CN113889989A - Link backup protection method suitable for power distribution network with double petal structure - Google Patents
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Abstract
The invention relates to a tie line backup protection method suitable for a power distribution network with a double-petal structure, which comprises the following steps of 1: acquiring line parameters of a double-petal power distribution network; step 2: judging a petal ring network where the fault is located; and step 3: determining a fault tie line; and 4, step 4: special case handling. Compared with the traditional backup protection, the protection method has the advantages that the fault isolation speed is higher, the number of the tie line acquisition equipment is reduced, the adaptability and the economy are good, and the method is suitable for being popularized and applied in a complex petal-shaped power distribution network.
Description
Technical Field
The invention belongs to the field of relay protection in the field of power engineering, and particularly relates to a tie line backup protection method suitable for a power distribution network with a double petal structure.
Background
With the rapid increase of the power load density of large and medium-sized cities, the power supply pressure of the cities is increasingly prominent. The power distribution network is directly connected with users, and faults of the power distribution network are main factors influencing the power supply quality and reliability of the urban power grid. In order to meet the requirement of users in urban high-density load areas on power supply reliability, the construction of a high-reliability intelligent power distribution network is actively promoted in various parts of China, a double-petal power distribution network which operates in a closed loop mode is constructed in urban areas with high load density and high power supply reliability, and the topological structure of the double-petal power distribution network is shown in the attached drawing 1 in detail. Under a normal operation mode, the power grid adopts a closed-loop operation mode and multi-power supply, the principle of 'N-1' is met, a single bus sectional wiring mode is adopted in each switch station, the spare power automatic switching function can be realized, and the power supply reliability reaches 99.9999%.
While searching for a power distribution network with a new topological structure, researchers also develop relevant research on relay protection aiming at the power distribution network. Because short-circuit faults in the network mostly occur on the tie lines, misoperation and refusal of the tie line protection can cause multi-path feeder tripping and large-range power failure events, so that the operation safety of the whole power grid is endangered, and therefore the tie lines among the switch stations play a vital role.
The traditional directional current protection is generally installed in a connecting line, time level difference exists in a petal-shaped power grid, so that when the connecting line close to a transformer substation fails, the fault can be completely isolated after a long time, and the current protection generally needs to install measuring elements such as a mutual inductor and the like and power directional elements on two sides of the connecting line, so that the configuration cost is high; in addition, most of the traditional power distribution network protection methods act on the basis of information of the installation position of the protection device, the information utilization is limited, great limitation exists, and the requirement of complex power distribution network protection control is difficult to meet.
In the prior art, for example, a patent of a backup protection method for a petal-type urban power grid area containing a distributed power supply in CN112595930A discloses a backup protection method for a petal-type urban power grid area based on analysis of fault characteristics of the petal-type urban power grid, and provides a backup protection method for a petal-type urban power grid area containing a distributed power supply based on multi-source information, which has the problems of complex protection logic, large calculation amount and the like. Also, for example, CN 201811054717.9 patent of a petal type power grid protection method suitable for distributed power access discloses a microcomputer protection method, which can effectively locate a fault location of a power grid node, but requires that acquisition devices be installed at both ends of a tie line, so that the protection configuration cost is high.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a tie line backup protection method suitable for a power distribution network with a double-petal structure.
The technical scheme adopted by the invention for solving the technical problems is as follows: comprises the following steps of (a) carrying out,
step 1: acquiring line parameters of a double-petal power distribution network;
step 2: judging a petal ring network where the fault is located;
and step 3: determining a fault tie line;
and 4, step 4: special case handling.
Further, in step 1, the parameters include unit line impedance and line length of each segment of the tie line and the feeder line in each petal ring network.
Further, step 2 comprises the steps of,
(1) determining generalized nodes of the double-petal ring network;
(2) arranging current transformers with the same transformation ratio and characteristics on a line connected with the generalized node area, processing measured current signals to obtain current phasors, and uploading the current phasors to a protection main station;
(3) constructing a protection criterion suitable for multi-end differential protection of a double-petal power distribution network;
(4) judging whether the protection master station meets the protection criterion of the multi-terminal differential protection, wherein the judgment result is represented by a multi-terminal differential protection starting signal Flag, setting the Flag signal to zero in the initial state, and setting the Flag to be 1 when the protection criterion is met.
