Abstract:
A cascade converter station and a multi-end cascade high-voltage direct current (HVDC) power transmission system. The converter station includes a low-voltage end converter station ( 11 ) and a high-voltage end converter station ( 12 ). The high-voltage end converter station ( 12 ) is connected in series with the low-voltage end converter station ( 11 ) through a medium-voltage direct current (DC) power transmission line ( 13 ) and connected to a HVDC power transmission line ( 14 ). With the cascade converter station and the multi-end cascade HVDC power transmission system, HVDC power transmission can be achieved in a flexible, reliable and economical manner.

Description:
TECHNICAL FIELD 
     This invention relates to the field of DC power transmission, and more particularly to a cascaded converter station used in cascaded multi-terminal HVDC power transmission and a cascaded multi-terminal HVDC power transmission system constructed by such cascaded converter stations. 
     BACKGROUND 
     With the development of the power and electrical techniques, especially, the development of high-power silicon controlled rectifier (SCR) manufacturing, DC power transmission has gain wider and wider applications in electric power systems. A cascaded multi-terminal HVDC power transmission system is composed of three or above converter stations and a DC power transmission line, wherein more than one converter stations operate as a rectifier station or an inverter station. As compared with a two-terminal HVDC power transmission system, in the following situations, for example, a cascaded multi-terminal HVDC power transmission system may operate in a more economical and flexible manner: collecting electric power from multiple electric power bases (for example, wind farms) located in a large area for outward transmission; transmitting a large amount of electricity from an energy base to several remote load centers; providing access to power supplies or loads on mid-branches of a DC line; realizing asynchronous networking of several independent AC systems through a DC line; for the power transmission of metropolis areas or industrial centers, transmitting power energy to several converter stations through DC power transmission, where cables must be used due to limits on overhead power line corridors, or AC power transmission is unsuitable due to limits on short-circuit capacity. 
     In a cascaded multi-terminal HVDC power transmission system, it is inevitable for high voltage devices, such as converters, smoothing reactors, DC filters, etc. suffering from the impacts of high voltage, large current, the natural environment and connected AC systems to have failures. In the case of faulty part of the system (such as, a converter on a certain stage), it is desired to cut such part off from the system reliably while keeping other parts of the system operating normally, so as to ensure the safety of the HVDC power transmission system and improve its energy availability. 
     SUMMARY 
     This invention is directed to overcome the above problem, and to provide a technique for realizing HVDC power transmission in a flexible, reliable and economical manner. 
     In order to achieve the above object, according to a first aspect of this invention, a cascaded converter station used in cascaded multi-terminal HVDC power transmission is provided, comprising: a low-voltage end converter station having a positive side and a negative side, each of which comprising a converter transformer coupled to a first alternating current (AC) network; a converter valve coupled to the converter transformer for realizing DC/AC conversion; and smoothing reactors provided on both ends of the converter valve; and a high-voltage end converter station, which is connected in series to the low-voltage end converter station through a middle-voltage DC power transmission line, and is connected to a high-voltage power transmission line, wherein the high-voltage end converter station comprises a positive side and a negative side, each of which comprising a converter transformer coupled to a second alternating current (AC) network; a converter valve coupled to the converter transformer for realizing DC/AC conversion; and smoothing reactors provided on both ends of the converter valve; wherein a grounding line coupled to a grounding electrode and a metal return line coupled between a positive line and a negative line are provided in the low-voltage end converter station. 
     According to a second aspect of this invention, a cascaded converter station used in cascaded multi-terminal HVDC power transmission is provided, comprising: a low-voltage end converter station comprising a positive side and a negative side, each of which comprising a converter transformer coupled to a first alternating current (AC) network; a converter valve coupled to the converter transformer for realizing DC/AC conversion; and smoothing reactors provided on both ends of the converter valve; and a high-voltage end converter station, which is connected in series to the low-voltage end converter station through a middle-voltage DC power transmission line, and is connected to a high-voltage DC power transmission line, wherein the high-voltage end converter station comprises a positive side and a negative side, each of which comprising a converter transformer coupled to a second alternating current (AC) network; a converter valve coupled to the converter transformer for realizing DC/AC conversion; and smoothing reactors provided on both ends of the converter valve; wherein a grounding line coupled to a grounding electrode and a metal return line coupled between a positive line and a negative line are provided in the low-voltage end converter station, and a grounding line coupled to the grounding electrode is provided in the high-voltage end converter station. 
     According to a third aspect of this invention, a cascaded converter station used in cascaded multi-terminal HVDC power transmission is provided, comprising: a low-voltage end converter station comprising a positive side and a negative side, each of which comprising a converter transformer coupled to a first alternating current (AC) network; a converter valve coupled to the converter transformer for realizing DC/AC conversion; and smoothing reactors provided on both ends of the converter valve; and a high-voltage end converter station, which is connected in series to the low-voltage end converter station through a middle-voltage DC power transmission line, and is connected to a high-voltage DC power transmission line, wherein the high-voltage end converter station comprises a positive side and a negative side, each of which comprising a converter transformer coupled to a second alternating current (AC) network; a converter valve coupled to the converter transformer for realizing DC/AC conversion; and smoothing reactors provided on both ends of the converter valve; wherein a grounding line coupled to the grounding electrode and a metal return line coupled between a positive line and a negative line are provided in the low-voltage end converter station; a grounding line coupled to the grounding electrode and a neutral bus switch are provided in the high-voltage end converter station. 
