WO2021233522A1

WO2021233522A1 - Closed loop control of modular-multilevel converter by means of wireless exchange between valve control unit and cell control units

Info

Publication number: WO2021233522A1
Application number: PCT/EP2020/063839
Authority: WO
Inventors: Michele LUVISOTTO; Yuhei OKAZAKI; Mikael Davidsson; Zhibo PANG; Roger JANSSON; Daniel Hallmans; Jimmy Öhman; Christer SJÖBERG
Original assignee: Abb Power Grids Switzerland Ag
Priority date: 2020-05-18
Filing date: 2020-05-18
Publication date: 2021-11-25

Abstract

There is provided mechanisms for controlling MMCs in a HVDC transmission system. A method is performed by a VCU of the MMCs. The method comprises wirelessly receiving, during a current update period, voltage measurements of a previous update period from PCUs of the MMCs. The current update period subsequently follows the previous update period. The method comprises determining a switching command for the PCUs for a next update period based on the voltage measurements of the previous update period. The next update period subsequently follows the current update period. The method comprises controlling the MMCs by, during the next update period, wirelessly transmitting the switching command to the PCUs.

Description

CLOSED LOOP CONTROL OF MODULAR-MULTILEVEL CONVERTER BY MEANS OF WIRELESS EXCHANGE BETWEEN VALVE CONTROL UNIT AND CELL CONTROL UNITS

TECHNICAL FIELD

Embodiments presented herein relate to methods, a PCU unit (VCU), position control units (PCUs), computer programs, and a computer program product for controlling modular multilevel converters (MMCs), in a high-voltage direct current (HVDC) transmission system.

BACKGROUND

In modern HVDC transmission systems, MMCs are employed for power conversion. In such topologies, a set of power electronics cells are connected in series, each of which comprising a circuit of semiconductor devices (e.g. insulated-gate bipolar transistors; IGBTs) and a PCU. A desired voltage waveform is generated by switching on/off individual PCUs according to a certain pattern, defined by a switching command, which is determined by a VCU.

In order to achieve a good resolution on the generated voltage waveform, the switching of the PCUs should be controlled with high update frequencies, which requires high-speed communication between the VCU and the PCUs. Such a communication is bidirectional: the VCU periodically sends the switching commands to the PCUs and receives voltage measurements of each PCU. These voltage measurements are used to compute the future switching commands. Communication links between VCU and PCUs are typically realized with optical fibers, which offer high speed and reliable transmission. However, significant costs are involved with the installation and commissioning of optical fibers in large HVDC transmission systems that might involve hundreds of power electronics cells and thousands of cables. Furthermore, since the optical fibers might be subject to high electric potential, the optical fibers might be damaged over time or even cause flammability issues, which requires tight and costly restrictions to the climate in the valve. For these reasons, the possibility to replace optical fibers with wireless links is attractive for both cost reduction and safety reasons.

The closed-loop control of PCUs in HVDC systems has very high demands on the communication. In particular, the communication system should ensure that within one update period the VCU sends a switching command to all the PCUs and receives an individual feedback from each PCU. Such an update period can be as short as a few tens of microseconds, with the number of PCUs being as high as a few tens.

While these performance figures can be achieved with optical fibers, it is challenging to guarantee the same performance when using wireless networks due to two main limitations of the latter. On one hand, the shared nature of the wireless channel imposes that not more than one wireless device can transmit at a given time on a given frequency band. Consequently, individual messages from PCUs containing voltage measurements cannot be parallelized, as it can be done with optical fiber links, and the voltage measurements must thus be transmitted one after the other, ending up taking a significant part of the update period. On the other hand, the unreliable nature of the wireless channel imposes to use complex modulation and coding schemes at the transmitter and receiver, which take additional processing time with respect to optical fibers transceivers and further contribute to increasing the update period. Hence, there is still a need for an improved control of MMCs in a wirelessly controlled HVDC transmission system.

SUMMARY

An object of embodiments herein is to provide efficient control of MMCs in a wirelessly controlled HVDC transmission system that does not suffer from the issues noted above or at least where the above noted issues have been mitigated or reduced..

