Fault ride-through control method and device for receiving end of isolated island through flexible direct current system

Abstract

The application discloses an island external DC transmission system fault ride-through control method and device, and the method comprises the following steps: decoupling a DC voltage from submodules of a sending-end converter module and a receiving-end converter module in a flexible DC transmission system, calculating the number of cut-out submodules in the flexible DC transmission system; calculating the maximum average operating power and the maximum power input into the flexible DC transmission system under the condition of safe operation of the flexible DC transmission system according to the number of submodules in use in the flexible DC transmission system after the submodules are cut out; and controlling to reduce the output power of the island. By continuously updating the calculation of the submodules in use in the flexible DC transmission system, the maximum average operating power and the maximum power input, the voltage of the flexible DC transmission system is controlled to be kept within the safe voltage, the DC blocking and shutdown of the flexible DC transmission system caused by the short-time fault of the receiving-end power grid is avoided, and the dependence on the energy dissipation resistor in the fault ride-through process of the flexible DC transmission system is reduced.

Description

Technical Field

[0001] This invention relates to the field of power system stability analysis and control, and in particular to a receiving-end fault ride-through control method and apparatus for islanded power transmission via a flexible DC system. Background Technology

[0002] New energy sources, represented by wind power and photovoltaics, are mainly distributed in the far-off coastal and western regions, far from load centers. Flexible DC transmission has become an important choice for new energy transmission due to its advantages such as flexibility and adjustability. Flexible DC transmission based on modular multilevel converters has attracted much attention because it is more conducive to long-distance high-voltage DC transmission.

[0003] In flexible DC transmission systems based on modular multilevel converters, a single short-circuit fault in the receiving-end power grid can easily cause power imbalance in the flexible DC system, leading to overvoltage problems. Specifically, as DC voltage levels increase, the flexible DC system is more susceptible to short-circuit faults originating from the receiving-end power grid, causing power imbalance between the sending and receiving ends and resulting in DC overvoltage. This DC overvoltage can trigger the flexible DC system's protection mechanisms, leading to DC blocking or outages.

[0004] In the existing technology, the protection measures for flexible DC systems adopt the method of switching energy-consuming resistors on the AC and DC sides. However, this method is not economical and is prone to causing system instability.

[0005] Therefore, there is an urgent need for a coordinated control scheme for receiving-end fault ride-through of a new energy islanded system via flexible DC transmission, which can fully mobilize the fault ride-through capability of the flexible DC system itself, prevent DC blocking or system shutdown, maintain stable operation under system faults, and avoid causing huge adverse effects on the power grid. Summary of the Invention

[0006] Objective of the Invention: This invention provides a receiving-end fault ride-through control method and apparatus for islanded power transmission via a flexible DC system. By continuously updating and calculating three interrelated parameters—the sub-modules used in the flexible DC system, the maximum average operating power, and the maximum power input—the voltage of the flexible DC system is controlled to remain within a safe voltage range. This avoids DC blocking and outages of the flexible DC system due to short-term faults in the receiving-end power grid. At the same time, it reduces the dependence on energy-consuming resistors during the fault ride-through process of the flexible DC system, thereby improving the economic efficiency of stable system operation.

[0007] Technical Solution: This invention provides a receiving-end fault ride-through control method for an islanded flexible DC system, comprising: after a short-circuit fault occurs in the receiving-end power grid, when the DC voltage of the flexible DC system reaches a warning safety threshold, fault ride-through control is performed; the DC voltage is decoupled from the sub-modules of the sending-end converter module and the receiving-end converter module in the flexible DC system, and the number of sub-modules cut out in the flexible DC system is calculated; the sub-modules are used for mutual conversion between AC and DC power; based on the number of sub-modules put into use in the flexible DC system after the sub-modules are cut out, the maximum average operating power and maximum power of the input flexible DC system under safe operation conditions are calculated; the output power of the islanded system is reduced according to the calculated maximum average operating power and maximum power; when the short-circuit fault in the receiving-end power grid is cleared, the fault ride-through control ends, and the number of sub-modules put into use in the flexible DC system increases.

[0008] Specifically, after a short-circuit fault occurs in the receiving-end power grid, the DC voltage of the flexible DC system reaches the first safety threshold, and the corresponding time t1 is recorded. Then, when the DC voltage of the flexible DC system rises to the second safety threshold, the corresponding time t2 is recorded, and fault ride-through control is performed.

