Power distribution network protection method and device suitable for high-proportion distributed power supply access

Abstract

The invention belongs to the field of power system protection, and relates to a power distribution network protection method and device suitable for high-proportion distributed power supply access, and the method comprises the steps: collecting the bus voltage of a power distribution network and the outlet current of a feeder line, and judging whether to start protection or not; after protection is started, carrying out equivalence on a plurality of distributed power supplies on a feeder line, and determining an initial value of short-circuit current calculation according to parameters of the equivalent distributed power supplies; judging whether short-circuit calculation is converged or not, if yes, calculating fault parameters of the equivalent distributed power supply when the tail end of the feeder line is in fault, and if not, calculating the fault parameters of the equivalent distributed power supply when the tail end of the feeder line is in fault by using an improved method; calculating the positive sequence current of a feeder outlet when the tail end of the feeder fails; current protection is set again according to the positive sequence current, whether the positive sequence current of the feeder line outlet under the fault meets the action criterion or not is judged, if yes, the protection device sends out a tripping signal to cut off the fault feeder line, and if not, protection is returned; the method is not affected by factors such as short-circuit transition resistance, fault positions and system operation modes, and has the advantages of being high in speed, good in reliability, high in adaptability and independent of communication.

Description

Technical Field

[0001] The present invention belongs to the field of power system protection, and in particular relates to a distribution network protection method and device adapted for access of a high proportion of distributed power sources. Background Art

[0002] With the widespread adoption of renewable energy sources such as solar and wind power, the proportion of distributed generation (DGs) integrated into distribution networks continues to rise. Their integration into power systems not only helps alleviate energy pressures but also improves the reliability and economic efficiency of power supply. The integration of a large number of DGs can trigger grid fluctuations and alter the grid's structure and operation. A high proportion of DGs integrated into distribution networks can reduce power losses and provide essential voltage support during distribution network faults. However, the output of DGs is random and volatile. When integrating DGs into new distribution networks with complex and variable network topologies, the protection performance based on traditional fault characteristics needs to be further evaluated.

[0003] Existing research has addressed the impact of inverter-based distributed generation (DGs) on grid protection through a series of measures: First, planning the integration of DGs, restricting their location and capacity, etc. However, such approaches fail to fully utilize the benefits of DGs. Second, improvements have been made to protection principles. Improvements to differential protection primarily address the topology after DG integration, reconstructing the action equations based on current amplitude, impedance, and impedance angle. However, such approaches require synchronized and accurate communication and the installation of additional measurement and communication equipment. Traditional distribution network protection devices are primarily based on centralized power supply models, making them ineffective in addressing the complexities of DG integration, such as multi-source power supply and bidirectional current flow, potentially leading to protection failure or malfunction. The integration of a high proportion of DGs can lead to changes in current characteristics, and current-based protection strategies may not be able to accurately identify faults. Therefore, the development of new current measurement and fault identification algorithms is necessary.

[0004] In summary, exploring a stable power grid protection method and device that can adapt to complex power grid structures and does not rely on communication has become an urgent problem that technicians in this field need to solve. Summary of the Invention

[0005] To solve the above problems in the prior art, the present invention proposes a distribution network protection method adapted to a high proportion of distributed power supply access, the method comprising:

[0006] S1. Collect the bus voltage and feeder outlet current of the distribution network, and determine whether the distribution network has a fault based on the bus voltage and feeder outlet current. If a fault occurs, execute step S2; otherwise, re-collect data.

[0007] S2. Equivalence multiple distributed power sources on the feeder to obtain equivalent distributed power source parameters; determine the initial positive sequence voltage and output positive sequence current of the equivalent distributed power source grid connection point for short-circuit current calculation based on the equivalent distributed power source parameters;

[0008] S3. Determine whether the short-circuit current calculation converges. If so, calculate the positive-sequence current at the feeder outlet when the feeder end fault occurs based on the initial positive-sequence voltage and output positive-sequence current of the equivalent distributed power supply grid connection point. Otherwise, execute step S4.

[0009] S4. Calculate the positive sequence voltage and output positive sequence current of the equivalent distributed generation grid connection point when a feeder end fault occurs using an improved method;

[0010] S5. Calculate the positive sequence current at the feeder outlet when a feeder end fault occurs based on the positive sequence voltage and output positive sequence current of the equivalent distributed generation grid connection point;

[0011] S6. Set the protection action criterion according to the positive sequence short-circuit current at the feeder outlet when the feeder end fails, and judge whether the positive sequence current at the feeder outlet meets the action criterion. If the action criterion is met, the protection device sends a trip signal to cut off the faulty feeder. Otherwise, re-collect the distribution network bus voltage and feeder outlet current.

