A flexible direct current power grid control and protection cooperation method based on a direct current circuit breaker

Abstract

The application relates to a flexible DC power grid control and protection collaborative method based on a DC circuit breaker. The DC circuit breaker is a source-grid matching voltage clamping type DC circuit breaker. The flexible DC power grid control and protection collaborative process comprises the following steps: 1) control and protection starting stage: configuring an improved voltage gradient algorithm as a starting criterion of control and protection; configuring the positive and negative of a current variation as a selection criterion of input of the DC circuit breaker; 2) fault current limiting stage: when a line side voltage is suddenly changed and a detected current variation is positive, the MMC and the DC circuit breaker enter the fault current limiting stage, first, the control mode of the converter station is changed, the number of capacitor sub-modules of the MMC bridge arm is actively controlled to be reduced, and the voltage clamping capacitor of the DC circuit breaker is input; 3) protection discrimination stage: using the mean value of the line current variation as a protection criterion to perform fault positioning; and 4) fault removal stage.

Description

Technical Field

[0001] This invention relates to the field of power system relay protection and automation, specifically to a flexible DC grid control and protection coordination method based on DC circuit breakers. Background Technology

[0002] Flexible DC power systems have gained widespread application in the DC transmission field due to their significant advantages, including precise commutation, flexible control methods, and ease of integrating renewable energy sources, and are a major direction for power grid development in the near future. However, when a short-circuit fault occurs in a DC transmission line, the short-circuit current rises rapidly, reaches a high peak value, and the fault propagates extremely quickly. Simultaneously, many power electronic devices in flexible DC systems have very weak overcurrent tolerance capabilities, thus placing extremely stringent requirements on relay protection for DC transmission systems.

[0003] In current DC transmission systems, traveling wave protection and differential undervoltage protection are conventionally used as primary protection, with differential protection as backup. Traditional traveling wave protection cannot guarantee high sensitivity under high-resistance faults and is extremely sensitive to interference. Differential undervoltage protection operates slightly slower than traveling wave protection, has poor tolerance to transition resistance, and is exceptionally difficult to set due to the very low damping in DC systems. In addition, differential protection is also a widely used protection principle, but its time delay limits its further development, and line distributed capacitance affects its reliability. In short, current protection principles cannot simultaneously meet the requirements of high speed and high reliability.

[0004] To date, the vast majority of DC line fault handling methods involve first locating the faulty line using protection criteria, and then isolating it using circuit breakers. If this fault handling mode of locating first and then isolating continues, it will be difficult for fault handling technology to achieve breakthroughs. Protection and control are two crucial processes that occur after a line fault, and they are inevitably closely related. Especially in flexible DC grids, the action time scales of protection and control are the same, and their mutual influence is certain. However, most current research has not deeply integrated the two, resulting in a lack of organic coordination between the two parts. Current protection principles cannot simultaneously meet the requirements of high speed and high reliability. To break through the bottleneck of protection principles, finding a new fault handling method becomes particularly important. Summary of the Invention

[0005] This invention, based on the concept of control and protection coordination, deeply integrates circuit breakers and protection systems. Utilizing a DC circuit breaker based on the source-grid coordinated voltage clamping principle, it provides a control and protection coordination method for flexible DC power grids. Upon a fault, the activation of the control circuit breaker and the adjustment of the converter station limit overcurrent in power electronic devices, addressing the vulnerability of MMC devices; simultaneously, it provides a protection boundary, creating conditions for precise fault location. Upon receiving an activation signal, the converter station resumes normal control and disconnects the faulty line via the circuit breaker. The technical solution is as follows:

[0006] A flexible DC grid control and protection coordination method based on DC circuit breakers, wherein the DC circuit breaker used is a source-grid coordinated voltage clamping DC circuit breaker, characterized in that the flexible DC grid control and protection coordination process includes:

[0007] 1) Configure the improved voltage gradient algorithm as the start criterion for control protection, and configure the positive or negative current change as the selection criterion for DC circuit breaker activation. When control protection is activated, the circuit breaker with positive current change is activated only if and only if the circuit breaker with negative current change is activated.