     According to a fourth aspect of this invention, a cascaded converter station used in cascaded multi-terminal HVDC power transmission is provided, comprising: a low-voltage end converter station comprising a positive side and a negative side, each of which comprising a converter transformer coupled to a first alternating current (AC) network; a converter valve coupled to the converter transformer for realizing DC/AC conversion; and smoothing reactors provided on both ends of the converter valve; and a high-voltage end converter station, which is connected in series to the low-voltage end converter station through a middle-voltage DC power transmission line, and is connected to a high-voltage DC power transmission line, wherein the high-voltage end converter station comprises a positive side and a negative side, each of which comprising a converter transformer coupled to a second alternating current (AC) network; a converter valve coupled to the converter transformer for realizing DC/AC conversion; and smoothing reactors provided on both ends of the converter valve; wherein a grounding line coupled to the grounding electrode and a metal return line coupled between a positive line and a negative line are provided in the low-voltage end converter station; a grounding line coupled to the grounding electrode, a neutral bus switch, and a neutral bus isolation knife switch are provided in the high-voltage end converter station, and a path for bypassing the high-voltage end cascaded converter station is coupled between the middle voltage DC power transmission line and the high voltage DC power transmission line. 
     According to a fifth aspect of this invention, a cascaded multi-terminal HVDC power transmission system is provided, comprising: a sending side converter station, a receiving side converter station, and a high-voltage DC power transmission line therebetween, wherein at least one of the sending side converter station and the receiving side converter station is constructed according to the cascaded converter station of the first to fourth aspects above. 
     With the cascaded converter station of this invention and the cascaded multi-terminal HVDC power transmission system formed by such cascaded converter stations, because various flexible combinations of a grounding line, a metal return line, a neutral bus device, and an isolation knife switch are provided in the wiring of the cascaded converter station, other parts of the system may continue operation if a failure occurs on a certain part of the system, so that safety of the HVDC power transmission system and its energy availability can be improved. In addition, because smoothing reactors are provided on both sides of the converter valve, the effect of lighting protection can be effectively achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In order to understand the above features and advantages of this invention more clearly, preferred embodiments of this invention are shown in accompanying drawings in an unrestrictive manner, wherein the same or similar reference labels denote the same or similar components. 
         FIG. 1  is a schematic diagram of the structure and wiring of a cascaded converter station used in cascaded multi-terminal HVDC power transmission according to a first embodiment of this invention; 
         FIG. 2  shows the full bipolar wiring of the cascaded converter station in its normal operation state according to the first embodiment of this invention; 
         FIG. 3A-FIG .  3 C show the ¾ bipolar wiring of the cascaded converter station of the first embodiment of this invention; 
         FIG. 4A-FIG .  4 B show the ½ bipolar wiring of the cascaded converter station of the first embodiment of this invention; 
         FIG. 5  shows the full monopole grounding return wiring of the cascaded converter station of the first embodiment of this invention; 
         FIG. 6A-6B  shows the ½ monopole grounding return wiring of the cascaded converter station of the first embodiment of this invention; 
         FIG. 7  shows the full monopole metal return wiring of the cascaded converter station of the first embodiment of this invention; 
         FIG. 8A  and  FIG. 8B  show the ½ monopole metal return wiring of the cascaded converter station of the first embodiment of this invention; 
         FIG. 9  is a schematic diagram of the structure and wiring of a cascaded converter station used in cascaded multi-terminal HVDC power transmission according to a second embodiment of this invention; 
         FIG. 10  shows the full bipolar wiring of the cascaded converter station in its normal operation state according to the second embodiment of this invention; 
         FIG. 11A-FIG .  11 C show the ¾ bipolar wiring of the cascaded converter station of the second embodiment of this invention; 
         FIG. 12A-FIG .  12 B show the ½ bipolar wiring of the cascaded converter station of the second embodiment of this invention; 
         FIG. 13  shows the full monopole grounding return wiring of the cascaded converter station of the second embodiment of this invention; 
         FIG. 14A-14C  show the ½ monopole grounding return wiring of the cascaded converter station of the second embodiment of this invention; 
         FIG. 15  shows the full monopole metal return wiring of the cascaded converter station of the second embodiment of this invention; 
         FIG. 16A-FIG .  16 C show the ½ monopole metal return wiring of the cascaded converter station of the second embodiment of this invention; 
         FIG. 17  shows a first expanded wiring scheme of the cascaded converter station of the second embodiment of this invention; 
         FIG. 18  shows the monopole metal return wiring of the high-voltage end converter station in the first expanded wiring scheme of the second embodiment of this invention; 
         FIG. 19  shows a second expanded wiring scheme of the cascaded converter station of the second embodiment of this invention; 
         FIG. 20  shows the monopole metal return wiring of the low-voltage end converter station in the second expanded wiring scheme of the second embodiment of this invention; 
         FIG. 21  is a schematic diagram of the structure and wiring of a cascaded converter station used in cascaded multi-terminal HVDC power transmission according to a third embodiment of this invention; 
         FIG. 22  shows the ¾ bipolar wiring of the cascaded converter station of the third embodiment of this invention; 
         FIG. 23  shows the bipolar wiring of the high-voltage end converter station in the cascaded converter station of the third embodiment of this invention; 
         FIG. 24  shows an expanded wiring scheme of the cascaded converter station of the third embodiment of this invention; 
         FIG. 25  is a schematic diagram of the structure and wiring of a cascaded converter station used in cascaded multi-terminal HVDC power transmission according to a fourth embodiment of this invention; 
         FIG. 26  shows the full bipolar wiring of the cascaded converter station in its normal operation state according to the fourth embodiment of this invention; 
         FIG. 27A-FIG .  27 B show the ¾ bipolar wiring of the cascaded converter station of the fourth embodiment of this invention; 
         FIG. 28A-FIG .  28 B show the ½ bipolar wiring of the cascaded converter station of the fourth embodiment of this invention; 
         FIG. 29  shows the full monopole grounding return wiring of the cascaded converter station of the fourth embodiment of this invention; 
         FIG. 30A  and  FIG. 30B  show the ½ monopole grounding return wiring of the cascaded converter station of the fourth embodiment of this invention; 
         FIG. 31  shows the full monopole metal return wiring of the cascaded converter station of the fourth embodiment of this invention; 
         FIG. 32A  and  FIG. 32B  show the ½ monopole metal return wiring of the cascaded converter station of the fourth embodiment of this invention; 
         FIG. 33  shows a first expanded wiring scheme of the cascaded converter station of the fourth embodiment of this invention; 
         FIG. 34  shows a second expanded wiring scheme of the cascaded converter station of the fourth embodiment of this invention; 
         FIG. 35  shows an optional DC filter configuration; 
         FIG. 36  shows another optional DC filter configuration; 
         FIG. 37  shows still another optional DC filter configuration; 
         FIG. 38  shows a cascaded multi-terminal HVDC power transmission system according to this invention. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of preferred embodiments of this invention will be given below with reference to the drawings, which are merely illustrative but not limitations on the scope of this invention. 