[0012] A distribution network protection device adapted to access of a high proportion of distributed power sources, the device comprising: an acquisition module, a first comparison module, a first calculation module, a second comparison module, a second calculation module, a third calculation module, a fourth calculation module, a third comparison module, and a control module;

[0013] The acquisition module is used to collect the distribution network bus voltage and feeder outlet current before and after the fault, and extract the corresponding positive sequence component;

[0014] The first comparison module is used to compare the sudden change amount of the phase voltage and phase current with the threshold value of the phase voltage and phase current, and if the sudden change amount of the phase voltage and phase current is greater than the threshold value of the phase voltage and phase current, the first calculation module is called;

[0015] The first calculation module is used to calculate the active reference value of the equivalent distributed power source and the equivalent line positive sequence impedance, determine the initial value of the short-circuit current calculation, and call the second comparison module;

[0016] The second comparison module is used to compare whether the derivative of the convergence function is less than one at a specific point, and if the derivative of the convergence function is less than one, call the second calculation module, otherwise call the third calculation module;

[0017] The second calculation module is used to calculate the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed power supply when a feeder end fault occurs, and call the fourth calculation module;

[0018] The third calculation module is used to calculate the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed power supply when a feeder end fault occurs using an improved method, and call the fourth calculation module;

[0019] The fourth calculation module is used to calculate the positive sequence current at the feeder outlet when a feeder end fault occurs, and call the third comparison module;

[0020] The third comparison module is used to compare whether the amplitude of the positive sequence current at the feeder outlet under the fault is greater than the current protection setting value, and if the amplitude of the positive sequence current at the feeder outlet is greater than the current protection setting value, the control module is called, otherwise the acquisition module is called;

[0021] The control module is used to control the protection device to send a trip signal to cut off the fault feeder.

[0022] Beneficial effects of the present invention:

[0023] This invention continues existing fault protection concepts for distribution networks, employing a current principle. It requires only the electrical quantities captured by the busbar protection device for protection, resulting in a simple principle, low cost, and high reliability. This protection system uses real-time calculation of short-circuit current to set the current. Faults are identified by comparing the real-time fault current with the set current, adapting to different fault types and varying numbers and capacities of distributed power sources. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a flow chart of a distribution network protection method adapted to a high proportion of distributed power access according to the present invention;

[0025] Figure 2 It is a structural schematic diagram of a multi-photovoltaic distribution network according to the present invention;

[0026] Figure 3 It is the effect diagram of the present invention. DETAILED DESCRIPTION

[0027] 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. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0028] The present invention provides a distribution network protection method and device suitable for a high proportion of distributed power sources. The method includes collecting the distribution network bus voltage and feeder outlet current, performing equivalence on multiple distributed power sources on the feeder, and determining the initial value for short-circuit current calculation based on the parameters of the equivalent distributed power sources. The fault parameters of the equivalent distributed power sources are then calculated based on the short-circuit calculation results. The positive-sequence current at the feeder outlet during a feeder end fault is calculated and the current protection is re-set. Finally, it is determined whether the positive-sequence current at the feeder outlet under the fault meets the action criteria. The method is unaffected by factors such as short-circuit transition resistance, fault location, and system operating mode, and has the advantages of high speed, good reliability, strong adaptability, and independence from communications.

[0029] A specific implementation method of a distribution network protection method adapted to high proportion of distributed power access, such as Figure 1 As shown, the method includes:

[0030] S101: Collect the bus voltage and feeder outlet current of the distribution network, and start protection when a fault occurs in the distribution network, and execute step S102;

[0031] S102: Equilibrium is performed on multiple distributed power sources on the feeder, and initial values ​​of the positive sequence voltage and output positive sequence current of the equivalent distributed power source grid connection point for short-circuit current calculation are determined based on the parameters of the equivalent distributed power source;

[0032] S103: Determine whether the short-circuit current calculation has converged. If so, calculate the equivalent distributed generation grid connection point positive sequence voltage and output positive sequence current at the time of the feeder end fault based on the initial values ​​of the equivalent distributed generation grid connection point positive sequence voltage and output positive sequence current, and execute step S105; otherwise, execute step S104;

[0033] S104: Calculate the positive sequence voltage and output positive sequence current of the equivalent distributed generation grid connection point when a feeder end fault occurs using an improved method;

[0034] S105: Calculate the positive sequence current at the feeder outlet when a feeder end fault occurs based on the positive sequence voltage and output positive sequence current of the equivalent distributed generation grid connection point;

[0035] S106: Using the positive sequence short-circuit current at the feeder outlet when the feeder end fault occurs, set the protection action criterion to determine whether the positive sequence current at the feeder outlet meets the action criterion. If so, execute step S107; otherwise, return to S101.

