Control method, device and system of photovoltaic through flexible direct current sending-out system

Abstract

The application provides a control method, device and system of a photovoltaic flexible DC sending-out system. The method makes the sending-end flexible DC converter station and the photovoltaic field station still work normally when the sending-end AC fault of the photovoltaic flexible DC sending-out system occurs, and restores the system after the sending-end AC fault of the photovoltaic flexible DC sending-out system is cleared, thereby solving the problem that the photovoltaic flexible DC sending-out system cannot be safely and stably operated when the sending-end power grid of the photovoltaic flexible DC sending-out system encounters a serious AC fault, and the photovoltaic flexible DC sending-out system loses voltage support.

Description

Technical Field

[0001] The present application relates to the technical field of new energy direct current transmission engineering, and in particular to a control method, device and system for a photovoltaic flexible direct current transmission system. Background Art

[0002] Existing renewable energy DC transmission projects primarily bundle renewable energy sources connected to the sending AC grid with conventional thermal power, then transmit them through the DC system. If the sending system experiences a severe AC fault, the system voltage will still be supported by conventional power sources such as thermal power, and surplus power from renewable energy can still be absorbed by the local AC grid. Therefore, for traditional renewable energy DC transmission systems that use wind and thermal power bundling, transient voltage issues on the sending grid are not severe during AC system failures and subsequent recovery.

[0003] However, in a system that transmits renewable energy via PV-HVDC Flexible, the AC voltage of the sending-end grid is entirely generated by HVDC Flexible, and the power generated by renewable energy can only be transmitted through the sending-end HVDC Flexible converter station. If the sending-end grid of a PV-HVDC Flexible system experiences a severe AC fault, the PV-HVDC Flexible system will lose voltage support and experience a large power surplus. This will not only affect the effective absorption of PV power but also seriously affect the system's transient voltage characteristics, threatening the safe and stable operation of the system. Summary of the Invention

[0004] The main purpose of this application is to provide a control method, device and system for a photovoltaic flexible DC transmission system, so as to at least solve the problem that when the sending-end power grid of the photovoltaic flexible DC transmission system encounters a serious AC fault, the photovoltaic flexible DC transmission system will lose voltage support, thereby making the photovoltaic flexible DC transmission system unable to operate safely and stably.

[0005] To achieve the above objectives, according to one aspect of the present application, a control method for a photovoltaic flexible direct current transmission system is provided, the method comprising:

[0006] At least based on the AC voltage and DC voltage at the photovoltaic grid connection point, determine whether there is an AC fault at the sending end of the photovoltaic flexible DC transmission system;

[0007] In the case where it is determined that the sending-end AC fault occurs in the photovoltaic via flexible DC transmission system, the sending-end flexible DC converter station of the photovoltaic via flexible DC transmission system is used to adjust the d-axis voltage command value of the sending-end flexible DC converter station to a reference voltage, and at the same time, the photovoltaic field station of the photovoltaic via flexible DC transmission system is used to keep the output of the outer loop integral control unchanged, and at the same time, the active current command value of the photovoltaic field station is adjusted to a preset active current, and at the same time, the reactive current command value of the photovoltaic field station is adjusted to a preset reactive current, and the output of the outer loop integral control is the steady-state value of the variable of the photovoltaic field station before the sending-end AC fault occurs in the photovoltaic via flexible DC transmission system;

[0008] When it is determined that the sending-end AC fault of the photovoltaic flexible DC transmission system has been cleared, the sending-end flexible DC converter station is used to increase the d-axis voltage command value at a first preset rate until the d-axis voltage command value reaches 1 p.u., and at the same time, the photovoltaic field station is used to increase the active current command value at a first preset rate until the active current command value returns to the size before the sending-end AC fault.

[0009] Optionally, before using the sending-end flexible DC converter station of the photovoltaic flexible DC transmission system to adjust the d-axis voltage command value of the sending-end flexible DC converter station to a reference voltage, the method further includes:

[0010] The reference voltage is determined to be a preset voltage corresponding to the minimum per-unit value of the effective value of the AC bus voltage of the sending-end flexible DC converter station.

[0011] Optionally, determining the reference voltage as a preset voltage corresponding to the minimum per-unit value of the effective value of the AC bus voltage of the sending-end flexible DC converter station includes:

[0012] according to determining the reference voltage,

[0013] Among them, U a is the reference voltage, U PMS_min It is the minimum per-unit value of the effective value of the AC bus voltage of the sending-end flexible DC converter station.

[0014] Optionally, before adjusting the reactive current command value of the photovoltaic station to a preset reactive current, the method further includes:

[0015] according to determining the preset reactive current,

[0016] in, is the preset reactive current, I N_PV is the rated current of the photovoltaic station, U PCCis the AC voltage of the photovoltaic grid-connected point, and a is a preset constant.

[0017] Optionally, before adjusting the active current command value of the photovoltaic station to a preset active current, the method further includes:

[0018] according to determining the preset active current,

[0019] in, is the preset active current, I vmax is the maximum current of the photovoltaic station, is the preset reactive current, The current command value for the photovoltaic additional control of the photovoltaic station during the sending-end AC fault.

[0020] Optionally, the method further includes:

[0021] according to determining a current command value for the photovoltaic additional control of the photovoltaic station during the AC fault at the sending end,

[0022] in, is the current instruction value of the photovoltaic additional control of the photovoltaic station during the AC fault at the sending end, is the current instruction value of the photovoltaic additional control of the photovoltaic station before the AC fault at the sending end, U PCC is the AC voltage of the photovoltaic grid-connected point, V DC is the photovoltaic DC voltage, V DCN is the rated value of the photovoltaic DC voltage.

[0023] Optionally, determining whether a sending-end AC fault occurs in the photovoltaic flexible DC transmission system based on at least the magnitude of the AC voltage and the DC voltage at the photovoltaic grid connection point includes:

[0024] If the duration of the first preset condition being met exceeds 20ms, it is determined that the sending-end AC fault occurs in the photovoltaic flexible DC transmission system, where the first preset condition indicates that the AC voltage at the photovoltaic grid connection point is less than 0.9pu and the photovoltaic DC voltage is greater than 1.05 times the rated photovoltaic DC voltage;

[0025] When the duration of satisfying the first preset condition does not exceed 20 ms, it is determined that the photovoltaic flexible DC transmission system does not have the sending-end AC fault.

