A network construction / following network type wind farm current and differential protection method based on equivalent short-circuit current

Abstract

The application discloses a network construction / following network type wind farm current and differential protection method based on equivalent short-circuit current in the field of power system relay protection, and aims to solve the technical problems of low short-circuit current level, strong nonlinearity caused by inverter amplitude limiting control of a wind farm, and insufficient sensitivity and reliability of traditional protection. The method comprises the following steps: calculating a pre-fault modulation coefficient and an AC internal potential based on pre-fault steady-state parameters and a pre-built equivalent model, and setting a three-section current protection value according to the pre-fault modulation coefficient and the AC internal potential; if the setting does not meet the requirements, starting protection, collecting post-fault parameters and calculating a post-fault modulation coefficient; restoring the post-fault AC internal potential to the pre-fault level by using the ratio of the pre-fault modulation coefficient to the post-fault modulation coefficient, and then generating an equivalent short-circuit current; finally, comparing the equivalent short-circuit current with a preset value to realize fault identification and judgment. The application can effectively improve the sensitivity and reliability of protection in a low short-circuit current scene, and accurately perform fault positioning and removal.

Description

TECHNICAL FIELD

[0001] The application relates to a network construction / following network type wind farm current and differential protection method based on equivalent short-circuit current, and belongs to the technical field of power system relay protection. BACKGROUND

[0002] With the large-scale wind farms with mixed access of network construction type and following network type converters, especially the rapid development of offshore wind farms, the short-circuit fault characteristics thereof are significantly different from those of traditional synchronous generator systems, and the main features are as follows: during fault ride-through, the short-circuit current provided by the wind farm is low and strongly nonlinear due to the current limiting effect of low-penetration and high-penetration control strategies; the fault response characteristics of the network construction type and following network type converters are different and coupled with each other. These characteristics result in the risk of insufficient sensitivity, decreased reliability and even failure of the existing sectional current protection and differential protection based on the short-circuit current model of the traditional synchronous power source, and the fault location and removal are difficult to be accurately and quickly performed. SUMMARY

[0003] The application aims to provide a network construction / following network type wind farm current and differential protection method based on equivalent short-circuit current, which can overcome the problem of low short-circuit current level caused by inverter power limiting control, thereby effectively improving the sensitivity and reliability of protection and accurately and quickly performing fault location.

[0004] To solve the above technical problems, the application is implemented by using the following technical scheme.

[0005] In a first aspect, the application provides a network construction / following network type wind farm current protection method based on equivalent short-circuit current, which comprises the following steps.

[0006] Obtaining steady-state parameters before fault, and determining a modulation coefficient before fault according to the steady-state parameters before fault;

[0007] Inputting the steady-state parameters before fault and the modulation coefficient before fault into a pre-constructed equivalent model of inverter power before fault to obtain an alternating current internal potential before inverter fault;

[0008] According to the alternating current internal potential before inverter fault and the internal impedance of the inverter itself, the three-section current protection setting value is set, and if the setting meets the requirements, the steady-state parameters before fault are continuously measured; otherwise, fault protection is started; wherein the starting of fault protection comprises the following steps.

[0009] Obtaining steady-state parameters after fault, and determining a modulation coefficient after fault based on the steady-state parameters after fault;

[0010] Using the ratio of the modulation coefficient before fault to the modulation coefficient after fault to restore the steady-state parameters after fault to the steady-state level before fault;

[0011] calculating an equivalent short-circuit current based on the steady-state parameters reduced to the pre-fault steady-state level;

[0012] comparing the equivalent short-circuit current with pre-set three-section current protection setting values, if the equivalent short-circuit current is greater than the I-section setting value, the protection device sends a trip signal; if the equivalent short-circuit current is greater than the II-section setting value and the II-section delay has arrived, the protection device sends a trip signal; if the equivalent short-circuit current is greater than the III-section setting value and the III-section delay has arrived, the protection device sends a trip signal.

[0013] With reference to the first aspect, further, the steady-state parameters before the fault include a direct current side voltage , an effective value of an inverter power supply grid-connected point voltage in the steady state before the fault , an effective value of an inverter grid-connected point current in the steady state before the fault , and an internal resistance of the inverter itself ;

[0014] the steady-state parameters after the fault include a direct current side voltage , an effective value of an inverter power supply grid-connected point voltage in the steady state after the fault , an effective value of an inverter grid-connected point current in the steady state after the fault , and an internal resistance of the inverter itself .

[0015] With reference to the first aspect, further, the equivalent model formula of the inverter power supply before the fault is:

[0016] ;

[0017] ;

[0018] wherein, represents a modulation coefficient before the fault; represents an effective value of an inverter grid-connected point current in the steady state before the fault; represents an internal resistance of the inverter itself; represents an effective value of an inverter power supply grid-connected point voltage in the steady state before the fault; represents a direct current side voltage; represents an alternating current internal potential before the fault.

[0019] With reference to the first aspect, further, the calculation of the equivalent short-circuit current includes:

[0020] based on the modulation coefficient after the fault and the direct current side voltage , calculating an alternating current internal potential after the fault of the inverter ;

[0021] based on the ratio of the modulation coefficient before the fault and the modulation coefficient after the fault , a modulation coefficient compression ratio is obtained ;

[0022] the AC internal voltage after the fault of the inverter , the RMS value of the inverter power grid connection point voltage in the steady state after the fault is restored by using the modulation coefficient compression ratio , to obtain the restored AC internal voltage and the restored grid connection point voltage ;

[0023] based on the restored AC internal voltage and the restored grid connection point voltage , an equivalent short-circuit current is generated .