[0008] 2) When a sudden change occurs in the line-side voltage and a positive change in current is detected, the control MMC adjustment and DC circuit breaker are activated to jointly limit the fault current. The method is as follows: First, switch the converter station control mode: After the fault, the source-side MMC active control module adopts non-adaptive control with a fixed preset value, that is, the active control reduces the number of capacitor sub-modules in the MMC bridge arm and lowers the reference value of the MMC bridge arm voltage, while activating the voltage clamping capacitor of the DC circuit breaker.

[0009] 3) Use the average value of line current change as a protection criterion for fault location. When it is determined that a line may be faulty after step 1), calculate the ratio of the average value of the actual measured fault current change to the average value of the minimum calculated current. Based on the size of the ratio, confirm whether a short circuit fault has occurred on the line.

[0010] 4) After fault location is completed, regardless of whether it is a faulty line or a non-faulty line, the control mode of MMC will be restored to the initial control mode; when it is determined that the fault occurs within the zone, the fault clearing function of the DC circuit breaker will be activated to discharge the energy in the clamping capacitor, thereby clearing the faulty line; when it is determined that the fault occurs outside the zone, the DC circuit breaker on the non-faulty line will be reset to keep the healthy system running normally.

[0011] Furthermore, in step 1), the improved voltage gradient algorithm is configured as the start criterion for control and protection, as shown in the following formula:

[0012]

[0013] In the formula: D(i) is the improved voltage gradient value at the current sampling time i, u(ij) is the sampled value of the j-th voltage before the current sampling point, D set The threshold value should be set to avoid the maximum improved voltage gradient that occurs during normal operation.

[0014] Furthermore, in step 3), the method for fault location using the average value of line current changes as a protection criterion is as follows:

[0015] The value of K is defined as follows: when the value of K satisfies this formula, a short-circuit fault is confirmed to have occurred on the line; otherwise, the line is considered a non-faulty line.

[0016]

[0017] In the formula, T is the protection time window length, n is the number of sampling points within the time window length T, and i j The sampled current value measured by the circuit breaker line, Δi j Δi represents the change in current when a short-circuit fault occurs in the actual line. min k represents the theoretical minimum change in short-circuit current when a short-circuit fault occurs in the line. rel This is the reliability coefficient.

[0018] Furthermore, in step 3), k rel =0.8.

[0019] On the other hand, the present invention provides a flexible DC grid control and protection coordination device based on a DC circuit breaker, wherein the DC circuit breaker used is a source-grid coordinated voltage clamping DC circuit breaker, characterized in that the flexible DC grid control and protection coordination device includes:

[0020] The startup module is used to configure the improved voltage gradient algorithm as the startup criterion for the control protection, and to configure the positive or negative current change as the selection criterion for the DC circuit breaker to be put into operation. When the control protection is started, the circuit breaker is put into operation only if and only if the current change is positive, while the circuit breaker with the current change is negative.

[0021] The current limiting module is used to control the MMC adjustment and the DC circuit breaker to limit the fault current when the line-side voltage changes abruptly and the detected current change is positive. The method is as follows: First, the converter station control mode is switched: after the fault, the source-side MMC active control module adopts non-adaptive control with a fixed preset value, that is, the active control reduces the number of capacitor sub-modules of the MMC bridge arm, lowers the reference value of the MMC bridge arm voltage, and at the same time, the voltage clamping capacitor of the DC circuit breaker is activated.

[0022] The fault confirmation module is used to locate faults by using the average value of line current changes as a protection criterion. When the configuration module determines that a line may be faulty, it calculates the ratio of the average value of the actual measured fault current changes to the average value of the minimum calculated current. Based on the value of the ratio, it confirms whether a short circuit fault has occurred on the line.

[0023] The recovery module is used to restore the MMC control mode to the initial control mode after fault location, regardless of whether it is a faulty line or a non-faulty line. When the fault is determined to occur within the zone, the fault clearing function of the DC circuit breaker is activated to discharge the energy in the clamping capacitor, thereby clearing the faulty line. When the fault is determined to occur outside the zone, the DC circuit breaker on the non-faulty line is reset to keep the healthy system running normally.

