WO2019161905A1

WO2019161905A1 - Method of controlling an hvdc converter station based on series connected converters

Info

Publication number: WO2019161905A1
Application number: PCT/EP2018/054487
Authority: WO
Inventors: Lekshman KG; Tomas Jonsson; Praveen Kumar Barupati
Original assignee: Abb Schweiz Ag
Priority date: 2018-02-23
Filing date: 2018-02-23
Publication date: 2019-08-29
Also published as: CN111758214A

Abstract

A method of controlling a high voltage direct current, HVDC, converter station is provided, wherein the converter station includes 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 a first DC terminal and a second DC terminal of the converter station. According to an embodiment, the method comprises determining a common DC voltage reference based on either one of a first DC voltage measured across the first VSC and a second DC voltage measured across the second VSC. The method further includes determining a first reference DC voltage for the first VSC based on the determined common DC voltage reference and a 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 voltage reference and a second DC voltage contribution from the second power controller. Operation of the first VSC and the second VSC may then be controlled at least partly based on the first reference DC voltage and the second reference DC voltage, respectively.

Description

METHOD OF CONTROLLING AN HVDC CONVERTER STATION BASED ON SERIES CONNECTED CONVERTERS

Technical field

The present disclosure generally relates to the field of power transmission systems to transfer or receive high-voltage direct current, HVDC, electrical power. More specifically, the present disclosure relates to methods of controlling an HVDC converter station based on converters connected in series.

Background

Due to their lower losses and costs, direct current (DC) power transmission systems have become the preferred option over their alternating current (AC) competitors for bulk transmission of high-voltage electrical power. In modern HVDC power transmission systems (with voltages of several hundred of kV), the power may reach several gigawatts in size and be transferred over distances of up to several thousands of kilometers.

At each end of an HVDC power transmission system, converter stations may be used to convert between AC and DC electrical power.

Converter stations based on current-source converters (CSCs) using thyristors as switching devices have been widely used in HVDC applications. However, with recent development of semiconductor technology, voltage- source converters (VSCs), using e.g. insulated-gate bipolar transistors (IGBTs) as switching devices, have gained in popularity as they are self- commutating and less sensitive to commutation failures.

Further, a converter station may include two or more converters connected in series to provide a higher availability and reliability, as only part of the total power supply capability may be lost if one of the converters malfunctions. Also, multiple converters connected in series may allow for e.g. transformers connected to the converters to be manufactured having a reduced size. This may offer both a reduced cost and space-requirement as well as an easier transportation of the transformers. Such types of converter stations require however improved methods of controlling the operation of the series-connected converters.

Summary

To at least partially fulfil the above requirements, the present disclosure seeks to provide at least an improved method of controlling an HVDC converter station including at least two series-connected converters, an improved control unit, an improved converter station and a high-voltage power system thereof.

To achieve this, 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 one aspect, a method of controlling a high voltage direct current, HVDC, converter station is provided, wherein the converter station includes 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 called“active power controller”) and the second VSC may be associated with a second power controller.

Each of the first and second VSCs may be a modular multilevel converter (MMC) including a certain number of full-bridge cells (or

submodules) and/or half-bridge cells (or submodules). However, the present inventive concept is applicable to any type of VSC.

The method may comprise determining a common DC voltage reference based on either one of a first DC voltage measured across the first VSC and a second DC voltage measured across the second VSC. The method may further include determining a first reference DC voltage for the first VSC based on the determined common DC voltage reference and a 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 voltage reference and a second DC voltage contribution from the second power controller. Operation of the first VSC and the second VSC may then be controlled at least partly based 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 not only be based on the first and second DC reference voltages but also based on other voltage references such as an AC voltage reference and circulating voltage references.

It will also be appreciated that, in an HVDC power transmission system with two converter stations connected at two ends of a DC transmission link (or DC transmission lines) of the transmission system, one converter station may be in power control mode and another station may be in DC voltage control mode, irrespective of whether it is a rectifier station or an inverter station. Both converter stations can normally not be in the same control mode simultaneously as it may lead to a conflict of control. If a station is in power control mode, a reference power value will be set by, for example, an operator. Similarly, if a station is in DC voltage control mode, the DC voltage reference will be set by the operator.

