Multi-port alternating-current and direct-current fault handling device for multi-port direct-current power transmission sending-out system

Abstract

The invention discloses a multi-port alternating current and direct current fault handling device for a multi-port direct current transmission sending-out system, and the device is provided with n ports which are recorded as Port1-Portn. Each port is respectively connected with one end of a bypass branch, one end of a feed flow branch and one end of an injection branch; the other end of the bypass branch, the other end of the feed current branch and the other end of the injection branch are connected to a direct-current bus Bus1, a direct-current bus Bus2 and a direct-current bus Bus3 respectively. In the fault current breaking process, the feed current branch, the injection branch and the valve group Valve1 form an H-bridge structure, so that the bidirectional fault current breaking capacity is realized; when the Bus1 or the Bus3 has a direct-current fault, the fault current is blocked by the H-bridge structure; the device has DC fault isolation and AC / DC fault ride-through capabilities, can quickly cut off a fault current when a DC short circuit occurs, and stabilizes a DC voltage during an AC fault or DC fault de-dissociation period.

Description

Technical Field

[0001] This invention relates to the field of power transmission, specifically to a multi-port AC / DC fault handling device for multi-terminal DC power transmission systems. Background Technology

[0002] Point-to-point DC transmission systems based on modular multilevel converters (MMCs) cannot meet the needs of offshore wind power sharing. Therefore, multi-terminal DC transmission systems based on MMCs have attracted widespread attention. However, handling AC / DC faults has long been considered a major challenge. DC faults can cause overcurrents, while power imbalances during AC and DC fault deionization can lead to DC-side overvoltages. Therefore, cost-effective and reliable DC fault isolation and AC / DC fault ride-through schemes are needed to ensure power transmission in multi-terminal DC transmission systems.

[0003] Currently, there are four main types of DC fault isolation schemes: AC circuit breakers, hybrid submodule converter valves, high-speed switches, and DC circuit breakers. There are five main types of AC / DC fault ride-through schemes: wind turbine converter built-in resistors, AC energy dissipation devices, energy self-balancing converter valves, and DC energy dissipation devices. However, the AC and DC voltages in the DC fault isolation scheme can affect the operation of the AC / DC fault ride-through scheme. Therefore, the selection of DC fault isolation and AC / DC fault ride-through schemes must consider their compatibility.

[0004] DC circuit breakers are preferred due to their ability to prevent AC / DC voltage collapse and ensure continuous power supply. Existing research proposes three types of DC circuit breakers: mechanical DC circuit breakers, all-solid-state DC circuit breakers, and hybrid DC circuit breakers. Among them, hybrid DC circuit breakers are favored because they combine the low cost of mechanical DC circuit breakers with the low conduction losses and fast isolation speed of all-solid-state DC circuit breakers. A typical structure offered by ABB includes a bypass branch and a main circuit breaker. DC power dissipation devices are considered a reliable choice due to their fast response, low DC voltage fluctuations, and high reliability. A typical DC power dissipation device consists of hundreds of sub-modules connected in series with a lumped resistor.

[0005] However, DC circuit breakers need to be installed at both ends of each DC transmission line, and each sending-end converter station requires a DC power dissipation device on the DC side. Existing research mainly focuses on the main circuit breaker topology of two-port DC circuit breakers. The number and cost of these devices remain major obstacles to their large-scale application. To address this issue, researchers have proposed multi-port hybrid DC circuit breakers with a shared main circuit breaker, thereby improving cost-effectiveness. Various multi-port hybrid DC circuit breaker structures have been proposed in the prior art to optimize fault current paths. In a typical structure, the bypass branches of each port are connected in a star configuration, forming a load current path, with a common DC bus as the convergence point. When a fault occurs, this bus is responsible for converging the fault current to the main circuit breaker, and then the current is directed to the faulty DC line via an injection branch composed of diodes. However, in this structure, the normally conducting bypass branches need to bear a high fault current, and the common bus itself is also at risk of DC faults, requiring additional protection. To reduce the current stress on the bypass branches, subsequent schemes have introduced dedicated feed-in branches. This branch, composed of diodes or unidirectional thyristor strings, provides an independent path for the converter to feed fault current into the main circuit breaker. Another improved scheme employs a ring-connected bypass branch structure. This design eliminates the central common bus, fundamentally avoiding the risk of DC faults at this node. Furthermore, its feed-in branch and injection branch together form an H-bridge structure, enabling the circuit breaker to bidirectionally interrupt fault current. Further research has simplified the structure, directly using the same branch design to handle both feed-in and injection functions, thus eliminating the need for separate load current branches. However, this scheme requires all switching devices to withstand high current stress and places extremely high demands on the synchronization of these switches' operation. However, the aforementioned multi-port hybrid DC circuit breakers primarily focus on how to configure multiple ports (their main circuit breaker is only compatible with ABB's structure). The scalability of the main circuit breaker has not been further investigated. In addition, DC power dissipation devices still need to be configured independently.