[0036] S107: The protection device sends a tripping signal to cut off the faulty feeder.

[0037] In the specific implementation, in step S102, the equivalent operation of multiple distributed power sources on the feeder specifically refers to equating all distributed power sources on the feeder and the line impedances therebetween to a two-port network in parallel of an equivalent distributed power source and an equivalent positive sequence impedance, specifically in the following manner:

[0038] The positive sequence voltage at the grid connection point of the equivalent distributed power source is determined as follows:

[0039]

[0040] in, is the positive sequence voltage of the grid connection point of the equivalent distributed power source;

[0041] It is the positive sequence voltage at the grid connection point of the first distributed generation on the feeder before the fault.

[0042] The active reference value of the equivalent distributed power source is determined as follows:

[0043]

[0044] in, is the active reference value of the equivalent distributed power source; is the positive sequence current output by the equivalent distributed power supply before the fault, which is determined as follows:

[0045]

[0046] in, is the positive sequence current flowing through the load before the fault; is the positive sequence current at the feeder outlet before the fault. The equivalent positive sequence impedance is determined as follows:

[0047]

[0048] in, is the equivalent line positive sequence impedance; is the positive sequence voltage across the load before the fault.

[0049] In the specific implementation, in step S102, the initial values ​​of the positive sequence voltage and output positive sequence current of the equivalent distributed power grid connection point for short-circuit current calculation are selected within the following ranges:

[0050]

[0051] in, are the initial values ​​of the positive sequence voltage and output positive sequence current of the equivalent distributed generation grid connection point respectively; is the rated voltage of the distribution network; K low K is the ratio of the starting voltage of the distributed generation low voltage ride-through to the rated voltage of the distribution network;t is the amplitude ratio of the grid connection point voltage to the rated voltage of the distribution network when the current amplitude of the distributed power supply is limited; i′ df 、i q ' f are the d-axis and q-axis components of the initial current, respectively, and are determined as follows:

[0052]

[0053] Among them, K LVRT is the low voltage ride-through coefficient; K d ' own is the amplitude ratio of the initial voltage value to the rated voltage of the distribution network; is the rated current of the equivalent distributed power supply.

[0054] In the specific implementation, in step S103, it is determined whether the short-circuit current calculation has converged according to the following steps:

[0055] S401: Establish a composite sequence network for the feeder end fault after the distributed power source on the feeder is equivalent to an equivalent distributed power source, and establish the first equation of the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed power source:

[0056]

[0057] in, k is the positive sequence voltage and output positive sequence current of the grid connection point of the equivalent distributed generation when the feeder end fails, U 、b U The fixed slope and intercept are determined according to the impedance of the composite sequence network. The specific calculation method is:

[0058]

[0059] in, is the equivalent positive-sequence and negative-sequence impedance of the upper power grid; It is the positive sequence impedance of the line between the protection device and the first distributed generation grid connection point; is the equivalent positive-sequence and negative-sequence impedance of the load; is the negative sequence impedance of the feeder; is the equivalent positive sequence voltage of the upper power grid.

[0060] S402: Based on the distributed generation low voltage ride-through control strategy, write the second equation of the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed generation:

[0061]

[0062] Among them, f LVRT () represents the low voltage ride-through control strategy of distributed power supply, and the specific expression is:

[0063]

[0064] Among them, I DGmax is the maximum output current of the distributed power supply. By combining the first equation, the convergence function is established as follows:

[0065]

[0066] S403: Determine whether the short-circuit current calculation has converged. If the following relationship is satisfied, the short-circuit current calculation has converged; otherwise, it has not converged:

[0067]

[0068] Among them, g′ LVRT () is the function g LVRT The derivative of (), is the positive sequence voltage of the distributed generation grid connection point corresponding to the intersection of the first equation and the second equation.