According to a first aspect there is presented a method for controlling MMCs in a HVDC transmission system. The method is performed by a VCU of the MMCs. The method comprises wirelessly receiving, during a current update period, voltage measurements of a previous update period from PCUs of the MMCs. The current update period subsequently follows the previous update period. The method comprises determining a switching command for the PCUs for a next update period based on the voltage measurements of the previous update period. The next update period subsequently follows the current update period. The method comprises controlling the MMCs by, during the next update period, wirelessly transmitting the switching command to the PCUs. According to a second aspect there is presented a VCU for controlling MMCs in a HVDC transmission system. The VCU comprises processing circuitry. The processing circuitry is configured to cause the VCU to wirelessly receive, during a current update period, voltage measurements of a previous update period from PCUs of the MMCs. The current update period subsequently follows the previous update period. The processing circuitry is configured to cause the VCU to determine a switching command for the PCUs for a next update period based on the voltage measurements of the previous update period. The next update period subsequently follows the current update period. The processing circuitry is configured to cause the VCU to control the MMCs by, during the next update period, wirelessly transmitting the switching command to the PCUs.

According to a third aspect there is presented a computer program for controlling MMCs in a HVDC transmission system. The computer program comprises computer program code which, when run on processing circuitry of a VCU, causes the VCU to perform a method according to the first aspect.

According to a fourth aspect there is presented a method for controlling a MMCs in a HVDC transmission system. The method is performed by a PCU of the MMC. The method comprises wirelessly transmitting, during a current update period, voltage measurements of a previous update period to a VCU. The current update period subsequently follows the previous update period. The method comprises controlling the MMC by acting according to a switching command wirelessly received from the VCU during a next update period. The next update period subsequently follows the current update period. The switching command is based on the voltage measurement of the previous update period. According to a fifth aspect there is presented a PCU for controlling a MMCs in a

HVDC transmission system. The PCU comprises processing circuitry. The processing circuitry is configured to cause the PCU to wirelessly transmit, during a current update period, voltage measurements of a previous update period to a VCU. The current update period subsequently follows the previous update period. The processing circuitry is configured to cause the PCU to control the MMC by acting according to a switching command wirelessly received from the VCU during a next update period. The next update period subsequently follows the current update period. The switching command is based on the voltage measurement of the previous update period.

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

According to a seventh aspect there is presented a computer program product comprising a computer program according to at least one of the third aspect and the sixth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium.

Advantageously these aspects provide efficient control of the MMCs in the HVDC transmission system

Advantageously these aspects enable a wirelessly controlled HVDC transmission system to meet required timing constraints for all the MMCs.

Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, 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, apparatus, component, means, module, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, 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.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which: Fig. l is a schematic diagram illustrating a HVDC transmission system according to embodiments;

Fig. 2 is a schematic illustration of the currently used flow of information between PCUs and VCU; Fig. 3 is a schematic illustration of the currently used sequence of operations performed by VCU and the PCUs;

Figs. 4 and 8 are flowcharts of methods according to embodiments;

Fig. 5 is a schematic illustration of a sequence of operations performed by VCU and the PCUs according to embodiments; Fig. 6 is a schematic illustration of a flow of information between PCUs and VCU according to embodiments;

Fig. 7 is a schematic illustration of prediction of voltage measurements according to embodiments;

Fig. 9 is a schematic diagram showing functional units of a VCU according to an embodiment;

Fig. 10 is a schematic diagram showing functional units of a PCU according to an embodiment; and

Fig. 11 shows one example of a computer program product comprising computer readable means according to an embodiment. DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This 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.

Wireless-controlled power electronic systems, such as MMC of HVDC transmission system, have tight constraints in terms of timeliness and reliability which are not compatible with some of the challenges of wireless communication, like shared channel and long processing delays. In some aspects, there is provided a dedicated protocol and a corresponding modification to the control architecture to fully parallelize the computing and communication with the goal of meeting the timing constraints in wireless-controlled HVDC transmission systems. Fig. l schematically illustrates a HVDC transmission system comprising one VCU 200 and IV PCUs 300a:300N, all operatively connected to a respective wireless transceiver (WTRX) no, i2oa:i2oN. Each PCU 300a:300N is connected to, integrated with, or part of, an MMC 130a: 130N. During every update period, of duration P, the VCU 200 sends a switching command to the PCUs 300a:300N and each PCU300a:300N sends back a feedback signal containing voltage measurements to the VCU 200.