Further, in the step (1), all the tie lines and the switchyard buses in each petal ring in the petal-shaped power distribution network are regarded as a virtual node and are defined as generalized nodes.
Further, in the step (2), a full-period Fourier algorithm is adopted to obtain the amplitude and the angle of the current.
Further, in step (3), the protection criterion of the multi-terminal differential protection is as follows:
in the formula (I), the compound is shown in the specification,
for current phasors of lines connected to the generalized nodal region, KresTo the coefficient of braking, IsetThe setting value is the action current setting value.
Further, the starting current of the multi-terminal differential protection, namely the action current setting value IsetMaximum unbalanced current I generated when short circuit of external circuit of generalized node needs to be avoidedkmaxThe setting value of the operating current IsetCalculated by the following formula
Iset＝Krel×Inub.max＝Krel×0.1Ikmax (2)；
In the formula, KrelIs the reliability factor (usually taken to be 1.3); i iskmaxThe maximum short-circuit current flows when any line outside the generalized node area is short-circuited.
Meanwhile, when judging the multi-terminal differential protection, the following formula of checking sensitivity coefficient is also satisfied, namely
In the formula, KsenReferred to as the sensitivity coefficient, IkminThe minimum short-circuit current threshold value is the minimum short-circuit current threshold value when the connecting line in the generalized node area has a fault.
Further, step 3 comprises the steps of,
s1, arranging a voltage and current transformer at one side of each segment of the tie line, measuring the voltage and current, and extracting the amplitude U, I and phase angle of the voltage and current by utilizing a Fourier algorithm
By using
Obtaining active power, transmitting the active power data to a protection master station, and recording the active power data by the protection master station;
s2, when a short-circuit fault occurs to a connecting line in the petal ring net, setting Flag to be 1, acquiring active power data which is recorded by the protection main station and transmitted last time before the Flag is set to be 1, and recording the active power data as Pi1I is 2,3 …, n, wherein, the subscript i represents the number of the tie line, and n is the number of the tie line segments contained in the petal ring net;
s3, the protection master station takes the active power data of each segment of the connecting line again after a certain time delay and records as P ″i1I 2,3 …, n and is compared with the active power data P before the faulti1Making a difference to obtain the active power variation quantity, and recording as delta Pi1，i＝2,3…n。
And S4, judging the positive and negative of the active power variation of each section of the tie line, determining the broken tie line, and sending a signal to the circuit breakers at the two ends of the broken tie line by the protection main station to trip the corresponding circuit breakers.
S5, observing the state of the multi-terminal differential protection starting signal Flag; if Flag returns to zero after the breaker acts, the protection logic is finished; and if the Flag is 1 after the breaker acts, additionally tripping off the bus fault of the breaker disconnecting switch station.
Further, in step S3, the delay time is 0.1S; in step S4, the active power data of each segment of tie line is taken and subtracted from the active power data before the fault, if the difference is less than or equal to zero, the segment of tie line is the tie line for which the fault is determined, and if the difference is greater than zero, the other tie lines are continuously determined.
Further, step 4 is a special case handling process, wherein the special case handling process is that when the bus fails, and in the case of failure of the bus main protection, the fault on the bus cannot be effectively isolated, and needs to be considered separately. The invention has the beneficial effects that:
the invention designs a backup protection configuration scheme aiming at short-circuit faults of a tie line in a double-petal power distribution network, and comprehensively judges the fault position of the tie line by utilizing multi-terminal differential protection based on regional fault information and the change of active power at the same end of the tie line. The method comprises the steps of firstly judging the petal ring network where a fault connecting line is located by utilizing generalized node external branch current, judging the fault connecting line by utilizing the change of the active power of the connecting line, and finally sending signals to circuit breakers at two ends of the fault connecting line by a protection main station to enable the corresponding circuit breakers to trip.