[0009] Specifically, during the decoupling process between the DC voltage and the sub-modules in the flexible DC system, it is necessary to satisfy the monotonicity condition between the sub-module capacitor voltage and the DC voltage, and to maintain both the sub-module capacitor voltage and the DC voltage below the safe voltage.

[0010] Specifically, the monotonicity condition, and the requirement that the submodule capacitor voltage and DC voltage remain below the safe voltage, are constrained by the following formula:

[0011]

[0012] Where N represents the number of submodules put into use on a single-phase bridge arm in the flexible DC system after the submodules are cut out, and U dc (t) represents the DC voltage of the flexible DC system at time t, U c S(t) represents the capacitor voltage of the submodule at time t, S(t) represents the number of submodules put into use by a single converter module at time t, and K s P represents the decoupling coefficient. in U represents the maximum input power under safe operating conditions of the flexible DC system, C represents the equivalent capacitance of all sub-modules used in the flexible DC system, and U represents the maximum input power. dcN and U cN K represents the rated values ​​of the DC voltage of the flexible DC system and the capacitor voltage of the submodule, respectively. dc U' represents the DC voltage withstand coefficient of a flexible DC system. dc (t) represents the DC voltage conversion efficiency of the flexible DC system at time t, Modu(x) represents the floor function of x, and S(t) represents the DC voltage conversion efficiency of x at time t. minThis represents the minimum number of submodules that are put into use in a single converter module.

[0013] Specifically, based on the number of sub-modules cut off during the total fault duration, the maximum number of sub-modules put into use in the flexible DC system after the sub-modules are cut off is calculated, and the maximum average operating power and maximum power of the input flexible DC system are calculated.

[0014] Specifically, the maximum average operating power P s The following formula is used for calculation:

[0015]

[0016] Among them, C sm U represents the capacitance of the submodule. c3 This indicates the maximum withstand voltage of the submodule capacitor, and T represents the total fault duration.

[0017] Specifically, the maximum power P in The following formula is used for calculation:

[0018]

[0019] Among them, U c2 t2 represents the capacitor voltage of the submodule at time t2, and t3 represents the time when the fault is cleared.

[0020] Specifically, during fault ride-through control, the control method for the wind turbines in the island is as follows:

[0021]

[0022] Among them, i wd i wq and i max P represents the components of the fan current on the d-axis and q-axis, respectively, and the maximum output current of the fan. 0_PMSG u represents the reactive power output of the wind turbine at the moment the fault occurs. w Indicates the fan voltage, u w0 Indicates the rated voltage at the fan port, u ac1 The boundary voltage, K, represents the reactive power output of the wind turbine. d Indicates the low penetration factor, u ac1 Satisfy the following formula:

[0023] P inmax ≥U ac1 ×i max ,

[0024] Among them, P inmax This represents the calculated maximum power P. in The maximum value in.

[0025] Specifically, during fault ride-through control, the energy loss of the wind turbines in the island reaches (P0 - P). s When (t3-t0), the output current of the fan is reduced to i. dref1 Where t0 represents the time when the fault occurs, and P0 represents the maximum power P at the time when the fault occurs. in i dref1 The calculation formula is as follows:

[0026]

[0027] Among them, P out U represents the output power of the flexible DC system during a fault. d This represents the component of the wind turbine voltage on the d-axis. It describes the calculation of the output power P of the flexible DC system on the wind turbine side of an isolated island without communication. out The following formula is used:

[0028]

[0029] Where P1 represents the active power of the islanded input flexible DC system after voltage reduction, W 1.05 This represents the charging energy of the flexible DC system when the DC voltage rises to 1.05 per unit value, in W. △VSC This indicates the energy that can be charged under safe operating conditions of the flexible DC system.

[0030] This invention also provides a receiving-end fault ride-through control device for an islanded flexible DC system, comprising: a judgment unit, a decoupling unit, a power calculation unit, a control unit, and a recovery unit, wherein: the judgment unit is used to perform fault ride-through control when the DC voltage of the flexible DC system reaches a warning safety threshold after a short-circuit fault occurs in the receiving-end power grid; the decoupling unit is used to decouple the DC voltage from the sub-modules of the sending-end converter module and the receiving-end converter module in the flexible DC system, and calculate the number of sub-modules cut out in the flexible DC system; the sub-modules are used for mutual conversion between AC and DC power; the power calculation unit is used to calculate the maximum average operating power and maximum power of the flexible DC system under safe operation conditions, based on the number of sub-modules put into use in the flexible DC system after the sub-modules are cut out; the control unit is used to reduce the output power of the islanded system according to the calculated maximum average operating power and maximum power; the recovery unit is used to end the fault ride-through control and increase the number of sub-modules put into use in the flexible DC system when the short-circuit fault in the receiving-end power grid is cleared.