[0026] Optionally, when it is determined that the sending-end AC fault occurs in the photovoltaic flexible direct current transmission system, the method further includes:

[0027] When the per-unit value of the AC bus voltage of the sending-end flexible DC converter station is greater than or equal to 0.9pu, and the duration of the per-unit value of the AC bus voltage being greater than or equal to 0.9pu exceeds 20ms, it is determined that the sending-end AC fault of the photovoltaic flexible DC transmission system has been cleared.

[0028] According to another aspect of the present application, a control device for a photovoltaic flexible direct current transmission system is provided, the device comprising:

[0029] A determination unit, configured to determine whether a sending-end AC fault occurs in the photovoltaic flexible DC transmission system based on at least the magnitude of the AC voltage and the photovoltaic DC voltage at the photovoltaic grid-connected point;

[0030] a first processing unit, configured to, upon determining that the sending-end AC fault occurs in the photovoltaic flexible direct current transmission system, use the sending-end flexible direct current converter station of the photovoltaic flexible direct current transmission system to adjust the d-axis voltage command value of the sending-end flexible direct current converter station to a reference voltage, and simultaneously use the photovoltaic field station of the photovoltaic flexible direct current transmission system to maintain the output of the outer loop integral control unchanged, and simultaneously adjust the active current command value of the photovoltaic field station to a preset active current, and simultaneously adjust the reactive current command value of the photovoltaic field station to a preset reactive current, wherein the output of the outer loop integral control is the steady-state value of the variable of the photovoltaic field station before the sending-end AC fault occurs in the photovoltaic flexible direct current transmission system;

[0031] The second processing unit is used to, when it is determined that the sending-end AC fault of the photovoltaic flexible DC transmission system has been cleared, use the sending-end flexible DC converter station to increase the d-axis voltage command value at a first preset rate until the d-axis voltage command value reaches 1 p.u., and at the same time use the photovoltaic field station to increase the active current command value at a first preset rate until the active current command value recovers to the size before the sending-end AC fault.

[0032] According to another aspect of the present application, a control system for a photovoltaic flexible direct current transmission system is provided, the system comprising: one or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, and the one or more programs include methods for executing any one of the methods described.

[0033] By applying the technical solution of the present application, when it is determined that the photovoltaic via flexible DC transmission system has the said sending-end AC fault, the sending-end flexible DC converter station of the photovoltaic via flexible DC transmission system is used to adjust the d-axis voltage command value of the sending-end flexible DC converter station to the reference voltage, and at the same time, the photovoltaic station of the photovoltaic via flexible DC transmission system is used to keep the output of the outer loop integral control unchanged, and at the same time, the active current command value of the photovoltaic station is adjusted to the preset active current, and at the same time, the reactive current command value of the photovoltaic station is adjusted to the preset reactive current, so that the sending-end flexible DC converter station and the photovoltaic station can still work normally when the photovoltaic via flexible DC transmission system has the said sending-end AC fault, and at the same time, when it is determined that the photovoltaic via flexible DC transmission system has the said sending-end AC fault When the sending-end AC fault of the system has been cleared, the sending-end flexible DC converter station is used to increase the d-axis voltage command value at a first preset rate until the d-axis voltage command value reaches 1 p.u., and at the same time, the photovoltaic station is used to increase the active current command value at a first preset rate until the active current command value is restored to the size before the sending-end AC fault, thereby achieving the purpose of restoring the system after determining that the sending-end AC fault of the photovoltaic flexible DC transmission system has been cleared, and further solving the problem that when the sending-end power grid of the photovoltaic flexible DC transmission system encounters a serious AC fault, the photovoltaic flexible DC transmission system will lose voltage support, thereby making the photovoltaic flexible DC transmission system unable to operate safely and stably. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The drawings that constitute part of this application are used to provide a further understanding of this application. The illustrative embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation on this application. In the drawings:

[0035] Figure 1 A schematic flow chart of a control method for a photovoltaic flexible direct current transmission system according to an embodiment of the present application is shown;

[0036] Figure 2 A schematic diagram of a photovoltaic station provided according to an embodiment of the present application is shown;

[0037] FIG3( a ) shows a waveform diagram of the effective voltage value of the sending-end power grid under an AC fault according to an embodiment of the present application;

[0038] FIG3( b ) shows a waveform diagram of the instantaneous voltage value of the sending-end power grid under an AC fault according to an embodiment of the present application;

[0039] FIG4( a ) shows a waveform diagram of the effective voltage value of the sending-end power grid under an AC fault according to an embodiment of the present application;

[0040] FIG4( b ) shows a waveform diagram of the instantaneous voltage value of the sending-end power grid under an AC fault according to an embodiment of the present application;

[0041] FIG4( c ) shows a waveform diagram of the d-axis voltage command value of the flexible DC converter station provided according to an embodiment of the present application;

[0042] FIG4( d ) shows a waveform diagram of the active current of a photovoltaic station provided according to an embodiment of the present application;

[0043] FIG4( e ) shows a waveform diagram of reactive current of a photovoltaic station provided according to an embodiment of the present application;

[0044] Figure 5 A structural block diagram of a control device for a photovoltaic flexible direct current transmission system provided according to an embodiment of the present application is shown. DETAILED DESCRIPTION

[0045] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0046] In order to enable those skilled in the art to better understand the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of this application.

[0047] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequential order. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present application described here. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0048] For ease of description, some nouns or terms involved in the embodiments of the present application are explained below:

[0049] A PV-HVDC Flexible transmission system utilizes the power generated by photovoltaic panels and converts it into a variable DC voltage. This system can adjust output voltage and frequency based on demand, while also enabling energy storage and management. The use of a PV-HVDC Flexible transmission system can improve the efficiency and reliability of PV power generation systems and better adapt to varying grid access conditions and requirements.