[0024] In combination with the first aspect, further, the expression of the equivalent short-circuit current is:

[0025] ;

[0026] ;

[0027] ;

[0028] wherein, represents the restored AC internal voltage; represents the modulation coefficient compression ratio; represents the AC internal voltage after the fault of the inverter; represents the restored grid connection point voltage; represents the equivalent short-circuit current; represents the RMS value of the inverter power grid connection point voltage in the steady state after the fault; represents the internal resistance of the inverter itself.

[0029] In combination with the first aspect, further, the starting fault protection further comprises:

[0030] based on the AC internal voltage before the fault of the inverter and the RMS value of the inverter power grid connection point voltage in the steady state before the fault , a virtual impedance is calculated ;

[0031] the virtual impedance Adjusting so that the grid-connected / following grid wind farm meets the constraint equation, the expression of which is:

[0032] ;

[0033] Wherein, represents the AC internal potential before inverter failure; represents the effective value of the inverter power supply grid connection point voltage in the steady state before failure; represents the virtual impedance; represents the line impedance.

[0034] In combination with the first aspect, further, the virtual impedance is adjusted by using a graph partitioning ordering algorithm Adjusting, including:

[0035] Based on the number of inverters in the grid-connected / following grid wind farm , an electrical correlation graph is constructed ;

[0036] The subgraph area in the electrical correlation graph is identified by recursive edge contraction, thereby generating a decreasing number of coarse graphs, until decreasing to the smallest coarse graph ; wherein each subgraph area is merged into a supernode;

[0037] A fast heuristic algorithm is used to preliminarily sort the smallest coarse graph , and the preliminary sorting result is projected back to the coarse graph of the previous level in reverse order;

[0038] In each level of projection process, the supernode is decompressed into the subgraph area corresponding to the supernode in the previous level of coarse graph, and an initial sequence is formed according to the decompressed subgraph area;

[0039] The initial sequence is adjusted by using a local optimization algorithm to obtain an adjusted optimized sequence;

[0040] According to the self priority of the optimized sequence, by adjusting the modulation coefficient in the sinusoidal pulse width modulation strategy, the AC internal potential and the effective value of the power supply grid connection point voltage of the inverter are changed, thereby adjusting the virtual impedance , until the constraint equation is met.

[0041] The second aspect is a grid-connected / following grid wind farm differential protection method based on equivalent short-circuit current, including:

[0042] Obtain the steady-state parameters before the fault, and determine the modulation coefficient before the fault based on the steady-state parameters before the fault;

[0043] The steady-state parameters and modulation coefficients before the fault are input into the pre-constructed equivalent model of the inverter power supply before the fault to obtain the AC internal potential of the inverter before the fault.

[0044] Based on the AC internal potential of the inverter before the fault and the inverter's own internal impedance, the protection setting is adjusted according to the maximum value between the maximum unbalanced current when avoiding an external short circuit and the maximum load current. If the setting meets the requirements, the measurement of the steady-state parameters before the fault continues; otherwise, the fault protection is activated. Activating the fault protection includes:

[0045] Obtain the steady-state parameters after the fault, and determine the modulation coefficients after the fault based on the steady-state parameters after the fault;

[0046] By using the ratio of the modulation coefficient before the fault to the modulation coefficient after the fault, the steady-state parameters after the fault are restored to the steady-state level before the fault.

[0047] Based on the steady-state parameters restored to the pre-fault steady-state level, the equivalent short-circuit current is calculated;

[0048] The differential current and braking current are calculated by replacing the measured current of the measured component with the equivalent short-circuit current.

[0049] The operation determination of non-braking differential protection and ratio braking differential protection is performed based on the differential current and braking current, respectively.

[0050] In conjunction with the second aspect, the steady-state parameters prior to the fault further include the DC-side voltage. Effective value of inverter power supply grid connection point voltage under steady state before fault Effective value of inverter grid-connected current under steady-state conditions before fault and the inverter's own internal resistance The steady-state parameters after the fault include the DC-side voltage. Effective value of inverter power supply grid connection point voltage under steady state after fault Effective value of inverter grid connection point current under steady state after fault and the internal resistance of the inverter itself ;

[0051] Based on the modulation coefficient after the fault and DC side voltage Calculate the AC internal potential after an inverter fault. ;

[0052] Modulation coefficient before the fault a ratio of the modulation coefficient after the fault and the modulation coefficient before the fault a modulation coefficient compression ratio ;

[0053] using the modulation coefficient compression ratio the AC internal voltage after the inverter fault the effective value of the inverter grid-connected point voltage in a steady state after the fault restore the AC internal voltage after the inverter fault and the grid-connected point voltage after the restoration ;

[0054] based on the restored internal voltage and the grid-connected point voltage after the restoration generate an equivalent short-circuit current .

[0055] In combination with the second aspect, further, the starting fault protection further comprises:

[0056] based on the AC internal voltage before the inverter fault and the effective value of the inverter grid-connected point voltage in a steady state before the fault calculate a virtual impedance ;

[0057] adjust the virtual impedance using a graph partitioning ordering algorithm, so that the grid-connected / followed grid type wind farm satisfies the following constraint equation, the expression of which is:

[0058] ;

[0059] wherein, represents the AC internal voltage before the inverter fault; represents the effective value of the inverter grid-connected point voltage in a steady state before the fault; represents the virtual impedance; represents the line impedance;

[0060] wherein, the virtual impedance is adjusted using a graph partitioning ordering algorithm, comprising:

[0061] based on the number of inverters in the grid-connected / followed grid type wind farm , construct an electrical correlation graph ;

[0062] identify subgraph areas in the electrical correlation graph in a recursive edge contraction manner, thereby generating a number of coarse graphs decreasing in size, until decreasing to a minimum coarse graph ; wherein each subgraph region is merged into a supernode;

[0063] A fast heuristic algorithm is adopted to preliminarily sort the supernodes in the minimum coarse graph , and the preliminarily sorted results are projected back to the coarse graph of the previous level in a reverse manner;

[0064] In each level of projection, the supernodes are decompressed into subgraph regions corresponding to the supernodes in the coarse graph of the previous level, and an initial sequence is formed according to the decompressed subgraph regions;

[0065] The initial sequence is adjusted by using a local optimization algorithm to obtain an adjusted optimized sequence;

[0066] According to the self priority of the optimized sequence, the modulation coefficient in the sinusoidal pulse width modulation strategy is adjusted , the AC internal potential of the inverter and the effective value of the voltage at the power grid connection point are changed , so as to adjust the virtual impedance until the constraint equation is satisfied.