Fig. 2 illustrates the currently used flow 400 of information between the PCUs 300a:300N and the VCU 200. The voltage measured by the PCUs 300a:300N (Measure Vi, Measure VN) during one update period P is sent to the VCU 200 during the following period (TX Vi, TX VN) and received by the VCU 200 (RX Vi-VN). In the same period, the VCU 200 computes the switching command (Comp. SS) and sends it to the PCUs 300a:300N (TX SS). The total information latency between voltage measurement and the PCU switching hence corresponds to one update period, as can be seen in the figure. With currently used flow 400 of information as illustrated in Fig. 2, each update period should be sufficiently long to accommodate for the transmission of N voltage measurements from the PCUs 300a:300N, their reception by the VCU 200, the computation of the switching command by the VCU 200, the transmission of the switching command by the VCU 200 and its reception by the PCUs 300a:300N. While this is feasible if optical fibers are used for the communication between the

VCU 200 and the PCUs 300a:300N, it becomes challenging if wireless links are used, due to the aforementioned issues. This is further illustrated in Fig. 3. Fig. 3 illustrates the currently used sequence of operations 500 performed by the VCU 200 and the PCUs 300a:300N. The required duration of the update period P can be estimated as:

where T_{proc tx PCU} and T_{proc rxycu} are the times (TX proc. PCU, RX proc. VCU)) needed for the WTRX of each PCU 300a:300N to process the voltage measurement into a packet and for the WTRX of the VCU 200 to extract the voltage measurements from the packet, respectively; T_{proc txycu} and T_{procrx PCU} are the times (TX proc. VCU, RX proc. VCU) needed for the WTRX of the VCU 200 to process the switching command into a packet and for the WTRX of each PCU 300a:300N to extract the sequence from the packet, respectively; T_{tx V} and T_{tx SS} are the times (TX Vi ... TX VN, TX SS) required for a packet containing a voltage measurement and a packet containing the switching command to travel over the wireless channel, respectively;

T '_compute ^ls the time (Compute SS) required to compute the switching command at the VCU 200 once all the voltage measurements have been collected.

It can be seen from Fig. 3 that the VCU 200 as well as the PCUs 300a:300N and the communication channel are often in an idle state during one update period. This is due to the fact that a specific order has to be followed among the operations carried out. For example, after all the voltage measurements have been transmitted over the wireless channel, the switching command cannot be sent immediately, because the VCU 200 must process the received packets, compute the switching command and process the packet that contains it before transmitting it. While such a constraint is needed with currently used control schemes, it leads to a non-efficient utilization of resources, which can impact the performance of a wireless-based HVDC transmission system 100.

The embodiments disclosed herein therefore relate to mechanisms for controlling MMCs in a HVDC transmission system 100. 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, for example in the form of a computer program, that 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, for example in the form of a computer program, that when run on processing circuitry of the PCU 300a:300N, causes the PCU 300a:300N to perform the method. Reference is now made to Fig. 4 illustrating a method for controlling MMCs in a HVDC transmission system as performed by the VCU 200 according to an embodiment.

S102: The VCU 200 wirelessly receives, during a current update period, voltage measurements of a previous update period from PCUs 300a:300N of the MMCs. The current update period subsequently follows the previous update period.

S104: The VCU 200 determines a switching command for the PCUs for a next update period based on the voltage measurements of the previous update period. The next update period subsequently follows the current update period.

S108: The VCU 200 controls the MMCs by, during the next update period, wirelessly transmitting the switching command to the PCUs 300a:300N.

Embodiments relating to further details of controlling MMCs in a HVDC transmission system as performed by the VCU 200 will now be disclosed.

In some embodiments, the switching command comprises individual instructions for each of the PCUs to perform a switch or not. In some aspects, it is assumed that the PCUs 300a:300N in each update period obtains voltage measurements which are then forwarded to the VCU 200. That is, according to an embodiment, the VCU 200 is configured to perform step S106:

S106: The VCU 200 wirelessly receives, during the next update period, voltage measurements of the current update period from the PCUs 300a:300N. The switching command is then wirelessly transmitted immediately after the last of the voltage measurements of the current update period has been received.

In some embodiments, transmission of the switching command is prepared in parallel with wirelessly receiving the voltage measurements of the current update period in step S106. Parallel reference is here made to Fig. 5 which illustrates the sequence of operations 600 performed by the VCU 200 and the PCUs 300a:300N according to at least some of the herein disclosed embodiments and where the notation is the same as in Figs. 2 and 3. In comparison to with Fig. 3, it can be observed that, according to Fig. 5, the transmission of the switching command starts before the sequence for the current period is actually computed and that the computation of the sequence ends in the following update period with respect to the one where the voltage values have been received. Consequently, the switching command transmitted to the PCUs 300a:300N in each period will be computed based on voltage measurements that are one update period old.