[0069] In the specific implementation, in step S103, the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed generation when the feeder end fails are calculated in the following manner:

[0070] S501: Set the number of short-circuit calculation rounds to k DG , the initial value is 1;

[0071] S502: The kth DG -1 round of positive sequence current for short-circuit calculation Substitute into the first equation and solve for the kth DG Positive sequence voltage at the grid connection point calculated by short circuit When k DG When 1,

[0072] S503: Substitute into the second equation and calculate the kth DG Positive sequence current for short-circuit calculation

[0073] S504: Determine the kth DG Whether the positive sequence voltage at the grid connection point calculated by the short-circuit calculation meets the convergence conditions:

[0074]

[0075] Among them, ε DGeq Is the convergence threshold value, which is an infinite decimal. If the above formula is established, it is determined that the short-circuit calculation converges. When the feeder end fault occurs, the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed power supply are: and Otherwise, let kDG =k DG +1, and return to S402.

[0076] In specific implementation, in step S104, the improved short-circuit calculation method is performed as follows:

[0077] S601: Set the number of short-circuit calculation rounds to k DG , the initial value is 1

[0078] S602: When k DG When 1, Will Substitute into the first equation and solve for the kth DG Positive sequence voltage at the grid connection point calculated by short circuit Will Substitute into the second equation and calculate the kth DG Positive sequence current for short-circuit calculation Will Substitute into the first equation and calculate the kth DG +1 round of short-circuit calculation of the grid connection point positive sequence voltage

[0079] S603: Determine the An improved positive sequence voltage:

[0080]

[0081] in, According to the k DG Round and kth DG +1 round of short-circuit calculation results determine the An improvement in positive sequence voltage.

[0082] S604: Substitute into the second equation to determine Improved positive sequence current Will Substitute into the first equation and calculate the kth DG +2 rounds of short-circuit calculation of the grid connection point positive sequence voltage Will Substitute into the second equation and calculate the kth DG +2 rounds of positive sequence current for short-circuit calculation Will Substitute into the first equation and calculate the kth DG +3 rounds of short-circuit calculation of the grid connection point positive sequence voltage

[0083] S605: Determine the An improved positive sequence voltage:

[0084]

[0085] S606: Determine whether two adjacent improved positive sequence voltages meet the convergence condition:

[0086]

[0087] If the above formula is established, it is determined that the short-circuit calculation has converged. Substituting into the second equation, we can calculate The positive sequence voltage and output positive sequence current of the equivalent distributed generation at the grid connection point under the feeder end fault are: and Otherwise, let k DG =k DG +3, and return to S502.

[0088] In the specific implementation, in step S105, the positive sequence current at the feeder outlet when a feeder end fault occurs is calculated according to the following method:

[0089]

[0090] in, is the positive sequence current at the feeder outlet when the feeder end fails, is the busbar positive sequence voltage when a fault occurs at the end of the feeder; It is the positive sequence impedance of the line between the protection device and the first distributed generation grid connection point.

[0091] In the specific implementation, in step S106, the current protection action criterion is formulated according to the following method:

[0092]

[0093] in, is the positive sequence current at the feeder outlet under fault; is the current protection setting value, calculated as follows:

[0094]

[0095] Among them, K rel is the reliability coefficient, which is generally taken as 1.2.

[0096] A distribution network protection device adapted to access of a high proportion of distributed power sources, the device comprising: an acquisition module, a first comparison module, a first calculation module, a second comparison module, a second calculation module, a third calculation module, a fourth calculation module, a third comparison module, and a control module;

[0097] The acquisition module is used to collect the distribution network bus voltage and feeder outlet current before and after the fault, and extract the corresponding positive sequence component;

[0098] The first comparison module is used to compare the sudden change amount of the phase voltage and phase current with the threshold value of the phase voltage and phase current, and if the sudden change amount of the phase voltage and phase current is greater than the threshold value of the phase voltage and phase current, the first calculation module is called;

[0099] The first calculation module is used to calculate the active reference value of the equivalent distributed power source and the equivalent line positive sequence impedance, determine the initial value of the short-circuit current calculation, and call the second comparison module;

[0100] The second comparison module is used to compare whether the derivative of the convergence function is less than one at a specific point, and if the derivative of the convergence function is less than one, call the second calculation module, otherwise call the third calculation module;

[0101] The second calculation module is used to calculate the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed power supply when a feeder end fault occurs, and call the fourth calculation module;

[0102] The third calculation module is used to calculate the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed power supply when a feeder end fault occurs using an improved method, and call the fourth calculation module;

[0103] The fourth calculation module is used to calculate the positive sequence current at the feeder outlet when a feeder end fault occurs, and call the third comparison module;

[0104] The third comparison module is used to compare whether the amplitude of the positive sequence current at the feeder outlet under the fault is greater than the current protection setting value, and if the amplitude of the positive sequence current at the feeder outlet is greater than the current protection setting value, the control module is called, otherwise the acquisition module is called;

[0105] The control module is used to control the protection device to send a trip signal to cut off the fault feeder.