[0050] A sending-end AC failure occurs when a fault occurs on the sending end during communication, preventing information from being correctly transmitted to the receiving end. This failure may be caused by a faulty sending device, network issues, or signal interference. To resolve this issue, it is necessary to diagnose the sending end and repair or replace the faulty device to ensure normal communication.

[0051] As introduced in the background technology, in the photovoltaic-via-flexible direct current transmission system for new energy, the AC voltage of the sending-end power grid is completely constructed by flexible direct current, and the power generated by the new energy can only be transmitted through the sending-end flexible direct current converter station. When the sending-end power grid of the photovoltaic-via-flexible direct current transmission system encounters a serious AC fault, the photovoltaic-via-flexible direct current transmission system will lose voltage support and a large amount of power surplus will appear, which will not only affect the effective absorption of photovoltaic power, but also seriously affect the transient voltage characteristics of the system, threatening the safe and stable operation of the system. In order to solve the problem that when the sending-end power grid of the photovoltaic-via-flexible direct current transmission system encounters a serious AC fault, the photovoltaic-via-flexible direct current transmission system will lose voltage support, thereby making the photovoltaic-via-flexible direct current transmission system unable to operate safely and stably, the embodiments of the present application provide a control method, device and system for a photovoltaic-via-flexible direct current transmission system.

[0052] The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0053] In this embodiment, a control method for a photovoltaic flexible direct current transmission system is provided. It should be noted that the steps shown in the flowchart of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and although a logical order is shown in the flowchart, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0054] Figure 1 FIG. 1 is a flow chart of a control method for a photovoltaic flexible direct current transmission system according to an embodiment of the present application. Figure 1 As shown, the method includes the following steps:

[0055] Step S101, determining whether a sending-end AC fault occurs in the photovoltaic flexible DC transmission system based on at least the magnitudes of the AC voltage and the DC voltage at the photovoltaic grid connection point;

[0056] Specifically, if the duration of the first preset condition being met exceeds 20ms, it is determined that the sending-end AC fault has occurred in the photovoltaic flexible DC transmission system, and the first preset condition indicates that the AC voltage at the photovoltaic grid connection point is less than 0.9pu, and the photovoltaic DC voltage is greater than 1.05 times the rated photovoltaic DC voltage;

[0057] When the duration of satisfying the first preset condition does not exceed 20ms, it is determined that the photovoltaic flexible DC transmission system does not have the above-mentioned sending-end AC fault.

[0058] In addition, it is also possible to determine whether a sending-end AC fault occurs in the photovoltaic flexible DC transmission system based solely on the size of the AC voltage and the photovoltaic DC voltage at the photovoltaic grid-connected point. When the AC voltage at the photovoltaic grid-connected point is less than 0.9pu and the photovoltaic DC voltage is greater than 1.05 times the rated value of the photovoltaic DC voltage, it is determined that the sending-end AC fault occurs in the photovoltaic flexible DC transmission system. Otherwise, it is determined that no sending-end AC fault occurs.

[0059] Step S102: When it is determined that the sending-end AC fault occurs in the photovoltaic flexible DC transmission system, the d-axis voltage command value of the sending-end flexible DC converter station of the photovoltaic flexible DC transmission system is adjusted to a reference voltage, and at the same time, the output of the outer loop integral control is maintained unchanged by the photovoltaic station of the photovoltaic flexible DC transmission system, and the active current command value of the photovoltaic station is adjusted to a preset active current, and the reactive current command value of the photovoltaic station is adjusted to a preset reactive current, and the output of the outer loop integral control is the steady-state value of the variable of the photovoltaic station before the sending-end AC fault occurs in the photovoltaic flexible DC transmission system;

[0060] In one embodiment of the present application, before using the sending-end flexible DC converter station of the photovoltaic flexible DC transmission system to adjust the d-axis voltage command value of the sending-end flexible DC converter station to a reference voltage, the method further includes:

[0061] The reference voltage is determined to be a preset voltage corresponding to the minimum per-unit value of the effective value of the AC bus voltage of the sending-end flexible DC converter station.

[0062] In one embodiment of the present application, determining the reference voltage as a preset voltage corresponding to the minimum per-unit value of the effective value of the AC bus voltage of the sending-end flexible DC converter station includes:

[0063] according to Determine the above reference voltage,

[0064] Among them, U a is the above reference voltage, U PMS_minIt is the minimum per-unit value of the effective value of the above-mentioned AC bus voltage of the above-mentioned sending-end flexible DC converter station.

[0065] In one embodiment of the present application, before adjusting the reactive current command value of the photovoltaic station to a preset reactive current, the method further includes:

[0066] according to Determine the above preset reactive current,

[0067] in, is the preset reactive current, I N_PV is the rated current of the above photovoltaic station, U PCC is the AC voltage at the photovoltaic grid-connected point, and a is a preset constant.

[0068] a can be 1.5.

[0069] In one embodiment of the present application, before adjusting the active current command value of the photovoltaic station to a preset active current, the method further includes:

[0070] according to Determine the above preset active current,

[0071] in, is the preset active current, I vmax is the maximum current of the above photovoltaic station, is the above preset reactive current, It is the current command value of the photovoltaic additional control of the above-mentioned photovoltaic station during the above-mentioned sending-end AC fault.

[0072] In one embodiment of the present application, the above method further includes:

[0073] according to Determine the current command value of the photovoltaic additional control of the photovoltaic station during the AC fault at the sending end,

[0074] in, is the current command value of the photovoltaic additional control during the AC fault at the sending end of the photovoltaic station, is the current command value of the photovoltaic additional control of the photovoltaic station before the AC fault at the sending end, U PCC is the AC voltage of the photovoltaic grid-connected point, V DC is the photovoltaic DC voltage mentioned above, V DCN is the rated value of the photovoltaic DC voltage.

[0075] The structure of the photovoltaic station is as follows: Figure 2As shown in the figure, the DC power generated by the photovoltaic array is inverted into 0.69kV AC power through the photovoltaic inverter, and then stepped up to 35kV, 230kV, and 525kV in sequence through the step-up transformer, and finally fed into the main grid at the sending end. A three-phase grounding fault occurred in the sending end system at 1.0s. After the fault occurred, the voltage at the PCC point dropped. The fault duration was set to 0.1s. After the fault was cleared, the voltage gradually recovered. When the system does not adopt the control strategy proposed by the present invention, the simulation results are shown in Figures 3(a) and 3(b). It can be seen from Figures 3(a) and 3(b) that after the fault occurs, the flexible DC system will experience a relatively serious overvoltage, which can reach up to 830kV, seriously threatening the safe operation of the system.

