A tie line backup protection method suitable for double petal structure distribution network

Abstract

The present invention relates to a tie line backup protection method suitable for a double-petal structure distribution network, which comprises the following steps: step 1: obtaining line parameters of the double-petal distribution network; step 2: identifying the petal ring network where the fault is located; step 3: determining the fault tie line; and step 4: handling special situations. Compared with traditional backup protection, the protection method has a faster fault isolation speed and reduces the number of tie line acquisition devices, has good adaptability and economy, and is suitable for promotion and application in complex petal-shaped distribution networks.

Description

Technical Field

[0001] The invention belongs to the field of relay protection in the field of electric power engineering, and in particular relates to a tie line backup protection method suitable for a double-petal structure distribution network. Background Art

[0002] With the rapid growth of power load density in large and medium-sized cities, the pressure on urban power supply is becoming increasingly prominent. The distribution network is directly connected to users, and its failure is the main factor affecting the quality and reliability of urban power supply. In order to meet the requirements of users in high-density load areas in cities for power supply reliability, various parts of my country are actively promoting the construction of high-reliability smart distribution networks, and building double-petal distribution networks with closed-loop operation in urban areas with high load density and high power supply reliability requirements. The topology structure is detailed in the attached Figure 1 In normal operation, the power grid adopts a closed-loop operation mode with multiple power supplies to meet the "N-1" principle. Each switch station adopts a single-mother segmented wiring method to achieve a standby automatic switching function, and its power supply reliability rate reaches 99.9999%.

[0003] While researchers are seeking new topological distribution networks, they are also conducting research on relay protection for these power grids. Since short-circuit faults in the network often occur on the tie lines, the misoperation and refusal of tie line protection may cause multiple feeder tripping and large-scale power outages, thus endangering the safe operation of the entire power grid. Therefore, the tie lines between switch stations play a vital role.

[0004] Traditional directional current protection commonly installed in interconnection lines has a time difference in the petal-shaped power grid, resulting in a long time to completely isolate the fault when a fault occurs in the interconnection line close to the substation. Current protection usually requires the installation of measuring elements such as mutual inductors and power direction elements on both sides of the interconnection line, which has a high configuration cost. In addition, traditional distribution network protection methods are mostly based on information from the protection installation site itself, with limited information utilization and great limitations, making it difficult to meet the requirements of protection and control of complex distribution networks.

[0005] In the prior art, for example, CN112595930A, a petal-type urban power grid regional backup protection method containing distributed power sources, discloses a petal-type urban power grid regional backup protection method containing distributed power sources based on multi-source information based on the analysis of the fault characteristics of the petal-type urban power grid, but there are problems such as complex protection logic and large amount of calculation. Another example is CN201811054717.9, a petal-type power grid protection method suitable for distributed power source access, which discloses a microcomputer protection method that can effectively locate the fault location of the power grid node, but it requires the installation of acquisition equipment at both ends of the interconnection line, and the protection configuration cost is relatively high. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies of the above-mentioned prior art and provide a tie line backup protection method suitable for a double-petal structure distribution network. Compared with traditional backup protection, this protection method has a faster fault isolation speed and reduces the number of tie line acquisition devices. It has good adaptability and economy and is suitable for promotion and application in complex petal-shaped distribution networks.

[0007] The technical solution adopted by the present invention to solve the technical problem is: comprising the following steps:

[0008] Step 1: Obtain the line parameters of the double petal distribution network;

[0009] Step 2: Identify the petal ring network where the fault is located;

[0010] Step 3: Determine the fault contact line;

[0011] Step 4: Handling special cases.

[0012] Furthermore, in step 1, the parameters include the unit line impedance and line length of each section of the connecting line and feeder line in each petal ring network.

[0013] Further, step 2 comprises the following steps,

[0014] (1) Determine the generalized nodes of the double-petal ring network;

[0015] (2) Install a current transformer with the same transformation ratio and characteristics on the line connected to the generalized node area, process the measured current signal to obtain the current phasor, and upload it to the protection master station;

[0016] (3) Construct the protection criteria for multi-terminal differential protection applicable to double-petal distribution network;

[0017] (4) Determine whether the protection master station meets the protection criteria of the multi-terminal differential protection. The judgment result is represented by the multi-terminal differential protection start signal Flag. In the initial state, the Flag signal is set to zero. When the protection criteria are met, the Flag is set to 1.

[0018] Furthermore, in step (1), all the tie lines and switch station buses in each petal ring in the petal-shaped distribution network are regarded as a virtual node and defined as a generalized node.

