CN110635501A

CN110635501A - Flexible direct current transmission system and providing method thereof

Info

Publication number: CN110635501A
Application number: CN201910967389.XA
Authority: CN
Inventors: 马茨·安德森; 蒋雯
Original assignee: ABB Schweiz AG
Current assignee: Hitachi Energy Co ltd
Priority date: 2019-10-12
Filing date: 2019-10-12
Publication date: 2019-12-31
Anticipated expiration: 2039-10-12
Also published as: CN110635501B

Abstract

The present disclosure relates to a flexible direct current transmission system and a method of providing a flexible direct current transmission system. The flexible direct current transmission system includes: a voltage source converter coupled to the first DC bus terminal; and an arrester arrangement, a first end of which is coupled to the first dc bus terminal, and a second end of which, different from the first end, is coupled to a second dc bus terminal, different from the first dc bus terminal, and/or to a neutral node of a valve-side star coil of a transformer coupled to the voltage source converter. By using the solution according to embodiments of the present disclosure, the voltage source converter can be effectively protected in case of an ac fault.

Description

Flexible direct current transmission system and providing method thereof

Technical Field

The present disclosure relates to the electrical field, and more particularly to protection of voltage source converters in a flexible direct current transmission system.

Background

Voltage Source Converter (VSC) high voltage direct current transmission (HVDC) transmission networks are being widely established. The transmission network typically transmits power from a transmitting end to a receiving end in a dc high voltage or ultra high voltage manner. The transmitting end typically converts ac power from an ac system to dc power for dc transmission. The receiving end converts the received direct current into alternating current for subsequent use.

In long distance (such as 800km) high and extra high voltage dc transmission, large dc current is used to transmit through long distance transmission lines. In this case, a large amount of energy is stored on the dc transmission line. However, when an intra-station ac fault occurs in the VSC, the energy stored on the dc transmission line can cause the energy to be quickly dumped into the capacitors in the VSC and create an overvoltage in the upper arm of the VSC. Such overvoltages may damage the VSC device.

In some conventional solutions, a surge arrester is used to limit the pole voltage of the dc transmission line, but this does not reduce the overvoltage of the upper arm of the VSC. In other conventional approaches, an energy absorbing device on the dc side is used to reduce the overvoltage in the upper arm of the VSC, but this adds cost and complexity to the system.

Disclosure of Invention

According to an embodiment of the present disclosure, a flexible direct current power transmission system and a providing method thereof are provided.

In a first aspect of the present disclosure, there is provided a flexible direct current power transmission system comprising: a voltage source converter coupled to the first DC bus terminal; and an arrester arrangement, a first end of which is coupled to the first dc bus terminal, and a second end of which, different from the first end, is coupled to a second dc bus terminal, different from the first dc bus terminal, and/or to a neutral node of a valve-side star coil of a transformer coupled to the voltage source converter.

In a second aspect of the present disclosure, there is provided a method for providing a flexible direct current power transmission system, comprising: providing a voltage source converter coupled to the first dc bus terminal; and providing a surge arrester device, a first end of the surge arrester device being coupled to the first dc bus terminal, and a second end of the surge arrester device, different from the first end, being coupled to a second dc bus terminal, different from the first dc bus terminal, and/or to a neutral node of a star winding of a valve side of a transformer coupled with the voltage source converter.

By using embodiments according to the present disclosure, voltage source converters can be effectively protected against ac faults and ac system faults within the converter station.

It should be understood that the statements herein reciting aspects are not intended to limit the critical or essential features of the embodiments of the present disclosure, nor are they intended to limit the scope of the present disclosure. Other features of the present disclosure will become apparent from the following description.

Drawings

The above and other features, advantages and aspects of various embodiments of the present disclosure will become more apparent by referring to the following detailed description when taken in conjunction with the accompanying drawings. In the drawings, like or similar reference characters designate like or similar elements, and wherein:

fig. 1 shows an example of a conventional symmetrical monopole VSC-HVDC system;

fig. 2 shows one example of a symmetric monopole VSC-HVDC system according to one embodiment of the present disclosure;

fig. 3 shows a schematic partial detail view of the symmetrical monopole VSC-HVDC system of fig. 2;

fig. 4 shows one example of an asymmetric monopole VSC-HVDC system according to one embodiment of the present disclosure;

fig. 5 shows one example of a symmetric bipolar VSC-HVDC system according to one embodiment of the present disclosure;

fig. 6 shows one example of a VSC-HVDC system according to one embodiment of the present disclosure;

fig. 7 shows another example of a VSC-HVDC system according to an embodiment of the present disclosure;

fig. 8 shows a further example of a VSC-HVDC system according to an embodiment of the present disclosure; and

fig. 9 shows a flow diagram of a method for providing a flexible direct current power transmission system according to an embodiment of the present disclosure.

