An Active Distribution Network Line Protection Method and System with Unmeasurable Branches

Abstract

The present invention provides a line protection method and system for an active distribution network with unmeasurable branches, belonging to the field of distribution network protection. The method includes: calculating the positive-sequence current fault component according to the three-phase currents of the branches before and after the occurrence of a fault in the active distribution network; judging whether a fault occurs on the bus according to the positive-sequence current fault components of the branches on the bus; if no fault occurs, judging whether the bus is in a light-fault area or a heavy-fault area; if the bus is in a light-fault area, judging whether a fault occurs on each branch according to the phase relationship between the positive-sequence current fault component of each branch on the bus and the positive-sequence voltage fault component of the bus; if the bus is in a heavy-fault area, judging whether an inverter-type branch has a fault according to an additional criterion; further judging whether a fault occurs on each connection branch according to the phase relationship value between each bus and each connection branch, and if a fault occurs, disconnecting the switch of the corresponding connection branch. The reliability of the line protection for the active distribution network with unmeasurable branches is improved.

Description

Technical Field

[0001] The present invention relates to the field of distribution network protection, and particularly to a line protection method and system for an active distribution network with unmeasurable branches. Background Art

[0002] With the emergence of the global energy crisis and the "dual carbon" goal, a large number of new energy sources are dispersed and distributed into the distribution network, resulting in a more complex topological structure and fault characteristics of the distribution network. The performance of traditional three-stage current protection based on local information has declined and can no longer meet the protection requirements of the distribution network.

[0003] Communication-based pilot protection constructs protection criteria using protection information at both ends of the line. For example, in the "New Sufficient-Type Protection Principle and Scheme for Distributed Generation Distribution Network" proposed by Xu Sixuan et al., a "sufficient-type" current amplitude comparison protection criterion is proposed to achieve differential protection under weak synchronization conditions and reduce the requirements for communication conditions. Zhou Chenghan et al. proposed a current differential protection criterion based on a compensation coefficient in the "Pilot Protection for Active Distribution Network Based on Positive Sequence Current Fault Component", which calculates the braking threshold using the current phase difference on both sides of the line to achieve adaptive differential protection. Han Haijuan et al. proposed a protection criterion using the phase relationship of positive sequence fault components of voltage and current on both sides of the line in the "Microgrid Protection Considering Low Voltage Ride-Through of IIDG" to achieve longitudinal direction protection of fault components.

[0004] However, the above protection schemes do not consider the impact of unmeasurable branches on protection. In the distribution network, as the line capacity increases, some DG (distributed generator) and load branches are directly connected to the line in a "T" shape, and the system cannot detect the operating state of this branch, so it is called an unmeasurable branch. The voltage and current of the unmeasurable branch are unmeasurable, making the fault characteristics of the distribution network more complex. For the active distribution network with unmeasurable branches connected, Li Juan et al. considered the impact of unmeasurable branches on differential protection in the "Inverse Time Current Differential Protection for Active Distribution Network Considering the Characteristics of Inverter-Type Distributed Generation", but only analyzed the impact of unmeasurable branches on protection during external faults. He Jinghan et al. added a node branch auxiliary criterion to the positive sequence current amplitude comparison protection in the "Differential Protection Principle for Distribution Network with DG Based on Node Branch Current Amplitude", but it is only applicable to the case where multiple branch nodes are located at the end of the line. Zhang Xuesong et al. proposed a new current amplitude differential protection criterion in the "New Current Amplitude Differential Protection Criterion for Lines with Unmeasurable Branches in Active Distribution Network", which approximates the current amplitude difference between both sides during normal operation of the line as the current level of the unmeasurable branch load and uses it as the braking quantity.

[0005] However, the above protection schemes only consider unmeasurable branches of the load type, and there is less research on unmeasurable branches of the DG type, resulting in low protection reliability for active distribution network lines. Summary of the Invention

[0006] The object of the present invention is to provide a line protection method and system for an active distribution network with unmeasurable branches, which can improve the reliability of line protection for an active distribution network with unmeasurable branches.

