System and method for managing faults in power transmission network

Abstract

There is provided a system (800A, 800B, 800C) and method for managing a fault (400) on a transmission line. The system includes an electrical branch (805), a diode arrangement (610), and a controller (200). The electrical branch includes an inductor (321) and a circuit breaker (311) connected in series between a first terminal (810) and a second terminal (820). The circuit breaker (311) is configured to connect or disconnect the first terminal (810) and the second terminal (820). The diode arrangement (610) comprises at least one diode, where a first end of the diode arrangement is connected to the electrical branch and a second end of the diode arrangement is connected to the third terminal (830). The controller (200) is configured to receive an indication signal, wherein the indication signal is indicative of a forward current in the diode arrangement (610). The controller is further configured to, in response to receiving the indication signal, issue a command to the circuit breaker (311) to open the first terminal (810) and the second terminal (820).

Description

Technical Field

[0001] The present invention relates to systems and methods for managing faults on transmission lines, and more particularly, to systems and methods for controlling circuit breakers in a power transmission network. Background Art

[0002] In high-voltage direct current (HVDC) power transmission networks, alternating current (AC) power is typically converted to direct current (DC) power for transmission via a power transmission medium, such as overhead lines, submarine cables, and / or underground cables. This conversion eliminates the need to compensate for the AC reactive / capacitive loading effects imposed by the power transmission medium (i.e., transmission lines or cables), reduces the cost per kilometer of line and / or cable, and therefore becomes cost-effective when power needs to be transmitted over long distances.

[0003] Conversion between DC power and AC power is used where DC and AC power need to be interconnected, such as between an AC grid and an HVDC transmission line. In a power transmission network, power conversion components, also known as converter stations (i.e., power converters, power inverters, etc. in converter stations), are required at each interface or interconnection between AC power and DC power to achieve the required conversion from AC to DC or from DC to AC.

[0004] An HVDC transmission network typically comprises an HVDC transmission line (i.e., the power transmission medium), with converter stations connected on either side of the HVDC transmission line. The converter stations are typically connected to the AC network via switchgear. Thus, a typical HVDC transmission network will have two converter stations, one on each side of the HVDC transmission line, interfacing the HVDC transmission line to respective AC networks or the same AC network. Summary of the Invention

[0005] Faults can occur in HVDC transmission lines. Over time, the fault current in the DC transmission line tends to continue to rise. In conventional HVDC systems, AC circuit breakers between converter stations and the corresponding AC network are opened to manage faults in the HVDC transmission line. The present inventors have recognized that in such systems, the response to faults tends to be slow. The present inventors have also recognized that DC circuit breakers between the converter stations and the DC transmission line can be used to isolate the fault more quickly.

[0006] Furthermore, in multi-terminal DC systems, conventional methods tend to require end-to-end communication in order to decide which circuit breaker(s) to open (often referred to as "discrimination"). Such end-to-end communication tends to be slow, and thus conventional methods for fault management tend to be unsuitable for detecting and distinguishing faults on transmission lines in multi-terminal DC systems.

[0007] In view of these considerations and emerging grid demands, it is desirable to develop methods that provide improved fault management of faults in multi-terminal DC systems.

[0008] According to a first aspect, a system for managing faults on an HVDC transmission line is provided. The system includes an electrical branch, a diode arrangement, and a controller. The electrical branch includes an inductor and a circuit breaker connected in series between a first terminal and a second terminal. The circuit breaker is configured to connect or disconnect the first terminal and the second terminal. The diode arrangement includes at least one diode, wherein a first end of the diode arrangement is connected to the electrical branch, and a second end of the diode arrangement is connected to a third terminal. The controller is configured to receive an indication signal, wherein the indication signal indicates a forward current in the diode arrangement. The controller is further configured to, in response to receiving the indication signal, issue a command to the circuit breaker to disconnect the first terminal and the second terminal.

[0009] In HVDC systems, faults should be detected and isolated as quickly as possible, otherwise the fault current will continue to rise, leading to higher demands on circuit breakers. Therefore, it is desirable to manage faults without transmitting information from one side of the HVDC transmission line to the other. Furthermore, in multi-terminal DC systems, where power converter stations can be connected to multiple transmission lines, it is desirable not only to detect faults, but also to determine which of the transmission lines contains the fault so that only the appropriate (one or more) circuit breakers are opened (triggered) in order to isolate the fault. Determining which line contains the fault can be referred to as selectivity or discrimination. A fault management system that cannot discriminate can cause an otherwise healthy transmission line to be unintentionally or accidentally disconnected due to opening the wrong circuit breaker. Therefore, the discrimination capability of the fault management system should be highly reliable.

[0010] The present inventors have realised that by connecting a diode arrangement to the ends of a transmission line, the forward conduction of the diode arrangement can be used as a means of detecting faults on the transmission line and determining (ie identifying) which transmission line contains the fault.

[0011] Due to the detection of forward current, a circuit breaker can be triggered. This advantageously tends to allow determination that a fault has occurred on a particular transmission line without requiring information to be transmitted from one side of the transmission line to the other. In other words, it tends to be possible to determine that a fault has occurred, and on which transmission line, based solely on one side of the transmission line. This tends to increase the speed of determining which circuit breaker to open, and thus allows for much faster fault isolation compared to the conventional methods described above.

[0012] Furthermore, because only the diodes connected to the faulty transmission line will conduct, the probability of triggering the wrong circuit breaker tends to be greatly reduced or even zero. Thus, determining which circuit breaker to open (i.e., selectivity or discrimination) tends to be highly reliable compared to other conventional methods.

[0013] The third terminal may be connected to ground.

[0014] The third terminal can be connected to the neutral wire.

[0015] The third terminal can be connected to the negative line.

