System and method for transmission of high voltage DC power

Abstract

The present invention provides a high voltage direct current (HVDC) transmission system (300, 600), comprising: a first station (102), the first station (102) comprising a first HVDC converter and a second HVDC converter (110, 130) connected in series; a second station (104), the second station (104) comprising a third HVDC converter and a fourth HVDC converter (150, 170) connected in series, wherein a neutral node (164) coupling the third HVDC converter (150) to the fourth HVDC converter (170) is coupled to ground; a first transmission line (200), the first transmission line (200) connecting a positive node (114) of the first HVDC converter (110) to a corresponding positive node (154) of the third HVDC converter (150), wherein the first transmission line (200) of the system (300, 600) The pole (240) comprises a first HVDC converter (110), a third HVDC converter (150) and a first transmission line (200); a second transmission line (210) connecting a negative node (138) of the second HVDC converter (130) to a corresponding negative node (178) of the fourth HVDC converter (170), wherein a second pole (250) of the system (300, 600) comprises the second HVDC converter (130), the fourth HVDC converter (170) and the second transmission line (210); characterized in that a neutral node (124) connecting the first HVDC converter (110) to the second HVDC converter (130) is coupled to a parallel combination of a resistor (310) and a neutral bus ground switch (312) for coupling the neutral node (124) to ground.

Description

Technical Field

[0001] The present disclosure relates to systems and methods for transmission of high voltage direct current (HVDC) power. Background Art

[0002] High voltage direct current (HVDC) power transmission is a cost-effective way to transmit electrical power over long distances. In an HVDC system, alternating current (AC) electrical power is converted to HVDC electrical power by an HVDC converter at a first station for transmission via overhead or submarine cables to a destination. At the destination, the HVDC power is converted back to AC power by an HVDC converter at a second station for onward distribution to end-user sites via a distribution network.

[0003] The first and second stations can be linked by a monopole link, in which a conductive (e.g. metal) transmission line links a first energized high voltage (relative to ground) node of the HVDC converter of the first station to a corresponding first node of the HVDC converter of the second station, while a second node of the HVDC converter of the first station and a corresponding second node of the HVDC converter of the second station are connected to ground using a grounding electrode. Thus, in this arrangement, grounding acts as a return path for the current. The second nodes of the HVDC converters of the first and second stations can also be connected by a second conductive (e.g. metal) transmission line, in which case the second transmission line provides a dedicated return path for the current.

[0004] Alternatively, the first and second stations can be linked by a bipolar link. In a bipolar link, two HVDC converters are connected in series in each of the first and second stations. A first conductive (e.g., metal) transmission line links a first energized high voltage (the high voltage is configured to be positive relative to the ground) node of the first HVDC converter of the first station to a corresponding first energized high voltage (also configured to be positive relative to the ground) node of the first HVDC converter of the second station, thereby forming a first pole (pole) of the system. A second conductive (e.g., metal) transmission line links a second energized high voltage (the high voltage is configured to be negative relative to the ground) node of the second HVDC converter of the first station to a corresponding second energized high voltage (also configured to be negative relative to the ground) node of the second HVDC converter of the second station, thereby forming a second pole of the system. Therefore, in this arrangement, the current flowing in the first transmission line and the second transmission line has opposite polarities. The third neutral node of the first station (the node connecting the first and second HVDC converters of the first station) is connected to ground, and the corresponding third neutral node of the second station (the node connecting the first and second HVDC converters of the second station) is also connected to ground. A return path is also provided by an additional conductive (eg metal) transmission line connecting the third nodes of the first and second stations or by a ground return (in which case a ground electrode is used to connect the third electrodes of the first and second stations to ground).

[0005] An advantage of a bipolar transmission system is that in the event of a fault in one of the poles, a return path (ground or dedicated return path) can be used to replace the faulty pole, which allows the system to continue to operate as a single pole link, albeit at a reduced capacity. The system can then be reconfigured to commutate current from the return path to the transmission line of the faulty pole, thereby employing the transmission line of the faulty pole as the current return path. Thus, the system can continue to operate as a single pole link while the faulty pole is repaired.

[0006] In one variation of a two-pole link system, called a rigid two-pole link, no return path is provided for the current; there is no additional conductive (e.g., metallic) transmission line or ground return using a connection to the station's ground electrode. In this variation, because there is no return path, in the event of a fault in one of the poles, the system must be reconfigured to use the transmission line of the faulted pole as the current return path before the system can continue to operate as a single-pole link. Thus, in the event of a fault in one of the poles, there will be a delay while the system is reconfigured before it can resume operation.

[0007] Figure 1 is a schematic representation of a system comprising a first and a second station linked by a rigid bipolar link. Figure 1 The system shown generally at 100 in FIG. 1 includes a first station 102 having first and second HVDC converters 110, 130 and a second station 104 having third and fourth HVDC converters 150, 170. The first station 102 includes or is associated with a first controller 106 configured to control the operation of components of the first station 102, and the second station 104 includes or is associated with a second controller 108 configured to control the operation of components of the second station 104. The first and second controllers 106, 108 are configured to communicate with each other, such as via a wired or wireless bidirectional telecommunication link between the first controller 106 and the second controller 108, so as to coordinate the operations required in each of the first and second stations 102, 104.

[0008] The first HVDC converter 110 is coupled to an AC transmission terminal of the first AC transmission system through a first AC breaker 112, and the second HVDC converter 130 is coupled to a second AC transmission terminal of the AC transmission system through a second AC breaker 132. Similarly, the third first HVDC converter 150 is coupled to an AC transmission terminal of the first AC transmission system through a third AC breaker 152, and the fourth HVDC converter 170 is coupled to a second AC transmission terminal of the AC transmission system through a fourth AC breaker 172.

