Converter control mode switching method

Abstract

The embodiment of the invention discloses a converter control mode switching method, which comprises the following steps of: performing fault analysis according to a first actual voltage deviation of an alternating current observation node, a second actual voltage deviation of a direct current observation node and a voltage deviation threshold value of the alternating current observation node and the direct current observation node at the current moment; determining a fault occurrence condition and a fault type of the grid-connected system; the actual short-circuit ratio is compared with a short-circuit ratio threshold value of the grid-connected system, and the power grid strength of the grid-connected system at the current moment is determined; and according to the fault occurrence condition, the fault type, the power grid strength and the initial converter control mode of the grid-connected system at the current moment, determining a switching strategy of the converter control mode at the current moment. According to the method, the power grid strength and the fault type are considered at the same time, the control mode of the converter of the grid-connected system is selected, and the converter control method is effectively switched under the condition of different power grid strength and when different types of faults occur, so that the safe operation of the grid-connected system is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of converters, and in particular to a method for switching converter control modes. Background Art

[0002] Large-scale renewable energy access to the grid often uses converters as access ports. Voltage source converters have two main control methods: grid-following and grid-forming.

[0003] The grid-following converter uses a phase-locked loop to achieve synchronization with the power grid. However, when the grid-connected system is in a weak grid condition, the grid-following converter will produce negative damping and other problems due to the existence of the phase-locked loop, thereby affecting the stable operation of the converter. The grid-building converter uses a power synchronization loop to complete synchronization with the grid. Although it overcomes the disadvantage of the grid-following converter's performance deterioration in a weak grid to a certain extent, when the grid-connected system is in a strong grid condition, the performance of the grid-building converter is not as good as that of the grid-following converter.

[0004] Furthermore, the performance of grid-following and grid-forming converters varies under different fault conditions. Under severe AC faults, a grid-following converter can lose precise synchronization with the grid due to a significant voltage drop at the common coupling point. Under DC faults, the DC-side voltage of a grid-forming converter can experience significant fluctuations.

[0005] Therefore, in order to ensure the safe operation of the grid-connected system, a suitable converter control method needs to be selected, but there is currently no effective method for selecting the converter control method. Summary of the Invention

[0006] Based on this, it is necessary to propose a method for switching the converter control mode to address the above problems, so as to achieve effective switching of the converter control method under different grid strength conditions and when different types of faults occur, thereby ensuring the safe operation of the grid-connected system.

[0007] To achieve the above objectives, the present application provides a method for switching a converter control mode, the method comprising:

[0008] Determining an AC observation node on the AC side and a DC observation node on the DC side of the converter based on voltage deviations of all nodes on the AC side and the DC side of the grid-connected system converter under different operating conditions;

[0009] Obtaining a first actual voltage deviation of the AC observation node and a second actual voltage deviation of the DC observation node at a current moment, an actual short-circuit ratio of the grid-connected system at a current moment, and an initial converter control mode of the grid-connected system at a current moment;

[0010] Performing a fault analysis based on the first actual voltage deviation, the second actual voltage deviation, and voltage deviation thresholds of the AC observation node and the DC observation node to determine a fault condition and a fault type of the grid-connected system;

[0011] Determining the grid strength of the grid-connected system at a current moment by comparing the actual short-circuit ratio with a short-circuit ratio threshold of the grid-connected system;

[0012] An analysis is performed based on the fault occurrence, fault type, grid strength, the initial converter control mode and the preset function switching table of the grid-connected system at the current moment to determine the switching strategy of the converter control mode at the current moment, wherein the function switching table contains the corresponding relationships between the switching strategy of the converter control mode and the fault occurrence, fault type, grid strength and initial converter control mode.

[0013] Furthermore, the performing of fault analysis based on the first actual voltage deviation, the second actual voltage deviation, and the voltage deviation thresholds of the AC observation node and the DC observation node to determine the occurrence and type of the fault of the grid-connected system specifically includes:

[0014] Obtaining maximum voltage deviations of the AC observation node and the DC observation node within the fault occurrence time under each working condition, and generating a first maximum voltage deviation set of the AC observation node and a second maximum voltage deviation set of the DC observation node;

[0015] Using the minimum value in the first maximum voltage deviation set as the AC voltage deviation threshold of the AC observation node, and using the minimum value in the second maximum voltage deviation set as the DC voltage deviation threshold of the DC observation node;

[0016] A maximum AC voltage deviation threshold and a minimum AC voltage deviation threshold are calculated based on the AC voltage deviation threshold and a preset AC detection margin, and a maximum DC voltage deviation threshold and a minimum DC voltage deviation threshold are calculated based on the DC voltage deviation threshold and a preset DC detection margin;

[0017] Based on the hysteresis comparison method, fault analysis is performed according to the first actual voltage deviation, the second actual voltage deviation, the maximum AC voltage deviation threshold, the minimum AC voltage deviation threshold, the maximum DC voltage deviation threshold and the minimum DC voltage deviation threshold to determine the fault occurrence and fault type of the grid-connected system.

[0018] Furthermore, the hysteresis comparison method is based on the first actual voltage deviation, the second actual voltage deviation, the maximum AC voltage deviation threshold, the minimum AC voltage deviation threshold, the maximum DC voltage deviation threshold, and the minimum DC voltage deviation threshold. Fault analysis is performed to determine the occurrence and type of a fault in the grid-connected system, specifically including:

[0019] Based on a hysteresis comparison method, performing an AC fault analysis according to the first actual voltage deviation, the maximum AC voltage deviation threshold, and the minimum AC voltage deviation threshold to determine whether an AC fault occurs in the grid-connected system;

[0020] Based on a hysteresis comparison method, a DC fault analysis is performed according to the second actual voltage deviation, the maximum DC voltage deviation threshold, and the minimum DC voltage deviation threshold to determine whether a DC fault occurs in the grid-connected system;

[0021] If an AC fault or a DC fault occurs in the grid-connected system, determining that the fault occurrence condition of the grid-connected system is a fault;

[0022] If no AC fault or DC fault occurs in the grid-connected system, it is determined that the fault occurrence condition of the grid-connected system is no fault.

[0023] Furthermore, the hysteresis comparison method is based on which AC fault analysis is performed according to the first actual voltage deviation, the maximum AC voltage deviation threshold, and the minimum AC voltage deviation threshold to determine whether an AC fault occurs in the grid-connected system, specifically including:

[0024] When the first actual voltage deviation is greater than the maximum AC voltage deviation threshold, it is determined that an AC fault occurs in the grid-connected system;

[0025] When the first actual voltage deviation is less than the minimum AC voltage deviation threshold, it is determined that no AC fault occurs in the grid-connected system;

[0026] When the first actual voltage deviation is not greater than the maximum AC voltage deviation threshold and not less than the minimum AC voltage deviation threshold, obtaining a first historical voltage deviation of the AC observation node before the current moment;

[0027] If the first historical voltage deviation is greater than the maximum AC voltage deviation threshold, determining that an AC fault occurs in the grid-connected system;

[0028] If the first historical voltage deviation is less than the minimum AC voltage deviation threshold, it is determined that no AC fault occurs in the grid-connected system.