[0106] The present invention provides a distribution network protection method and device suitable for a high proportion of distributed power sources. The method includes collecting the distribution network bus voltage and feeder outlet current, performing equivalence on multiple distributed power sources on the feeder, and determining the initial value for short-circuit current calculation based on the parameters of the equivalent distributed power sources. The fault parameters of the equivalent distributed power sources are then calculated based on the short-circuit calculation results. The positive-sequence current at the feeder outlet during a feeder end fault is calculated and the current protection is re-set. Finally, it is determined whether the positive-sequence current at the feeder outlet under the fault meets the action criteria. The method is unaffected by factors such as short-circuit transition resistance, fault location, and system operating mode, and has the advantages of high speed, good reliability, strong adaptability, and independence from communications.

[0107] To verify the effectiveness of the method of the present invention, Figure 2 The analysis is carried out by taking the schematic diagram of the distribution network structure containing multiple distributed power sources shown in FIG.

[0108] The upstream grid voltage level is 110 kV; the transformer capacity is 100 MVA with a transformation ratio of 11:1. The high-voltage side uses a delta connection, the low-voltage side uses a star connection, and the neutral point is grounded via a 10 Ω resistor. Each of the three feeders is 10 km long, with an impedance per unit length of (0.35 + 0.76 j) Ω. Feeder 1 connects to distributed power sources DG1, DG2, and DG3 at 2 km, 4 km, and 8 km from the busbar, respectively. During normal operation, the reference active power of each PV generator is 1 MW. Feeder 3 connects to distributed PV generator DG4 at 5 km from the busbar. During normal operation, the reference active power is 2 MW. Load LD1 is 10 MVA with a power factor of 0.879; load LD2 is 5 MVA with a power factor of 0.826; load LD3 is 3 MVA with a power factor of 0.741; load LD4 is 2 MVA with a power factor of 0.729; and load LD5 is 2 MVA with a power factor of 0.815.

[0109] In this embodiment, a two-phase ground fault and a three-phase short circuit fault are set at the end of feeder 1, and the fault occurrence time is set to 0.1s. Equivalent values ​​are calculated using the formula in step S102 to obtain a reference value for the equivalent photovoltaic active power of feeder 1 of 2.935MW and an equivalent line positive sequence impedance of (2.63 + j5.70)Ω.

[0110] Figure 3 (a) to (c) are the simulation and calculated values ​​of the fault current output by DG1, DG2, and DG3 when a two-phase ground fault occurs on feeder 1. The red dotted line represents the iterative calculated value of the fault current, and the blue solid line represents the corresponding simulation value. Figure 3 The simulated fault current values ​​for DG1, DG2, and DG3 are 139∠-8.58°A, 150∠-14.0°A, and 150∠-20.2°A, respectively, while the calculated values ​​are 141.8∠-9.1°A, 150∠-15.1°A, and 150∠-21.3°A. The maximum amplitude error is 2.8A; the minimum phase angle error is only 0.52°, and the maximum error is 1.1°. The calculated results accurately reflect the PV output short-circuit current.

[0111] Figure 3 (d) to (f) are the simulated and calculated values ​​of the fault current output by DG1, DG2, and DG3 when a three-phase short circuit fault occurs at the end of feeder 1. Figure 3The simulated fault current values ​​for DG1, DG2, and DG3 are 150∠-10.5°A, 150∠-20.0°A, and 150∠-30.0°A, respectively, while the calculated values ​​are 150∠-11.5°A, 150∠-21.9°A, and 150∠-33.1°A. The maximum amplitude error is 6.2A; the minimum phase angle error is only 1°, and the maximum error is 3.1°. The proposed method for calculating distribution network short-circuit current has high accuracy when symmetrical or asymmetrical faults occur.

