WO2014153729A1

WO2014153729A1 - Bipolar high/low voltage converter station for series mtdc system

Info

Publication number: WO2014153729A1
Application number: PCT/CN2013/073242
Authority: WO
Inventors: Xiaobo Yang; Chunming YUAN; Dawei YAO; Chengyan YUE
Original assignee: Abb Technology Ltd.
Priority date: 2013-03-27
Filing date: 2013-03-27
Publication date: 2014-10-02
Also published as: CN104471815B; CN104471815A

Abstract

A bipolar converter station and series MTDC system thereof are disclosed. The higher voltage converter station comprises: two pole areas, each pole area comprising a valve group; a DC smoothing reactor connected at the higher voltage side of said valve group; a bypass isolator, a bypass switch and two disconnectors; a line disconnector connected between lower DC voltage conductor and lower DC voltage line; a lower DC voltage conductors, one terminal of said lower DC voltage conductor is connected to the low voltage side of said pole area; the other terminal of said lower DC voltage conductor is connected to the lower voltage line via a line disconnector; a neutral bus switches connected between the neutral area and the lower DC voltage conductor at the lower voltage side of the pole area; and four DC voltage dividers, two of which are configured at the lower DC voltage lines respectively; another two of the said DC voltage dividers are configured at the higher DC voltage lines respectively.

Description

Bipolar High/Low Voltage Converter Station for Series MTDC

System

FIELD OF THE INVENTION

The invention relates to the field of MTDC (Muiti-terminal Direct Current HVDC) technology, and more particularly to a bipolar converter station and series MTDC system thereof.

BACKGROUND OF THE INVENTION

HVDC is usually a point to point transmission system. However, an MTDC system with more than two converter stations can be more attractive for grid companies for the following reasons:

1 ) MTDC is cost efficient to transmit bulk distributed or renewable powers which consisted of several scatted power plants and at the same time ensure AC system security at the receiving ends.

2) MTDC can realize large scale wind-hydro compensated system.

3) Tapping of HVDC to provide partial power supply for some areas along the line.

4) For some kind of MTDC (series MTDC), it can mitigate the challenge of high altitude HVDC line construction.

5) MTDC solution can decrease initial investment of the construction.

Today there are two kinds of MTDC technologies, i.e. line commutated converter based MTDC (LCC MTDC) and voltage source converter based MTDC (VSC MTDC).

Compared with VSC MTDC, the LCC MTDC is capable of achieving high power rating with low cost and low losses while it inherits limitations from LCC HVDC, such as high reactive power requirement.

LCC MTDC (Hereafter abbreviated as MTDC) can be a parallel or series connection system. A simplified series MTDC is shown in Fig.1. Fig.1 iilustrates a diagram of the simplified 4-terminal series MTDC with 2 rectifier converter and 2 inverter converter stations.

Generally speaking, series MTDC is suitable for tapping application. Besides, compared with parallel MTDC, series MTDC is more prompt for power flow reverse operation. For certain application, series MTDC has lower cost than parallel MTDC or two-terminal HVDC, in particular when the scattered terminals are not far away from each other. However, the series connected converter stations have different voltage levels which bring challenges for the application of series MTDC. Take example of Fig.1 , the converter station Rectifier 2 has higher voltage insulation level than that of converter station Rectifier 1. The Insulation coordination at the converter stations; and deployment of DC voltage measurement points need to be considered during the design of series MTDC. Besides, due to the different physical locations of terminals, the block and de-block of series MTDC and operation mode transition subsequently would be much more complex than that of two-terminal HVDC. Therefore the bipolar configuration of series MTDC converter station is very important to realize the required operation modes and the transition between them.