[0006] In summary, the existing fault handling schemes for DC fault isolation and AC / DC fault ride-through have the following disadvantages: (1) When dealing with permanent or temporary DC short-circuit faults, the DC fault isolation technology is used to clear the fault current, which causes the AC or DC voltage to become disordered or to be maintained at zero potential, and cannot provide the AC / DC voltage for the operation of the fault ride-through scheme. In severe cases, it can lead to the shutdown of the entire system; (2) DC circuit breakers + DC energy dissipation devices can ensure the normal operation of DC fault isolation and AC / DC fault ride-through. However, the individual cost of both is high, and the number of them configured in the system is large, which reduces the economic construction feasibility of the system. Summary of the Invention

[0007] The purpose of this invention is to provide a multi-port AC / DC fault handling device for multi-terminal DC power transmission systems. The multi-port AC / DC fault handling device has n ports, denoted as Port1–Port2. n ;

[0008] Each port is connected to one end of the bypass branch, feed branch, and injection branch respectively; the other end of the bypass branch, feed branch, and injection branch is connected to the DC bus Bus1, Bus2, and Bus3 respectively.

[0009] The bypass branch includes a series-connected fast mechanical switch (UFD) and a load converter switch (LCS).

[0010] Each feed branch consists of n diode strings, denoted as DS1–D Sn ;

[0011] Each injection branch consists of n diode strings, denoted as D. R1 –D Rn ;

[0012] Valve1 and Valve2 are both composed of sub-modules connected in series. Valve1 is connected to DC bus Bus1 and Bus3.

[0013] The submodules in Valve2 contain nested diode full-bridge resistors, and the lumped resistor R... m Reverse parallel diode series D m ;

[0014] Under DC short-circuit faults, the multi-port AC / DC fault handling device intervenes in the fault current interruption process. The feed branch, injection branch, and valve group Valve1 form an H-bridge structure, thereby achieving bidirectional fault current interruption capability. When a DC fault occurs in Bus1 or Bus3, the fault current is blocked by the H-bridge structure. Valve1 is used to clear the fault current on the valve side, and Valve2 is used to clear the fault current on the DC side.

[0015] During the deionization of DC short-circuit fault lines and AC short-circuit faults, the multi-port AC / DC fault handling device operates in a power dissipation state. The valve group Valve1 and valve group Valve2 sub-modules switch on and off, adjusting the voltage of the centralized resistor so that the dissipated power matches the surplus power of the system.

[0016] Furthermore, all the aforementioned components (diodes, IGBTs, UFDs, capacitors, and resistors) are configured in series and parallel, with the specific number depending on the electrical stress actually operating in the system and the specifications of the selected devices.

[0017] Furthermore, the multi-port AC / DC fault handling device is used to handle AC / DC faults in three-port and above DC systems.

[0018] Furthermore, the number of ports of the multi-port AC / DC fault handling device is replaced with n+1, with one port connected to the DC side of the converter station on the wind farm side, and the remaining ports connected to the DC transmission line of the DC system.

[0019] Furthermore, the multi-port AC / DC fault handling device is used to handle DC short-circuit faults, symmetrical AC short-circuit faults, and asymmetrical AC short-circuit faults;

[0020] The DC short-circuit faults include pole-to-ground short-circuit faults and pole-to-pole short-circuit faults.

[0021] The symmetrical AC short-circuit fault is a three-phase short-circuit fault.

[0022] The asymmetrical AC short-circuit fault includes single-phase-to-ground short-circuit fault, two-phase short-circuit fault, and two-phase-to-ground short-circuit fault;

[0023] Furthermore, the operating modes of the multi-port AC / DC fault handling device include startup mode, normal conduction mode, current interruption mode, power dissipation mode, and reclosing mode.

[0024] Furthermore, when a DC short-circuit fault occurs on the DC pole, the multi-port AC / DC fault handling device sequentially enters the current interruption mode, power dissipation mode, and reclosing mode; when an AC short-circuit fault occurs on the DC pole, the multi-port AC / DC fault handling device enters the power dissipation mode.

[0025] Furthermore, when the multi-port AC / DC fault handling device is in startup mode, diode D... R1 –D Rn Keep off, disconnect switch D S1 –D Sn And the bypass branch remains conducting, the capacitor in the submodule flows through the lumped resistor current i m Charge to the rated value;

[0026] When the multi-port AC / DC fault handling device is in normal conduction mode, the total voltage of the cascaded capacitors is equal to the rated voltage of the DC transmission line, and the load current between each converter station flows only through the DC transmission line.

[0027] Furthermore, when the multi-port AC / DC fault handling device is in current interruption mode, the fault current interruption process sequentially goes through the fault occurrence stage, the fault current commutation stage, the valve-side current clearing stage, and the DC-side current clearing stage.