       FIG. 1  is a schematic diagram of the structure and wiring of a cascaded converter station used in cascaded multi-terminal HVDC power transmission according to a first embodiment of this invention. For the purpose of simplifying this description,  FIG. 1  only shows the sending side of the HVDC power transmission system, that is, a schematic diagram of the rectification side. However, those skilled in the art may understand that the receiving side of the HVDC power transmission system, that is, the inverter side may have substantially the same structure and wiring as that of the sending side, but a converter station on the inverter side in an inverting working condition, and there is a slightly different between the filter configuration with the rectification side. 
     As shown in  FIG. 1 , the cascaded converter station according to the first embodiment comprises a low-voltage end converter station  11  and a high-voltage end converter station  12 , which may be located at different geographic positions. The high-voltage end converter station  12  is connected to the low-voltage end converter station  11  in series through the middle-voltage DC power transmission line  13 . The high-voltage end converter station  12  is further connected to a high-voltage DC power transmission line  14 . 
     The low-voltage end converter station  11  is used to convert an alternating current generated by a first alternating current power supply  110  to a direct current, and input it into the high-voltage end converter station  12  through the middle-voltage DC power transmission line  13 . The high-voltage end converter station  12  converts an alternating current generated by a second alternating current power supply  120  to a direct current, and superimpose it with the direct current outputted from the low-voltage end converter station  11  to generate a high-voltage direct current, which is then transmitted to the receiving side, i.e., the inverter side (not shown in  FIG. 1 ) of the HVDC power transmission system through the high-voltage DC power transmission line  14 . The first alternating current power supply  110  and the second alternating current power supply  120  may be wind farms located in different locations. Such that, electric energy collected from multiple AC power supplies can be sent out in the DC manner. 
     The voltage of the high voltage direct current outputted from the high-voltage end converter station  12  may be in a range above ±750 KV, for example, the voltage of the high voltage direct current may be ±800 KV or ±1000 KV. The present description will be given herein with ±800 KV high voltage direct current as an example. In this case, the voltage range of the direct current outputted from the low-voltage end converter station  11  is preferably half of that of the high voltage direct current, that is, ±400 KV. The voltage of the current of the second alternating current power supply  120  rectified by high-voltage end converter station  12  is also ±400 KV, so that the voltage of a high voltage direct current obtained through superimposing the two alternating currents is ±800 KV. 
     The negative side of the low-voltage end converter station  11  comprises a converter transformer  111   a  coupled to the first AC power supply  110 . The converter transformer  111   a  is used to change AC voltage and realizes electric isolation between the AC part and the DC part in the power transmission system. 
     A converter valve  112   a  is coupled to the converter transformer  111   a , which is used to realize AC/DC conversion. In the embodiment of this invention, the converter valve  112   a  is preferably a 12-pulse converter valve. 
     On each side of the converter valve  112   a , a smoothing reactor  115   a  is provided. The smoothing reactors  115   a  are used to smooth DC ripples in DC and prevent DC interruption. The smoothing reactor  115   a  may also prevent impulse steep waves generated by DC lines or DC devices from entering the valve hall, and thereby prevent over-current damages to the converter valve  112   a . Through arranging smoothing reactors  115   a  on both sides of the converter valve  112   a , the effect of lighting protection can be effectively achieved, so that the safety of the power transmission system can be improved. 
     In the scheme shown in  FIG. 1 , a DC filter  117   a  is further connected across the two ends of the smoothing reactors  115   a , for filtering out harmonic current generated in the conversion process of the converter valve, so as to prevent interference on the system caused by the harmonic current. According to another optional scheme, isolation knife switches can be provided on both sides of the DC filter  117   a.    
     A bypass isolation knife switch  116   a  is arranged between the smoothing reactors  115   a , for provide a bypass when a failure occurs on the converter valve  112   a . A bypass AC switch  113   a  and isolation knife switches  114   a  are further provided near the converter valve  112   a.    
     The positive side of the low-voltage end converter station  11  has a structure symmetric to the structure of the negative side, and comprises a converter transformer  111   b , a converter valve  112   b , smoothing reactors  115   b , a DC filter  117   a , a bypass isolation knife switch  116   b , a bypass AC switch  113   b  and isolation knife switches  114   a , which will not be described in detail herein as they have the same functions as that components on the negative side. 
     The high-voltage end converter station  12  has a bipolar structure similar to the low-voltage end converter station  11 . Particularly, the high-voltage end converter station  12  comprises: converter transformers  121   a ,  121   b  coupled to a second AC power supply  120 ; converter valves  122   a ,  122   b  coupled to the converter transformers  121   a ,  121   b , smoothing reactors  125   a  and smoothing reactors  125   b  arranged on both sides of the converter valves  122   a ,  122   b  respectively; DC filters  127   a ,  127   b  across both ends of the smoothing reactors  125   a  and the smoothing reactors  125   b  respectively; bypass isolation knife switches  126   a ,  126   b  provided between the smoothing reactors  125   a  and the smoothing reactors  125   b  respectively; and bypass AC switches  123   a ,  123   b  and isolation knife switches  124   a ,  124   b , which will not be described in detail herein as they have the same functions as that components of the low-voltage end converter station  11 . 