[0029] Furthermore, the hysteresis comparison method is based on which a DC fault analysis is performed according to the second actual voltage deviation, the maximum DC voltage deviation threshold, and the minimum DC voltage deviation threshold to determine whether a DC fault occurs in the grid-connected system, specifically including:

[0030] When the second actual voltage deviation is greater than the maximum DC voltage deviation threshold, it is determined that a DC fault occurs in the grid-connected system;

[0031] When the second actual voltage deviation is less than the minimum DC voltage deviation threshold, it is determined that no DC fault occurs in the grid-connected system;

[0032] When the second actual voltage deviation is not greater than the maximum DC voltage deviation threshold and not less than the minimum DC voltage deviation threshold, obtaining a second historical voltage deviation of the DC observation node before the current moment;

[0033] If the second historical voltage deviation is greater than the maximum DC voltage deviation threshold, determining that a DC fault occurs in the grid-connected system;

[0034] If the second historical voltage deviation is less than the minimum DC voltage deviation threshold, it is determined that no DC fault occurs in the grid-connected system.

[0035] Furthermore, the analysis is performed based on the current fault situation, fault type, grid strength, initial converter control mode, and a preset function switching table of the grid-connected system to determine the switching strategy of the converter control mode at the current moment, specifically including:

[0036] When the initial converter control mode of the grid-connected system at the current moment is grid-type control;

[0037] If the fault condition is that no fault has occurred and the grid strength is a strong grid, then the switching strategy of the converter control mode at the current moment is to switch from grid-forming control to grid-following control;

[0038] If the fault condition is that no fault has occurred and the grid strength is a weak grid, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode;

[0039] If the fault type is a DC fault and the grid strength is a strong grid, the switching strategy of the converter control mode at the current moment is to switch from grid-forming control to grid-following control;

[0040] If the fault type is a DC fault and the grid strength is a weak grid, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode;

[0041] If the fault type is an AC fault, the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode.

[0042] Furthermore, the analysis is performed based on the current fault situation, fault type, grid strength, initial converter control mode, and a preset function switching table of the grid-connected system to determine the switching strategy of the converter control mode at the current moment, specifically including:

[0043] When the initial converter control mode of the grid-connected system at the current moment is grid-following control;

[0044] If the fault condition is that no fault has occurred and the grid strength is a weak grid, then the switching strategy of the converter control mode at the current moment is to switch from grid-following control to grid-forming control;

[0045] If the fault condition is that no fault has occurred and the grid strength is a strong grid, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode;

[0046] If the fault type is a DC fault and the grid strength is a weak grid, the switching strategy of the converter control mode at the current moment is to switch from grid-following control to grid-forming control;

[0047] If the fault type is a DC fault and the grid strength is a strong grid, the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode;

[0048] If the fault type is an AC fault, the switching strategy of the converter control mode at the current moment is to switch from grid-following control to grid-forming control.

[0049] Furthermore, the converter control mode of the grid-connected system includes grid-forming control and grid-following control, and the method further includes:

[0050] When the switching strategy of the converter control mode is to switch from the initial converter control mode to another converter control mode, obtaining a first output phase angle of a grid-connected power synchronization loop and a second output phase angle of a grid-following phase-locked loop synchronization link of the grid-connected system;

[0051] Calculating a difference between the first output phase angle and the second output phase angle to obtain a phase angle difference;

[0052] Inputting the phase angle difference into a preset first PI controller to obtain the instantaneous power output by the first PI controller;

[0053] Within a preset time before or after executing the converter control mode switching operation, the preset input power reference value in the grid-type synchronous control loop is corrected according to the instantaneous power to obtain a corrected input power;

[0054] The second PI controller in the grid-type synchronous control loop is used to output a corrected first output phase angle according to the corrected input power, so as to smoothly switch the converter control mode based on the corrected first output phase angle within the preset time.

[0055] Furthermore, the determining of the AC observation node on the AC side and the DC observation node on the DC side of the converter based on the voltage deviation of all nodes on the AC side and the DC side of the grid-connected system converter under different working conditions specifically includes:

[0056] Adding different types of faults at different locations on the AC side of the grid-connected system converter to obtain first test voltage deviations of various nodes on the AC side of the converter under different working conditions;

[0057] Using the node corresponding to the maximum value of all the first test voltage deviations as an AC observation node;

[0058] Adding different types of faults at different positions on the DC side of the grid-connected system converter to obtain second test voltage deviations of various nodes on the DC side of the converter under different working conditions;

[0059] The node corresponding to the maximum value of all the second test voltage deviations is used as a DC observation node.

[0060] Furthermore, obtaining the actual short-circuit ratio of the grid-connected system at the current moment specifically includes:

[0061] Obtaining the real-time impedance of the grid-connected system at the current moment;

[0062] The actual short-circuit ratio of the grid-connected system is obtained according to the ratio of the real-time impedance to a preset impedance reference value.

[0063] The embodiments of the present invention have the following beneficial effects:

[0064] An embodiment of the present invention proposes a method for switching a converter control mode, the method comprising: determining an AC observation node on the AC side and a DC observation node on the DC side of the converter based on the magnitude of the voltage deviation of all nodes on the AC side and the DC side of the converter of a grid-connected system under different operating conditions; obtaining a first actual voltage deviation of the AC observation node and a second actual voltage deviation of the DC observation node at the current moment, the actual short-circuit ratio of the grid-connected system at the current moment, and an initial converter control mode of the grid-connected system at the current moment; performing a fault analysis based on the first actual voltage deviation, the second actual voltage deviation, and the voltage deviation thresholds of the AC observation node and the DC observation node to determine a fault occurrence and a fault type of the grid-connected system; determining a grid strength of the grid-connected system at the current moment by comparing the actual short-circuit ratio with the short-circuit ratio threshold of the grid-connected system; and determining a switching strategy for the converter control mode at the current moment by analyzing the fault occurrence, fault type, grid strength, initial converter control mode, and a preset function switching table of the grid-connected system, wherein the function switching table contains corresponding relationships between the switching strategies of the converter control modes and the fault occurrence, fault type, grid strength, and initial converter control mode. The present invention selects the control mode of the grid-connected system converter by taking into account the strength of the grid and the type of fault. It can effectively switch the converter control method under different grid strength conditions and when different types of faults occur, so as to meet the requirements for different converter control types in different time and space scenarios and ensure the safe operation of the grid-connected system. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0066] in:

[0067] Figure 1 Schematic diagram of a flow chart of a method for switching a converter control mode in an embodiment of the present invention;

[0068] Figure 2 A schematic diagram of a hysteresis comparison method for determining an AC fault occurrence in an embodiment of the present invention;

[0069] Figure 3 A schematic diagram of a hysteresis loop comparison method for determining the occurrence of a DC fault according to an embodiment of the present invention;

[0070] Figure 4 Schematic diagram of control switching strategies under different short-circuit ratios in an embodiment of the present invention;

[0071] Figure 5 This is an overall flow chart of the hysteresis switching method of the converter control method according to an embodiment of the present invention;

[0072] Figure 6 1 is a diagram of a smooth switching structure in an embodiment of the present invention;

[0073] Figure 7 FIG. 1 is a structural diagram of instantaneous power calculation in an embodiment of the present invention;

[0074] Figure 8 Schematic diagram of the structure of a switching device for a converter control mode in an embodiment of the present invention;

[0075] Figure 9 2 is a diagram showing the internal structure of a computer device in an embodiment of the present invention. DETAILED DESCRIPTION

[0076] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0077] Since the performance of different converter control modes varies greatly under different grid strengths and fault types, in order to achieve optimal performance of the converter in different working conditions and to ensure the safe operation of the grid-connected system, the present invention proposes a method for switching the converter control mode. Figure 1 , Figure 1 FIG. 1 is a flow chart of a method for switching a converter control mode in an embodiment of the present invention, the method comprising:

[0078] Step 100: Determine an AC observation node on the AC side and a DC observation node on the DC side of the grid-connected system converter based on voltage deviations of all nodes on the AC side and the DC side under different operating conditions.