[0028] During the fault occurrence phase, based on diode D in the Valve1 and Valve2 submodules... arm1 and D arm2 Due to its unidirectional conductivity, the cascaded capacitor in the converter valve only feeds discharge current i to the grounding point. dc0 and idci DC side fault current i f Ascend; i=2,…,n;

[0029] During the fault current commutation phase, after detecting the fault current at time t1, the fast mechanical switch LCS1 connected to the DC fault line is triggered to disconnect, and Q... arm1,1 Q arm2,1 The closing action; current flows through diode D S0 and D Si current i dc0 and i dci The current i of the combined valve group Valve1 arm1 The fault current passes through diode D. R1 Flow into the fault short-circuit point; based on the unidirectional nature of the diode, flow through D S1 D R0 and D R The current is zero, triggering the action of the fast mechanical switch UFD1 connected to the DC fault line;

[0030] During the valve-side current clearing phase, after the mechanical switch action delay, the fast mechanical switch UFD1 is fully open at time t2, triggering the main IGBT of the submodule in Valve1 to disconnect, thus enabling the submodule to be engaged. At this time, the pre-charge capacitor of the submodule in Valve1 is connected in series with the fault circuit to absorb current i. arm1 lumped resistor R m Voltage u at both ends m decline;

[0031] When u m When the voltage drops to zero at time t3, diode D m Turning on the circuit will transfer the voltage u of the lumped resistor. m Clamp to zero position, i arm1 It continues to be absorbed by the pre-charge capacitor until it drops to zero; at the same time, the DC side fault current i f Through diode D m The main IGBT and D in the Valve2 valve group submodule R1 A pathway is formed to achieve continuous flow;

[0032] During the DC-side current clearing phase, the DC-side current i at time t4 arm1 Clear the Q of the submodule in Valve2 of the trigger valve group arm2,1 The shutdown action and Q arm2,2 The conduction action activates the capacitors and distributed resistors of the Valve2 submodule to absorb the DC-side fault current i. f At the same time, close Q. arm1,2The distributed resistor is used to dissipate overvoltage in the capacitor of the Valve1 submodule; when the capacitor voltage drops to the normal level, it triggers Q in all submodules. arm1,2 and Q arm2,2 Turn off.

[0033] Furthermore, when the multi-port AC / DC fault handling device is in surplus power dissipation mode, the residual current switch RCB1 connected to the DC fault line is disconnected. By adjusting the number of sub-modules in the valve groups Valve1 and Valve2 that are put into operation, the dissipated power is made equal to the system's surplus power.

[0034] Furthermore, when the multi-port AC / DC fault handling device is in reclosing mode, it detects whether the voltage / current still exists. If it does not exist, the fault is diagnosed as a permanent fault, and the residual current switch RCB1 remains open. If it exists, the fault is diagnosed as a transient fault, and the device recloses after the deionization is completed.

[0035] The multi-port AC / DC fault handling device enables the reclosing of the residual current switch RCB1 and the fast mechanical switch UFD1 under zero current and zero voltage conditions by controlling the switching of the sub-module.

[0036] The technical effects of this invention are undeniable. By proposing an integrated device that combines multi-port fault isolation and dynamic energy dissipation capabilities, this invention addresses the core challenges of high equipment costs, complex structures, and difficult coordinated control in multi-terminal DC systems when dealing with DC short circuits and AC faults. It provides key technical support for building a more economical, compact, and reliable DC grid protection scheme.

[0037] This invention integrates the functions of a multi-port DC circuit breaker and a DC energy-consuming device into a single device, sharing a sub-module, thus eliminating the need for a separate main circuit breaker and DC energy-consuming device valve group.

[0038] This invention adopts a multi-port structure, which is suitable for multi-terminal DC systems, saves installation space, and the sub-module reuse design improves the modularity and scalability of the system.

[0039] This invention has the capability of DC fault isolation and AC / DC fault ride-through. It can quickly cut off the fault current during DC short circuit and stabilize the DC voltage during AC or DC fault deionization (valve-side fault current clearing time <5ms, DC voltage fluctuation <10%), avoiding the system shutdown problem caused by the DC voltage returning to zero in existing technical solutions. Attached Figure Description

[0040] Figure 1 This is a topology diagram of a new AC / DC fault handling device;

[0041] Figure 2(a) Schematic diagram of the installation of a protection scheme for DC circuit breakers and DC energy-consuming devices; Figure 2 (b) is a schematic diagram of the installation of the new fault handling device;

[0042] Figure 3 The working principle of a new type of fault handling device for handling DC faults;

[0043] Figure 4 For DC fault isolation and AC / DC fault ride-through effects; Figure 4 (a) Clearing DC fault current; Figure 4 (b) DC voltage stability during DC faults; Figure 4 (c) DC voltage stability during AC faults. Detailed Implementation

[0044] The present invention will be further described below with reference to embodiments, but it should not be construed that the scope of the present invention is limited to the following embodiments. Various substitutions and modifications made based on ordinary technical knowledge and common practices in the art without departing from the above-described technical concept of the present invention should be included within the scope of protection of the present invention.