Parallel reference is here made to Fig. 6 which illustrates the flow of information 700 between the PCUs 300a:300N and the VCU 200 for wireless links between the VCU 200 and the PCUs 300a:300N according to at least some of the herein disclosed embodiments and where the notation is the same as in Figs. 2, 3, and 5. In comparison to with Fig. 2, it can be observed that, according to Fig. 6, the latency between voltage measurements and switching commands is effectively increased from one update period to two update periods.

In some aspects, in order to decrease the effects of the latency being increased, the VCU 200 might by itself predict values of the voltage measurements. Particularly, in some embodiments, the VCU 200 is configured to perform step Si04a.

Si04a: The VCU 200 predicts values of the voltage measurements for the current update period based on the voltage measurements of the previous update period.

Step Si04a could be performed as part of the determining in step S104. That is, in some embodiments, the switching command for the next time period is based on the predicted values of the voltage measurements for the current update period.

In some aspects, there is provided a model of the MMCs which enables the VCU 200 to predict the voltage measurements with one period delay, based on the voltages measured two periods earlier, the command sequence at previous period and the current flowing through each cell (denoted arm current), which is the same for each of the MMCs and can be measured separately. Parallel reference is here made to Fig. 7 which illustrates an implementation at the VCU 200 for predicting the voltage measurements. A control module 240 is configured for predicting a switching command for update period k based on a reference voltage and the predicted voltage value at update period k- 1, as provided by an MMC model 250. The switching command for update period A: is in a l-period delay module 260 delayed during one update period before being fed to the MMC model 250. In one version of such an MMC model 250, the voltage value of the n:th PCU during the (/c-i):th period is predicted as:

In equation (2), t½(/c - 1) is the predicted voltage value at update period k- 1, where u_n(k - 2) is the measured voltage at update period k- 2, where s_n(k - 1) is the switching command at update period k- 1 (equal to 1 if device is on), where i_arm(k - 1) is the measured arm current at update period k- 1, where C is the capacitor voltage, and where P is the update period. That is, in some embodiments, the values of the voltage measurements for the current update period further are predicted based on the switching command for the current update period, an arm current value of the MMCs for the current update period, and/ or on a capacitor voltage value of the MMCs.

A prediction as in equation (2) derives from a simple numerical integration. More sophisticated methods, such as Newton-Cotes formulas, can be used if required. In practical systems, the capacity C will differ from the nominal value and vary over time. Since in each period the voltage at each PCU 300a:300N that was previously estimated is measured, a comparison of the two values would allow to estimate the real value of the capacity, which can be used to correct equation (2). This system would give information on the capacitor aging and possibly allow predictive maintenance capabilities. Finally, even if a prediction error persists, it the different scale between the update period (50-100 ps) and the time constant of the capacitor (30-40 ms) can guarantee that small errors will not affect the behavior of the system in a critical way.

Reference is now made to Fig. 8 illustrating a method for controlling an MMC in a HVDC transmission system as performed by the PCU 300a:300N according to an embodiment. S202: The PCU 300a:300N wirelessly transmits, during a current update period, voltage measurements of a previous update period to the VCU 200. As above, the current update period subsequently follows the previous update period.

S204: The PCU 300a:300N controls the MMC by acting according to a switching command wirelessly received from the VCU 200 during a next update period. As above, the next update period subsequently follows the current update period. The switching command is based on the voltage measurement of the previous update period.

Embodiments relating to further details of controlling an MMC in a HVDC transmission system as performed by the PCU 300a:300N will now be disclosed.

In some embodiments, the switching command comprises instructions for the PCU 400a:300N to perform a switch or not, and the PCU 300a:300N acts according to the switching command by performing the switch or not.

In some embodiments, transmission of the voltage measurements of the previous update period is performed in parallel with acting according to a switching command wirelessly received from the VCU 200 during the previous update period.

