CN111758214A

CN111758214A - Method for controlling an HVDC converter station based on series connected converters

Info

Publication number: CN111758214A
Application number: CN201880090091.9A
Authority: CN
Inventors: L·科基; T·琼森; P·K·巴鲁帕蒂
Original assignee: ABB Grid Switzerland AG
Current assignee: Hitachi Energy Co ltd
Priority date: 2018-02-23
Filing date: 2018-02-23
Publication date: 2020-10-09
Also published as: WO2019161905A1

Abstract

A method of controlling a high voltage direct current, HVDC, converter station is provided, wherein the converter station comprises a first Voltage Source Converter (VSC) associated with a first power controller and a second VSC associated with a second power controller. The first VSC and the second VSC are connected in series between the first DC terminal and the second DC terminal of the converter station. According to an embodiment, the method comprises: a common DC reference voltage is determined based on either of a first DC voltage measured across the first VSC and a second DC voltage measured across the second VSC. The method further comprises: a first reference DC voltage for the first VSC is determined based on the determined common DC reference voltage and the first DC voltage contribution from the first power controller, and a second reference DC voltage for the second VSC is determined based on the determined common DC reference voltage and the second DC voltage contribution from the second power controller. The operation of the first VSC and the second VSC may then be controlled based at least in part on the first reference DC voltage and the second reference DC voltage, respectively.

Description

Method for controlling an HVDC converter station based on series connected converters

Technical Field

The present disclosure relates generally to the field of power transmission systems for transmitting or receiving high voltage direct current, HVDC, power. More specifically, the present disclosure relates to a method of controlling an HVDC converter station based on series connected converters.

Background

Direct Current (DC) power transmission systems have become a preferred choice for transmitting high voltage power in large quantities over their Alternating Current (AC) competitors due to their lower losses and lower cost. In modern HVDC power transmission systems (with voltages of several hundred kV), power can reach the magnitude of several gigawatts, and can be transmitted over distances of up to several kilometres.

At each end of the HVDC power transmission system, a converter station may be used to convert between AC power and DC power. Converter stations based on Current Source Converters (CSCs) using thyristors as switching means have been widely used in HVDC applications. However, with the development of semiconductor technology in recent years, Voltage Source Converters (VSCs) using, for example, Insulated Gate Bipolar Transistors (IGBTs) as switching devices have become increasingly popular because they are self-rectifying and less sensitive to rectification faults.

Further, the converter station may comprise two or more converters connected in series to provide higher availability and reliability, since only a part of the total supply capacity may be lost if one of the converters fails. Furthermore, a plurality of converters connected in series enables, for example, the manufacture of transformers connected to the converters with reduced size. This may reduce both cost and space requirements and make the transformer easier to transport.

However, this type of converter station requires an improved method of controlling the operation of the converters connected in series.

Disclosure of Invention

In order to at least partly meet the above requirements, the present disclosure seeks to at least provide an improved method of controlling an HVDC converter station comprising at least two converters connected in series, an improved control unit, an improved converter station and a high voltage power system thereof.

To achieve this object, a method, a control unit, a converter station and a high voltage power system thereof as defined in the independent claims are provided. Further embodiments are provided in the dependent claims.

According to an aspect, a method of controlling a high voltage direct current, HVDC, converter station is provided, wherein the converter station comprises a first VSC connected in series with a second VSC between a first DC terminal and a second DC terminal of the converter station.

The converter station may be operated in a power control mode. The first VSC may be associated with a first power controller (also referred to as an "active power controller") and the second VSC may be associated with a second power controller.

Each of the first VSC and the second VSC may be a Modular Multilevel Converter (MMC) comprising a number of full-bridge cells (or sub-modules) and/or half-bridge cells (or sub-modules). However, the inventive concept is applicable to any type of VSC.