[0009] The first live high voltage node 114 of the first HVDC converter 110 (which node 114 is configured to be positive relative to ground) is linked to the corresponding first live high voltage node 154 of the third HVDC converter 150 (also configured to be positive relative to ground) via the first and second disconnectors 116, 118 of the first station and the first and second disconnectors 156, 158 of the second station 104 through the first transmission line 200, which may be, for example, an overhead, underground or submarine conductive (e.g., metallic) cable. The combination of the first HVDC converter 110, the third HVDC converter 150, the first transmission line 200 and the associated disconnectors 116, 118, 156, 158 form a first pole 240 of the system 100.

[0010] The second live high voltage node 120 of the first HVDC converter 110 (which node 120 is configured to be negative with respect to ground) is coupled in series to the first high voltage node 134 of the second HVDC converter 130 (which node 134 is configured to be negative with respect to ground) via the third and fourth disconnects 122, 136 of the first station 102. The neutral node 124 between the third and fourth disconnects 122, 136 of the first station 102 (which couples the first HVDC converter 110 to the second HVDC converter 130) is coupled to ground and to the link 126, which will be described in more detail below.

[0011] The second live high voltage node 160 of the third HVDC converter 150 (the node 160 is configured to be negative with respect to ground) is coupled in series to the first high voltage node 174 of the fourth HVDC converter 170 (the node 174 is configured to be positive with respect to ground) via the third and fourth disconnects 162, 176 of the second station 104. The neutral node 164 between the third and fourth disconnects 162, 176 of the second station 104 (the neutral node 164 couples the first HVDC converter 110 to the second HVDC converter 130) is coupled to ground and to the link 166 via a surge arrester 186, which will be described in more detail below.

[0012] The second live high voltage node 138 of the second HVDC converter 130 (which node 138 is configured to be negative relative to ground) is linked to the corresponding second live high voltage node 178 of the fourth HVDC converter 170 (which node 178 is also configured to be negative relative to ground) via the fifth and sixth disconnectors 140, 142 of the first station and the fifth and sixth disconnectors 180, 182 of the second station 104 through the second transmission line 210, which can be, for example, an overhead, underground or submarine conductive (e.g., metallic) cable. The combination of the second HVDC converter 130, the fourth HVDC converter 170, the second transmission line 210 and the associated disconnectors 140, 142, 180, 182 forms a second pole 250 of the system 100.

[0013] To facilitate reconfiguration of the system 100 in the event of a fault, the first station includes a link 126 between a first node intermediate the first and second disconnects 116 and 118 of the first station 102 and a second node intermediate the fifth and sixth disconnects 140 and 142 of the first station 102. The link 126 includes a seventh and eighth disconnects 128, 144 of the first station 102. The node intermediate the seventh and eighth disconnects 128, 144 of the link 126 is coupled to the neutral node 124 of the first station 102 and is therefore coupled to ground. The seventh and eighth disconnects 128, 144 are capable of selectively opening and closing to selectively couple the first or second transmission line 200, 210 to the neutral node 124 in order to reconfigure the system 100.

[0014] Similarly, the second station 104 includes a link 166 between a first node intermediate the first and second disconnects 156 and 158 of the second station 104 and a second node intermediate the fifth and sixth disconnects 180 and 182 of the second station 104. The link 166 includes seventh and eighth disconnects 168, 184 of the second station 104. The node intermediate the seventh and eighth disconnects 168, 184 of the link 166 is coupled to the neutral node 164 of the second station 104 and is therefore coupled to ground via the surge arrester 186. The seventh and eighth disconnects 168, 184 are capable of selectively opening and closing to selectively couple the first or second transmission line 200, 210 to the neutral node 124 in order to reconfigure the system 100.

[0015] In normal operation of the system 100, the first, second, third and fourth AC breakers 112, 132, 152, 172 are closed, coupling the first, second, third and fourth HVDC converters 110, 130, 150, 170 to their respective AC transmission terminals.

[0016] The first and second disconnectors 116 , 118 of the first station 102 are closed, and the first and second disconnectors 156 , 158 of the second station 104 are also closed, thereby establishing a path for positive current flow between the first HVDC converter 110 and the third HVDC converter 150 .

[0017] The third and fourth disconnectors 122 , 136 of the first station 102 are closed, thereby connecting the node 120 of the first HVDC converter 110 to the node 134 of the second HVDC converter 130 , while the third and fourth disconnectors 162 , 176 of the second station 104 are also closed, thereby connecting the node 160 of the third HVDC converter 150 to the node 174 of the fourth HVDC converter 170 .

[0018] The fifth and sixth disconnectors 140 , 142 of the first station 102 are closed, and the fifth and sixth disconnectors 180 , 182 of the second station 104 are also closed, thereby establishing a path for negative current flow between the second HVDC converter 130 and the fourth HVDC converter 170 .

[0019] The seventh and eighth disconnects 128 , 144 of the first station 102 are opened, and the seventh and eighth disconnects 168 , 184 of the second station 104 are also opened.

[0020] If a single-phase fault to ground is formed in one of the HVDC converters 110, 130, 150, 170 during operation of the system 100, the first and second controllers 106, 108 issue appropriate commands to the components of the first and second stations 102, 104 to implement a number of actions, which will be described below with reference to Figure 2 To describe, Figure 2 is after the detection of the ground fault in the third HVDC converter 150 Figure 1 A schematic representation of a system 100 is provided.