[0031] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: By continuously updating and calculating three interrelated parameters, the sub-modules used in the flexible DC system, the maximum average operating power, and the maximum power input, the voltage of the flexible DC system is controlled to remain within a safe voltage range, avoiding DC blocking and shutdown of the flexible DC system due to short-term faults in the receiving-end power grid. At the same time, it reduces the dependence on energy-consuming resistors during fault ride-through of the flexible DC system and improves the economic efficiency of stable system operation. Attached Figure Description

[0032] Figure 1 This is a flowchart illustrating the receiving-end fault ride-through control method of the islanded flexible direct current transmission system of the present invention.

[0033] Figure 2 A schematic diagram of the system and control method of the flexible DC transmission system provided by the present invention;

[0034] Figure 3 This is a schematic diagram of power equal area for fault ride-through provided by the present invention. Detailed Implementation

[0035] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0036] "Isolated island" usually refers to a new energy island, which is a new energy power generation grid represented by wind power and photovoltaic power. The flexible DC system includes a sending-end converter module and a receiving-end converter module. The sending-end converter module is set at the new energy power generation grid end to convert AC power into DC power for external transmission. The receiving-end converter module is set at the load grid end to convert the transmitted DC power into AC power for the load grid to use.

[0037] See Figure 1 and Figure 2 This is a flowchart illustrating the receiving-end fault crossing control method of the islanded flexible DC transmission system of the present invention, and a schematic diagram illustrating the system and control method of the flexible DC transmission system provided by the present invention.

[0038] The receiving-end converter station is set with two safe DC voltage values ​​U dc1 and U dc2 , there is U dc1 <U dc2 When a short-circuit fault occurs in the receiving-end power grid, the DC voltage in the flexible DC system is recorded as reaching the first safety threshold U. dc1 At time t1, the DC voltage reaches the second safety threshold U. dc2 At time t2.

[0039] The DC circuit in a flexible DC system can be equivalent to a series of capacitors in parallel. The calculation method for the receiving-end output power sensed by the sending-end converter module is as follows:

[0040]

[0041] Among them, P f P0 represents the output power of the receiving-end converter module during a fault, P0 represents the system's transmission power (rated transmission power) in steady state, and C represents the equivalent capacitance of all sub-modules used in the flexible DC system.

[0042] See Figure 3 This is a power equal-area schematic diagram of fault ride-through provided by the present invention.

[0043] Calculate the maximum input power P of the flexible DC system during a fault. in This ensures that the DC voltage of the flexible DC system is always maintained at the safe voltage U. dc3 (U dc2 <U dc3 Under this process, the input power of the flexible DC system should satisfy the following power curve: Figure 3 As shown.

[0044] Figure 3 In the diagram, S1, S2, S3, and S4 correspond to four regions formed by curves, P s P represents the maximum average operating power of the system during safe operation under fault conditions. Ⅱ This represents the power operating curve without any fault ride-through measures, P. Ⅰ The curve representing the optimal sending-end power for achieving flexible DC fault ride-through. P Ⅰ The constraints are:

[0045]

[0046] That is:

[0047] S1≤S3(3)

[0048] Among them, P s The calculation formula is as follows:

[0049]

[0050] Where C represents the equivalent capacitance of all sub-modules used in the flexible DC system, and U dcN U represents the rated DC voltage of the flexible DC system. dc3 This represents the safe voltage of the flexible DC system, and T represents the total fault duration, which is generally the main protection operation time of the relay protection under AC three-phase short circuit conditions.

[0051] Power curve P Ⅰ The calculation method is as follows:

[0052]

[0053] Based on the rapid power adjustability of the flexible DC system, formula (5) can be expressed as:

[0054]

[0055] Where t3 represents the fault clearing time, which is generally determined by the fault start time and the total fault duration. The total fault duration is the main protection operation time of the relay protection under AC three-phase short circuit conditions. t2 represents the response time of the sending-end converter module, when U is detected. dc2 At that moment.