The control system of a converter station may be structured in such a way that a DC voltage control block comes in cascade with a power control block. For the converter station in DC voltage control mode, the DC voltage reference value is directly set by the operator. However, for the station in power control mode, the reference value for the DC voltage control block is generated from the power control block whose reference (in terms of power) would be set by the operator or the like. The control system then provides reference DC voltage values for controlling the converters of the converter station operated in power control mode. The reference DC voltage values generated by the power control block (also referred to as the control unit, in the following) may be based on the reference power value set by the operator to operate the converter station and, for example, a measure of an actual DC voltage between the DC terminals of the converter station.

In the present inventive concept, a method is provided for controlling a converter station including series connected converters and being operated in power control mode. The inventor has realized that, by determining a common DC voltage reference based on either one of the first DC voltage measured across the first VSC and the second DC voltage measured across the second VSC, one of the converters will act as a master and the other one will act as a slave. In other words, one of the two converters will be regulated in accordance with the DC voltage measured upon the other converter.

Voltage sharing between the two converters connected in series in the converter station may thereby be improved in that the values of the first reference DC voltage and the second reference DC voltage will then to be closer to each other.

It will be appreciated that the voltage measurement and the control of series connected VSCs (or MMCs) in the converter station are not dependent on the voltage measured in a converter station connected at another end of the DC transmission system. The converter station can therefore work more or less independently without knowledge of measurements performed by the other converter station.

The present inventive concept provides a technique to more correctly measure, or rather determine, the positive pole to negative pole DC voltage during normal conditions with all the four VSCs (or MMCs) in operation and also after one of the VSCs (or MMCs) is bypassed following a fault 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 pole voltage of one MMC (given the other MMC is bypassed) or the DC pole voltage of the two MMCs connected in series.

Further, when one of the MMCs is bypassed following a fault, the DC link voltage may be reduced to match the DC voltage rating of the MMC remaining in operation, i.e. the MMC that is still connected, to continue transfer of electrical power. The present technique ensures equal voltage sharing between the other two MMCs (at the healthy converter) when the DC link voltage is reduced in such a scenario.

According to an embodiment, the determining may include identifying a maximum value between the first DC voltage and the second DC voltage, wherein the common DC voltage reference is based on the maximum value. The present embodiment is therefore 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 voltage reference is then based on this highest value. In some embodiments, the common DC voltage reference may be equal (or approximately equal) to the highest value, i.e. either the first DC voltage or the second DC voltage.

In this embodiment, the converter at which the highest DC voltage is measured is the master and the other one is the slave.

It will also be appreciated that the determining may, for example, include the calculation of a sum of the first DC voltage, the second DC voltage and an absolute value of the difference between the second DC voltage and the first DC voltage. 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 (because the absolute value of the difference between the second DC voltage and the first DC voltage will then be negative and the two values of the first DC voltage in the sum will cancel each other) while, if the first DC voltage is lower than the second DC voltage then the sum will be equal to twice the first DC voltage.

The sum may then be multiplied by a factor in order to obtain a common reference DC value relevant for the controlling method. In particular, the sum may be multiplied by a factor being larger 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 it is for example half or full pole to pole voltage) is used as input in the control method for determining the first reference DC voltage and the second reference DC voltage.

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

According to another embodiment, the determining may include identifying a minimum value between the first DC voltage and the second DC voltage, wherein the common DC voltage reference is based on the minimum value. The present embodiment is therefore 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 the common DC voltage reference is then based on this lowest value. In some embodiments, the common DC voltage reference may be equal (or approximately equal) to the lowest value, i.e. either the first DC voltage or the second DC voltage.

In this embodiment, the converter at which the lowest DC voltage is measured is the master and the other one is the slave.