     Incidentally, in the first embodiment shown in  FIG. 1 , there are DC filters connected across both sides of the smoothing reactors in the low-voltage end converter station  11  and the high-voltage end converter station  12  respectively, and harmonic current throughout the system can be eliminated with such a configuration. However, it should be noted that when selecting a wiring scheme for the cascaded multi-terminal HVDC power transmission system, a DC filter configuration can be selected reasonably depending on equivalent interference current requirements of a project. In the case that it is required to meet a standard about equivalent interference current all along the line, the configuration of providing a DC filter across both sides of the smoothing reactors is adopted; on the other hand, in the case of permitting substandard whole-line equivalent interference main current be not in standard, the DC filters can be canceled. Hereinafter, the configuration of the DC filters will be described in more detail below. 
     In the cascaded converter station according to the first embodiment, a grounding line  126  coupled to a grounding electrode and a metal return line  128  which is coupled between the positive line and the negative line are provided in the low-voltage end converter station  11 . The grounding electrode may be provided at a distance of 40-50 km from the low-voltage end converter station  11 . In addition, neutral bus switches (NBS)  119   a ,  119   b , a neutral bus grounding switch (NBGS)  121 , a grounding return transfer switch (GRTS)  120  and a metal return transfer switch (MRTS)  125  are provided in the wiring of the low-voltage end converter station  11 . NBS  119   a ,  119   b  are used to rapidly isolate a pole which is locked to quit and a normal pole. NBGS  121  is used to switch the neutral bus to a temporal grounding grid of the low-voltage end converter station  11  when the grounding electrode quits in a bi-pole mode. MRTS  125  and GRTS  120  cooperate with each other to realize switching between monopole grounding return and monopole metal return. 
     The high-voltage end converter station  12  does not have a grounding line coupled to the grounding electrode and a metal return line set up therein. 
       FIG. 2  to  FIG. 8  show seven operation wiring manners of the above cascaded converter station according to the first embodiment of this invention respectively: 
     (1) full bi-pole operation wiring; 
     (2) ¾ bi-pole operation wiring; 
     (3) ½ bi-pole operation wiring; 
     (4) full monopole grounding return wiring; 
     (5) ½ monopole grounding return wiring; 
     (6) full monopole metal return wiring; 
     (7) ½ monopole metal return wiring; 
     In these seven operation wiring modes, the full bi-pole operation wiring is the wiring manner in a normal operation condition, and the others are those in faulty conditions. 
     Referring to  FIG. 2 , in which the full bi-pole operation wiring in the normal operation condition is shown. The live portions of the cascaded converter station are illustrated by heavy lines. Four converter valves  112   a ,  112   b ,  122   a ,  122   b  in the positive and negative poles of the low-voltage end converter station  11  and the high-voltage end converter station  12  are all put into operation. 
       FIG. 3A  to  FIG. 3C  show the ¾ bi-pole operation wiring. This operation manner means that, among the four converter valves  112   a ,  112   b ,  122   a ,  122   b  in the positive and negative poles of the low-voltage end converter station  11  and the high-voltage end converter station  12 , a certain faulty converter valve quits operation, while other three converter valves keep running. 
       FIG. 3A  and  FIG. 3B  show a schematic diagram of the operation wiring when the converter valve  112   a  of the low-voltage end quits operation. As shown in  FIG. 3A  and  FIG. 3B , there are two bypass paths for the converter valve  112   a  out of service: a GRTS and metal return circuit, or a circuit with bypass isolation knife switches. When a failure occurs on the smoothing reactor  115   a  or the DC filter  117   a  of the low-voltage end converter valve  112   a , it can be bypassed using the GRTS  120  and the metal return line  128 . In this case, the converter valves  122   a ,  122   b  on the high-voltage end are still in operation. Because DC breakers are provided for both of these return circuits, switching can be performed on line. 
       FIG. 3C  shows a schematic diagram of the operation wiring when the converter valve  122   a  on the high-voltage end quits operation. As shown in  FIG. 3C , when the converter valve  122   a  on the high-voltage end quits operation, the smoothing reactors  125   a  on both sides of the converter valve are still connected in the operation circuit and do not quit. 
       FIG. 4A  and  FIG. 4B  show the ½ bi-pole operation wiring. This operation manner means that one converter station of the low-voltage end converter station  11  and the high-voltage end converter station  12  quits operation due to a failure, while the positive and negative poles of the other converter station still keep in operation. 
       FIG. 4A  shows a schematic diagram of the operation wiring when the converter valves  122   a  and  122   b  at the high-voltage end quit operation. As shown in  FIG. 4A , when the converter valves  122   a  and  122   b  at the high-voltage end quit operation, the smoothing reactors  125   a  and  125   b  on both sides of the converter valves are still connected in the operation circuit and do not quit. 
       FIG. 4B  shows a schematic diagram of the operation wiring when the converter valves  112   a  and  112   b  at the low-voltage end quit operation. As shown in  FIG. 4B , when the converter valve  112   b  at the low-voltage end quits operation, the smoothing reactors  115   b  on both sides of the converter valve are still connected in the operation circuit and do not quit. 
       FIG. 5  shows the full monopole grounding return wiring. This operation manner means that among the positive and negative poles of the low-voltage end converter station  11  and the high-voltage end converter station  12 , converter valves of a pole quit operation due to a failure, while converter valves of the other pole (including the high-voltage end and the low-voltage end) still keep in operation, and a return circuit is formed through the grounding.  FIG. 5  shows a condition in which the converter valve  112   b  at the low-voltage end and the converter valve  122   b  at the high-voltage end of the positive pole quit operation, while the converter valve  112   a  at the low-voltage end and the converter valve  122   a  at the high-voltage end of the negative pole keep in operation. 