[0112] The above embodiments further illustrate the purpose, technical solutions and advantages of the present invention in detail. It should be understood that the above embodiments are only preferred implementation plans of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made to the present invention within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A distribution network protection method adapted to high proportion of distributed power access, characterized in that: include: S1, collecting the bus voltage and feeder outlet current of the distribution network, judging whether the distribution network has a fault according to the bus voltage and feeder outlet current, if a fault occurs, executing step S2, otherwise re-collecting data; S2. Equivalence multiple distributed power sources on the feeder to obtain equivalent distributed power source parameters; determine the initial positive sequence voltage value and output positive sequence current value of the equivalent distributed power source grid connection point for short-circuit current calculation according to the equivalent distributed power source parameters; S3, judging whether the short-circuit current calculation converges, if converged, then calculating the positive sequence current at the feeder outlet when the feeder end fails according to the initial value of the positive sequence voltage at the grid connection point of the equivalent distributed power source and the initial value of the output positive sequence current; Otherwise, execute step S4; S4. Use the improved method to calculate the equivalent distributed generation grid connection point positive sequence voltage and output positive sequence current when the feeder end fails; S5. Calculate the positive sequence current at the feeder outlet when the feeder end fails based on the positive sequence voltage and output positive sequence current of the equivalent distributed power grid connection point; S6. Set the protection action criterion according to the positive-sequence short-circuit current at the feeder outlet when the feeder end fails, and judge whether the positive-sequence current at the feeder outlet meets the action criterion. If the action criterion is met, the protection device sends a tripping signal to cut off the faulty feeder, otherwise the distribution network bus voltage and feeder outlet current are collected again.

2. A distribution network protection method adapted to high proportion of distributed power access according to claim 1, characterized in that: Equivalence of multiple distributed power sources on the feeder includes: equating all distributed power sources on the feeder and the line impedances therebetween to a two-port network in which an equivalent distributed power source and an equivalent positive-sequence impedance are connected in parallel.

3. A distribution network protection method adapted to high proportion of distributed power access according to claim 1, characterized in that: Determining the initial values ​​of the positive-sequence voltage and output positive-sequence current of the equivalent distributed power supply grid connection point for short-circuit current calculation includes: obtaining the rated voltage of the distribution network; obtaining the amplitude ratio of the starting voltage to the rated voltage of the distribution network and the amplitude ratio of the grid connection point voltage to the rated voltage of the distribution network; calculating the initial value of the positive-sequence voltage of the equivalent distributed power supply grid connection point according to the rated voltage, the amplitude ratio of the starting voltage to the rated voltage of the distribution network and the amplitude ratio of the grid connection point voltage to the rated voltage of the distribution network; setting the low voltage ride-through coefficient, the amplitude ratio of the initial voltage value to the rated voltage of the distribution network, and the rated current of the equivalent distributed power supply; calculating the initial value of the output positive-sequence current of the equivalent distributed power supply grid connection point according to the initial value of the positive-sequence voltage of the grid connection point, the low voltage ride-through coefficient, the amplitude ratio of the initial voltage value to the rated voltage of the distribution network, and the rated current of the equivalent distributed power supply.

4. A distribution network protection method adapted to high proportion of distributed power access according to claim 1, characterized in that: Judging whether the short-circuit current calculation converges includes: establishing a first equation of the grid-connected point positive-sequence voltage and the output positive-sequence current of the equivalent distributed power source; constructing a distributed power source low-voltage ride-through control strategy, and establishing a second equation of the grid-connected point positive-sequence voltage and the output positive-sequence current of the equivalent distributed power source based on the distributed power source low-voltage ride-through control strategy; establishing a convergence function according to the first equation and the second equation; when the convergence function meets the convergence condition, the short-circuit current calculation converges, otherwise it does not converge.

5. A distribution network protection method adapted to high proportion of distributed power access according to claim 4, characterized in that: The calculation of the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed generation when the feeder end fails includes: Step 1: Set the number of short-circuit calculation rounds k DG , initialize the number of calculation rounds; Step 2: Set the kth DG -1 round of short-circuit calculation of positive sequence current Substitute into the first equation and solve for the kth DG Positive sequence voltage at the grid connection point calculated by short circuit Step 3: Substitute into the second equation and calculate the kth DG Positive sequence current for short circuit calculation Step 4: Determine the kth DG The positive sequence voltage at the grid connection point of the short-circuit calculation meets the convergence condition. If the convergence condition is met, the short-circuit calculation converges. When the feeder end fails, the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed power source are and Otherwise, let k DG =k DG +1, and return to step 2.