Detailed Description

Embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. While certain embodiments of the present disclosure are shown in the drawings, it is to be understood that the present disclosure may be embodied in various forms and should not be construed as limited to the embodiments set forth herein, but rather are provided for a more thorough and complete understanding of the present disclosure. It should be understood that the drawings and embodiments of the disclosure are for illustration purposes only and are not intended to limit the scope of the disclosure.

In describing embodiments of the present disclosure, the terms "include" and its derivatives should be interpreted as being inclusive, i.e., "including but not limited to. The term "based on" should be understood as "based at least in part on". The term "one embodiment" or "the embodiment" should be understood as "at least one embodiment". The terms "first," "second," and the like may refer to different or the same object. Other explicit and implicit definitions are also possible below.

In the description of the embodiments of the present disclosure, the transformer may have a valve side and a grid side, and the valve side and the grid side may independently have a star coil (Y) arrangement or a delta coil (Δ) arrangement corresponding to three phases, where in the star coil arrangement, the common node of the three coils coupled to each other is a neutral node. In the following, the valve side and/or the mesh side of the transformer may have a star coil arrangement or a delta coil arrangement, unless otherwise defined in the specification.

Fig. 1 shows an example of a conventional symmetrical monopole VSC-HVDC system 10. The symmetrical monopole VSC-HVDC system 10 comprises a transmitting end, a direct current transmission line and a receiving end. For example, the transmitting end includes a transformer 11 and a VSC 13 coupled thereto. The transformer 11 transforms Alternating Current (AC) after receiving it from an AC source, and sends the transformed three-phase AC to the VSC 13.

The VSC 13 converts the AC current into a high voltage Direct Current (DC) current. The DC current is transmitted to the receiving terminal through the DC transmission line. In the example of fig. 1, the dc transmission line 19A is coupled with the arrester 12 for protection. The receiving end includes the VSC 14 and the transformer 16 coupled thereto. VSC 14 converts the DC current to three-phase AC current and sends the three-phase AC current to transformer 16. The transformer 16 transforms the received ac power for subsequent use.

In the example of fig. 1, the system in fig. 1 is in a symmetrical monopole VSC-HVDC configuration, due to the symmetrical arrangement of the transmission lines 19A and 19B, with positive and negative symmetrical dc voltages. Although only a single VSC is shown in fig. 1 at both the transmitting and receiving ends, this is merely an example and not a limitation on the scope of the present disclosure.

For example, a VSC-HVDC system may have multiple VSCs on the transmitting and/or receiving side. In another example, one of the transmission lines 19A and 19B may be grounded to form an asymmetric VSC-HVDC configuration.

In the example of fig. 1, the arrester is coupled between the dc bus and ground, and may provide a certain degree of protection for the converter of the VSC-HVDC system. However, this configuration does not address fault conditions of the upper arm overvoltage of the VSC due to rapid flooding of the energy stored on the dc transmission line into the capacitors in the VSC, and such overvoltage may damage the VSC devices. Although there are some conventional schemes to cope with such cases, the conventional schemes for protecting the VSC devices are high in cost and complicated in system, as described above.

Embodiments of the present disclosure will be described below in detail with reference to the accompanying drawings. Fig. 2 shows one example of a symmetric monopole VSC-HVDC system 20 according to one embodiment of the present disclosure. Similar to the system 10 of fig. 1, the symmetric monopole VSC-HVDC system 20 includes a transmitting end, a direct current transmission line, and a receiving end.

For example, the transmitting end includes a transformer 21 and a VSC 23 coupled thereto. The transformer 21 transforms the alternating current after receiving the alternating current from the AC source, and sends the transformed three-phase alternating current to the VSC 23.

The VSC 23 converts the AC current into a high voltage DC current. The DC current is transmitted to the receiving terminal through the DC transmission line. The receiving end includes a VSC24 and a transformer 26 coupled thereto. VSC24 is coupled to first and second

DC bus terminals

29A, 29B and converts the DC current to three-phase AC current for delivery to transformer 26. The transformer 26 transforms the received ac power for subsequent use.