[0079] In this embodiment, appropriate observation nodes need to be selected on both sides of the converter of the grid-connected system to record the operating status of the grid-connected system under different converter control modes.

[0080] Specifically, the voltage deviations of different nodes on the AC side of the converter under different operating conditions are obtained, and an appropriate AC observation node is selected based on the voltage deviations of all nodes on the AC side. Alternatively, the node corresponding to the largest voltage deviation among all the voltage deviations of the AC side nodes can be used as the AC observation node; alternatively, the node corresponding to any one of the top N largest voltage deviations among all the voltage deviations of the AC side nodes can be used as the AC observation node, without limitation.

[0081] Obtain the voltage deviations of different nodes on the DC side of the converter under different operating conditions, and determine the appropriate DC observation node based on the voltage deviations of all nodes on the DC side. Optionally, the node corresponding to the largest voltage deviation among all DC side node voltage deviations can be used as the DC observation node; alternatively, the node corresponding to any of the top N largest voltage deviations among all DC side node voltage deviations can be used as the DC observation node, without limitation.

[0082] Step 200: Obtain a first actual voltage deviation of the AC observation node and a second actual voltage deviation of the DC observation node at the current moment, an actual short-circuit ratio of the grid-connected system at the current moment, and an initial converter control mode of the grid-connected system at the current moment.

[0083] In this embodiment, the current operating state of the grid-connected system is monitored to adjust the inverter control mode based on the current operating state of the grid-connected system. The operating state of the grid-connected system may include data such as the voltage deviation between the AC and DC sides, the short-circuit ratio, and the current inverter control mode.

[0084] Step 300: Perform fault analysis based on the first actual voltage deviation, the second actual voltage deviation, and the voltage deviation thresholds of the AC observation node and the DC observation node to determine the occurrence and type of the fault in the grid-connected system.

[0085] In this embodiment, both the AC observation node and the DC observation node have voltage deviation thresholds, which are used to determine whether a fault exists on the AC or DC side of the converter. The voltage deviation thresholds can be the maximum or minimum voltage deviation allowed during normal operation of the AC or DC side.

[0086] Specifically, a first actual voltage deviation and a second actual voltage deviation of the AC observation node and the DC observation node are obtained in real time, so as to judge the current fault condition of the grid-connected system based on the first actual voltage deviation and the second actual voltage deviation.

[0087] The first actual voltage deviation and the second actual voltage deviation are compared with the voltage deviation threshold, respectively. If the first actual voltage deviation is greater than the voltage deviation threshold, an AC fault is considered to have occurred; if the second actual voltage deviation is greater than the voltage deviation threshold, a DC fault is considered to have occurred. When the fault type is an AC fault or a DC fault, the fault condition of the grid-connected system is considered to have occurred. When the fault type is "none" (neither an AC fault nor a DC fault has occurred), the fault condition of the grid-connected system is considered to have occurred. In this embodiment, it is assumed that the grid-connected system is either fault-free or can only experience one of the AC and DC faults.

[0088] Step 400 : Determine the grid strength of the grid-connected system at the current moment by comparing the actual short-circuit ratio with the short-circuit ratio threshold of the grid-connected system.

[0089] In this embodiment, the grid-connected system has a short-circuit ratio threshold, which is used to judge the current grid strength of the grid-connected system. The short-circuit ratio threshold can be the maximum short-circuit ratio of the grid-connected system under a weak grid, or the minimum short-circuit ratio of the grid-connected system under a strong grid.

[0090] Specifically, the actual short-circuit ratio of the grid-connected system is obtained in real time, so as to determine the grid strength of the grid-connected system at the current moment based on the actual short-circuit ratio.

[0091] The actual short circuit ratio is compared with the short circuit ratio threshold. If the actual short circuit ratio is less than the short circuit ratio threshold, the grid-connected system is currently a weak grid; if the actual short circuit ratio is not less than the short circuit ratio threshold, the grid-connected system is currently a strong grid.

[0092] Step 500: Analyze the current fault situation, fault type, grid strength, initial converter control mode and preset function switching table of the grid-connected system to determine the switching strategy of the converter control mode at the current moment, wherein the function switching table includes the corresponding relationships between the switching strategy of the converter control mode and the fault situation, fault type, grid strength and initial converter control mode.

[0093] Because grid-forming control has strong AC / DC coupling capabilities, a DC fault can cause significant fluctuations in the DC grid voltage, which is detrimental to the stability of the DC grid. Therefore, when a DC line fault occurs, it is necessary to switch to grid-following control. Furthermore, different types of converters have varying stability at different grid strengths, with grid-following control being more susceptible to instability at weak grid strengths, while grid-forming control is more susceptible to instability at strong grid strengths. Therefore, based on the aforementioned switching principles, this embodiment analyzes the grid-connected system's current fault status, fault type, grid strength, and initial converter control mode to determine the current converter control mode switching strategy.

[0094] Specifically, a function switching table is set in advance, which contains switching strategies corresponding to different fault conditions, different fault types, different grid strengths and different initial converter control methods. According to the acquired fault conditions, fault types, grid strengths and initial converter control methods of the grid-connected system at the current moment, a search is performed in the function switching table to obtain the switching strategy of the grid-connected system at the current moment.

[0095] The present invention selects appropriate AC and DC observation nodes to more accurately observe the operating status of the grid-connected system. By considering the grid strength and fault type during the grid-connected system's operating state, and also considering the grid-connected system's converter control method, the present invention effectively switches the converter control method under varying grid strength conditions and when different fault types occur. This allows for different converter control types to be used in different spatial and temporal scenarios, ensuring the safe operation of the grid-connected system.

[0096] In one embodiment of the present invention, step 100 of determining an AC observation node on the AC side and a DC observation node on the DC side of the converter based on voltage deviations of all nodes on the AC side and the DC side of the grid-connected system converter under different operating conditions specifically includes:

[0097] Step 110: Add different types of faults at different locations on the AC side of the grid-connected system converter to obtain first test voltage deviations of various nodes on the AC side of the converter under different working conditions.

[0098] Step 120: Nodes corresponding to the maximum values ​​of all first test voltage deviations are used as AC observation nodes.

[0099] Step 110 to Step 120 describe how to determine the AC observation node.

[0100] In this embodiment, different types of AC faults are first added at different positions on the AC side of the grid-connected system converter to obtain the voltage amplitudes of different nodes during the fault occurrence time under different working conditions and the node voltages when the grid-connected system enters a steady state under different working conditions.