Existing solutions for bipolar configuration, for example prior art CN102082432A "Cascading converter station and cascading multi-terminal high-voltage direct-current (MTHVDC) transmission system", proposed a bipolar configuration which is based on 2-terminal HVDC converter station. In the prior art, the configuration for lower voltage converter station has no difference from an existing bipolar configuration of a converter station in the 2-terminal HVDC; the configuration for higher voltage converter station includes DC smoothing reactors arranged at both sides of said converter valve, parallel connected NBS between middle voltage line and neutral bus, and additional isolators to bypass the DC smoothing reactors and DC filters. The bipolar configuration for the high voltage converter station in the prior art increase the operation complexity. in the present invention, a new bipolar configuration method for series MTDC is proposed, which can be used for either rectifier stations or inverter stations. With the proposed configuration and coordination operation between lower voltage converter station and higher voltage converter station, various operation modes and their transitions can be realized.

SUMMARY OF THE INVENTION To solve the above mentioned problems, one object of the present invention is to propose a new bipolar configuration for series MTDC, which can be either rectifier stations or inverter stations. With the proposed configuration and coordination operation method between lower converter station and higher voltage converter station, various operation modes and their transitions can be easily realized.

According to an aspect of the present invention, it provides a bipolar higher voltage converter station for series MTDC. The higher voltage converter station comprises: two pole areas, each pole area comprising a valve group; a DC smoothing reactor connected at the higher voltage side of said valve group; a bypass isolator, a bypass switch and two disconnectors; a line disconnector connected between lower DC voltage conductor and lower DC voltage line; a lower DC voltage conductors, one terminal of said lower DC voltage conductor is connected to the low voltage side of said pole area; the other terminal of said lower DC voltage conductor is connected to the lower voltage line via a line disconnector; a neutral bus switches connected between the neutral area and the lower DC voltage conductor at the lower voltage side of the pole area; and four DC voltage dividers, two of which are configured at the lower DC voltage lines respectively; another two of the said DC voltage dividers are configured at the higher DC voltage lines respectively. According to a preferred embodiment of the present invention, the higher voltage converter station further comprises: a metallic return transit breaker and a neutral bus grounding switch in said neutral area.

According to a preferred embodiment of the present invention, said higher said neutral bus switches are configured to implement mono-polar metallic return operation mode of higher voltage converter station.

According to a preferred embodiment of the present invention, said higher voltage converter station further comprises two DC filters, each of which is connected between one terminal of DC smoothing reactor and the low voltage side of valve group within the respective pole area.

According to a preferred embodiment of the present invention, said bypass isolator, bypass switch and two isolators within respective poie area are configured for bypass or reconnecting operation of the DC filter, DC smoothing reactor and valve group within the corresponding pole area.

According to a preferred embodiment of the present invention, said bipolar higher voltage converter station further comprises two line disconnectors, each of which is configured between iower DC voltage conductor and lower DC voltage line.

According to a preferred embodiment of the present invention, said bipolar higher voitage converter station further comprises two Iower voltage surge arrestors connected to the lower DC voltage lines and two higher voltage surge arrestors connected to the higher DC voitage lines respectively.

According to another aspect of the present invention, a bipolar low voltage converter station for series MTDC is provided. The bipolar low voltage converter station comprises: two pole areas, each pole area comprising a valve group, a DC smoothing reactor connected at the higher voitage side of said valve group, a bypass switch, a bypass isolator and two disconnectors; two neutral bus switches, one of which is connected between the low voltage side of the pole area and the neutral area, the other one is connected between the low voltage side of the second pole area and said neutral area; and two DC voltage dividers, which are configured at Iower DC voitage lines respectively.

According to a preferred embodiment of the present invention, each pole area further comprises an additional DC smoothing reactor installed at the Iower voltage side of said pole area respectively.

According to a preferred embodiment of the present invention, said bipolar low voltage converter station further comprises a metallic return transit breaker, a neutral bus grounding switch for said neutral area.

According to a preferred embodiment of the present invention, said neutral bus switches are configured to implement monopolar metallic return operation of said bipolar low voltage converter station.