[0019] Furthermore, in step (2), a full-cycle Fourier algorithm is used to obtain the amplitude and angle of the current.

[0020] Furthermore, in step (3), the protection criterion of the multi-terminal differential protection is:

[0021]

[0022] In the formula, is the current phasor of the line connected to the generalized node area, K res is the braking coefficient, I set It is the setting value of action current.

[0023] Furthermore, the starting current of the multi-terminal differential protection, i.e., the action current setting value I set The maximum unbalanced current I generated when the external line of the generalized node is short-circuited needs to be avoided kmax , the action current setting value I set Calculate according to the following formula

[0024] I set =K rel ×I nub.max =K rel ×0.1I kmax (2);

[0025] In the formula, K rel is the reliability coefficient (usually 1.3); I kmax It is the maximum short-circuit current flowing when any line outside the generalized node area is short-circuited.

[0026] At the same time, when determining the multi-terminal differential protection, the following calibration sensitivity coefficient should also be satisfied, that is:

[0027]

[0028] In the formula, K sen It is called the sensitivity coefficient, I kmin It is the minimum short-circuit current threshold when a tie line fault occurs in the generalized node area.

[0029] Further, step 3 includes the following steps,

[0030] S1, install voltage and current transformers on one side of each section of the tie line, measure its voltage and current, and then use the Fourier algorithm to extract the voltage and current amplitude U, I and phase angle Utilization

[0031]

[0032] Obtain active power, and transmit the active power data to the protection master station, which records it;

[0033] S2, when a short circuit occurs in the tie line in the petal ring network, the Flag is set to 1, and the active power data recorded by the protection master before the Flag is set to 1 is obtained, which is recorded as P i1 , i = 2, 3…, n, where the subscript i represents the tie line number, and n is the number of tie line segments contained in the petal ring network;

[0034] S3, the protection master station will obtain the active power data of each section of the tie line again after a certain delay, recorded as P` i1 , i = 2, 3…, n, and compare it with the active power data P before the fault i1 The difference is obtained to obtain the change in active power, which is recorded as △P i1 , i=2,3…n.

[0035] S4, judge the positive or negative of the active power change of each section of the interconnection line, determine the faulty interconnection line, and the protection master station sends a signal to the circuit breakers at both ends of the faulty interconnection line to trip the corresponding circuit breakers.

[0036] S5, observe the status of the multi-terminal differential protection start signal Flag; if the Flag returns to zero after the circuit breaker is actuated, the protection logic ends; if the Flag is 1 after the circuit breaker is actuated, the circuit breaker is additionally tripped to isolate the switch station bus fault.

[0037] Furthermore, in step S3, the delay is 0.1s; in step S4, the active power data of each section of the tie line is obtained and subtracted from the active power data before the fault. If the difference is less than or equal to zero, the section of the tie line is the tie line with the fault. If the difference is greater than zero, continue to judge other tie lines.

[0038] Furthermore, step 4 is a special case handling, wherein when a busbar fault occurs and the busbar main protection fails, the fault on the busbar cannot be effectively isolated and needs to be considered separately.

[0039] The present invention designs a backup protection configuration scheme for the short-circuit fault of the tie line in the double-petal distribution network, and comprehensively determines the tie line fault location by using the multi-terminal differential protection based on regional fault information and the change of the active power at the same end of the tie line. The idea of ​​the present invention is to first use the generalized node external branch current to identify the petal ring network where the faulty tie line is located, then use the change of the active power of the tie line to determine the faulty tie line, and finally protect the main station to send a signal to the circuit breakers at both ends of the faulty tie line to trip the corresponding circuit breaker.

[0040] Compared with traditional backup protection, the present invention has a faster fault isolation speed and reduces the number of tie line acquisition devices. It has good adaptability and economy and is suitable for promotion and application in complex double-petal distribution networks. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic diagram of the topological structure of a double-petal type distribution network;

[0042] Figure 2 It is a schematic diagram of the process of the protection method of the present invention;

[0043] Figure 3It is a schematic diagram of the process of step 2 of the present invention;

[0044] Figure 4 It is a schematic diagram of the process of step 3 of the present invention;

[0045] Figure 5 It is a diagram of the left petal ring network topology according to an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The present invention is further described in detail below in conjunction with embodiments, but the scope of the present invention is not limited thereto, and the scope of protection required is recorded in the claims.