[0101] Secondly, a first test voltage deviation is calculated according to the difference between the voltage amplitude of each node during the fault occurrence time under each working condition and the node voltage when the grid-connected system enters a steady state.

[0102] Specifically, the voltage deviation is the difference between the voltage amplitude of the node at any moment during the fault occurrence time under a certain working condition and the voltage amplitude of the node after the grid-connected system enters a steady state under the working condition. The voltage deviation can be calculated using the following formula:

[0103]

[0104] Where, is the voltage deviation of node i under the kth working condition, is the voltage amplitude of node i at time t under the kth working condition, where t is any time within the fault occurrence time, k is any one of all working conditions, is the voltage amplitude of node i after the grid-connected system enters steady state under the kth operating condition. The voltage deviation of all nodes under all operating conditions can be obtained using the above formula.

[0105] Finally, the AC observation node is determined according to the magnitudes of the first test voltage deviations of all nodes on the AC side. In this embodiment, the node corresponding to the maximum value of all first test voltage deviations is used as the AC observation node.

[0106] Step 130, adding different types of faults at different locations on the DC side of the grid-connected system converter to obtain second test voltage deviations of each node on the DC side of the converter under different working conditions;

[0107] Step 140: Nodes corresponding to the maximum values ​​of all second test voltage deviations are used as DC observation nodes.

[0108] Step 130 to Step 140 describe how to determine the DC observation node.

[0109] In this embodiment, different types of ground faults are first added at different positions on the DC side of the grid-connected system converter to obtain the voltage amplitudes of different nodes during the fault occurrence time under different working conditions and the node voltages when the grid-connected system enters a steady state under different working conditions.

[0110] Next, a second test voltage deviation is calculated based on the difference between the voltage amplitude at each node during the fault occurrence time and the node voltage when the grid-connected system enters steady state under each operating condition. The voltage deviation can also be calculated using the voltage deviation calculation formula mentioned above and will not be repeated here.

[0111] Finally, the DC observation node is determined according to the magnitudes of the second test voltage deviations of all nodes on the DC side. In this embodiment, the node corresponding to the maximum value of all the second test voltage deviations is used as the DC observation node.

[0112] This embodiment of the present invention proactively injects multiple fault types on both the AC and DC sides and calculates the voltage deviation at each node during the fault period, enabling a quantitative assessment of the sensitivity of different nodes to faults. Selecting the node with the largest voltage deviation as the observation node allows for more accurate capture of subtle changes in system operating status, avoiding potential blind spots associated with traditional monitoring methods.

[0113] In one embodiment of the present invention, step 200 of obtaining a first actual voltage deviation of an AC observation node and a second actual voltage deviation of a DC observation node at a current moment, an actual short-circuit ratio of the grid-connected system at a current moment, and an initial converter control mode of the grid-connected system at a current moment specifically includes:

[0114] Step 210: Calculate the first actual voltage deviation of the AC observation node and the second actual voltage deviation of the DC observation node at the current moment according to the voltage deviation calculation formula.

[0115] Step 220: Obtain the real-time impedance of the grid-connected system at the current moment; and obtain the actual short-circuit ratio of the grid-connected system based on the ratio of the real-time impedance to the preset impedance reference value.

[0116] In the embodiment of the present invention, first, the real-time impedance of the grid-connected system is calculated based on the grid resistance, grid inductance, and system frequency of the grid-connected system, which can be specifically calculated using the following formula:

[0117] Z s =|R g +j2πfL g |

[0118] Where Z s is the real-time impedance, R g is the grid resistance, L g is the grid inductance, f is the system frequency, and || represents the size of the module.

[0119] Secondly, the impedance per unit value of the grid-connected system is calculated based on the real-time impedance and the preset impedance reference value, which can be calculated using the following formula:

[0120]

[0121] Where Z spu is the per-unit impedance value, Z s is the real-time impedance, Z base is the impedance reference value.

[0122] In one embodiment, the per-unit impedance value is calculated according to the following formula:

[0123]

[0124] Where V pcc is the rated voltage at the point of common coupling (PCC), S N is the rated DC power of the grid-connected system.

[0125] Finally, the actual short-circuit ratio is calculated based on the impedance mark value, which can be calculated using the following formula:

[0126]

[0127] In one embodiment of the present invention, step 300, performing fault analysis based on the first actual voltage deviation, the second actual voltage deviation, and the voltage deviation thresholds of the AC observation node and the DC observation node to determine the occurrence and type of the fault in the grid-connected system, specifically includes:

[0128] Step 310: Obtain the maximum voltage deviations of the AC observation nodes and the DC observation nodes within the fault occurrence time under each working condition, and generate a first maximum voltage deviation set of the AC observation nodes and a second maximum voltage deviation set of the DC observation nodes.

[0129] After determining the AC observation node and the DC observation node in step 100, the maximum voltage deviation of the AC observation node within the fault occurrence time under each working condition is obtained, and a first maximum voltage deviation set of the AC observation node is generated. in, is the voltage deviation of node i in the AC side under the kth working condition, 1≤k≤K; the maximum voltage deviation of the DC observation node within the fault occurrence time under each working condition is also obtained, and the second maximum voltage deviation set of the DC observation node is generated in, is the voltage deviation of node i in the DC side under the kth working condition.

[0130] Step 320: Use the minimum value in the first maximum voltage deviation set as the AC voltage deviation threshold of the AC observation node, and use the minimum value in the second maximum voltage deviation set as the DC voltage deviation threshold of the DC observation node.

[0131] Specifically, the AC voltage deviation threshold is selected from the first maximum voltage deviation set, and the DC voltage deviation threshold is selected from the second maximum voltage deviation set. In the embodiment of the present invention, the minimum value in the maximum voltage deviation set is selected as the voltage deviation threshold.

[0132] This embodiment selects the minimum value in the maximum voltage deviation set as the threshold value. When the grid-connected system is in the mildest fault condition, that is, the maximum voltage deviation of the observation node is relatively small under this condition, it can also be captured by the threshold value, thereby avoiding missed detection.

[0133] Step 330: Calculate the maximum AC voltage deviation threshold and the minimum AC voltage deviation threshold based on the AC voltage deviation threshold and the preset AC detection margin, and calculate the maximum DC voltage deviation threshold and the minimum DC voltage deviation threshold based on the DC voltage deviation threshold and the preset DC detection margin.

[0134] In this embodiment, the AC detection margin and the DC detection margin are pre-set, and the maximum AC voltage deviation threshold and the minimum AC voltage deviation threshold are calculated based on the AC voltage deviation threshold and the AC detection margin; the maximum DC voltage deviation threshold and the minimum DC voltage deviation threshold are calculated based on the DC voltage deviation threshold and the DC detection margin.

[0135] Specifically, the maximum AC voltage deviation threshold is ε ac +ξ ac and the minimum AC voltage deviation threshold ε ac -ξ ac , maximum DC voltage deviation threshold ε dc +ξ dc and the minimum DC voltage deviation threshold ε dc -ξ dc , where ε ac is the AC voltage deviation threshold, ξ ac is the AC detection margin, ε dc is the DC voltage deviation threshold, ξ dc is the DC detection margin.

[0136] The obtained maximum DC voltage deviation threshold and minimum DC voltage deviation threshold as well as the maximum AC voltage deviation threshold and minimum AC voltage deviation threshold are used as comparison thresholds for fault judgment based on the hysteresis comparison method.