The method may comprise determining a common DC reference voltage based on any of a first DC voltage measured across the first VSC and a second DC voltage measured across the second VSC. The method further comprises determining a first reference DC voltage for the first VSC based on the determined common DC reference voltage and the first DC voltage contribution from the first power controller, and determining a second reference DC voltage for the second VSC based on the determined common DC reference voltage and the second DC voltage contribution from the second power controller. The operation of the first VSC and the second VSC may then be controlled based at least in part on the first reference DC voltage and the second reference DC voltage, respectively.

It will be appreciated that the operation of the first VSC and the second VSC may be based not only on the first DC reference voltage and the second DC reference voltage, but also on other reference voltages, such as an AC reference voltage and a cyclic reference voltage.

It will also be appreciated that in an HVDC power transmission system having two converter stations connected at both ends of a DC transmission link (or DC transmission line) of the transmission system, one converter station may be in a power control mode and the other in a DC voltage control mode, whether the converter station is a rectifier station or an inverter station. Normally, two converter stations cannot be in the same control mode at the same time, as this may lead to control conflicts. The reference power value will be set by e.g. an operator if the converter station is in power control mode. Similarly, if the converter station is in DC voltage control mode, the DC reference voltage will be set by the operator.

The control system of the converter station may be constructed in such a way that a DC voltage control block is cascaded with a power control block. For a converter station in DC voltage control mode, the DC reference voltage value is set directly by the operator. However, for a converter station in power control mode, a reference value for the DC voltage control block is generated from the power control block, the reference of which (in terms of power) will be set by an operator or the like. The control system then provides a reference DC voltage value for controlling the converters of the converter station operating in the power control mode. The reference DC voltage value generated by the power control block (hereinafter also referred to as control unit) may be based on a reference power value set by an operator for operating the converter station and may be based on, for example, measurements of the actual DC voltage between the DC terminals of the converter station.

In the inventive concept, a method for controlling a converter station comprising series connected converters and operating in a power control mode is provided. The inventors have realized that by determining a common DC reference voltage based on either of a first DC voltage measured across the first VSC and a second DC voltage measured across the second VSC, one of the converters will act as a master converter and the other will act as a slave converter. In other words, one of the two converters will be regulated depending on the DC voltage measured on the other converter. Since the value of the first reference DC voltage and the value of the second reference DC voltage will be close to each other, the voltage distribution between two converters connected in series in the converter station may be improved.

It will be understood that the voltage measurement and control of the series connected VSCs (or MMCs) in the converter station is not dependent on the voltage measured in the converter station connected at the other end of the DC transmission system. Thus, the converter station may operate more or less independently without knowledge of the measurements performed by the other converter station.

The present inventive concept provides a technique to more accurately measure or rather determine the positive to negative DC voltage during normal conditions when all four VSCs (or MMCs) are in operation, and after one of the VSCs (or MMCs) is bypassed after a fault occurs at any one of the converters. The DC voltage measurement technique ensures that the voltage fed back to the control system (or control unit) corresponds to the DC voltage of one MMC (assuming the other MMC is bypassed) or the DC voltages of two MMCs connected in series.

Further, when one of the MMCs is bypassed after a fault occurs, the DC link voltage may be lowered to match the DC rated voltage of the MMC remaining in operation (i.e., the MMC still connected) to continue to transfer power. When the DC link voltage decreases in this scenario, the present technique ensures that the voltage distribution between the other two MMCs (converters in normal state) is equal.

According to one embodiment, the determining may comprise: a maximum value between the first DC voltage and the second DC voltage is identified, wherein the common DC reference voltage is based on the maximum value. Thus, the present embodiment is based on a maximum criterion, wherein it is determined which of the first DC voltage and the second DC voltage has the highest value, and the common DC reference voltage is then based on the highest value. In some embodiments, the common DC reference voltage may be equal (or approximately equal) to the highest value, i.e., the first DC voltage or the second DC voltage.

In this embodiment, the converter that measures the highest DC voltage is the master converter and the other is the slave converter.

