Direct current transmission system and its fault protection method

Abstract

This application provides a DC transmission system and its fault protection method, relating to the field of flexible DC transmission technology. The system includes a renewable energy power station and energy storage system on the generation side, a diode rectifier unit, and a modular multilevel converter. The diode rectifier unit is connected to the generation side via an AC transmission line and to the modular multilevel converter via a DC transmission line. The modular multilevel converter is connected to the power grid. The method includes: detecting that the current in the DC transmission line exceeds a threshold, determining that a system fault has occurred; controlling the modular multilevel converter to reduce the voltage of the DC transmission line, blocking the energy feed from the grid to the DC transmission line; controlling the energy storage system to adjust the voltage of the AC transmission line, reducing the current fed from the generation side to the diode rectifier unit; and controlling the DC transmission system to restart after a first preset time, restoring energy transmission. This application effectively protects the system during faults without significantly increasing costs.

Description

Technical Field

[0001] This application relates to the field of flexible DC transmission technology, and in particular to a DC transmission system and its fault protection method. Background Technology

[0002] With the rapid development of offshore wind power and onshore new energy bases, series transmission systems based on diode rectifier units (DRUs) and modular multilevel converters (MMCs) offer advantages over the more commonly used pure flexible DC solutions, including smaller size, lighter weight, lower cost, and better economic benefits. Therefore, in new energy DC transmission projects, the adoption of DRU-MMC DC transmission systems has become a development trend due to the comprehensive consideration of economic efficiency and technological advancement.

[0003] However, in the DRU-MMC DC transmission system, the diode valves used are inherently uncontrollable. Unlike fully controlled devices, diode valves cannot be actively shut off and will continue to conduct during system faults, which can easily lead to system damage. Therefore, fault handling is particularly important in the DRU-MMC DC transmission system.

[0004] Considering that diode valves cannot actively shut off, the current common fault handling strategy for DRU-MMC DC transmission systems is to use DC circuit breakers combined with single-point overcurrent protection. Specifically, DC circuit breakers are added at different nodes in the system, and current sensors are independently installed at multiple key locations in the system. When any current sensor detects an overcurrent, the corresponding DC circuit breaker at that location is used to cut off the short-circuit current or isolate the fault.

[0005] However, this approach requires additional DC circuit breakers and current sensors. DC circuit breakers themselves are expensive and have significant operating losses, making them unsuitable for large-scale engineering applications. Current sensors also contribute to increased costs. Overall, the current strategy of combining DC circuit breakers with single-point overcurrent protection in the fault protection of DRU-MMC DC transmission systems suffers from high costs. Summary of the Invention

[0006] In view of this, this application provides a DC transmission system and its fault protection method to solve the problem that the current strategy of combining DC circuit breakers with single-point overcurrent protection has high cost.

[0007] This application provides a DC transmission system and a fault protection method thereof through several embodiments. The following description covers multiple aspects, and the embodiments and beneficial effects described herein can be referenced interchangeably.

[0008] Firstly, this application provides a fault protection method for a DC transmission system. This method is applied to a DC transmission system, which includes a generator side, a diode rectifier unit, and a modular multilevel converter. The generator side includes renewable energy power plants and energy storage systems. The AC side of the diode rectifier unit is connected to the generator side via an AC transmission line, and the DC side of the diode rectifier unit is connected to the DC side of the modular multilevel converter via a DC transmission line. The AC side of the modular multilevel converter is connected to the power grid.

[0009] The fault protection method includes:

[0010] During the operation of a DC transmission system, the current in the DC transmission lines is monitored.

[0011] When the current in a DC transmission line is detected to exceed the current threshold, a fault is determined to have occurred in the DC transmission system, and the following actions are performed.

[0012] The modular multilevel converter is controlled to reduce the voltage of the DC transmission line and block the energy feed from the grid to the DC transmission line.

[0013] The energy storage system is controlled to reduce the voltage of the AC transmission line and decrease the current fed into the diode rectifier unit on the generator side.

[0014] After the energy storage system reduces the voltage of the AC transmission line, the DC transmission system is restarted after a first preset time to restore the energy transmission from the DC transmission system to the grid.

[0015] In one possible implementation of the first aspect above, controlling the energy storage system to reduce the voltage of the AC transmission line includes:

[0016] Calculate the target value U of the AC bus voltage according to the preset AC bus voltage calculation model. S .

[0017] Based on the target value U of AC bus voltage S The energy storage system is controlled to reduce the voltage of AC transmission lines.

[0018] The AC bus voltage calculation model includes:

[0019]

[0020] In the formula, I d U represents the actual current value of the DC transmission line. con X represents the rated voltage of the DC transmission line. t U is the leakage reactance of the converter transformer in the diode rectifier unit. S This is the target value for the AC bus voltage.

[0021] In one possible implementation of the first aspect above, controlling the restart of the DC transmission system includes:

[0022] The control modular multilevel converter adjusts the voltage of the DC transmission line to a first voltage value to establish a connection between the modular multilevel converter and the power grid.

[0023] The control energy storage system adjusts the voltage of the AC transmission line to the second voltage value.

[0024] The system detects the output of the new energy power station. When power output is detected, the system controls the energy storage system to adjust the voltage of the AC transmission line to the third voltage value so that the energy output from the generation side can be transmitted to the grid through the diode rectifier unit and the modular multilevel converter.

[0025] In one possible implementation of the first aspect above, controlling the energy storage system to adjust the voltage of the AC transmission line to the third voltage value includes:

[0026] The third voltage value is calculated based on the power-voltage coordinated control model.

[0027] The power-voltage coordinated control model includes:

[0028]

[0029] In the formula, P r For the target output power of the AC transmission line, U con U is the rated voltage value of the DC transmission line. S-R X is the third voltage value. t This is the leakage reactance of the converter transformer in the diode rectifier unit.

[0030] In one possible implementation of the first aspect described above, the method further includes:

[0031] During the restart process of the controlled DC transmission system, the current of the DC transmission line is detected.

[0032] If the current of the DC transmission line is detected to exceed the current threshold, the restart is determined to have failed. After the current of the DC transmission line exceeds the current threshold, the DC transmission system is restarted again after a second preset time. During the restart process, the current of the DC transmission line is monitored to determine whether the restart has failed.

[0033] Once the number of restarts exceeds the preset threshold, the restart control of the DC transmission system will be stopped, and the path between the generator side and the diode rectifier unit will be disconnected.

[0034] In one possible implementation of the first aspect described above, the DC transmission system further includes reactors installed on the DC transmission lines. The reactors can reduce the rate of rise and peak value of the current on the DC transmission lines in the event of a fault in the DC transmission system.

[0035] Secondly, this application provides a DC power transmission system, which includes: a power generation side, a diode rectifier unit, a modular multilevel converter, and a DC control and protection system.