The symmetric monopole VSC-HVDC system 20 further comprises a lightning arrester arrangement 22. In one example, the surge arrester device 22 is located near the VSC 24. A first end of the surge arrester device 22 is coupled to the first dc bus terminal 29A, and a second end of the surge arrester device 22 is coupled to the second dc bus terminal 29B.

The surge arrester device 22 includes a first surge arrester unit 22A and a second surge arrester unit 22B. A first end of the first arrester unit 22A is coupled to the first dc bus terminal 29A, and a second end of the first arrester unit 22A is coupled to the midpoint 22N. A first end of the second arrester unit 22B is coupled to the midpoint 22N, and a second end of the second arrester unit 22B is coupled to the second dc bus terminal 29B.

In the example of fig. 2, the first arrester unit 22A and the second arrester unit 22B are substantially identical. It will be understood that this is by way of example only and is not limiting as to the scope of the disclosure. For example, the performance of the first and second arrester units may be different.

In the example of fig. 2, the midpoint 22N is coupled to a neutral node 26N of a valve-side star winding of a transformer 26 coupled with the VSC 24.

Fig. 3 shows a schematic partial detail view of the symmetric monopole VSC-HVDC system 20 of fig. 2. VSC24 includes three legs corresponding to three phases, with a first leg including upper leg 24UA and lower leg 24LA, a second leg including upper leg 24UB and lower leg 24LB, and a third leg including upper leg 24UC and lower leg 24 LC. In this configuration, the first arrester unit 22A is connected in parallel with the upper arm, and the second arrester unit 22B is connected in parallel with the lower arm.

The inventors have found through research that by coupling the midpoint 22N to the neutral node 26N of the valve-side star coil of the transformer 26, the voltage of the upper arm can be limited using the first arrester unit 22A, and the voltage of the lower arm can be limited using the second arrester unit 22B, so that the upper arm does not withstand the overvoltage as described above. Therefore, the intra-station alternating current fault of the flexible direct current transmission system can be protected.

On the other hand, since the surge arrester device 22 is coupled between the first dc bus terminal 29A and the second dc bus terminal 29B, protection against a fault of the ac system of the flexible dc power transmission system can also be achieved. Even under the serious three-phase grounding short circuit fault, the VSC can still be protected from locking.

It is to be understood that although in the examples of fig. 2 and 3, the configurations and principles of the embodiments of the present disclosure are described for the VSC on the receiving end, the described configurations and principles may also be applied to the case of the VSC on the transmitting end.

Hereinafter, the configurations and principles of the embodiments of the present disclosure described for the VSC of the receiving end may be applied to the case of the VSC of the transmitting end, and the configurations and principles of the embodiments of the present disclosure described for the VSC of the transmitting end may also be applied to the case of the VSC of the receiving end unless otherwise expressly defined. Furthermore, in some embodiments, features according to embodiments of the present disclosure may be applied to both the receiving and transmitting VSCs.

Fig. 4 shows one example of an asymmetric monopole VSC-HVDC system 40 according to one embodiment of the present disclosure. The asymmetric monopole VSC-HVDC system 40 comprises a transmitting end, a direct current transmission line and a receiving end.

For example, the transmitting end includes a transformer 41 and a VSC 43 coupled thereto. The transformer 41 transforms the alternating current after receiving the alternating current from the AC source, and sends the transformed three-phase alternating current to the VSC 43.

The VSC 43 converts the AC current to a high voltage DC current. The DC current is transmitted to the receiving end through a DC transmission line, wherein a second DC transmission line is coupled to ground to form an asymmetric system. The receiving end includes a VSC44 and a transformer 46 coupled thereto. The VSC44 is coupled to a first direct current bus terminal 49A and a second direct current bus terminal 49B, and converts the DC current to a three-phase AC current to be sent to the transformer 46. The transformer 46 transforms the received ac power for subsequent use.

The asymmetric monopole VSC-HVDC system 40 further comprises a lightning arrester arrangement 42. In one example, the surge arrester device 42 is near the VSC 44. A first end of the surge arrester device 42 is coupled to the first dc bus terminal 49A, and a second end of the surge arrester device 42 is coupled to the neutral node 46N of the valve-side star coil of the transformer 46.

As described above, the present inventors have found through studies that the upper arm of the VSC44 is not subjected to the overvoltage as described above by limiting the voltage of the upper arm by connecting the surge arrester device 42 in parallel with the upper arm. Therefore, the intra-station alternating current fault of the flexible direct current transmission system can be protected.