[0047] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present application and its application or use. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0048] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0049] Unless otherwise specifically stated, the relative arrangement, numerical expressions and numerical values ​​of the parts and steps set forth in these embodiments do not limit the scope of the present application. At the same time, it should be understood that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to the actual proportional relationship. The technology, method and equipment known to those of ordinary skill in the relevant field may not be discussed in detail, but in appropriate cases, the technology, method and equipment should be considered as a part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following drawings, so that once a certain item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0050] The idea of ​​the present invention is to first use the branch current outside the generalized node to identify the petal ring network where the faulty tie line is located, then use the change in the active power of the tie line to determine the faulty tie line, and finally the protection master station sends a signal to the circuit breakers at both ends of the faulty tie line to trip the corresponding circuit breaker. Figure 2 The specific steps are as follows:

[0051] The present invention provides a tie line backup protection method applicable to a double petal structure distribution network. Since the left and right petal ring network parameters are symmetrical and equal, this embodiment takes the left petal ring network as the research object. Figure 2 This embodiment includes the following steps.

[0052] Step 1: Obtain the line parameters of the double-petal distribution network. In step 1, the parameters include the unit line impedance and line length of each section of the tie line and feeder line in each petal ring network. The topological structure of the double-petal distribution network in this embodiment is as follows: Figure 5 Among them, the 110 / 10kV substation adopts three-phase voltage source simulation; the interconnection line between each switch station adopts 400mm 2 The cable can be simplified to a π model in the simulation model; the line distance between each two switch stations is considered to be 1km, the total load carried by each switch station is about 4MW, and the load power factor is 0.9. The grounding method is to ground the neutral point through a small resistor, and the resistance value is set to 10Ω.

[0053] Step 2: Identify the petal ring network where the fault is located; refer to Figure 3 Step 2 includes the following steps, see step (1) to step (4).

[0054] (1) Determine the generalized nodes of the double-petal ring network; all the tie lines and switch station buses in each petal ring of the petal-shaped distribution network are regarded as a virtual node and defined as a generalized node.

[0055] (2) Install a current transformer with the same transformation ratio and characteristics on the line connected to the generalized node area, process the measured current signal to obtain the current phasor, and transmit it to the protection master station; the protection master station is a part of the digital relay protection system, and the specific protection master station is not explicitly mentioned here. In step (2), the full-cycle Fourier algorithm is used to obtain the current amplitude and angle.

[0056] (3) Constructing a protection criterion for multi-terminal differential protection applicable to the double-petal distribution network; in step (3), the protection criterion for the multi-terminal differential protection is:

[0057]

[0058] In the formula, is the current phasor of the line connected to the generalized node area, K resis the braking coefficient, I set It is the setting value of action current.

[0059] The starting current of the multi-terminal differential protection is the setting value of the operating current I set The maximum unbalanced current I generated when the external line of the generalized node is short-circuited needs to be avoided kmax In engineering, it can be calculated as follows:

[0060] I set =K rel ×I nub.max =K rel ×0.1I kmax (2);

[0061] In the formula, K rel is the reliability coefficient, generally taken as 1.3; I kmax It is the maximum short-circuit current flowing when any line outside the generalized node area is short-circuited.

[0062] At the same time, when determining the multi-terminal differential protection, the following calibration sensitivity coefficient should also be satisfied, that is:

[0063]

[0064] In the formula, K sen It is called the sensitivity coefficient, I kmin It is the minimum short-circuit current threshold when a tie line fault occurs in the generalized node area.

[0065] In this embodiment, using the distribution network line parameters in step 1, according to formula (2) and formula (3), when a three-phase short circuit fault occurs outside the generalized node, according to ① avoiding the maximum unbalanced current generated when the generalized node area is short-circuited externally, it is known that the unbalanced current generated is 0.5 kA. According to formula (2), I set It is 1.3*0.1*0.5=0.065kA, and the operating current of the multi-terminal differential protection is 0.065kA.

[0066] In addition, the minimum short-circuit current I occurring in the generalized node kmin The current is 10kA. According to formula (4), the sensitivity coefficient is about 153, which is much larger than 2 and meets the sensitivity requirement.

[0067] (4) Determine whether the protection master station meets the protection criteria of the multi-terminal differential protection. The judgment result is represented by the multi-terminal differential protection start signal Flag. In the initial state, the Flag signal is set to zero. When the protection criteria are met, the Flag is set to 1.

[0068] Step 3: After determining the location of the faulty petal ring network, it is necessary to determine the specific location of the fault. The specific steps are as follows: Figure 4As shown, it specifically includes steps S1-S4.