[0067] Compared with the prior art, the present application has the following beneficial effects:

[0068] (1) It has strong universality, and the equivalent short-circuit current construction method as the core is based on the sinusoidal pulse width modulation (SPWM) control mechanism in the inverter, which is independent of whether the inverter is a grid-forming type or a grid-following type, the mixing ratio of the two in the wind farm, and the specific fault ride-through current limiting multiple.

[0069] (2) Compared with the traditional protection method, the sensitivity and reliability are improved, and by equivalent to the actual limited short-circuit current to the traditional generator short-circuit current which is greatly improved and has linear characteristics, the traditional current protection and differential protection can be reliably and sensitively actuated in the low short-circuit current scene of the wind farm.

[0070] (3) It has good compatibility, and does not need to change the existing fault ride-through control strategy of the inverter, and can directly use the voltage and current measurement signals at the grid connection point.

[0071] (4) It is simple and economical, mainly relies on local measurement signals and calculation, has low dependence on communication, does not need complex hardware modification, and is suitable for harsh environments such as offshore wind farms and high maintenance cost scenarios.

[0072] (5) It has excellent protection performance, and effectively solves the problems of fault positioning difficulty and traditional protection failure caused by low short-circuit current level and strong nonlinearity in the low ride / high ride condition of the grid-forming / grid-following hybrid wind farm. BRIEF DESCRIPTION OF DRAWINGS

[0073] Figure 1 A framework diagram of a method for constructing an equivalent short-circuit current provided by an embodiment of the application is shown.

[0074] Figure 2 A process for constructing an equivalent short-circuit current provided by an embodiment of the application is shown.

[0075] Figure 3 A flowchart of a graph partitioning ordering algorithm (METIS algorithm) provided by an embodiment of the application is shown.

[0076] Figure 4 A flowchart of a differential protection method based on an equivalent short-circuit current provided by an embodiment of the application is shown.

[0077] Figure 5 A flowchart of a current protection method based on an equivalent short-circuit current provided by an embodiment of the application is shown.

[0078] Figure 6 A schematic diagram of a typical power system simulation scenario provided by an embodiment of the application is shown.

[0079] Figure 7 A schematic diagram of five typical power system simulation scenarios of differential protection provided by an embodiment of the application is shown. DETAILED DESCRIPTION

[0080] The technical solutions of the present application will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific features of the embodiments and the specific features in the embodiments are detailed descriptions of the technical solutions of the present application, and are not limitations of the technical solutions of the present application. In the case of no conflict, the technical features of the embodiments and the embodiments can be combined with each other.

[0081] The term "and / or", only describes the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / ", generally represents that the associated objects before and after are in an "or" relationship.

[0082] Embodiment 1

[0083] Referring to Figure 1 and Figure 5 , the present application introduces a network construction / following network type wind farm current protection method based on equivalent short-circuit current, which comprises:

[0084] Step S1, obtaining the steady-state parameters before the fault, and determining the modulation coefficient before the fault based on the steady-state parameters before the fault ;

[0085] Specifically, the modulation coefficient before the fault a calculation process, comprising:

[0086] Step S11, real-time monitoring and acquiring the steady-state parameters of the grid-connection / following grid type wind farm before the fault, including the DC side voltage , the effective value of the inverter power source grid-connected point voltage in the steady state before the fault , the effective value of the inverter grid-connected point current in the steady state before the fault , and the internal resistance of the inverter itself ;

[0087] Step S12, calculating the modulation coefficient before the fault according to the acquired steady-state parameters before the fault ;

[0088] In the embodiment of the present application, according to the control response characteristics of the grid-connection type and following grid type inverters at the moment of the power grid fault, the two types of inverters are unified and equivalent to an equivalent model of the inverter power source before the fault and an equivalent model of the inverter power source after the fault;

[0089] Based on the equivalent model of the inverter power source before the fault, the modulation coefficient before the fault is calculated , and the expression is:

[0090] (1)

[0091] Wherein, represents the modulation coefficient before the fault; represents the effective value of the inverter grid-connected point current in the steady state before the fault; represents the internal resistance of the inverter itself; represents the effective value of the inverter power source grid-connected point voltage in the steady state before the fault; represents the DC side voltage.

[0092] Step S2, inputting the steady-state parameters before the fault and the modulation coefficient before the fault to the pre-constructed equivalent model of the inverter power source before the fault, and outputting the AC internal potential of the inverter ;

[0093] Referring to Figure 2 (a) in the figure, based on the equivalent model of the inverter power source before the fault, the DC side voltage in the steady-state parameters before the fault is converted into the AC internal potential of the inverter by using the modulation coefficient before the fault , and the expression is:

[0094] (2)

[0095] Wherein, represents the AC internal potential before the fault.

[0096] Step S3, according to the AC internal potential before the inverter fault and the internal impedance of the inverter itself, the three-stage current protection setting value is set, if the setting meets the requirements, return to continue measuring the steady-state parameters before the fault; otherwise, start the fault protection; wherein, the steps after the fault protection is started are as follows:

[0097] Step S31, obtain the steady-state parameters after the fault enters the steady-state stage, and determine the modulation coefficient after the fault based on the steady-state parameters after the fault and the equivalent model of the inverter power supply before the fault ; wherein, the steady-state parameters after the fault include the DC side voltage , the effective value of the inverter power supply grid connection point voltage under the steady-state after the fault , the effective value of the inverter grid connection point current under the steady-state after the fault , and the internal resistance of the inverter itself .