       FIG. 6A  and  FIG. 6B  show the ½ monopole grounding return wiring. This operation manner means that among the low-voltage end converter station  11  and the high-voltage end converter station  12 , the converter valves of one converter station (including the positive and negative poles) quit operation due to a failure, while the converter valve of a pole in the other converter station keeps in operation, and a return circuit is formed through the earth. 
       FIG. 6A  shows a schematic diagram of the operation wiring when the converter valves  122   a  and  122   b  of the high-voltage end converter station  12  quit operation, while only the converter valve  112   a  of the negative pole in the low-voltage end converter station  11  keeps in operation. As shown in  FIG. 6A , when converter valves  122   a  of the high-voltage end quits operation, the smoothing reactors  125   a  on both sides of the converter valve are still connected in the operation circuit and do not quit. 
       FIG. 6B  shows a schematic diagram of the operation wiring when the converter valves  112   a  and  112   b  of the low-voltage end converter station  11  quit operation, while only the converter valve  122   a  of the negative pole in the high-voltage end converter station  12  keeps in operation. 
       FIG. 7  shows the full monopole metal return wiring. This operation manner means that among the positive and negative poles of the low-voltage end converter station  11  and the high-voltage end converter station  12 , converter valves of a pole quit operation due to a failure, while converter valves of the other pole (including the high-voltage end and the low-voltage end) still keep in operation, and a return circuit is formed through a metal line.  FIG. 7  shows a condition in which the converter valve  112   b  at the low-voltage end and the converter valve  122   b  at the high-voltage end of the positive pole quit operation, while the converter valve  112   a  at the low-voltage end and the converter valve  122   a  at the high-voltage end of the negative pole keep in operation. 
       FIG. 8A  and  FIG. 8B  show the ½ monopole metal return wiring. This operation manner means that among the low-voltage end converter station  11  and the high-voltage end converter station  12 , the converter valves of one converter station (including the positive and negative poles) quit operation due to a failure, while only the converter valve of one pole in the other converter station keeps in operation, and a return circuit is formed through a metal line. 
       FIG. 8A  shows a schematic diagram of the operation wiring when the converter valves  122   a  and  122   b  of the high-voltage end converter station  12  quit operation, while only the converter valve  112   a  of the negative pole in the low-voltage end converter station  11  keeps in operation. As shown in  FIG. 8A , when the high-voltage end converter station  122   a  and  122   b  quit operation, the smoothing reactors  125   a  and  125   b  on both sides of the converter valves are still connected in the operation circuit and do not quit. 
       FIG. 8B  shows a schematic diagram of the operation wiring when the converter valves  112   a  and  112   b  of the low-voltage end converter station  11  quit operation, while only the converter valve  122   a  of the negative pole in the high-voltage end converter station  12  keeps in operation. As shown in  FIG. 8B , when the high-voltage end converter valve  122   b  and the low-voltage end converter valve  112   a  quit operation, the smoothing reactors  125   b  and  115   a  on both sides of the converter valves are still connected in the operation circuit and do not quit. 
     The advantages of the wiring schemes of the cascaded converter station according to the first embodiment of this invention as depicted with reference to  FIG. 1  to  FIG. 8  are: when a converter valve in the low-voltage end converter station  11  stop working, online bypass is achieved using a metal return line or a bypass isolation knife switch, to provide control flexibility. The circuit has a less number of elements, and thus higher reliability. In addition, as compared with the Xiang jiaba-Shang hai HVDC power transmission system in the prior art, because the metal return line  128  is provided in the low-voltage end converter station  11 , the same function is realized with a lower isolation level that is required for the devices. 
     In the wiring schemes of the cascaded converter station according to the first embodiment of this invention, if a failure occurs on the middle-voltage DC line of a single pole, or on neutral bus devices of a single pole in the low-voltage converter station (NBS, the isolation knife switch, CT, PT and other devices), only monopole grounding return operation is possible. If a failure occurs on the middle-voltage lines of the two poles, the two poles have to stop operation. In order to improve energy availability, according to a second embodiment of this invention, another cascaded converter station is provided. 
       FIG. 9  is a schematic diagram of the structure and wiring of a cascaded converter station used in cascaded multi-terminal HVDC power transmission according to a second embodiment of this invention. 
     As compared with the first embodiment, in the cascaded converter station according to the second embodiment, a grounding line  133  coupled to a grounding electrode is set up in the high-voltage end converter station  12 . In addition, neutral bus isolation knife switches  137   a ,  137   b  are added. 
     Similar to the first embodiment,  FIG. 10  to  FIG. 16  show seven operation wiring manners of the cascaded converter station according to the third embodiment of this invention respectively: 
     (1) full bi-pole operation wiring; 
     (2) ¾ bi-pole operation wiring; 
     (3) ½ bi-pole operation wiring; 
     (4) full monopole grounding return wiring; 
     (5) ½ monopole grounding return wiring; 
     (6) full monopole metal return wiring; 
     (7) ½ monopole metal return wiring. 
     Referring to  FIG. 10 , in which the full bi-pole operation wiring in a normal operation condition is shown. Four converter valves  112   a ,  112   b ,  122   a ,  122   b  in the positive and negative poles of the low-voltage end converter station  11  and the high-voltage end converter station  12  are all put into operation. 
       FIG. 11A  to  FIG. 11C  show the ¾ bi-pole operation wiring. 
       FIG. 11A  shows a schematic diagram of the operation wiring when the high-voltage end converter valve  122   a  quits operation. As shown in  FIG. 11A , when the high-voltage end converter valve  122   a  quits operation, the smoothing reactors  125   a  on both sides of the converter valve are still connected in the operation circuit and do not quit. 
       FIG. 11B  and  FIG. 11C  show a schematic diagram of the operation wiring when the low-voltage end converter valve  112   a  quits operation. As shown in  FIG. 11B  and  FIG. 11C , there are two bypass paths for the converter valve  112   a  out of service: a GRTS and metal return circuit, or a circuit with bypass isolation knife switches. When a failure occurs on the smoothing reactor  115   a  or the DC filter  117   a  of the low-voltage end converter valve  112   a , it can be bypassed using the GRTS  120  and the metal return circuit  128 . 