[0137] Step 340: Based on the hysteresis comparison method, a fault analysis is performed according to the first actual voltage deviation, the second actual voltage deviation, the maximum AC voltage deviation threshold, the minimum AC voltage deviation threshold, the maximum DC voltage deviation threshold, and the minimum DC voltage deviation threshold to determine the occurrence and type of the fault in the grid-connected system.

[0138] In this embodiment, a fault analysis is performed based on the first actual voltage deviation, the second actual voltage deviation, the maximum AC voltage deviation threshold, the minimum AC voltage deviation threshold, the maximum DC voltage deviation threshold, and the minimum DC voltage deviation threshold to determine the fault type of the grid-connected system, and then the fault occurrence of the grid-connected system is determined based on the fault type of the grid-connected system.

[0139] In one embodiment of the present invention, Step 340, based on the hysteresis comparison method, performs fault analysis based on the first actual voltage deviation, the second actual voltage deviation, the maximum AC voltage deviation threshold, the minimum AC voltage deviation threshold, the maximum DC voltage deviation threshold, and the minimum DC voltage deviation threshold to determine the occurrence and type of the fault in the grid-connected system, specifically including:

[0140] Step 341: Based on the hysteresis comparison method, perform AC fault analysis according to the first actual voltage deviation, the maximum AC voltage deviation threshold, and the minimum AC voltage deviation threshold to determine whether an AC fault occurs in the grid-connected system.

[0141] The hysteresis comparison method is based on the hysteresis characteristic. Its core principle lies in the existence of two thresholds to form a hysteresis interval. For example, a common hysteresis comparator has two threshold voltages: an upper and a lower threshold. When the input signal exceeds the upper threshold, the output state changes, such as from a low level to a high level. When the input signal drops to the lower threshold, the output changes from a high level to a low level. In the region between the upper and lower thresholds, the output state remains unchanged, forming a hysteresis interval. In this embodiment, the hysteresis interval is formed by the maximum AC voltage deviation threshold and the minimum AC voltage deviation threshold.

[0142] The dual thresholds of hysteresis comparator are used for transition to avoid instability caused by voltage deviation fluctuating around a single threshold.

[0143] Specifically, the method for determining whether an AC fault occurs in the grid-connected system is as follows:

[0144] Step 3411. When the first actual voltage deviation is greater than the maximum AC voltage deviation threshold, it is determined that an AC fault has occurred in the grid-connected system; when the first actual voltage deviation is less than the minimum AC voltage deviation threshold, it is determined that no AC fault has occurred in the grid-connected system; when the first actual voltage deviation is not greater than the maximum AC voltage deviation threshold and not less than the minimum AC voltage deviation threshold, the first historical voltage deviation of the AC observation node before the current moment is obtained.

[0145] For reference Figure 2 , Figure 2 This is a schematic diagram of the hysteresis comparison method for judging the occurrence of AC faults in an embodiment of the present invention. ac Greater than the maximum AC voltage deviation threshold ε ac +ξ ac , it is determined that an AC fault occurs in the grid-connected system; when the first actual voltage deviation ΔV ac Less than the minimum AC voltage deviation threshold ε ac -ξ ac , it is determined that there is no AC fault in the grid-connected system.

[0146] When the first actual voltage deviation ΔV ac At the minimum AC voltage deviation threshold ε ac -ξ ac and the maximum AC voltage deviation threshold ε ac +ξ acWhen the voltage is between , it is necessary to judge based on the voltage deviation before entering the interval. Therefore, it is necessary to obtain the current moment as the starting point to search forward for the voltage that is not within the minimum AC voltage deviation threshold ε. ac -ξ ac and the maximum AC voltage deviation threshold ε ac +ξ ac The voltage deviation between the two will be searched for the first one that is not within the minimum AC voltage deviation threshold ε ac -ξ ac and the maximum AC voltage deviation threshold ε ac +ξ ac The voltage deviation between them is taken as the first historical voltage deviation.

[0147] Step 3412: If the first historical voltage deviation is greater than the maximum AC voltage deviation threshold, it is determined that an AC fault has occurred in the grid-connected system; if the first historical voltage deviation is less than the minimum AC voltage deviation threshold, it is determined that no AC fault has occurred in the grid-connected system.

[0148] In this embodiment, if the first historical voltage deviation before entering the interval is less than the minimum AC voltage deviation threshold ε ac -ξ ac , it is still considered that no AC fault has occurred; if the first historical voltage deviation value before entering the interval is greater than the maximum AC voltage deviation threshold ε ac +ξ ac If the value of , it is considered that an AC fault has occurred.

[0149] Step 342: Based on the hysteresis comparison method, a DC fault analysis is performed according to the second actual voltage deviation, the maximum DC voltage deviation threshold, and the minimum DC voltage deviation threshold to determine whether a DC fault occurs in the grid-connected system.

[0150] Specifically, the method for determining whether a DC fault occurs in the grid-connected system is as follows:

[0151] Step 3421. When the second actual voltage deviation is greater than the maximum DC voltage deviation threshold, it is determined that a DC fault has occurred in the grid-connected system; when the second actual voltage deviation is less than the minimum DC voltage deviation threshold, it is determined that no DC fault has occurred in the grid-connected system; when the second actual voltage deviation is not greater than the maximum DC voltage deviation threshold and not less than the minimum DC voltage deviation threshold, the second historical voltage deviation of the DC observation node before the current moment is obtained.

[0152] For reference Figure 3 , Figure 3 This is a schematic diagram of the hysteresis comparison method for judging the occurrence of a DC fault in an embodiment of the present invention. dc Greater than the maximum DC voltage deviation threshold ε dc +ξ dc, it is determined that a DC fault occurs in the grid-connected system; when the second actual voltage deviation ΔV dc Less than the minimum DC voltage deviation threshold ε dc -ξ dc , it is determined that a DC fault occurs in the grid-connected system. It is determined that no DC fault occurs in the grid-connected system.

[0153] When the second actual voltage deviation ΔV dc At the minimum DC voltage deviation threshold ε dc -ξ dc and the maximum DC voltage deviation threshold ε dc +ξ dc When the voltage is between , it is necessary to judge based on the voltage deviation before entering the interval. Therefore, it is necessary to obtain the current moment as the starting point to search forward for the voltage that is not within the minimum DC voltage deviation threshold ε. dc -ξ dc and the maximum DC voltage deviation threshold ε dc +ξ dc The voltage deviation between the two will be searched for the first one that is not within the minimum DC voltage deviation threshold ε dc -ξ dc and the maximum DC voltage deviation threshold ε dc +ξ dc The voltage deviation between them is taken as the second historical voltage deviation.

[0154] Step 3422: If the second historical voltage deviation is greater than the maximum DC voltage deviation threshold, it is determined that a DC fault has occurred in the grid-connected system; if the second historical voltage deviation is less than the minimum DC voltage deviation threshold, it is determined that no DC fault has occurred in the grid-connected system.

[0155] In this embodiment, if the second historical voltage deviation before entering the interval is less than the minimum DC voltage deviation threshold ε dc -ξ dc , it is still considered that no DC fault has occurred; if the second historical voltage deviation value before entering the interval is greater than the maximum DC voltage deviation threshold ε dc +ξ dc If the value of , a DC fault is considered to have occurred.