[0076] The following describes the control and effects of the proposed method: 1.0s after a fault occurs, the PCC voltage drops rapidly. When the detection device detects that the per-unit value of the AC bus voltage at the sending-end converter station has fallen below 0.9 pu for a duration exceeding 20ms, the sending-end flexible DC converter station determines that an AC fault has occurred in the system and switches to fault control mode. Because the per-unit value of the AC bus voltage at the sending-end converter station, URMS_min, is detected to be less than 0.3 pu during the fault period, the flexible DC converter station fault handling module sets the d-axis voltage command value, Ua, to 0.4 pu during the fault period.

[0077] At the same time, the detection module for the AC fault of the sending-end system of the photovoltaic station detects that the AC voltage at the photovoltaic grid-connected point is less than 0.9pu, the photovoltaic DC voltage is greater than 1.05 times the rated value of the photovoltaic DC voltage, and the duration exceeds 20ms. The photovoltaic station determines that an AC fault has occurred. The active and reactive current control module during the photovoltaic station fault is used to control the active and reactive currents of the photovoltaic station during the sending-end AC fault. The photovoltaic station determines that a sending-end AC fault has occurred. The photovoltaic station freezes the outer loop integral controller of the photovoltaic station and disables the outer loop controller (the photovoltaic station using the above-mentioned photovoltaic flexible DC transmission system maintains the output of the outer loop integral control unchanged) to directly control the active and reactive currents of the photovoltaic. During the photovoltaic station fault, the reactive current is controlled according to the formula for determining the above-mentioned preset reactive current, and the active current is controlled according to the formula for determining the above-mentioned preset active current.

[0078] Step S103: When it is determined that the sending-end AC fault of the photovoltaic flexible DC transmission system has been cleared, the sending-end flexible DC converter station is used to increase the d-axis voltage command value at a first preset rate until the d-axis voltage command value reaches 1 p.u., and at the same time, the photovoltaic field station is used to increase the active current command value at a first preset rate until the active current command value returns to the value before the sending-end AC fault.

[0079] In the above steps, when it is determined that the above-mentioned photovoltaic via flexible DC transmission system has the above-mentioned AC fault at the sending end, the sending-end flexible DC converter station of the above-mentioned photovoltaic via flexible DC transmission system is used to adjust the d-axis voltage command value of the above-mentioned flexible DC converter station to the reference voltage, and at the same time, the photovoltaic station of the above-mentioned photovoltaic via flexible DC transmission system is used to keep the output of the outer loop integral control unchanged, and at the same time, the active current command value of the above-mentioned photovoltaic station is adjusted to the preset active current, and at the same time, the reactive current command value of the above-mentioned photovoltaic station is adjusted to the preset reactive current, so that the sending-end flexible DC converter station and the photovoltaic station can still work normally when the above-mentioned AC fault at the sending end of the photovoltaic via flexible DC transmission system occurs, and at the same time, when it is determined that the above-mentioned photovoltaic via flexible DC transmission system has the above-mentioned AC fault at the sending end, the sending-end flexible DC converter station and the photovoltaic station can still work normally. When the sending-end AC fault has been cleared, the sending-end flexible DC converter station is used to increase the d-axis voltage command value at a first preset rate until the d-axis voltage command value reaches 1 p.u., and at the same time, the photovoltaic station is used to increase the active current command value at a first preset rate until the active current command value is restored to the size before the sending-end AC fault, thereby achieving the purpose of restoring the system after determining that the sending-end AC fault of the photovoltaic flexible DC transmission system has been cleared, and further solving the problem that when the sending-end power grid of the photovoltaic flexible DC transmission system encounters a serious AC fault, the photovoltaic flexible DC transmission system will lose voltage support, thereby making the photovoltaic flexible DC transmission system unable to operate safely and stably.

[0080] In one embodiment of the present application, when it is determined that the photovoltaic flexible direct current transmission system has the sending-end AC fault, the method further includes:

[0081] If the per-unit value of the AC bus voltage at the sending-end flexible DC converter station is greater than or equal to 0.9 pu, and the duration of the per-unit value of the AC bus voltage being greater than or equal to 0.9 pu exceeds 20 ms, it is determined that the sending-end AC fault of the PV-V flexible DC transmission system has been cleared. Otherwise, it is determined that the sending-end AC fault has not been cleared.

[0082] When the sending-end system AC fault clearance detection module detects that the per-unit value of the AC bus voltage at the sending-end converter station exceeds 0.9 pu and lasts for more than 20ms, the sending-end system AC fault is determined to be cleared. The D-axis voltage command value of the sending-end flexible DC converter station is gradually increased at a rate of 3 p.u / s until it returns to 1.0 pu; the active current command value of the photovoltaic station is gradually increased at a specific rate of 2 p.u / s until it returns to its pre-fault value. As can be seen from Figures 4(a), 4(b), 4(c), 4(e), and 4(d), the control strategy proposed in this invention can effectively improve the fault ride-through capability and voltage stability of the sending-end system.

[0083] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0084] The embodiments of the present application also provide a control device for a photovoltaic flexible DC transmission system. It should be noted that the control device for a photovoltaic flexible DC transmission system in the embodiments of the present application can be used to execute the control method for a photovoltaic flexible DC transmission system provided in the embodiments of the present application. The device is used to implement the above-mentioned embodiments and preferred implementations, and those that have been explained will not be repeated. As used below, the term "module" can be a combination of software and / or hardware that implements a predetermined function. Although the devices described in the following embodiments are preferably implemented in software, implementation in hardware, or a combination of software and hardware, is also possible and conceivable.

[0085] The following introduces the control device of the photovoltaic flexible direct current transmission system provided in the embodiment of the present application.