[0036] The power generation side includes new energy power plants and energy storage systems. The AC side of the diode rectifier unit is connected to the power generation side via an AC transmission line, and the DC side of the diode rectifier unit is connected to the DC side of the modular multilevel converter via a DC transmission line. The AC side of the modular multilevel converter is connected to the power grid. The DC control and protection system executes the DC transmission system fault protection method disclosed in the first aspect and any possible implementation thereof.

[0037] Thirdly, this application provides a fault protection device for a DC transmission system. This device is applied to a DC transmission system, which includes a generator side, a diode rectifier unit, and a modular multilevel converter. The generator side includes renewable energy power plants and energy storage systems. The AC side of the diode rectifier unit is connected to the generator side via an AC transmission line, and the DC side of the diode rectifier unit is connected to the DC side of the modular multilevel converter via a DC transmission line. The AC side of the modular multilevel converter is connected to the power grid.

[0038] The fault protection device includes a fault detection module, a fault determination module, a receiving end control module, a sending end control module, and a system restart control module.

[0039] The fault detection module detects the current in the DC transmission lines during operation. The fault determination module determines a fault in the DC transmission system when the current exceeds a threshold. The receiving-end control module controls the modular multilevel converter to reduce the voltage of the DC transmission lines, blocking energy feed from the grid. The sending-end control module controls the energy storage system to reduce the voltage of the AC transmission lines, decreasing the current fed into the diode rectifier unit on the generator side. The system restart control module, after controlling the energy storage system to reduce the voltage of the AC transmission lines, restarts the DC transmission system after a first preset time to restore energy transmission from the DC transmission system to the grid.

[0040] Fourthly, this application provides an electronic device. The electronic device includes a processor and a memory, the memory storing at least one instruction or at least one program segment, which is loaded and executed by the processor to implement the DC transmission system fault protection method disclosed in the first aspect and any possible implementation thereof.

[0041] Fifthly, this application provides a computer-readable storage medium storing at least one instruction or at least one program, wherein the at least one instruction or at least one program is loaded and executed by a processor to implement the DC transmission system fault protection method disclosed in the first aspect and any possible implementation thereof.

[0042] The above-mentioned technical solution of this application has at least one of the following beneficial effects:

[0043] The fault protection method proposed in this application is based on the original structure of the system and addresses the issue from two aspects: the energy storage system on the generation side and the modular multilevel converter on the receiving side. This reduces the energy feed into the DC transmission line, blocks the energy source of the fault current, and avoids further damage to the system by the fault current. It achieves active fault suppression and energy blocking, and can effectively protect the entire system during a fault without requiring additional components, thus avoiding increased costs. Compared with the traditional strategy of combining DC circuit breakers with single-point overcurrent protection, it has the advantage of lower cost.

[0044] In addition, after detecting the fault and handling it from the generation and receiving sides respectively, this application waits for a period of time and then promptly controls the DC transmission system to restart. It properly considers the black start problem of the system after the fault is cleared, realizes the whole chain solution of "detection-isolation-recovery" of the fault, provides a complete fault handling solution, improves the reliability of the system, and makes the system applicable to a variety of complex scenarios. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the DC power transmission system structure provided in the embodiments of this application;

[0046] Figure 2 This is a flowchart illustrating a fault protection method for a DC transmission system provided in an embodiment of this application.

[0047] Figure 3 (a) in the figure is a schematic diagram of the full-bridge submodule provided in the embodiment of this application being in the active state;

[0048] Figure 3 (b) is a schematic diagram of the full-bridge submodule provided in the embodiment of this application when it is in a negative input state;

[0049] Figure 3 (c) is a schematic diagram of the full-bridge submodule provided in the embodiment of this application in the bypass state;

[0050] Figure 4 This is a schematic diagram of the structure of a modular multilevel converter provided in an embodiment of this application;

[0051] Figure 5 A waveform diagram of the AC side voltage of a modular multilevel converter provided in an embodiment of this application within one cycle;

[0052] Figure 6 This is another schematic flowchart of the fault protection method for DC transmission systems provided in the embodiments of this application;

[0053] Figure 7 This is a schematic diagram of the structure of a fault protection device for a DC transmission system provided in an embodiment of this application;

[0054] Figure 8 A block diagram of an electronic device provided in an embodiment of this application;

[0055] Figure 9 This is a block diagram of a SoC (System on Chip) provided for an embodiment of this application. Detailed Implementation

[0056] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0057] In the existing technology, the fault handling of DRU-MMC DC transmission systems generally adopts a strategy of DC circuit breaker combined with single-point overcurrent protection, but this strategy has the problem of high cost.

[0058] In view of this, the present application provides a DC transmission system and its fault protection method. The fault protection method is based on the original structure of the system and is processed from both the generation side and the receiving side of the modular multilevel converter, so as to realize active suppression of faults and energy blocking, without the need for additional devices and thus not easily increasing costs.

[0059] The DC transmission system and its fault protection method according to the embodiments of this application will be described below with reference to specific embodiments and accompanying drawings. It should be understood that the DC transmission system in the embodiments of this application refers to the DRU-MMC DC transmission system.

[0060] First, let's introduce the DC power transmission system.

[0061] refer to Figure 1 , Figure 1 This is a schematic diagram of a DC power transmission system provided in an embodiment of this application.

[0062] like Figure 1 As shown, the DC transmission system includes a power generation side, a diode rectifier unit, and a modular multilevel converter. The power generation side includes new energy power plants and energy storage systems.

[0063] The diode rectifier unit's AC side is connected to the generator side via an AC transmission line, the diode rectifier unit's DC side is connected to the modular multilevel converter's DC side via a DC transmission line, and the modular multilevel converter's AC side is connected to the power grid.

[0064] Continue to refer to Figure 1 The DC transmission system may also include a sending-end circuit breaker connected between the diode rectifier unit and the generator side, and a receiving-end circuit breaker connected between the modular multilevel converter and the power grid. Specifically, two sending-end circuit breakers may be used, respectively installed on the positive AC transmission line and the negative AC transmission line between the diode rectifier unit and the generator side.

[0065] DC transmission systems can also include DC control and protection systems, which can communicate with new energy power plants, energy storage systems, diode rectifier units, modular multilevel converters, and the power grid. They are responsible for detecting faults in the DC transmission system and protecting it.

[0066] The following is a detailed explanation of each module of the DC transmission system.