[0098] The expression of the modulation coefficient after the fault is:

[0099] (3);

[0100] Wherein, represents the modulation coefficient after the fault; represents the effective value of the inverter grid connection point current under the steady-state after the fault; represents the effective value of the inverter power supply grid connection point voltage under the steady-state after the fault; represents the DC side voltage.

[0101] Further, referring to (b) in Figure 2 , the modulation coefficient after the fault and the DC side voltage are input into the equivalent model of the inverter power supply after the fault, the AC internal potential of the inverter after the fault is calculated and output, and the expression is:

[0102] (4);

[0103] Wherein, represents the AC internal potential of the inverter after the fault;

[0104] Step S32, using the ratio of the modulation coefficient before the fault and the modulation coefficient after the fault, restore the steady-state parameters after the fault to the steady-state level before the fault;

[0105] Specifically, in order to intuitively show the difference between the modulation coefficients before and after the fault, the modulation coefficient compression ratio is introduced, and the expression is as follows:

[0106] (5);

[0107] in, This indicates the modulation index compression ratio.

[0108] In the stable state after a fault, to eliminate the impact of the fault on the inverter's internal potential, the modulation coefficient compression ratio is utilized. The AC internal potential after an inverter failure To restore the internal potential to the level before the fault, that is, to amplify the internal potential that was suppressed after the fault, the restored AC internal potential. The expression is as follows:

[0109] (6);

[0110] in, This represents the reduced AC internal potential.

[0111] Furthermore, the relationship between the inverter's grid connection point voltage and internal potential is analyzed. Based on the connection between the external line impedance and the internal impedance of the new energy source, the effective value of the inverter's grid connection point voltage under steady-state conditions after a fault is determined. If the equivalent grid connection point voltage is restored to the steady-state equivalent grid connection point voltage of a traditional synchronous generator model with the same AC internal potential and internal impedance as the inverter after a fault, then the restored grid connection point voltage is... The expression is as follows:

[0112] (7);

[0113] in, This represents the restored grid connection point voltage.

[0114] Based on this, see Figure 2 In Figure (c), using formulas (6) and (7), the controlled new energy inverter after the fault is equivalent to a reduced AC internal potential with a constant value. and the known internal resistance of the inverter itself The equivalent short-circuit current calculation model. After the equivalent short-circuit current calculation model is connected to the power grid, its port electrical characteristics ( and The relationship between the two states (i.e., the virtual operating state of the inverter, which is equivalent to that of a synchronous generator of the same capacity, without the influence of current-limited control) represents the inverter's operating state.

[0115] Step S33: Restore the steady-state parameters (restored AC internal potential) to the pre-fault steady-state level. and the restored grid connection point voltage The input is fed into the equivalent short-circuit current calculation model to calculate and output the equivalent short-circuit current. .

[0116] Among them, the equivalent short-circuit current The expression is:

[0117] (8);

[0118] in, This represents the equivalent short-circuit current.

[0119] It should be noted that the steady-state parameters used in this invention to restore the steady-state level before the fault are not directly equal to the instantaneous measurement value at a certain moment before the fault occurred, but refer to the steady-state parameters corresponding to the stable operating stage of the inverter before the fault.

[0120] Step S34: Compare the equivalent short-circuit current with the pre-set three-stage current protection settings. If the equivalent short-circuit current is greater than the settings for stage I, stage II, and stage III respectively, the protection device will issue a trip signal. The specific determination process includes:

[0121] The calculated equivalent short-circuit current The current protection action is determined by comparing the settings with the three-stage current protection settings (Stage I, Stage II, Stage III).

[0122] If the equivalent short-circuit current If the first stage setting (instantaneous overcurrent protection setting) is set, the protection device will immediately issue a trip signal;

[0123] If the equivalent short-circuit current If the second-stage setting (time-limited instantaneous overcurrent protection setting) is set, the protection device will start with a short delay (short delay). If the current still exceeds the setting after the delay ends, a trip signal will be issued.

[0124] If the equivalent short-circuit current >Section III setting (overcurrent protection setting): The protection device starts with a relatively long delay (long delay). If the current still exceeds the setting after the delay ends, a trip signal is issued.

[0125] On the other hand, the process of activating fault protection also includes the following steps:

[0126] Step S41: Based on the AC internal potential before the inverter fault Effective value of inverter power supply grid connection point voltage under steady state before fault and the effective value of the inverter grid connection point current under steady state before the fault Calculate virtual impedance ;

[0127] The virtual impedance The expressions include:

[0128] (9);

[0129] wherein, represents a virtual impedance.

[0130] Step S42, adjust the virtual impedance using METIS (graph partitioning and ordering algorithm) so that the grid-following wind farm meets the following constraint equation:

[0131] (10);

[0132] wherein, represents a line impedance.

[0133] Specifically, referring to Figure 3 adjust the virtual impedance using the METIS ordering algorithm to meet the constraint equation, thereby achieving control of the inverter ride-through current by enhancing the voltage support capability of the grid-forming inverter during faults and correcting the phase output of the grid-following inverter phase-locked loop. Specifically, it includes:

[0134] Step S421, based on multiple inverters in the wind farm, construct an electrical correlation graph ; wherein the nodes represent inverters and the edges represent electrical connection relationships.