[0016] The system may further include a second electrical branch comprising a second inductor and a second circuit breaker connected in series between the fourth terminal and the fifth terminal. The second circuit breaker may be configured to connect or disconnect the fourth terminal and the fifth terminal. The third terminal may be connected to the second electrical branch. The controller may be further configured to, in response to receiving the indication signal, issue a command to the second circuit breaker to disconnect the third terminal and the fourth terminal.

[0017] The diode arrangement may be oriented such that when a positive voltage is present on the electrical branch, the diode arrangement is reverse biased and current blocking.

[0018] The diode arrangement may be oriented such that when a negative voltage is present on the electrical branch, the diode arrangement is reverse biased and current blocking.

[0019] The system may further comprise a measuring device connected to the diode arrangement and the controller.The measuring device may be configured to measure a forward current through the diode arrangement; determine an indication signal based on the forward current; and output the indication signal.

[0020] The measuring device may comprise a current transformer or a Rogowski coil, which is used to measure the forward current.

[0021] The indication signal may be a signal based on the forward current through the diode arrangement or a time derivative of said forward current.

[0022] The indication signal may be a signal proportional to the forward current.

[0023] The indication signal may be a signal determined by comparing the forward current with a threshold value.

[0024] The indication signal may be an analog signal.

[0025] The indication signal may be a digital signal.

[0026] The controller may be configured to issue the command in less than 150 microseconds from receipt of the indication signal.

[0027] In the electrical branch, the inductor may be connected in series between the first terminal and the circuit breaker, the circuit breaker may be connected in series between the inductor and the first end of the diode arrangement, and the first end of the diode arrangement may be connected in series between the circuit breaker and the second terminal.

[0028] In the electrical branch, the inductor may be connected in series between the first end and the first terminal of the diode arrangement, the first end of the diode arrangement may be connected in series between the inductor and the circuit breaker, and the circuit breaker may be connected in series between the first end and the second terminal of the diode arrangement.

[0029] In the electrical branch, the circuit breaker may be connected in series between the first terminal and the inductor, the inductor may be connected in series between the circuit breaker and the first end of the diode arrangement, and the first end of the diode arrangement may be connected in series between the inductor and the second terminal.

[0030] The diode arrangement may comprise a plurality of diodes connected in series.

[0031] The diode arrangement may be configured to operate at a voltage of at least 100 kV.

[0032] The controller may be further configured to issue a second command to another circuit breaker external to the system in response to receiving the indication signal.

[0033] The second command may configure the additional circuit breaker from the normal state to an armed state, wherein in the armed state the additional circuit breaker is configured to open faster than in the normal state.

[0034] A second command may control the opening of another circuit breaker.

[0035] In a second aspect, there is provided an HVDC system comprising a converter station, an HVDC transmission line, and the system of any preceding aspect. A first end of the HVDC transmission line is connected to a second terminal, and the converter station is connected to the first terminal.

[0036] The HVDC transmission line may have a length of at least 100 km.

[0037] The HVDC transmission line may be configured to operate at a voltage of at least 300 kV.

[0038] In a third aspect, there is provided a multi-terminal HVDC transmission network, comprising a first converter station; a second converter station; a third converter station; a first HVDC transmission line; and a second HVDC transmission line. The first converter station is connected to a first end of the first HVDC transmission line via a first subsystem; the second converter station is connected to a second end of the first HVDC transmission line via a second subsystem; the first converter station is connected to a first end of the second HVDC transmission line via a third subsystem; and the third converter station is connected to a second end of the second HVDC transmission line via a fourth subsystem; wherein the first, second, third, and fourth subsystems each comprise a system according to any of the preceding aspects.

[0039] In a fourth aspect, a method for managing faults on an HVDC transmission line using a system is provided, the system comprising an electrical branch and a diode arrangement, wherein the electrical branch comprises an inductor and a circuit breaker connected in series between a first terminal and a second terminal, wherein the circuit breaker is configured to connect or disconnect the first terminal and the second terminal, and wherein the diode arrangement comprises at least one diode, wherein a first end of the diode arrangement is connected to the electrical branch and a second end of the diode arrangement is connected to a third terminal. The method comprises receiving, by a controller, an indication signal, wherein the indication signal indicates a forward current in the diode arrangement; and, in response to receiving the indication signal, issuing, by the controller, a command to the circuit breaker to disconnect the first terminal and the second terminal.

[0040] In general, the controllers disclosed herein are intended to be configured to perform the methods described herein.

[0041] According to a fifth aspect, there is provided a computer program comprising instructions which, when executed by a processor of a controller, cause the controller to perform the method of the fourth aspect.

[0042] According to a sixth aspect, there is provided a non-transitory computer-readable storage medium comprising the computer program of the fifth aspect.

[0043] It will be appreciated that specific features of different aspects of the present invention share the technical effects and benefits of corresponding features of other aspects of the present invention. More particularly, the technical effects and benefits of the system, HVDC system, multi-terminal HVDC transmission network, computer program and non-transitory computer-readable medium are shared by the method of the present invention.

[0044] It will also be appreciated that the use of the terms "first," "second," etc., are merely intended to help distinguish between similar features and are not intended to indicate the relative importance of one feature to another, unless otherwise stated.

