Method for controlling HVDC converter station based on series connected converters

Abstract

A method for 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 includes: determining a common DC reference voltage based on any 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 reference voltage 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 reference voltage and a second DC voltage contribution from the second power controller. The operation of the first VSC and the second VSC can then be controlled, respectively, based at least in part on the first reference DC voltage and the second reference DC voltage.

Description

Technical Field

[0001] The present disclosure generally relates 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 Art

[0002] Direct current (DC) power transmission systems have become the preferred choice over their alternating current (AC) competitors for transmitting large amounts of high-voltage power due to their lower losses and costs. In modern HVDC power transmission systems (with voltages of several hundred kV), the power can reach several thousand megawatts and can be transmitted over distances of up to several thousand kilometers.

[0003] At each end of an 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 devices have been widely used in HVDC applications. However, in recent years, with the development of semiconductor technology, 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 failures.

[0004] Further, a converter station may comprise two or more converters connected in series to provide higher availability and reliability, since if one of the converters fails, only a portion of the total power supply capacity is lost. Moreover, multiple converters connected in series make it possible, for example, to manufacture transformers connected to the converters with reduced size. This can reduce both costs and space requirements, and make the transformers easier to transport.

[0005] However, this type of converter station requires an improved method of controlling the operation of the series-connected converters. Summary of the invention

[0006] To at least partially meet the above requirements, the present disclosure seeks to at least provide an improved method of controlling a HVDC converter station comprising at least two series connected converters, an improved control unit, an improved converter station and a high voltage power system thereof.

[0007] In order to achieve this object, a method, a control unit, a converter station and a high voltage power system thereof are provided as defined in the independent claims. Further embodiments are provided in the dependent claims.

[0008] According to one 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.

[0009] 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.

[0010] Each of the first VSC and the second VSC 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 inventive concept is applicable to any type of VSC.

[0011] The method may include determining a common DC reference voltage based on either a first DC voltage measured across the first VSC and a second DC voltage measured across the second VSC. The method also includes determining a first reference DC voltage for the first VSC based on the determined common DC reference voltage 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 reference voltage and a second DC voltage contribution from the second power controller. Operations of the first VSC and the second VSC may then be controlled, respectively, based at least in part on the first reference DC voltage and the second reference DC voltage.

[0012] 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.

[0013] It will also be understood that in an HVDC power transmission system having two converter stations, which are connected at the two ends of a DC transmission link (or DC transmission line) of the transmission system, one converter station may be in power control mode, and the other converter station may be in DC voltage control mode, regardless of whether the converter station is a rectifier station or an inverter station. Typically, two converter stations cannot be in the same control mode at the same time, as this may result in control conflicts. If the converter station is in power control mode, the reference power value will be set by, for example, an operator. Similarly, if the converter station is in DC voltage control mode, the DC reference voltage will be set by the operator.

[0014] The control system of the converter station can be constructed in such a way that the DC voltage control block is cascaded with the power control block. For a converter station in a DC voltage control mode, the DC reference voltage value is set directly by the operator. However, for a converter station in a power control mode, a reference value for the DC voltage control block is generated from the power control block, and the reference (in terms of power) of the power control block will be set by an operator or the like. The control system then provides a reference DC voltage value for controlling the converter 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 a control unit) can be based on a reference power value set by an operator to operate the converter station, and can be based on, for example, a measurement of the actual DC voltage between the DC terminals of the converter station.

[0015] In the present inventive concept, a method for controlling a converter station comprising converters connected in series and operating in a power control mode is provided. The inventors have realized that by determining a common DC reference voltage based on either a first DC voltage measured across a first VSC and a second DC voltage measured across a 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 adjusted according to 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 the two converters connected in series in the converter station can be improved.

[0016] It will be appreciated that the voltage measurement and control of the VSCs (or MMCs) connected in series in a converter station does not depend on the voltage measured in the converter station connected at the other end of the DC transmission system. Thus, the converter station can operate more or less independently without knowledge of the measurements performed by another converter station.

[0017] The present inventive concept provides a technique to more accurately measure or more precisely determine the positive-to-negative DC voltage during normal conditions when all four VSCs (or MMCs) are in operation, and after a fault occurs at any one of the converters after one of the VSCs (or MMCs) is bypassed. 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 (assuming that the other MMC is bypassed) or the DC pole voltages of two MMCs connected in series.

[0018] Further, when one of the MMCs is bypassed after a fault occurs, the DC link voltage can be reduced to match the DC rated voltage of the MMC that remains in operation (i.e., the MMC that is still connected) to continue transmitting power. When the DC link voltage is reduced in this scenario, the present technology ensures that the voltage distribution between the other two MMCs (converters in normal state) is equal.