Fig. 5 shows one example of a symmetric bipolar VSC-HVDC system 50 according to one embodiment of the present disclosure. The symmetric bipolar VSC-HVDC system 50 comprises a transmitting end, a direct current transmission line and a receiving end.

For example, the transmitting end includes a transformer 51A and a VSC 53A coupled thereto, and a transformer 51B and a VSC53B coupled thereto, where a common node between the VSC 53A and the VSC53B is grounded. The

transformers

51A and 51B transform the AC power after receiving it from a single or multiple AC sources, and send the transformed three-phase AC power to the VSC 53A and the VSC53B, respectively.

The

VSCs

53A and 53B convert the AC current into a high-voltage DC current. The DC current is transmitted to the receiving terminal through the DC transmission line. The receiving end includes VSC 54A and transformer 56A coupled thereto, and VSC 54B and transformer 56B coupled thereto.

VSC 54A is coupled to a first dc bus terminal 59A and ground, and VSC 54B is coupled to ground and a second dc bus terminal 59B.

VSCs

54A and 54B convert the DC current to three-phase AC current for delivery to

transformers

56A and 56B, respectively.

Transformers

56A and 56B transform the received ac power for subsequent use.

In one example, the symmetric bipolar VSC-HVDC system 50 further includes a surge arrester device 52A. In one example, the surge arrester device 52A is located near the VSC 54A. A first end of the surge arrester device 52A is coupled to the first dc bus terminal 59A, and a second end of the surge arrester device 52A is coupled to the neutral node 56N of the valve-side star coil of the transformer 56A.

As described above, the present inventors have found through studies that the upper arm of the VSC 54A is not subjected to the overvoltage as described above by limiting the voltage of the upper arm by connecting the surge arrester device 52A in parallel with the upper arm. Therefore, the intra-station alternating current fault of the flexible direct current transmission system can be protected.

In another example, the symmetric bipolar VSC-HVDC system 50 further comprises yet another surge arrester device. The other arrester device is located near VSC 54B. The first end of the further arrester device is coupled to the second dc bus terminal 59B and the second end is coupled to the neutral node of the valve-side star winding of the transformer 56B. Similarly, this may further protect against intra-site ac faults of the flexible dc power transmission system.

Fig. 6 shows one example of a VSC-HVDC system 60 according to one embodiment of the present disclosure. The VSC-HVDC system 60 may be a receiving end or a transmitting end. In case the VSC-HVDC system 60 is a receiving terminal, the VSC-HVDC system 60 comprises a transformer 66 and a VSC 64 coupled thereto. VSC 64 converts the DC current to three-phase AC current to transformer 66. The transformer 66 transforms the received ac power for subsequent use.

The VSC-HVDC system 60 further comprises a lightning arrester arrangement 62. In one example, the surge arrester device 62 is located near the VSC 64. A first end of the surge arrester device 62 is coupled to the first dc bus terminal 69A, and a second end of the surge arrester device 62 is coupled to the second dc bus terminal 69B.

As described above, the present inventors have found through studies that protection against a fault in the ac system of the flexible dc power transmission system can also be achieved because the surge arrester device 62 is coupled between the first dc bus terminal 69A and the second dc bus terminal 69B. Even under the serious three-phase grounding short circuit fault, the VSC can still be protected from locking.

Fig. 7 shows another example of a VSC-HVDC system according to an embodiment of the present disclosure. The VSC-HVDC system 70 may be a receiving end or a transmitting end. In case the VSC-HVDC system 70 is a receiving terminal, the VSC-HVDC system 70 comprises a transformer 76 and a VSC74 coupled thereto. The VSC74 converts the DC current to three phase AC current to the transformer 76. The transformer 76 transforms the received ac power for subsequent use.

The VSC-HVDC system 70 further comprises a lightning arrester arrangement 72. In one example, the surge arrester device 72 is located near the VSC 74. A first end of the surge arrester device 72 is coupled to the first dc bus terminal 79A, and a second end of the surge arrester device 72 is coupled to the second dc bus terminal 79B.

As described above, the present inventors have found through studies that protection against a fault in the ac system of the flexible dc power transmission system can also be achieved because the surge arrester device 72 is coupled between the first dc bus terminal 79A and the second dc bus terminal 79B. Even under the serious three-phase grounding short circuit fault, the VSC can still be protected from locking.

The VSC-HVDC system 70 further comprises a further arrester arrangement 78. In one example, another arrester device 78 is located near the VSC74 and/or the arrester device 72. The arrester device 78 may be the same as or different from the arrester device 72. One end of the surge arrester device 78 is coupled to the first dc bus terminal 79A, and the other end is coupled to the neutral point 76N of the valve-side star coil of the transformer 76.