Compared with the traditional backup protection, the method has the advantages of higher fault isolation speed, reduction in the number of the junctor acquisition equipment, good adaptability and economy, and suitability for popularization and application in the complex double-petal power distribution network.
Drawings
Fig. 1 is a schematic view of a topological structure of a double-petal power distribution network;
FIG. 2 is a schematic flow chart of the protection method of the present invention;
FIG. 3 is a schematic flow chart of step 2 of the present invention;
FIG. 4 is a schematic flow chart of step 3 of the present invention;
fig. 5 is a topology structure diagram of a left petal ring network in the embodiment of the invention.
Detailed Description
The present invention is further described in detail below with reference to examples, but the scope of the present invention is not limited thereto, and the scope of the invention is set forth in the claims.
The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are only a part of the embodiments of the present application, and not all of the embodiments. The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the application, its application, or uses. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
The relative arrangement of the components and steps, the numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the present application unless specifically stated otherwise. Meanwhile, it should be understood that the sizes of the respective portions shown in the drawings are not drawn in an actual proportional relationship for the convenience of description. Techniques, methods, and apparatus known to those of ordinary skill in the relevant art may not be discussed in detail but are intended to be part of the specification where appropriate. In all examples shown and discussed herein, any particular value should be construed as merely illustrative, and not limiting. Thus, other examples of the exemplary embodiments may have different values. It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, further discussion thereof is not required in subsequent figures.
The method comprises the steps of firstly judging the petal ring network where a fault connecting line is located by utilizing generalized node external branch current, judging the fault connecting line by utilizing the change of the active power of the connecting line, and finally sending signals to circuit breakers at two ends of the fault connecting line by a protection main station to enable the corresponding circuit breakers to trip. The protection flow chart is shown in the attached figure 2 in detail, and the specific scheme comprises the following steps:
the invention provides a tie line backup protection method suitable for a power distribution network with a double-petal structure, wherein left and right petal ring networks have symmetrical and equal parameters, the left petal ring network is taken as a research object in the embodiment, and the embodiment with reference to fig. 2 comprises the following steps.
Step 1: and acquiring the line parameters of the double-petal power distribution network. In step 1, the parameters include unit line impedance and line length of each segment of connecting lines and feeder lines in each petal ring net. The topology structure of the double-petal power distribution network of the embodiment is shown in fig. 5. The 110/10kV transformer substation is simulated by a three-phase voltage source; the tie line between each switch station adopts 400mm2The cable can be simplified into a pi model in the simulation model; the line distance between every two switch stations is considered according to 1km, the total load carried by each switch station is about 4MW, and the load power factor is 0.9. The grounding mode is that a neutral point is grounded through a small resistor, and the resistance value of the resistor is set to be 10 omega.
Step 2: judging a petal ring network where the fault is located; referring to fig. 3, step 2 includes the following steps, see step (1) -step (4).
(1) Determining generalized nodes of the double-petal ring network; all tie lines and switch station buses in each petal ring in the petal-shaped power distribution network are regarded as a virtual node and defined as a generalized node.
(2) Arranging current transformers with the same transformation ratio and characteristics on a line connected with the generalized node area, processing measured current signals to obtain current phasors, and transmitting the current phasors to a protection master station; the protection master station is a part of a digital relay protection system, and specific protection master stations are not explicitly specified. In the step (2), the amplitude and the angle of the current are obtained by adopting a full-period Fourier algorithm.
(3) Constructing a protection criterion suitable for multi-end differential protection of a double-petal power distribution network; in the step (3), the protection criterion of the multi-terminal differential protection is as follows:
in the formula (I), the compound is shown in the specification,
for current phasors of lines connected to the generalized nodal region, KresTo the coefficient of braking, IsetThe setting value is the action current setting value.
Starting current of multi-terminal differential protection, namely the action current setting value IsetMaximum unbalanced current I generated when short circuit of external circuit of generalized node needs to be avoidedkmax. In engineering terms, it can be calculated as follows:
Iset＝Krel×Inub.max＝Krel×0.1Ikmax (2)；
in the formula, KrelIs a reliability coefficient, generally 1.3; i iskmaxThe maximum short-circuit current flows when any line outside the generalized node area is short-circuited.