It will also be appreciated that the determination may, for example, comprise: the first DC voltage, the second DC voltage, and a sum of absolute values of differences between the second DC voltage and the first DC voltage are calculated. In this example, if the first DC voltage is higher than the second DC voltage, the sum will be equal to twice the second DC voltage (since the absolute value of the difference between the second DC voltage and the first DC voltage will be negative and the two values of the first DC voltage in the sum will cancel each other), whereas if the first DC voltage is lower than the second DC voltage, the sum will be equal to twice the first DC voltage.

The sum may then be multiplied by a factor to obtain a common reference DC value related to the control method. In particular, the sum may be multiplied by a factor greater than 0 but less than 1. The factor may take into account the number of converters connected in series in the converter station and which "voltage" (i.e. whether the "voltage" is e.g. a half or a full pole-to-pole voltage) is used as input in the control method for determining the first and second reference DC voltages.

More specifically, if half of the voltage between the DC positive and negative electrodes is used as an input to the control method, the sum may be multiplied by a factor equal to 0.25. In another example, if a full positive-to-negative voltage is used to control the method or system, the factor would be 0.5.

According to another embodiment, the determining may comprise: a minimum value between the first DC voltage and the second DC voltage is identified, wherein the common DC reference voltage is based on the minimum value. Thus, the present embodiment is based on a minimum criterion, wherein it is determined which of the first DC voltage and the second DC voltage has the lowest value, and then the common DC reference voltage is based on this lowest value. In some embodiments, the common DC reference voltage may be equal (or approximately equal) to the lowest value, i.e., the first DC voltage or the second DC voltage.

In this embodiment, the converter that measures the lowest DC voltage is the master converter and the other is the slave converter.

The common DC reference voltage may be determined based on calculations as described above in the previous embodiments, which indirectly includes determining (or identifying) which of the two converters is the master converter and which is the slave converter, but determining that the common DC reference voltage is adapted to get a value based on the lowest of the two measured DC voltages.

According to an embodiment, the DC voltage contribution from the first power controller and the DC voltage contribution from the second power controller may be used for operating the first VSC and the second VSC, respectively, in the power control mode. With the inventive concept, a reference DC voltage value for each of the two converters is obtained based at least in part on a common DC reference voltage.

It will be appreciated that a certain power requirement may be used to control the commutation station in the power control mode. However, the power to be realized by the converter station may then be shared by the different converters constituting the converter station, such that for example in case of two converters connected in series, a first converter is controlled based on a first reference DC voltage and a second converter is controlled based on a second reference DC voltage for realizing the required power.

Further, it will be appreciated that the inventive method provides a more accurate control of the converter station, since the reference DC voltage for one converter is based not only on a measurement of the DC voltage across the converter in question, but also on a measurement of the DC voltage across the DC terminals of the converter station.

According to an embodiment, the method may further comprise: the value of the measured DC voltage used for determining the common DC reference voltage is low-pass filtered. The low-pass filtering is performed in order to extract a ripple-free DC voltage value, which further improves the control method. Eliminating or at least reducing ripple in the measured DC voltage facilitates comparing the extracted value with a reference value.

According to one embodiment, the operation of the first VSC and the second VSC may be controlled by controlling switching devices of the first VSC and the second VSC.

According to an embodiment, the first VSC and/or the second VSC may comprise at least one of a modular multi-level converter, a full-bridge MMC and a half-bridge MMC.

Specifically, the switching devices of the half-bridge cells and the full-bridge cells of the MMC may be Insulated Gate Bipolar Transistors (IGBTs). However, the switching device of the MMC may not be limited to the IGBT, and the switching device may be, for example, an Integrated Gate Commutated Thyristor (IGCT), a dual-mode insulated gate transistor (BIGT), or the like.

