs 300a:300N.

[0060] At a given time instant (determined by higher-level control), the VCU 200 starts preparing a start-up pulse. A certain amount of time is needed to process the packet to be transmitted (“TX processing FP” block), and then a certain amount of time is needed to transmit the packet (“start-up pulse” block).

[0061] The start-up pulse packet is broadcast to all PCUs 300a:300N. Depending on the propagation time between the VCU 200 and each PCU 300a:300N, the PCUs 300a:300N receive the start-up pulse packet at different time instants. The maximum propagation time between a PCU 300a:300N and the VCU 200 is denoted by T prop，max , and the minimum propagation time between a PCU 300a:300N and the VCU 200 is denoted by T prop，diffto express the maximum difference between the propagation times experienced by the two PCUs 300a:300N.

[0062] As soon as a PCU 300a:300N has completed the reception of the start impulse packet, it starts the reception processing ("RX processing FP" block). Once this processing is completed, the PCU 300a:300N waits for a fixed amount of time ("idle time" block), during which internal measurements are implemented.

[0063] After the idle time, the PCU 300a:300N sends an indication impulse to the VCU 200. An amount of time is required to process the packet to be transmitted ("TX processing IP" block). An amount of time is required to actually transmit the packet ("indication impulse" block).

[0064] The VCU 200 receives the indication impulses (even if these indication impulses collide with each other) and processes them. A new start impulse can be triggered after a time interval from the previous start impulse, where T pulse is the minimum value of such time interval.

[0065] In order to respect the timing requirements, all these operations should be implemented within T pulse . In particular, the following conditions should be met:

[0066] 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)

[0067] T proc，tx，FP and T proc，tx，IP are the times required for the transmission processing of the impulse packets. Since the content of these packets is fixed and will not change, these packets can be pre-processed by the VCU 200 and therefore T proc，tx，FP = T proc，tx，IP = 0.

[0068] T proc，rx，FP and T proc，rx，IP are the times required for the reception processing of the impulse packets in the PCU 300a:300N. Unlike the transmission processing times, these times cannot be skipped.

[0069] T FP and T IPis the time required for over the air wireless transmission of the start pulse packet and of the indication pulse packets.

[0070] T prop，max is the maximum propagation time, which is equal to the maximum distance between the VCU 200 and the PCU 300a:300N divided by the speed of light. In common HVDC power transmission systems, such distances can be up to several tens of meters, so the value of T prop，max will be lower than 1 microsecond (ps).

[0071] T idle is the idle time that the PCU 300a:300N waits for the internal measurement. The value of T idle is hardware dependent.

[0072] The signal transmitted by the first communication channel for the indication pulses is not structured like a regular packet, i.e. it comprises an uncoded preamble (for transmitter-receiver synchronization) and a coded payload. Instead, each indication pulse is sent as an uncoded sequence. This sequence can be provided as a carrier at a specific carrier frequency, or it can be provided as a pulse with a wider bandwidth generated by an uncoded sequence of symbols (similar to the preamble of a regular packet). This allows to greatly reduce the reception processing time (as complex decoding / demodulation operations are avoided), and the transmission time. Indeed, the quantities T proc，rx，IP and T IP can be very short (e.g. of the order of one microsecond if the channel bandwidth B pulse is of the order of a few MHz).

[0073] A further advantage of using uncoded sequences for the indication pulses is that several indication pulses colliding with each other at the VCU 200 is not a problem, as Figure 4 shown in Fig. 2. Indeed, the WRTX of the VCU 200 just needs to detect that a sequence, and not noise, is received to determine that at least one indication pulse has been received. It does not matter that the WRTX of the VCU 200 decodes the sequence and understands its content. In detail, after sending the start pulse, the WRTX of the VCU 200 can start correlating the unmodulated symbols with known symbols corresponding to the indication pulses. If no correlation peak is detected within a certain amount of time (i.e. within a detection window), the VCU 200 concludes that no indication pulse has been sent. The duration of the detection window DW can be defined as:

[0074] DW = 2T prop，max + T proc，rx，FP + T idle + T proc，tx，IP+ T IP (2)

[0075] A further requirement can be to transmit the indication pulses with a maximum deviation Apulse The start pulse is received by all PCUs 300a:300N. From Figure 4 It can be seen that such time interval is equal to T prop，diff which in turn can be calculated as:

[0076]

[0077] Here, d max is the maximum distance between any of the PCUs 300a:300N and the VCU 200, d min is the minimum distance between any of the PCUs 300a:300N and the VCU 200, and c is the speed of light. Under the assumption that the distance between the VCU 200 and the PCUs 300a:300N can vary between a few meters and a hundred meters, T prop，diff is approximately half a microsecond.