[0021] After detection of a fault (which in this example is a ground fault in the third HVDC converter 150), the first, second, third and fourth AC circuit breakers 112, 132, 152, 172 are opened in response to commands issued by the first and second controllers 106, 108 to isolate the first, second, third and fourth HVDC converters 110, 130, 150, 170 from their respective AC transmission terminals.

[0022] By isolating the first, second, third and fourth HVDC converters 110, 130, 150, 170 in this manner, by opening the first and third disconnects 116, 122 of the first station 102 and opening the second and third disconnects 158, 162 of the second station in response to commands issued by the first and second controllers 106, 108, the faulty pole (in this example, the first pole 240) containing the faulty HVDC converter is isolated from the healthy pole (in this example, the second pole 250) and from the transmission line of the faulty pole (in this example, the first transmission line 200).

[0023] Once the faulty pole has been isolated from the healthy pole and from the transmission line of the faulty pole, the system 100 can be reconfigured to operate in a single pole link configuration, wherein the transmission line of the faulty pole serves as a return path for the healthy pole. In the present example, this is accomplished by closing the seventh disconnector 128 of the link 126 of the first station 102 and closing the seventh disconnector 168 of the link 166 of the second station 104 in response to commands issued by the first and second controllers 106, 108 to couple the first transmission line 210 to the neutral nodes 124, 166 of the first and second stations 102, 104.

[0024] The second, fourth, fifth and sixth disconnectors 118, 136, 140, 142 of the first station 102 and the first, fourth, fifth and sixth disconnectors 156, 176, 180, 182 of the second station 104 remain closed. Thus, the transmission line 210 of the (faulty) first pole 240 can be used as a current return path for the (healthy) second pole 250.

[0025] Once the system 100 is reconfigured in this manner, the second and fourth AC breakers 132, 172 can be closed in response to commands issued by the first and second controllers 106, 108 to recouple the second and fourth HVDC converters 130, 170 to their respective AC transmission terminals, thereby allowing current to flow in the system 100 at the AC terminals 110 and 111. Figure 2 Flow in the direction shown by the arrow.

[0026] As will be appreciated, due to the large number of AC breakers and disconnects involved, the process described above of isolating the faulty pole and reconfiguring the system 100 can take a significant amount of time (on the order of 20-30 seconds) to complete. In some applications where the reconfiguration must be completed quickly (e.g., in about 2 seconds), this delay may be unacceptable.

[0027] One way to reduce the time taken for the above reconfiguration process is to replace the first, second, third, fourth, fifth and sixth disconnectors 116, 118, 122, 136, 140, 142 of the first station 102 and the first, second, third, fourth, fifth and sixth disconnectors 156, 158, 162, 176, 180, 182 of the second station 104 with high-speed switches or circuit breakers. Such high-speed switches or circuit breakers must be designed to be able to open and close quickly and must be able to withstand DC voltage / current stress. However, switches or circuit breakers that meet these requirements tend to be high-cost, and therefore replacing many disconnectors of the system 100 with such switches or circuit breakers involves significant costs.

[0028] Accordingly, it is desirable to have a cost-effective HVDC transmission system that can be quickly reconfigured in the event of a failure. Summary of the invention

[0029] According to a first aspect, the present invention provides a high voltage direct current (HVDC) transmission system comprising: a first station comprising first and second HVDC converters connected in series; a second station comprising third and fourth HVDC converters connected in series, wherein a neutral node coupling the third HVDC converter to the fourth HVDC converter is coupled to ground; a first transmission line connecting a positive node of the first HVDC converter to a corresponding positive node of the third HVDC converter, wherein a first pole of the system comprises the first HVDC converter, the third HVDC converter and the first transmission line; a second transmission line connecting a negative node of the second HVDC converter to a corresponding negative node of the fourth HVDC converter, wherein a second pole of the system comprises the second HVDC converter, the fourth HVDC converter and the second transmission line; characterised in that a neutral node connecting the first HVDC converter to the second HVDC converter is coupled to a parallel combination of a resistor and a neutral bus ground switch for coupling the neutral node to ground.

[0030] The HVDC transmission system of the present invention provides a bipolar HVDC transmission scheme which is rapidly reconfigurable in the event of a failure in one of its HVDC converters to a unipolar scheme using the transmission line of the pole in which the failure has occurred as a return path.

[0031] The first station may include a first link coupled to a first transmission line at a first end and coupled to a second transmission line at a second end, the first link including first and second high-speed switches connected in series. A third node connecting the first and second high-speed switches may be coupled to a neutral node of the first station, and the first and second high-speed switches may be selectively operable to couple the first transmission line or the second transmission line to the neutral node of the first station. The second station may include a second link coupled to the first transmission line at a first end and coupled to the second transmission line at a second end, the second link including third and fourth high-speed switches. A fourth node connecting the third and fourth high-speed switches may be coupled to a neutral node of the second station, and the second and third high-speed switches may be selectively operable to couple the first transmission line or the second transmission line to the neutral node of the second station.

[0032] The neutral node of the second station may be coupled to a surge arrester for coupling the neutral node to ground.

[0033] The first, second, third and fourth HVDC converters may comprise line-commutated converters.

[0034] Alternatively, the first, second, third and fourth HVDC converters may comprise voltage source converters.

[0035] The HVDC transmission system may further include a dedicated metallic return coupling the neutral node of the first station to the neutral node of the second station.

[0036] According to a second aspect of the invention, there is provided a HVDC converter station comprising: first and second HVDC converters connected in series, the first HVDC converter comprising a positive node configured to be coupled to a first transmission line, the second HVDC converter comprising a negative node configured to be coupled to a second transmission line, characterised in that a neutral node connecting the first HVDC converter to the second HVDC converter is coupled to a parallel combination of a resistor and a neutral bus ground switch for coupling the neutral node to ground.