[0069] S1, install voltage and current transformers on one side of each section of the tie line, measure its voltage and current, and then use the Fourier algorithm to extract the voltage and current amplitude U, I and phase angle Utilization

[0070]

[0071] Obtain active power, and transmit the active power data to the protection master station, which records it;

[0072] S2, when a short circuit occurs in the connecting line in the petal ring network, the Flag is set to 1, and the active power data P of the last transmission before the Flag is set to 1 recorded by the protection master station is obtained. i1 , i=2,3…n; , n is 5 in this embodiment.

[0073] S3, the protection master station will take the active power data of each section of the tie line again after a certain delay, and make a difference with the active power data P1 before the fault to obtain the active power change. In step S3, the delay is 0.1s; in step S4, take the active power data of each section of the tie line, and make a difference with the active power data P1 before the fault. When the difference is less than or equal to zero, the section of the tie line is the tie line with the confirmed fault. When the difference is greater than zero, continue to judge other tie lines.

[0074] S4, judge the positive or negative of the active power change of each section of the interconnection line, determine the faulty interconnection line, and the protection master station sends a signal to the circuit breakers at both ends of the faulty interconnection line to trip the corresponding circuit breakers.

[0075] The following will refer to Figure 4 , further explain step 2 and step 3,

[0076] This embodiment takes the left petal ring network as the research object, the initial time is t=0, and the power at the left end of each section of the connecting line is collected.

[0077] ① If a short circuit fault occurs at k1, the protection master station meets the protection criteria of multi-terminal differential protection, the Flag is set to 1, and the active power data P recorded by the protection master station before the Flag is set to 1 is obtained. 21 , P 31 , P 41 and P 51 , the protection master station will obtain the active power data P' of each section of the tie line again after a delay of △t1 = 0.1s 21 , P' 31 , P' 41 and P' 51 , and subtract it from the first data to get the active power change, that is, △Pi1 (i=2,3,4,5), these power data are shown in Appendix 1. First, determine △P 21 Is it greater than zero? If it is less than zero, the circuit breakers at both ends of the tie line L1 will trip after a delay of △t2 (which can be set to 0.2s according to the change characteristics of the active power of the tie line after the fault).

[0078] ② If a short circuit fault occurs at k2, the protection master station meets the protection criteria of multi-terminal differential protection, the Flag is set to 1, and the active power data P recorded by the protection master station before the Flag is set to 1 is obtained. i1 , P 31 , P 41 and P 51 , the protection master station will obtain the active power data P' of each section of the tie line again after a delay of △t1 = 0.1s 21 , P' 31 , P' 41 and P' 51 , and subtract it from the first data to get the active power change, that is, △P i1 (i=2,3,4,5), these power data are shown in Appendix 1. First, determine △P 21 Is it greater than zero? If the result is greater than zero, then △P 31 Is it greater than zero? If it is less than zero, the circuit breakers at both ends of the tie line L2 will trip after a delay of △t2 (which can be set to 0.2s according to the change characteristics of the active power of the tie line after the fault).

[0079] ③ If a short circuit fault occurs at k3, the protection master station meets the protection criteria of multi-terminal differential protection, the Flag is set to 1, and the active power data P recorded by the protection master station before the Flag is set to 1 is obtained. 21 , P 31 , P 41 and P 51 , the protection master station will take the active power data of each section of the tie line again after a delay of △t1=0.1s, and make a difference with the data taken for the first time to obtain the active power change, that is, △P i1 (i=2,3,4,5), these power data are shown in Appendix 1. First, determine △P 21 Is it greater than zero? If the result is greater than zero, then △P 31 Is it greater than zero? If the result is greater than zero, then △P 41 Is it greater than zero? If it is less than zero, the circuit breakers at both ends of the tie line L3 will trip after a delay of △t2 (which can be set to 0.2s according to the change characteristics of the active power of the tie line after the fault).

[0080] ④ If a short circuit fault occurs at k4, the protection master station meets the protection criteria of multi-terminal differential protection, the Flag is set to 1, and the active power data P recorded by the protection master station before the Flag is set to 1 is obtained. 21 , P 31 , P 41 and P 51 , the protection master station will take the active power data of each section of the tie line again after a delay of △t1=0.1s, and make a difference with the data taken for the first time to obtain the active power change, that is, △P i1 (i=2,3,4,5), these power data are shown in Appendix 1. First, determine △P 21 Is it greater than zero? If the result is greater than zero, then △P 31 Is it greater than zero? If the result is greater than zero, then △P 41 Is it greater than zero? If the result is greater than zero, then △P 51 Is it greater than zero? If it is less than zero, the circuit breakers at both ends of the tie line L4 will trip after a delay of △t2 (which can be set to 0.2s according to the change characteristics of the active power of the tie line after the fault).