As described above, the present inventors have found through studies that the upper arm of the VSC74 is not subjected to the overvoltage as described above by limiting the voltage of the upper arm by connecting the surge arrester device 78 in parallel with the upper arm. Therefore, the intra-station alternating current fault of the flexible direct current transmission system can be protected.

Fig. 8 shows a further example of a VSC-HVDC system according to an embodiment of the present disclosure. The VSC-HVDC system 80 may be a receiving end or a transmitting end. In case the VSC-HVDC system 80 is a receiving terminal, the VSC-HVDC system 80 comprises a transformer 86 and a VSC84 coupled thereto. VSC84 converts the DC current to three-phase AC current to transformer 86. The transformer 86 transforms the received ac power for subsequent use.

The VSC-HVDC system 80 further comprises a lightning arrester arrangement 82. In one example, the surge arrester device 82 is located near the VSC 84. A first end of the surge arrester device 82 is coupled to the first dc bus terminal 89A, and a second end of the surge arrester device 82 is coupled to the second dc bus terminal 89B.

Furthermore, the VSC-HVDC system 80 comprises a surge arrester arrangement 87 coupled between the first dc bus terminal 89A and ground for conventional protection of the converter.

As described above, the present inventors have found through studies that protection against a fault in the ac system of the flexible dc power transmission system can also be achieved because the surge arrester device 82 is coupled between the first dc bus terminal 89A and the second dc bus terminal 89B. Even under the serious three-phase grounding short circuit fault, the VSC can still be protected from locking.

The VSC-HVDC system 80 further comprises a further arrester arrangement 88. The further arrester device 88 may be the same as or different from the arrester device 82. In one example, the surge arrester device 82 is located near the VSC84 and/or the surge arrester device 82. One end of the surge arrester device 88 is coupled to the first dc bus terminal 89A, and the other end of the surge arrester device 88 is coupled to the neutral point 86N of the valve-side star coil of the transformer 86.

As described above, the present inventors have found through studies that the upper arm of the VSC84 is not subjected to the overvoltage as described above by limiting the voltage of the upper arm by connecting the surge arrester device 88 in parallel with the upper arm. Therefore, the intra-station alternating current fault of the flexible direct current transmission system can be protected.

Fig. 9 shows a flow diagram of a method 100 for providing a flexible direct current power transmission system according to an embodiment of the present disclosure. It is to be appreciated that various features described with respect to fig. 2-8 may be applied to the method 100 of fig. 9.

In 102, a voltage source converter is provided. The voltage source converter is coupled to a first dc bus terminal.

At 104, a lightning arrester device is provided. A first end of the surge arrester arrangement is coupled to a first dc bus terminal and a second end of the surge arrester arrangement, different from the first end, is coupled to a second dc bus terminal, different from the first dc bus terminal and/or to a neutral node of a valve-side star winding of a transformer coupled with the voltage source converter.

In one example, a surge arrester device includes: a first surge arrester unit having a first end coupled to the first dc bus terminal and a second end coupled to the neutral node; and a second arrester unit, a first end of the second arrester unit being coupled to the neutral node, and a second end of the second arrester unit being coupled to the second direct current bus terminal.

A flexible direct current power transmission system and a method of providing the same according to embodiments of the present disclosure are generally described above. Some exemplary embodiments according to the present disclosure are listed below.

Item 1 provides a flexible direct current transmission system comprising: a voltage source converter coupled to the first DC bus terminal; and an arrester arrangement, a first end of which is coupled to the first dc bus terminal, and a second end of which, different from the first end, is coupled to a second dc bus terminal, different from the first dc bus terminal, and/or to a neutral node of a valve-side star coil of a transformer coupled to the voltage source converter.

Item 2: the flexible direct current transmission system according to item 1, wherein the arrester device includes: a first surge arrester unit having a first end coupled to the first dc bus terminal and a second end coupled to the neutral node; and a second arrester unit, a first end of the second arrester unit being coupled to the neutral node and a second end of the second arrester unit being coupled to the second dc bus terminal.

Item 3: the flexible direct current transmission system according to item 1 or 2, wherein the first arrester unit and the second arrester unit are the same arrester unit.

Entry 4: the flexible direct current transmission system according to any one of clauses 1-3, wherein the power system comprises an asymmetric unipolar voltage source converter direct current system, and the second direct current bus terminal is coupled to ground, and the second end of the arrester device is coupled to the neutral node.