[0045] Example 1:

[0046] See Figures 1 to 4 A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, wherein the multi-port AC / DC fault handling device has n ports, denoted as Port1–Port2. n ;

[0047] Each port is connected to one end of the bypass branch, feed branch, and injection branch respectively; the other end of the bypass branch, feed branch, and injection branch is connected to the DC bus Bus1, Bus2, and Bus3 respectively.

[0048] The bypass branch includes a series-connected fast mechanical switch (UFD) and a load converter switch (LCS).

[0049] Each feed branch consists of n diode strings, denoted as DS1–D Sn ;

[0050] Each injection branch consists of n diode strings, denoted as D. R1 –D Rn ;

[0051] Valve1 and Valve2 are both composed of sub-modules connected in series. Valve1 is connected to DC bus Bus1 and Bus3.

[0052] The submodules in Valve2 contain nested diode full-bridge resistors, and the lumped resistor R... m Reverse parallel diode series Dm ;

[0053] Under DC short-circuit faults, the multi-port AC / DC fault handling device intervenes in the fault current interruption process. The feed branch, injection branch, and valve group Valve1 form an H-bridge structure, thereby achieving bidirectional fault current interruption capability. When a DC fault occurs in Bus1 or Bus3, the fault current is blocked by the H-bridge structure. Valve1 is used to clear the fault current on the valve side, and Valve2 is used to clear the fault current on the DC side.

[0054] During the deionization of DC short-circuit fault lines and AC short-circuit faults, the multi-port AC / DC fault handling device operates in a power dissipation state. The valve group Valve1 and valve group Valve2 sub-modules switch on and off, adjusting the voltage of the centralized resistor so that the dissipated power matches the surplus power of the system.

[0055] Example 2:

[0056] A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, with the same technical content as Embodiment 1, further wherein all the aforementioned components (diodes, IGBTs, UFDs, capacitors, resistors) are configured in series and parallel, and the specific number depends on the electrical stress actually operating in the system and the specifications of the selected devices.

[0057] Example 3:

[0058] A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, with the same technical content as any one of Embodiments 1-2, further wherein the multi-port AC / DC fault handling device is used to handle AC / DC faults in three-port or higher DC systems.

[0059] Example 4:

[0060] A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, with the same technical content as any one of embodiments 1-3, further wherein the number of ports of the multi-port AC / DC fault handling device is replaced with n+1, one of which is connected to the DC side of the converter station on the wind farm side, and the remaining ports are connected to the DC transmission line of the DC system.

[0061] Example 5:

[0062] A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, with the same technical content as any one of embodiments 1-4, further wherein the multi-port AC / DC fault handling device is used to handle DC short-circuit faults, symmetrical AC short-circuit faults, and asymmetrical AC short-circuit faults.

[0063] The DC short-circuit faults include pole-to-ground short-circuit faults and pole-to-pole short-circuit faults.

[0064] The symmetrical AC short-circuit fault is a three-phase short-circuit fault.

[0065] The asymmetrical AC short-circuit fault includes single-phase-to-ground short-circuit fault, two-phase short-circuit fault, and two-phase-to-ground short-circuit fault;

[0066] Example 6:

[0067] A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, with the same technical content as any one of embodiments 1-5, further comprising the following operating modes: startup mode, normal conduction mode, current interruption mode, power dissipation mode and reclosing mode.

[0068] Example 7:

[0069] A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, with the same technical content as any one of embodiments 1-6, further wherein when a DC short-circuit fault occurs in the DC pole line, the multi-port AC / DC fault handling device sequentially enters the current interruption mode, the power dissipation mode and the reclosing mode; when an AC short-circuit fault occurs in the DC pole line, the multi-port AC / DC fault handling device enters the power dissipation mode.

[0070] Example 8:

[0071] A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, with the same technical content as any one of embodiments 1-7, further comprising the following: when the multi-port AC / DC fault handling device is in the start-up mode, diode D... R1 –D Rn Keep off, disconnect switch D S1 –D Sn And the bypass branch remains conducting, the capacitor in the submodule flows through the lumped resistor current i m Charge to the rated value;

[0072] When the multi-port AC / DC fault handling device is in normal conduction mode, the total voltage of the cascaded capacitors is equal to the rated voltage of the DC transmission line, and the load current between each converter station flows only through the DC transmission line.