[0086] Figure 5 This is a structural block diagram of a control device for a photovoltaic flexible direct current transmission system according to an embodiment of the present application. Figure 5 As shown, the device includes:

[0087] A determination unit 51 is configured to determine whether a sending-end AC fault occurs in the photovoltaic flexible DC transmission system based on at least the magnitude of the AC voltage and the photovoltaic DC voltage at the photovoltaic grid connection point;

[0088] The first processing unit 52 is configured to, upon determining that the sending-end AC fault has occurred in the photovoltaic flexible DC transmission system, adjust the d-axis voltage command value of the sending-end flexible DC converter station to a reference voltage using the sending-end flexible DC converter station of the photovoltaic flexible DC transmission system, and simultaneously maintain the output of the outer-loop integral control by the photovoltaic station of the photovoltaic flexible DC transmission system, and simultaneously adjust the active current command value of the photovoltaic station to a preset active current, and simultaneously adjust the reactive current command value of the photovoltaic station to a preset reactive current, wherein the output of the outer-loop integral control is the steady-state value of the variable of the photovoltaic station before the sending-end AC fault occurred in the photovoltaic flexible DC transmission system;

[0089] The second processing unit 53 is used to, when it is determined that the sending-end AC fault of the above-mentioned photovoltaic flexible DC transmission system has been cleared, use the above-mentioned sending-end flexible DC converter station to increase the above-mentioned d-axis voltage command value at a first preset rate until the above-mentioned d-axis voltage command value reaches 1 p.u., and at the same time use the above-mentioned photovoltaic field station to increase the above-mentioned active current command value at a first preset rate until the above-mentioned active current command value recovers to the value before the above-mentioned sending-end AC fault.

[0090] In the above-mentioned device, when it is determined that the above-mentioned sending-end AC fault occurs in the above-mentioned photovoltaic via flexible DC transmission system, the sending-end flexible DC converter station of the above-mentioned photovoltaic via flexible DC transmission system is used to adjust the d-axis voltage command value of the above-mentioned sending-end flexible DC converter station to the reference voltage, and at the same time, the photovoltaic station of the above-mentioned photovoltaic via flexible DC transmission system is used to keep the output of the outer loop integral control unchanged, and at the same time, the active current command value of the above-mentioned photovoltaic station is adjusted to the preset active current, and at the same time, the reactive current command value of the above-mentioned photovoltaic station is adjusted to the preset reactive current, so that the sending-end flexible DC converter station and the photovoltaic station can still work normally when the above-mentioned sending-end AC fault occurs in the photovoltaic via flexible DC transmission system, and at the same time, when it is determined that the above-mentioned photovoltaic via flexible DC transmission system When the sending-end AC fault has been cleared, the sending-end flexible DC converter station is used to increase the d-axis voltage command value at a first preset rate until the d-axis voltage command value reaches 1 p.u., and at the same time, the photovoltaic station is used to increase the active current command value at a first preset rate until the active current command value is restored to the size before the sending-end AC fault, thereby achieving the purpose of restoring the system after determining that the sending-end AC fault of the photovoltaic flexible DC transmission system has been cleared, and further solving the problem that when the sending-end power grid of the photovoltaic flexible DC transmission system encounters a serious AC fault, the photovoltaic flexible DC transmission system will lose voltage support, thereby making the photovoltaic flexible DC transmission system unable to operate safely and stably.

[0091] In one embodiment of the present application, the above-mentioned device also includes a third processing unit. Before the sending-end flexible DC converter station of the above-mentioned photovoltaic flexible DC transmission system is used to adjust the d-axis voltage command value of the above-mentioned sending-end flexible DC converter station to a reference voltage, the third processing unit is used to determine that the above-mentioned reference voltage is a preset voltage corresponding to the minimum value of the per-unit value of the effective value of the AC bus voltage of the above-mentioned sending-end flexible DC converter station.

[0092] In one embodiment of the present application, the third processing unit includes a first determining module;

[0093] The first determining module is used to Determine the above reference voltage,

[0094] Among them, U a is the above reference voltage, U PMS_minIt is the minimum per-unit value of the effective value of the above-mentioned AC bus voltage of the above-mentioned sending-end flexible DC converter station.

[0095] In one embodiment of the present application, the apparatus further includes a fourth processing unit, which, before adjusting the reactive current command value of the photovoltaic station to a preset reactive current,

[0096] The fourth processing unit is used to Determine the above preset reactive current,

[0097] in, is the preset reactive current, I N_PV is the rated current of the above photovoltaic station, U PCC is the AC voltage at the photovoltaic grid-connected point, and a is a preset constant.

[0098] In one embodiment of the present application, the apparatus further includes a fifth processing unit, which, before adjusting the active current command value of the photovoltaic station to a preset active current,

[0099] The fifth processing unit is used to Determine the above preset active current,

[0100] in, is the preset active current, I vmax is the maximum current of the above photovoltaic station, is the above preset reactive current, It is the current command value of the photovoltaic additional control of the above-mentioned photovoltaic station during the above-mentioned sending-end AC fault.

[0101] In one embodiment of the present application, the apparatus further includes a sixth processing unit.

[0102] The sixth processing unit is used to Determine the current command value of the photovoltaic additional control of the photovoltaic station during the AC fault at the sending end,

[0103] in, is the current command value of the photovoltaic additional control during the AC fault at the sending end of the photovoltaic station, is the current command value of the photovoltaic additional control of the photovoltaic station before the AC fault at the sending end, U PCC is the AC voltage of the photovoltaic grid-connected point, V DC is the photovoltaic DC voltage mentioned above, V DCN is the rated value of the photovoltaic DC voltage.

[0104] In one embodiment of the present application, the determining unit includes a second determining module and a third determining module.

[0105] The second determining module is configured to determine that the sending-end AC fault occurs in the photovoltaic flexible DC transmission system when a first preset condition has been met for a duration exceeding 20ms, wherein the first preset condition indicates that the AC voltage at the photovoltaic grid connection point is less than 0.9pu and the photovoltaic DC voltage is greater than 1.05 times the rated photovoltaic DC voltage;

[0106] The third determination module is used to determine that the photovoltaic flexible DC transmission system does not have the above-mentioned sending-end AC fault when the duration of the first preset condition being met does not exceed 20ms.