According to a preferred embodiment of the present invention, said bipolar low voltage converter station further comprises two DC filters, each of which is connected between one terminal of said DC smoothing reactor and the low voltage side of said valve group within respective pole area.

According to a preferred embodiment of the present invention, said bipolar low voltage converter station further comprises two DC filters, each of which is connected between one terminal of DC smoothing reactor at the higher voltage side of said va!ve group and one terminal of another DC smoothing reactor at lower voltage side of the valve group within respective pole area.

According to a preferred embodiment of the present invention, said bypass isolator, bypass switch and two disconnectors within respective pole area can be used for bypass or reconnecting operation of the DC filter, DC smoothing reactor and valve group within respective pole area.

According to a preferred embodiment of the present invention, said bipolar low voltage converter station further comprises two lower voltage surge arrestors connected at the lower DC voltage lines respectively.

According to another aspect of the present invention, a bipolar configuration of higher voltage converter station is provided. The bipolar configuration of higher voltage converter station comprises additional disconnectors configured in the higher voltage converter station mentioned above and the bipolar area can be reconnected to the higher voltage side.

Embodiments of the present invention provide a bipolar high or low voltage converter station and series MTDC system thereof and achieve reliable and flexible operations and cost reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more details in the following description with reference to preferred exemplary embodiments which are illustrated in the drawings, in which:

Fig.1 illustrates an example of a 4-terminal series MTDC with 2 rectifier converter stations and 2 inverter converter stations; Fig. 2 illustrates a bipolar configuration of higher voltage converter station 1 in a series MTDC system;

Fig.3a, Fig.3b, Fig.3c and Fig.3d iliustrate the operation mode transition process between normal bipolar operation mode and higher voltage converter station local grounding return mode;

Fig.4a, Fig.4b, Fig.4c and Fig.4d illustrate the operation mode transition of higher voltage converter station from bipolar local grounding mode to monopolar local metallic return mode, and bipolar local grounding mode to monopolar local grounding return mode; Fig. 5 illustrates the DC voltage measurement points at higher voltage converter station;

Fig. 6 illustrates the lower voltage converter station configuration in a series MTDC system;

Fig. 7 illustrates the lower voltage converter station configuration with two DC smoothing reactor s at each pole;

Fig. 8 illustrates the higher voltage converter station configuration with additional disconnectors to realize special operation modes; and

Fig. 9 illustrates a simplified 4-terminal bipolar MTDC configuration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are described in conjunction with the accompanying drawings hereinafter. For the sake of clarity and conciseness, not all the features of actual implementations are described in the specification. Fig. 2 illustrates a bipolar configuration of higher voltage converter station 1 in a series MTDC system. For normal full voltage bipolar operation, the said high voltage bipolar converter station connects to DC lines 22 (the positive lower voltage pole line) and 22' (the negative lower voltage pole line), DC lines 23 (the positive higher voltage pole line) and 23' (the negative higher voltage pole line), and local AC grid 2.

In the said higher voltage converter station, DC smoothing reactor 106 and 106' are series connected at higher voltage side of valve group 107 and 107' respectively. DC filter 05 and 105' are connected between one terminal of said DC smoothing reactor and lower voltage side of said valve group respectively.

Bypass circuit breaker (BPS) 102 and 102^!, bypass isolation switch (BPI) 101 and 101', disconnector 103 and 103', and disconnector 104 and 104' are used for bypass operation of said DC smoothing reactor s, DC filters and valve groups. Lower voltage surge arrestor (SA) 13 and 13' are shunt connected at the lower DC voltage line 22 and 22' respectively; higher voltage SA 4 and 14' are shunt connected at the higher DC voltage line 23 and 23' respectively.

Switch 115 and 115' are line disconnectors to connect/disconnect the lower voltage pole lines 22 and 22' to/from the lower voltage conductor 24 and 24' of higher voltage converter stations.