[0007] To achieve the above object, the present invention provides the following solutions:

[0008] A line protection method for an active distribution network with unmeasurable branches, comprising:

[0009] Real-time collect the three-phase currents of each branch and the three-phase voltages of each bus in the active distribution network;

[0010] Judge whether the active distribution network has a fault according to the three-phase currents of each branch, and determine the time when the fault occurs when the active distribution network has a fault;

[0011] For any branch, calculate the positive-sequence current fault component of the branch according to the three-phase currents of the branch before and after the fault of the active distribution network;

[0012] For any bus, judge whether the bus has a fault according to the positive-sequence current fault components of each branch on the bus; if the bus has a fault, disconnect the switches of each branch on the bus;

[0013] If the bus has no fault, calculate the positive-sequence voltage fault component of the bus according to the three-phase voltages of the bus before and after the fault of the active distribution network;

[0014] Determine the phase relationship values between each branch on the bus and the bus according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of each branch on the bus;

[0015] Determine the fault category of the area where the bus is located according to the three-phase voltages of the bus; the fault category is a light-fault area or a heavy-fault area;

[0016] If the bus is in the light-fault area, judge whether each branch has a fault according to the phase relationship values between each branch on the bus and the bus. If a fault occurs, disconnect the switch of the corresponding branch;

[0017] If the busbar is in a heavy fault area, for any non-connection branch on the busbar, if the non-connection branch is the branch where the load or motor-type distributed power source is located, then according to the phase relationship value between the non-connection branch and the busbar, it is determined whether the non-connection branch has a fault; if the non-connection branch is the branch where the inverter-type distributed power source is located, then according to the phase relationship value between the non-connection branch and the busbar and the three-phase current of the non-connection branch, it is determined whether the non-connection branch has a fault; if a fault occurs, the switch of the corresponding non-connection branch is disconnected; according to the phase relationship values between each busbar and each connection branch, it is determined whether each connection branch has a fault, and if a fault occurs, the switch of the corresponding connection branch is disconnected.

[0018] Optionally, the method for determining whether the active distribution network has a fault according to the three-phase current of each branch and determining the fault occurrence time when the active distribution network has a fault specifically includes:

[0019] For any branch, according to the three-phase current of the branch collected within three cycles, it is determined whether the formula ||i(t)-i(t-N)|-|i(t-N)-i(t-2N)||≥0.1I ave holds. If it holds, the active distribution network has a fault, and the t moment is the fault occurrence time;

[0020] wherein, i(t) is the three-phase current of the branch collected at the t moment, i(t-N) is the three-phase current of the branch collected at the t-N moment, i(t-2N) is the three-phase current of the branch collected at the t-2N moment, N is the number of sampling points in one cycle, and I ave is the current amplitude of the branch when the active distribution network operates normally.

[0021] Optionally, according to the three-phase current of the branch before and after the fault occurrence of the active distribution network, the positive-sequence current fault component of the branch is calculated, specifically including:

[0022] According to the three-phase current of the branch before the fault occurrence of the active distribution network, the vector values of the phase currents of the branch before the fault occurrence of the active distribution network are calculated by the full-cycle Fourier algorithm;

[0023] According to the vector values of the phase currents of the branch before the fault occurrence of the active distribution network, the positive-sequence current of the branch before the fault occurrence of the active distribution network is calculated;

[0024] According to the three-phase current of the branch after the fault occurrence of the active distribution network, the vector values of the phase currents of the branch after the fault occurrence of the active distribution network are calculated by the full-cycle Fourier algorithm;

[0025] According to the vector values of the phase currents of the branch after the fault occurrence of the active distribution network, the positive-sequence current of the branch after the fault occurrence of the active distribution network is calculated;

[0026] Calculate the positive-sequence current fault component of the branch according to the positive-sequence current of the branch before and after the occurrence of the active distribution network fault.