Meanwhile, when judging the multi-terminal differential protection, the following formula of checking sensitivity coefficient is also satisfied, namely
In the formula, KsenReferred to as the sensitivity coefficient, IkminThe minimum short-circuit current threshold value is the minimum short-circuit current threshold value when the connecting line in the generalized node area has a fault.
In this embodiment, when a three-phase short-circuit fault occurs outside the generalized node, according to the equations (2) and (3) and using the power distribution network line parameters in the step 1, the maximum unbalanced current generated when the external short-circuit of the generalized node area is avoided according to the formula (2), and the generated unbalanced current is known to be 0.5kA, and I can be obtained according to the formula (2)setThe operating current of the multi-terminal differential protection is 0.065kA at 1.3 × 0.1 × 0.5.
In addition, the minimum short-circuit current I occurring in the generalized nodekminThe sensitivity coefficient is about 153 and is far more than 2 according to the formula (4), which is 10kA, and meets the sensitivity requirement.
(4) Judging whether the protection master station meets the protection criterion of the multi-terminal differential protection, wherein the judgment result is represented by a multi-terminal differential protection starting signal Flag, setting the Flag signal to zero in the initial state, and setting the Flag to be 1 when the protection criterion is met.
And step 3: after the petal ring network where the fault is located is judged, the specific position where the fault occurs needs to be judged, and the specific steps are as follows: as shown in FIG. 4, the method specifically comprises steps S1-S4.
S1, arranging a voltage and current transformer at one side of each segment of the tie line, measuring the voltage and current, and extracting the amplitude U, I and phase angle of the voltage and current by utilizing a Fourier algorithm
By using
Obtaining active power, transmitting the active power data to a protection master station, and recording the active power data by the protection master station;
s2, when a short-circuit fault occurs to a connecting line in the petal ring net, setting Flag to be 1, and acquiring active power data P recorded by the protection main station and transmitted last time before the Flag is set to be 1i1I is 2,3 … n; and n is 5 in this embodiment.
S3, the protection master station takes the active power data of each segment of the connecting line again after a certain time delay and compares the active power data with the dataActive power data P before failure1And performing difference to obtain the active power variation. In step S3, the delay time is 0.1S; in step S4, active power data of each segment of tie line is taken and compared with active power data P before fault1And (4) making a difference, when the difference value is less than or equal to zero, the section of the connecting line is the connecting line with the determined fault, and when the difference value is more than zero, continuously judging other connecting lines.
And S4, judging the positive and negative of the active power variation of each section of the tie line, determining the broken tie line, and sending a signal to the circuit breakers at the two ends of the broken tie line by the protection main station to trip the corresponding circuit breakers.
Referring now to fig. 4, steps 2 and 3 will be further described,
in this embodiment, the left petal ring network is used as a research object, and the initial time t is 0, and the left end power of each segment of tie line is started to be collected.
If a short-circuit fault occurs at the position k1, the protection main station is a protection criterion meeting multi-terminal differential protection, Flag is set to be 1, and active power data P transmitted last time before the Flag is set to be 1 and recorded by the protection main station is obtained21，P31，P41And P51The protecting master station will be delayed by Δ t1After 0.1s, each segment of tie line active power data P 'is taken again'21，P’31，P’41And P'51And subtracting the data obtained for the first time to obtain the active power variation quantity, namely obtaining the delta Pi1(i is 2,3, 4, 5), and these power data are shown in the attached table 1, and Δ P is first determined21If the time is greater than zero, the judgment result is less than zero, and then the time is delayed by delta t2(which can be set to 0.2s according to the change in active power of the link after a fault) the circuit breakers across link L1 trip.
If a short-circuit fault occurs at the k2 position, the protection master station is a protection criterion meeting multi-terminal differential protection, Flag is set to be 1, and active power data P transmitted last time before the Flag is set to be 1 and recorded by the protection master station is obtainedi1，P31，P41And P51The protecting master station will be delayed by Δ t1After 0.1s, each segment of tie line active power data P 'is taken again'21，P’31，P’41And P'51And subtracting the data obtained for the first time to obtain the active power variation quantity, namely obtaining the delta Pi1(i is 2,3, 4, 5), and these power data are shown in the attached table 1, and Δ P is first determined21If the value is greater than zero, the judgment result is greater than zero, and then the value delta P is judged31If the time is greater than zero, the judgment result is less than zero, and then the time is delayed by delta t2After time (which can be set to 0.2s according to the change characteristic of the active power of the link after the fault), the circuit breakers at both ends of the link L2 trip.