[0024] Furthermore, the startup module is configured with an improved voltage gradient algorithm as the startup criterion for the control and protection system, as shown in the following formula:

[0025]

[0026] In the formula: D(i) is the improved voltage gradient value at the current sampling time i, and u(ij) is the sampled value of the j-th voltage before the current sampling point; D set The threshold value should be set to avoid the maximum improved voltage gradient that occurs during normal operation.

[0027] Furthermore, the fault confirmation module is specifically used for:

[0028] The value of K is defined as follows: when the value of K satisfies this formula, a short-circuit fault is confirmed to have occurred on the line; otherwise, the line is considered a non-faulty line.

[0029]

[0030] In the formula, T is the protection time window length, n is the number of sampling points within the time window length T, and i j The sampled current value measured by the circuit breaker line, Δi j Δi represents the change in current when a short-circuit fault occurs in the actual line. min k represents the theoretical minimum change in short-circuit current when a short-circuit fault occurs in the line. rel This is the reliability coefficient.

[0031] Furthermore, in the fault confirmation module, k rel =0.8.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] 1. It breaks away from the traditional fault handling mode of first locating the faulty line using protection criteria and then disconnecting the faulty line using a circuit breaker.

[0034] 2. By utilizing the control of circuit breakers and converter stations, three functions are simultaneously realized: fault current limiting, protection boundary, and fault isolation.

[0035] 3. This protection principle is a single-ended quantity protection, which does not require communication. It can achieve accurate fault location by relying solely on the magnitude of the current change measured at the protection installation point.

[0036] 4. It limits the overcurrent of power electronic devices during faults, reduces the system's requirements for protection speed, and has a certain ability to withstand transition resistance. Attached Figure Description

[0037] Figure 1 This is the topology of a voltage-clamped DC circuit breaker;

[0038] Figure 2 A flowchart illustrating the action flow of the control and protection coordination strategy;

[0039] Figure 3 This is the topology of a four-terminal flexible DC transmission system;

[0040] Figure 4 The current flow path when charging the clamping capacitor;

[0041] Figure 5 This represents the current flow path during the fault current limiting phase.

[0042] Figure 6 This represents the current flow path during the fault clearing phase on the faulty line.

[0043] Figure 7 This represents the current flow path for fault recovery on a non-faulty line. Detailed Implementation

[0044] Protection and control are two crucial processes that occur after a line fault, and they are inevitably closely related. Especially in flexible DC power grids, protection and control have the same timescale, and their mutual influence is unavoidable. DC circuit breakers are the core equipment for clearing DC line faults, possessing numerous power electronic components and offering the advantage of flexible control; therefore, DC circuit breakers are the optimal control target after a fault occurs.

[0045] This invention provides a control and protection coordination method for flexible DC power grids based on DC circuit breakers. The circuit breaker used is a source-grid coordinated voltage clamping DC circuit breaker, and its control and protection coordination process is divided into the following four stages:

[0046] 1) Control and protection start-up phase

[0047] The present invention configures an improved voltage gradient algorithm as the start criterion for control and protection, as shown in the following formula. Control and protection will only start if the formula is satisfied at least three consecutive sampling times under the sliding window length.

[0048]

[0049] In the formula: D(i) is the improved voltage gradient value at the current sampling time i, and u(ij) is the sampled value of the j-th voltage before the current sampling point; D set The startup threshold should avoid the maximum improved voltage gradient that occurs during normal operation.

[0050] Furthermore, this invention configures the sign of the current change as the selection criterion for circuit breaker activation. When the control protection is activated, the circuit breaker is activated only if and only if the current change is positive, while the circuit breaker with the negative current change is not activated.