[0037] The HVDC converter station may further include: a link configured to couple to the first transmission line at a first end and configured to couple to the second transmission line at a second end, the first link comprising first and second high-speed switches connected in series, wherein a node connecting the first and second high-speed switches is coupled to a neutral node, and wherein the first and second high-speed switches are selectively operable to couple the first transmission line or the second transmission line to the neutral node of the first station.

[0038] The first and second HVDC converters may comprise line commutated converters.

[0039] Alternatively, the first and second HVDC converters may comprise voltage source converters.

[0040] According to a third aspect of the invention, there is provided a method for reconfiguring the system of the first aspect upon detection of a to-earth fault in one of the first, second, third or fourth HVDC converters, the method comprising: detecting the fault; isolating at least the HVDC converter of the faulty pole comprising the HVDC converter in which the fault occurred from the respective AC transmission terminals; opening the neutral bus to earth switch so that leakage current in the system flows through the resistor; coupling the transmission line of the faulty pole to the neutral points of the first and second stations such that the coupled transmission line provides a return path for current in a healthy pole in which the fault did not occur; and isolating the HVDC converter of the faulty pole from the HVDC converters of the healthy poles and from the coupled transmission line.

[0041] The method may further comprise: after isolating the HVDC converter of the faulty pole from the HVDC of the healthy pole and from the coupled transmission line.

[0042] According to a fourth aspect of the invention, there is provided a method for reconfiguring the system of the second aspect upon detection of a to-ground fault in one of the first, second, third or fourth HVDC converters, the method comprising: detecting the fault; isolating the HVDC converter comprising the faulty pole of the HVDC converter in which the fault occurred from the respective AC transmission terminals; and opening the neutral bus to ground switch so as to allow leakage current in the system to flow through the resistor.

[0043] The method according to the invention may further comprise: coupling the transmission line of the faulty pole to the neutral nodes (124, 164) of the first and second stations such that the coupled transmission line provides a return path for current in the healthy pole; and isolating the HVDC converter in which the fault occurs from the HVDC converter of the healthy pole and from the coupled transmission line.

[0044] The method may further include closing the neutral bus ground switch. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the invention will now be described strictly by way of example only, with reference to the accompanying drawings, in which:

[0046] Figure 1 is a schematic representation of a bipolar link HVDC transmission system;

[0047] Figure 2 is used to act as a single-pole link HVDC transmission system after reconfiguration Figure 1 Schematic representation of the HVDC transmission system;

[0048] Figure 3 is a schematic representation of an alternative bipolar link HVDC transmission system;

[0049] Figures 4a-4d After the fault is detected Figure 3 Schematic representation of the HVDC transmission system;

[0050] Figure 5 is a diagram for reconfiguration Figure 3 A flow chart of the steps in a method of an HVDC transmission system;

[0051] Figure 6 is a schematic representation of a bipolar link HVDC transmission system with a dedicated metallic return (DMR); and

[0052] Figures 7a-7d After the fault is detected Figure 5 A schematic representation of an HVDC transmission system; and

[0053] Figure 8 is a diagram for reconfiguration Figure 6 Flow chart of the steps in a method of an HVDC transmission system. DETAILED DESCRIPTION

[0054] Now refer to Figure 3 , a HVDC transmission system is generally shown at 300. The system 300 is similar to the system described above and Figure 1 The system 100 illustrated in FIG. 1 shares many common elements, and therefore common reference numerals have been used to represent common elements of the systems 100 and 300. For the sake of clarity and brevity, only those features of the system 300 that differ from the system 100 will be described herein.

[0055] The system 300 is similar to the system 100, except that the link 126 of the first station 102 includes first and second high-speed switches 302, 304 instead of Figure 1 Similarly, the link 166 of the second station 104 includes third and fourth high-speed switches 306, 308, in place of Figure 1 The high-speed switches 302, 304 are capable of selectively opening and closing to selectively couple the first or second transmission line 200, 210 to the neutral node 124 of the first station 102 so as to reconfigure the system 300, and the high-speed switches 306, 308 are capable of selectively opening and closing to selectively couple the first or second transmission line 200, 210 to the neutral node 164 of the second station 104 so as to reconfigure the system 300.

[0056] In addition, the first station 102 includes a high resistance resistor 310 (e.g., about 50-1000Ω) and a neutral bus ground switch (NBGS) 312 connected in parallel between the neutral node 124 thereof and ground. The resistor 310 provides a first high resistance path to ground, while the NBGS 312, when closed, provides a second path to ground that is much lower than the resistor 310, such that the first path is ineffective when the NBGS 312 is closed. In normal operation of the system 300, the NBGS 312 is closed.

[0057] In normal operation, the system 300 operates in the same manner as the system 100 described above, except that the first and second high-speed switches 302, 304 of the link 126 of the first station 102 (the high-speed switches 302, 304 replace the Figure 1 The seventh and eighth disconnectors 128, 144 of the link 126 of the first station 102 of the system 100 are opened, and the third and fourth high-speed switches 306, 308 of the link 166 of the second station 104 (the high-speed switches 306, 308 replace Figure 1 The seventh and eighth disconnects 168, 184 of the link 166 of the second station 104 of the system 100 are also opened.

[0058] As described above, the NBGS 312 is closed during normal operation of the system.

[0059] If a single-phase fault to ground is formed in one of the HVDC converters 110, 130, 150, 170 during operation of the system 300, a number of actions are taken, as will be described below with reference to Figures 4a-4d As stated, Figures 4a-4d is after the detection of the ground fault in the third HVDC converter 150 Figure 3 It will of course be appreciated that the system 300 operates in the same manner when a fault is detected in any of the other HVDC converters 110, 130, 170.