       FIG. 12A  and  FIG. 12B  show the ½ bi-pole operation wiring. 
       FIG. 12A  shows a schematic diagram of the operation wiring when the high-voltage end converter valves  122   a  and  122   b  quit operation. As shown in  FIG. 12A , when the high-voltage end converter valves  122   a  and  122   b  quit operation, the smoothing reactors  125   a  and  125   b  on both sides of the converter valves are still connected in the operation circuit and do not quit. 
       FIG. 12B  shows a schematic diagram of the operation wiring when the low-voltage end converter valves  112   a  and  112   b  quit operation. As shown in  FIG. 12B , when the low-voltage end converter valves  112   a  and  112   b  quit operation, the smoothing reactors  115   a  and  115   b  on both sides of the converter valves are still connected in the operation circuit and do not quit. 
       FIG. 13  shows the full monopole grounding return wiring, in which the low-voltage end converter valve  112   b  and the high-voltage end converter valve  122   b  of the positive pole quit operation, while the low-voltage end converter valve  112   a  and the high-voltage end converter valve  122   a  of the negative pole keep in operation. 
       FIG. 14A  to  FIG. 14C  show the ½ monopole grounding return wiring. 
     FIG. ANA shows a schematic diagram of the operation wiring when the converter valves  122   a  and  122   b  of the high-voltage end converter station  12  quit operation, while only the converter valve  112   a  of the negative pole in the low-voltage end converter station  11  keeps in operation. As shown in  FIG. 14A , when the high-voltage end converter station  122   a  quits operation, the smoothing reactors  125   a  on both sides of the converter valve are still connected in the operation circuit and do not quit. 
       FIG. 14B  and  FIG. 14C  show a schematic diagram of the operation wiring when the converter valves  112   a  and  112   b  of the low-voltage end converter station  11  quit operation, while only the converter valve  122   a  of the negative pole in the high-voltage end converter station  12  keeps in operation. 
       FIG. 15  shows the full monopole metal return wiring, in which the low-voltage end converter valve  112   b  and the high-voltage end converter valve  122   b  of the positive pole quit operation, while the low-voltage end converter valve  112   a  and the high-voltage end converter valve  122   a  of the negative pole keep in operation. 
       FIG. 16A  to  FIG. 16C  show the ½ monopole metal return wiring. 
       FIG. 16A  shows a schematic diagram of the operation wiring when the converter valves  122   a  and  122   b  of the high-voltage end converter station  12  quit operation, while only the converter valve  112   a  of the negative pole in the low-voltage end converter station  11  keeps in operation. As shown in  FIG. 16A , when the high-voltage end converter station  122   a  and  122   b  quit operation, the smoothing reactors  125   a  and  125   b  on both sides of the converter valves are still connected in the operation circuit and do not quit. 
       FIG. 16B ,  FIG. 16C  show a schematic diagram of the operation wiring when the converter valves  112   a  and  112   b  of the low-voltage end converter station  11  quit operation, while only the converter valve  122   a  of the negative pole in the high-voltage end converter station  12  keeps in operation. As shown in  FIG. 16B  and  FIG. 16C , when the high-voltage end converter valve  122   b  quits operation, the smoothing reactors  125   b  on both sides of the converter valve are still connected in the operation circuit and do not quit. 
     The energy availability of the cascaded converter station according to the second embodiment is higher than that of the first embodiment. When a failure occurs on the middle-voltage lines of the two poles or low-voltage end converter station on neutral bus devices (NBS, NBGS, isolation knife switch, and other devices) of the two poles, the low-voltage end converter station  11  quits operation, and the high-voltage end converter station  12  operates by means of a metal return line of a single pole or a grounding return line of a single pole. 
     Based on the wiring schemes of the cascaded converter station of the second embodiment, other wiring schemes can be obtained through expansion according to particular project requirements. 
       FIG. 17  shows a first expanded wiring scheme based on the second embodiment, in which a metal return line  138  is added in the high-voltage end converter station  12 . When the high-voltage end converter station  12  is in monopole metal return operation, the smoothing reactors  125   b  and the DC filter  127   b  of the other pole in the station can be bypassed, as shown in  FIG. 18 . 
       FIG. 19  shows a second expanded wiring scheme based on the second embodiment, in which converter station bypass paths  139   a  and  139   b  are added in the high-voltage end converter station  12 . The low-voltage end converter station  11  may operate even if a failure occurs on the smoothing reactors or the DC filter of the high-voltage end converter station  12 , as shown in  FIG. 20 , in which the monopole grounding return wiring of the low-voltage end converter station  11  is shown. 
     In the first embodiment, in the double pole or monopole grounding return state, if a failure occurs on the smoothing reactors, the DC filter, or the bypass isolation knife switch of a single pole of the low-voltage end converter station  11 , it can be bypassed using the metal return line and the GRTS; however, if a single pole failure occurs on the middle voltage 400 kV DC power transmission line or on neutral bus devices of single pole such as NBS, CT, PT and the isolation knife switch of a single pole (N−1 failure), that pole has to be stopped, and double pole DC operation cannot be realized. In order to further improve energy availability, according to a third embodiment of this invention, another cascaded converter station is provided. 
       FIG. 21  is a schematic diagram of the structure and wiring of a cascaded converter station used in cascaded multi-terminal HVDC power transmission according to a third embodiment of this invention. 
     As compared with the second embodiment, on the basis of adding a neutral bus isolation knife switch, two NBS breakers  140   a ,  140   b  and two neutral bus isolation knife switches  141   a ,  141   b  are further added. 