Thirdly, if a short-circuit fault occurs at the k3 position, the protection master station meets the protection criterion of multi-terminal differential protection, the Flag is set to be 1, and the active power data P which are recorded by the protection master station and transmitted last before the Flag is set to be 1 are obtained21，P31，P41And P51The protecting master station will be delayed by Δ t1After 0.1s, taking the active power data of each segment of the connecting line again, and subtracting the active power data from the data taken for the first time to obtain the active power variation, namely obtaining the delta Pi1(i is 2,3, 4, 5), and these power data are shown in the attached table 1, and Δ P is first determined21If the value is greater than zero, the judgment result is greater than zero, and then the value delta P is judged31If the value is greater than zero, the judgment result is greater than zero, and then the value delta P is judged41If the time is greater than zero, the judgment result is less than zero, and then the time is delayed by delta t2After time (which can be set to 0.2s according to the change characteristic of the active power of the link after the fault), the circuit breakers at both ends of the link L3 trip.
If a short-circuit fault occurs at the k4 position, the protection master station meets the protection criterion of multi-terminal differential protection, the Flag is set to be 1, and the active power data P which are recorded by the protection master station and transmitted last before the Flag is set to be 1 are obtained21，P31，P41And P51The protecting master station will be delayed by Δ t1After 0.1s, taking the active power data of each segment of the connecting line again, and subtracting the active power data from the data taken for the first time to obtain the active power variation, namely obtaining the delta Pi1(i is 2,3, 4, 5), and these power data are shown in the attached table 1, and Δ P is first determined21If the value is greater than zero, the judgment result is greater than zero, and then the value delta P is judged31Whether it is greater than zero, judging the junctionIf the value is greater than zero, determining the value of delta P41If the value is greater than zero, the judgment result is greater than zero, and then the value delta P is judged51If the time is greater than zero, the judgment result is less than zero, and then the time is delayed by delta t2After time (which can be set to 0.2s according to the change characteristic of the active power of the link after the fault), the circuit breakers at both ends of the link L4 trip.
If a short-circuit fault occurs at the k5 position, the protection master station meets the protection criterion of multi-terminal differential protection, the Flag is set to be 1, and the active power data P which are recorded by the protection master station and transmitted last before the Flag is set to be 1 are obtained21，P31，P41And P51The protecting master station will be delayed by Δ t1After 0.1s, taking the active power data of each segment of the connecting line again, and subtracting the active power data from the data taken for the first time to obtain the active power variation, namely obtaining the delta Pi1(i is 2,3, 4, 5), and these power data are shown in the attached table 1, and Δ P is first determined21If the value is greater than zero, the judgment result is greater than zero, and then the value delta P is judged31If the value is greater than zero, the judgment result is greater than zero, and then the value delta P is judged41If the value is greater than zero, the judgment result is greater than zero, and then the value delta P is judged51If the time is greater than zero, the judgment result is greater than zero, and then the time is delayed by delta t2After time (which can be set to 0.2s according to the change characteristic of the active power of the link after the fault), the circuit breakers at both ends of the link L5 trip.
Sixthly, if the bus B of the switch station has a short-circuit fault, the protection main station meets the protection criterion of multi-terminal differential protection, the Flag is set to be 1, and the active power data P transmitted last time before the Flag is set to be 1 and recorded by the protection main station is obtained21，P31，P41And P51The protecting master station will be delayed by Δ t1After 0.1s, taking the active power data of each segment of the connecting line again, and subtracting the active power data from the data taken for the first time to obtain the active power variation, namely obtaining the delta Pi1(i is 2,3, 4, 5), and these power data are shown in the attached table 1, and Δ P is first determined21If the value is greater than zero, the judgment result is greater than zero, and then the value delta P is judged31If the time is greater than zero, the judgment result is less than zero, and then the time is delayed by delta t2Time (the change characteristic according to the active power of the tie after the fault can be set to 0.2s), the circuit breakers across the tie line L2 are tripped.