[0135] Step S422, identify densely connected subgraph regions of the electrical correlation graph by recursive edge contraction, and merge each subgraph region into a supernode, thereby generating a series of coarse graphs , ,..., . The purpose is to quickly reduce the size while still retaining important graph structure information, until it is reduced to the smallest coarse graph ;

[0136] In the recursive edge contraction process, the algorithm dynamically identifies "densely connected subgraph regions" (i.e., regions where the connection relationships between a portion of inverter nodes through equivalent electrical branches are very close, the electrical distance is close, and the mutual influence is strong) in the current graph, and merges each such subgraph region into a supernode. This process is recursive, starting from the initial graph , each contraction generates a coarser graph (such as comes from contraction, comes from contraction), until it reaches the smallest coarse graph .

[0137] Step S423, use a fast heuristic algorithm in the smallest coarse graph The initial sorting is performed, and the initial sorting results are projected back to the previous level's rough map in reverse order. , , ..., )middle;

[0138] It should be noted that the next level rough map refers to a more detailed map than the next level rough map, hereinafter referred to as the fine map.

[0139] Step S424: During each level of projection, the supernodes in the current coarse graph are decompressed into supernodes and corresponding densely connected subgraph regions in the previous level of coarse graph, and an initial sequence is formed based on the decompressed subgraph regions.

[0140] Step S425: Adjust the initial sequence using a local optimization algorithm. Optimize the sequence structure by swapping or adjusting node positions, and finally obtain the adjusted optimized sequence.

[0141] Step S426: Based on the priority of the optimized sequence, change the inverter's SPWM modulation strategy (i.e., adjust the modulation coefficient). Change the AC internal potential of the inverter and the effective value of the power supply grid connection point voltage To dynamically adjust the virtual impedance This continues until the above constraint equations are satisfied.

[0142] For example: in the smallest rough map A division is obtained from the above (for example, by...) Divide into P parts). At each step of projection and optimization, the nodes within each part on the rough map are assigned an internal order (e.g., by node degree, coordinates, space-fill curve, etc.), and each part itself is also sorted according to some rule (e.g., by part ID, or according to the part's position in the original map).

[0143] The final node sequence is: sorted part 1 + sorted part 2 + ... + sorted part P. This ensures that nodes within the same part are adjacent in the final sequence, and adjacent parts are also adjacent after sorting, greatly enhancing the locality of the graph structure. During decompression, a sequence is generated for the nodes within each supernode. This applies to the minimum coarse graph. An initial sorting algorithm is applied. During backprojection, multiple nodes corresponding to a supernode in the coarse map are assigned an order (e.g., based on their connections to the supernode's neighbors) and inserted into the sequence. The positions of the nodes in the sequence are further adjusted through local optimization. The final sequence is formed by superimposing the node insertion order from each decompression stage with the local optimization results.

[0144] Example 2

[0145] Referring to Figure 1 and Figure 4 A network configuration / following network type wind farm differential protection method based on equivalent short-circuit current, comprising the following steps:

[0146] Step 1, obtaining the steady-state parameters before the fault, and determining the modulation coefficient before the fault according to the steady-state parameters before the fault;

[0147] Step 2, inputting the modulation coefficient before the fault and the steady-state parameters before the fault into the pre-constructed inverter power source equivalent model before the fault, calculating and outputting the AC internal potential before the inverter fault;

[0148] It should be noted that the specific implementation process of steps 1-2 is the same as steps S1-S2 in embodiment 1, and will not be repeated here.

[0149] Step 3, according to the AC internal potential before the inverter fault and the internal impedance of the inverter itself, the maximum value between the maximum unbalanced current when avoiding external short circuit and the maximum load current is used for protection setting, if the setting meets the requirements, return to continue measuring the steady-state parameters before the fault; otherwise, start fault protection; wherein, starting fault protection includes:

[0150] Step 331, obtaining the steady-state parameters after the fault, and inputting the steady-state parameters after the fault into the inverter power source equivalent model after the fault to obtain the modulation coefficient after the fault;

[0151] Step 332, using the ratio of the modulation coefficient before the fault and the modulation coefficient after the fault to restore the steady-state parameters after the fault to the steady-state level before the fault;

[0152] Step 333, calculating the equivalent short-circuit current based on the steady-state parameters restored to the steady-state level before the fault;

[0153] It should be noted that the calculation process of the equivalent short-circuit current is the same as steps S31-S33 in embodiment 1, and will not be repeated again.

[0154] Step 334, using the equivalent short-circuit current to replace the measured current of the measured element, executing the differential protection algorithm, i.e. calculating the differential current and the braking current, and determining whether to act according to the differential protection criterion, thereby respectively performing the action determination of the non-braking differential protection and the ratio braking differential protection.

[0155] Specifically, the action determination process of the ratio braking differential protection is as follows:

[0156] The traditional ratio braking differential protection action is performed by collecting the measured currents on both sides of the protected device, calculating the differential current and the braking current, and the expression is:

[0157] (11);

[0158] (12);

[0159] Action condition: (13);

[0160] wherein, represents the differential current; represents the braking current; represents the measured current on one side; represents the measured current on the other side; is a braking coefficient; represents a starting threshold.

[0161] To solve the problem of measured current distortion caused by current transformer saturation under serious faults, the application makes key improvements to the above criterion: the calculation basis is replaced from the easily affected measured current to the equivalent short-circuit current and , the expression of which is:

[0162] (14);

[0163] (15);

[0164] (16);

[0165] wherein, represents the equivalent short-circuit differential current; represents the equivalent short-circuit current on one side; represents the equivalent short-circuit current on the other side; represents the equivalent short-circuit braking current.

[0166] It should be noted that the braking coefficient and the starting threshold are recalibrated to match the equivalent current characteristics, thereby forming a more reliable action determination logic under complex fault conditions.