[0107] In one embodiment of the present application, the apparatus further includes a seventh processing unit, which, upon determining that the photovoltaic flexible DC transmission system has the AC fault at the sending end,

[0108] The seventh processing unit is used to determine that the above-mentioned sending-end AC fault of the above-mentioned photovoltaic flexible DC transmission system has been cleared when the per-unit value of the AC bus voltage of the above-mentioned sending-end flexible DC converter station is greater than or equal to 0.9pu, and the duration of the per-unit value of the above-mentioned AC bus voltage being greater than or equal to 0.9pu exceeds 20ms.

[0109] The control device for the photovoltaic flexible direct current transmission system includes a processor and memory. The determination unit, first processing unit, and second processing unit are stored in the memory as program units, and the processor executes the program units stored in the memory to implement the corresponding functions. The modules are all located in the same processor; alternatively, the modules may be located in different processors in any combination.

[0110] The processor contains a kernel, which retrieves the corresponding program unit from memory. One or more kernels can be configured. By adjusting kernel parameters, the system can address the issue of losing voltage support when the PV-HVDC transmission system's sending-end grid experiences a severe AC fault, preventing the system from operating safely and stably.

[0111] The memory may include non-permanent memory in a computer-readable medium, random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM, and the memory includes at least one memory chip.

[0112] An embodiment of the present invention provides a computer-readable storage medium, which includes a stored program. When the program is running, the device where the computer-readable storage medium is located is controlled to execute the control method of the photovoltaic flexible direct current transmission system.

[0113] An embodiment of the present invention provides a processor, which is used to run a program, wherein the control method of the photovoltaic flexible direct current transmission system is executed when the program is run.

[0114] An embodiment of the present invention provides a device, which includes a processor, a memory, and a program stored in the memory and executable on the processor. When the processor executes the program, at least the following steps are implemented: determining whether a sending-end AC fault occurs in a photovoltaic flexible direct current transmission system based at least on the magnitude of the AC voltage and the photovoltaic DC voltage at the photovoltaic grid-connected point; in the event that it is determined that the above-mentioned sending-end AC fault occurs in the above-mentioned photovoltaic flexible direct current transmission system, adjusting the d-axis voltage command value of the above-mentioned sending-end flexible direct current converter station to a reference voltage by using the sending-end flexible direct current converter station of the above-mentioned photovoltaic flexible direct current transmission system, and at the same time maintaining the output of the outer loop integral control unchanged by using the photovoltaic field station of the above-mentioned photovoltaic flexible direct current transmission system, and at the same time adjusting the above-mentioned photovoltaic field station to a reference voltage. The active current command value is adjusted to a preset active current, and the reactive current command value of the above-mentioned photovoltaic station is adjusted to a preset reactive current. The output of the above-mentioned outer loop integral control is the steady-state value of the variable of the above-mentioned photovoltaic station before the above-mentioned sending-end AC fault occurs in the above-mentioned photovoltaic flexible DC transmission system. When it is determined that the above-mentioned sending-end AC fault of the above-mentioned photovoltaic flexible DC transmission system has been cleared, the above-mentioned sending-end flexible DC converter station is used to increase the above-mentioned d-axis voltage command value at a first preset rate until the above-mentioned d-axis voltage command value reaches 1 p.u., and the above-mentioned photovoltaic station is used to increase the above-mentioned active current command value at a first preset rate until the above-mentioned active current command value returns to the size before the above-mentioned sending-end AC fault. The device in this article can be a server, PC, PAD, mobile phone, etc.

[0115] The present application also provides a computer program product, which, when executed on a data processing device, is suitable for executing a program initialized with at least the following method steps: determining whether a sending-end AC fault occurs in the photovoltaic flexible DC transmission system at least based on the magnitude of the AC voltage and the photovoltaic DC voltage at the photovoltaic grid-connected point; in the case of determining that the above-mentioned sending-end AC fault occurs in the above-mentioned photovoltaic flexible DC transmission system, using the sending-end flexible DC converter station of the above-mentioned photovoltaic flexible DC transmission system to adjust the d-axis voltage command value of the above-mentioned sending-end flexible DC converter station to a reference voltage, and at the same time using the above-mentioned photovoltaic field station of the photovoltaic flexible DC transmission system to keep the output of the outer loop integral control unchanged, and at the same time adjusting the active current command value of the above-mentioned photovoltaic field station to a reference voltage. The value is adjusted to a preset active current, and the reactive current command value of the above-mentioned photovoltaic station is adjusted to a preset reactive current. The output of the above-mentioned outer loop integral control is the steady-state value of the variable of the above-mentioned photovoltaic station before the above-mentioned sending-end AC fault occurs in the above-mentioned photovoltaic flexible DC transmission system; when it is determined that the above-mentioned sending-end AC fault of the above-mentioned photovoltaic flexible DC transmission system has been cleared, the above-mentioned sending-end flexible DC converter station is used to increase the above-mentioned d-axis voltage command value at a first preset rate until the above-mentioned d-axis voltage command value reaches 1p.u., and at the same time, the above-mentioned photovoltaic station is used to increase the above-mentioned active current command value at a first preset rate until the above-mentioned active current command value returns to the size before the above-mentioned sending-end AC fault.