[0051] 2) Fault current limiting phase

[0052] When a sudden change occurs in the line-side voltage, and a positive current change is detected simultaneously, the control MMC and circuit breaker enter the fault current-limiting phase. First, the converter station control mode is switched: after the fault, the source-side MMC active control module adopts non-adaptive control with a fixed preset value, that is, actively reducing the number of capacitor sub-modules in the MMC arm and setting the reference value of the MMC arm voltage to kU. N (k is a constant, and k≤1). U N This serves as a reference value for the bridge arm voltage. Simultaneously, the voltage clamping capacitor of the DC circuit breaker is activated, and the coordinated control of the power source and grid jointly suppresses the rise of the short-circuit current, ensuring the safety of power electronic devices. At the same time, the early activation of the circuit breaker provides a boundary for relay protection, collaboratively achieving fault location and accelerating fault clearing.

[0053] 3) Protection Judgment Stage

[0054] By utilizing the boundary created by the circuit breaker's activation, different fault characteristics are generated between faulty and non-faulty lines, thus identifying the faulty line. On the faulty line, the voltage clamping capacitor activated by the circuit breaker discharges to the MMC and the fault point, and the faulty line current is close to the short-circuit current without any current limiting measures, resulting in a very large current variation. On the non-faulty line, regardless of whether the circuit breaker's clamping capacitor is activated, the voltage across it is equal, both being the rated clamping voltage, so the current variation on the healthy line is relatively small. Therefore, using the change in line current as a protection criterion can easily distinguish faulty and non-faulty lines. However, due to the distributed parameter characteristics of high-voltage lines, the short-circuit current can fluctuate significantly or even drop. To reduce the impact of this fluctuation on fault location, this invention uses the average value of the line current variation as a protection criterion for fault location. The K value is defined as follows: when the K value satisfies this formula, a short-circuit fault is confirmed in the line; otherwise, the line is considered a non-faulty line.

[0055]

[0056] In the formula, T is the protection time window length, n is the number of sampling points within the time window length T, and i j The sampled current value measured by the circuit breaker line, Δi j Δi represents the change in current when a short-circuit fault occurs in the actual line. min k represents the theoretical minimum change in short-circuit current when a short-circuit fault occurs in the line. rel As the reliability coefficient, this invention takes k. rel =0.8. As defined, K is essentially the ratio of the fault current to the minimum calculated fault current, so K≥1. To improve the reliability of the protection, the data window slides point by point. When the above formula is satisfied three times consecutively, a short-circuit fault is confirmed to have occurred on the line; otherwise, the line is considered intact.

[0057] 4) Fault clearance phase

[0058] After the protection discrimination phase is completed, regardless of whether it is a faulty or non-faulty line, the MMC control mode will be restored to the initial control mode, i.e., the bridge arm voltage reference value is U. N When a fault is determined to occur within the fault zone, the fault-clearing function of the circuit breaker is activated to discharge the energy in the clamping capacitor, thereby disconnecting the faulty line. When a fault is determined to occur outside the fault zone, the circuit breakers on the non-faulty lines are reset, ultimately ensuring the normal operation of the intact system.

[0059] The present invention will be further described below with reference to embodiments.

[0060] The circuit breaker used in the control and protection coordination method of this invention is a source-grid coordinated voltage clamping DC circuit breaker, and its typical topology is as follows: Figure 1As shown, it consists of five parts: main branch, transfer branch, voltage clamping capacitor, fast discharge branch, and MMC active control module.

[0061] Figure 2 The sequence diagram for the control and protection coordination process is as follows:

[0062] 1. For example Figure 3 As shown, a clamping capacitor DC circuit breaker is installed on each side of each line, a current-limiting inductor is installed on the line side of the circuit breaker, and a voltage transformer is installed on the line side of the current-limiting inductor.

[0063] 2. During normal system operation, the capacitor of the voltage clamping module needs to be charged, which triggers thyristor T1 to conduct. The current flow path is as follows: Figure 4 As shown, as the voltage of the clamping capacitor rises to the system voltage, thyristor T1 automatically turns off, and the charging process is complete.