[0167] Further, the non-braking differential protection, as a supplement to the ratio braking protection, does not introduce the concept of braking current, and its traditional criterion is directly based on the comparison between the differential current and the preset setting value When is met, the protection immediately acts. This improvement enables the non-braking differential protection to maintain extremely high action sensitivity and reliability for serious faults in the area even when dealing with conditions such as current transformer saturation that cause inaccurate regular measurements.

[0168] Example 3

[0169] Participation Figure 6, multiple typical wind farm scenarios are built on the simulation platform to verify the current / differential protection method for grid-forming and grid-following wind farms based on equivalent short-circuit current.

[0170] (a) Verification results of current protection method.

[0171] The key scenarios are as follows:

[0172] Scenario one: single grid-forming inverter power supply configuration, total line length 1.15 km, load 10 kV·A;

[0173] Scenario two: double grid-forming inverter power supply configuration, load 20 kV·A;

[0174] Scenario three: mixed power supply configuration of grid-forming and grid-following inverters, load 20 kV·A;

[0175] Scenario four: ring power supply structure, line AB length 0.43 km, BC length 0.57 km, AC length 0.43 km, load distribution is diverse.

[0176] Among them, Figure 6 The numbers 1, 2, 3, and 4 in the middle box are the numbers of "protection sections". They represent the logical protection intervals defined and acted by the protection method of the present application.

[0177] Key parameters of inverter:

[0178] Grid-forming converter: rated capacity 10 kV·A, DC bus voltage 800 V, switching frequency 20 kHz, filter inductance 2 mH, filter capacitance 10 μF;

[0179] Grid-following converter: parameters are basically the same as those of grid-forming converter, additionally including PLL proportion coefficient 50 and integral coefficient 1200.

[0180] The key parameters of the above-mentioned inverter provide a basis for subsequent protection setting and short-circuit current calculation. In order to facilitate understanding of the present embodiment and Figure 6 , the key parameters and abbreviations in the present application will be explained as follows:

[0181] DC (Direct Current): in the embodiment of the present application, it refers to the DC side voltage .

[0182] P_control (Power Control): power control, refers to the control loop in the grid-forming (GFM) inverter for adjusting active power output.

[0183] GFM (Grid-Forming Inverter): Grid-forming inverter. A control mode of inverter that can autonomously establish and maintain grid voltage and frequency, with similar "voltage source" characteristics to synchronous generators.

[0184] MPPT (Maximum Power Point Tracking): Maximum power point tracking, a control algorithm used for grid-following (GFL) inverters or fan converters.

[0185] Load: Load. Refers to the power-consuming equipment or energy-consuming elements connected to the power system.

[0186] Protection setting value setting results:

[0187] The traditional current protection setting value is based on the equivalent synchronous generator model setting. The protection I and II segment values of each scenario are as follows in Table 1:

[0188] Table 1: Traditional current protection segment line protection setting values

[0189]

[0190] The above setting values are used for subsequent comparison with equivalent short-circuit current to verify protection action.

[0191] Short-circuit current verification and protection action analysis: By setting different location short-circuit faults, the protection performance of measured short-circuit current and equivalent short-circuit current is compared. The key results are described as follows:

[0192] Protection I segment verification (20% line fault):

[0193] In scenario one, when a three-phase short circuit occurs at 20% of the AB line, the measured short-circuit current is 45.00A, and the equivalent short-circuit current is 297.00A. Based on the measured current, the protection I segment cannot act, while based on the equivalent current, it can act reliably. Similarly, in all scenarios, the equivalent short-circuit current makes the protection I segment meet the requirement of 15%-20% of the line length, while the measured current causes the protection to fail due to low amplitude. Conclusion: The equivalent short-circuit current can accurately reflect the zone fault for all fault types (three-phase, two-phase) in the four scenarios, while the measured current generally fails.

[0194] Protection II segment verification (line end fault):

[0195] In scenario one, when a three-phase short circuit occurs at the end of the AB line, the measured current is 45.00A, and the equivalent current is 162.90A. The protection II segment does not act under the measured current, but acts reliably under the equivalent current.

[0196] In the ring structure of scenario four, when the end of the BA line is short-circuited in two phases, the measured current is 99.24 A, and the equivalent current is 127.68 A; the equivalent current ensures that protection II covers the entire length of the line.

[0197] Overall, the equivalent short-circuit current solves the problem of protection range mismatch caused by inverter current limiting.

[0198] Sensitivity improvement analysis:

[0199] The sensitivity coefficient of protection II (end of two-phase short-circuit current / protection setting value) is compared and displayed:

[0200] Scenario one protection 1: the measured current sensitivity coefficient is 0.367, and the equivalent current is increased to 1.005;

[0201] Scenario two protection 1: measured 0.277, equivalent short-circuit current increased to 1.027;

[0202] Scenario three protection 1: measured 0.263, equivalent short-circuit current increased to 1.032;

[0203] Scenario four protection 1: measured 1.023, equivalent short-circuit current increased to 4.831.

[0204] According to the comparison of the sensitivity coefficient of protection II, the equivalent short-circuit current makes the sensitivity coefficient reach more than 1.0, which is significantly better than the 0.2-0.4 level of the measured current, proving the superiority of the method in low short-circuit current scenarios.

[0205] (b) Verification results of differential protection method

[0206] Reference Figure 7 Taking line BC as the protected line, for the five scenarios, three-phase short-circuit, two-phase short-circuit and single-phase short-circuit are set at the midpoint of line AB. The short-circuit currents flowing through the two sides of line BC are measured 、 , and the corresponding virtual short-circuit currents 、 are generated, as shown in Table 2 (in Table 2, “x” represents that both types of differential protection cannot reflect the fault in the interval, and the same applies hereinafter).

[0207] Table 2 Measured short-circuit current and equivalent short-circuit current of line BC when fault occurs at the midpoint of AB

[0208]

[0209] As can be seen from Table 2, whether based on the measured short-circuit current 、 or the virtual short-circuit current , When a fault occurs outside the protection zone, the differential protection can reliably not act.