[0060] as Figure 1 As in the system 100, after detection of a fault in the third HVDC converter 150, the first, second, third and fourth AC circuit breakers 112, 132, 152, 172 are opened in response to commands issued by the first and second controllers 106, 108 to isolate the first, second, third and fourth HVDC converters 110, 130, 150, 170 from their respective AC transmission terminals.

[0061] Alternatively, instead of opening all AC breakers 112, 132, 152, 172, only the AC breakers associated with the faulty pole are opened (in this case the first and third AC breakers 112, 152 of the first pole 240). The AC breakers associated with the healthy poles (in this case the second and fourth AC breakers of the second pole 250) remain closed, but the DC power in the healthy pole is ramped down to minimize the DC current flowing in the healthy pole.

[0062] By isolating the associated HVDC converters of the first, second, third and fourth HVDC converters 110, 130, 150, 170 in this manner, the stored energy circulates in the system 300. Due to the ground fault in the third HVDC converter 150, the leakage current flows through the ground, the first and second disconnectors 116 and 118 of the first station 102, the fourth disconnector 122, the first HVDC converter 110, the second low resistance path provided by the closed NBGS 312, the first transmission line 200, and the first and second disconnectors 156 and 158 of the second station 104, as shown by Figure 4a Indicated by the dotted arrow.

[0063] After the associated AC breakers of the first, second, third and fourth AC breakers 112, 132, 152, 172 have been opened to isolate the associated HVDC converters of the first, second, third and fourth HVDC converters 110, 130, 150, 170 from their respective AC transmission terminals, the NBGS 312 opens in response to a command issued by the first controller 106, thereby effectively activating the first high resistance path to ground including the resistor 310, as shown. Figure 4b as shown in .

[0064] The system 300 is then reconfigured to operate in a single-pole link configuration by closing the first high-speed switch 302 of the link 126 of the first station 102 and closing the third high-speed switch 306 of the link 166 of the second station 104 in response to commands issued by the first and second controllers 106, 108 to couple the first transmission line 200 to the neutral nodes 124, 164 of the first and second stations 102, 104.

[0065] The second, fourth, fifth and sixth disconnectors 118, 136, 140, 142 of the first station 102 and the first, fourth, fifth and sixth disconnectors 156, 176, 180, 182 of the second station 104 remain closed so that the first transmission line 200 can be used as a current return path for the second pole 250, as shown in FIG. Figure 4c as shown in .

[0066] Once the system 100 has been reconfigured in this manner, the second and fourth AC breakers 132, 172 are closed (if they were previously open) or the DC power in the healthy pole is ramped up again (if it was previously ramped down) in response to commands issued by the first and second controllers 106, 108 to recouple the second and fourth HVDC converters 130, 170 to their respective AC transmission terminals (if necessary) and allow current to flow in the system 100 at Figure 4c The current flows in the direction indicated by the solid arrow, wherein the first transmission line 200 serves as a current return path.

[0067] The leakage current generated by the fault can now flow through the ground, the first and second disconnectors 116 and 118 of the first station 102, the fourth disconnector 122, the first HVDC converter 110, the first high resistance path provided by the resistor 310, the first transmission line 200, and the first and second disconnectors 156 and 158 of the second station 104, as shown in FIG. Figure 4c However, due to the high resistance of resistor 310, the leakage current that flows is extremely small.

[0068] Since the leakage current flowing in the first disconnector 116 and the third disconnector 122 of the first station 102 and in the second disconnector 158 and the third disconnector 162 of the second station 104 is small once the NBGS 312 has been opened, the second disconnector 158 of the second station 104 can now be opened in response to the command issued by the second controller 108, such as Figure 4d , so as to isolate the faulty third HVDC converter 150 from the first transmission line 200. The third disconnector 162 of the second station is also opened in response to the command issued by the second controller 108, so as to isolate the faulty third HVDC converter 150 from the fourth HVDC converter 170. The first and third disconnectors 116, 122 of the first station 102 are also opened in response to the command issued by the first controller 106, so as to isolate the first HVDC converter 110 from the first transmission line 200 and the second HVDC converter 130. With the second and third disconnectors 158, 162 and the first and third disconnectors 116, 122 of the second station 104 opened, no leakage current flows, and the NBGS 312 can be restored to its normally closed state in response to the command issued by the first controller 106.

[0069] In the system 300, the healthy pole (the second pole 250 in the above example) can be used to provide a fault-free power supply very quickly after the fault is detected. Figure 4bThis is because fewer disconnects are required to be opened before the healthy pole 250 is available for use, because the leakage current in the system 300 is greatly reduced by the resistor 310 and can therefore be safely handled by the second disconnect 158 ​​of the second station 104 after the system 300 has been reconfigured to operate in the single-pole configuration, meaning that the first disconnect 116 and the third disconnect 122 of the first station 102 and the second disconnect 158 ​​and the third disconnect 162 of the second station 104 can be opened even while the healthy pole 250 is conducting. Therefore, the process of returning the healthy pole to service is not overly constrained by the time required to open the first disconnect 116 and the third disconnect 122 of the first station 102 and the second disconnect 158 ​​and the third disconnect 162 of the second station 104, because the healthy pole can be returned to service while the first disconnect 116 and the third disconnect 122 of the first station 102 and the second disconnect 158 ​​and the third disconnect 162 of the second station 104 are closed, with the second disconnect 158 ​​subsequently opening. Furthermore, the use of high speed switches 302, 306 in the transfer busses 126, 166 facilitates rapid reconfiguration of the system 300 to a single pole configuration.