Neutral bus switch (NBS) 12 is series connected between lower voltage conductor 24 of first pole area 10 and bipolar area 11 ; NBS 12' is series connected between lower voltage conductor 24' of second pole area 10' and bipolar area 11. Switch 111 are metallic return transit breaker (MRTB), switch 113 are neutral bus grounding switch (NBGS). Switch 116 and 116', 117 and 117' are auxiliary disconnectors to realize operation mode transition.

AC filter 15 is connected to the AC bus of the local grid. 108 an 108' are converter transformers of first pole area and second pole area separately. Fig. 3a, Fig. 3b, Fig. 3c and Fig. 3d illustrate the operation mode transition from remote grounding mode to local grounding mode sequentially, after the lower voltage converter station is bypassed.

Fig. 3a is the normal bipolar operation mode of MTDC system, the system is grounded at earth electrode 412 of lower voltage converter station. Fig. 3b illustrates the bypass status of lower voltage converter station via BPI 401 and 401' respectively. The higher converter station operates at bipolar remote grounding return mode.

Fig. 3c illustrates the transient mode that the system is grounded by both remote earth electrode 412 at lower voltage converter station and local earth electrode 112 at higher voltage converter station, after the NBS 12 and 12', disconnector 117 and 11 , and MRTB 111 are closed.

Fig. 3d illustrates the bipolar local grounding mode of higher voltage converter station, after the opening of NBS 42 and 42' at iower voltage converter station and line disconnector 115 and 115' at higher voltage converter station.

The whole transition process described above from Fig.3a to Fig.3d can be reversed to realize mode transition from bipolar local grounding return mode to normal bipolar operation mode: Firstly, after energizing of Iower voltage converter station and closing NBS 42, 42' and MRTB 412, the local grounding mode shown in Fig.3d is transmitted to Fig. 3c; secondly, NBS 12 and 12' of higher voltage converter station are opened, which blocks the grounding current path at higher voltage converter station and the operation is transmitted to remote grounding mode, as shown in Fig. 3b; finally, Iower voltage converter station is de-blocked and consequently the normal bipolar operation is re-established, as shown in Fig. 3a.

Fig. 4a, Fig. 4b, Fig. 4c illustrate the operation mode transition of higher voltage converter station from bipolar local grounding mode to monopolar local metallic return mode, and bipolar local grounding mode to monopolar local grounding return mode, with example of bypass of first pole 10. It should be noted that the same method can be applied for case of bypass of second pole 10'.

Fig. 4a illustrates the bipolar local grounding mode of higher voltage converter station.

Fig. 4b illustrates the bypass status of Pole 10 of higher voltage converter station via BPI 101. The grounding current of higher voltage converter station has two circuit paths: one is pole line 23 and the other is grounding path via earth electrode 112.

Fig. 4c illustrates the monopolar local grounding return mode of higher voltage converter station, which can be transited from the operation mode shown in Fig. 4b, by opening NBS 12. Fig. 4d illustrates the monopolar local metallic return mode of higher voltage converter station, which can be transited from the mode shown in Fig. 4b, by opening MRTB 111.

Monopolar local metallic return mode of higher voltage converter station as shown in Fig.4d can be transmitted to monopolar local grounding return mode as shown in Fig.4c, by closing MRTB 111 , and subsequently opening NBS12; monopolar local grounding return mode of higher voltage converter station as shown in Fig.4c can be transmitted to monopolar local metallic return mode as shown in Fig.4d by closing NBS 12 and BPI 101 , and subsequently opening MRTB 111. Fig. 5 illustrates DC voltage measurement points of higher voltage converter station. The DC voltages of line to ground are measured at both higher voltage pole lines and lower voltage pole lines. DC divider 16 and 16' are used for DC voltage measurement of lower voltage pole line 22 and 22' respectively; DC divider 17 and 17' are used for DC voltage measurement of higher voltage pole line 23 and 23' respectively.