[0045] Within the scope of the present application, it is expressly intended that the various aspects, embodiments, examples and alternatives set forth in the preceding paragraphs and claims and / or the following description and drawings, and in particular the individual features thereof, may be employed independently or in any combination. That is, all embodiments and all features of any embodiment may be combined in any manner and / or combination, unless such features are incompatible. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 is a schematic diagram of an example of a DC power transmission network;

[0047] Figure 2 is a schematic diagram of a controller for controlling a circuit breaker and / or a power transmission network;

[0048] Figure 3 is a schematic diagram of an example of a multi-terminal DC power transmission network;

[0049] Figure 4 is a schematic diagram of a portion of a multi-terminal DC power transmission network;

[0050] Figure 5 is a graph showing voltage at various points in a conventional multi-terminal DC power transmission network plotted against time before and after a fault;

[0051] Figure 6 is a schematic diagram of an improved portion of a multi-terminal DC power transmission network;

[0052] Figure 7 is a graph showing voltage and current plotted against time at various points in an improved multi-terminal DC power transmission network before and after a fault;

[0053] Figure 8A is a schematic diagram of an example of the first system;

[0054] Figure 8B is a schematic diagram of an example of the second system;

[0055] Figure 8C is a schematic diagram of an example of a third system;

[0056] Figure 9 are schematic diagrams of various examples of the system; and

[0057] Figure 10 is a schematic diagram of an example of a multi-terminal power transmission network divided into zones. DETAILED DESCRIPTION

[0058] Figure 1An example of a power transmission network 100 is generally shown. The illustration is not intended to be limited to representing a particular power transmission scheme, such as a unipolar or bipolar high voltage direct current (HVDC) transmission network, but is also provided as an illustration to assist in understanding the operating principles of the power transmission network of the present invention. In this manner, for example, the power transmission network 100 may generally represent a unipolar or bipolar scheme, or may represent a multi-terminal power transmission scheme. Thus, while certain features in the illustration are shown as being connected to one another using a particular number of connections, it will be understood that this is not intended to be limiting, but rather illustrates the general connections between features / components. Relatedly, the relative sizes or distances between components understood in the illustration are also not intended to be limiting. Thus, it will be understood that, for example, the principles and features discussed in the power transmission network 100 and herein may be applied to a variety of power transmission schemes including Figure 2 The controller 200 of the network.

[0059] Power transmission network 100 includes a first power converter 110 (also referred to as a converter, inverter, inverter station, etc.), a second power converter 120 , a transmission medium 130 , a first AC network 140 , and a second AC network 150 .

[0060] The power converters 110, 120 are configured to convert AC power to DC power, essentially acting as rectifiers, or to convert DC power to AC power, essentially acting as inverters. In the case of a unipolar system, the power converters 110, 120 may each include a single converter, or in the case of a bipolar system, two converters. The power converters 110, 120 may represent multiple converter stations arranged as a multi-terminal power transmission system. Generally speaking, the first power converter 110 includes a first AC side 110a and a first DC side 110b. Generally speaking, the second power converter 120 includes a second AC side 120a and a second DC side 120b.

[0061] The first power converter 110 is connected to a first AC network 140. The first AC network 140 is connected to a first AC side 110a of the first power converter 110. The second power converter 120 is connected to a second AC network 150. The second AC network 150 is connected to a second AC side 120a of the second power converter 120.

[0062] The first AC network 140 and / or the second AC network 150 may be an electric power transmission system including power generation equipment, transmission equipment, distribution equipment, and electrical loads. The first AC network 140 and / or the second AC network 150 may include a renewable power generation network, such as a wind power generation network, a solar power generation network, or a biomass power generation network. The first AC network 140 or the second AC network 150 may be a consumer network, or a network comprising a mix of consumers and generators. As a non-limiting example, for example, the first AC network 140 may be a power generation network, wherein the second AC network 150 is a network comprising a mix of consumers and generators. In a specific example, the power converters 110 and 120 may be geographically remote. For example, the first power converter 110 may be located on an offshore platform connected to a wind farm, while the second power converter 120 may be located onshore.

[0063] A power transmission medium 130 connects first power converter 110 and second power converter 120. Power transmission medium 130 is connected between first DC side 110b of first power converter 110 and second DC side 120b of second power converter 120. Power transmission medium 130 may include cables and other electrical components used to connect first and second power converters 110, 120. For example, power transmission medium 130 may include a conductor that provides a first electrode and / or a conductor that provides a second electrode. A neutral arrangement may also be provided to interconnect first and second power converters 110, 120. Power transmission medium 130 provides a medium through which DC power is transferred between power converters 110, 120.

[0064] The operation of power transmission network 100 can be generally described as follows. First AC network 140 generates AC power that is provided to first power converter 110 at first AC side 110a. First power converter 110 converts the received AC power into DC power for transmission to transmission medium 130. DC power is transmitted from first DC side 110b of first power converter 110 to transmission medium 130. Second DC side 120b of second power converter 120 receives DC power from transmission medium 130. Second power converter 120 converts the received DC power into AC power. For example, AC power is then provided from second AC side 120a of second power converter 120 to second AC network 150 for consumption.

[0065] When second power converter 120 receives power from transmission medium 130 , first power converter 110 transmits power from AC network 140 to transmission medium 130 such that a nominal voltage of transmission medium 130 is maintained.

[0066] Additionally, in some cases, first power converter 110 may also receive power from transmission medium 130. Thus, first power converter 110 may be configured to transfer active power or reactive power in either direction into or out of first AC network 140. Second power converter 120 may also be configured to transfer active power or reactive power in either direction into or out of second AC network 150.

[0067] It will be appreciated that various other electrical components may be located at any particular location or with any particular feature / component in the example power transmission network 100. These may include switches, transformers, resistors, reactors, surge arresters, harmonic filters, and other components known in the art.

[0068] It will be appreciated that the converter or power conversion component may comprise a variety of different technologies, such as a voltage source converter (e.g. using insulated gate bipolar transistor (IGBT) valves). Such a converter may generally be considered to use 'power electronics'. For example, a power electronics converter may comprise a multi-level voltage source converter.

[0069] It will be appreciated that cables used as power transmission media may include the following non-limiting examples of cross-linked polyethylene (XLPE) and / or heavily impregnated (MI) insulated cables. Such cables may include a conductor (e.g., copper or aluminum) surrounded by an insulating layer. The dimensions of the cable and its associated layers may vary depending on the specific application (and particularly the operating voltage requirements). In applications such as submarine installations, the cable may also include reinforcement or 'armoring'. The cable may also include a sheath / shield that is grounded at one or more locations.