[0067] A renewable energy power station can include large-scale onshore and offshore wind and solar power generation systems. For example, a renewable energy power station may include wind power plants, photovoltaic power plants, substations, SVG (Static Var Generator), and dispatching systems. The connection is as follows: the input of the photovoltaic converter is connected to the photovoltaic power plant, the input of the wind turbine converter is connected to the wind turbine power plant, the outputs of the photovoltaic and wind turbine converters are combined and connected to the input of the substation, and the output of the substation is connected to the AC transmission line. Inside the renewable energy power station, the photovoltaic and wind turbine converters typically operate in grid-connected current source mode, possessing phase-locked loop (PLL) and high-voltage ride-through (HV R-through) capabilities, and can receive system dispatch and respond quickly. The functions of each component are as follows: the photovoltaic converter outputs the active power of the photovoltaic power station, the wind turbine converter outputs the active power of the wind turbine, and then the power is boosted by the step-up substation and transmitted to the AC grid, thereby reducing energy loss and increasing transmission distance. The SVG is used to compensate for the reactive power of the new energy power station. The dispatching system is mainly used to realize the functions of information collection and access, power prediction and power control, monitoring and control, and comprehensive statistical analysis of the new energy power station.

[0068] Energy storage systems can be specifically configured as grid-connected energy storage systems. Grid-connected energy storage systems can provide supporting power for renewable energy power plants, constructing a power grid to support large-scale renewable energy grid connection and transmission. These include centralized, clustered, and high-voltage direct-connected energy storage systems. Grid-connected energy storage systems can autonomously construct AC grid voltage, enabling black start of renewable energy power plants, and can support multiple units operating in parallel. They also feature high reactive power balance, automatic adjustable grid voltage, and low-voltage fault ride-through capabilities.

[0069] The diode rectifier unit may include a converter transformer and a three-phase bridge rectifier circuit based on diodes. Considering the quality of the AC power grid, the diode rectifier unit may adopt 6-pulse, 12-pulse, 24-pulse rectification, etc. The embodiments of this application do not impose specific limitations on this.

[0070] DC transmission lines can include DC submarine cables and DC overhead lines, and are used to achieve long-distance power transmission.

[0071] Modular multilevel converters can employ flexible DC valves based on a hybrid of full-bridge and half-bridge sub-modules to invert power from DC transmission lines and transmit it to the power grid.

[0072] The following example uses a DC control and protection system as the subject of the fault protection method to illustrate the specific fault protection method for DC transmission systems.

[0073] refer to Figure 2 , Figure 2 This is a flowchart illustrating a fault protection method for a DC transmission system provided in an embodiment of this application.

[0074] like Figure 2 As shown, the fault protection method includes the following steps S100~S500.

[0075] Step S100: During the operation of the DC transmission system, the current of the DC transmission line is detected.

[0076] Step S200: When the current of the DC transmission line is detected to exceed the current threshold, it is determined that a fault has occurred in the DC transmission system.

[0077] It is understood that the current threshold is higher than the normal current value on the DC transmission line during normal system operation. Those skilled in the art can set the specific value of the current threshold themselves, and this application does not specifically limit this. For example, the current threshold can be set to 1.5 times the normal current value, such as 3kA.

[0078] In step S300, the modular multilevel converter is controlled to reduce the voltage of the DC transmission line, thereby blocking the energy feed from the grid to the DC transmission line.

[0079] Specifically, in step S300, the DC control and protection system can control the modular multilevel converter to reduce the voltage of the DC transmission line to zero, thereby blocking the power grid from feeding energy into the DC transmission line. In order to further improve system safety, after the voltage of the DC transmission line is reduced to zero, the modular multilevel converter can continue to be controlled so that the DC transmission line remains at zero voltage until the fault is cleared.

[0080] It is understood that in this embodiment, the modular multilevel converter can be composed of multiple cascaded sub-modules (SMs). These sub-modules may include full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs), meaning the modular multilevel converter can be a hybrid type. Based on this, by controlling the number of sub-modules in operation and bypass, the modular multilevel converter can output positive, negative, and zero voltage. During normal system operation, the sub-modules in the modular multilevel converter cooperate to ensure normal AC output on the AC side. When a system fault occurs, the DC protection system quickly controls a portion of the sub-modules in the modular multilevel converter to output negative voltage, thereby canceling out the positive voltage output by another portion of the sub-modules. This rapidly reduces the voltage of the DC transmission line. When the voltage of the DC transmission line drops to zero, the potential difference between the fault point and ground is zero, thus blocking the energy feed from the grid to the DC transmission line.

[0081] To facilitate understanding, the following will be combined with... Figure 3 Sections (a), (b), and (c) provide specific descriptions of the various operating states of the full-bridge submodule.

[0082] Figure 3 (a) in the figure is a schematic diagram of the full-bridge submodule provided in the embodiment of this application when it is in the active state. Figure 3 (b) in the figure is a schematic diagram of the full-bridge submodule provided in the embodiment of this application when it is in a negative input state. Figure 3 (c) in the figure is a schematic diagram of the full-bridge submodule provided in the embodiment of this application in the bypass state, as shown in the figure. Figure 3 As shown in (a), (b), and (c), the full-bridge submodule includes four Insulated Gate Bipolar Transistors (IGBTs) (T1, T2, T3, and T4 in the figure) and one capacitor (capacitor C in the figure), and each IGBT includes a corresponding body diode (D1, D2, D3, and D4 in the figure). It has three operating states: positive input state, negative input state, and bypass state. These three operating states are explained in detail below.

[0083] Specifically, such as Figure 3 As shown in (a), when a PWM signal is applied to T1 and T4 to turn them on, the full-bridge submodule enters the active state. This state includes the following two stages: in the first stage, the current starts from port A, passes through D1 and D4 in sequence, and flows to port B; in the second stage, the current starts from port B, passes through T4 and T1 in sequence, and flows to port A, making the output voltage of the full-bridge submodule +Uc.

[0084] like Figure 3 As shown in (b), when a PWM signal is applied to T2 and T3 to turn them on, the full-bridge submodule enters the negative input state. This state includes the following two stages: in the first stage, the current starts from port A, passes through T2 and T3 in sequence, and flows to port B; in the second stage, the current starts from port B, passes through D3 and D2 in sequence, and flows to port A, making the output voltage of the full-bridge submodule -Uc.

[0085] like Figure 3 As shown in (c), applying a PWM signal to T1, T2, T3, and T4 turns them on, and the full-bridge submodule enters a bypass state. This state includes the following two stages: in the first stage, the current starts from port A, passes through D1 and T3 in sequence, and flows to port B; in the second stage, the current starts from port B, passes through T4 and D2 in sequence, and flows to port A, making the output voltage of the full-bridge submodule zero.

[0086] As can be seen, in the positive and negative input states, the capacitor is connected to the circuit through different IGBT combinations, and the full-bridge submodule outputs a positive or negative voltage. In the bypass state, the output voltage of the full-bridge submodule is zero.

[0087] It is understandable that half-bridge modules typically only have two states: positive input and bypass, meaning they can only output positive voltage and zero voltage, which will not be elaborated further here.