[0156] Step 330: If an AC fault or a DC fault occurs in the grid-connected system, the fault condition of the grid-connected system is determined to be a fault; if no AC fault or DC fault occurs in the grid-connected system, the fault condition of the grid-connected system is determined to be no fault.

[0157] In an embodiment of the present invention, if any one of an AC fault and a DC fault occurs in the grid-connected system, the fault occurrence condition of the grid-connected system is determined to be a fault occurrence; if neither an AC fault nor a DC fault occurs in the grid-connected system, the fault occurrence condition of the grid-connected system is determined to be no fault occurrence.

[0158] In traditional single-threshold judgment, when the voltage deviation approaches the threshold, noise or transient fluctuations may cause the fault state to repeatedly jump, such as oscillating from "normal to faulty to normal", leading to frequent switching of control modes. However, this embodiment utilizes a hysteresis comparison method, setting a hysteresis interval by setting maximum and minimum thresholds. Action is triggered only when the voltage deviation exceeds this interval, effectively reducing false switching caused by critical fluctuations.

[0159] In one embodiment of the present invention, step 400, based on comparing the actual short-circuit ratio with the short-circuit ratio threshold of the grid-connected system, determines the grid strength of the grid-connected system at the current moment, specifically including: based on the hysteresis comparison method, based on comparing the actual short-circuit ratio with the short-circuit ratio threshold of the grid-connected system, determines the grid strength of the grid-connected system at the current moment.

[0160] In this embodiment, first, the grid-following type and grid-forming type control systems are respectively operated under grid-connected systems with different short-circuit ratios to determine the minimum short-circuit ratio at which the grid-following type control system can operate stably, and the maximum short-circuit ratio at which the grid-forming type control system can operate stably.

[0161] Secondly, a hysteresis interval is formed based on the maximum short-circuit ratio and the minimum short-circuit ratio, which can be referred to Figure 4 , Figure 4 Schematic diagram of the control switching strategy under different short circuit ratios in the embodiment of the present invention. max , it is determined that the grid-connected system is a strong grid at the current moment; when the actual short-circuit ratio SCR is less than the minimum short-circuit ratio SCR min , it is determined that the grid-connected system is a weak grid at the current moment; when the actual short-circuit ratio SCR is at the minimum short-circuit ratio SCR min and maximum short circuit ratio SCR max If the historical short circuit ratio before the current moment is greater than the maximum short circuit ratio SCR, max , it is determined that the grid-connected system is a strong grid at the current moment; when the historical short-circuit ratio is less than the minimum short-circuit ratio SCR min , it is determined that the grid-connected system is a weak grid at the current moment.

[0162] When the short-circuit ratio approaches the threshold, traditional single-threshold judgments can cause repeated jumps in grid strength determinations due to grid fluctuations or measurement noise, such as oscillations from "strong grid to weak grid to strong grid," leading to frequent switching between grid-following and grid-forming control strategies. Hysteresis comparison establishes a hysteresis interval by setting maximum and minimum short-circuit ratios. A change in grid strength determination is triggered only when the short-circuit ratio exceeds this range, effectively reducing false switching caused by critical fluctuations.

[0163] In one embodiment of the present invention, step 500 analyzes the current fault status, fault type, grid strength, initial converter control mode, and a preset function switching table of the grid-connected system to determine a current converter control mode switching strategy, specifically including:

[0164] Step 510. When the initial converter control mode of the grid-connected system at the current moment is grid-forming control; if the fault condition is that no fault occurs and the grid strength is a strong grid, then the switching strategy of the converter control mode at the current moment is to switch from grid-forming control to grid-following control; if the fault condition is that no fault occurs and the grid strength is a weak grid, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode; if the fault type is a DC fault and the grid strength is a strong grid, then the switching strategy of the converter control mode at the current moment is to switch from grid-forming control to grid-following control; if the fault type is a DC fault and the grid strength is a weak grid, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode; if the fault type is an AC fault, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode.

[0165] Step 520. When the initial converter control mode of the grid-connected system at the current moment is grid-following control; if the fault condition is that no fault occurs and the grid strength is a weak grid, then the switching strategy of the converter control mode at the current moment is to switch from grid-following control to grid-forming control; if the fault condition is that no fault occurs and the grid strength is a strong grid, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode; if the fault type is a DC fault and the grid strength is a weak grid, then the switching strategy of the converter control mode at the current moment is to switch from grid-following control to grid-forming control; if the fault type is a DC fault and the grid strength is a strong grid, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode; if the fault type is an AC fault, then the switching strategy of the converter control mode at the current moment is to switch from grid-following control to grid-forming control.

[0166] For reference Figure 5 , Figure 5 This is an overall flow chart of the hysteresis switching method of the converter control method according to an embodiment of the present invention. You may also refer to the function switching table, as shown in Table 1.

[0167] Table 1 Function switching table

[0168]

[0169] After generating the switching strategy for the converter control mode, when the switching strategy is to maintain the current converter control mode, no adjustment is required; when the switching strategy is to switch from grid-following control to grid-forming control, or from grid-forming control to grid-following control, a switching control instruction is generated to perform the switching operation according to the switching control instruction.

[0170] An embodiment of the present invention further provides a switching method to achieve smooth switching of the converter control mode. The specific method includes:

[0171] Step 610: When the switching strategy of the converter control mode is to switch from the initial converter control mode to other converter control modes, obtain the first output phase angle of the grid-connected system's grid-forming power synchronization loop and the second output phase angle of the grid-following phase-locked loop synchronization link.

[0172] In this embodiment, when the switching strategy is to switch from grid-following control to grid-forming control, or from grid-forming control to grid-following control, the control mode of the converter needs to be switched. At this time, a switching control instruction will be received.

[0173] After receiving the switching control instruction, a first output phase angle of the grid-connected system's grid-forming power synchronization loop and a second output phase angle of the grid-following phase-locked loop synchronization link are obtained.

[0174] For details, please refer to Figure 6 , Figure 6 is a diagram of a smooth switching structure in an embodiment of the present invention, Figure 6 It includes a network-building type synchronization circuit and a network-following type synchronization circuit, and the first output phase angle θ is obtained according to the synchronization control link in the network-building type synchronization circuit. psl , and the second output phase angle θ is obtained from the synchronization control link in the grid-type synchronization loop pll .

[0175] Step 620: Calculate the difference between the first output phase angle and the second output phase angle to obtain the phase angle difference.

[0176] In this embodiment, a difference is calculated based on the obtained first output phase angle and the second output phase angle to obtain a phase angle difference.

[0177] Step 630: Input the phase angle difference into a preset first PI controller to obtain the instantaneous power output by the first PI controller.

[0178] For reference Figure 7 , Figure 7This is a structural diagram of the instantaneous power calculation in an embodiment of the present invention. The phase angle difference is subjected to the mod link to obtain the 2π congruence. Since the phase angle range is radians by default, it is taken from 0 to 2π; then the first PI controller in the second link performs PI proportional integral control to obtain the instantaneous power output by the second link.

[0179] Step 640: Within a preset time before or after executing the converter control mode switching operation, the preset input power reference value in the grid-type synchronous control loop is corrected according to the instantaneous power to obtain the corrected input power.