     With the wiring of the third embodiment, when a N−1 failure occurs, that is, a single pole failure of the 400 kV middle voltage DC power transmission line or a failure on neutral bus devices such as NBS, CT, PT and the isolation knife switch of a single pole, the cascaded converter station can operate in a ¾ double pole state, as shown in  FIG. 22 . 
     When a N−2 failure occurs, that is, when a failure occurs on the middle-voltage lines of the two poles or the low-voltage end converter station  11  is in power-off service, the high-voltage end converter station  12  may operate in the double pole, the monopole metal return, or monopole grounding return state, to improve energy availability of the system. Because NBS  140   a ,  140   b  are provided in the neutral bus line of the high-voltage end converter station  12 , when the low-voltage end converter station  11  is in maintenance and the high-voltage end converter station  12  operates in the double pole state, it is not necessary to stop double pole operation if a single pole failure occurs, as shown in  FIG. 23 , in which the double pole operation wiring of the high-voltage end converter station  12  is shown. 
     Based on the wiring scheme of the third embodiment, if it is required for the high-voltage end converter station  12  to switch online between single pole grounding return and single pole metal line return, to operate without passing through the separate metal return line of the other converter station and operate in the bi-pole state using a converter station temporal grounding, the expanded wiring scheme shown in  FIG. 24  can be adopted, in which a metal return line  138  and MRTB  143 , GRTS  142 , NGBS  144  are added in the high-voltage end converter station  12 . 
       FIG. 25  is a schematic diagram of the structure and wiring of a cascaded converter station used in cascaded multi-terminal HVDC power transmission according to a fourth embodiment of this invention. 
     As compared with the third embodiment, in the cascaded converter station of the fourth embodiment, bypass paths  139   a ,  139   b  for bypassing the high-voltage end converter station  12  are coupled between the middle-voltage DC power transmission line  13  and the high-voltage DC power transmission line  14 . 800 KV isolation knife switches are provided between the smoothing reactors  125   a ,  125   b  and the high-voltage DC power transmission line  14 , and in the bypass paths  139   a ,  139   b.    
       FIG. 26  to  FIG. 32  show seven operation wiring modes of the above cascaded converter station according to the fourth embodiment of this invention respectively: 
     (1) full bi-pole operation wiring; 
     (2) ¾ bi-pole operation wiring; 
     (3) ½ bi-pole operation wiring; 
     (4) full monopole grounding return wiring; 
     (5) ½ monopole grounding return wiring; 
     (6) full monopole metal return wiring; 
     (7) ½ monopole metal return wiring. 
     In these seven operation wiring modes, the full bi-pole operation wiring is a wiring mode in the normal operation condition, and other operation wiring modes are those in faulty conditions. 
     Referring to  FIG. 26 , in which the full bi-pole operation wiring in the normal operation condition is shown. Four converter valves  112   a ,  112   b ,  122   a ,  122   b  in the positive and negative poles of the low-voltage end converter station  11  and the high-voltage end converter station  12  are all put into operation. 
       FIG. 27A ,  FIG. 27B  show the ¾ bi-pole operation wiring.  FIG. 27A  shows a schematic diagram of the operation wiring when the low-voltage end converter valve  122   a  quits operation.  FIG. 27B  shows a schematic diagram of the operation wiring when the high-voltage end converter valve  122   a  quits operation. As shown in  FIG. 27B , when the high-voltage end converter valve  122   a  quits operation, a return loop is formed through the bypass path  139   a , and the smoothing reactors  125   a , etc. are not connected in the operation circuit. 
       FIG. 28A  and  FIG. 28B  show the ½ bi-pole operation wiring.  FIG. 28A  shows a schematic diagram of the operation wiring when the high-voltage end converter valves  122   a  and  122   b  quit operation. As shown in  FIG. 28A , when the high-voltage end converter valves  122   a  and  122   b  quit operation, a return loop is formed through the bypass paths  139   a ,  139   b , and the smoothing reactors  125   a ,  125   b  etc. are not connected in the operation circuit.  FIG. 28B  shows a schematic diagram of the operation wiring when the low-voltage end converter valves  122   a  and  122   b  quit operation. 
       FIG. 29  shows the full monopole grounding return wiring mode, in which the low-voltage end converter valve  112   b  and the high-voltage end converter valve  122   b  of the positive pole quit operation, while the low-voltage end converter valve  112   a  and the high-voltage end converter valve  122   a  of the negative pole keep in operation. 
       FIG. 30A  and  FIG. 30B  show the ½ monopole grounding return wiring mode. 
       FIG. 30A  shows a schematic diagram of the operation wiring when the converter valves  122   a  and  122   b  of the high-voltage end converter station  12  quit operation, while only the converter valve  112   a  of the negative pole in the low-voltage end converter station  11  keeps in operation. As shown in  FIG. 30A , when the high-voltage end converter station  122   a  quits operation, a return loop is formed through the bypass path  139   a  and the grounding line  126 , and the smoothing reactors  125   a , etc. are not connected in the operation circuit. 
       FIG. 30B  shows a schematic diagram of the operation wiring when the converter valves  112   a  and  112   b  of the low-voltage end converter station  11  quit operation, while only the converter valve  122   a  of the negative pole in the high-voltage end converter station  12  keeps in operation. 
       FIG. 31  shows the full monopole metal return wiring, in which the low-voltage end converter valve  112   b  and the high-voltage end converter valve  122   b  of the positive pole quit operation, while the low-voltage end converter valve  112   a  and the high-voltage end converter valve  122   a  of the negative pole keep in operation. As shown in  FIG. 31 , when the high-voltage end converter valve  122   b  quits operation, a return loop is formed through the bypass path  139   b  and the metal return line  128 , and the smoothing reactors  125   b , etc. are not connected in the operation circuit. 