[0064] 3. When a sudden change in line voltage and a positive change in current are detected simultaneously, the circuit breaker on that side is activated, and the circuit breaker's voltage clamping module is engaged. Taking a fault on line 12 as an example... Figure 3 As shown, the circuit breakers on the B12 and B21 sides are activated, and simultaneously the control mode of MMC1 and MMC2 is switched to non-adaptive control with a fixed preset value, that is, the number of sub-modules activated is reduced to the preset value kN. N However, B14 and B23 will not start because the current surge is negative.

[0065] 4. The specific activation process of circuit breakers B12 and B21 during the fault current limiting stage is as follows: For the sending-end line circuit breaker B21, a trigger pulse signal is first provided to the thyristor Tp2 of the transfer branch (providing a path for the short-circuit current; due to the activation of the clamping capacitor, when the current direction of the transfer branch changes from the line to the bus, its thyristor naturally turns off, so its conduction time is very short; subsequently, the fast mechanical switch Sp3 is opened to achieve disconnection). At the same time, the thyristor group T2 of the conduction voltage clamping branch and the thyristor group Tp1 of the transfer branch are triggered, immediately turning off the IGBT of the main branch, so that the current is transferred from the main branch to the transfer branch module, and then the fast mechanical switch Sp1 is opened. For the receiving-end line circuit breaker B12, a trigger signal is first provided to Tp1, so that the current flows through the transfer branch module; at the same time, the thyristor group T2 is triggered, clamping the system voltage at the MMC output and providing a blocking signal to the IGBT of the main branch to turn it off; then the fast mechanical switch Sp1 is opened. The current flow path at this stage is as follows Figure 5 As shown.

[0066] 5. After the protection discrimination phase, the number of converter submodules returns to normal switching. Based on the judgment in the previous phase, if a line is a faulty line (e.g., Line 12), the mechanical switch S1 of the discharge branch of the circuit breakers at both ends (B12 and B21) is closed, opening the fast discharge branch and causing the voltage of the clamping capacitor to drop rapidly. When the current in the transfer branch reverses, the thyristor group Tp1 naturally turns off, and then immediately the corresponding fast switch Sp2 of the transfer branch is opened, isolating the MMC from the faulty line. The duration of this phase depends on system parameters and normal operating conditions, and is generally 1-3 ms in actual engineering. After disconnecting the faulty line, the natural discharge phase begins. The current flow path in this phase is as follows: Figure 6 As shown.

[0067] For non-faulty lines (such as Line 23), if the protection activation criterion at the circuit breaker (B32) is activated, the voltage clamping capacitor of B32 will be engaged and the reference voltage value of the converter station (MMC3) bridge arm will be adjusted to the preset value. However, if the current value of the line does not meet the protection criterion, it is confirmed as a non-faulty line, and the circuit breaker clamping module needs to be disconnected to restore the system to normal operation. The converter station bridge arm reference voltage is adjusted to the system rated voltage, and then the mechanical switch S2 of the fast discharge module is closed to discharge the energy of capacitor C2, causing T2 to withstand reverse voltage and turn off, thus disconnecting the clamping module. Discharging only the energy of capacitor C2 reduces the impact of discharging all the circuit breaker capacitors on the healthy system. Simultaneously, the IGBTs of the main branch, which are adapted to the normal operating current direction, are turned on, allowing current to flow from the main branch. At this point, the non-faulty line will return to normal operation. The current flow path in this stage is as follows: Figure 7 As shown.

[0068] On the other hand, the present invention provides a flexible DC grid control and protection coordination device based on a DC circuit breaker. The DC circuit breaker used is a source-grid coordinated voltage clamping DC circuit breaker. The flexible DC grid control and protection coordination device includes:

[0069] The startup module is used to configure the improved voltage gradient algorithm as the startup criterion for the control protection, and to configure the positive or negative current change as the selection criterion for the DC circuit breaker to be put into operation. When the control protection is started, the circuit breaker is put into operation only if and only if the current change is positive, while the circuit breaker with the current change is negative.