Further, it will be understood that the switching devices may be arranged in a full bridge FB sub-module or FB cell, where four switching devices or cells, each comprising an Insulated Gate Bipolar Transistor (IGBT) and a parallel freewheeling diode, are connected together with a charging capacitor in an H-bridge configuration, or in a half bridge HB sub-module or HB cell, where two switching devices (or cells) are connected together in series with a parallel charging capacitor. Multiple FB monomers connected in series can result in an FBMMC, while multiple HB monomers connected in series can result in an HB MMC.

As mentioned above, the inventive concept is applicable to any VSC connected in series. However, it will only be possible to reduce the DC voltage (e.g. after one bypass in the converter) if each of the series-connected VSCs can generate an AC voltage even after reducing the DC reference voltage. This is possible with FB MMC with a normal rating or with HB MMC with a sufficiently high rating (i.e. the AC voltage generated in normal operation is much smaller than the DC voltage).

According to another aspect, a control unit adapted to control a converter station is provided. The converter station may comprise at least two series connected VSCs, wherein each of the VSCs is associated with a power controller. The control unit may be configured to operate according to a method as defined in any of the preceding embodiments.

According to another aspect, a converter station for a high voltage power system is provided, said converter station being adapted to transmit power to another converter station. The converter station may comprise a first voltage source converter VSC and a second VSC connected in series between a first DC terminal and a second DC terminal of the converter station. The first VSC may be associated with a first power controller, and the second VSC may be associated with a second power controller. The converter station may further comprise a control unit (or controller) adapted to control the first VSC and the second VSC by performing the method as defined in any one of the preceding embodiments.

According to another aspect, a high voltage power system adapted to transmit power is provided. The system may comprise a first converter station and a second converter station, wherein at least one of the first converter station and the second converter station is a converter station as defined in any of the previous embodiments.

It will be understood that in the context of the present invention, the converter station may operate as an inverter and/or as a rectifier, depending on the situation and/or application.

It will be understood that all embodiments described with reference to the first aspect of the disclosure may be combined with any embodiments described with reference to the other aspects of the disclosure, and vice versa.

The disclosure relates to all possible combinations of features recited in the claims. Further objects and advantages of various embodiments of the present disclosure will be described below by way of exemplary embodiments.

Drawings

Exemplary embodiments will be described below with reference to the accompanying drawings, in which:

fig. 1 is a schematic diagram of a high voltage power transmission system according to an embodiment;

fig. 2 schematically shows an embodiment of a measuring module/step for determining an actual DC voltage between the DC terminals of the converter station; and is

Fig. 3 schematically shows an embodiment of a control unit/step for determining a reference DC voltage value for controlling a converter of a converter station.

In the drawings, like reference numerals will be used for like elements unless otherwise specified. Unless explicitly stated to the contrary, the drawings only show those elements necessary to illustrate example embodiments, and other elements may be omitted or only suggested for clarity. As shown in the drawings, the sizes of elements and regions may be exaggerated for illustrative purposes, and thus, are provided to illustrate the overall structure of the embodiments.

Detailed Description

Fig. 1 shows a schematic diagram of a high voltage power transmission system according to an embodiment.

Fig. 1 shows a high voltage transmission system 100 comprising two

converter stations

110 and 120. For example, the first converter station 110 may operate in a power control mode, while the other converter station 120 may operate in a voltage control mode. Each of the

converter stations

110 and 120 may be based on Voltage Source Converters (VSCs). In particular, the VSC may be an MMC.

The first VSC station 110 and the second VSC station 120 are connected via a DC transmission system or link 130 and are arranged in a monopole configuration, with one of the DC transmission lines being connected to ground. Each

VSC station

110 and 120 may also be connected to an AC grid (not shown). It is envisaged that the power transmission system 100 may be arranged using other configurations, for example a bipolar configuration which may use different symmetries (e.g. symmetrical or asymmetrical) and a bipolar configuration which may include more than two VSC stations. At least one of the first VSC station 110 and the second VSC station 120 may be a VSC station as described herein.

The first converter station 110 may comprise at least two converters or

MMCs

115 and 116 connected in series. Similarly, the second converter station 120 may include a first VSC 125 connected in series with a second VSC 126.