[0088] Based on this, during normal system operation, the DC control and protection system can control the operating status of each sub-module in the modular multilevel converter, enabling the modular multilevel converter to output AC power. The specific details of its AC output are explained below.

[0089] Figure 4 This is a schematic diagram of the modular multilevel converter provided in the embodiments of this application, as shown below. Figure 4 As shown, this modular multilevel converter includes upper and lower bridge arms, and each bridge arm includes four sub-modules. These four sub-modules are a mixture of full-bridge and half-bridge sub-modules, which can generate five voltage levels. The following section uses... Figure 4 Taking the hybrid MMC structure shown as an example, its output AC power will be explained in detail.

[0090] To ensure the stable operation of this modular multilevel converter and its ability to perform rectification and inversion, the following two conditions must be met:

[0091] First, it is necessary to ensure a constant DC-side voltage. This condition can be achieved by keeping the sum of the number of submodules deployed in the upper and lower bridge arms constant. In other words, the following relationship must be satisfied:

[0092] u pj +u nj =U dc

[0093] N p +N n =N

[0094] In the formula, u pj Indicates the output voltage of the upper bridge arm, u nj U represents the output voltage of the lower bridge arm. dc N represents the DC side voltage. p N represents the number of submodules deployed on the upper bridge arm, that is, the number of submodules in the upper bridge arm that are currently deployed. n This indicates the number of sub-modules deployed in the lower bridge arm, that is, the number of sub-modules in the lower bridge arm that are in active deployment. N represents the total number deployed.

[0095] Secondly, in order to maintain a sinusoidal output voltage on the AC side, the upper and lower bridge arms need to satisfy the following relationship:

[0096] u pj =0.5*U dc -u j

[0097] u nj =0.5*U dc +u j

[0098] -0.5*(u pj -u nj )=u j

[0099] In the formula, u j This indicates the AC side voltage.

[0100] In this structure, by changing the number of upper and lower bridge arm sub-modules engaged, the output voltage of the upper and lower bridge arms can be changed, thereby meeting the requirement of outputting a sinusoidal stepped wave on the AC side. The specific explanation is as follows.

[0101] Figure 5 The AC side voltage waveform of the modular multilevel converter provided in this application embodiment is shown in Table 1, which illustrates the submodule switching status within one cycle. In Table 1, the number of submodules connected in the upper arm represents the number of submodules in the upper arm that are actively connected, while the other submodules in the upper arm are in a bypass state. Similarly, the number of submodules connected in the lower arm represents the number of submodules in the lower arm that are actively connected, while the other submodules in the lower arm are in a bypass state.

[0102] like Figure 5 As shown in Table 1, within one cycle, when the DC-side voltage of the modular multilevel converter is always U... dc During different time periods of this cycle, as the number of submodules engaged in the upper and lower bridge arms changes, the AC side voltage gradually increases from 0 to U. dc / 4、U dc / 2, and from U dc / 2 gradually decreases to U dc / 4, 0, -U dc / 4、-U dc / 2, finally from -U dc / 2 increases to -U dc / 4. As can be seen, within one cycle, the modular multilevel converter completes the output of a sinusoidal stepped wave, that is, it completes the output of alternating current.

[0103]

[0104] Therefore, based on the foregoing, the FBSM is a submodule with three states: positive voltage, zero voltage, and negative voltage. When a system fault occurs at a certain moment, the method of controlling the modular multilevel converter to reduce the DC side voltage to zero can be referred to the description of the non-blocking fault ride-through function of the hybrid MMC in the prior art. Its core is to use a submodule like the FBSM with fault self-clearing capability to work in the negative input state and output a negative voltage to offset or counteract the system voltage. This will not be elaborated further here.

[0105] In step S400, the energy storage system is controlled to reduce the voltage of the AC transmission line and reduce the current fed into the diode rectifier unit on the generation side.

[0106] It is understandable that the higher the voltage of the AC transmission line, the greater the power transmitted by the diode rectifier unit, and the more current is fed into the diode rectifier unit from the generator side. Therefore, in the event of a system fault, this step reduces the voltage of the AC transmission line on the generator side, thereby reducing the active current fed into the diode rectifier unit input terminal from the generator side, which can significantly reduce the current fed into the DC transmission line.

[0107] In step S500, after controlling the energy storage system to reduce the voltage of the AC transmission line, after a first preset time, the DC transmission system is restarted to restore the energy transmission from the DC transmission system to the grid.

[0108] Understandably, DC transmission systems typically possess a self-clearing fault function, allowing short-term fault currents to clear automatically. That is, after the fault handling operations described in S300 and S400, short-term faults in the DC transmission system can be cleared automatically. Therefore, after controlling the energy storage system to reduce the voltage of the AC transmission line, a first preset time can be waited for, i.e., for the fault on the DC transmission line to automatically recover. The first preset time can be set according to the length of the DC transmission line; generally, the longer the DC transmission line, the longer the first preset time, for example, it can be set to 100~500ms.

[0109] In addition, during the process of restarting the DC transmission system in step S500 above, it is also possible to determine whether the fault is permanent and to take further measures, including the following process.

[0110] During the restart process of the controlled DC transmission system, the current of the DC transmission line is detected.

[0111] If the current in the DC transmission line exceeds the current threshold, the restart is deemed a failure. After a second preset time has elapsed since the current exceeds the threshold, the DC transmission system is restarted again. During the restart process, the current in the DC transmission line is monitored to determine whether the restart has failed. For example, the second preset time can be 500ms.

[0112] The DC transmission line is considered to have a permanent fault once the number of restarts exceeds a preset threshold (e.g., 3 times). Therefore, restarting the DC transmission system is stopped, and the connection between the generator side and the diode rectifier unit is disconnected to prevent further damage. Personnel are promptly alerted to address the issue. This process can be achieved by controlling the sending-end circuit breaker to disconnect the connection between the generator side and the diode rectifier unit.

[0113] Through the aforementioned steps S100~S500, the fault protection method of this application embodiment, when a fault occurs in a DC transmission line, firstly, handles the issue from the receiving end's modular multilevel converter, blocking the energy feed from the grid to the DC transmission line; secondly, it handles the issue from the generation side, reducing the energy feed from the generation side to the sending end's diode rectifier unit, and then promptly restarts the control system after the fault current is automatically cleared. It can be seen that the fault protection method of this application embodiment, based on the original system structure, handles the issue from both the generation-side energy storage system and the receiving-side modular multilevel converter, reducing the energy feed to the DC transmission line, blocking the energy source of the fault current, and preventing further damage to the system from the fault current. This achieves active fault suppression and energy blocking, effectively protecting the entire system during a fault without requiring additional components, thus minimizing cost increases. Compared to the traditional strategy of combining DC circuit breakers with single-point overcurrent protection, it has the advantage of lower cost.