[0070] Furthermore, it will be appreciated that the power transmission network 100 can be used with a three-phase power system. In a three-phase power system, three conductors supply first, second, and third phase AC power to consumers, respectively. Each of the first, second, and third phases will typically have a voltage or current of equal magnitude, being 120° out of phase with one another.

[0071] In a three-phase power system, phase currents and voltages can be represented by three single-phase components: a positive-sequence component; a negative-sequence component; and a zero-sequence component. This is the positive-sequence component that rotates in phase with the power system. Therefore, in the preferred case, only positive-sequence voltages / currents will be present. It will be appreciated that imbalances in the magnitude or phase angle of the voltages or currents between the first, second, and third phases of the three-phase system can result in undesirable negative-sequence or zero-sequence components. Such imbalances can be caused, for example, by a fault condition in the first and second AC networks 140, 150.

[0072] The power transmission network 100 may operate using a method such as synchronized grid formation (SGFM), in which either or both of the power converters 110 , 120 act as a three-phase positive phase sequence AC voltage source after an impedance that operates at a frequency synchronized with the other SGFM sources connected to the power transmission network 100 .

[0073] The power transmission network 100 may also include a controller for controlling the operation of components of the power transmission network 100. For example, a controller may be provided to perform the methods described herein. For example, such a controller may control components associated with one or both of the first and second power converters 110, 120. Such a controller may be referred to as a controller component or a control component. The controller may be Figure 2 Controller 200.

[0074] Figure 2 An embodiment of a controller 200 is shown as it may be used to implement the methods described herein.

[0075] The controller 200 includes a memory 210 and at least one processor 220. The memory 210 includes computer-readable instructions that, when executed by the at least one processor 220, cause the controller 200 to perform the methods described herein.

[0076] The controller 200 includes a transceiver arrangement 230, which may include a separate transmitter 231 and a receiver 232. The transceiver arrangement 230 may be used to operatively communicate with other components or features of the embodiments described herein, either directly or via another interface such as a network interface. The transceiver arrangement 230 may, for example, use the transmitter 231 and the receiver 232 to send and receive control signals. The control signals may include or define electrical control parameters, such as a reference current or a reference voltage.

[0077] At least one processor 220 is configured to execute computer-readable instructions and / or perform logical operations. The at least one processor 220 may be a microcontroller, a microprocessor, a central processing unit (CPU), a field programmable gate array (FPGA), or a similar programmable controller. The controller may also include a user input device and / or an output device. The processor 220 is communicatively coupled to the memory 210 and may be coupled to the transceiver 230.

[0078] The memory 210 may be a computer-readable storage medium. For example, the memory 210 may include a non-volatile computer storage medium. For example, the memory 210 may include a hard disk drive, a flash memory, etc.

[0079] Although Figure 2Not shown, but the controller 200 may further include a user input device interface and / or a user output device interface, which may allow visual, auditory or tactile input / output. Examples of user input devices include, but are not limited to, interfaces for electronic displays, touch screens, keyboards, mice, speakers, and microphones.

[0080] Figure 3 An example of a multi-terminal power transmission network 300 is shown. The multi-terminal power transmission network may be a high voltage direct current (HVDC) transmission network 300.

[0081] Multi-terminal HVDC transmission network 300 includes a first converter station 310, a second converter station 320, a third converter station 330, and a fourth converter station 340. As described above, each converter station 310, 320, 330, 340 includes a power converter having an AC side and a DC side. Specifically, first converter station 310 includes first power converter 110, and second converter station 320 includes second power converter 120. Third converter station 330 includes third power converter 350. Fourth converter station 340 includes fourth power converter 360.

[0082] The multi-terminal HVDC transmission network 300 further includes a first transmission line 331, a second transmission line 332, a third transmission line 333, and a fourth transmission line 334. Each of the transmission lines 331, 332, 333, 334 is a power transmission medium 130 as described above. Each transmission line 331, 332, 333, 334 has a first end 331a, 332a, 333a, 334a and a second end 331b, 332b, 333b, 334b.

[0083] A circuit breaker and an inductor are connected to each end (or end portion) of each transmission line 331, 332, 333, and 334. Specifically, a first circuit breaker 311 and a first inductor 321 are connected to the first end 331a of the first transmission line 331. A second circuit breaker 312 and a second inductor 322 are connected to the second end 331b of the first transmission line 331. A third circuit breaker 313 and a third inductor 323 are connected to the first end 333a of the third transmission line 333. A fourth circuit breaker 314 and a fourth inductor 324 are connected to the first end 332a of the second transmission line 332. A fifth circuit breaker 315 and a fifth inductor 325 are connected to the second end 333b of the third transmission line 333. A sixth circuit breaker 316 and a sixth inductor 326 are connected to the second end 332b of the second transmission line 332. The seventh circuit breaker 317 and the seventh inductor 327 are connected to the first end 334a of the fourth transmission line 334. The eighth circuit breaker 318 and the eighth inductor 328 are connected to the second end 334b of the fourth transmission line 334.

[0084] 326. The first converter station 310 is connected to the second converter station 320 via a series connection of a first inductor 321, a first circuit breaker 311, a first transmission line 331, a second circuit breaker 312, and a second inductor 322. The first converter station 310 is also connected to the third converter station 330 via a series connection of a third inductor 323, a third circuit breaker 313, a third transmission line 333, a fifth circuit breaker 315, and a fifth inductor 325. The second converter station 320 is connected to the fourth converter station 340 via a series connection of a fourth inductor 324, a fourth circuit breaker 314, a second transmission line 332, a sixth circuit breaker 316, and a sixth inductor 326. The third converter station 330 is connected to the fourth converter station 340 via a series connection of a seventh inductor 327, a seventh circuit breaker 317, a fourth transmission line 334, an eighth circuit breaker 318, and an eighth inductor 328. Each converter station 310 , 320 , 330 , 340 is thus connected to two other converter stations. In this way, the converter stations 310 , 320 , 330 , 340 and the transmission lines 331 , 332 , 333 , 334 form a multi-terminal HVDC transmission network 300 .