The first converter station 110 has

DC terminals

112 and 114 with which the first converter station 110 can be connected to one or more DC lines of, for example, a DC transmission system 130. The first converter station 110 may also have AC terminals 140 and 142 with which the converter station 110 may be connected to an AC grid (not shown) via, for example, one or

more transformers

160, 162. In some embodiments, at least one of the

VSCs

115 and 116 of the first converter station 110 may comprise at least one full bridge sub-module. Further, the first VSC115 may be associated with, or may comprise, a first active power controller (not shown in fig. 1, see fig. 3), and the second VSC 116 may be associated with, or may comprise, a second (active) power controller (not shown in fig. 1, see fig. 3).

Similarly, the second converter station 120 has

DC terminals

122 and 124 with which the second converter station 120 can be connected to, for example, one or more DC lines of a DC transmission system 130. The second converter station 120 may also have

AC terminals

170 and 172 with which the second converter station 120 may be connected to an AC grid (not shown) via, for example, one or

more transformers

180, 182. In some embodiments, at least one of the

VSCs

125 and 126 of the second converter station 120 may include at least one full bridge sub-module.

The first converter station 110 may further comprise a controller or control unit 150 for implementing the control method for controlling the first converter and the second converter. Alternatively, the external controller or control unit 150 may be configured to control the converter station 110.

The controller 150 may control the

VSCs

115 and 116 of the first converter station 110 by providing, for example, a DC reference voltage (and an AC reference voltage), or may generate a control signal for each VSC115 and 116 of the converter station 110 based on such a DC reference voltage (and such an AC reference voltage). The controller 150 may be adapted to control the VSC by performing a control method according to embodiments described herein below (e.g., with reference to fig. 2 and 3).

Fig. 2 schematically shows an embodiment of a DC voltage measurement module/step for determining a common DC reference voltage for the

converters

115 and 116 of the first converter station 110.

As shown in fig. 2, the measurement module or measurement step 200 for determining the common DC reference voltage may comprise adding a first DC voltage 215 measured across the first VSC115 and a second DC voltage 216 measured across the second VSC 116 (see also fig. 1, where these voltages are represented by arrows) to obtain a first (sub-) sum 217 of these two values.

The measurement module or step 200 also includes subtracting the second DC voltage 216 measured across the second VSC 116 from the first DC voltage 215 measured across the first VSC115 to obtain a difference 218. The absolute value 219 of the difference 218 between the second DC voltage 216 and the first DC voltage 215 is then extracted. This absolute value 219 is then added to the first (sub) sum 217 to obtain a sum 220 of the first DC voltage 215 measured across the first VSC115, the second DC voltage 216 measured across the second VSC 116, and the absolute value 219 of the difference between the second DC voltage and the first DC voltage.

In practice, the result of this summation will be twice the first DC voltage or twice the second DC voltage, depending on whether the first DC voltage is greater than the second DC voltage. Such a calculation will result in identifying a maximum value between the first DC voltage and the second DC voltage. The common DC reference voltage is then based on this maximum value. This alternative is therefore based on a "maximum criterion", in which it is determined which of the first DC voltage and the second DC voltage has the highest value, and the common DC reference voltage is then based on this highest value. The common DC reference voltage may be equal (or approximately equal) to the highest value, i.e. the first DC voltage or the second DC voltage. In this embodiment, the converter that measures the highest DC voltage is the master converter and the other is the slave converter.

In other words, if the first DC voltage 215 is higher than the second DC voltage 216, the first VSC115 is identified or determined to be the main converter and the common DC reference voltage is based on the first DC voltage 215. Similarly, if the first DC voltage 215 is lower than the second DC voltage 216, the second VSC 116 is identified or determined to be the main converter and the common DC reference voltage is based on the second DC voltage 216.

