CN117638868B - Distribution network direction current protection applicable boundary assessment method under distributed photovoltaic access - Google Patents

Distribution network direction current protection applicable boundary assessment method under distributed photovoltaic access Download PDF

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CN117638868B
CN117638868B CN202311375500.9A CN202311375500A CN117638868B CN 117638868 B CN117638868 B CN 117638868B CN 202311375500 A CN202311375500 A CN 202311375500A CN 117638868 B CN117638868 B CN 117638868B
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CN117638868A (en
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何佳伟
牟锐
李斌
盛亚如
谢仲润
周博昊
王文博
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Tianjin University
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Abstract

The invention discloses a method for evaluating a direction current protection applicable boundary of a distribution network under distributed photovoltaic access. Compared with the prior art, the invention improves the capacity of the power distribution network for absorbing the distributed photovoltaic so as to guide the installation of the distributed photovoltaic in actual engineering and ensure the safe operation of the power grid.

Description

Distribution network direction current protection applicable boundary assessment method under distributed photovoltaic access
Technical Field
The invention relates to the field of relay protection of new energy power systems, in particular to a method for evaluating a current protection applicable boundary in a direction of a distribution network under distributed photovoltaic access.
Background
The traditional distribution network is generally a single-power radial power supply network, and only nondirectional current protection is configured on a circuit breaker. The distributed photovoltaic access makes the power distribution network become a multi-terminal active complex network, the size and the direction of fault current can be changed, so that the reverse fault current fed out by the photovoltaic in the original current protection can cause the current protection to malfunction without selectivity, and the technical problem of malfunction is solved by adding a direction criterion in the power distribution network. However, the equivalent impedance of the photovoltaic is completely different from that of the synchronous machine power supply, and the misjudgment of the traditional power direction criterion is possibly caused, so that the problem of unreliable action exists due to the influence of the distributed photovoltaic impedance on the traditional power direction criterion.
The existing evaluation method of the distribution network protection applicable boundary under the distributed photovoltaic access is only aimed at current protection, but lacks an analysis method of the directional current protection applicable boundary. Therefore, the applicable boundary of current protection in the direction of the distribution network under the distributed photovoltaic access, namely the maximum admittance capacity of the distributed photovoltaic in the distribution network, is widely paid attention to. The existing evaluation method of the protection applicable boundary of the distribution network under the distributed photovoltaic access is only designed aiming at the current protection, such as the document "Relay protection coordination integrated optimal placement and sizing of distributed generation sources in distribution networks", considering the current protection selectivity and reliability requirements, and a current protection applicable boundary calculation model is designed, so that the maximum admittance capacity of the distributed photovoltaic in the distribution network is calculated. But this capacity is very limited. And for the problems of adaptability of the traditional power direction criterion in the distributed photovoltaic power distribution network and the applicable boundary of directional current protection, a corresponding analysis method is lacking at present. Thereby causing the original current protection to malfunction in the reverse fault. Thus, direction criteria are applied in the distribution network to solve this problem, but conventional power direction criteria are affected by distributed photovoltaic impedance and have the problem of unreliable operation. The existing evaluation method of the distribution network protection applicable boundary under the distributed photovoltaic access is only aimed at current protection, but lacks an analysis method of the directional current protection applicable boundary. Therefore, the invention provides a boundary evaluation method for current protection in the direction of a distribution network under distributed photovoltaic access.
Disclosure of Invention
Aiming at a distribution network accessed by distributed photovoltaic, the invention provides a method for evaluating the applicable boundary of current protection in the direction of the distribution network under the access of the distributed photovoltaic, analyzes the traditional direction criterion to improve the capacity of the distribution network for absorbing the distributed photovoltaic, and provides a calculation method for the maximum capacity of the distributed photovoltaic which is allowed to be accessed by the distribution network in actual engineering.