The determination of the common DC voltage reference, which indirectly includes a determination (or identification) of which of the two converters is the master and which is the slave, may be based on a calculation as explained above in the preceding embodiment but adapted to result in a value based on the lowest DC voltage 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 a power control mode. With the present inventive concept, reference DC voltage values are obtained for each of the two converters based on, at least partly, the common DC voltage reference.

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

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

measurement of the DC voltage across the DC terminals of the converter station.

According to an embodiment, the method may further comprise low- pass filtering of the value of the measured DC voltage used for determining the common DC voltage reference. The low pass filtering is done to extract a DC voltage value without ripples, which further improves the control method. Eliminating, or at least reducing, ripples in the measured DC voltage facilitate the comparison of the extracted value to the reference value.

According to an embodiment, 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 include at least one of a modular multi-level converter, a full-bridge MMC, and a half-bridge MMC.

In particular, the switching devices of the half-bridge cells and full- bridge cells of the MMCs may be insulated gate bipolar transistors (IGBTs). The switching devices of the MMCs may however not be limited to IGBTs and they may for example be integrated gate-commutated thyristors (IGCT), Bi- mode insulated gate transistor (BIGT), or the like.

Further it will be appreciated that the switching devices may be arranged in a full-bridge, FB, submodule or FB cell, in which four switching devices or units, each including an insulated-gate bipolar transistor (IGBT) together with a parallel freewheeling diode, are connected in an H-bridge configuration together with a charge up capacitor, or in a half-bridge, FIB, submodule or FIB cell, in which two switching devices (or units) are connected in series together with a parallel charge up capacitor. The connection of a plurality of FB cells in series may result in a FB MMC while the connection of a plurality of FIB cells in series may result in an FIB MMC.

As mentioned above, the present inventive concept is applicable to any VSCs connected in series. Flowever, reducing the DC voltage (after for example bypass of one of the converters) will only be possible if each of the series-connected VSCs can generate the ac voltage even after the DC voltage references are reduced. This is possible with FB MMC with normal rating or with FIB MMCs with sufficient over rating (i.e. the AC voltage generated in normal operation is much less than the DC voltage).

According to another aspect, a control unit adapted to control a converter station is provided. The converter station may include 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 in accordance with a method as defined in any one of the preceding embodiments.

According to another aspect, a converter station for a high- voltage power system adapted to transfer electrical power to another converter station is provided. 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 include a control unit (or controller) adapted to control the first VSC and the second VSC by performing a method as defined in any one of the preceding embodiments.

According to another aspect, a high-voltage power system

adapted to transfer electrical 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 one of the

preceding embodiments.

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

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

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

Brief descript of the drawings Exemplifying embodiments will be described below with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of a high-voltage power transmission system according to an embodiment;

Figure 2 illustrates schematically an embodiment of a measurement module/step for determining an actual DC voltage between DC terminals of a converter station; and

Figure 3 illustrates schematically an embodiment of a control unit/step for determining reference DC voltage values to control converters of a converter station.

In the drawings, like reference numerals will be used for like elements unless stated otherwise. Unless explicitly stated to the contrary, the drawings show only such elements that are necessary to illustrate the example embodiments, while other elements, in the interest of clarity, may be omitted or merely suggested. As illustrated in the figures, the sizes of elements and regions may be exaggerated for illustrative purposes and, thus, are provided to illustrate the general structure of the embodiments.

Detailed description

Figure 1 illustrates a schematic view of a high-voltage power transmission system according to an embodiment.

Figure 1 shows a high-voltage transmission system 100 comprising two converter stations 110 and 120. A first converter station 110 may be operated in a power control mode while the other converter station 120 may be operated in a voltage control mode, for example. Each of the converter stations 110 and 120 may be based on voltage source converters (VSCs). In particular, the VSCs may be MMCs.

The first VSC station 110 and the second VSC station 120 are connected via a DC transmission system or link 130, and arranged in a monopole configuration, in which one of the DC transmission lines is 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 (such as a bipolar configuration, that different symmetries, such as symmetric or asymmetric, may be used, and that more than two VSC stations may be included). 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 include 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 may be connected to e.g. one or more DC lines of the 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, via for example one or more transformers 160, 162, to an AC grid (not shown). At least one of the VSCs 115 and 116 of the first converter station 110 may, in some embodiments, include at least one full- bridge submodule. Further, the first VSC 115 may be associated with, or may include, a first active power controller (not shown in figure 1 , see figure 3) and the second VSC 116 may be associated with, or may include, a second (active) power controller (not shown in figure 1 , see figure 3).