[0070] The current limiting module is used to control the MMC adjustment and the DC circuit breaker to limit the fault current when the line-side voltage changes abruptly and the detected current change is positive. The method is as follows: First, the converter station control mode is switched: after the fault, the source-side MMC active control module adopts non-adaptive control with a fixed preset value, that is, the active control reduces the number of capacitor sub-modules of the MMC bridge arm, lowers the reference value of the MMC bridge arm voltage, and at the same time, the voltage clamping capacitor of the DC circuit breaker is activated.

[0071] The fault confirmation module is used to locate faults by using the average value of line current changes as a protection criterion. When the configuration module determines that a line may be faulty, it calculates the ratio of the average value of the actual measured fault current changes to the average value of the minimum calculated current. Based on the value of the ratio, it confirms whether a short circuit fault has occurred on the line.

[0072] The recovery module is used to restore the MMC control mode to the initial control mode after fault location, regardless of whether it is a faulty line or a non-faulty line. When the fault is determined to occur within the zone, the fault clearing function of the DC circuit breaker is activated to discharge the energy in the clamping capacitor, thereby clearing the faulty line. When the fault is determined to occur outside the zone, the DC circuit breaker on the non-faulty line is reset to keep the healthy system running normally.

[0073] Furthermore, the startup module is configured with an improved voltage gradient algorithm as the startup criterion for the control and protection system, as shown in the following formula:

[0074]

[0075] In the formula: D(i) is the improved voltage gradient value at the current sampling time i, and u(ij) is the sampled value of the j-th voltage before the current sampling point; D set The threshold value should be set to avoid the maximum improved voltage gradient that occurs during normal operation.

[0076] Furthermore, the fault confirmation module is specifically used for:

[0077] The value of K is defined as follows: when the value of K satisfies this formula, a short-circuit fault is confirmed to have occurred on the line; otherwise, the line is considered a non-faulty line.

[0078]

[0079] In the formula, T is the protection time window length, n is the number of sampling points within the time window length T, and i j The sampled current value measured by the circuit breaker line, Δi j Δi represents the change in current when a short-circuit fault occurs in the actual line. min k represents the theoretical minimum change in short-circuit current when a short-circuit fault occurs in the line. rel This is the reliability coefficient.

[0080] Furthermore, in the fault confirmation module, k rel =0.8.

[0081] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. The solutions in the embodiments of this application can be implemented in various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0082] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0083] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0084] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0085] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0086] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A flexible DC grid control and protection coordination method based on a DC circuit breaker, wherein the DC circuit breaker used is a source-grid coordinated voltage clamping DC circuit breaker, characterized in that, The flexible DC grid control and protection coordination process includes: 1) Configure the improved voltage gradient algorithm as the start criterion for control protection, and configure the positive or negative current change as the selection criterion for DC circuit breaker activation. When control protection is activated, the circuit breaker with positive current change is activated only if and only if the circuit breaker with negative current change is activated. 2) When a sudden change occurs in the line-side voltage and a positive change in current is detected, the control MMC adjustment and DC circuit breaker are activated to jointly limit the fault current. The method is as follows: First, switch the converter station control mode: After the fault, the source-side MMC active control module adopts non-adaptive control with a fixed preset value, that is, the active control reduces the number of capacitor sub-modules in the MMC bridge arm and lowers the reference value of the MMC bridge arm voltage, while activating the voltage clamping capacitor of the DC circuit breaker. 3) Use the average value of line current change as a protection criterion for fault location. When it is determined that a line may be faulty after step 1), calculate the ratio of the average value of the actual measured fault current change to the average value of the minimum calculated fault current. Based on the size of the ratio, confirm whether a short circuit fault has occurred on the line. 4) After fault location is completed, regardless of whether it is a faulty line or a non-faulty line, the control mode of MMC will be restored to the initial control mode; when it is determined that the fault occurs within the zone, the fault clearing function of the DC circuit breaker will be activated to discharge the energy in the clamping capacitor, thereby clearing the faulty line; when it is determined that the fault occurs outside the zone, the DC circuit breaker on the non-faulty line will be reset to keep the healthy system running normally.