[0114] In addition, the fault protection method of this application embodiment continuously monitors whether the fault is cleared after the fault is detected and handled from the generation side and the receiving side respectively. After the fault is cleared, the DC transmission system is restarted in a timely manner, and the black start problem of the system is properly considered. It realizes the whole chain solution of fault "detection-isolation-recovery", provides a complete fault handling solution, improves the reliability of the system, and makes the system applicable to a variety of complex scenarios, promoting the development of DC transmission system towards high efficiency, high power density and high reliability.

[0115] The following section details the design work done on DC transmission lines to further address fault issues in DC transmission systems.

[0116] Continue to refer to Figure 1 ,like Figure 1 As shown, the DC transmission system also includes reactors installed on the DC transmission lines. These reactors can reduce the rate of rise and peak value of the current on the DC transmission lines when a fault occurs, further reducing the damage caused by the fault current to the system. Specifically, the DC transmission system may include two reactors, one installed on the positive DC transmission line and the other on the negative DC transmission line.

[0117] Understandably, during normal system operation, the current on the DC transmission line is within the normal range, and the reactor is at a low inductance value. When a fault occurs in the DC transmission line, the current on the DC transmission line increases. At this time, the reactor, according to its own characteristics, rapidly increases its equivalent reactance, thereby effectively reducing the rate of rise and peak value of the current on the DC transmission line. The reactor can be a superconducting current-limiting or saturated iron core type DC current-limiting reactor, and this application does not impose specific limitations on this.

[0118] Therefore, installing reactors on DC transmission lines serves two purposes. First, based on the inherent characteristics of reactors, which do not require commands, they can quickly reduce the rise rate and peak value of fault current when a fault in the DC transmission line causes a sudden current change. Utilizing their rapid response, this further improves the speed and efficiency of fault protection. Second, building upon the dual protection achieved through coordinated processing from the sending end to the receiving end, an additional layer of protection is added to the DC transmission line itself, realizing coordinated processing from the sending end, the DC transmission line, and the receiving end, resulting in more comprehensive protection.

[0119] The process of controlling the energy storage system to reduce the voltage of the AC transmission line in step S400 above will be described below. Step S400 may specifically include the following steps S410 to S420.

[0120] Step S410: Calculate the target value U of the AC bus voltage according to the preset AC bus voltage calculation model. S .

[0121] The calculation model for AC bus voltage is as follows:

[0122]

[0123] In the formula, I d U represents the actual current value of the DC transmission line. con U is the rated voltage value of the DC transmission line. S X is the target value for AC bus voltage. t This is the leakage reactance of the converter transformer in the diode rectifier unit.

[0124] In this AC bus voltage calculation model, I d The value is obtained in real time from the DC control and protection system, U con The value is preset, for example, it can be set to ±200kV, X t The value can be manually entered into the DC control and protection system based on information such as the converter transformer model, or the DC control and protection system can automatically query the information based on the converter transformer model. This application embodiment does not impose specific limitations on this. Based on these data in the AC bus voltage calculation model, the target value U of the AC bus voltage can be calculated.S .

[0125] Step S420, based on the target value U of the AC bus voltage S The energy storage system is controlled to reduce the voltage of AC transmission lines.

[0126] Through steps S410~S420, the target value U of the AC bus voltage is calculated based on the voltage and current data of the DC transmission line. S This controls the energy storage system to operate according to the target AC bus voltage value U. S The voltage of the AC transmission line is adjusted by actively reducing it to U using feedforward control. S This reduces the current fed into the DC transmission line.

[0127] The derivation process of the AC bus voltage calculation model used in step S410 is explained in detail below.

[0128] Based on the DC voltage drop formula and the DC voltage control target value formula, the AC bus voltage calculation model can be derived.

[0129] The formula for DC voltage drop is as follows:

[0130]

[0131] In the formula, U d U represents the voltage value of a DC transmission line. S I is the target value of AC bus voltage. d This represents the actual current value of the DC transmission line.

[0132] The formula for the DC voltage control target value is:

[0133] U d =U con

[0134] In the formula, U con This refers to the rated voltage value of a DC transmission line.

[0135] It is understandable that in the DC voltage control target value formula, the voltage value U of the DC transmission line is set... d Equal to the rated voltage value U of the DC transmission line con Substituting this into the DC voltage drop formula aims to force the DC transmission line voltage to reach this value during fault protection through the cooperation of modular multilevel converters and energy storage systems, thereby achieving rapid current reduction.

[0136] The process of restarting the DC transmission system in step S500 is described below.

[0137] Reference Figure 6 , Figure 6 This is another schematic flowchart of the fault protection method for DC transmission systems provided in the embodiments of this application.

[0138] like Figure 6 As shown, the process of controlling the restart of the DC transmission system may specifically include the following steps S510~S530.

[0139] Step S510: Control the modular multilevel converter to adjust the voltage of the DC transmission line to a preset first voltage value so that a connection is established between the modular multilevel converter and the power grid.

[0140] Through this step, after the fault current is cleared, the modular multilevel converter is first started. The modular multilevel converter is then controlled to adjust the voltage of the DC transmission line to a preset first voltage value, establishing a connection between the modular multilevel converter and the grid. At this point, the grid reverse-charges the modular multilevel converter, thereby quickly restoring the voltage of the DC transmission line. For example, the first voltage value can be set to ±200kV, that is, this first voltage value is equal to the aforementioned rated voltage U of the DC transmission line. con Consistent.

[0141] It is understandable that, as mentioned above, during the restart process, the DC control and protection system can also re-detect the current of the DC transmission line, which can be done during the process of establishing a connection between the modular multilevel converter and the grid in this step.

[0142] Step S520: Control the energy storage system to adjust the voltage of the AC transmission line to a preset second voltage value.

[0143] In this step, specifically, the energy storage system can be controlled to enter the discharge mode, and then the voltage of the AC transmission line can be adjusted to a preset second voltage value.

[0144] Understandably, during system restart, the renewable energy power station is an islanded system, not connected to the grid. Therefore, the energy storage system needs to first establish the AC voltage and frequency for grid connection to facilitate the startup of the renewable energy power station. Thus, after restoring the DC transmission line voltage in step S510, this step S520 controls the energy storage system to stabilize the AC transmission line voltage to a second voltage value, providing a foundation for the startup of the renewable energy power station and facilitating its rapid subsequent startup. For example, the second voltage value can be set to 35kV.

[0145] Specifically, the energy storage system may include a power conversion system (PCS), a battery management system (BMS), and an AC / DC converter, wherein the power flow of the AC / DC converter is bidirectional. In this step, a command may be sent from the DC protection system to the PCS, causing the PCS to control the battery to enter a discharge mode and control the AC / DC converter to adjust the voltage of the AC transmission line to a preset second voltage value.