[0085] Figure 3 is an example of a meshed multi-terminal HVDC transmission network. However, if one of the transmission lines 331, 332, 333, 334 is removed, the multi-terminal HVDC transmission network 300 may be a radial multi-terminal HVDC transmission network. The multi-terminal HVDC transmission network 300 may have three, four, five, or more converter stations.

[0086] Generally, the operation of the multi-terminal HVDC transmission network 300 is similar to the power transmission network 100 discussed above, except that each power converter 110, 120, 350, 360 in each converter station 310, 320, 330, 340 is configured to supply two transmission lines 331, 332, 333, 334, rather than one transmission line. However, as also described above, the present disclosure should not be limited to converter stations or power converters that supply two transmission lines. In particular, each power converter 110, 120, 350, 360 or converter station 310, 320, 330, 340 can be configured to supply three, four, five, or more than five transmission lines.

[0087] Figure 3 The disclosure of is equally applicable to the positive or negative pole of a bipolar system. Similarly, by removing the ground connection and having a pair of DC lines or cables of equal and opposite voltages, Figure 3 The disclosure can be applied to symmetrical monopole systems.

[0088] As mentioned above, faults in DC systems tend to cause fault currents that rise rapidly over time, unlike fault currents in AC systems, which have frequent periods of near-zero current. The traditional approach to managing faults in AC systems is to isolate the fault by opening circuit breakers on both sides of the faulty transmission line. To isolate the fault, information related to the voltage and current on either side of the transmission line is transmitted from one side of the transmission line to the other. By comparing the voltage and current information from both sides of the transmission line, it can be determined whether a fault has occurred in the respective lines. After determining that a fault has occurred in the respective lines, the circuit breakers on either side of the faulty transmission line can be opened.

[0089] For long transmission lines, e.g., greater than or equal to 300 km, transmitting information from one side of the line to the other can be time-consuming (e.g., several milliseconds). This can be a problem for DC systems because the magnitude of the fault current will tend to rise during the several milliseconds that it can take to transmit the information. When the circuit breakers finally receive an open command, they may be required to interrupt a relatively large fault current. Consequently, conventional methods for fault management in AC systems (e.g., fault detection and identification) tend to be unsuitable for fault management in multi-terminal DC systems.

[0090] It can be appreciated, therefore, that, generally speaking, for multi-terminal DC systems, the faster a fault can be detected and the faulty line identified, the faster the circuit breaker can be opened, and the lower the fault current the circuit breaker must interrupt. Interrupting lower fault currents offers numerous advantages over interrupting higher fault currents, such as the tendency for lower voltage and current requirements on the circuit breaker. The lower the voltage and current ratings of a circuit breaker, the smaller and less expensive the circuit breaker. As will now be discussed, such advantages are achieved when using the systems and methods of the present disclosure.

[0091] Furthermore, in multi-terminal transmission systems, there tends to be an additional need to ensure that only circuit breakers connected to faulty transmission lines are opened and that circuit breakers connected to otherwise healthy transmission lines do not accidentally open (i.e., the fault management system should be able to distinguish between faulty transmission lines and normal, non-faulty transmission lines).

[0092] As will now be discussed, these and other problems tend to be addressed when using the systems and methods of the present disclosure.

[0093] Figure 4 Shown Figure 3 A portion 410 of a conventional multi-terminal power transmission network 300 is shown in FIG. 4 and described above. Fault 400 may occur in any portion of any of transmission lines 331, 332, 333, 334. In this example, Figure 4As shown in FIG, a fault 400 has occurred at a first point 401 on a first transmission line 331. The first transmission line 331 may be referred to as a faulty transmission line 331. A second point 402 is located between the first circuit breaker 311 and the first converter station 310. The first inductor 321 is also located between the second point 402 and the first converter station 310. A third point 403 is located between the third circuit breaker 313 and the first converter station 310. The third inductor 323 is also located between the third point 403 and the first converter station 310.

[0094] Figure 5 403 in the portion 410 before and after the fault 400. A first waveform 510 shows the voltage at the first point 401. A second waveform 520 shows the voltage at the second point 402. A third waveform 530 shows the voltage at the third point 403.

[0095] As in Figure 5 As can be seen in FIG. 4 , before time T1 , the voltages at the first, second, and third points 401 , 402 , 403 are 1 per unit (pu). This indicates normal operation of the portion 410 .

[0096] At time T1, a fault 400 occurs at a first point 401 on the first transmission line 331. As a result, the voltage at the first point 401 changes from 1 pu to 0 pu or close to 0 pu, as shown in the first waveform 510. Due to the length of the first transmission line 331, the voltages at the second point 402 and the third point 403 do not change at time T1.

[0097] Between times T1 and T2, the voltage variation at first point 401 propagates through first transmission line 331 as a traveling wave. At time T2, the wavefront of the traveling wave reaches first converter station 310. As a result of the traveling wave arriving at first converter station 310, the voltage at second point 402 begins to drop after time T2, as shown in second waveform 520. Furthermore, although third point 403 is not directly connected to faulty transmission line 331, the voltage at third point 403 also drops slightly. The voltage drop at third point 403 (from time T2) is a result of third point 403 being indirectly connected to faulty transmission line 331 via first converter station 310. Therefore, as a result of the traveling wave, the voltages at second and third points 402, 403 vary.

[0098] Between times T2 and T3, there is a period in which the voltage at point 402 becomes negative. This is due to the traveling wave being reflected after reaching converter station 310. This reflection causes the voltage at point 402 to temporarily stabilize at an amplitude that is theoretically limited to -1 pu, but in practice, this voltage can stabilize at a value less than this, for example, -0.5 pu or less. Point 403 also stabilizes at a voltage closer to the normal operating voltage than the voltage at point 402. In other words, compared to point 402, point 403 sees a more attenuated form of the transient.