[0070] Figure 5 is a flow chart illustrating the steps of a method 500 performed by the first and second controllers 106 , 108 to reconfigure the system 300 to operate in a unipolar configuration.

[0071] In a first step 502, a fault to ground is detected by the first controller 106 or the second controller 108 in one of the HVDC converters 110, 130, 150, 170 of the system 300. In the above example, the fault occurs in the third HVDC converter 150, which is an integral part of the second station 104 and can therefore be detected by the second controller 108. It will be appreciated that corresponding steps will be performed using appropriate AC breakers, disconnectors and high-speed switches of the system 300 in the event that a fault is detected in one of the other HVDC converters 110, 130, 170.

[0072] At step 504, the first and second controllers 106, 108 issue commands to the first, second, third and fourth AC breakers 112, 132, 152, 172 to open them. Alternatively, the first and second controllers 106, 108 may issue commands only to the AC breakers associated with the faulty pole (in this case, the first and third AC breakers 112, 152 of the first pole 240) to open them, while issuing commands to the HVDC converters 130, 170 of the healthy poles to ramp down their DC power to minimize the DC current flowing in the healthy poles.

[0073] At step 506, the first controller 106 issues a command to the NBGS 312 to cause it to open.

[0074] At step 508, the first and second controllers 106, 108 send control signals to the first high-speed switch 302 of the link 126 of the first station 102 and the third high-speed switch 306 of the link 166 of the second station 104 to close those switches, thereby reconfiguring the system 300 to operate in a single-pole link configuration using the first transmission line 200 as a current return path as described above.

[0075] At step 510, the first and second controllers 106, 108 issue commands to the second and fourth AC breakers 132, 172 to close them, thereby re-coupling the second and fourth HVDC converters 130, 170 to their respective AC transmission terminals. Alternatively, if the second and fourth AC breakers are not opened, but the DC power in the healthy pole is ramped down after the detection of the fault, the first and second controllers 106, 108 may issue commands to the HVDC converters 130, 170 of the healthy poles to ramp up their DC power, thereby increasing the current in the healthy poles.

[0076] At step 512, the second controller 108 issues a control signal to the second and third disconnects 158, 162 of the second station 104 so as to open the second and third disconnects 158, 162, thereby isolating the third HVDC converter 150 from the first transmission line 200 and from the fourth HVDC converter 170. The first controller 106 also issues a control signal to the first and third disconnects 116, 122 of the first station 102 so as to open the first and third disconnects 116, 122, thereby isolating the first HVDC controller 110 from the first transmission line 200 and from the second HVDC converter 130.

[0077] At step 510, the first controller 106 issues a command to the NBGS 312 to close it.

[0078] The above principles can also be applied to a bipolar configuration with a dedicated metal return (DMR), as will now be referred to. Figure 6-8 To explain.

[0079] Now refer to Figure 6 , a HVDC transmission system with dedicated metallic loops is generally shown at 600. System 600 is similar to that described above and Figure 3 The system 300 shown in FIG. 600 shares many common elements, and therefore common reference numerals are used to represent common elements of systems 300 and 600. For the sake of clarity and brevity, only those features of system 600 that differ from system 600 will be described herein.

[0080] System 600 is similar to system 300 except that system 600 includes a dedicated metallic return (DMR) 610 operable to couple the neutral node 124 of the first station 102 to the corresponding neutral node 164 of the second station. DMR 610 may be, for example, an overhead, underground, or submarine transmission line of a conductive metallic material.

[0081] A DMR transfer breaker (DMRTB) 612 is provided between the link 126 of the first station 102 and the neutral node 124, and a metallic return transfer breaker (MRTB) 614 is provided between the neutral node 124 of the first station 102 and the second end of the DMR 610. In normal operation of the system 600 (before any fault in the system 600), the DMRTB 612 is closed, and the MRTB 614 is open.

[0082] In normal operation, system 600 operates in the same manner as system 300 described above.

[0083] If a single-phase fault to ground is formed in one of the HVDC converters 110, 130, 150, 170 during operation of the system 600, a number of actions are taken, as will be described below with reference to Figures 7a-7d As stated, Figures 7a-7d is after the detection of the ground fault in the third HVDC converter 150 Figure 6 It will of course be appreciated that the system 600 operates in the same manner as when a fault is detected in any of the other HVDC converters 110, 130, 170.

[0084] After detection of a fault in the third HVDC converter 150, the first and third AC breakers are opened in response to commands issued by the first and second controllers 106, 108 to isolate the first and third HVDC converters 110, 130 from their respective AC transmission terminals, thereby disabling the first pole 240 in which the fault occurred. Figure 3 Unlike in the system 300, the second and fourth AC breakers 132, 172 are not opened, but remain closed, thereby permitting the healthy second pole 250 to continue to operate with the return current flowing in the DRM 510, as shown in FIG. Figure 7a Indicated by solid arrows.

[0085] By isolating the first and third HVDC converters 110, 150 in this manner, the stored energy circulates in the system 600. Due to the ground fault in the third HVDC converter 150, the leakage current flows through the ground, the first and second disconnectors 116 and 118 of the first station 102, the fourth disconnector 122, the first HVDC converter 110, the second low resistance path provided by the closed NBGS 312, the first transmission line 200, and the first and second disconnectors 156 and 158 of the second station 104, as shown in FIG. Figure 7a Indicated by the dotted arrow.

[0086] Healthy pole 250 can now be reconfigured to operate as a single pole with a metal return (using DMR 610 as a return path), such as Figure 7b Thus, the first controller 106 may issue control signals to the MRTB 614 and the DMRTB 612 to close the DMRTB 612 and then open it, thus enabling the DMR 610 to provide a dedicated current return path between the first and second stations 102 , 104 .