[0056] To mitigate the power reduction caused by new energy islanding while achieving fault ride-through of the flexible DC system, during the fault period, the decoupling method between the DC voltage and the flexible DC submodule is adopted to fully exploit the energy storage capacity of the submodule capacitor. The P value in formula (4) is... s The calculation method can be expressed as formula (7), and P in formula (6) in The calculation method can be expressed as formula (8), and its calculation formula is as follows:

[0057]

[0058]

[0059] The circuit structures of the sending-end converter module and the receiving-end converter module include multiple bridge arms, where N is the number of sub-modules in a single bridge arm. In the above formula, the circuit structures of the sending-end converter module and the receiving-end converter module each include 6 bridge arms. This invention does not impose any particular limitations on the sending-end converter module, the receiving-end converter module, or their sub-modules. The flexible DC system can be a flexible DC power transmission system based on a modular multilevel converter. The flexible DC system, the converter module, and the sub-modules within the converter module have already been disclosed in the prior art.

[0060] In practical implementation, if it is necessary to use the maximum input power P under safe operating conditions during the decoupling process of DC voltage and submodules in the flexible DC system, in At this point, the relevant formula can be used to calculate P in the current state. in The value is calculated and then continuously updated.

[0061] Among them, U cN This represents the voltage across the capacitor during rated operation of the system, i.e., the capacitor's rated voltage; U c3 U represents the maximum voltage that the submodule capacitor can withstand. c2 This indicates that the DC voltage of the flexible DC system reaches U. dc2 When the capacitor voltage of the submodule is C sm This indicates the capacitance value of the submodule capacitor, t3 indicates the fault clearing time, t2 indicates the response time of the sending-end converter module, and U is detected. dc2 At that moment.

[0062] During the decoupling process between DC voltage and submodule, it is necessary to satisfy the monotonicity condition between submodule capacitor voltage and DC voltage, and to maintain both submodule capacitor voltage and DC voltage below the safe voltage, i.e., the constraint relationship of formula (9) exists:

[0063]

[0064] Where t represents a certain moment during a receiving-end fault, N represents the number of sub-modules in use on a single-phase bridge arm in the converter module, and U dc (t) represents the DC voltage of the flexible DC system at time t, U c (t) represents the submodule capacitor voltage at time t; U dcN and U cN These represent the rated DC voltage of the flexible DC system and the rated capacitor voltage of the submodule, respectively; U' dc K represents the DC voltage conversion rate of the flexible DC system at time t. dc K represents the withstand voltage coefficient of the DC voltage in the flexible DC system; S(t) represents the number of submodules put into use by a single converter module at time t; K s represents the decoupling coefficient, and Modu(x) represents rounding down x.

[0065] During the fault, the sending-end power instantly dropped to P. in It is necessary to use flexible DC converter modules to reduce the voltage of wind farms in renewable energy islands, and to reduce the input power of the flexible DC system in conjunction with the active current reduction of wind turbines.

[0066] To improve the low-voltage ride-through scheme of the wind turbine and address the impact of reactive power generated by the wind turbine on the voltage drop of the flexible DC system during fault ride-through, the improved low-voltage ride-through scheme of the wind turbine is expressed by formula (10):

[0067]

[0068] Among them, i wd i wq and i max P represents the components of the fan current on the d-axis and q-axis, respectively, and the maximum output current of the fan. 0_PMSG u represents the reactive power output of the wind turbine at the moment the fault occurs. w Indicates the fan voltage, u w0 Indicates the rated voltage at the fan port, K d Indicates the low penetration factor, u ac1 The boundary voltage representing the reactive power output of the wind turbine.

[0069] During the fault period t3, according to Figure 3 Given the equal area requirement, the maximum input power of the flexible DC system is P.inmax u ac1 Satisfy the following formula:

[0070] P inmax ≥U ac1 ×i max (11)

[0071] Among them, P inmax This represents the calculated maximum power P. in The maximum value in.