[0019] According to one embodiment, the determining may include: 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. 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 then the common DC reference voltage is based on the highest value. In some embodiments, the common DC reference voltage may be equal to (or approximately equal to) the highest value, i.e., the first DC voltage or the second DC voltage.

[0020] In this embodiment, the converter measuring the highest DC voltage is the master converter, and the other is the slave converter.

[0021] It will also be appreciated that the determination may, for example, include calculating the sum of the absolute values ​​of the first DC voltage, the second DC voltage, and 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 be negative and the two values ​​of the first DC voltage in the sum will cancel each other out), and if the first DC voltage is lower than the second DC voltage, the sum will be equal to twice the first DC voltage.

[0022] The sum may then be multiplied by a factor to obtain a common reference DC value relevant 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 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.

[0023] More specifically, if half of the DC positive and negative voltage is used as input to the control method, the sum can be multiplied by a factor equal to 0.25. In another example, if the full positive to negative voltage is used for the control method or system, the factor would be 0.5.

[0024] 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 reference voltage is based on the minimum value. Thus, this 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 the lowest value. In some embodiments, the common DC reference voltage may be equal to (or approximately equal to) the lowest value, i.e., the first DC voltage or the second DC voltage.

[0025] In this embodiment, the converter measuring the lowest DC voltage is the master converter, and the other is the slave converter.

[0026] A common DC reference voltage can be determined based on the calculations described above in the aforementioned 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 obtain a value based on the lowest DC voltage of the two measured DC voltages.

[0027] According to one embodiment, the DC voltage contribution from the first power controller and the DC voltage contribution from the second power controller can be used to operate the first VSC and the second VSC in the power control mode, respectively. 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.

[0028] It will be appreciated that a certain power requirement may be used to control the converter station in a power control mode. However, the power to be achieved by the converter station may then be shared by the different converters constituting the converter station, such that, for example, in the case of two converters connected in series, 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.

[0029] Further, it will be appreciated that since the reference DC voltage for a converter is based not only on measurements of the DC voltage across the converter in question, but also not only on measurements of the DC voltage across the DC terminals of the converter station, the method of the present invention provides more accurate control of the converter station.

[0030] According to one embodiment, the method may further comprise: low pass filtering the value of the measured DC voltage used to determine the common DC reference voltage. 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 the ripple in the measured DC voltage facilitates comparing the extracted value with the reference value.

[0031] According to one embodiment, operations of the first VSC and the second VSC may be controlled by controlling switching devices of the first VSC and the second VSC.

[0032] According to one embodiment, the first VSC and / or the second VSC may include at least one of a modular multilevel converter, a full-bridge MMC, and a half-bridge MMC.

[0033] Specifically, the switching devices of the half-bridge monomer and the full-bridge monomer of the MMC can be insulated gate bipolar transistors (IGBT). However, the switching devices of the MMC may not be limited to IGBTs, and the switching devices may be, for example, integrated gate commutated thyristors (IGCTs), dual-mode insulated gate transistors (BIGTs), etc.

[0034] Further, it will be understood that the switching devices can be arranged in a full-bridge FB submodule or FB single unit, wherein four switching devices or units (each including an insulated gate bipolar transistor (IGBT) and a parallel freewheeling diode) are connected together with a charging capacitor in an H-bridge configuration, or the switching devices can be arranged in a half-bridge HB submodule or HB single unit, wherein two switching devices (or units) are connected in series with a parallel charging capacitor. A plurality of FB single units connected in series can obtain a FBMMC, while a plurality of HB single units connected in series can obtain a HB MMC.

[0035] As mentioned above, the inventive concept is applicable to any VSCs connected in series. However, it will be possible to reduce the DC voltage only if each VSC in the series connected VSCs can generate an AC voltage even after reducing the DC reference voltage (e.g. after one bypass in the converter). This is possible if a FB MMC with a normal rating is employed or if a HB MMC with a sufficiently high rating is employed (i.e. the AC voltage generated in normal operation is much smaller than the DC voltage).

[0036] According to another aspect, a control unit adapted to control a converter station is provided. The converter station may comprise at least two VSCs connected in series, 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.

[0037] According to another aspect, a converter station for a high voltage power system is provided, the converter station being adapted to transfer 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), the control unit (or controller) being adapted to control the first VSC and the second VSC by performing a method as defined in any one of the aforementioned embodiments.

[0038] According to another aspect, a high voltage power system adapted for transmitting electric power is provided. The system may include 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.

[0039] It will be understood that in the context of the present invention, the converter station may be operated as an inverter and / or as a rectifier, depending on the circumstances and / or application.