The invention is realized by the following technical scheme:
A distributed photovoltaic access lower distribution network direction current protection applicable boundary assessment method comprises the following steps:
step 1, initializing particle swarm information, wherein the particle swarm information comprises the number PsoSize of particles of a particle swarm, the number PsoDim of particles, namely the number PsoDim of distribution network access distributed photovoltaics, the maximum iteration number K max of an algorithm, and the photovoltaic capacity combination x and the photovoltaic capacity variation combination v of each particle;
Step 2, calculating to obtain a photovoltaic capacity combination x j and a photovoltaic capacity variation combination v j of the iteratively updated particles j, wherein the photovoltaic capacity combination x j and the photovoltaic capacity variation combination v j are shown in the following formula:
Where K represents the current iteration number, K max represents the maximum iteration number, j represents the number of particles, ω represents the inertial weight, ω max and ω min represent their maximum and minimum values, respectively, c 1、c2 represents the normal number for measuring the importance of the self-cognition factor and the social influence factor, r 1、r2 represents the random number uniformly distributed in the region [0,1], And/>Respectively representing the position of the photovoltaic capacity combination and the change speed of the photovoltaic capacity combination under the current iteration number k of the particle j, wherein Pbest j represents the optimal photovoltaic capacity combination currently found by the particle j, and Gbest represents the optimal photovoltaic capacity combination found by all particles so far;
step 3, checking whether the photovoltaic capacity combination of the particles j meets a constraint condition I of current protection sensitivity, wherein the constraint condition I is shown in the following formula:
In the method, in the process of the invention, And/>Respectively representing threshold values of I section, II section and III section of current protection of a line i based on a current protection setting principle,/>Represents the fault current flowing through line i when the photovoltaic of line i upstream and other feeders is out of operation and a two-phase short circuit fault occurs 15% from the head end of line i,/>Represents the fault current flowing through line i when the photovoltaic of line i upstream and other feeders is out of operation and a two-phase short circuit fault occurs at the end of line i,/>Representing a fault current flowing through line i when photovoltaic of line i upstream and other feeders is out of operation and a two-phase short circuit fault occurs at the end of the line i's lower line; n and m respectively represent the number of lines in the power distribution network and the number of other lines except for the tail end line of each feeder line;
if the constraint condition I is met, executing the step 4; if the constraint condition I is not met, executing the step 7;
Step 4, checking whether the particle j meets a constraint condition II of the direction criterion reverse fault non-misoperation, wherein the constraint condition II is shown in the following formula:
Wherein l represents the number of bus lines equipped with directional current protection in a power distribution network, K d.i represents the voltage range of a photovoltaic grid-connected point allowed under the condition that the directional criterion of a line i is reverse fault and is not misoperation, K r.i represents the voltage range of an actual grid-connected point of a downstream photovoltaic when the fault occurs in the overcurrent criterion reverse misoperation area of the line i, K umin.i、kumax.i respectively represents the minimum value and the maximum value of the range, K umin.i is the voltage of the downstream photovoltaic when the three-phase short circuit fault occurs at the near end of the back side of the line i, and K umax.i is the voltage of the downstream photovoltaic at the fault position p max_bound of the power distribution network;
If the constraint condition II is met, executing the step 5; if the constraint condition II is not satisfied, executing the step 7;
Step 5, checking whether the particle j meets a constraint condition III of the forward fault non-refusal of the direction criterion, wherein the constraint condition III is shown in the following formula:
ku.i(α)∈K'd.i.(α)i＝1,2…c；α∈(0,1]
wherein c represents the number of lines with the photovoltaic at the tail end, alpha represents the ratio of the fault distance of a forward adjacent line to the total length of the line, K u.i (alpha) represents the voltage of the photovoltaic at the tail end of the line i when the fault occurs at the position of the adjacent line alpha, and K' d.i. (alpha) represents the voltage range of the photovoltaic grid-connected point allowed under the condition that the direction criterion of the line i is forward fault;
If the constraint condition III is met, executing the step 6, and if the constraint condition III is not met, executing the step 7;
step 6, updating the individual optimum and the global optimum of the photovoltaic capacity combination, wherein the process specifically comprises the following operations:
first, the fitness of the current particle j is calculated as follows:
fitness＝SPV_1+SPV_2+…+SPV_PSODim
wherein S PV_1、SPV_2、...、SPV_PSODim represents the capacities of the photovoltaic cells pv_1, pv_2, pv_ PsoDim;
Then, comparing the fitness value of the particle j with the fitness value of the current optimal position of the particle j, and updating the individual optimal combination Pbestj of the particle j when the fitness value is larger; comparing the fitness value of the particle j with the fitness value of the global optimal position of the particle swarm, and updating the global optimal combination Gbest of the particle swarm when the fitness value is larger;
Step 7, checking whether all particles are updated, specifically, checking whether the number j of the particles which are updated reaches the total number PsoSize of the particles, if so, executing step 8; if not, j=j+1 is made, and then the step 2 is returned;
Step 8, checking whether the iteration number K reaches the maximum iteration number K max: if so, obtaining the current optimal photovoltaic capacity combination Gbest, wherein the sum of all photovoltaic capacities is the maximum admittance capacity of the photovoltaic, namely the applicable boundary of current protection; if not, let k=k+1 and j=1, and then return to step 2 for further iteration.