[0099] At time T3, the voltages at the second and third points 402 and 403 begin to rise. This is because the second wavefront has reached the first converter station 310, due to the traveling wave being reflected from the impedance discontinuity at the converter station and returning toward the fault, where it will be reflected again. After time T3, the voltages at the second and third points 402 and 403 continue to rise. The voltage at the second point 402 temporarily stabilizes.

[0100] At time T4, the voltages at the second and third points 402, 403 begin to drop again. This is because the third wave front has reached the first converter station 310 as a result of the traveling wave. After time T4, the voltages at the second and third points 402, 403 continue to drop. The voltage at the second point 402 temporarily stabilizes.

[0101] In this manner, as a result of the traveling wave caused by fault 400, the voltages at second and third points 402 and 403 oscillate. Notably, the voltage oscillations at third point 403 are smaller than those at second point 402. Eventually, the amplitude of the oscillations decreases, and the voltages at second and third points 402 and 403 stabilize. However, starting at time T1, the fault current in faulty transmission line 331 continues to rise, and as described above, this can be problematic for circuit breakers on either side of faulty transmission line 331. Also as described above, the voltage variations at second and third points 402 and 403 can make it difficult to distinguish which of the transmission lines in section 410 is faulty.

[0102] Figure 6 Shown Figure 3 An improved portion 600 of the multi-terminal power transmission network 300 shown in and described above is provided. Figure 6 The improved portion 600 shown in FIG. Figure 4, except that a diode is connected to the first end of each transmission line. Specifically, a first diode 610 is connected to the first end (or end portion) 331a of the first transmission line 331. A second diode 620 is connected to the first end (or end portion) 333a of the third transmission line 333. The first end of the first diode 610 is connected to the first end 331a of the first transmission line 331, and the second end of the first diode 610 is connected to ground. The first end of the second diode 620 is connected to the first end 333a of the third transmission line 333, and the second end of the second diode 620 is connected to ground. The first diode 610 is oriented such that when a positive voltage is applied to the first transmission line 331, the first diode 610 is reverse biased and current-blocking. The second diode 620 is oriented such that when a positive voltage is applied to the third transmission line 333, the second diode 620 is reverse biased and current-blocking.

[0103] The arrangement of the first and second diodes 610, 620 in the improved portion 600 tends to be particularly well-suited for bipolar HVDC transmission schemes. However, the present disclosure should not be limited thereto.

[0104] Specifically, the second ends of the first and second diodes 610 , 620 may be connected to the neutral line, for example in the case of a unipolar solution or a bipolar solution with a dedicated metal return.

[0105] Alternatively, the orientation of the first and second diodes 610, 620 can be reversed so that when a negative voltage is present on the first and second transmission lines 331, 333, respectively, for example, in the case of a negative polarity, the first and second diodes 610, 620 are reverse biased and current blocking. Therefore, whether it is a positive voltage or a negative voltage, it may be appropriate to refer to the transmission line voltage as a normal operating voltage.

[0106] Additionally, in some arrangements, the first and second diodes 610, 620 may be connected between the positive and negative lines, such as in the case of a symmetrical unipolar arrangement that does not include a neutral or ground connection.

[0107] As a result of the first diode 610, the voltage at the second point 402 does not oscillate as a result of the traveling wave generated by the fault 400, as described below. Figure 7 As shown in Figure 7 The voltages at the first, second and third points 401, 402, 403 are shown plotted against time before and after a fault, as well as the current through the first diode 610. The first diode 610 clamps the voltage at point 402 to zero, which means that the voltage at point 403 must therefore be above zero and the second diode 620 cannot conduct.

[0108] exist Figure 7, a fourth waveform 710 shows a voltage at the first point 401 , a fifth waveform 720 shows a voltage at the second point 402 , a sixth waveform 730 shows a voltage at the third point 403 , and a seventh waveform 740 shows a current through the first diode 610 .

[0109] exist Figure 7 In the case of a fault 400, a Figure 5 At time T5 of time T1 in FIG. 4 . At time T6, the wavefront of the traveling wave generated by the fault 400 reaches the first converter station 310. After time T6, the voltages at the second and third points 402, 403 begin to drop, similar to the above description of FIG. Figure 5 Describe the situation.

[0110] At time T7, as a result of the reflected wave, the voltage at second point 402 is driven to a negative value. However, as a result of the reflected wave, first diode 610 changes from reverse bias and current blocking to forward bias and begins conducting. The forward conduction of first diode 610 prevents the voltage at second point 402 from becoming negative between time periods T6 and T8, and instead maintains the voltage at or near zero. In this way, first diode 610 prevents oscillations in the traveling wave generated by fault 400 at first converter station 310.

[0111] To illustrate the comparison of the voltages at the second and third points 402, 403 with and without the first and second diodes 610, 620, the second and third waveforms 520, 530 (from Figure 5 ) is also shown in dotted form in Figure 7 . Thus, the fifth waveform 720 shows the voltage at the second point 402 during the fault 400 with the first and second diodes 610, 620 connected, while the second waveform 520 shows the voltage at the second point 402 during the fault 400 with the first and second diodes 610, 620 not connected. Similarly, the sixth waveform 730 shows the voltage at the third point 403 during the fault 400 with the first and second diodes 610, 620 connected, while the third waveform 530 shows the voltage at the third point 403 during the fault 400 with the first and second diodes 610, 620 not connected.