6. A distribution network protection method adapted to high proportion of distributed power access according to claim 4, characterized in that: The improved method for calculating the equivalent distributed generation grid connection point positive sequence voltage and output positive sequence current when the feeder end fault occurs includes: Step 1: Set the number of short-circuit calculation rounds k DG ; Initialize k DG ; Step 2: Input the positive sequence current into the first equation and solve the kth DG The positive sequence voltage of the grid connection point calculated by the short circuit calculation; Substitute the positive sequence voltage of the grid connection point into the second equation to calculate the kth DG The positive sequence current of the round short circuit calculation; DG Substitute the positive sequence current into the first equation and calculate the kth DG +1 round of short-circuit calculation of the grid-connected point positive sequence voltage; Step 3: According to the kth DG Round and kth DG +1 round of short circuit calculation results Improved positive sequence voltage; Step 4: Substituting the improved positive sequence voltage into the second equation, we get Improved positive sequence current Will Substituting into the first equation, calculate the kth DG +2 rounds of short-circuit calculation of the grid-connected point positive sequence voltage Will Substitute into the second equation and calculate the kth DG +2 rounds of positive sequence current for short-circuit calculation Will Substituting into the first equation, calculate the kth DG +3 rounds of short-circuit calculation of the grid-connected point positive sequence voltage Step 5: According to and Determine An improved positive sequence voltage; Step 6: Determine whether two adjacent improved positive sequence voltages meet the convergence condition. If so, Substituting into the second equation, we calculate The positive sequence voltage and output positive sequence current of the grid connection point of the equivalent distributed generation under the feeder end fault are calculated as follows: and Otherwise, let k DG =k DG +3 and return to step 2.

7. A distribution network protection method adapted to high proportion of distributed power access according to claim 1, characterized in that: Calculating the positive sequence current at the feeder outlet when a feeder end fault occurs includes: calculating the difference between the bus positive sequence voltage at the feeder end fault and the positive sequence voltage of the grid connection point of the equivalent distributed power source; obtaining the line positive sequence impedance between the protection device and the first distributed power source grid connection point; and taking the ratio of the calculated difference to the line positive sequence impedance as the positive sequence current at the feeder outlet when the feeder end fault occurs.

8. A distribution network protection method adapted to high proportion of distributed power access according to claim 1, characterized in that: The protection action criteria are: in, is the positive sequence current at the feeder outlet under fault; is the current protection setting value, K rel is the reliability coefficient.

9. A distribution network protection device adapted to high proportion of distributed power access, the device is used to execute a distribution network protection method adapted to high proportion of distributed power access as claimed in any one of claims 1 to 8, characterized in that: include: An acquisition module, a first comparison module, a first calculation module, a second comparison module, a second calculation module, a third calculation module, a fourth calculation module, a third comparison module and a control module; The acquisition module is used to collect the distribution network bus voltage and feeder outlet current before and after the fault, and extract the corresponding positive sequence component; The first comparison module is used to compare the sudden change amount of the phase voltage and the phase current with the threshold value of the phase voltage and the phase current, and if the sudden change amount of the phase voltage and the phase current is greater than the threshold value of the phase voltage and the phase current, the first calculation module is called; The first calculation module is used to calculate the active reference value of the equivalent distributed power source and the equivalent line positive sequence impedance, determine the initial value of the short-circuit current calculation, and call the second comparison module; The second comparison module is used to compare whether the derivative of the convergence function is less than one at a specific point, and if the derivative of the convergence function is less than one, call the second calculation module, otherwise call the third calculation module; The second calculation module is used to calculate the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed power source when the feeder end fails, and call the fourth calculation module; The third calculation module is used to calculate the grid connection point positive sequence voltage and output positive sequence current of the equivalent distributed power source when the feeder end fails by using an improved method, and call the fourth calculation module; The fourth calculation module is used to calculate the positive sequence current at the feeder outlet when the feeder end fails, and call the third comparison module; The third comparison module is used to compare whether the amplitude of the positive sequence current at the feeder outlet under the fault is greater than the current protection setting value, if the amplitude of the positive sequence current at the feeder outlet is greater than the current protection setting value, then call the control module, otherwise call the acquisition module; The control module is used to control the protection device to send a trip signal to cut off the faulty feeder.

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