Compared with the prior art, the invention has the following beneficial effects:
the capacity of the power distribution network for absorbing the distributed photovoltaic is improved, so that the installation of the distributed photovoltaic in actual engineering is guided, and the safe operation of the power grid is guaranteed.
Drawings
FIG. 1 is a block diagram of a typical two-feeder distribution system;
FIG. 2 is a positive sequence fault equivalent circuit diagram of a two feeder distribution system;
fig. 3 is a flowchart of a method for evaluating the applicable boundary of the direction current protection of the distribution network under the distributed photovoltaic access.
Detailed Description
The technical scheme of the invention is further described in detail below with reference to the attached drawings and specific embodiments.
As shown in fig. 1, in a typical two-feeder distribution system, nodes C and E are connected to photovoltaic PV 1 and PV 2.CB1～CB5, respectively, representing circuit breakers installed on each segment of line.
Wherein, CB 3 and CB 5 are only provided with current protection, and the rest CB 4、CB1、CB2 adds a power direction criterion to form directional current protection on the basis of the current protection.
Taking the fault f 1 as an example, in a positive sequence equivalent circuit of the two feeder distribution system shown in fig. 2:
The power direction protection of CB 2 measures a phase angle difference between the positive sequence voltage and the positive sequence current as θ CB2, as shown in the following equation:
In the method, in the process of the invention, And/>Representing the positive sequence voltage and positive sequence current measured by CB 2, Z PV、RPV、XPV represents the equivalent impedance, resistance, and reactance of photovoltaic PV 1, respectively, and Z PV＝RPV+jXPV (j is an imaginary unit), Z BC、RBC、XBC represents the impedance, resistance, and reactance of line BC, respectively, and Z BC＝RBC+jXBC,ZΣ is the sum of the series impedance of the line and load downstream of photovoltaic PV 1, i.e., Z Σ＝ZCD+ZL1, whose magnitudes are a Σ,ZCD and Z L1 represent the impedance of line CD and load L 1, respectively.
In the case of network line parameter and photovoltaic access location determination, the numerator and denominator of formula (1) are functions C 1(RPV,XPV) and C 2(RPV,XPV) of R PV and X PV, respectively), as shown in the following formula:
Action zone and line impedance due to direction criteria In the event of a reverse failure, the method can satisfy The direction criterion does not misjudge, and the impedance Z PV of the light Fu Dengxiao at the moment falls into one of the following three areas:
The equivalent impedance Z PV of the photovoltaic after failure is shown as follows:
In the method, in the process of the invention, And/>Respectively representing the photovoltaic output fault current and the photovoltaic grid-connected point voltage, U N and I N respectively represent rated values of the photovoltaic output fault current and the photovoltaic grid-connected point voltage, I d(1) and I q(1) represent dq components of the photovoltaic output positive sequence current, k u represents the ratio of the photovoltaic grid-connected point voltage to the rated voltage, and S represents the photovoltaic capacity; as can be seen from the formula (3), when the photovoltaic capacity S is fixed, the impedance of the light Fu Dengxiao is related to the grid-connected point voltage k u after the fault;
Substituting the formula (3) into the formula (2) can calculate K u set which meets the formula (2) and is used as the allowable photovoltaic grid-connected point voltage range K d under the condition that the direction criterion is not malfunction in the reverse fault. When the protected reverse line fails, if the ratio k u∈Kd of the grid-connected point voltage of the photovoltaic at the downstream of the protection to the rated voltage is met, the reverse criterion is judged to be not false.
According to the principle, a constraint condition of reliable action of a direction criterion is designed, and an evaluation model of a direction current protection applicable boundary is established by combining typical constraint conditions of current protection sensitivity and selectivity. In consideration of the advantages of simplicity, easiness in implementation, strong adaptability, good robustness, better adaptation to a nonlinear model and the like of the particle swarm algorithm, the method adopts the particle swarm algorithm to calculate the applicable boundary of the current protection in the direction of the distribution network under the distributed photovoltaic access, namely the maximum admittance capacity of the distributed photovoltaic.