[0112] Therefore, when there would otherwise be a reflected wave causing a negative voltage at the first end 331a of the transmission line 331, the first diode 610 conducts in the forward direction. The inventors have recognized that the forward conduction of the first diode 610 can be used as a means of determining (meaning detecting and identifying) a fault on the transmission line, and as a result of the determination, a circuit breaker can be triggered. This advantageously allows for determination that a fault has occurred on a particular transmission line without requiring information to be transmitted from one side of the transmission line to the other. In other words, it is possible to determine that a fault has occurred on a particular transmission line based solely on one side of the transmission line. This increases the speed at which the determination can be made and, therefore, allows the circuit breaker to be opened (triggered) much faster than the conventional methods described above. Therefore, as described above, it is possible to achieve the advantages of using circuit breakers with relatively low current and voltage ratings to interrupt lower fault currents and, therefore, relatively low cost and less.

[0113] Furthermore, in some cases, it may be desirable to improve the reliability of circuit breakers. This can be achieved by using circuit breakers with relatively high voltage and current ratings. Because the fault current will be relatively low, the circuit breaker's reliability in interrupting lower fault currents tends to increase. In other words, because the circuit breaker is "overrated" for the application, the circuit breaker tends to be less likely to fail or experience a fault over an extended period of time.

[0114] Furthermore, because only the diode connected to the faulty transmission line will conduct, the probability of triggering the wrong circuit breaker in a multi-terminal power transmission network is greatly reduced or even eliminated. Therefore, determining which circuit breaker to open is highly reliable compared to other conventional methods.

[0115] Figures 8A to 8C Examples of first, second, and third subsystems 800A, 800B, 800C that can be used to implement the present invention in a multi-terminal power transmission network 300 are shown. Each of the subsystems 800A, 800B, 800C includes an electrical branch 805 having a first terminal 810 and a second terminal 820. An inductor 321 and a circuit breaker 311 are connected in series between the first terminal 810 and the second terminal 820. Each of the subsystems 800A, 800B, 800C also includes a diode arrangement 610 including at least one diode; a measuring device 840, and a controller 200. The controller 200 is combined with the above. Figure 2 The controller 200 described is the same.

[0116] A first end of the diode arrangement 610 is connected to the electrical branch 805. A measuring device 840 is connected between the diode arrangement 610 and the controller 200. The controller 200 is configured to receive information or data from the measuring device 840 and use the information or data to control the circuit breaker 311. The circuit breaker 311 is configured to connect or disconnect the first terminal 810 and the second terminal 820, as discussed further below.

[0117] A second end of the diode arrangement 610 is connected to a third terminal 830. In each of the subsystems 800A, 800B, 800C, the diode arrangement 610 or the third terminal 830 may be configured differently, as discussed further below.

[0118] In the first subsystem 800A, the third terminal 830 is connected to ground or the neutral line of the multi-terminal power transmission network 300. The diode arrangement 610 is oriented so that when there is a positive voltage on the electrical branch 805, the diode arrangement 610 is reverse biased and current is blocked. This arrangement is suitable for the positive pole of a unipolar or bipolar transmission scheme.

[0119] In the second subsystem 800B, the third terminal 830 is connected to ground or the neutral line of the multi-terminal power transmission network 300. The diode arrangement 610 is oriented so that when a negative voltage is present on the electrical branch 805, the diode arrangement 610 is reverse biased and current is blocked. This arrangement is suitable for the negative pole of a unipolar or bipolar transmission scheme.

[0120] The third subsystem 800C further includes a second electrical branch 850 having a fourth terminal 860 and a fifth terminal 870. A second inductor 880 and a ninth circuit breaker 890 are connected in series between the fourth terminal 860 and the fifth terminal 870. The third terminal 830 is connected to the second electrical branch 850 between the ninth circuit breaker 890 and the fifth terminal 870.

[0121] In this embodiment, during normal operation of third subsystem 800C, electrical branch 805 is configured to be connected to the positive pole of the power converter and thus has a positive voltage, and second electrical branch 850 is configured to be connected to the negative pole of the power converter and thus has a negative voltage. Diode arrangement 610 is oriented such that when a positive voltage is present on electrical branch 805, diode arrangement 610 is reverse biased and current blocking. Controller 200 is also configured to be communicatively coupled to both circuit breakers 311 and 890. Therefore, this arrangement can be applied to both the positive and negative poles of a symmetrical unipolar transmission scheme.

[0122] In each of the subsystems 800A, 800B, 800C, the controller 200 is configured to receive an indication signal indicative of a forward current in the diode arrangement 610 .

[0123] In the first and second subsystems 800A, 800B, the controller is configured to issue a command to the circuit breaker 311 to disconnect the first terminal 810 and the second terminal 820 in response to receiving the indication signal.

[0124] In the third subsystem 800C, the controller is configured to, in response to receiving the indication signal, issue a command to both the circuit breakers 311 and 890. The command is configured to cause the circuit breaker 311 to disconnect the first terminal 810 and the second terminal 820, and to cause the ninth circuit breaker 890 to disconnect the fourth terminal 860 and the fifth terminal 870.

[0125] In this manner, each of subsystems 800A, 800B, and 800C is capable of detecting a fault on a transmission line, identifying which specific line the fault is on from among the multiple lines in a multi-terminal transmission system, and isolating the fault without having to transmit information from one side of the transmission line to the other. This is because only the circuit breaker connected to the faulty transmission line will open, and only the diode arrangement connected to the faulty line will conduct forward current. In other words, adjacent diodes on adjacent lines will not conduct forward current unless there is a fault on the adjacent line.

[0126] The controller 200 can issue a command in less than 150 microseconds from the time the indication signal is received. This makes it possible to determine that a fault has occurred and issue a command to open a circuit breaker much faster than conventional methods, which can take, for example, several milliseconds.

[0127] The measuring device 840 is configured to measure a forward current through the diode arrangement 610 , determine an indication signal based on the forward current; and output the indication signal.

[0128] The measuring device 840 can determine the indicator signal as a signal based on the forward current through the diode arrangement or the time derivative of the forward current; or as an (analog) signal proportional to the forward current; or as a (digital) signal determined by comparing the forward current to a threshold value. The threshold value can be set at a tolerance or fault limit defined by an external controller, system, or user.