[0210] Further, the zone-out fault and zone-in fault are analyzed as follows:

[0211] 1) Zone-out fault:

[0212] When a fault occurs at the midpoint of line AB, the current flowing through the two sides of BC line (such as scenario one three-phase short circuit: B side measured current 63.06A, equivalent 111.01A; C side measured 63.69A, equivalent 111.07A), the differential protection can reliably not act, proving the selectivity of the method.

[0213] 2) Zone-in fault:

[0214] When a three-phase short circuit occurs at 20% of BC line, the equivalent current of B side of scenario one is 148.53A, and the equivalent current of C side is 165.19A, the differential protection acts sensitively; the sensitivity coefficient of the non-braking differential protection is improved to 24.25 based on the equivalent current, while the measured current is only 9.80.

[0215] At 40%, 60% and 80% fault positions, the equivalent short-circuit current makes the differential protection act reliably, and the ratio braking characteristic is further optimized, for example, in scenario five, when a two-phase short circuit occurs at 80%, the braking sensitivity of the equivalent current is 9.13, which is much higher than the 2.78 of the measured current.

[0216] Conclusion: The equivalent short-circuit current solves the protection direction confusion caused by multiple power supply feeders, and maintains high sensitivity at low limit amplitude current.

[0217] The embodiments of the application are described above in combination with the drawings, but the application is not limited to the above specific embodiments, and the above specific embodiments are only illustrative, not restrictive, and those skilled in the art can make many forms under the inspiration of the application without departing from the purpose of the application and the scope protected by the claims, which are all within the protection of the application.

Claims

1. A network configuration / following network type wind farm current protection method based on equivalent short-circuit current, characterized in that, The method comprises the following steps: obtaining steady-state parameters before the fault, and determining modulation coefficients before the fault according to the steady-state parameters before the fault; inputting the steady-state parameters before the fault and the modulation coefficients before the fault into a pre-constructed equivalent model of an inverter power supply before the fault to obtain an AC internal electromotive force of the inverter before the fault; according to the AC internal electromotive force of the inverter before the fault and an internal impedance of the inverter itself, setting a three-section current protection value, and if the setting meets the requirements, returning to continue measuring the steady-state parameters before the fault; otherwise, starting the fault protection; wherein the starting of the fault protection comprises: obtaining steady-state parameters after the fault, and determining modulation coefficients after the fault based on the steady-state parameters after the fault; restoring the steady-state parameters after the fault to a steady-state level before the fault by using a ratio of the modulation coefficients before the fault to the modulation coefficients after the fault; calculating an equivalent short-circuit current based on the steady-state parameters restored to the steady-state level before the fault; comparing the equivalent short-circuit current with a pre-set three-section current protection value, and if the equivalent short-circuit current is greater than a first-section value, the protection device sends a trip signal; if the equivalent short-circuit current is greater than a second-section value and a second-section delay time has arrived, the protection device sends a trip signal; and if the equivalent short-circuit current is greater than a third-section value and a third-section delay time has arrived, the protection device sends a trip signal.

2. The network configuration / following network type wind farm current protection method based on equivalent short-circuit current according to claim 1, characterized in that, The steady-state parameter before the fault includes a direct current side voltage , an effective value of an inverter power grid connection point voltage in a steady state before the fault , an effective value of an inverter grid connection point current in a steady state before the fault , and an internal resistance of the inverter itself ; The steady-state parameter after the fault includes a direct current side voltage , an effective value of the inverter power grid connection point voltage in the steady state after the fault , an effective value of the inverter grid connection point current in the steady state after the fault , and an internal resistance of the inverter itself .

3. The network configuration / following network type wind farm current protection method based on equivalent short-circuit current according to claim 1, characterized in that, The equivalent model of the inverter power supply before the fault is as follows: ； ； wherein, represents the modulation factor before the fault; represents the effective value of the inverter grid-connection point current in the steady state before the fault; represents the internal resistance of the inverter itself; represents the effective value of the inverter power supply grid-connection point voltage in the steady state before the fault; represents the DC-side voltage; represents the AC internal potential before the fault.

4. The network configuration / following network type wind farm current protection method based on equivalent short-circuit current according to claim 1, characterized in that, The calculation of the equivalent short-circuit current comprises: based on the modulation factor after the fault and the dc-side voltage , calculate the ac internal voltage after the inverter fault ; Based on the ratio of the modulation factor before failure and the modulation factor after failure , the modulation factor compression ratio is obtained; The modulation coefficient compression ratio The AC internal potential after the inverter fails The effective value of the inverter power grid point voltage in the steady state after the failure The reduction is carried out to obtain the reduced AC internal potential And the reduced grid point voltage ; based on the reduced ac internal potential and the reduced point of common coupling voltage , generating an equivalent short circuit current .

5. The network configuration / following network type wind farm current protection method based on equivalent short-circuit current according to claim 1, characterized in that, The equivalent short circuit current The expression is: ； ； ； wherein, represents the AC internal potential after reduction; represents the modulation factor compression ratio; represents the AC internal potential after inverter failure; represents the grid point voltage after reduction; represents the equivalent short-circuit current; represents the inverter power grid point voltage effective value in steady state after failure; represents the internal resistance of the inverter itself.

6. The equivalent short circuit current based network configuration follow / network type wind farm current protection method according to claim 1, characterized in that, The starting of the fault protection further comprises: based on the ac internal potential before the inverter fault and the effective value of the inverter mains point voltage in the steady state before the fault , calculating a virtual impedance ; The virtual impedance is sorted by a graph partitioning algorithm The network configuration / following network type wind farm is adjusted to satisfy the constraint equation, and the expression of the constraint equation is: ； wherein, represents the AC internal potential before the inverter fault; represents the effective value of the inverter power grid connection point voltage in the steady state before the fault; represents the virtual impedance; represents the line impedance.