[0072] When the energy loss detected by the wind farm reaches (P0-P... s When (t3-t0), the output current of the wind farm drops to i dref1 Where t0 represents the time when the fault occurs, and P0 represents the maximum power P at the time when the fault occurs. in i dref1 The calculation formula is as follows:

[0073]

[0074] Among them, P out U represents the output power of the flexible DC system during a fault. d This represents the component of the wind turbine voltage on the d-axis. It describes the calculation of the output power P of the flexible DC system on the wind turbine side of an isolated island without communication. out The following formula is used:

[0075]

[0076] Where P1 represents the active power of the islanded input flexible DC system after voltage reduction, W 1.05 This represents the charging energy (constant) of the flexible DC system when the DC voltage rises to 1.05 per unit value, in W. △VSC This represents the amount of energy (a fixed value) that can be charged under safe operating conditions of the flexible DC system.

[0077] When the fault in the receiving-end power grid is detected to be cleared, the output power of the receiving-end converter module is restored, the voltage of the sub-module capacitor drops, the power transmitted by the sending-end converter module begins to recover, and the number of sub-modules put into use in the bridge arm begins to increase until it is restored to the rated number for stable operation.

[0078] In practical implementation, by continuously updating and calculating three interrelated parameters—namely, the sub-modules used in the flexible DC system, the maximum average operating power, and the maximum power input—and by calculating the number of sub-modules used after decoupling, the maximum average operating power and maximum power input to the flexible DC system are continuously updated and calculated. This ensures that the voltage of the flexible DC system is always kept within a safe voltage range, fully coordinating and utilizing the fault ride-through capability of the flexible DC system itself and the islanded system, avoiding DC blocking of the flexible DC system due to short-term faults in the receiving-end power grid, which could lead to outages. At the same time, it reduces the dependence of the flexible DC system on energy-consuming resistors during fault ride-through, improving the economic efficiency of stable system operation.

[0079] In practical implementation, flexible DC converter modules are used to reduce the voltage of wind farms in renewable energy islands. This, combined with the active current reduction of wind turbines, lowers the input power of the flexible DC system. This further reduces the dependence of the flexible DC system on energy-consuming resistors during fault ride-through, ensuring the stability of the flexible DC system operation.

[0080] This invention also provides a receiving-end fault ride-through control device for an islanded flexible DC system, comprising: a judgment unit, a decoupling unit, a power calculation unit, a control unit, and a recovery unit, wherein: the judgment unit is used to perform fault ride-through control when the DC voltage of the flexible DC system reaches a warning safety threshold after a short-circuit fault occurs in the receiving-end power grid; the decoupling unit is used to decouple the DC voltage from the sub-modules of the sending-end converter module and the receiving-end converter module in the flexible DC system, and calculate the number of sub-modules cut out in the flexible DC system; the sub-modules are used for mutual conversion between AC and DC power; the power calculation unit is used to calculate the maximum average operating power and maximum power of the flexible DC system under safe operation conditions, based on the number of sub-modules put into use in the flexible DC system after the sub-modules are cut out; the control unit is used to reduce the output power of the islanded system according to the calculated maximum average operating power and maximum power; the recovery unit is used to end the fault ride-through control and increase the number of sub-modules put into use in the flexible DC system when the short-circuit fault in the receiving-end power grid is cleared.

[0081] In specific implementation, the calculation formulas, steps and functions executed by each execution unit of the islanded flexible DC system receiving-end fault ride-through control device provided by the present invention can refer to the receiving-end fault ride-through control method of the islanded flexible DC system provided by the present invention.