As disclosed above, according to at least some of the herein disclosed embodiments, the first PCU 300a starts transmitting its voltage measurements immediately at the beginning of the update period and the packet to be transmitted is prepared in advance at the end of the previous update period. As soon as the transmission of the last packet containing the voltage measurement ends, the transmission of the packet containing the switching command starts and this packet has already been prepared by the VCU 200 in parallel with the processing of the received packets of voltage measurements from the PCUs 300a:300N. The proposed sequence of operations allows to use more efficiently both the devices (i.e., the VCU 200 and the PCUs 300a:300N) and the communication channel and to significantly reduce the minimum update period duration P, which can now be estimated as:

P = N T_{tX V} + T_{tx SS} (3)

Such a performance gain can bring great benefits, effectively making feasible to control PCUs 300a:300N over wireless links. Fig. 9 schematically illustrates, in terms of a number of functional units, the components of a VCU 200 according to an embodiment. 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 1110a (as in Fig. 11), e.g. in the form of a storage medium 230. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, 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 may store the set of operations, and the processing circuitry 210 maybe 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 maybe provided as a set of executable instructions. Thus the processing circuitry 210 is thereby arranged to execute methods as herein disclosed.

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

The VCU 200 may further comprise a communications interface 220 for communications with the PCUs 300a:300N. As such the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components.

The processing circuitry 210 controls the general operation of the VCU 200 e.g. by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and instructions from the storage medium 230. Other components, as well as the related functionality, of the VCU 200 are omitted in order not to obscure the concepts presented herein.

Fig. 10 schematically illustrates, in terms of a number of functional units, the components of a PCU 300a:300N according to an embodiment. Processing circuitry 310 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 1110b (as in Fig. 11), e.g. in the form of a storage medium 330. The processing circuitry 310 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, 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 may store the set of operations, and the processing circuitry 310 maybe 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 may be provided as a set of executable instructions. Thus the processing circuitry 310 is thereby arranged to execute methods as herein disclosed.

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

The PCU 300a:300N may further comprise a communications interface 320 for communications with the VCU 200. As such the communications interface 320 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 310 controls the general operation of the PCU 300a:300N e.g. by sending data and control signals to the communications interface 320 and the storage medium 330, by receiving data and reports from the communications interface 320, and by retrieving data and instructions from the storage medium 330. Other components, as well as the related functionality, of the PCU 300a:300N are omitted in order not to obscure the concepts presented herein.

Fig. 11 shows one example of a computer program product 1110a, 1110b comprising computer readable means 1130. On this computer readable means 1130, a computer program 1120a can be stored, which computer program 1120a 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 1120a and/or computer program product 1110a may thus provide means for performing any steps of the VCU 200 as herein disclosed. On this computer readable means 1130, a computer program 1120b can be stored, which computer program 1120b 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 1120b and/or computer program product 1110b may thus provide means for performing any steps of the PCU 300a:300N as herein disclosed.

In the example of Fig. 11, the computer program product 1110a, 1110b 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 1110a, 1110b 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 in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 1120a, 1120b is here schematically shown as a track on the depicted optical disk, the computer program 1120a, 1120b can be stored in anyway which is suitable for the computer program product 1110a, 1110b. 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 controlling modular multilevel converters, MMCs (130a: 130N), in a high-voltage direct current, HVDC, transmission system, the method being performed by a valve control unit, VCU (200), of the MMCs (130a: 130N), the method comprising: wirelessly receiving (S102), during a current update period, voltage measurements of a previous update period from position control units, PCUs (300a:300N), of the MMCs (i30a:i30N), wherein the current update period subsequently follows the previous update period; determining (S104) a switching command for the PCUs (300a:300N) for a next update period based on the voltage measurements of the previous update period, wherein the next update period subsequently follows the current update period; and controlling (S108) the MMCs (130a: 130N) by, during the next update period, wirelessly transmitting the switching command to the PCUs (300a:300N). 2. The method according to claim 1, further comprising: wirelessly receiving (S106), during the next update period, voltage measurements of the current update period from the PCUs (300a:300N), and wherein the switching command is wirelessly transmitted immediately after a last of the voltage measurements of the current update period has been received. 3. The method according to claim 2, wherein transmission of the switching command is prepared in parallel with wirelessly receiving the voltage measurements of the current update period.

4. The method according to claim 1, further comprising: predicting (Si04a) values of the voltage measurements for the current update period based on the voltage measurements of the previous update period.

5. The method according to claim 4, wherein the switching command for the next time period is based on the predicted values of the voltage measurements for the current update period.

6. The method according to claim 4, wherein the values of the voltage measurements for the current update period further are predicted based on the switching command for the current update period.

7. The method according to claim 4, wherein the values of the voltage measurements for the current update period further are predicted based on an arm current value of the MMCs (130a: 130N) for the current update period.

8. The method according to claim 4, wherein the values of the voltage measurements for the current update period further are predicted based on a capacitor voltage value of the MMCs (130a: 130N). 9. The method according to claim 1, wherein the switching command comprises individual instructions for each of the PCUs (300a:300N) to perform a switch or not.