Fig.6 illustrates a bipolar configuration of lower voltage converter station in a series MTDC system. For normal bipolar operation, the said high voltage bipolar converter station connects to DC lines 22 (the positive lower voltage pole line) and 22' (the negative lower voltage pole line), and local AC grid 5. In the said lower voltage converter station, DC smoothing reactor 406 and

406' are series connected at higher voltage side of valve group 407 and 407' respectively; DC filter 405 and 405' are connected between one terminal of said DC smoothing reactor and lower voltage side of said valve group respectively.

Bypass circuit breaker (BPS) 402 and 402', bypass isolation switch (BPI) 401 and 401 ', disconnector 403 and 403', and disconnector 404 and 404' are used for bypass operation of said DC smoothing reactors, DC filters and valve groups.

Lower voltage surge arrestor (SA) 43 and 43' are shunt connected at the lower DC voltage line 22 and 22' respectively.

Neutral bus switch (NBS) 42 is series connected between lower voltage side of disconnector 401 of first pole 40 and bipolar area 41 ; NBS 42' is series connected between lower voltage side of disconnector 401 ' of second pole 40' and bipolar area 41 '.

Switch 411 is metallic return transit breaker (MRTB), switch 413 is neutral bus grounding switch (NBGS). Switch 416 and 416', 417 and 417' are auxiliary disconnectors to realize operation mode transition.

AC filter 45 is connected to the AC bus of the local grid. 408 an 408' are converter transformers of first pole area 40 and second pole area 40' separately.

Fig. 7 illustrates the lower voltage converter station configuration with two DC smoothing reactors at each pole. The configuration is simulator to that shown in Fig. 6, except that additional DC smoothing reactor 409 and 409' are series connected at lower voltage side of valve group 407 and 407' respectively, and the DC filter 405 and 405' are connected between terminal of said DC smoothing reactor 406 and 409, and 406' and 409' respectively. Fig. 8 illustrates the higher voltage converter station configuration with additional disconnectors to realize special operation modes. With additional disconnectors (119,119'), (120, 120') and (121 ,121 '), the bipolar area 11 can be reconnected to higher voltage side of each converters, to realize special operation modes such as split mode, in which the series TDC system is splitted into two 2-terminal HVDC systems, or ¼ cross modes, in which the higher voltage line can be reconnected to the earth electrode of a higher voltage converter station.

Fig.9 illustrates a simplified 4-termian! bipolar MTDC single line diagram. Each block represents a converter station of one pole. The (1_,1-) and (2+, 2-) are respective lower and higher voltage rectifier converter stations; the (3+, 3-) and (4+, 4-) are respective higher and lower inverter converter stations. Table 1 , Table 2, Table 3 and Table 4 list the operation mode realized by the invention, based on the variations of Fig.9. In these drawing in Table 1 , Table 2, Table 3 and Table 4, the dark block means the station is on operation, and the non-filled block means the station out of operation or bypassed. The dark lines are energized lines; the dash lines are de-energized lines. The symbol "=>" in the table denotes the current flow direction, the number 1 to 4 represent the 4 converter stations respectively; superscript "+" or "-" of the number represents positive or negative pole of each converter station. For example, "1⁺=>2⁺=>3⁺=>4⁺" represents that the current flows the respective positive pole of converter 1 , 2, 3 and 4 subsequently. The annotations of the converter stations in the drawing are removed for simplicity reason.

Table 1 lists the basic operation modes that can be realized by the bipolar configuration method proposed in the invention.,

In Table 1 , the full voltage operation is the normal mode, in which the full voltage bipolar operation and the full voltage monopolar operation are listed.