[0180] Step 650: Utilize the second PI controller in the meshed synchronous control loop to output the corrected first output phase angle according to the corrected input power, so as to smoothly switch the converter control mode based on the corrected first output phase angle within a preset time.

[0181] In this embodiment, if the switching strategy is to switch from grid-following control to grid-forming control, the switch switches from "1" to "0" according to the switching instruction, where "0" represents the output phase angle of grid-forming control, and "1" represents the output phase angle of grid-following control. Within a preset time while the switch performs the switching operation, the preset input power reference value in the grid-forming synchronous control loop is corrected based on the instantaneous power to obtain a corrected input power. A second PI controller in the grid-forming synchronous control loop outputs a corrected first output phase angle based on the corrected input power, thereby smoothly switching the inverter control mode based on the corrected first output phase angle within a preset time. The smooth switching process ends after the preset time.

[0182] If the switching strategy is to switch from grid-forming control to grid-following control, the input power reference value preset in the grid-forming synchronous control loop is corrected according to the instantaneous power within the preset time to obtain the corrected input power, and the second PI controller in the grid-forming synchronous control loop is used to output the corrected first output phase angle according to the corrected input power, so as to smoothly switch the converter control mode based on the corrected first output phase angle within the preset time. The smooth switching process ends after the preset time. After completing the smooth switching process of the preset time, the switching switch performs the switching operation and switches from "0" to "1".

[0183] In one embodiment of the present invention, the corrected input power is obtained by subtracting the instantaneous power from the input power reference value. This subtraction of the instantaneous power from the input power reference value forms a negative feedback loop. As the meshing-type output phase angle increases, the phase angle difference between the meshing-type output phase angle and the following-type output phase angle increases, resulting in excessive instantaneous power, which can lead to unstable switching. The corrected meshing-type output phase angle can reduce the phase angle difference, minimizing power surges and achieving smooth switching.

[0184] In one embodiment of the present invention, a switching device for a converter control mode is also provided. Please refer to Figure 8 , Figure 9 9 is a schematic structural diagram of a switching device for a converter control mode in an embodiment of the present invention. The device includes a data acquisition module 901 , a data processing module 902 and a strategy generation module 903 .

[0185] The data acquisition module 901 is configured to determine an AC observation node on the AC side and a DC observation node on the DC side of the converter based on the voltage deviations of all nodes on the AC side and the DC side of the grid-connected system converter under different operating conditions;

[0186] Obtaining a first actual voltage deviation of the AC observation node and a second actual voltage deviation of the DC observation node at a current moment, an actual short-circuit ratio of the grid-connected system at a current moment, and an initial converter control mode of the grid-connected system at a current moment;

[0187] The data processing module 902 is configured to perform fault analysis based on the first actual voltage deviation, the second actual voltage deviation, and the voltage deviation thresholds of the AC observation node and the DC observation node to determine the occurrence and type of the fault of the grid-connected system;

[0188] Determining the grid strength of the grid-connected system at a current moment by comparing the actual short-circuit ratio with a short-circuit ratio threshold of the grid-connected system;

[0189] The strategy generation module 903 is used to analyze the fault occurrence, fault type, grid strength, initial converter control mode and preset function switching table of the grid-connected system at the current moment, and determine the switching strategy of the converter control mode at the current moment, wherein the function switching table contains the corresponding relationships between the switching strategy of the converter control mode and the fault occurrence, fault type, grid strength and initial converter control mode.

[0190] The converter control mode switching device proposed in this embodiment selects appropriate AC and DC observation nodes to more accurately observe the grid-connected system's operating status. This device considers the grid strength and fault type of the grid-connected system's converter control mode, effectively switching converter control methods under varying grid strengths and when different fault types occur. This ensures the safe operation of the grid-connected system by meeting the requirements for different converter control types in different spatial and temporal scenarios.

[0191] Figure 9 FIG1 shows the internal structure of a computer device in one embodiment of the present invention. The computer device can be a terminal or a system. Figure 9 As shown, the computer device includes a processor, a memory, and a network interface connected via a system bus. The memory includes a non-volatile storage medium and an internal memory. The non-volatile storage medium of the computer device stores an operating system and may also store a computer program. When the computer program is executed by the processor, the processor can implement the various steps in the above method embodiment. The internal memory may also store a computer program. When the computer program is executed by the processor, the processor can implement the various steps in the above method embodiment. It will be understood by those skilled in the art that Figure 9 The structure shown in the figure is only a block diagram of a part of the structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than shown in the figure, or combine certain components, or have a different component arrangement.

[0192] In one embodiment, a computer device is provided, including a memory and a processor. The memory stores a computer program. When the computer program is executed by the processor, the processor executes each step in the above method embodiment.

[0193] In one embodiment, a computer-readable storage medium is provided, which stores a computer program. When the computer program is executed by a processor, the processor executes the steps in the above method embodiment.

[0194] Those skilled in the art will appreciate that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program. The program can be stored in a non-volatile computer-readable storage medium. When the program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to memory, storage, database or other media used in the embodiments provided in this application can include non-volatile and / or volatile memory. Non-volatile memory can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) or flash memory. Volatile memory can include random access memory (RAM) or external cache memory. By way of illustration and not limitation, RAM is available in various forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM).

[0195] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0196] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for switching a converter control mode, characterized in that: The method comprises: Determining an AC observation node on the AC side and a DC observation node on the DC side of the converter based on voltage deviations of all nodes on the AC side and the DC side of the grid-connected system converter under different operating conditions; Obtaining a first actual voltage deviation of the AC observation node and a second actual voltage deviation of the DC observation node at a current moment, an actual short-circuit ratio of the grid-connected system at a current moment, and an initial converter control mode of the grid-connected system at a current moment; Performing a fault analysis based on the first actual voltage deviation, the second actual voltage deviation, and voltage deviation thresholds of the AC observation node and the DC observation node to determine a fault condition and a fault type of the grid-connected system; Determining the grid strength of the grid-connected system at a current moment by comparing the actual short-circuit ratio with a short-circuit ratio threshold of the grid-connected system; An analysis is performed based on the fault occurrence, fault type, grid strength, the initial converter control mode and the preset function switching table of the grid-connected system at the current moment to determine the switching strategy of the converter control mode at the current moment, wherein the function switching table contains the corresponding relationships between the switching strategy of the converter control mode and the fault occurrence, fault type, grid strength and initial converter control mode.

2. The method according to claim 1, wherein The performing fault analysis based on the first actual voltage deviation, the second actual voltage deviation, and the voltage deviation thresholds of the AC observation node and the DC observation node to determine the fault occurrence and fault type of the grid-connected system specifically includes: Obtaining maximum voltage deviations of the AC observation node and the DC observation node within the fault occurrence time under each working condition, and generating a first maximum voltage deviation set of the AC observation node and a second maximum voltage deviation set of the DC observation node; Using the minimum value in the first maximum voltage deviation set as the AC voltage deviation threshold of the AC observation node, and using the minimum value in the second maximum voltage deviation set as the DC voltage deviation threshold of the DC observation node; A maximum AC voltage deviation threshold and a minimum AC voltage deviation threshold are calculated based on the AC voltage deviation threshold and a preset AC detection margin, and a maximum DC voltage deviation threshold and a minimum DC voltage deviation threshold are calculated based on the DC voltage deviation threshold and a preset DC detection margin; Based on the hysteresis comparison method, fault analysis is performed according to the first actual voltage deviation, the second actual voltage deviation, the maximum AC voltage deviation threshold, the minimum AC voltage deviation threshold, the maximum DC voltage deviation threshold and the minimum DC voltage deviation threshold to determine the fault occurrence and fault type of the grid-connected system.