[0040] It will be understood that all embodiments described with reference to the first aspect of the present disclosure may be combined with any embodiments described with reference to other aspects of the present disclosure, and vice versa.

[0041] The present disclosure relates to all possible combinations of features recited in the claims. Other objects and advantages of various embodiments of the present disclosure will be described below with the aid of exemplary embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Exemplary embodiments will be described below with reference to the accompanying drawings, in which:

[0043] Figure 1 is a schematic diagram of a high voltage power transmission system according to one embodiment;

[0044] Figure 2 schematically illustrates an embodiment of a measurement module / step for determining an actual DC voltage between DC terminals of a converter station; and

[0045] Figure 3 An embodiment of a control unit / step for determining a reference DC voltage value for controlling a converter of a converter station is schematically shown.

[0046] In the drawings, similar reference numerals will be used for similar elements unless otherwise specified. Unless otherwise explicitly stated, the drawings show only those elements necessary to illustrate the example embodiments, and other elements may be omitted or only presented for clarity. As shown in the figures, the sizes of elements and regions may be exaggerated for illustrative purposes, and therefore, the sizes of elements and regions are provided to illustrate the overall structure of the embodiments. DETAILED DESCRIPTION

[0047] Figure 1 A schematic diagram of a high voltage power transmission system according to one embodiment is shown.

[0048] Figure 1 A high voltage transmission system 100 is shown that includes two converter stations 110 and 120. For example, the first converter station 110 can operate in a power control mode, while the other converter station 120 can operate in a voltage control mode. Each of the converter stations 110 and 120 can be based on a voltage source converter (VSC). Specifically, the VSC can be an MMC.

[0049] 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, wherein 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 contemplated that the power transmission system 100 may be arranged using other configurations, such as bipole configurations that may use different symmetries (e.g., symmetrical or asymmetrical) and bipole configurations that 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.

[0050] 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.

[0051] 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 can also have AC terminals 140 and 142, with which the converter station 110 can 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 can include at least one full-bridge submodule. Further, the first VSC 115 can be connected to a first active power controller ( Figure 1 Not shown, see Figure 3 ) may be associated with, or may include, a first active power controller, and the second VSC 116 may be associated with a second (active) power controller ( Figure 1 Not shown, see Figure 3 ) is associated with, or may include, a second (active) power controller.

[0052] 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 the DC transmission system 130. The second converter station 120 can also have AC terminals 170 and 172, with which the second converter station 120 can 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 can include at least one full-bridge submodule.

[0053] The first converter station 110 may further include a controller or control unit 150 for implementing a control method for controlling the first converter and the second converter. Alternatively, an external controller or control unit 150 may be configured to control the converter station 110.

[0054] 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 control signals for each VSC 115 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 VSCs 115 and 116 of the first converter station 110 by performing the following operations as described herein (e.g., with reference to Figure 2 and Figure 3 ) is used to control the VSC.

[0055] Figure 2 An embodiment of a DC voltage measurement module / step for determining a common DC reference voltage for converters 115 and 116 of a first converter station 110 is schematically shown.

[0056] like Figure 2 As shown, the measurement module or measurement step 200 for determining a common DC reference voltage may include comparing a first DC voltage 215 measured across the first VSC 115 with a second DC voltage 216 measured across the second VSC 116 (see also Figure 1 , where these voltages are represented by arrows) are added to obtain a first (sub-)sum 217 of these two values.

[0057] 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 a 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.

[0058] In fact, depending on whether the first DC voltage is greater than the second DC voltage, the result of the sum will be twice the first DC voltage or twice the second DC voltage. Such a calculation will lead to the identification of the maximum value between the first DC voltage and the second DC voltage. The common DC reference voltage is then based on this maximum value. Therefore, this alternative is based on the "maximum criterion", in which it is determined which of the first DC voltage and the second DC voltage has the highest value, and then the common DC reference voltage is based on this highest value. The common DC reference voltage can be equal to (or approximately equal to) the highest value, that is, the first DC voltage or the second DC voltage. In this embodiment, the converter that measures the highest DC voltage is the main converter, and the other is the slave converter.

[0059] In other words, if the first DC voltage 215 is higher than the second DC voltage 216, the first VSC 115 is identified or determined as the master 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 as the master converter, and the common DC reference voltage is based on the second DC voltage 216.

[0060] Alternatively, the master converter may be identified based on a "minimum criterion", wherein the measurement module or measurement step 200 for determining the common DC reference voltage may include adding the first DC voltage 215 measured across the first VSC 115 and 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, the first VSC 115 is identified or determined to be the master 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 master 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 the minimum value between the first DC voltage 215 and the second DC voltage 216 .