2. The flexible DC grid control and protection coordination method according to claim 1, characterized in that, In step 1), the improved voltage gradient algorithm is configured as the start criterion for control and protection, as shown in the following formula: In the formula: D(i) is the improved voltage gradient value at the current sampling time i, u(ij) is the sampled value of the j-th voltage before the current sampling point, D set The threshold value should be set to avoid the maximum improved voltage gradient that occurs during normal operation.

3. The flexible DC grid control and protection coordination method according to claim 1, characterized in that, In step 3), the method of using the average value of line current changes as a protection criterion for fault location is as follows: The value of K is defined as follows: when the value of K satisfies this formula, a short-circuit fault is confirmed to have occurred on the line; otherwise, the line is considered a non-faulty line. In the formula, T is the protection time window length, n is the number of sampling points within the time window length T, and i j The sampled current value measured by the circuit breaker line, Δi j Δi represents the change in current when a short-circuit fault occurs in the actual line. min k represents the theoretical minimum change in short-circuit current when a short-circuit fault occurs in the line. rel For reliability coefficient, .

4. The flexible DC grid control and protection coordination method according to claim 3, characterized in that, In step 3), k rel =0.

8.

5. A flexible DC grid control and protection coordination device based on a DC circuit breaker, wherein the DC circuit breaker used is a source-grid coordinated voltage clamping DC circuit breaker, characterized in that, The flexible DC grid control and protection coordination device includes: The startup module is used to configure the improved voltage gradient algorithm as the startup criterion for the control protection, and to configure the positive or negative current change as the selection criterion for the DC circuit breaker to be put into operation. When the control protection is started, the circuit breaker is put into operation only if and only if the current change is positive, while the circuit breaker with the current change is negative. The current limiting module is used to control the MMC adjustment and the DC circuit breaker to limit the fault current when the line-side voltage changes abruptly and the detected current change is positive. The method is as follows: First, the converter station control mode is switched: after the fault, the source-side MMC active control module adopts non-adaptive control with a fixed preset value, that is, the active control reduces the number of capacitor sub-modules of the MMC bridge arm, lowers the reference value of the MMC bridge arm voltage, and at the same time, the voltage clamping capacitor of the DC circuit breaker is activated. The fault confirmation module is used to locate faults by using the average value of line current changes as a protection criterion. When the configuration module determines that a line may be faulty, it calculates the ratio of the average value of the actual measured fault current changes to the average value of the minimum calculated fault current. Based on the value of the ratio, it confirms whether a short circuit fault has occurred on the line. The recovery module is used to restore the MMC control mode to the initial control mode after fault location, regardless of whether it is a faulty line or a non-faulty line. When the fault is determined to occur within the zone, the fault clearing function of the DC circuit breaker is activated to discharge the energy in the clamping capacitor, thereby clearing the faulty line. When the fault is determined to occur outside the zone, the DC circuit breaker on the non-faulty line is reset to keep the healthy system running normally.

6. The flexible DC grid control and protection coordination device according to claim 5, characterized in that, The aforementioned startup module is configured with an improved voltage gradient algorithm as the startup criterion for the control and protection system, as shown in the following formula: In the formula: D(i) is the improved voltage gradient value at the current sampling time i, and u(ij) is the sampled value of the j-th voltage before the current sampling point; D set The threshold value should be set to avoid the maximum improved voltage gradient that occurs during normal operation.

7. The flexible DC grid control and protection coordination device according to claim 5, characterized in that, The fault confirmation module is specifically used for: The value of K is defined as follows: when the value of K satisfies this formula, a short-circuit fault is confirmed to have occurred on the line; otherwise, the line is considered a non-faulty line. In the formula, T is the protection time window length, n is the number of sampling points within the time window length T, and i j The sampled current value measured by the circuit breaker line, Δi j Δi represents the change in current when a short-circuit fault occurs in the actual line. min k represents the theoretical minimum change in short-circuit current when a short-circuit fault occurs in the line. rel For reliability coefficient, .

8. The flexible DC grid control and protection coordination device according to claim 7, characterized in that, In the fault confirmation module, k rel =0.8.

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