[0087] After the first and third AC breakers 112 , 152 have been opened to isolate the first and third HVDC converters 110 , 150 from their respective AC transmission terminals, the NBGS 312 opens in response to a command issued by the first controller 106 , effectively activating a first high resistance path to ground including the resistor 310 .

[0088] The system 600 can now be reconfigured to use the first transmission line 200 as a current return path if necessary. To reconfigure the system 600 in this manner, the first and second controllers 106, 108 issue commands to close the first high-speed switch 302 of the link 126 of the first station 102 and the third high-speed switch 306 of the link 166 of the second station 104. In addition, the first controller 106 issues a command to the MRTB 614 to open it, thereby isolating the DMR 610 from the first station 102 so that it can no longer serve as a current return path. The first controller 106 then issues a command to the DMRTB 612 to close the DMRTB 612.

[0089] The second, fourth, fifth and sixth disconnectors 118, 136, 140, 142 of the first station 102 and the first, fourth, fifth and sixth disconnectors 156, 176, 180, 182 of the second station 104 remain closed so that the first transmission line 200 can be used as a current return path for the second pole 250, as shown in FIG. Figure 7d Thus, the system 600 operates in a unipolar link configuration.

[0090] The leakage current can now flow through ground, the first and second disconnects 116 and 118 of the first station 102, the fourth disconnect 122, the first HVDC converter 110, the first high resistance path provided by the resistor 310, the first transmission line 200, and the first and second disconnects 156 and 158 of the second station 104, as shown in FIG. Figure 7c However, due to the high resistance of resistor 310, the leakage current that flows is extremely small.

[0091] Because the leakage current flowing in the first disconnector 116 and the third disconnector 122 of the first station 102 and in the second disconnector 158 and the third disconnector 162 of the second station 104 is small once the NBGS 312 has been opened, the second disconnector 158 of the second station 104 can now be opened in response to the command issued by the second controller 108 to isolate the faulty third HVDC converter 150 from the first transmission line 200. The third disconnector 162 of the second station is also opened in response to the command issued by the second controller 108 to isolate the third HVDC converter 150 from the fourth HVDC converter 130. The first and third disconnectors 116, 122 of the first station 102 are also opened in response to the command issued by the first controller 106 to isolate the first HVDC converter 110 from the first transmission line 200 and from the second HVDC converter 130. With the second and third disconnectors 158, 162 of the second station 104 and the first and third disconnectors 116, 122 of the first station 102 open, no leakage current flows, as Figure 7d , and the NBGS 312 is capable of returning to its normally closed state in response to a command issued by the first controller 106.

[0092] Figure 8 is a diagram illustrating the steps performed by the first and second controllers 106, 108 to reconfigure the system 600 to follow Figure 7d Flow chart of the steps of method 800 operating in a unipolar configuration of the present invention (wherein the first transmission line 200 is used as a current return path).

[0093] In a first step 802, a fault to ground is detected by the first controller 106 or the second controller 108 in one of the HVDC converters 110, 130, 150, 170 of the system 300. In the above example, the fault occurs in the third HVDC converter 150, which is an integral part of the second station 104 and can therefore be detected by the second controller 108. It will be appreciated that corresponding steps will be performed using appropriate AC breakers, disconnectors and high-speed switches of the system 600 in the event that a fault is detected in one of the other HVDC converters 110, 130, 170.

[0094] At step 804, the first and second controllers 106, 108 issue commands to the first and third AC circuit breakers 112, 152 to open them.

[0095] At step 806, the first controller 106 issues a command to the NBGS 312 to cause it to open.

[0096] To reconfigure the system 600 to use the first transmission line 200 as a current return path, at step 808, the first and second controllers 106, 108 issue commands to close the first high-speed switch 302 of the link 126 of the first station 102 and the third high-speed switch 306 of the link 166 of the second station 104. The first controller 106 also issues commands to the DMRTB 612 and the MRTB 614 to open them.

[0097] At step 810, the second controller 108 sends a control signal to the second and third disconnects 158, 162 of the second station 104 so as to open the second and third disconnects 158, 162, thereby isolating the third HVDC converter 150. The first controller 106 may also send a control signal to the first and third disconnects 116, 122 of the first station 102 so as to open the first and third disconnects 116, 122, thereby isolating the first HVDC converter 110.

[0098] At step 812, the first controller 106 issues a command to the NBGS 312 to close it.

[0099] The systems and methods described above are suitable for use with different types of HVDC converters. For example, the HVDC converters 110, 130, 150, 170 may be line-commutated converters (LCCs) or voltage-source converters (VSCs).

[0100] As will be appreciated from the foregoing, the systems and methods described above provide a cost-saving mechanism for rapidly reconfiguring a bipolar HVDC transmission system in the event of a detected fault.

[0101] It should be noted that the above embodiments illustrate rather than limit the present invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in the claims, and "a" or "an" does not exclude a plurality. Any reference in the claims will not be construed as limiting the scope thereof.