[0116] The present application also provides a control system for a photovoltaic flexible direct current transmission system, the system comprising: one or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, and the one or more programs include methods for executing any of the above methods. By, in the case of determining that the photovoltaic flexible direct current transmission system has the above-mentioned AC fault at the sending end, the sending-end flexible direct current converter station of the photovoltaic flexible direct current transmission system is used to adjust the d-axis voltage command value of the sending-end flexible direct current converter station to a reference voltage, and at the same time, the photovoltaic station of the photovoltaic flexible direct current transmission system is used to keep the output of the outer loop integral control unchanged, and at the same time, the active current command value of the photovoltaic station is adjusted to a preset active current, and at the same time, the reactive current command value of the photovoltaic station is adjusted to a preset reactive current, so that the sending-end flexible direct current converter station and the photovoltaic station can still work normally when the photovoltaic flexible direct current transmission system has the above-mentioned AC fault at the sending end, and at the same time, when determining the above-mentioned sending-end flexible direct current transmission system When the AC fault at the sending end has been cleared, the above-mentioned sending-end flexible DC converter station is used to increase the above-mentioned d-axis voltage command value at a first preset rate until the above-mentioned d-axis voltage command value reaches 1 p.u., and at the same time, the above-mentioned photovoltaic station is used to increase the above-mentioned active current command value at a first preset rate until the above-mentioned active current command value is restored to the size before the above-mentioned sending-end AC fault, thereby achieving the purpose of restoring the system after determining that the above-mentioned sending-end AC fault of the above-mentioned photovoltaic flexible DC transmission system has been cleared, and further solving the problem that when the sending-end power grid of the photovoltaic flexible DC transmission system encounters a serious AC fault, the photovoltaic flexible DC transmission system will lose voltage support, thereby making the photovoltaic flexible DC transmission system unable to operate safely and stably.

[0117] Obviously, those skilled in the art will appreciate that the various modules or steps of the present invention described above can be implemented using a general-purpose computing device, can be centralized on a single computing device, or can be distributed across a network of multiple computing devices. They can be implemented using program code executable by the computing device, and thus, can be stored in a storage device and executed by the computing device. In some cases, the steps shown or described herein can be performed in a different order than that shown, or can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. Thus, the present invention is not limited to any particular combination of hardware and software.

[0118] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0119] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the steps in the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0120] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0121] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0122] In a typical configuration, a computing device includes one or more processors (CPUs), input / output interfaces, network interfaces, and memory.

[0123] The memory may include non-permanent memory in a computer-readable medium, random access memory (RAM) and / or non-volatile memory in the form of read-only memory (ROM) or flash RAM. The memory is an example of a computer-readable medium.

[0124] Computer-readable media includes permanent and non-permanent, removable and non-removable media that can be implemented by any method or technology to store information. The information can be computer-readable instructions, data structures, program modules, or other data. Examples of computer storage media include, but are not limited to, phase change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technology, compact disc read-only memory (CD-ROM), digital versatile disc (DVD) or other optical storage, magnetic cassettes, magnetic disk storage or other magnetic storage devices, or any other non-transmission media that can be used to store information that can be accessed by a computing device. As defined herein, computer-readable media does not include transitory media such as modulated data signals and carrier waves.

[0125] It should also be noted that the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, commodity, or apparatus that includes a series of elements includes not only those elements but also other elements not explicitly listed, or includes elements inherent to such process, method, commodity, or apparatus. In the absence of further limitations, an element defined by the phrase "comprises a ..." does not exclude the presence of other identical elements in the process, method, commodity, or apparatus that includes the element.

[0126] From the above description, it can be seen that the above embodiments of the present application achieve the following technical effects:

[0127] 1) The control method of the photovoltaic flexible DC transmission system of the present application, when it is determined that the above-mentioned sending-end AC fault occurs in the above-mentioned photovoltaic flexible DC transmission system, the sending-end flexible DC converter station of the above-mentioned photovoltaic flexible DC transmission system is used to adjust the d-axis voltage command value of the above-mentioned sending-end flexible DC converter station to a reference voltage, and at the same time, the photovoltaic station of the above-mentioned photovoltaic flexible DC transmission system is used to keep the output of the outer loop integral control unchanged, and at the same time adjust the active current command value of the above-mentioned photovoltaic station to a preset active current, and at the same time adjust the reactive current command value of the above-mentioned photovoltaic station to a preset reactive current, so that the sending-end flexible DC converter station and the photovoltaic station can still work normally when the above-mentioned sending-end AC fault occurs in the photovoltaic flexible DC transmission system, and at the same time, when it is determined that the above-mentioned photovoltaic When the AC fault at the sending end of the flexible DC transmission system has been cleared, the sending-end flexible DC converter station is used to increase the d-axis voltage command value at a first preset rate until the d-axis voltage command value reaches 1 p.u., and at the same time, the photovoltaic station is used to increase the active current command value at a first preset rate until the active current command value is restored to the size before the sending-end AC fault, thereby achieving the purpose of restoring the system after determining that the AC fault at the sending end of the photovoltaic flexible DC transmission system has been cleared, and further solving the problem that when the sending-end power grid of the photovoltaic flexible DC transmission system encounters a serious AC fault, the photovoltaic flexible DC transmission system will lose voltage support, thereby making the photovoltaic flexible DC transmission system unable to operate safely and stably.

[0128] 2) The control device of the photovoltaic flexible DC transmission system of the present application, when it is determined that the above-mentioned sending-end AC fault occurs in the above-mentioned photovoltaic flexible DC transmission system, uses the sending-end flexible DC converter station of the above-mentioned photovoltaic flexible DC transmission system to adjust the d-axis voltage command value of the above-mentioned sending-end flexible DC converter station to a reference voltage, and at the same time uses the photovoltaic station of the above-mentioned photovoltaic flexible DC transmission system to keep the output of the outer loop integral control unchanged, and at the same time adjusts the active current command value of the above-mentioned photovoltaic station to a preset active current, and at the same time adjusts the reactive current command value of the above-mentioned photovoltaic station to a preset reactive current, so that the sending-end flexible DC converter station and the photovoltaic station can still work normally when the above-mentioned sending-end AC fault occurs in the photovoltaic flexible DC transmission system, and at the same time, when it is determined that the above-mentioned photovoltaic When the AC fault at the sending end of the flexible DC transmission system has been cleared, the sending-end flexible DC converter station is used to increase the d-axis voltage command value at a first preset rate until the d-axis voltage command value reaches 1 p.u., and at the same time, the photovoltaic station is used to increase the active current command value at a first preset rate until the active current command value is restored to the size before the sending-end AC fault, thereby achieving the purpose of restoring the system after determining that the AC fault at the sending end of the photovoltaic flexible DC transmission system has been cleared, and further solving the problem that when the sending-end power grid of the photovoltaic flexible DC transmission system encounters a serious AC fault, the photovoltaic flexible DC transmission system will lose voltage support, thereby making the photovoltaic flexible DC transmission system unable to operate safely and stably.