Half voltage operation with various modes should be allowed during equipment failure, permanent line fault, maintenance and so forth. Half voltage operation modes can be categorized as ½ bipolar mode, ½ cross mode, ¾ cross mode and ¼ monopolar mode for the 4 terminal series MTDC system. Furthermore, for each operation mode, three types of wirings are identified according to the return method of high voltage converters, as shown below:

V Local earth return method

Transmission line return method (metallic return)

Mixed return method

Operation modes of 4-terminal series MTDC

2^"<=4' Symmetric configuration of 2⁺=>4⁺

Table 2 lists some special operation modes that can be realized by the bipolar configuration method proposed in the invention. Each operation mode shown in Table 2 has an earth electrode which is connected to only one pole. Besides full voltage monopolar mode and ½ bipolar mode, another two new operation modes are identified, i.e. ½ cross mode and ¾ cross mode. Table 2

Γ =3^' Symmetric configuration of l⁺=>3^÷ Γ<=4^" Symmetric configuration of 1 ⁺=>4⁺

2^"<=3^' Symmetric configuration of 2⁺=>3⁺ r<=4^" Symmetric configuration of 2⁺=>4^÷

Table 3 lists another special operation mode, named as ¼ cross mode. In ¼ cross mode, the voltage polarity of a higher voltage converter station in one pole will be changed. For example, in the case the 1⁺=>3^" , the DC voltages at both sides of higher converter station 3^" is changed from (-400kV, -800kV) to (400kV, OV). The mode transition can be realized by the special bipolar configuration method proposed in the invention.

In principle, it is also possible to realize ¼ cross mode with voltage polarity changing at a lower voltage station, such as 1⁺=>4^" case and 2⁺=>4^~ case, while the insolation level of the low voltage side of the lower converter station will be increased significantly and additional disconnectors are needed.

Table 3 ¼ cross mode

Tabie 4 lists another special operation mode, the split mode, in which the series MTDC system is separated into two 2-terminai HVDC systems. The split mode can be realized by the special bipolar configuration method proposed in the invention.

Table 4 Split Mode

Compared with the existing prior arts, the proposed solution of the present invention is much more practical and easier for implementation on the series MTDC system. Referring to the description of the exemplary embodiments, those skilled in the art appreciate the advantages of the present invention that, only one DC smoothing reactor at HV converter stations need to be optimized, and various operation modes and their transitions can be easily realized according to a bipolar converter station and series MTDC system thereof proposed in the present invention.

Though the present invention has been described on the basis of some preferred embodiments, those skilled in the art should appreciate that those embodiments should by no means limit the scope of the present invention. Without departing from the spirit and concept of the present invention, any variations and modifications to the embodiments should be within the apprehension of those with ordinary knowledge and skills in the art, and therefore fall in the scope of the present invention which is defined by the accompanied claims.

Claims

1. A bipolar higher voltage converter station for series MTDC, wherein said higher voltage converter station comprises:

two pole areas (10, 10'), each pole area comprising a valve group (107, 107'); a DC smoothing reactor (106, 106') connected at the higher voltage side of said valve group (107, 107'); a bypass isolator (101 , 101'), a bypass switch ( 02, 02') and two disconnectors (103 and 104; 103' and 104'); a line disconnector (115, 115') connected between lower DC voltage conductor (24, 24') and lower DC voltage line (22, 22'); a lower DC voltage conductors (24, 24'), one terminal of said lower DC voltage conductor is connected to the low voltage side of said pole area (10, 10'); the other terminal of said lower DC voltage conductor is connected to the lower voltage line (22, 22') via a line disconnector (115, 115'); a neutral bus switches (12, 12') connected between the neutral area (11) and the lower DC voltage conductor (24, 24') at the lower voltage side of the pole area (10, 10'); and

four DC voltage dividers (16, 16' and 17, 17'), two of which (16, 16') are configured at the lower DC voltage lines (22, 22') respectively; another two of the said DC voltage dividers (17, 17') are configured at the higher DC voltage lines (23, 23') respectively.

2. The bipolar higher voltage converter station according to Claim 1 , said higher voltage converter station further comprises: a metallic return transit breaker (111 ) and a neutrai bus grounding switch (113) in said bipolar area (11 ).