[0078] Details of the communication through the second communication channel, as applicable to both the VCU 200 and the PCUs 300a:300N, will now be disclosed with reference to Figure 5 Figure 5 is a schematic illustration of a communication protocol 500 for communicating between the VCU 200 and the PCUs 300a:300N through the second communication channel.

[0079] The signals transmitted through the second communication channel can be transmitted in the form of standard modulated packets. In particular, keep-alive messages can be transmitted by each PCU 300a:300N in unicast mode as packets in a contention-free mode, i.e. without collisions at the WRTX of the VCU 200. In order to ensure that the communication channel access is collision-free, each PCU 300a:300N delays its transmission by a specific amount of time from the reception of an initial packet transmitted by the VCU 200 in broadcast mode, e.g. containing the start pulse.

[0080] The VCU 200 starts the communication by broadcasting a start keep-alive (SKA) message in the form of a packet. Such packet needs to be first processed by the VCU 200 and then transmitted through the second communication channel.

[0081] The SKA packet is received and processed by each PCU 300a:300N. Then, each PCU 300a:300N waits for a specific amount of time, which is different for each PCU 300a:300N.

[0082] Once the waiting time is over, the PCUs 300a:300N transmit keep-alive (KA) messages to the VCU 200 in the form of packets using unicast.

[0083] ​The waiting time is inserted so that KA packets do not collide at the VCU 200. The waiting time of the PCU n, with 1 ≤ n ≤ N, can be estimated as:

[0084] T wait，n = (n - 1) · T KA + T margin (4)

[0085] Here, T KA is the time needed for the over-the-air transmission of a KA message, and T margin is a margin to account for differences in propagation times (which can be estimated as T prop，diff ).

[0086] To respect the timing requirements, all these operations should be implemented within T alive . In particular, the following conditions should be met:

[0087] 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)

[0088] T proc，tx，SKA and T proc，tx，KA are the times needed for the transmission processing of a SKA packet and a KA packet, respectively. These times can be negligible, as the SKA packet (whose content does not change over time) can be prepared in advance, and the KA packet can be prepared during the waiting time of each PCU 300a:300N. T proc，rx，SKA and T proc，rx，KA are the times needed for the reception processing of a SKA packet and a KA packet, respectively. Unlike the transmission processing times, these times cannot be skipped and can be as long as tens of microseconds. T SKA and T KA are the times needed for the over-the-air transmission of a SKA packet and a KA packet. Under the assumption that the length of these packets is on the order of a few bytes and that the channel bandwidth B active is on the order of a few MHz, the values of T SKA and T KA are on the order of tens of microseconds. T wait，N is given by equation (4) and is also on the order of tens of microseconds. Under these assumptions, it is feasible that equation (5) holds if T alive is on the order of 10 milliseconds.

[0089] The communication between the two communication channels does not need to be synchronized. However, since the operation in both communication channels is initiated by the VCU 200, the communication protocol can be designed such that the SKA message is only sent after all start-up pulses have been sent.

[0090] Figure 6 The components of the VCU 200 according to an embodiment are schematically illustrated in terms of a number of functional units. The processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 810a (as in Figure 8 ) in the form of a storage medium 230. The processing circuitry 210 can further be provided as at least one application-specific integrated circuit (ASIC), or field programmable gate array (FPGA).

[0091] In particular, the processing circuitry 210 is configured to cause the VCU 200 to perform a set of operations or steps as disclosed above. For example, the storage medium 230 can store the set of operations, and the processing circuitry 210 can be configured to retrieve the set of operations from the storage medium 230 to cause the VCU 200 to perform the set of operations. The set of operations can be provided as a set of executable instructions. Hence, the processing circuitry 210 is thereby arranged to perform the methods as disclosed herein.

[0092] The storage medium 230 can also include a non-volatile storage such as, for example, a magnetic storage, an optical storage, a solid state storage, or even a remotely installed storage, any single one of or a combination of.

[0093] The VCU 200 can further comprise a communication interface 220 for communication with the PCU 300a:300N. The communication interface 220 can thus comprise one or more transmitters and receivers, including analog and digital components.