Claims

1. A receiving-end fault ride-through control method for islanded transmission via a flexible DC system, characterized in that, include: After a short-circuit fault occurs in the receiving-end power grid, when the DC voltage of the flexible DC system reaches the early warning safety threshold, fault ride-through control is implemented. Decouple the DC voltage from the sub-modules of the sending-end converter module and the receiving-end converter module in the flexible DC system, and calculate the number of sub-modules cut out in the flexible DC system. The submodule is used for mutual conversion between AC and DC power. During the decoupling process between the DC voltage and the submodule in the flexible DC system, a monotonicity condition must be met between the submodule capacitor voltage and the DC voltage, and both the submodule capacitor voltage and the DC voltage must be maintained below a safe voltage. The monotonicity condition and the maintenance of the submodule capacitor voltage and the DC voltage below a safe voltage are constrained by the following formula: ， Where N represents the number of submodules put into use on a single-phase bridge arm in the flexible DC system after the submodules are cut out, and U dc (t) represents the DC voltage of the flexible DC system at time t, U c S(t) represents the capacitor voltage of the submodule at time t, S(t) represents the number of submodules put into use by a single converter module at time t, and K s P represents the decoupling coefficient. in U represents the maximum input power under safe operating conditions of the flexible DC system, C represents the equivalent capacitance of all sub-modules used in the flexible DC system, and U represents the maximum input power. dcN and U cN K represents the rated values ​​of the DC voltage of the flexible DC system and the capacitor voltage of the submodule, respectively. dc U represents the DC voltage withstand coefficient of a flexible DC system. ’ dc (t) represents the DC voltage conversion efficiency of the flexible DC system at time t, Modu(x) represents the floor function of x, and S(t) represents the DC voltage conversion efficiency of x at time t. min This represents the minimum number of submodules that are put into use in a single converter module. Based on the number of submodules deployed in the flexible DC system after the submodules are cut out, calculate the maximum average operating power and maximum power of the flexible DC system under safe operation conditions; the maximum average operating power P... s The following formula is used for calculation: ， Among them, C sm U represents the capacitance of the submodule. c3 This indicates the maximum withstand voltage of the submodule capacitor, and T represents the total fault duration. The maximum power P in The following formula is used for calculation: ， Among them, U c2 t2 represents the capacitor voltage of the submodule at time t2, and t3 represents the time when the fault is cleared; Reduce the island's output power based on the calculated maximum average operating power and maximum power; When the short-circuit fault in the receiving-end power grid is cleared, the fault ride-through control ends, and the number of sub-modules put into use in the flexible DC system increases.

2. The receiving-end fault ride-through control method for islanded flexible DC transmission systems according to claim 1, characterized in that, When the DC voltage of the flexible DC system reaches the warning safety threshold, fault ride-through control is performed, including: After a short-circuit fault occurs in the receiving-end power grid, the DC voltage of the flexible DC system reaches the first safety threshold, and the corresponding time t1 is recorded. Then, when the DC voltage of the flexible DC system rises to the second safety threshold, the corresponding time t2 is recorded, and fault ride-through control is performed.

3. The receiving-end fault ride-through control method for islanded flexible DC transmission systems according to claim 2, characterized in that, The calculation of the maximum average operating power and maximum power of the flexible DC system under safe operation conditions, based on the number of submodules used in the flexible DC system after the submodules are cut out, includes: Based on the number of sub-modules disconnected from the flexible DC system during the total fault duration, calculate the maximum number of sub-modules that can be put into use in the flexible DC system after the sub-modules are disconnected, and calculate the maximum average operating power and maximum power of the input flexible DC system.

4. The receiving-end fault ride-through control method for islanded flexible DC transmission systems according to claim 3, characterized in that, During fault ride-through control, the wind turbines in the isolated system are controlled as follows: ， Among them, i wd i wq and i max P represents the components of the fan current on the d-axis and q-axis, respectively, and the maximum output current of the fan. 0_PMSG u represents the reactive power output of the wind turbine at the moment the fault occurs. w Indicates the fan voltage, u w0 Indicates the rated voltage at the fan port, u ac1 The boundary voltage, K, represents the reactive power output of the wind turbine. d Indicates the low penetration factor, u ac1 Satisfy the following formula: ， Among them, P inmax This represents the calculated maximum power P. in The maximum value in.

5. The receiving-end fault ride-through control method for islanded flexible DC transmission systems according to claim 4, characterized in that, The reduction of islanded output power based on the calculated maximum average operating power and maximum power includes: During fault ride-through control, the energy loss of the wind turbines in the island reaches (P0-P). s When (t3-t0), the output current of the fan is reduced to i. dref1 Where t0 represents the time when the fault occurs, and P0 represents the maximum power P at the time when the fault occurs. in i dref1 The calculation formula is as follows: ， Among them, P out U represents the output power of the flexible DC system during a fault. d This represents the component of the wind turbine voltage on the d-axis. It describes the calculation of the output power P of the flexible DC system on the wind turbine side of an isolated island without communication. out The following formula is used: ， Where P1 represents the active power of the islanded input flexible DC system after voltage reduction, W 1.05 This represents the charging energy of the flexible DC system when the DC voltage rises to 1.05 per unit value, in W. △VSC This indicates the energy that can be charged under safe operating conditions of the flexible DC system.

6. A receiving-end fault ride-through control device for islanded transmission via a flexible DC system, characterized in that, This method is used to implement the receiving-end fault ride-through control method for islanded flexible direct current transmission systems according to any one of claims 1 to 5.

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