Claims

1. A high voltage direct current (HVDC) transmission system, comprising: a first station comprising a first high voltage direct current (HVDC) converter and a second high voltage direct current (HVDC) converter coupled in series via a first disconnect and a second disconnect; a second station comprising a third high voltage direct current (HVDC) converter and a fourth high voltage direct current (HVDC) converter connected in series, wherein a neutral node coupling the third high voltage direct current (HVDC) converter to the fourth high voltage direct current (HVDC) converter is coupled to ground; a first transmission line connecting a positive node of the first HVDC converter to a corresponding positive node of the third HVDC converter, wherein a first pole of the system comprises the first HVDC converter, the third HVDC converter and the first transmission line; a second transmission line connecting a negative node of the second HVDC converter to a corresponding negative node of the fourth HVDC converter, wherein a second pole of the system comprises the second HVDC converter, the fourth HVDC converter and the second transmission line; Features: A neutral node between the first disconnect and the second disconnect is coupled to a parallel combination of a resistor and a neutral bus ground switch for coupling the neutral node to ground.

2. The high voltage direct current (HVDC) transmission system of claim 1, wherein: the first station comprising a first link coupled to the first transmission line at a first end and to the second transmission line at a second end, the first link comprising a first high-speed switch and a second high-speed switch connected in series, wherein a third node connecting the first high-speed switch and the second high-speed switch is coupled to the neutral node of the first station, and wherein the first high-speed switch and the second high-speed switch are selectively operable to couple the first transmission line or the second transmission line to the neutral node of the first station; The second station includes a second link coupled to the first transmission line at a first end and to the second transmission line at a second end, the second link including a third high-speed switch and a fourth high-speed switch, wherein a fourth node connecting the third high-speed switch and the fourth high-speed switch is coupled to the neutral node of the second station, and wherein the second high-speed switch and the third high-speed switch are selectively operable to couple the first transmission line or the second transmission line to the neutral node of the second station.

3. The high voltage direct current (HVDC) transmission system according to claim 1 or 2, wherein: The neutral node of the second station is coupled to a surge arrester for coupling the neutral node to ground.

4. The high voltage direct current (HVDC) transmission system according to any one of claims 1 to 2, wherein: The first, second, third and fourth HVDC converters include line-commutated converters.

5. The high voltage direct current (HVDC) transmission system according to any one of claims 1 to 2, wherein: The first, second, third and fourth HVDC converters include voltage source converters.

6. The high voltage direct current (HVDC) transmission system of any one of claims 1-2, further comprising a dedicated metallic return coupling the neutral node of the first station to the neutral node of the second station.

7. A high voltage direct current (HVDC) converter station (102), comprising: A first high voltage direct current (HVDC) converter and a second high voltage direct current (HVDC) converter, which are coupled in series via a first disconnect and a second disconnect, the first high voltage direct current (HVDC) converter comprising a positive node configured to be coupled to a first transmission line, the second high voltage direct current (HVDC) converter comprising a negative node configured to be coupled to a second transmission line, characterized in that: A neutral node between the first disconnect and the second disconnect is coupled to a parallel combination of a resistor and a neutral bus ground switch for coupling the neutral node to ground.

8. The high voltage direct current (HVDC) converter station of claim 7, further comprising: A link configured to be coupled to the first transmission line at a first end and to be coupled to the second transmission line at a second end, the link comprising a first high-speed switch and a second high-speed switch connected in series, wherein a node connecting the first high-speed switch and the second high-speed switch is coupled to the neutral node, and wherein the first high-speed switch and the second high-speed switch are selectively operable to couple the first transmission line or the second transmission line to the neutral node of the high-voltage direct current (HVDC) converter station.

9. A high voltage direct current (HVDC) converter station according to claim 7 or 8, wherein: The first high voltage direct current (HVDC) converter and the second high voltage direct current (HVDC) converter include line commutated converters.

10. A high voltage direct current (HVDC) converter station according to any one of claims 7 to 8, wherein: The first high voltage direct current (HVDC) converter and the second high voltage direct current (HVDC) converter include voltage source converters.

11. A method for reconfiguring a system according to any one of claims 1 to 5 upon detection of a fault to ground in one of a first high voltage direct current (HVDC) converter, a second high voltage direct current (HVDC) converter, a third high voltage direct current (HVDC) converter or a fourth high voltage direct current (HVDC) converter, the method comprising: detecting the fault; isolating at least the high voltage direct current (HVDC) converter including the faulty pole of the high voltage direct current (HVDC) converter in which the fault occurs from the corresponding AC transmission terminal; opening the neutral bus ground switch so that the leakage current in the system flows through the resistor; coupling the transmission line of the faulted pole to the neutral nodes of the first station and the second station such that the coupled transmission line provides a return path for current in a healthy pole where the fault did not occur; as well as The high voltage direct current (HVDC) converter of the faulty pole is isolated from the high voltage direct current (HVDC) converter of the healthy pole and from the coupled transmission line.

12. The method of claim 11, further comprising: After isolating the high voltage direct current (HVDC) converter of the faulty pole from the high voltage direct current (HVDC) converter of the healthy pole and from the coupled transmission line, the neutral bus ground switch is closed.

13. A method for reconfiguring the system of claim 6 upon detection of a fault to ground in one of a first high voltage direct current (HVDC) converter, a second high voltage direct current (HVDC) converter, a third high voltage direct current (HVDC) converter or a fourth high voltage direct current (HVDC) converter, the method comprising: detecting the fault; isolating the high voltage direct current (HVDC) converter including the faulty pole of the high voltage direct current (HVDC) converter in which the fault occurs from the corresponding AC transmission terminal; as well as The neutral bus ground switch is opened to allow leakage current in the system to flow through the resistor.

14. The method of claim 13, further comprising: coupling the transmission line of the faulty pole to the neutral nodes of the first station and the second station such that the coupled transmission line provides a return path for current in a healthy pole; as well as The high voltage direct current (HVDC) converter in which the fault occurs is isolated from the high voltage direct current (HVDC) converter of the healthy pole and from the coupled transmission line.

15. The method of claim 14, further comprising: Close the neutral bus ground switch.

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