3. The bipolar higher voltage converter station according to Claim 1 or 2, said higher said neutral bus switches (12, 12') are configured to implement metallic return operation mode of higher voltage converter station or mode transition between monopolar metallic return operation mode and grounding return mode.

4. The bipolar higher voltage converter station according to Claim 1 or 2, said higher voltage converter station further comprises two DC filters (105, 105'), each of which is connected between one terminal of DC smoothing reactor (106, 106') and the lower voltage side of valve group ( 07, 107') within the respective pole area.

5. The bipolar higher voltage converter station according to Claim 1 or 2, said bypass isolator (101 , 101'), bypass switch (102, 102') and two disconnectors (103 and 104; 103' and 104') within respective pole area are configured for bypass or reconnecting operation of the DC filter, DC smoothing reactor and valve group within the corresponding pole area.

6. The bipolar higher voltage converter station according to Claim 1 or 2, said bipolar higher voltage converter station further comprises two line disconnectors (115, 115'), each of which is configured between lower DC voltage conductor (24, 24') and lower DC voltage fine (22, 22').

7. The bipolar higher voltage converter station according to Claim 1 or 2, said bipolar higher voltage converter station further comprises two lower voltage surge arrestors (13, 13') connected to the lower DC voltage lines and two higher voltage surge arrestors (14, 14') connected to the higher DC voltage lines (23, 23') respectively.

8. A bipolar lower voltage converter station for series MTDC, wherein said lower voltage converter station comprises:

two pole areas (40, 40'), each pole area comprising a valve group (407, 407'), a DC smoothing reactor (406, 406') connected at the higher voltage side of said valve group, a bypass switch (402, 402'), a bypass isolator (401 , 401') and two disconnectors (403 and 404, 403' and 404'), a neutral bus switches (42, 42') connected between the lower voltage side of the pole area (40, 40') and the bipolar area (41); and

two DC voltage dividers (47, 47'), which are configured at lower DC voltage lines (22, 22') respectively.

9. The bipolar lower voltage converter station according to Claim 8, each pole area further comprises an additional DC smoothing reactor (409, 409') installed at the lower voltage side of said pole area respectively.

10. The bipolar lower voltage converter station according to Claim 8 or 9, said bipolar lower voltage converter station further comprises a metallic return transit breaker (411), a neutral bus grounding switch (413) for said neutral area (4 ).

11. The bipolar lower voltage converter station according to Claim 8 or 9, said neutral bus switches (42, 42') are configured to implement metallic return operation of said bipolar lower voltage converter station or transition between monopolar metallic return operation mode and grounding return mode.

12. The bipolar lower voltage converter station according to Claim 8, said bipolar lower voltage converter station further comprises two DC filters (405, 405'), each of which is connected between one terminal of said DC smoothing reactor (406, 406') and the low voltage side of said valve group within respective pole area.

13. The bipolar lower voltage converter station according to Claim 9, said bipolar lower voltage converter station further comprises two DC filters (405, 405'), each of which is connected between one terminal of DC smoothing reactor (406, 406') at the higher voltage side of said valve group and one terminal of another DC smoothing reactor (409, 409') at lower voltage side of the valve group within respective pole area.

14. The bipolar lower voltage converter station according to Claim 8 or 9, said bypass isolator (101 , 101 '), bypass switch (102, 102') and two disconnectors (103 and 104; 103' and 104') within respective pole area can be used for bypass or reconnecting operation of the DC filter, DC smoothing reactor and valve group within respective pole area.

15. The bipolar lower voltage converter station according to Claim 8 or 9, said bipolar lower voltage converter station further comprises two lower voltage surge arrestor (43, 43') connected at the lower DC voltage lines (22, 22') respectively.

16. A bipolar configuration of higher voltage converter station, wherein additional disconnectors (119,119', 120, 120' and 121 ,121') are configured in said higher voltage converter station according to any one of Claims 1-7, and the bipolar area (11 ) can be reconnected to the higher voltage side.