[0129] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A control method for a photovoltaic flexible direct current transmission system, characterized in that: include: At least based on the AC voltage and DC voltage at the photovoltaic grid connection point, determine whether there is an AC fault at the sending end of the photovoltaic flexible DC transmission system; In the case where it is determined that the sending-end AC fault occurs in the photovoltaic via flexible DC transmission system, the sending-end flexible DC converter station of the photovoltaic via flexible DC transmission system is used to adjust the d-axis voltage command value of the sending-end flexible DC converter station to a reference voltage, and at the same time, the photovoltaic field station of the photovoltaic via flexible DC transmission system is used to keep the output of the outer loop integral control unchanged, and at the same time, the active current command value of the photovoltaic field station is adjusted to a preset active current, and at the same time, the reactive current command value of the photovoltaic field station is adjusted to a preset reactive current, and the output of the outer loop integral control is the steady-state value of the variable of the photovoltaic field station before the sending-end AC fault occurs in the photovoltaic via flexible DC transmission system; When it is determined that the sending-end AC fault of the photovoltaic flexible DC transmission system has been cleared, the sending-end flexible DC converter station is used to increase the d-axis voltage command value at a first preset rate until the d-axis voltage command value reaches 1 p.u., and at the same time, the photovoltaic field station is used to increase the active current command value at a first preset rate until the active current command value returns to the size before the sending-end AC fault.

2. The method according to claim 1, characterized in that Before using the sending-end flexible DC converter station of the photovoltaic flexible DC transmission system to adjust the d-axis voltage command value of the sending-end flexible DC converter station to a reference voltage, the method further includes: The reference voltage is determined to be a preset voltage corresponding to the minimum per-unit value of the effective value of the AC bus voltage of the sending-end flexible DC converter station.

3. The method according to claim 2, characterized in that Determining the reference voltage as a preset voltage corresponding to the minimum per-unit value of the effective value of the AC bus voltage of the sending-end flexible DC converter station includes: according to determining the reference voltage, Among them, U a is the reference voltage, U PMS_min It is the minimum per-unit value of the effective value of the AC bus voltage of the sending-end flexible DC converter station.

4. The method according to claim 1, wherein Before adjusting the reactive current command value of the photovoltaic station to a preset reactive current, the method further includes: according to determining the preset reactive current, in, is the preset reactive current, I N_PV is the rated current of the photovoltaic station, U PCC is the AC voltage of the photovoltaic grid-connected point, and a is a preset constant.

5. The method according to claim 1, wherein Before adjusting the active current command value of the photovoltaic station to a preset active current, the method further includes: according to determining the preset active current, in, is the preset active current, I vmax is the maximum current of the photovoltaic station, is the preset reactive current, The current command value for the photovoltaic additional control of the photovoltaic station during the sending-end AC fault.

6. The method according to claim 5, characterized in that The method further comprises: according to determining a current command value for the photovoltaic additional control of the photovoltaic station during the AC fault at the sending end, in, is the current instruction value of the photovoltaic additional control of the photovoltaic station during the AC fault at the sending end, is the current instruction value of the photovoltaic additional control of the photovoltaic station before the AC fault at the sending end, U PCC is the AC voltage of the photovoltaic grid-connected point, V DC is the photovoltaic DC voltage, V DCN is the rated value of the photovoltaic DC voltage.

7. The method according to claim 1, characterized in that At least based on the AC voltage and DC voltage at the PV grid connection point, determine whether there is an AC fault at the sending end of the PV flexible DC transmission system, including: If the duration of the first preset condition being met exceeds 20ms, it is determined that the sending-end AC fault occurs in the photovoltaic flexible DC transmission system, where the first preset condition indicates that the AC voltage at the photovoltaic grid connection point is less than 0.9pu and the photovoltaic DC voltage is greater than 1.05 times the rated photovoltaic DC voltage; When the duration of satisfying the first preset condition does not exceed 20 ms, it is determined that the photovoltaic flexible DC transmission system does not have the sending-end AC fault.

8. The method according to any one of claims 1 to 7, characterized in that When it is determined that the photovoltaic flexible direct current transmission system has the sending-end AC fault, the method further includes: When the per-unit value of the AC bus voltage of the sending-end flexible DC converter station is greater than or equal to 0.9pu, and the duration of the per-unit value of the AC bus voltage being greater than or equal to 0.9pu exceeds 20ms, it is determined that the sending-end AC fault of the photovoltaic flexible DC transmission system has been cleared.

9. A control device for a photovoltaic flexible direct current transmission system, characterized in that: include: A determination unit, configured to determine whether a sending-end AC fault occurs in the photovoltaic flexible DC transmission system based on at least the magnitude of the AC voltage and the photovoltaic DC voltage at the photovoltaic grid-connected point; a first processing unit, configured to, upon determining that the sending-end AC fault occurs in the photovoltaic flexible direct current transmission system, use the sending-end flexible direct current converter station of the photovoltaic flexible direct current transmission system to adjust the d-axis voltage command value of the sending-end flexible direct current converter station to a reference voltage, and simultaneously use the photovoltaic field station of the photovoltaic flexible direct current transmission system to maintain the output of the outer loop integral control unchanged, and simultaneously adjust the active current command value of the photovoltaic field station to a preset active current, and simultaneously adjust the reactive current command value of the photovoltaic field station to a preset reactive current, wherein the output of the outer loop integral control is the steady-state value of the variable of the photovoltaic field station before the sending-end AC fault occurs in the photovoltaic flexible direct current transmission system; The second processing unit is used to, when it is determined that the sending-end AC fault of the photovoltaic flexible DC transmission system has been cleared, use the sending-end flexible DC converter station to increase the d-axis voltage command value at a first preset rate until the d-axis voltage command value reaches 1 p.u., and at the same time use the photovoltaic field station to increase the active current command value at a first preset rate until the active current command value recovers to the size before the sending-end AC fault.

10. A control system for a photovoltaic flexible direct current transmission system, characterized in that: include: One or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, and the one or more programs include instructions for executing the method of any one of claims 1 to 8.

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