[0081] ⑤ If a short circuit fault occurs at k5, the protection master station meets the protection criteria of multi-terminal differential protection, the Flag is set to 1, and the active power data P recorded by the protection master station before the Flag is set to 1 is obtained. 21 , P 31 , P 41 and P 51 , the protection master station will take the active power data of each section of the tie line again after a delay of △t1=0.1s, and make a difference with the data taken for the first time to obtain the active power change, that is, △P i1 (i=2,3,4,5), these power data are shown in Appendix 1. First, determine △P 21 Is it greater than zero? If the result is greater than zero, then △P 31 Is it greater than zero? If the result is greater than zero, then △P 41 Is it greater than zero? If the result is greater than zero, then △P 51 Is it greater than zero? If it is greater than zero, the circuit breakers at both ends of the tie line L5 will trip after a delay of △t2 (which can be set to 0.2s according to the change characteristics of the active power of the tie line after the fault).

[0082] ⑥ If a short circuit occurs on the busbar of switch station B, the protection master station meets the protection criteria of multi-terminal differential protection, the Flag is set to 1, and the active power data P recorded by the protection master station before the Flag is set to 1 is obtained. 21 , P 31 , P 41 and P 51, the protection master station will take the active power data of each section of the tie line again after a delay of △t1=0.1s, and make a difference with the data taken for the first time to obtain the active power change, that is, △P i1 (i=2,3,4,5), these power data are shown in Appendix 1. First, determine △P 21 Is it greater than zero? If the result is greater than zero, then △P 31 Is it greater than zero? If it is less than zero, the circuit breakers at both ends of the tie line L2 will trip after a delay of △t2 (which can be set to 0.2s according to the change characteristics of the active power of the tie line after the fault).

[0083] Step 4: Determine and handle special situations; the special situation described in step 4 is that when a busbar fault occurs and the busbar main protection fails, the fault on the busbar cannot be effectively isolated and needs to be considered separately.

[0084] After the circuit breaker trips, observe the status of the multi-terminal differential protection start signal Flag; if the Flag returns to zero after the circuit breaker is actuated, the protection logic ends; if the Flag is 1 after the circuit breaker is actuated, the circuit breaker is additionally tripped to isolate the switch station busbar fault.

[0085] In this embodiment,

[0086] ① If a short circuit fault occurs at k1, the circuit breakers at both ends of the interconnection line L1 will trip, and the Flag will return to zero after the circuit breaker is actuated, and the protection logic ends.

[0087] ② If a short circuit fault occurs at k2, the circuit breakers at both ends of the interconnection line L2 will trip, and the Flag will return to zero after the circuit breaker is actuated, and the protection logic ends.

[0088] ③ If a short circuit fault occurs at k3, the circuit breakers at both ends of the interconnection line L3 will trip, and the Flag will return to zero after the circuit breaker is actuated, and the protection logic ends.

[0089] ④ If a short circuit fault occurs at k3, the circuit breakers at both ends of the interconnection line L4 will trip, and the Flag will return to zero after the circuit breaker is actuated, and the protection logic ends.

[0090] ⑤ If a short circuit fault occurs at k3, the circuit breakers at both ends of the interconnection line L5 will trip, and the Flag will return to zero after the circuit breaker is actuated, and the protection logic ends.

[0091] ⑥ If a short circuit occurs in the busbar of switch station B, the circuit breakers at both ends of the tie line L2 will trip. After the circuit breaker is actuated, Flag is 1, and the circuit breaker connecting the busbar B of switch station and the tie line L3 will trip.

[0092] Table 1 Setting and sensitivity verification results of longitudinal current differential protection

[0093] Unit: MVA

[0094]

[0095] The backup protection action result in this embodiment is shown in Appendix 2. It can be seen from the table that the fault occurs at 0.4s. When the main protection fails to operate, the present invention can trip the corresponding circuit breaker within 0.3s. The fault isolation time is independent of the fault type and the location where the fault occurs.

[0096] Table 2 Setting and sensitivity verification results of definite time overcurrent protection

[0097]

[0098] Based on the double-petal distribution network structure, the present invention proposes an improved backup protection scheme for various faults on the tie line. The multi-terminal differential protection based on regional fault information and the change of active power at one end of the tie line are used to comprehensively determine the fault location of the tie line. Compared with the traditional backup protection, the protection scheme has a faster fault isolation speed and reduces the number of tie line acquisition equipment. It has good adaptability and economy and is suitable for promotion and application in complex petal-shaped distribution networks.