[0094] The processing circuitry 210 controls the general operation of the VCU 200, e.g. 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 the VCU 200, as well as their

[0095] Figure 7The components of the PCU 300a:300N according to an embodiment are schematically illustrated with respect to a number of functional units. Processing circuitry 310, which can comprise any combination of one or more of a microprocessor, a microcontroller, a digital signal processor (DSP), a graphics processing unit (GPU), a central processing unit (CPU), a state machine, and / or other processing structures, is used to provide functionality to the PCU 300a:300N. The processing circuitry 310 is capable of executing software instructions stored in a computer program product 810b (as in Figure 8

[0096] In particular, the processing circuitry 310 is configured to cause the PCU 300a:300N to perform a set of operations or steps as disclosed above. For example, the storage medium 330 can store the set of operations, and the processing circuitry 310 can be configured to retrieve the set of operations from the storage medium 330 to cause the PCU 300a:300N to perform the set of operations. The set of operations can be provided as a set of executable instructions. Thus, the processing circuitry 310 is thereby arranged to perform the methods as disclosed herein.

[0097] The storage medium 330 can further include a non-transitory storage device, which can be any single one or combination of magnetic storage, optical storage, solid state storage, or even remotely mounted storage.

[0098] The PCU 300a:300N can further include a communication interface 320 for communication with the VCU 200 and the thyristors 130a:130N. As such, the communication interface 320 can include one or more transmitters and receivers, including analog and digital components.

[0099] The processing circuitry 310 controls the general operation of the PCU 300a:300N, e.g., 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 and related functionality of the PCU 300a:300N are omitted in order not to obscure the concepts presented herein.

[0100] Figure 8 ​One example of a computer program product 810a, 810b comprising a computer readable means 830 is shown. On this computer readable means 830, a computer program 820a can be stored, which computer program 820a can cause the processing circuitry 210, and thereto operatively coupled entities and devices such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein. The computer program 820a and / or computer program product 810a can thus provide means for performing any steps of a VCU 200 as disclosed herein. On this computer readable means 830, a computer program 820b can be stored, which computer program 820b can cause the processing circuitry 310, and thereto operatively coupled entities and devices such as the communications interface 320 and the storage medium 330, to execute methods according to embodiments described herein. The computer program 820b and / or computer program product 810b can thus provide means for performing any steps of a PCU 300a:300N as disclosed herein.

[0101] In Figure 8 In the example shown, the computer program product 810a, 810b is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 810a, 810b could 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 a device, e.g., an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Therefore, although the computer program 820a, 820b is here schematically illustrated as a track on the depicted optical disk, the computer program 820a, 820b can be stored in any way which is suitable for a computer program product 810a, 810b.

[0102] The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by 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:300N) in a high voltage direct current, HVDC, power transmission system, the method being performed by a valve control unit, VCU, (200), the method comprising: transmitting start pulses and indication pulses wirelessly with the PCU (300a:300N) over a first communication channel, wherein the indication pulses are transmitted as unmodulated symbols over the first communication channel; and transmitting keep-alive messages wirelessly with the PCU (300a:300N) 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 unencoded symbol sequences.

3. The method of claim 1, wherein, modulated symbols are transmitted for the start pulses over the first communication channel and for the keep-alive messages over the second communication channel.

4. The method of claim 1, wherein, the start pulses are transmitted from the VCU (200) to the PCU (300a:300N).

5. The method of claim 4, wherein, the start pulses are transmitted by broadcasting in packets to the PCU (300a:300N).

6. The method of claim 1, wherein, the start pulses define triggers for switching PECs (130a:130N) operatively connected to the PCU (300a:300N).

7. The method of claim 1, wherein, the indication pulses are transmitted from the PCU (300a:300N) to the VCU (200).

8. The method of claim 1, wherein, the indication pulses are transmitted in response to the PCU (300a:300N) having received the start pulses.

9. The method of claim 1, wherein, the keep-alive messages are transmitted from the PCU (300a:300N) to the VCU (200).

10. The method of claim 1, wherein, each keep-alive message comprises identity information of that PCU (300a:300N) which has transmitted the keep-alive message.

11. The method of claim 1, wherein, an open keep-alive message is transmitted wirelessly from the VCU (200) to the PCU (300a:300N) over the second communication channel.