And 4, step 4: judging and processing special conditions; and 4, under the special condition that when the bus fails and the main protection of the bus fails, the bus fault cannot be effectively isolated, and the bus fault needs to be considered independently.
Observing the state of a multi-terminal differential protection starting signal Flag after the circuit breaker trips; if Flag returns to zero after the breaker acts, the protection logic is finished; and if the Flag is 1 after the breaker acts, additionally tripping off the bus fault of the breaker disconnecting switch station.
In the present embodiment, it is preferred that,
if a short-circuit fault occurs at the position k1, after the circuit breakers at the two ends of the connecting line L1 trip, the Flag returns to zero after the circuit breakers act, and the protection logic is ended.
If a short-circuit fault occurs at the k2 position, after the circuit breakers at the two ends of the connecting line L2 trip, the Flag returns to zero after the circuit breakers act, and the protection logic is finished.
And thirdly, if a short-circuit fault occurs at the k3 position, after the circuit breakers at the two ends of the connecting line L3 trip, the Flag returns to zero after the circuit breakers act, and the protection logic is finished.
If a short-circuit fault occurs at the position k3, after the circuit breakers at the two ends of the connecting line L4 trip, the Flag returns to zero after the circuit breakers act, and the protection logic is finished.
And fifthly, if a short-circuit fault occurs at the position k3, after the circuit breakers at the two ends of the connecting line L5 trip, the Flag returns to zero after the circuit breakers act, and the protection logic is finished.
When the bus of the switch station B has a short-circuit fault, after the circuit breakers at the two ends of the connecting line L2 trip, the Flag is 1 after the circuit breakers act, and then the circuit breaker connected with the bus of the switch station B and the connecting line L3 trips.
TABLE 1 tuning and sensitivity calibration results for pilot current differential protection
Unit: MVA
The backup protection action result in the embodiment is shown in the attached table 2, and the occurrence time of the fault is 0.4s as can be seen from the table, when the main protection refuses to act, the circuit breaker can be tripped in 0.3s, and the fault isolation time is independent of the fault type and the fault occurrence position.
TABLE 2 setting and sensitivity check results for timing-limited over-current protection
The invention provides a backup protection improvement scheme aiming at various faults on a tie line based on a double-petal power distribution network structure, and comprehensively judges the fault position of the tie line by utilizing multi-terminal differential protection based on regional fault information and the change of active power at one end of the tie line. Compared with the traditional backup protection, the protection scheme has the advantages that the fault isolation speed is higher, the number of the junctor collecting devices is reduced, the good adaptability and the good economical efficiency are realized, and the protection scheme is suitable for being popularized and applied in a complex petal-shaped power distribution network.
Claims (10)
1. The utility model provides a tie line reserve protection method suitable for two petal structure distribution networks which characterized in that: which comprises the following steps of,
step 1: acquiring line parameters of a double-petal power distribution network;
step 2: judging a petal ring network where the fault is located;
and step 3: determining a fault tie line;
and 4, step 4: special case handling.
2. The tie line backup protection method suitable for the power distribution network with the double petal structure according to claim 1, wherein the method comprises the following steps: in step 1, the parameters include unit line impedance and line length of each segment of connecting lines and feeder lines in each petal ring net.
3. The tie line backup protection method suitable for the power distribution network with the double petal structure according to claim 1, wherein the method comprises the following steps: the step 2 comprises the following steps of,
(1) determining generalized nodes of the double-petal ring network;
(2) arranging current transformers with the same transformation ratio and characteristics on a line connected with the generalized node area, processing measured current signals to obtain current phasors, and uploading the current phasors to a protection main station;
(3) constructing a protection criterion suitable for multi-end differential protection of a double-petal power distribution network;
(4) judging whether the protection master station meets the protection criterion of the multi-terminal differential protection, wherein the judgment result is represented by a multi-terminal differential protection starting signal Flag, setting the Flag signal to zero in the initial state, and setting the Flag to be 1 when the protection criterion is met.