[0146] Step S530: Detect the output of the new energy power station. When power output is detected from the new energy power station, control the energy storage system to adjust the voltage of the AC transmission line to the third voltage value so that the energy output from the generation side can be transmitted to the grid through the diode rectifier unit and the modular multilevel converter.

[0147] Understandably, since the renewable energy power station can adaptively adjust its output based on the voltage of the AC transmission line, after the energy storage system reduces the voltage of the AC transmission line in step S400, the renewable energy power station enters a low-voltage ride-through mode, meaning it stops outputting active power. When the energy storage system raises the voltage of the AC transmission line back to the second voltage value in step S520, the renewable energy power station automatically adjusts and outputs active power based on the increased voltage, thus participating in the operation of the DC transmission system.

[0148] Understandably, after detecting output from a renewable energy power station, it is necessary to increase the voltage of the AC transmission line based on the output of the renewable energy power station to complete the power transmission from the generation side to the grid via the DC transmission system. For example, the third voltage value can be set to 35.5kV.

[0149] In this step, the voltage of the AC transmission line can still be adjusted to the third voltage value using an AC / DC converter.

[0150] The calculation method for the third voltage value is explained in detail below. That is, before the energy storage system adjusts the voltage of the AC transmission line to the third voltage value in step S530, the method also includes the following steps.

[0151] The third voltage value is calculated based on the power-voltage coordinated control model.

[0152] The power-voltage coordinated control model includes:

[0153]

[0154] In the formula, P r For the target output power of the AC transmission line, Ucon U is the rated voltage value of the DC transmission line. S-R X is the third voltage value. t This is the leakage reactance of the converter transformer in the diode rectifier unit.

[0155] In this formula, the target output power P of the AC transmission line r This indicates the amount of power transmitted to the power grid. Based on this, those skilled in the art can set it according to actual needs, for example, it can be set to 200MW. Similarly, for the rated voltage value U of DC transmission lines... con Those skilled in the art can also set it themselves according to actual needs, for example, it can be set to ±200kV, X t The value can be manually entered into the DC control and protection system based on the converter transformer model, or the DC control and protection system can automatically query the value based on the converter transformer model. This application embodiment does not impose specific limitations on this. Based on these known data in the formula, the third voltage value U can be calculated. S-R .

[0156] It is understood that the step of calculating the third voltage value can be pre-calculated by the DC control and protection system before the DC transmission system starts and runs, or it can be calculated when the power output of the new energy power station is detected, or it can be calculated at other time points. As long as it is before the DC control and protection system controls the energy storage system to adjust the voltage of the AC transmission line to the third voltage value, this application embodiment does not impose specific restrictions on this.

[0157] Through S520~S530, the system smoothly transitions from a startup mode where the energy storage system "solely supports" the system to a normal operation mode where the new energy power station and the energy storage system "share power supply", ensuring a smooth system recovery process and reducing the impact on the power grid and the generation side.

[0158] Therefore, through the startup process described in S510~S530, a highly controllable modular multilevel converter is first used to quickly restore the voltage of the DC transmission line, establishing a stable foundation for system startup. Then, the energy storage system is started, autonomously constructing voltage and frequency to quickly restore the voltage of the AC transmission line and provide a basis for the connection of renewable energy power plants. Finally, the renewable energy power plant is connected, and the voltage of the AC transmission line is appropriately boosted through the energy storage system, enabling stable power transmission from the generation side to the grid. This series of operations not only achieves a black start for the DC transmission system, restoring normal system operation and bringing the system to a stable working state, but also strictly adheres to the "receiving end first, then sending end" restart procedure, avoiding system impact before readiness and preventing secondary faults or equipment damage that may occur during the restart process.

[0159] Now refer to Figure 7 , Figure 7 This is a schematic diagram of the structure of a DC transmission system fault protection device 1100 provided in an embodiment of this application. The fault protection device 1100 can be applied to a DC transmission system, and the specific structure of the DC transmission system can be referred to in the above embodiment. Figures 1-6 The corresponding explanations will not be repeated here.

[0160] like Figure 7 As shown, the fault protection device 1100 may specifically include a fault detection module 1101, a fault determination module 1102, a receiving end control module 1103, a sending end control module 1104, and a system restart control module 1105. The functions of each module are explained in detail below.

[0161] The fault detection module 1101 is used to detect the current of the DC transmission line during the operation of the DC transmission system.

[0162] The function of the fault determination module 1102 is to determine that a fault has occurred in the DC transmission system when the current of the DC transmission line exceeds the current threshold.

[0163] The function of the receiving end control module 1103 is to control the modular multilevel converter to reduce the voltage of the DC transmission line and block the energy feed from the grid to the DC transmission line.

[0164] The function of the sending-end control module 1104 is to control the energy storage system to reduce the voltage of the AC transmission line and reduce the current fed into the diode rectifier unit on the generation side.

[0165] The function of the system restart control module 1105 is to control the DC transmission system to restart after the energy storage system reduces the voltage of the AC transmission line and after a first preset time, so as to restore the energy transmission of the DC transmission system to the grid.

[0166] It is understood that in the apparatus of this application embodiment, each module executes the method of the above embodiment, and its specific functions and corresponding technical effects can be referred to the above embodiments. Figures 1-6 The methods explained will not be elaborated here.

[0167] Now for reference Figure 8 The diagram shown is a block diagram of an electronic device 1200 according to an embodiment of this application. The electronic device 1200 may include one or more processors (corresponding to...) coupled to a controller hub 1203. Figure 8The first processor 1201 is described above. In at least one embodiment, the controller hub 1203 communicates with the first processor 1201 via a multi-branch bus such as a Front Side Bus (FSB), a point-to-point interface such as a Quick Path Interconnect (QPI), or a similar connection. The first processor 1201 executes instructions that control general types of data processing operations. In one embodiment, the controller hub 1203 includes, but is not limited to, a Graphics Memory Controller Hub (GMCH) (not shown) and an Input / Output Hub (IOH) (which may be on a separate chip) (not shown), wherein the GMCH includes memory and a graphics controller and is coupled to the IOH.

[0168] Electronic device 1200 may also include a coprocessor coupled to controller hub 1203 (corresponding to...) Figure 8 The processor 1202 and memory 1204 are integrated within the processor (as described in this application). Alternatively, one or both of the memory and GMCH can be integrated within the processor (as described in this application), with memory 1204 and the first coprocessor 1202 directly coupled to the first processor 1201 and the controller hub 1203, which is located on a single chip with the IOH. Memory 1204 can be, for example, Dynamic Random Access Memory (DRAM), Phase Change Memory (PCM), or a combination of both. In one embodiment, the first coprocessor 1202 is a dedicated processor, such as a high-throughput MIC processor (Many Integerated Core, MIC), a network or communication processor, a compression engine, a graphics processor, a general-purpose graphics processor (GPGPU), or an embedded processor, etc. Optional properties of the first coprocessor 1202 are indicated by dashed lines. Figure 8 middle.