Claims

1. A tie line backup protection method applicable to a double-petal structure distribution network, characterized in that: It includes the following steps, Step 1: Obtain the line parameters of the double petal distribution network; Step 2: Identify the petal ring network where the fault is located; The following steps are included: (1) Determine the generalized nodes of the double-petal ring network; (2) Install current transformers with the same transformation ratio and characteristics on the lines connected to the generalized node area, process the measured current signals to obtain current phasors, and upload them to the protection master station; (3) Construct the protection criteria for multi-terminal differential protection applicable to double-petal distribution network; (4) Determine whether the protection master station meets the protection criteria of the multi-terminal differential protection. The judgment result is represented by the multi-terminal differential protection start signal Flag. In the initial state, the Flag signal is set to zero. When the protection criteria are met, the Flag is set to 1. Step 3: Determine the fault contact line; The following steps are included: S1, install voltage and current transformers on one side of each section of the tie line, measure its voltage and current, and then use the Fourier algorithm to extract the voltage and current amplitude U, I and phase angle , using the formula （4） Obtain active power, and transmit the active power data to the protection master station, which records it; S2, when a short circuit occurs in the tie line in the petal ring network, the Flag is set to 1, and the active power data recorded by the protection master before the Flag is set to 1 is obtained, which is recorded as , where the subscript i represents the tie line number, and n is the number of tie line segments contained in the petal ring network; S3, the protection master station will obtain the active power data of each section of the tie line again after a certain delay, recorded as and compared with the active power data before the fault Difference is made to obtain the change in active power, which is recorded as ; S4, judging the positive and negative of the active power change of each section of the tie line, determining the faulty tie line, and the protection master station sends a signal to the circuit breakers at both ends of the faulty tie line to trip the corresponding circuit breaker; S5, observe the state of the multi-terminal differential protection start signal Flag; if the Flag returns to zero after the circuit breaker is actuated, the protection logic ends; if the Flag is 1 after the circuit breaker is actuated, the circuit breaker is additionally tripped to isolate the switch station bus fault; Step 4: Special case handling.

2. A tie line backup protection method suitable for a double-petal structure distribution network according to claim 1, characterized in that: In step 1, the parameters include the unit line impedance and line length of each section of the connecting line and feeder line in each petal ring network.

3. A tie line backup protection method suitable for a double-petal structure distribution network according to claim 1, characterized in that: In step (1), all the tie lines and switch station buses in each petal ring of the petal-shaped distribution network are regarded as a virtual node and defined as a generalized node.

4. The tie line backup protection method applicable to a double-petal structure distribution network according to claim 1, characterized in that: In step (2), a full-cycle Fourier algorithm is used to obtain the amplitude and angle of the current.

5. The tie line backup protection method applicable to a double-petal structure distribution network according to claim 1, characterized in that: In step (3), the protection criterion of the multi-terminal differential protection is: （1） In the formula, is the current phasor of the line connected to the generalized node area, is the braking coefficient, is the action current setting value; the starting current of the multi-terminal differential protection is the action current setting value The maximum unbalanced current generated when the external line of the generalized node is short-circuited must be avoided ; The action current setting value is calculated as follows: （2） In the formula, is the reliability coefficient; is the maximum unbalanced current generated when the external line of the generalized node is short-circuited, To define symbols, used to represent .

6. The tie line backup protection method applicable to a double-petal structure distribution network according to claim 1, characterized in that: In step (3), when determining the multi-terminal differential protection, the following calibration sensitivity coefficient should also be satisfied, that is: （3） In the formula, is called the sensitivity coefficient, It is the minimum short-circuit current threshold when a tie line fault occurs in the generalized node area.

7. The tie line backup protection method applicable to a double-petal structure distribution network according to claim 1, characterized in that: In step S3, the delay is 0.1s; In step S4, the active power data of each section of the tie line is obtained and subtracted from the active power data before the fault. If the difference is less than or equal to zero, the previous section of the tie line is the tie line with the fault. If the difference is greater than zero, continue to judge other tie lines.

8. The tie line backup protection method applicable to a double-petal structure distribution network according to claim 1, characterized in that: Step 4 is for handling special situations. The special situation is that when a busbar fault occurs and the busbar main protection fails, the fault on the busbar cannot be effectively isolated and needs to be considered separately.

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