Alternatively, the main converters may be identified based on a "minimum criterion", wherein the measuring module or step 200 for determining the common DC reference voltage may comprise adding the first DC voltage 215 measured across the first VSC115 and the second DC voltage 216 measured across the second VSC 116 to obtain the first (sub-) output or (sub-) sum 217 based on these two values. The absolute value of the difference between the first DC voltage 215 and the second DC voltage 216 may then be subtracted from the (sub) output 217. If the first DC voltage 215 is lower than the second DC voltage 216, the first VSC115 is identified or determined to be the main converter and the common DC reference voltage is based on the first DC voltage 215. Similarly, if the first DC voltage 215 is higher than the second DC voltage 216, the second VSC 116 is identified or determined to be the main converter and the common DC reference voltage is based on the second DC voltage 216. In this alternative, the common DC reference voltage is based on a minimum value between the first DC voltage 215 and the second DC voltage 216.

It will be appreciated that other ways of calculating the common DC reference voltage or identifying which of the two converters is the main converter may be employed based on the principles described above.

The common DC reference voltage may be determined based on calculations as explained in the above embodiments, which indirectly includes determining (or identifying) which of the two converters is the master converter and which is the slave converter, but determining that the common DC reference voltage is adapted to get a value based on the lowest of the two measured DC voltages.

Specifically, as further shown in fig. 2, the sum or output 220 may be multiplied by a factor f (e.g., equal to 0.25) to obtain a value 222 of the actual DC voltage. Using 0.25 as a factor, the sum or output 220 is adjusted to half the voltage between the DC positive and negative electrodes, which can then be used as an input for the rest of the control method. If a full positive to negative voltage is to be used, the sum or output 220 may be multiplied by 0.5.

Further, as represented in fig. 2, the values 222 may be low pass filtered using a low pass filter or low pass filtering method 250, resulting in a filtered value 224 of the sum or output 220, i.e., of either the first DC voltage or the second DC voltage.

It will be appreciated that although fig. 2 shows different steps to be performed in a certain order, the steps may also be performed in another order, for example applying filtering to the measured DC voltage before applying the factor.

The filtered value 224 may then be used as an input in the control method, as further described with reference to fig. 3.

Fig. 3 schematically shows an embodiment of a control unit/step for determining a reference DC voltage value for controlling each of the

VSCs

115 and 116 of the first converter station 110.

By way of example, it can be said that fig. 3 shows the determination of the first reference DC voltage 310 for controlling the first VSC115 of the first converter station 110. A similar determination may be performed to obtain a reference DC voltage value for controlling the second VSC 116.

It will be appreciated that the generated DC reference voltage 310 may then be used to switch one or more FB cells and/or HB cells in the VSC115 of the first converter station 110. For example, the DC reference voltage 310 may be fed directly to the switching devices of the cells of the first VSC 115.

In a converter station, the total cell capacitor voltage corresponding to the voltage over the cells of one or more MMCs (each cell comprising a capacitor as an energy storage device) may start to deviate from the reference voltage of the total cell capacitor voltage due to disturbances, and the generated DC voltage may also differ from its reference value. To solve these problems it is conceivable to obtain the reference DC voltage to be used for controlling the converters of the converter stations comprising series connected converters, as shown in fig. 3, based on the common DC reference voltage 224 determined by the measurement modules/steps described with reference to fig. 2 and the DC voltage contribution obtained from the power controller of the converter in question.

In fig. 3, the reference DC current 302 is subtracted from the actual DC current 303 to determine a DC current error 304. The actual DC current may be measured at one of the DC terminals of the converter station 110. The reference DC current 302 may be determined based on the power value 301 (e.g., input by an operator for operating the converter station 110 to deliver a certain power).

The DC current error 304 may then be input to the controller module 350. The controller module 350 may be, for example, a PI controller, although other types of controller modules are contemplated. The controller module 350 may be configured to determine a DC voltage contribution or voltage correction 305 based on the DC current error 304. The DC voltage contribution or voltage correction 305 may then be added to the common DC reference voltage 224 and the sum of the DC voltage contribution 305 and the common DC reference voltage 224 output as a first DC reference voltage 310 to control the converters 115 of the first converter station 110.