Similarly, the second converter station 120 has DC terminals 122 and

124 with which the second converter station 120 may be connected to e.g. one or more DC lines of the 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, via for example one or more transformers 180, 182, to an AC grid (not shown). At least one of the VSCs

125 and 126 of the second converter station 120 may, in some embodiments, include at least one full-bridge submodule.

The first converter station 110 may also include a controller or control unit 150 for implementing a control method controlling the first and second converters. Alternatively, an 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 voltage reference (and also an AC voltage reference), or generate control signals based on such a DC voltage reference (and such an AC voltage reference) to each VSC 115 and 116 of the converter station 110. The controller 150 may be adapted to control the VSCs by performing a control method according to the

embodiments as described herein, for example with reference to Figures 2 and 3, as follows.

Figure 2 illustrates schematically an embodiment of a DC voltage measurement module/step for determining a common DC voltage reference for the converters 115 and 116 of the first converter station 110.

As illustrated in Figure 2, the measurement module or measurement step 200 for determining a common DC voltage reference may include adding a first DC voltage 215 measured across the first VSC 115 and a second DC voltage 216 measured across the second VSC 116 (see also Figure 1 in which 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 VSC 115 to obtain the difference 218. An 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 VSC 115, 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 effect, this sum will result in either twice the first DC voltage or twice the second DC voltage depending on whether the first DC voltage is larger 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 voltage reference is then based on this maximum value. This alternative is therefore based on the“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 voltage reference is then based on this highest value. The common DC voltage reference may be equal (or approximately equal) to the highest value, i.e. either the first DC voltage or the second DC voltage. In this embodiment, the converter at which the highest DC voltage is measured is the master and the other one is the slave.

Expressed differently, if the first DC voltage 215 is higher than the second DC voltage 216, then the first VSC 115 is identified, or determined, to be the master and the common DC voltage reference is based on the first DC voltage 215. Analogously, if the first DC voltage 215 is lower than the second DC voltage 216, then the second VSC 116 is identified, or determined, to be the master and the common DC voltage reference is based on the second DC voltage 216.

Alternatively, the identification of the master converter may be based on a“minimum criterion”, wherein the measurement module or measurement step 200 for determining a common DC voltage reference may include adding the first DC voltage 215 measured across the first VSC 115 to the second DC voltage 216 measured across the second VSC 116 to obtain a 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, then the first VSC 115 is identified, or determined, to be the master and the common DC voltage reference is based on the first DC voltage 215. Analogously, if the first DC voltage 215 is higher than the second DC voltage 216, then the second VSC 116 is identified, or determined, to be the master and the common DC voltage reference is based on the second DC voltage 216. In this alternative, the common DC voltage reference is then based on the 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 voltage reference, or identifying which of the two converters is the master converter, may be employed based on the above described principles.

The determination of the common DC voltage reference, which indirectly includes a determination (or identification) of which of the two converters is the master and which is the slave, may be based on a calculation as explained in the above embodiments but adapted to result in a value based on the lowest DC voltage of the two measured DC voltages.

In particular, as further illustrated in Figure 2, the sum or output 220 may be multiplied by a factor f, such as for instance 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 represent half of the voltage between the DC positive pole and the negative pole, which may then be used as input for the remaining part of the control method. If a full positive pole to negative pole voltage is to be used, the sum or output 220 may be multiplied by 0.5.

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

It will be appreciated that, although Figure 2 shows the different steps to be performed in a certain order, these steps may be performed in another order too, with for example the filtering being applied to the measured DC voltage before the factor is applied.

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

Figure 3 illustrates schematically an embodiment of a control unit/step for determining reference DC voltage values for controlling each one of the VSCs 115 and 116 of the first converter station 110.