[0169] As a computer-readable storage medium, memory 1204 may include one or more tangible, non-transitory computer-readable media for storing programs and / or instructions. For example, memory 1204 may include any suitable non-volatile memory such as flash memory and / or any suitable non-volatile storage device such as one or more hard-disk drives (HDDs), one or more compact disc (CD) drives, and / or one or more digital versatile disc (DVD) drives.

[0170] In one embodiment, electronic device 1200 may further include a Network Interface Controller (NIC) 1206. The network interface 1206 may include a transceiver for providing a radio interface for electronic device 1200 to communicate with any other suitable device, such as a front-end module, antenna, etc. In various embodiments, the network interface 1206 may be integrated with other components of electronic device 1200. The network interface 1206 can implement the functions of the communication unit in the above embodiments.

[0171] Electronic device 1200 may further include input / output (I / O) device 1205. I / O device 1205 may include: a user interface designed to enable a user to interact with electronic device 1200; a peripheral component interface designed to enable peripheral components to also interact with electronic device 1200; and / or sensors designed to determine environmental conditions and / or location information related to electronic device 1200.

[0172] It is worth noting that, Figure 8 This is merely an example. That is, although... Figure 8 The electronic device 1200 is shown to include multiple devices such as a first processor 1201, a controller hub 1203, and a memory 1204. However, in practical applications, devices using the methods of this application may include only a portion of the devices in the electronic device 1200. For example, it may include only the first processor 1201 and the network interface 1206. Figure 8 The properties of the optional devices are shown in dashed lines. According to some embodiments of this application, the memory 1204, which is a computer-readable storage medium, stores instructions that, when executed on a computer, cause the electronic device 1200 to perform the DC transmission system fault protection method according to the above embodiments. Specific details can be found in the methods described in the above embodiments, and will not be repeated here.

[0173] This application also provides a computer-readable storage medium storing at least one instruction or at least one program. The at least one instruction or at least one program is loaded and executed by a processor to implement the DC transmission system fault protection method as described in the above embodiments. For details, please refer to the methods in the above embodiments, which will not be repeated here.

[0174] Now for reference Figure 9 The diagram shown is a block diagram of a SoC (System on Chip) 1300 according to an embodiment of this application. Figure 9 In the diagram, similar components share the same reference numerals. Additionally, dashed boxes are an optional feature for more advanced SoCs. Figure 6 In the SoC1300, interconnect unit 1350 is included, which is coupled to the processor (corresponding to...). Figure 9 The system includes a second processor 1310, a system agent unit 1380, a bus controller unit 1390, an integrated memory controller unit 1340, and one or more coprocessors (corresponding to...). Figure 9 The second coprocessor 1320 may include integrated graphics logic, an image processor, an audio processor, and a video processor; a static random access memory (SRAM) unit 1330; and a direct memory access (DMA) unit 1360. In one embodiment, the second coprocessor 1320 includes a dedicated processor, such as a network or communication processor, a compression engine, a GPGPU, a high-throughput MIC processor, or an embedded processor.

[0175] The static random access memory (SRAM) cell 1330 may include one or more computer-readable media for storing data and / or instructions. The computer-readable storage medium may store instructions, specifically, temporary and permanent copies of those instructions. These instructions may include, when executed by at least one unit in the processor, causing the SoC 1300 to perform a control method for the fuel cell power generation system according to the above embodiments, as specifically described in the above embodiments, which will not be repeated here.

[0176] Various embodiments of the mechanisms disclosed in this application can be implemented in hardware, software, firmware, or combinations of these implementation methods. Embodiments of this application can be implemented as computer programs or program code executable on a programmable system, the programmable system including at least one processor, a storage system (including volatile and non-volatile memory and / or storage elements), at least one input device, and at least one output device.

[0177] Program code can be applied to input instructions to execute the functions described in this application and generate output information. The output information can be applied to one or more output devices in a known manner. For the purposes of this application, the processing system includes any system having a processor such as a Digital Signal Processor (DSP), a microcontroller, an Application Specific Integrated Circuit (ASIC), or a microprocessor.

[0178] The program code can be implemented using a high-level procedural language or an object-oriented programming language to communicate with the processing system. Assembly language or machine language can also be used when needed. In fact, the mechanisms described in this application are not limited to any particular programming language. In either case, the language can be a compiled language or an interpreted language.

[0179] In some cases, the disclosed embodiments may be implemented in hardware, firmware, software, or any combination thereof. The disclosed embodiments may also be implemented as instructions carried or stored thereon on one or more temporary or non-temporary machine-readable (e.g., computer-readable) storage media, which may be read and executed by one or more processors. For example, the instructions may be distributed via a network or through other computer-readable media. Therefore, machine-readable media may include any mechanism for storing or transmitting information in a machine-readable (e.g., computer-readable) form, including but not limited to floppy disks, optical disks, CD-ROMs, compact disc read-only memory (CD-ROMs), magneto-optical disks, read-only memory (ROM), random access memory (RAM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), magnetic cards or optical cards, flash memory, or tangible machine-readable storage for transmitting information (e.g., carrier waves, infrared signals, digital signals, etc.) using the Internet in the form of electrical, optical, acoustic, or other forms of propagated signals. Therefore, machine-readable media include any type of machine-readable medium suitable for storing or transmitting electronic instructions or information in a machine-readable (e.g., computer-readable) form.

[0180] In the accompanying drawings, some structural or methodological features may be shown in a specific arrangement and / or order. However, it should be understood that such a specific arrangement and / or order may not be necessary. Rather, in some embodiments, these features may be arranged in a manner and / or order different from that shown in the accompanying drawings. Furthermore, including structural or methodological features in a particular figure does not imply that such features are required in all embodiments, and in some embodiments, these features may be omitted or may be combined with other features.

[0181] It should be noted that the order of the embodiments described above is merely for descriptive purposes and does not represent the superiority or inferiority of the embodiments. Furthermore, specific embodiments have been described above. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps described in the claims can be performed in a different order than that shown in the embodiments and still achieve the desired result. Additionally, the processes depicted in the drawings do not necessarily require a specific or sequential order to achieve the desired result. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0182] It should be noted that all units / modules mentioned in the device embodiments of this application are logical units / modules. Physically, a logical unit / module can be a physical unit / module, a part of a physical unit / module, or a combination of multiple physical units / modules. The physical implementation of these logical units / modules themselves is not the most important factor; the combination of functions implemented by these logical units / modules is the key to solving the technical problems proposed in this application. Furthermore, to highlight the innovative aspects of this application, the above-described device embodiments of this application have not introduced units / modules that are not closely related to solving the technical problems proposed in this application. This does not mean that the above-described device embodiments do not contain other units / modules.