[0073] Example 9:

[0074] A multi-port AC / DC fault handling device for a multi-terminal DC power transmission system, with the same technical content as any one of embodiments 1-8, further wherein when the multi-port AC / DC fault handling device is in the current interruption mode, the fault current interruption process sequentially passes through the fault occurrence stage, the fault current commutation stage, the valve side current clearing stage, and the DC side current clearing stage.

[0075] During the fault occurrence phase, based on diode D in the Valve1 and Valve2 submodules... arm1and D arm2 Due to its unidirectional conductivity, the cascaded capacitor in the converter valve only feeds discharge current i to the grounding point. dc0 and i dci DC side fault current i f Ascend; i=2,…,n;

[0076] During the fault current commutation phase, after detecting the fault current at time t1, the fast mechanical switch LCS1 connected to the DC fault line is triggered to disconnect, and Q... arm1,1 Q arm2,1 The closing action; current flows through diode D S0 and D Si current i dc0 and i dci The current i of the combined valve group Valve1 arm1 The fault current passes through diode D. R1 Flow into the fault short-circuit point; based on the unidirectional nature of the diode, flow through D S1 D R0 and D R The current is zero, triggering the action of the fast mechanical switch UFD1 connected to the DC fault line;

[0077] During the valve-side current clearing phase, after the mechanical switch action delay, the fast mechanical switch UFD1 is fully open at time t2, triggering the main IGBT of the submodule in Valve1 to disconnect, thus enabling the submodule to be engaged. At this time, the pre-charge capacitor of the submodule in Valve1 is connected in series with the fault circuit to absorb current i. arm1 lumped resistor R m Voltage u at both ends m decline;

[0078] When u m When the voltage drops to zero at time t3, diode D m Turning on the circuit will transfer the voltage u of the lumped resistor. m Clamp to zero position, i arm1 It continues to be absorbed by the pre-charge capacitor until it drops to zero; at the same time, the DC side fault current i f Through diode D m The main IGBT and D in the Valve2 valve group submodule R1 A pathway is formed to achieve continuous flow;

[0079] During the DC-side current clearing phase, the DC-side current i at time t4 arm1 Clear the Q of the submodule in Valve2 of the trigger valve group arm2,1 The shutdown action and Q arm2,2 The conduction action activates the capacitors and distributed resistors of the Valve2 submodule to absorb the DC-side fault current i. fAt the same time, close Q. arm1,2 The distributed resistor is used to dissipate overvoltage in the capacitor of the Valve1 submodule; when the capacitor voltage drops to the normal level, it triggers Q in all submodules. arm1,2 and Q arm2,2 Turn off.

[0080] Example 10:

[0081] A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, with the same technical content as any one of embodiments 1-9, further wherein when the multi-port AC / DC fault handling device is in surplus power dissipation mode, the residual current switch RCB1 connected to the DC fault line is disconnected, and the number of sub-modules in the valve groups Valve1 and Valve2 that are put into operation is adjusted so that the dissipated power is equal to the surplus power of the system.

[0082] Example 11:

[0083] A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, with the same technical content as any one of embodiments 1-10, further wherein when the multi-port AC / DC fault handling device is in reclosing mode, it detects whether the voltage / current still exists. If it does not exist, the fault is diagnosed as a permanent fault, and the residual current switch RCB1 remains in the open state; if it exists, it is diagnosed as a transient fault, and the device recloses after the deionization is completed.

[0084] The multi-port AC / DC fault handling device enables the reclosing of the residual current switch RCB1 and the fast mechanical switch UFD1 under zero current and zero voltage conditions by controlling the switching of the sub-module.

[0085] Example 12:

[0086] A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, the contents of which are as follows:

[0087] Figure 1 The basic structure of the proposed novel n-port (n being the number of ports) fault handling device is demonstrated (applicable to three-terminal and above multi-terminal DC systems). Each port (Port1–Port2) n It connects a bypass branch, a feed branch, and an injection branch. The bypass branch consists of a high-speed mechanical switch (UFD) and a load converter switch (LCS); the feed branch and the injection branch are both diode strings (DS1–D...). Sn With D R1 –D Rn ).

[0088] The other ends of the bypass branch, feed branch, and injection branch are connected to DC buses Bus1, Bus2, and Bus3, respectively. The remaining part can be considered as a valve group, divided into two groups, Valve1 and Valve2. Valve1 connects to DC buses Bus1 and Bus3, while Valve2 connects to DC bus Bus3 and a lumped resistor. The sub-modules of Valve1 and Valve2 are identical, both consisting of diodes, IGBTs, DC capacitors, and resistors. The difference lies in the fact that the sub-modules in Valve2 contain nested diode full-bridge resistors. In addition, the lumped resistor R... m A diode string D is connected in reverse parallel. m During the fault current interruption process, D S1 –D Sn D R1 –D Rn Together with the shared Valve1, they form an H-bridge structure, thereby achieving bidirectional fault current interruption capability. Due to D S1 –D Sn With D R1 –D Rn Due to its unidirectional conduction characteristic, the fault current will be blocked when a DC fault occurs in Bus1 or Bus3.