       FIG. 32A  and  FIG. 32B  show the ½ monopole metal return wiring.  FIG. 32A  shows a schematic diagram of the operation wiring when the converter valves  122   a  and  122   b  of the high-voltage end converter station  12  quit operation, while only the converter valve  112   a  of the negative pole in the low-voltage end converter station  11  keeps in operation. As shown in  FIG. 32A , when the high-voltage end converter station  122   a  and  122   b  quit operation, a return loop is formed through the bypass paths  139   a ,  139   b  and the metal return line  128 , and the smoothing reactors  125   a  and  125   b , etc. are not connected in the operation circuit. 
       FIG. 32B  shows a schematic diagram of the operation wiring when the converter valves  112   a  and  112   b  of the low-voltage end converter station  11  quit operation, while only the converter valve  122   a  of the negative pole in the high-voltage end converter station  12  keeps in operation. 
     The advantage of the fourth embodiment is that the low-voltage end converter station  11  and the high-voltage end converter station  12  can operate independently without interference with each other (for example, in converter station overhauling), so that energy availability can be improved. When a failure occurs on the smoothing reactors and the DC filter of the high-voltage end converter station  12 , the converter station of the low-voltage end converter station  11  of the same pole can operate continuously, without one pole interruption. 
     Based on the wiring scheme of the cascaded converter station of the fourth embodiment, other expanded wiring schemes may be further obtained, as shown in  FIG. 33  and  FIG. 34 . 
       FIG. 33  shows a first expanded wiring scheme based on the above embodiment, in which MRTB  143  and NBGS  144  are installed in the grounding line of the high-voltage end converter station  12 , and isolation knife switches  130   a ,  130   b  are provided near the smoothing reactors. According to this wiring scheme, online switching between the monopole grounding return mode and the monopole metal return mode of the high-voltage end converter station  12  can be achieved without passing through the smoothing reactors of the other converter station, and double pole operation can be achieved using a converter station temporal grounding. 
       FIG. 34  shows a second expanded wiring scheme based on the above embodiment, in which MRTB  143  and NBGS  144  are installed in the grounding line of the high-voltage end converter station  12 . According to this wiring scheme, online switching between the monopole grounding return mode and the monopole metal return mode of the high-voltage end converter station  12  can be achieved, and double pole operation can be achieved using a converter station temporal grounding. Different to  FIG. 33 , no isolation knife switches  130   a  and  130   b  are provided near the smoothing reactors, in the monopole metal operation of a pole&#39;s converter station, a smoothing reactor branch of the other converter station is required. 
     In the cascaded converter stations of the first to fourth embodiments and their expanded structures combination with  FIGS. 1-34 , DC filters are connected across the two ends of the smoothing reactors in the low-voltage end converter station  11  and the high-voltage end converter station  12 . However, this DC filter configuration is merely a preferable scheme but not a limitation.  FIGS. 35-37  show other three alternative DC filter configurations, which may be combined with various wiring manners of the cascaded converter stations of the first to third embodiments shown in  FIG. 1  to  FIG. 34  appropriately (to substitute the DC filters therein). When selecting a wiring scheme for a cascaded multi-terminal HVDC power transmission system, a DC filter configuration can be selected reasonably according to project requirements on equivalent interference current. 
     Equivalent interference current is defines as: a single-frequency harmonic current, which produces the same interference effect on adjacent parallel or crossed communication lines as the combined interference effect produced by harmonic currents of all frequencies on a line. According to particular project requirements, the threshold of equivalent interference current may be adjusted appropriately, to balance the cost of harmonic management and the cost of harmonic interference compensation, so as to minimize the harmonic management and compensation costs. 
     There are three following situations: 
     (1) In the case that it is required to meet a standard about equivalent interference current all along the line, the DC filter is connected across the two ends of the smoothing reactors in a converter station-dependent configuration as shown in  FIGS. 1-34 ; 
     (2) in the case of permitting substandard equivalent interference current on the 400 kV middle-voltage line, DC filters  142   a ,  142   b  to the ground can be provided in the high-voltage end converter station  12 , and DC filters across converter stations can be canceled, as shown in  FIG. 35  and  FIG. 36 .  FIG. 35  shows a situation that has a grounding line  133  provided in the high-voltage end converter station  12 .  FIG. 36  shows a situation that does not have grounding line  133  provided in the high-voltage end converter station  12 . In this case, harmonic current produced by the converter returns through the grounding grid of the high-voltage end converter station  12  via the grounding electrode of the low-voltage end converter station  11 ; 
     (3) in the case of permitting substandard equivalent interference current all along the line, the DC filters can be canceled as shown in  FIG. 37 . 
     A cascaded multi-terminal HVDC power transmission system is further provided in this invention. As shown in  FIG. 38 , the system comprises a converter station on the sending side, a converter station on the receiving side, and a HVDC power transmission line therebetween. The converter station on the sending side and the converter station on the receiving side connected to an AC power source and a load area respectively. Wherein, one or both of the converter station on the sending side and the converter station on the receiving side is constructed according to the cascaded converter station of the first to fourth embodiments described above. Correspondingly, the AC power source and the load area may comprise one or more AC power sources and load areas. 
     Note that, in this description, for example, the value of high voltage direct voltage, the number of isolation knife switches and the type of converter station are all illustrative. Those skilled in the art may make modifications thereto according to practical project requirements. In addition, the terms “first”, “second”, etc. in this description are merely used to distinguish one entity or operation from another entity or operation, and it is not necessary to require or imply any such specific relationship or sequence of those entities or operations. In addition, the terms “comprise,” “include,” and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. In the case of without a further limitation, the expression “comprising an element” does not preclude the addition of other identical elements in the process, method, article, or apparatus comprising that element. 
     Preferred embodiments of this invention have been described above with reference to drawings. It is apparent that, however, those embodiments are merely for the purpose of illustration, but are not intended to be limitations on the scope of this invention. Those skilled in the art may make various modifications, substitutions and improvements to those embodiments without departing from the spirit and scope of this invention. The scope of this invention is only defined by the accompanying claims.