[0061] It will be appreciated that other ways may be employed based on the above principles to calculate a common DC reference voltage or to identify which of the two converters is the master converter.

[0062] A common DC reference voltage can be determined based on calculations as described 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 obtain a value based on the lowest DC voltage of the two measured DC voltages.

[0063] Specifically, Figure 2 As further shown, the sum or output 220 may be multiplied by a factor f (e.g., equal to 0.25) to obtain an actual DC voltage value 222. Using 0.25 as a factor, the sum or output 220 is adjusted to half the voltage between the DC positive and negative poles, which may then be used as an input for the remainder of the control method. If the full positive to negative voltage is to be used, the sum or output 220 may be multiplied by 0.5.

[0064] Furthermore, if Figure 2 As shown, a low pass filter or low pass filtering method 250 may be used to low pass filter the value 222 to obtain a filtered value 224 of the sum or output 220, ie, a filtered value of either the first DC voltage or the second DC voltage.

[0065] It will be understood that although Figure 2 The different steps are shown to be performed in a certain order, but they may also be performed in another order, for example applying a filter to the measured DC voltage before applying the factor.

[0066] As reference Figure 3 As further described, the filtered value 224 may then be used as an input in a control method.

[0067] Figure 3 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 is schematically shown.

[0068] By way of example, it can be considered Figure 3 It is shown determining a first reference DC voltage 310 for controlling the first VSC 115 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.

[0069] 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 VSC 115 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.

[0070] In a converter station, the total cell capacitor voltage corresponding to the voltage on the cells of one or more MMCs (each cell including a capacitor as an energy storage device) may start to deviate from the reference voltage of the total cell capacitor voltage due to interference, and the generated DC voltage may also differ from its reference value. In order to solve these problems, it is conceivable to generate a DC voltage based on the reference voltage. Figure 2 The common DC reference voltage 224 determined by the described measurement module / step and the DC voltage contribution obtained from the power controller of the converter in question are used to obtain a reference DC voltage to be used for controlling the converters of a converter station comprising series-connected converters, such as Figure 3 shown.

[0071] exist Figure 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 a power value 301 (e.g., input by an operator for operating the converter station 110 to deliver a certain power).

[0072] The DC current error 304 may then be input to a controller module 350. The controller module 350 may be, for example, a PI controller, but other types of controller modules may also be 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 may be output as a first DC reference voltage 310 to control the converter 115 of the first converter station 110.

[0073] For illustration purposes only, using arbitrary units, if applying the maximum criterion the first DC voltage 215 on the first VSC 115 is measured as 1 and the second DC voltage 216 on the second VSC 116 is measured as 0.8, the first DC voltage 215 will be determined as the common DC reference voltage 224. If the DC voltage contribution output by the first power controller via the PI controller 350 is -0.1, the first reference DC voltage 310 for the first VSC 115 may be 0.9. Continuing with this example, if the DC voltage contribution output by the second power controller via its own PI controller 350 is -0.15, the second reference DC voltage for the second VSC 116 may be 0.85.

[0074] Reference Figure 2 and Figure 3The described control method or the controller 150 operating according to this method can ensure that the reference DC voltages to be applied to the first VSC 115 and the second VSC 116 move toward each other (i.e., become close to each other), thereby improving the stability of the converter station 110 (e.g., operating in the DC power control mode). The switch device of the first VSC 115 can then be controlled accordingly.

[0075] Those skilled in the art realize 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.

[0076] 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 the other features and elements.

[0077] Additionally, variations to the disclosed embodiments may be understood and effected by the skilled person in practicing the claimed invention, by studying 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 for controlling a high voltage direct current (HVDC) converter station (110), the converter station 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, and the method comprises: determining a common DC reference voltage (224) based on either a first DC voltage (215) measured across the first VSC or 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 Operation of the first VSC and the second VSC are controlled based at least in part on the first reference DC voltage and the second reference DC voltage, respectively.

2. The method according to claim 1, wherein: Determining a common DC reference voltage includes 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 according to claim 1, wherein: Determining a common DC reference voltage includes 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. The method according to any one of claims 1 to 3, 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 claims 1-3, further comprising low pass filtering (250) the value of the measured DC voltage used for determining the common DC reference voltage.

6. The method according to any one of claims 1 to 3, 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 according to any one of claims 1 to 3, wherein: The first VSC and / or the second VSC includes 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), the converter station comprising at least two voltage source converters (115, 116) connected in series, each voltage source converter being associated with a power controller, wherein: The control unit is configured to operate according to the method as defined in any of claims 1-7.

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), the first VSC and the second VSC being 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 executing a method as defined in any one of claims 1-7.

10. A high voltage power system, the high voltage power system being 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 the converter station according to claim 9.

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