[0089] 3.2 Installation Method

[0090] Taking a typical multi-terminal DC system as an example, the installation method includes a wind farm-side converter station that connects n DC transmission lines. Figure 2 This paper presents a comparative installation method using a two-port DC circuit breaker, a DC energy dissipation device, and the proposed device. In a typical protection scheme, a two-port hybrid DC circuit breaker is installed at both ends of each DC transmission line, and a DC energy dissipation device is installed on the DC side of the wind farm-side converter station. To replace this scheme, the proposed device adopts an (n+1)-port structure, with one port connected to the wind farm-side converter station and the remaining ports connected to each of the DC transmission lines. Thus, the proposed device can replace the original n two-port DC circuit breakers and one DC energy dissipation device.

[0091] 3.3 Operating Principle

[0092] When a DC pole-to-ground short-circuit fault occurs, this device sequentially enters current interruption mode, power dissipation mode, and reclosing mode. In addition, the device also has a startup mode and a normal conduction mode. To clarify its working principle, an (n+1)-terminal multi-terminal DC system will be used as an example (e.g., Figure 3 As shown in the figure, the black solid line represents the current conduction path, and the gray dashed line represents the current blocking path.

[0093] 1) Startup Mode

[0094] Under normal circumstances, the AC power grid feeds back power to the modular multilevel converter and wind farm to achieve black start. During this period, diode D... R1 –D Rn Keep off, disconnect switch D S1 –D Sn And the bypass branch remains conducting, the capacitor in the submodule flows through the lumped resistor current i m Charge to the rated value.

[0095] 2) Normal conduction mode

[0096] When the system is running stably, the total voltage of the cascaded capacitors is equal to the rated voltage of the DC transmission line, and at this time the current i in the lumped resistor is... m It can be ignored. The load current between each converter station flows only through the DC transmission line.

[0097] 3) Current interruption mode

[0098] Assuming a pole-to-ground short-circuit fault occurs on DC line DCTL1 at time t0, the fault current interruption process is described in four stages as follows: fault occurrence stage, fault current commutation stage, valve-side current clearing stage, and DC-side current clearing stage (see [link to relevant documentation]). Figure 4 Port 0 connects to the wind farm-side converter station, while the remaining ports connect to the grid-side converter station. Here, Q... arm1,1 Q arm1,2 Q arm2,1 and Q arm2,2 The actions represent the actions of the IGBTs in the corresponding submodules.

[0099] Stage I (Fault Occurrence): Due to diode D arm1 and D arm2 Due to its unidirectional conductivity, the cascaded capacitor in the converter valve only feeds discharge current i to the grounding point. dc0 and i dci (i=2,…,n), resulting in DC-side fault current i f It rose rapidly.

[0100] Phase II (Fault Current Commutation): After a fault current is detected at time t1, the fast mechanical switch LCS1 connected to the DC fault line is triggered to disconnect, and Q... arm1,1 Q arm2,1 The closing action of diode D. Current flows through diode D. S0 and D Si The current i of (i=2,…,n) dc0 and i dci The current i of the combined valve group Valve1 arm1 The fault current passes through diode D. R1 The current flows into the fault short-circuit point. Due to the unidirectional nature of the diode, the current flows through D.S1 D R0 and D Ri The current in (i=2,…,n) is zero. After the above commutation process is completed, the fast mechanical switch UFD1 connected to the DC fault line is triggered to operate.

[0101] Phase III (Valve-Side Current Clearing): After the mechanical switch action delay, the fast mechanical switch UFD1 is fully open at time t2, triggering the main IGBT of the submodule in Valve1 to disconnect, thus enabling the submodule to be engaged. At this time, the pre-charge capacitor of the submodule in Valve1 is connected in series with the fault circuit to absorb current i. arm1 lumped resistor R m Voltage u at both ends m Rapidly decreasing. When u m When the voltage drops to zero at time t3, diode D m Turning on the circuit will transfer the voltage u of the lumped resistor. m Clamp to zero position, i arm1 It continues to be absorbed by the pre-charge capacitor until it drops to zero. Meanwhile, the DC-side fault current i f Through diode D m The main IGBT and D in the Valve2 valve group submodule R1 A pathway is formed to achieve continuous flow.

[0102] Phase IV (DC-side current clearing): At time t4, the DC-side current i arm1 Clear the Q of the submodule in Valve2 of the trigger valve group arm2,1 The shutdown action and Q arm2,2 The conduction action activates the capacitors and distributed resistors of the Valve2 submodule to absorb the DC-side fault current i. f At the same time, close Q. arm1,2 The distributed resistors dissipate overvoltage in the Valve1 submodule. When the capacitor voltage drops to a normal level, it triggers Q in all submodules. arm1,2 and Q arm2,2 Turn off.