7. The network configuration / following network type wind farm current protection method based on equivalent short-circuit current according to claim 6, characterized in that, The virtual impedance is adjusted using a graph partitioning ordering algorithm including: Based on the network / following type wind farm in The electrical correlation graph is constructed ; The electrical correlation graph is identified by recursive edge contraction. Subgraph regions within the graph, thus generating decreasing-size subgraphs. A rough sketch is generated, and then reduced to the smallest rough sketch. In this process, each subgraph region is merged into a single supernode. using a fast heuristic algorithm on the smallest coarse map performing a preliminary ranking and projecting the results of the preliminary ranking back into the coarse map of the previous level in reverse. in each level of projection, decompressing the supernode into a subgraph region corresponding to the supernode in a previous level of a rough graph, and forming an initial sequence according to the decompressed subgraph region; adjusting the initial sequence by using a local optimization algorithm to obtain an adjusted optimized sequence; adjusting a modulation factor in a sinusoidal pulse width modulation strategy , changing an ac internal potential of the inverter and a voltage root mean square of a point of common coupling of the power source , thereby adjusting the virtual impedance until the constraint equation is satisfied.

8. A method for differential protection of network-forming / following wind farms based on equivalent short-circuit current, characterized in that, The method comprises the following steps: obtaining steady-state parameters before the fault, and determining modulation coefficients before the fault according to the steady-state parameters before the fault; inputting the steady-state parameters before the fault and the modulation coefficients before the fault into a pre-constructed equivalent model of an inverter power supply before the fault to obtain an AC internal electromotive force of the inverter before the fault; according to the AC internal electromotive force of the inverter before the fault and an internal impedance of the inverter itself, setting a protection value according to a maximum value between a maximum unbalanced current when external short-circuit is avoided and a maximum load current when maximum load is avoided, and if the setting meets the requirements, returning to continue measuring the steady-state parameters before the fault; otherwise, starting the fault protection; wherein the starting of the fault protection comprises: obtaining steady-state parameters after the fault, and determining modulation coefficients after the fault based on the steady-state parameters after the fault; restoring the steady-state parameters after the fault to a steady-state level before the fault by using a ratio of the modulation coefficients before the fault to the modulation coefficients after the fault; calculating an equivalent short-circuit current based on the steady-state parameters restored to the steady-state level before the fault; replacing a measured current of a measured element with the equivalent short-circuit current to calculate a differential current and a braking current; according to the differential current and the braking current, respectively performing action determination of a non-braking differential protection and a ratio braking differential protection.

9. The network configuration / following network type wind farm differential protection method based on equivalent short circuit current according to claim 8, characterized in that, The steady-state parameters before the fault include a DC side voltage , an effective value of the inverter power grid connection point voltage in a steady state before the fault , an effective value of the inverter grid connection point current in a steady state before the fault , and an internal resistance of the inverter itself ; the steady-state parameters after the fault include a DC side voltage , an effective value of the inverter power grid connection point voltage in a steady state after the fault , an effective value of the inverter grid connection point current in a steady state after the fault , and an internal resistance of the inverter itself ; based on the post-fault modulation factor and the dc-side voltage , calculating the ac internal voltage of the inverter post-fault ; pre-fault modulation index and post-fault modulation index ratio, resulting in a modulation index compression ratio ; compressing the modulation coefficient ratio the ac internal voltage after the inverter fault the effective value of the inverter grid point voltage in steady state after the fault reducing to obtain the reduced ac internal voltage and the reduced grid point voltage ; based on the reduced ac internal potential and the reduced point of common coupling voltage , generating an equivalent short circuit current .

10. The method of claim 8, wherein the method is based on the equivalent short circuit current of the network / following network type wind farm differential protection method, characterized in that, The starting of the fault protection further comprises: in each level of projection, decompressing the supernode into a subgraph region corresponding to the supernode in a previous level of a rough graph, and forming an initial sequence according to the decompressed subgraph region; adjusting the initial sequence by using a local optimization algorithm to obtain an adjusted optimized sequence; based on the ac internal potential before the inverter fault and the effective value of the inverter mains point voltage in the steady state before the fault , calculating a virtual impedance ; The virtual impedance is sorted by a graph partitioning algorithm The network construction / following network type wind farm is adjusted to satisfy the constraint equation, and the expression of the constraint equation is: ； wherein, represents the AC internal potential before the inverter fault; represents the effective value of the inverter power grid connection point voltage in the steady state before the fault; represents the virtual impedance; represents the line impedance; Wherein, the virtual impedance is adjusted by using a graph partitioning ordering algorithm comprising: based on the number of inverters in the network-constructing / following network type wind farm , an electrical correlation graph is constructed ; The electrical correlation graph is identified by recursive edge contraction. Subgraph regions within the graph, thus generating decreasing-size subgraphs. A rough sketch is generated, and then reduced to the smallest rough sketch. In this process, each subgraph region is merged into a single supernode. A fast heuristic algorithm is used to perform a preliminary ranking of the coarse maps and the results of the preliminary ranking are progressively back-projected into the coarse maps of the previous level. In each stage of projection, the supernode is decompressed into a subgraph region corresponding to the supernode in the previous stage of coarse graph, and an initial sequence is formed according to the decompressed subgraph region; An initial sequence is adjusted by using a local optimization algorithm to obtain an adjusted optimized sequence; adjusting a modulation factor in a sinusoidal pulse width modulation strategy according to a self priority of the optimization sequence , changing an ac internal potential of the inverter and a voltage root mean square of a point of common coupling of the power source , thereby adjusting the virtual impedance until the constraint equation is satisfied.

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