4. The tie line backup protection method suitable for the power distribution network with the double petal structure according to claim 3, wherein the method comprises the following steps: in the step (1), all tie lines and switch station buses in each petal ring in the petal-shaped power distribution network are regarded as a virtual node and defined as a generalized node.
5. The tie line backup protection method suitable for the power distribution network with the double petal structure according to claim 3, wherein the method comprises the following steps: in the step (2), the amplitude and the angle of the current are obtained by adopting a full-period Fourier algorithm.
6. The tie line backup protection method suitable for the power distribution network with the double petal structure according to claim 3, wherein the method comprises the following steps: in the step (3), the protection criterion of the multi-terminal differential protection is as follows:
in the formula (I), the compound is shown in the specification,
for current phasors of lines connected to the generalized nodal region, KresTo the coefficient of braking, IsetSetting the action current; starting current of multi-terminal differential protectionI.e. the setting value of the operating current IsetMaximum unbalanced current I generated when short circuit of external circuit of generalized node needs to be avoidedkmax；
The setting value I of the action currentsetCalculated by the following formula
Iset＝Krel×Inub.max＝Krel×0.1Ikmax (2)；
In the formula, KrelIs the reliability factor; i iskmaxThe maximum short-circuit current flows when any line outside the generalized node area is short-circuited.
7. The tie line backup protection method suitable for the power distribution network with the double petal structure according to claim 3, wherein the method comprises the following steps: in step (3), when multi-terminal differential protection is judged, the following formula of checking sensitivity coefficient is also satisfied, namely
In the formula, KsenReferred to as the sensitivity coefficient, IkminThe minimum short-circuit current threshold value is the minimum short-circuit current threshold value when the connecting line in the generalized node area has a fault.
8. The tie line backup protection method suitable for the power distribution network with the double petal structure according to claim 1, wherein the method comprises the following steps: the step 3 comprises the following steps of,
s1, arranging a voltage and current transformer at one side of each segment of the tie line, measuring the voltage and current, and extracting the amplitude U, I and phase angle of the voltage and current by utilizing a Fourier algorithm
By using
Obtaining active power, transmitting the active power data to a protection master station, and recording the active power data by the protection master station;
s2, when a short-circuit fault occurs to a connecting line in the petal ring net, setting Flag to be 1, acquiring active power data which is recorded by the protection main station and transmitted last time before the Flag is set to be 1, and recording the active power data as Pi1I is 2,3 …, n, wherein, the subscript i represents the number of the tie line, and n is the number of the tie line segments contained in the petal ring net;
s3, the protection master station takes the active power data of each segment of the connecting line again after a certain time delay and records as P ″i1I 2,3 …, n and is compared with the active power data P before the faulti1Making a difference to obtain the active power variation quantity, and recording as delta Pi1，i＝2,3…n；
S4, judging the positive and negative of the active power variation of each section of the tie line, determining the broken tie line, and sending a signal to the circuit breakers at the two ends of the broken tie line by the protection main station to trip the corresponding circuit breakers;
s5, observing the state of the multi-terminal differential protection starting signal Flag; if Flag returns to zero after the breaker acts, the protection logic is finished; and if the Flag is 1 after the breaker acts, additionally tripping off the bus fault of the breaker disconnecting switch station.
9. The tie line backup protection method suitable for the power distribution network with the double petal structure according to claim 8, wherein the method comprises the following steps: in step S3, the delay time is 0.1S; in step S4, the active power data of each segment of tie line is taken and subtracted from the active power data before the fault, if the difference is less than or equal to zero, the segment of tie line is the tie line for which the fault is determined, and if the difference is greater than zero, the other tie lines are continuously determined.
10. The tie line backup protection method suitable for the power distribution network with the double petal structure according to claim 1, wherein the method comprises the following steps: and 4, processing special conditions, wherein the special conditions are that when the bus fails and the bus main protection fails, the bus fault cannot be effectively isolated, and the bus fault needs to be considered independently.
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