[0103] 4) Surplus power dissipation mode

[0104] When the DC side fault current i f After clearing, the residual current circuit breaker RCB1 connected to the DC fault line is triggered to disconnect (see...). Figure 4 During the deionization of the faulty line, the device operates as a DC chopper, adjusting the number of sub-modules in operation to ensure that its power dissipation equals the system's excess power.

[0105] 5) Reclosing mode

[0106] If the fault is identified as permanent, the residual current switch RCB1 remains open. Conversely, if it is a transient fault, the device will reclose after deionization. By switching the control submodule, reclosing of the residual current switch RCB1 and the fast mechanical switch UFD1 under zero current and zero voltage conditions can be achieved.

[0107] The proposed multi-port hybrid DC circuit breaker with integrated energy dissipation capability was applied to fault handling in a ±400 kV four-terminal flexible DC transmission system. The system's rated power is 600 MW, with each of the wind farm side and three grid-side converter stations having a capacity of 200 MW. Simulation verification was conducted for the most severe near-end pole-to-ground short-circuit fault and AC side three-phase short-circuit fault. The fault detection time was set to 1 ms, and the circuit breaker action delay was set to 2 ms. As shown in the figure, the simulation results show that the device can quickly clear the valve-side fault current within 4.93 ms; during AC / DC faults, the DC voltage fluctuation is always controlled within ±10% of the rated value (maximum 1.08 pu, minimum 0.97 pu), and the submodule capacitor overvoltage during DC faults is only 1.33 pu. In terms of economics, as shown in Table 1, compared with the traditional "two-port DC circuit breaker + DC chopper" scheme, this device can achieve cost reductions of 58.8%, 66.5%, and 71.3% in three-terminal, four-terminal, and five-terminal systems, respectively, while eliminating a large number of independent components and resulting in a more compact structure. Simulations verify that this integrated device has significant performance advantages in maintaining system stability, achieving rapid fault isolation, and dissipating surplus power, combining high reliability with excellent economic efficiency, making it suitable for large-scale offshore wind power DC transmission projects.

[0108] Table 1 Cost Comparison

[0109]

Claims

1. A multi-port AC / DC fault handling device for multi-terminal DC power transmission systems, characterized in that: The multi-port AC / DC fault handling device has n ports, denoted as Port1–Port2. n ; Each port is connected to one end of the bypass branch, feed branch, and injection branch respectively; the other end of the bypass branch, feed branch, and injection branch is connected to the DC bus Bus1, Bus2, and Bus3 respectively. The bypass branch includes a series-connected fast mechanical switch (UFD) and a load converter switch (LCS). Each feed branch consists of n diode strings, denoted as DS1–D Sn ; Each injection branch consists of n diode strings, denoted as D. R1 –D Rn ; Valve1 and Valve2 are both composed of sub-modules connected in series. Valve1 is connected to DC bus Bus1 and Bus3. The submodules in Valve2 contain nested diode full-bridge resistors, and the lumped resistor R... m Reverse parallel diode series D m ; Under DC short-circuit faults, the multi-port AC / DC fault handling device intervenes in the fault current interruption process. The feed branch, injection branch, and valve group Valve1 form an H-bridge structure, thereby achieving bidirectional fault current interruption capability. When a DC fault occurs in Bus1 or Bus3, the fault current is blocked by the H-bridge structure. Valve1 is used to clear the fault current on the valve side, and Valve2 is used to clear the fault current on the DC side. During the deionization of DC short-circuit fault lines and AC short-circuit faults, the multi-port AC / DC fault handling device operates in a power dissipation state. The valve group Valve1 and valve group Valve2 sub-modules switch on and off, adjusting the voltage of the centralized resistor so that the dissipated power matches the surplus power of the system.

2. The multi-port AC / DC fault handling device for a multi-terminal DC power transmission system according to claim 1, characterized in that: Multi-port AC / DC fault handling device is used to handle AC / DC faults in three-port and above DC systems.

3. The multi-port AC / DC fault handling device for a multi-terminal DC power transmission system according to claim 1, characterized in that: The number of ports of the multi-port AC / DC fault handling device is replaced with n+1, with one port connected to the DC side of the converter station on the wind farm side, and the remaining ports connected to the DC transmission line of the DC system.

4. A multi-port AC / DC fault handling device for a multi-terminal DC power transmission system according to claim 1, characterized in that: The multi-port AC / DC fault handling device is used to handle DC short circuit faults, symmetrical AC short circuit faults, and asymmetrical AC short circuit faults. The DC short-circuit faults include pole-to-ground short-circuit faults and pole-to-pole short-circuit faults. The symmetrical AC short-circuit fault is a three-phase short-circuit fault. The asymmetrical AC short-circuit faults include single-phase-to-ground short-circuit faults, two-phase short-circuit faults, and two-phase-to-ground short-circuit faults.