As shown in fig. 3, in the overall flow of the present invention, the specific steps are as follows:
Step 1, initializing particle swarm information, specifically, initializing the number PsoSize of particles of the particle swarm, the number PsoDim of particle dimension, namely the number of distribution network access distributed photovoltaic, and the maximum iteration number K max of an algorithm; then, initializing a position matrix x (photovoltaic capacity combination) and a speed matrix v (photovoltaic capacity variation combination) of each particle according to a formula (4), setting a current particle number j=1 and a current iteration number k=1, and starting an iteration process;
Wherein S and delta S respectively represent the capacity and capacity variation of the photovoltaic, and subscripts represent corresponding distributed photovoltaic;
Step 2, obtaining a photovoltaic capacity combination x j and a photovoltaic capacity variation combination v j of the particle j which are updated iteratively, wherein the photovoltaic capacity combination x j and the photovoltaic capacity variation combination v j are shown in the following formula:
Where K represents the current iteration number, K max represents the maximum iteration number, j represents the number of particles, ω represents the inertial weight, ω max and ω min represent their maximum and minimum values, respectively, c 1、c2 represents the normal number for measuring the importance of the self-cognition factor and the social influence factor, r 1、r2 represents the random number uniformly distributed in the region [0,1], And/>Respectively representing the position of the photovoltaic capacity combination and the change speed of the photovoltaic capacity combination under the current iteration number k of the particle j, wherein Pbest j represents the optimal photovoltaic capacity combination currently found by the particle j, and Gbest represents the optimal photovoltaic capacity combination found by all particles so far;
step 3, checking whether the photovoltaic capacity combination of the particles j meets a constraint condition I of current protection sensitivity, wherein the constraint condition I is shown in the following formula:
In the method, in the process of the invention, And/>Respectively representing threshold values of I section, II section and III section of current protection of a line i based on a current protection setting principle,/>Represents the fault current flowing through line i when the photovoltaic of line i upstream and other feeders is out of operation and a two-phase short circuit fault occurs 15% from the head end of line i,/>Represents the fault current flowing through line i when the photovoltaic of line i upstream and other feeders is out of operation and a two-phase short circuit fault occurs at the end of line i,/>Representing a fault current flowing through line i when photovoltaic of line i upstream and other feeders is out of operation and a two-phase short circuit fault occurs at the end of the line i's lower line; n and m respectively represent the number of lines in the power distribution network and the number of other lines except for the tail end line of each feeder line;
If the constraint condition I of the formula (6) is satisfied, executing the step 4; if the constraint condition I of the formula (6) is not satisfied, executing the step 7;
step 4, in order to ensure that all the directional current protection does not malfunction when the reverse circuit of the directional current protection fails, whether the particle j meets the constraint condition II of the directional criterion reverse failure and the constraint condition II is shown in the following formula:
In the formula, l represents the number of bus lines equipped with directional current protection in a power distribution network, K d.i represents the allowable photovoltaic grid-connected point voltage range under the condition that the directional criterion of a line i is reverse in fault and is not misoperation, the calculation method is obtained by substituting an equivalent impedance formula (3) of photovoltaic at the downstream of the line i into a region shown in a formula (2) as described in the formulas (1) to (3), K r.i represents the actual grid-connected point voltage range of the photovoltaic at the downstream of the fault in the overcurrent criterion reverse in the line i, K umin.i、kumax.i respectively represents the minimum value and the maximum value of the range, wherein K umin.i is the voltage of the photovoltaic at the downstream of the three-phase short-circuit fault at the near-end of the back side of the line i, and K umax.i is the voltage of the photovoltaic at the downstream of the distribution network fault position p max_bound;
if the constraint condition II of the formula (7) is satisfied, executing the step 5; if the constraint condition II of the formula (7) is not satisfied, executing the step (7);
the calculation model of the fault location p max_bound is shown as follows:
where p backward denotes the total reverse fault region of the line, Represents the fault current flowing through line i when a three-phase short-circuit fault occurs at p max_bound,/>Representing a current protection III section threshold value of the line i;
Step 5, in order to ensure that all the direction criteria are not moved when the forward adjacent lines of the direction criteria are failed, whether the particles j meet the constraint condition III of the forward failure rejection of the direction criteria is checked, and the constraint condition III is shown as the following formula: ;