Item 5: the flexible direct current transmission system according to any one of items 1-4, wherein the power system comprises a symmetrical bipolar voltage source converter direct current system and the second direct current bus terminal is coupled to a first end of another voltage source converter and the second end of the arrester device is coupled to the neutral node; and the second terminal of the voltage source converter and the second terminal of the further voltage source converter are both coupled to ground.

Item 6: the flexible direct current transmission system according to any one of items 1 to 5, wherein the arrester device includes: a first surge arrester unit having a first end coupled to the first dc bus terminal and a second end coupled to the neutral node; and a second surge arrester unit, a first end of the second surge arrester unit being coupled to the first direct current bus terminal, and a second end of the second surge arrester unit being coupled to the second direct current bus terminal.

Item 7: the flexible direct current power transmission system according to any one of items 1 to 6, further comprising: and another arrester device coupled between the first dc bus terminal and ground.

Entry 8: the electric power system according to any one of items 1 to 7, wherein the arrester device is different from another arrester device.

Item 9: there is provided a method for providing a flexible direct current power transmission system, comprising: providing a voltage source converter coupled to the first dc bus terminal; and providing a surge arrester device, a first end of the surge arrester device being coupled to the first dc bus terminal, and a second end of the surge arrester device, different from the first end, being coupled to a second dc bus terminal, different from the first dc bus terminal, and/or to a neutral node of a star winding of a valve side of a transformer coupled with the voltage source converter.

Item 10: the method according to item 9, wherein the arrester device comprises: a first surge arrester unit having a first end coupled to the first dc bus terminal and a second end coupled to the neutral node; and a second arrester unit, a first end of the second arrester unit being coupled to the neutral node, and a second end of the second arrester unit being coupled to the second direct current bus terminal.

Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims

1. A flexible direct current transmission system (20), comprising:

a voltage source converter (24) coupled to a first DC bus terminal (29A); and

-an arrester arrangement (22) having a first end coupled to the first dc bus terminal (29A), and a second end of the arrester arrangement (22), different from the first end, coupled to a second dc bus terminal (29B), different from the first dc bus terminal, and/or to a neutral node (26N) of a valve-side star winding of a transformer coupled to the voltage source converter.

2. The flexible direct current transmission system according to claim 1, wherein the arrester arrangement comprises:

a first arrester unit (22A) having a first end coupled to the first DC bus terminal and a second end coupled to the neutral node; and

a second arrester unit (22B) having a first end coupled to the neutral node and a second end coupled to the second DC bus terminal.

3. The flexible direct current transmission system according to claim 2, wherein the first and second surge arrester units are the same surge arrester unit.

4. The flexible direct current transmission system according to claim 1, wherein the power system comprises an asymmetric unipolar voltage source converter direct current system and the second direct current bus terminal is coupled to ground and the second end of the arrester device is coupled to the neutral node.

5. The flexible direct current transmission system of claim 1, wherein the power system comprises a symmetrical bipolar voltage source converter direct current system and the second direct current bus terminal is coupled to a first end of another voltage source converter and the second end of the arrester device is coupled to the neutral node; and

the second terminal of the voltage source converter and the second terminal of the further voltage source converter are both coupled to ground.

6. The flexible direct current transmission system according to claim 1, wherein the arrester arrangement comprises:

a first arrester unit (78) having a first end coupled to the first DC bus terminal and a second end coupled to the neutral node; and

a second arrester unit (72) having a first end coupled to the first DC bus terminal and a second end coupled to the second DC bus terminal.

7. The flexible direct current transmission system according to claim 6, further comprising:

another surge arrester device (87) coupled between the first dc bus terminal and ground.

8. The power system of claim 7, wherein the arrester device and the another arrester device are different.

9. A method (100) for providing a flexible direct current power transmission system, comprising:

providing a voltage source converter (102) coupled to a first direct current bus terminal (29A); and

providing a surge arrester device (104), a first end of the surge arrester device being coupled to the first direct current bus terminal and a second end of the surge arrester device, different from the first end, being coupled to a second direct current bus terminal, different from the first direct current bus terminal and/or to a neutral node of a valve side star coil of a transformer coupled with the voltage source converter.

10. The method (100) according to claim 9, wherein the lightning arrester arrangement comprises:

a first arrester unit having a first end coupled to the first DC bus terminal and a second end coupled to the neutral node; and

a second arrester unit having a first end coupled to the neutral node and a second end coupled to the second DC bus terminal.