For illustration purposes only, if the maximum criterion is applied, the first DC voltage 215 on the first VSC115 will be determined to be a common DC reference voltage 224 if the first DC voltage 215 is measured as 1 and the second DC voltage 216 on the second VSC 116 is measured as 0.8, using arbitrary units. The first reference DC voltage 310 for the first VSC115 may be 0.9 if the DC voltage contribution output by the first power controller via the PI controller 350 is-0.1. Continuing with this example, if the DC voltage contribution output by the second power controller via its own PI controller 350 is-0.15, then the second reference DC voltage for the second VSC 116 can be 0.85.

The control methods described with reference to fig. 2 and 3 or the controller 150 operating according to such methods may ensure that the reference DC voltages to be applied to the first VSC115 and the second VSC 116 are moved towards each other (i.e. brought closer to each other), thereby improving the stability of the converter station 110 (e.g. operating in a DC power control mode). The switching devices of the first VSC115 can then be controlled accordingly.

The person skilled in the art realizes that the present disclosure by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements.

In addition, variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A method of controlling a high voltage direct current, HVDC, converter station (110) comprising a first voltage source converter, VSC, (115) associated with a first power controller and a second VSC associated with a second power controller, wherein the first VSC and the second VSC are connected in series between a first DC terminal (112) and a second DC terminal (114) of the converter station, the method comprising:

determining a common DC reference voltage (224) based on any one of a first DC voltage (215) measured across the first VSC and a second DC voltage (216) measured across the second VSC;

determining a first reference DC voltage (310) for the first VSC based on the determined common DC reference voltage and a first DC voltage contribution from the first power controller (301);

determining a second reference DC voltage for the second VSC based on the determined common DC reference voltage and a second DC voltage contribution from the second power controller; and

controlling operation of the first VSC and the second VSC based at least in part on the first reference DC voltage and the second reference DC voltage, respectively.

2. The method of claim 1, wherein the determining comprises: identifying a maximum value between the first DC voltage and the second DC voltage, wherein the common DC reference voltage is based on the maximum value.

3. The method of claim 1, wherein the determining comprises: identifying a minimum value between the first DC voltage and the second DC voltage, wherein the common DC reference voltage is based on the minimum value.

4. A method according to any of the preceding claims, wherein a DC voltage contribution from the first power controller and a DC voltage contribution from the second power controller are used to operate the first VSC and the second VSC, respectively, in a power control mode.

5. The method according to any one of the preceding claims, the method further comprising low pass filtering (250) the value of the measured DC voltage used for determining the common DC reference voltage.

6. The method of any preceding claim, wherein controlling operation of the first VSC and the second VSC comprises controlling switching devices of the first VSC and the second VSC.

7. A method according to any of the preceding claims, wherein the first VSC and/or the second VSC comprises at least one of a modular multilevel converter, a full-bridge MMC and a half-bridge MMC.

8. A control unit (150) adapted to control a converter station (110) comprising at least two series-connected voltage source converters (115, 116), each associated with a power controller, wherein the control unit is configured to operate according to a method as defined in any of the preceding claims.

9. A converter station (110) for a high voltage power system, the converter station being adapted to transmit power to another converter station, the converter station comprising:

a first voltage source converter, VSC, (115) and a second VSC, (116) connected in series between a first DC terminal (112) and a second DC terminal (114) of the converter station, wherein the first VSC is associated with a first power controller and the second VSC is associated with a second power controller; and

a control unit (150) adapted to control the first VSC and the second VSC by performing a method as defined in any one of the preceding claims.

10. A high voltage power system adapted to transmit power, the system comprising a first converter station and a second converter station, wherein at least one of the first converter station and the second converter station is a converter station according to claim 9.