ku.i(α)∈K'd.i.(α) i＝1,2…c；α∈(0,1] (9)
Wherein c represents the number of lines with the photovoltaic at the tail end, alpha represents the ratio of the fault distance of the forward adjacent line to the total length of the line, K u.i (alpha) represents the voltage of the photovoltaic at the tail end of the line i when the fault occurs at the position of the adjacent line alpha, K' d.i. (alpha) represents the voltage range of the photovoltaic grid-connected point allowed under the condition that the direction criterion of the line i is forward fault, and the calculation method refers to the calculation method of K d.i in the step 4;
If the constraint condition III of the formula (9) is satisfied, executing the step 6, and if the constraint condition III of the formula (9) is not satisfied, executing the step 7;
step 6, updating the individual optimum and the global optimum of the photovoltaic capacity combination, wherein the process specifically comprises the following operations:
first, the fitness of the current particle j is calculated as follows:
fitness＝SPV_1+SPV_2+…+SPV_PSODim (10)
wherein S PV_1、SPV_2、...、SPV_PSODim represents the capacities of the photovoltaic cells pv_1, pv_2, pv_ PsoDim;
then, comparing the fitness value of the particle j with the fitness value of the current optimal position of the particle j, and updating the optimal combination Pbestj of the particle j when the fitness value is better; comparing the fitness value of the particle j with the fitness value of the global optimal position of the particle swarm, and updating the global optimal combination Gbest of the particle swarm when the fitness value is larger;
Step 7, checking whether all particles are updated, specifically, checking whether the number j of the particles which are updated reaches the total number PsoSize of the particles, if so, executing step 8; if not, j=j+1 is made, and then the step 2 is returned;
step 8, terminating the iteration process, specifically, checking whether the termination condition is satisfied, that is, whether the iteration number K reaches the maximum iteration number K max: if so, the Gbest is the optimal photovoltaic capacity combination, wherein the sum of all photovoltaic capacities is the maximum admittance capacity of the photovoltaic, namely the applicable boundary of current protection; if not, let k=k+1 and j=1, and then return to step 2 for further iteration.
In summary, the invention provides a method for evaluating the applicable boundary of the direction current protection of the distribution network under the distributed photovoltaic access, so as to analyze the traditional direction criteria and improve the capacity of the distribution network for absorbing the distributed photovoltaic, and provide a calculation method for the maximum capacity of the distributed photovoltaic which is allowed to be accessed by the distribution network in the actual engineering, so as to guide the installation of the distributed photovoltaic in the actual engineering and ensure the safe operation of the power network.

Claims (3)

1. The utility model provides a distribution network direction current protection applicable boundary evaluation method under distributed photovoltaic access, which is characterized by comprising the following steps:
Step 1, initializing particle swarm information, wherein the particle swarm information comprises the number PsoSize of particles of the particle swarm, the number PsoDim of particles, namely the number PsoDim of distributed photovoltaic access to a power distribution network, the maximum iteration number K max of an algorithm, the photovoltaic capacity combination x and the photovoltaic capacity variation combination v of each particle, and the iteration number K is set;
Step 2, calculating to obtain the position of the photovoltaic capacity combination of the particles j with the iterative update times of k+1 Speed/>, combined with photovoltaic capacity variationThe following formula is shown:
Where K represents the current iteration number, K max represents the maximum iteration number, j represents the number of particles, ω represents the inertial weight, ω max and ω min represent their maximum and minimum values, respectively, c 1、c2 represents the normal number for measuring the importance of the self-cognition factor and the social influence factor, r 1、r2 represents the random number uniformly distributed in the region [0,1], And/>Respectively representing the position of the photovoltaic capacity combination and the change speed of the photovoltaic capacity combination under the current iteration number k of the particle j, wherein Pbest j represents the optimal photovoltaic capacity combination currently found by the particle j, and Gbest represents the optimal photovoltaic capacity combination found by all particles so far;
step 3, checking whether the photovoltaic capacity combination of the particles j meets a constraint condition I of current protection sensitivity, wherein the constraint condition I is shown in the following formula:
In the method, in the process of the invention, And/>Respectively representing threshold values of I section, II section and III section of current protection of a line i based on a current protection setting principle,/>Represents the fault current flowing through line i when the photovoltaic of line i upstream and other feeders is out of operation and a two-phase short circuit fault occurs 15% from the head end of line i,/>Represents the fault current flowing through line i when the photovoltaic of line i upstream and other feeders is out of operation and a two-phase short circuit fault occurs at the end of line i,/>The method comprises the steps that fault current flowing through a line i when photovoltaic of other feeder lines on the upstream of the line i is out of operation and a two-phase short circuit fault occurs at the tail end of the next line of the line i is represented, and n and m respectively represent the number of lines in the power distribution network and the number of other lines except for the tail end line of each feeder line;
if the constraint condition I is met, executing the step 4; if the constraint condition I is not met, executing the step 7;
Step 4, checking whether the particle j meets a constraint condition II of the direction criterion reverse fault non-misoperation, wherein the constraint condition II is shown in the following formula:
Wherein l represents the number of bus lines equipped with directional current protection in a power distribution network, K d.i represents the voltage range of a photovoltaic grid-connected point allowed under the condition that the directional criterion of a line i is reverse fault and is not misoperation, K r.i represents the voltage range of an actual grid-connected point of a downstream photovoltaic when the fault occurs in the overcurrent criterion reverse misoperation area of the line i, K umin.i、kumax.i respectively represents the minimum value and the maximum value of the range, K umin.i is the voltage of the downstream photovoltaic when the three-phase short circuit fault occurs at the near end of the back side of the line i, and K umax.i is the voltage of the downstream photovoltaic at the fault position p max_bound of the power distribution network;
If the constraint condition II is met, executing the step 5; if the constraint condition II is not satisfied, executing the step 7;
Step 5, checking whether the particle j meets a constraint condition III of the forward fault non-refusal of the direction criterion, wherein the constraint condition III is shown in the following formula:
ku.i(α)∈K'd.i.(α)i＝1,2…c；α∈(0,1]
wherein c represents the number of lines with the photovoltaic at the tail end, alpha represents the ratio of the fault distance of a forward adjacent line to the total length of the line, K u.i (alpha) represents the voltage of the photovoltaic at the tail end of the line i when the fault occurs at the position of the adjacent line alpha, and K' d.i. (alpha) represents the voltage range of the photovoltaic grid-connected point allowed under the condition that the direction criterion of the line i is forward fault;
If the constraint condition III is met, executing the step 6, and if the constraint condition III is not met, executing the step 7;
step 6, updating the individual optimum and the global optimum of the photovoltaic capacity combination, wherein the process specifically comprises the following operations:
first, the fitness of the current particle j is calculated as follows:
fitness＝SPV_1+SPV_2+…+SPV_PSODim
wherein S PV_1、SPV_2、...、SPV_PSODim represents the capacities of the photovoltaic cells pv_1, pv_2, pv_ PsoDim;
Then, comparing the fitness value of the particle j with the fitness value of the current optimal position of the particle j, and updating the individual optimal combination Pbestj of the particle j when the fitness value is larger; comparing the fitness value of the particle j with the fitness value of the global optimal position of the particle swarm, and updating the global optimal combination Gbest of the particle swarm when the fitness value is larger;
Step 7, checking whether all particles are updated, specifically, checking whether the number j of the particles which are updated reaches the total number PsoSize of the particles, if so, executing step 8; if not, j=j+1 is made, and then the step 2 is returned;
Step 8, checking whether the iteration number K reaches the maximum iteration number K max: if so, obtaining the current optimal photovoltaic capacity combination Gbest, wherein the sum of all photovoltaic capacities is the maximum admittance capacity of the photovoltaic, namely the applicable boundary of current protection; if not, let k=k+1 and j=1, and then return to step 2 for further iteration.
2. The method for evaluating the applicable boundary of the directional current protection of the distribution network under the distributed photovoltaic access according to claim 1, wherein the photovoltaic capacity combination x and the photovoltaic capacity variation combination v are represented by the following formula:
In the formula, S and delta S respectively represent the capacity and capacity variation of the photovoltaic, and subscripts represent corresponding distributed photovoltaic.
3. The method for evaluating the directional current protection applicable boundary of a distributed photovoltaic access lower power distribution network according to claim 1,
The calculation model of the fault location p max_bound is shown as follows:
where p backward denotes the total reverse fault region of the line, Represents the fault current flowing through line i when a three-phase short-circuit fault occurs at p max_bound,/>Representing the current protection segment iii threshold for line i.
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