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:300N), the method comprising: transmitting start pulses and indication pulses wirelessly with the VCU (200) over a first communication channel, wherein the indication pulses are transmitted as unmodulated symbols over the first communication channel; and transmitting keep-alive messages wirelessly 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 unencoded symbol sequences.

14. The method of claim 12, wherein, modulated symbols are transmitted for the start pulses over the first communication channel and for the keep-alive messages over the second communication channel.

15. The method of claim 12, wherein, the start pulses are transmitted from the VCU (200) to the PCU (300a:300N).

16. The method of claim 12, wherein, one of the start pulses defines a trigger for switching PECs (130a:130N) operatively connected to the PCU (300a:300N).

17. The method of claim 12, wherein, The indication pulse is transmitted from the PCU (300a:300N) to the VCU (200).

18. The method of claim 17, wherein, transmitting an indication pulse in response to the PCU (300a:300N) having received a start pulse.

19. The method of claim 12, wherein, The keep-alive message is transmitted from the PCU (300a:300N) to the VCU (200).

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

21. The method of claim 12, wherein, The keep-alive message is transmitted in unicast mode.

22. The method of claim 12, wherein, The start keep-alive message is wirelessly transmitted from the VCU (200) to the PCU (300a:300N) over the second communication channel.

23. The method of claim 17, wherein, The start keep-alive message is wirelessly transmitted from the VCU (200) to the PCU (300a:300N) over the second communication channel; and wherein the keep-alive message is transmitted only in response to the PCU (300a:300N) having received the start keep-alive message from the VCU (200).

24. The method of claim 12, wherein, The second communication channel is logically separate from the first communication channel.

25. The method of claim 12, wherein, The first communication channel is centered at a first carrier frequency and has a first bandwidth.

26. The method of claim 12, wherein, The second communication channel is centered at a second carrier frequency and has a second bandwidth.

27. The method of claim 25, wherein, The second communication channel is centered at a second carrier frequency and has a second bandwidth; and The first carrier frequency is the same as or different from the second carrier frequency.

28. The method of claim 25, wherein, The second communication channel is centered at a second carrier frequency and has a second bandwidth; and 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:300N) in a high voltage direct current, HVDC, power transmission system, the VCU (200) comprising processing circuitry (210) configured to cause the VCU (200) to: transmitting start pulses and indication pulses wirelessly with the PCUs (300a:300N) via the first communication channel, wherein The indication pulse is transmitted as an unmodulated symbol over the first communication channel; and wirelessly transmit a keep-alive message with the PCU (300a:300N) over a second communication channel, wherein the second communication channel is separate from the first communication channel.

30. The VCU (200) of claim 29, 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:300N) comprising processing circuitry (310) configured to cause the PCU (300a:300N) to: transmitting start pulses and indication pulses wirelessly 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 transmit 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.

32. The PCU (300a:300N) of claim 31, further configured to perform the method of any one of claims 13-28.

33. A computer-readable storage medium having stored thereon a computer program (820a), wherein the computer program is for communicating with a position control unit, PCU, (300a:300N) in a high voltage direct current, HVDC, power transmission system, and the computer program comprises computer code which, when run on processing circuitry (210) of a valve control unit, VCU, (200), causes the VCU (200) to: transmit the indication pulse as an unmodulated symbol over the first communication channel; and transmit keep-alive messages wirelessly with the PCU (300a:300N) over a second communication channel, wherein the second communication channel is separate from the first communication channel. transmitting start pulses and indication pulses wirelessly with the PCUs (300a:300N) via the first communication channel, wherein 34. A computer-readable storage medium having stored thereon a computer program (820b), wherein the computer program is for communicating with a valve control unit, VCU, (200) in a high voltage direct current, HVDC, power transmission system, and the computer program comprises computer code which, when run on processing circuitry (310) of a position control unit, PCU, (300a:300N), causes the PCU (300a:300N) to: transmit the indication pulse as an unmodulated symbol over the first communication channel; and transmit keep-alive messages wirelessly with the VCU (200) over a second communication channel, wherein the second communication channel is separate from the first communication channel.

35. A computer program product (810a, 810b) comprising a computer-readable storage medium according to any one of claims 33 and 34, having stored thereon a computer program (820a, 820b). ​ ​ transmitting start pulses and indication pulses wirelessly with the VCU (200) over a first communication channel, wherein ​ ​ ​ ​

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