10. A method for controlling a modular multilevel converter, MMC (i3oA:i3oN), in a high-voltage direct current, HVDC, transmission system, the method being performed by a position control unit, PCU (300A:300N), of the MMC (i3oA:i3oN), the method comprising: wirelessly transmitting (S202), during a current update period, voltage measurements of a previous update period to a valve position control unit, VCU (200), wherein the current update period subsequently follows the previous update period; and controlling (S204) the MMC (i3oA:i3oN) by acting according to a switching command wirelessly received from the VCU (200) during a next update period, wherein the next update period subsequently follows the current update period, and wherein the switching command is based on the voltage measurement of the previous update period. 11. The method according to claim 10, wherein transmission of the voltage measurements of the previous update period is performed in parallel with acting according to a switching command wirelessly received from the VCU (200) during the previous update period.

12. The method according to claim 10, wherein the switching command comprises instructions for the PCUs (300a:300N) to perform a switch or not, and wherein the PCU (300A:300N) acts according to the switching command by performing the switch or not. 13. A valve control unit, VCU (200), for controlling modular multilevel converters,

MMCs (130a: 130N), in a high-voltage direct current, HVDC, transmission system, the VCU (200) comprising processing circuitry (210), the processing circuitry being configured to cause the VCU (200) to: wirelessly receive, during a current update period, voltage measurements of a previous update period from position control units, PCUs (300a:300N), of the MMCs (130a: 130N), wherein the current update period subsequently follows the previous update period; determine a switching command for the PCUs (300a:300N) for a next update period based on the voltage measurements of the previous update period, wherein the next update period subsequently follows the current update period; and control the MMCs (130a: 130N) by, during the next update period, wirelessly transmitting the switching command to the PCUs (300a:300N).

14. The VCU (200) according to claim 13, further being configured to perform the method according to any of claims 2 to 9. 15. A position control unit, PCU (300A:300N), for controlling a modular multilevel converter, MMC (i3oA:i3oN), in a high-voltage direct current, HVDC, transmission system, the PCU (300A:300N) comprising processing circuitry (310), the processing circuitry being configured to cause the PCU (300A:300N) to: wirelessly transmit, during a current update period, voltage measurements of a previous update period to a valve position control unit, VCU (200), wherein the current update period subsequently follows the previous update period; and control the MMC (i3oA:i3oN) by acting according to a switching command wirelessly received from the VCU (200) during a next update period, wherein the next update period subsequently follows the current update period, and wherein the switching command is based on the voltage measurement of the previous update period.

16. The PCU (300A:300N) according to claim 15, further being configured to perform the method according to any of claims 11 or 12. 17. A computer program (1120a) for controlling modular multilevel converters,

MMCs (130a: 130N), in a high-voltage direct current, HVDC, transmission system, the computer program comprising computer code which, when run on processing circuitry (210) of a valve control unit, VCU (200), of the MMCs (130a: 130N), causes the VCU (200) to: wirelessly receive (S102), during a current update period, voltage measurements of a previous update period from position control units, PCUs (300a:300N), of the MMCs (i3oa:i3oN), wherein the current update period subsequently follows the previous update period; determine (S104) a switching command for the PCUs (300a:300N) for a next update period based on the voltage measurements of the previous update period, wherein the next update period subsequently follows the current update period; and control (S108) the MMCs (130a: 130N) by, during the next update period, wirelessly transmitting the switching command to the PCUs (300a:300N).

18. A computer program (1120b) for controlling a modular multilevel converter, MMC (i3oA:i3oN), in a high-voltage direct current, HVDC, transmission system, the computer program comprising computer code which, when run on processing circuitry (310) of a position control unit, PCU (300A:300N), of the MMC (i30A:i30N), causes the PCU (300A:300N) to: wirelessly transmit (S202), during a current update period, voltage measurements of a previous update period to a valve position control unit, VCU

(200), wherein the current update period subsequently follows the previous update period; and control (S204) the MMC (i3oA:i3oN) by acting according to a switching command wirelessly received from the VCU (200) during a next update period, wherein the next update period subsequently follows the current update period, and wherein the switching command is based on the voltage measurement of the previous update period.

19. A computer program product (1110a, 1110b) comprising a computer program (1120a, 1120b) according to at least one of claims 17 and 18, and a computer readable storage medium (1130) on which the computer program is stored.