[0183] It should be noted that in the examples and description of this application, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes the element.

[0184] Although this application has been illustrated and described with reference to certain preferred embodiments thereof, those skilled in the art should understand that various changes in form and detail may be made thereto without departing from the spirit and scope of this application.

Claims

1. A fault protection method for a DC transmission system, characterized in that, This invention is applied to a DC transmission system, which includes a generator side, a diode rectifier unit, and a modular multilevel converter. The generator side includes a new energy power plant and an energy storage system. The AC side of the diode rectifier unit is connected to the generator side via an AC transmission line, and the DC side of the diode rectifier unit is connected to the DC side of the modular multilevel converter via a DC transmission line. The AC side of the modular multilevel converter is connected to the power grid. The fault protection method includes: During the operation of the DC transmission system, the current of the DC transmission line is detected; When the current in the DC transmission line is detected to exceed the current threshold, a fault is determined to have occurred in the DC transmission system; and, The modular multilevel converter is controlled to reduce the voltage of the DC transmission line, thereby blocking the energy feed from the power grid to the DC transmission line. The energy storage system is controlled to reduce the voltage of the AC transmission line, thereby reducing the current fed into the diode rectifier unit from the power generation side; After controlling the energy storage system to reduce the voltage of the AC transmission line, after a first preset time, the DC transmission system is restarted to restore the energy transmission from the DC transmission system to the power grid; The method of controlling the restart of the DC transmission system includes: The modular multilevel converter is controlled to adjust the voltage of the DC transmission line to a first voltage value, so as to establish a connection between the modular multilevel converter and the power grid; The energy storage system is controlled to adjust the voltage of the AC transmission line to a second voltage value; The system detects the output of the new energy power station. When power output is detected, the system controls the energy storage system to adjust the voltage of the AC transmission line to a third voltage value so that the energy output from the power generation side is transmitted to the power grid through the diode rectifier unit and the modular multilevel converter.

2. The fault protection method for DC transmission systems according to claim 1, characterized in that, The control of the energy storage system to reduce the voltage of the AC transmission line includes: According to the preset AC bus voltage calculation model, the AC bus voltage target value U is calculated S ; Based on the AC bus voltage target value U S The energy storage system is controlled to reduce the voltage of the AC transmission line. The AC bus voltage calculation model includes: ， In the formula, I d U is the actual current value of the DC transmission line. con X is the rated voltage value of the DC transmission line. t U is the leakage reactance of the converter transformer in the diode rectifier unit. S The target value for the AC bus voltage.

3. The fault protection method for a DC transmission system according to claim 1, characterized in that, Before controlling the energy storage system to adjust the voltage of the AC transmission line to the third voltage value, the following steps are included: The third voltage value is calculated based on the power-voltage coordination control model; The power-voltage coordinated control model includes: ， In the formula, P r U is the target output power of the AC transmission line. con U is the rated voltage value of the DC transmission line. S-R X is the third voltage value. t This refers to the leakage reactance of the converter transformer in the diode rectifier unit.

4. The fault protection method for DC transmission systems according to claim 1, characterized in that, Also includes: During the restart process of the controlled DC transmission system, the current of the DC transmission line is detected; If the current of the DC transmission line is detected to exceed the current threshold, the restart is determined to have failed. After the current of the DC transmission line is detected to exceed the current threshold, the DC transmission system is restarted again after a second preset time. During the restart process, the current of the DC transmission line is detected to determine whether the restart has failed. Once the number of restarts exceeds a preset threshold, the restart control of the DC transmission system will be stopped, and the path between the power generation side and the diode rectifier unit will be disconnected.

5. The fault protection method for a DC transmission system according to claim 1, characterized in that, The DC transmission system also includes reactors installed on the DC transmission lines; The reactor is used to reduce the rate of rise and peak value of current on the DC transmission line when a fault occurs in the DC transmission system.

6. A direct current transmission system, characterized in that, include: On the power generation side, the power generation side includes new energy power plants and energy storage systems; A diode rectifier unit, wherein the AC side of the diode rectifier unit is connected to the power generation side via an AC transmission line; A modular multilevel converter, wherein the DC side of the diode rectifier unit is connected to the DC side of the modular multilevel converter via a DC transmission line, and the AC side of the modular multilevel converter is connected to the power grid; A DC control and protection system, wherein the DC control and protection system is used to execute the DC transmission system fault protection method according to any one of claims 1-5.

7. A fault protection device for a DC transmission system, characterized in that, This invention is applied to a DC transmission system, which includes a generator side, a diode rectifier unit, and a modular multilevel converter. The generator side includes a new energy power plant and an energy storage system. The AC side of the diode rectifier unit is connected to the generator side via an AC transmission line, and the DC side of the diode rectifier unit is connected to the DC side of the modular multilevel converter via a DC transmission line. The AC side of the modular multilevel converter is connected to the power grid. The fault protection device includes: The fault detection module is used to detect the current of the DC transmission line during the operation of the DC transmission system. The fault determination module is used to determine that a fault has occurred in the DC transmission system when the current of the DC transmission line exceeds the current threshold. The receiving-end control module is used to control the modular multilevel converter to reduce the voltage of the DC transmission line and block the energy feed from the power grid to the DC transmission line; The sending-end control module is used to control the energy storage system to reduce the voltage of the AC transmission line and reduce the current fed into the diode rectifier unit from the power generation side; The system restart control module is used to control the DC transmission system to restart after a first preset time following the control of the energy storage system to reduce the voltage of the AC transmission line, so as to restore the energy transmission of the DC transmission system to the power grid; The system restart control module controls the restart of the DC transmission system in the following ways: The modular multilevel converter is controlled to adjust the voltage of the DC transmission line to a first voltage value, so as to establish a connection between the modular multilevel converter and the power grid; The energy storage system is controlled to adjust the voltage of the AC transmission line to a second voltage value; The system detects the output of the new energy power station. When power output is detected, the system controls the energy storage system to adjust the voltage of the AC transmission line to a third voltage value so that the energy output from the power generation side is transmitted to the power grid through the diode rectifier unit and the modular multilevel converter.

8. An electronic device, characterized in that, The electronic device includes a processor and a memory, wherein the memory stores at least one instruction or at least one program, and the at least one instruction or at least one program is loaded and executed by the processor to implement the DC transmission system fault protection method as described in any one of claims 1-5.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores at least one instruction or at least one program, which is loaded and executed by a processor to implement the DC transmission system fault protection method as described in any one of claims 1-5.

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