5. A multi-port AC / DC fault handling device for a multi-terminal DC power transmission system according to claim 1, characterized in that: The operating modes of the multi-port AC / DC fault handling device include startup mode, normal conduction mode, current interruption mode, power dissipation mode and reclosing mode.

6. A multi-port AC / DC fault handling device for a multi-terminal DC power transmission system according to claim 5, characterized in that: When a DC short-circuit fault occurs on the DC pole, the multi-port AC / DC fault handling device sequentially enters the current interruption mode, power dissipation mode, and reclosing mode; when an AC short-circuit fault occurs on the DC pole, the multi-port AC / DC fault handling device enters the power dissipation mode.

7. A multi-port AC / DC fault handling device for a multi-terminal DC power transmission system according to claim 5, characterized in that: When the multi-port AC / DC fault handling device is in startup mode, diode D R1 –D Rn Keep off, disconnect switch D S1 –D Sn And the bypass branch remains conducting, the capacitor in the submodule flows through the lumped resistor current i m Charge to the rated value; When the multi-port AC / DC fault handling device is in normal conduction mode, the total voltage of the cascaded capacitors is equal to the rated voltage of the DC transmission line, and the load current between each converter station flows only through the DC transmission line.

8. A multi-port AC / DC fault handling device for a multi-terminal DC power transmission system according to claim 5, characterized in that: When the multi-port AC / DC fault handling device is in current interruption mode, the fault current interruption process sequentially goes through the fault occurrence stage, the fault current commutation stage, the valve side current clearing stage, and the DC side current clearing stage. During the fault occurrence phase, based on diode D in the Valve1 and Valve2 submodules... arm1 and D arm2 Due to its unidirectional conductivity, the cascaded capacitor in the converter valve only feeds discharge current i to the grounding point. dc0 and i dci DC side fault current i f Ascend; i=2,…,n; During the fault current commutation phase, after detecting the fault current at time t1, the fast mechanical switch LCS1 connected to the DC fault line is triggered to disconnect, and Q... arm1,1 Q arm2,1 The closing action; current flows through diode D S0 and D Si current i dc0 and i dci The current i of the combined valve group Valve1 arm1 The fault current passes through diode D. R1 Flow into the fault short-circuit point; based on the unidirectional nature of the diode, flow through D S1 D R0 and D R The current is zero, triggering the action of the fast mechanical switch UFD1 connected to the DC fault line; During the valve-side current clearing phase, after the mechanical switch action delay, the fast mechanical switch UFD1 is fully open at time t2, triggering the main IGBT of the submodule in Valve1 to disconnect, thus enabling the submodule to be engaged. At this time, the pre-charge capacitor of the submodule in Valve1 is connected in series with the fault circuit to absorb current i. arm1 lumped resistor R m Voltage u at both ends m decline; When u m When the voltage drops to zero at time t3, diode D m Turning on the circuit will transfer the voltage u of the lumped resistor. m Clamp to zero position, i arm1 It continues to be absorbed by the pre-charge capacitor until it drops to zero; at the same time, the DC side fault current i f Through diode D m The main IGBT and D in the Valve2 valve group submodule R1 A pathway is formed to achieve continuous flow; During the DC-side current clearing phase, the DC-side current i at time t4 arm1 Clear the Q of the submodule in Valve2 of the trigger valve group arm2,1 The shutdown action and Q arm2,2 The conduction action activates the capacitors and distributed resistors of the Valve2 submodule to absorb the DC-side fault current i. f At the same time, close Q. arm1,2 The distributed resistor is used to dissipate overvoltage in the capacitor of the Valve1 submodule; when the capacitor voltage drops to the normal level, it triggers Q in all submodules. arm1,2 and Q arm2,2 Turn off.

9. A multi-port AC / DC fault handling device for a multi-terminal DC power transmission system according to claim 5, characterized in that: When the multi-port AC / DC fault handling device is in surplus power dissipation mode, the residual current switch RCB1 connected to the DC fault line is disconnected. By adjusting the number of sub-modules in the valve groups Valve1 and Valve2 that are put into operation, the dissipated power is equal to the system's surplus power.

10. A multi-port AC / DC fault handling device for a multi-terminal DC power transmission system according to claim 5, characterized in that: When the multi-port AC / DC fault handling device is in reclosing mode, it detects whether the voltage / current still exists. If it does not exist, the fault is diagnosed as a permanent fault, and the residual current switch RCB1 remains open. If it exists, the fault is diagnosed as a transient fault, and the device recloses after the deionization is completed. The multi-port AC / DC fault handling device enables the reclosing of the residual current switch RCB1 and the fast mechanical switch UFD1 under zero current and zero voltage conditions by controlling the switching of the sub-module.

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