3. The method according to claim 2, wherein The method of performing fault analysis based on the hysteresis comparison method according to the first actual voltage deviation, the second actual voltage deviation, the maximum AC voltage deviation threshold, the minimum AC voltage deviation threshold, the maximum DC voltage deviation threshold, and the minimum DC voltage deviation threshold to determine the occurrence and type of a fault in the grid-connected system specifically includes: Based on a hysteresis comparison method, performing an AC fault analysis according to the first actual voltage deviation, the maximum AC voltage deviation threshold, and the minimum AC voltage deviation threshold to determine whether an AC fault occurs in the grid-connected system; Based on a hysteresis comparison method, a DC fault analysis is performed according to the second actual voltage deviation, the maximum DC voltage deviation threshold, and the minimum DC voltage deviation threshold to determine whether a DC fault occurs in the grid-connected system; If an AC fault or a DC fault occurs in the grid-connected system, determining that the fault occurrence condition of the grid-connected system is a fault; If no AC fault or DC fault occurs in the grid-connected system, it is determined that the fault occurrence condition of the grid-connected system is no fault.

4. The method according to claim 3, wherein The hysteresis comparison method is used to perform AC fault analysis according to the first actual voltage deviation, the maximum AC voltage deviation threshold, and the minimum AC voltage deviation threshold to determine whether an AC fault occurs in the grid-connected system. Specifically, the method includes: When the first actual voltage deviation is greater than the maximum AC voltage deviation threshold, it is determined that an AC fault occurs in the grid-connected system; When the first actual voltage deviation is less than the minimum AC voltage deviation threshold, it is determined that no AC fault occurs in the grid-connected system; When the first actual voltage deviation is not greater than the maximum AC voltage deviation threshold and not less than the minimum AC voltage deviation threshold, obtaining a first historical voltage deviation of the AC observation node before the current moment; If the first historical voltage deviation is greater than the maximum AC voltage deviation threshold, determining that an AC fault occurs in the grid-connected system; If the first historical voltage deviation is less than the minimum AC voltage deviation threshold, it is determined that no AC fault occurs in the grid-connected system.

5. The method according to claim 3, wherein The hysteresis comparison method is used to perform a DC fault analysis based on the second actual voltage deviation, the maximum DC voltage deviation threshold, and the minimum DC voltage deviation threshold to determine whether a DC fault occurs in the grid-connected system. Specifically, the method includes: When the second actual voltage deviation is greater than the maximum DC voltage deviation threshold, it is determined that a DC fault occurs in the grid-connected system; When the second actual voltage deviation is less than the minimum DC voltage deviation threshold, it is determined that no DC fault occurs in the grid-connected system; When the second actual voltage deviation is not greater than the maximum DC voltage deviation threshold and not less than the minimum DC voltage deviation threshold, obtaining a second historical voltage deviation of the DC observation node before the current moment; If the second historical voltage deviation is greater than the maximum DC voltage deviation threshold, determining that a DC fault occurs in the grid-connected system; If the second historical voltage deviation is less than the minimum DC voltage deviation threshold, it is determined that no DC fault occurs in the grid-connected system.

6. The method according to claim 1, wherein The determining of a switching strategy for the converter control mode at the current moment by analyzing the current fault condition, fault type, grid strength, the initial converter control mode, and a preset function switching table of the grid-connected system specifically includes: When the initial converter control mode of the grid-connected system at the current moment is grid-type control; If the fault condition is that no fault has occurred and the grid strength is a strong grid, then the switching strategy of the converter control mode at the current moment is to switch from grid-forming control to grid-following control; If the fault condition is that no fault has occurred and the grid strength is a weak grid, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode; If the fault type is a DC fault and the grid strength is a strong grid, the switching strategy of the converter control mode at the current moment is to switch from grid-forming control to grid-following control; If the fault type is a DC fault and the grid strength is a weak grid, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode; If the fault type is an AC fault, the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode.

7. The method according to claim 1, wherein The determining of a switching strategy for the converter control mode at the current moment by analyzing the current fault condition, fault type, grid strength, the initial converter control mode, and a preset function switching table of the grid-connected system specifically includes: When the initial converter control mode of the grid-connected system at the current moment is grid-following control; If the fault condition is that no fault has occurred and the grid strength is a weak grid, then the switching strategy of the converter control mode at the current moment is to switch from grid-following control to grid-forming control; If the fault condition is that no fault has occurred and the grid strength is a strong grid, then the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode; If the fault type is a DC fault and the grid strength is a weak grid, the switching strategy of the converter control mode at the current moment is to switch from grid-following control to grid-forming control; If the fault type is a DC fault and the grid strength is a strong grid, the switching strategy of the converter control mode at the current moment is to maintain the current converter control mode; If the fault type is an AC fault, the switching strategy of the converter control mode at the current moment is to switch from grid-following control to grid-forming control.

8. The method according to claim 1, wherein The converter control mode of the grid-connected system includes grid-forming control and grid-following control, and the method further includes: When the switching strategy of the converter control mode is to switch from the initial converter control mode to another converter control mode, obtaining a first output phase angle of a grid-connected power synchronization loop and a second output phase angle of a grid-following phase-locked loop synchronization link of the grid-connected system; Calculating a difference between the first output phase angle and the second output phase angle to obtain a phase angle difference; Inputting the phase angle difference into a preset first PI controller to obtain the instantaneous power output by the first PI controller; Within a preset time before or after executing the converter control mode switching operation, the preset input power reference value in the grid-type synchronous control loop is corrected according to the instantaneous power to obtain a corrected input power; The second PI controller in the grid-type synchronous control loop is used to output a corrected first output phase angle according to the corrected input power, so as to smoothly switch the converter control mode based on the corrected first output phase angle within the preset time.

9. The method according to claim 1, wherein The determining of the AC observation node on the AC side and the DC observation node on the DC side of the converter based on the voltage deviation of all nodes on the AC side and the DC side of the grid-connected system converter under different working conditions specifically includes: Adding different types of faults at different locations on the AC side of the grid-connected system converter to obtain first test voltage deviations of various nodes on the AC side of the converter under different working conditions; Using the node corresponding to the maximum value of all the first test voltage deviations as an AC observation node; Adding different types of faults at different positions on the DC side of the grid-connected system converter to obtain second test voltage deviations of various nodes on the DC side of the converter under different working conditions; The node corresponding to the maximum value of all the second test voltage deviations is used as a DC observation node.

10. The method according to claim 1, wherein Obtaining the actual short-circuit ratio of the grid-connected system at the current moment, specifically including: Obtaining the real-time impedance of the grid-connected system at the current moment; The actual short-circuit ratio of the grid-connected system is obtained according to the ratio of the real-time impedance to a preset impedance reference value.

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