By way of example, Figure 3 may be considered to illustrate the determination of the first reference DC voltage 310 to control the first VSC 115 of the first converter station 110. A similar determination may be performed to obtain the reference DC voltage value for controlling the second VSC 116.

It will be appreciated that the generated DC voltage reference 310 may then be used for switching one or more FB and/or FIB cells s in the VSC 115 of the first converter station 110. The DC voltage reference 310 may for example be fed directly to the switching devices of the cells of the first VSC 115. In a converter station, a sum cell capacitor voltage, which corresponds to the voltage upon the cells (each cell including a capacitor as an energy storage device) of the one or more MMCs, may, because of disturbances, start to deviate from its reference value and the generated DC voltage may also be different from its reference value. To counter such issues, it is envisaged that the reference DC voltage to be used for controlling a converter of a converter station including series connected converters is obtained based on the common DC voltage reference 224 as determined by the

measurement module/step described with reference to Figure 2 and a DC voltage contribution obtained from the power controller of the converter in question, as illustrated in Figure 3.

In Figure 3, a reference DC current 302 is subtracted from an actual DC current 303 in order 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 a power value 301 , for example input by an operator for operating the converter station 110 in order to deliver a certain electrical power.

The DC current error 304 may then be input to a controller module 350. The controller module 350 may for example be a Pl-controller, although it is envisaged also that other types of controller modules may be used. The controller module 350 may be configured to determine a DC voltage

contribution, or correction, 305 based on the DC current error 304. The DC voltage contribution, or correction, 305 may then be added to the common DC voltage reference 224, and the sum of the DC voltage contribution 305 and the common DC voltage reference 224 is output as the first DC voltage reference 310 to control the converter 115 of the first converter station 110.

For illustration purposes only, using arbitrary units, if the first DC voltage 215 upon the first VSC 115 is measured to be 1 and the second DC voltage 216 upon the second VSC 116 is measured to be 0.8, applying a maximum criterion, the first DC voltage 215 will be determined to be the common DC voltage reference 224. If the first power controller, via the PI controller 350, outputs a DC voltage contribution of - 0.1 , the first reference DC voltage 310 for the first VSC 115 may then be 0.9. Following on this example, if the second power controller, via its own PI controller 350, outputs a DC voltage contribution of -0.15, the second reference DC voltage for the second VSC 116 may then be 0.85.

The control method as described with reference to Figures 2 and 3, or the controller 150 operating in accordance with such a method, may ensure that the reference DC voltages to be applied to the first VSC 115 and the second VSC 116 move towards each other (i.e. becomes closer to each other), thereby improving the stability of the converter station 110 (operating e.g. in a DC power control mode). The switching devices of the first VSC 115 may then be controlled accordingly.

The person skilled in the art realizes that the present disclosure is by no means 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 may be used alone without the other features and elements or in various combinations with or without other features and elements.

Additionally, variations to the disclosed embodiments can be

understood and effected by the skilled person 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 features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to

advantage.

Claims

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

determining a common DC voltage reference (224) based on either 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 voltage reference 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 voltage reference and a second DC voltage contribution from the second power controller; and

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

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

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

4. The method of any one of the preceding claims, wherein the DC voltage contribution from the first power controller and the DC voltage contribution from the second power controller are used for operating the first VSC and the second VSC, respectively, in a power control mode.

5. The method of any one of the preceding claims, further

comprising low-pass filtering (250) of the value of the measured DC voltage used for determining the common DC voltage reference.

6. The method of any one of the preceding claims, wherein

controlling operation of the first VSC and the second VSC includes

controlling switching devices of the first VSC and the second VSC.

7. The method of any one of the preceding claims, wherein the first VSC and/or the second VSC includes at least one of a modular multi- level converter, a full-bridge MMC and a half-bridge MMC.

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

9. A converter station (110) for a high-voltage power system

adapted to transfer electrical power to another 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 said 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 said first VSC and said second VSC by performing a method as defined in any one of the preceding claims. 10. A high-voltage power system adapted to transfer electrical power, said 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.