[0129] Figure 91 . The diagrams illustrate first, second, and third arrangements 910, 920, 930 (for clarity, repeated reference numerals have been omitted) of an inductor 321, a circuit breaker 311, and a first end of a diode arrangement 610 in an electrical branch 805 when subsystem 800A is connected to transmission line 311. In each of the first, second, and third arrangements 910, 920, 930, a first terminal 810 is connected to a converter station 310, and a second terminal 820 is connected to transmission line 331. In the first arrangement 910, from the station 310 to the transmission line 331, the inductor 321 is connected in series with the circuit breaker 311, which is connected in series with the first end of the diode arrangement 610. In the second arrangement 920, from the converter station 310 to the transmission line 331, the inductor 321 is connected in series with the first end of the diode arrangement 610, which is connected in series with the circuit breaker 311. In a third arrangement 930 , from the power station 310 to the transmission line 331 , a circuit breaker 311 is connected in series with an inductor 321 , which is connected in series with a first end of the diode arrangement 610 .

[0130] The diode arrangement 610 may include a plurality of diodes connected in series.The diode arrangement 610 may be configured to operate at a voltage of at least 100 kV.

[0131] The controller 200 may also be configured to issue a second command to a second circuit breaker 312 external to the subsystems 800A, 800B, and 800C in response to receiving the indication signal. In some embodiments, the second command configures the second circuit breaker 312 from the normal state to the armed state, wherein in the armed state, the second circuit breaker 312 is configured to open faster than in the normal state. In other embodiments, the second command configures the second circuit breaker to open.

[0132] Subsystems 800A, 800B, and 800C may be implemented in a transmission network or a multi-terminal transmission network. For example, subsystem 800 may be implemented in the above Figure 3 334a, 334b of each transmission line 331, 332, 333, 334 of the depicted multi-terminal power transmission network 300.

[0133] When using the subsystems 800A, 800B, 800C in a multi-terminal power transmission network 300, each transmission line 331, 332, 333, 334 can be divided into separate "zones" for control and fault management purposes. Figure 10As shown in FIG, a first zone 1010 may include a first transmission line 331 and subsystems 800A located on either side of the first transmission line 331. A second zone 1020 may include a second transmission line 332 and subsystems 800A located on either side of the second transmission line 332. A third protection zone 1030 may include a third transmission line 333 and subsystems 800A located on either side of the third transmission line 333.

[0134] In this manner, the subsystems 800A, 800B, 800C, the controller 200, and the methods disclosed herein may be used to improve fault management on transmission lines.

Claims

1. A system (800A, 800B, 800C) for managing a fault (400) on an HVDC transmission line, the system comprising: an electrical branch (805) comprising an inductor (321) and a circuit breaker (311) connected in series between a first terminal (810) and a second terminal (820), wherein the circuit breaker (311) is configured to connect or disconnect the first terminal (810) and the second terminal (820); a diode arrangement (610) comprising at least one diode, wherein a first end of the diode arrangement is connected to the electrical branch and a second end of the diode arrangement is connected to a third terminal (830); and The controller (200) is configured to: receiving an indication signal, wherein the indication signal indicates a forward current in the diode arrangement (610); and In response to receiving the indication signal, a command is issued to the circuit breaker (311) to disconnect the first terminal (810) and the second terminal (820).

2. The system according to claim 1, wherein: The third terminal (830) is connected to the ground, or to the neutral line, or to the negative line.

3. The system (800C) according to claim 1, further comprising: a second electrical branch (850) comprising a second inductor (880) and a second circuit breaker (890) connected in series between a fourth terminal (860) and a fifth terminal (870), wherein the second circuit breaker (880) is configured to connect or disconnect the fourth terminal (860) and the fifth terminal (870); wherein the third terminal (830) is connected to the second electrical branch (850); and The controller (200) is further configured to issue the command to the second circuit breaker (890) to disconnect the third terminal (860) and the fourth terminal (870) in response to receiving the indication signal.

4. The system according to claim 2 or 3, wherein: The diode arrangement (610) is oriented such that when a positive voltage is present on the electrical branch, the diode arrangement (610) is reverse biased and current blocking.

5. The system according to claim 2 or 3, wherein: The diode arrangement (610) is oriented such that when a negative voltage is present on the electrical branch, the diode arrangement (610) is reverse biased and current blocking.

6. The system of any preceding claim, further comprising: a measuring device (840) connected to the diode arrangement (610) and the controller (200); Wherein the measuring device is configured to: measuring the forward current through the diode arrangement (610); determining the indication signal based on the forward current; and The indication signal is output.

7. The system according to claim 6, wherein: The indication signal is: a signal based on the forward current through the diode arrangement or a time derivative of the forward current; a signal proportional to the forward current; or A signal is determined by comparing the forward current to a threshold value.

8. A system according to any preceding claim, wherein The controller is configured to issue the command in less than 150 microseconds from receiving the indication signal.

9. A system according to any preceding claim, wherein: In the electrical branch: the inductor is connected in series between the first terminal and the circuit breaker, the circuit breaker is connected in series between the inductor and the first end of the diode arrangement, and the first end of the diode arrangement is connected in series between the circuit breaker and the second terminal; or the inductor is connected in series between the first end and the first terminal of the diode arrangement, the first end of the diode arrangement is connected in series between the inductor and the circuit breaker, and the circuit breaker is connected in series between the first end and the second terminal of the diode arrangement; or The circuit breaker is connected in series between the first terminal and the inductor, the inductor is connected in series between the circuit breaker and the first end of the diode arrangement, and the first end of the diode arrangement is connected in series between the inductor and the second terminal.

10. A system according to any preceding claim, wherein The diode arrangement comprises a plurality of diodes connected in series; and The diode arrangement is configured to operate at a voltage of at least 100 kV.

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