[0027] Optionally, use the following formula to calculate the positive-sequence current of branch n:

[0028]

[0029] where I n,a is the positive-sequence current of branch n, is the vector value of the current of phase a of branch n, is the vector value of the current of phase b of branch n, is the vector value of the current of phase c of branch n, j is the imaginary unit.

[0030] Optionally, according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of the branches on the bus, determine the phase relationship values between the branches on the bus and the bus, specifically including:

[0031] Calculate the fault component phase angles between the branches on the bus and the bus according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of the branches on the bus;

[0032] Determine the phase relationship values between the branches on the bus and the bus according to the fault component phase angles between the branches on the bus and the bus.

[0033] Optionally, define the positive direction as the direction from the bus to the protected device; the protected device is the branch and the devices connected to the branch; use the following formula to determine the fault component phase angle between branch n on bus m and bus m:

[0034]

[0035] where θ mn is the fault component phase angle between branch n and bus m, is the positive-sequence voltage fault component of bus m, is the positive-sequence current fault component of branch n;

[0036] Use the following formula to determine the phase relationship value between branch n on bus m and bus m:

[0037]

[0038] where k mn is the phase relationship value between branch n and bus m.

[0039] Optionally, the phase relationship value is 0, 1 or -1;

[0040] Judge whether each branch has a fault according to the phase relationship value between each branch on the bus and the bus, specifically including:

[0041] For any connection branch on the bus, if the phase relationship values between the connection branch and the two side buses are both 0, then disconnect the switch of the connection branch; the connection branch is the branch between the two buses;

[0042] For any non-connection branch on the bus, if the phase relationship value between the non-connection branch and the bus is 0, then disconnect the switch of the non-connection branch.

[0043] Optionally, the phase relationship value is 0, 1 or -1; judge whether the non-connection branch has a fault according to the phase relationship value between the non-connection branch and the bus and the three-phase current of the non-connection branch, specifically including:

[0044] Judge whether the phase relationship value between the non-connection branch and the bus is 0. If so, calculate the positive-sequence current of the non-connection branch according to the three-phase current of the non-connection branch, and judge the formula I n.IIDG >k IIDG I n.IIDG.ave Whether it holds. If it holds, the non-connection branch has a fault; where I n.IIDG Is the positive-sequence current of the branch where the inverter-type distributed power source is located, I n.IIDG.ave Is the effective value of the average current of the branch where the inverter-type distributed power source is located within 15 minutes during the normal operation of the active distribution network, k IIDG Is the current coefficient of the inverter-type distributed power source.

[0045] Optionally, the active distribution network includes a feeder agent and multiple regional agents. Each regional agent corresponds to a bus, and each regional agent is used to control the disconnection of the switches of each branch on the corresponding bus;

[0046] The method for judging whether each connection branch has a fault according to the phase relationship values between each bus and each connection branch specifically includes:

[0047] Send the phase relationship values between the corresponding bus and each connection branch to the feeder agent through each regional agent;

[0048] The feeder agent judges whether each connection branch has a fault according to the phase relationship values sent by each regional agent. If a connection branch has a fault, a fault isolation instruction is sent to the corresponding regional agent;

[0049] The regional agent disconnects the switch of the corresponding connection branch according to the fault isolation instruction.

[0050] To achieve the above object, the present invention also provides the following solutions:

[0051] An active distribution network line protection system with unmeasurable branches, comprising:

[0052] A current and voltage acquisition unit, configured to collect the three-phase currents of each branch and the three-phase voltages of each bus in the active distribution network in real time;

[0053] A power grid fault discrimination unit, connected to the current and voltage acquisition unit, configured to determine whether a fault occurs in the active distribution network according to the three-phase currents of each branch, and determine the moment when the fault occurs when a fault occurs in the active distribution network;

[0054] A current component determination unit, connected to the power grid fault discrimination unit, configured to calculate the positive-sequence current fault component of any branch according to the three-phase currents of the branch before and after the occurrence of the active distribution network fault;

[0055] A bus fault discrimination unit, connected to the current component determination unit, configured to determine whether a fault occurs in any bus according to the positive-sequence current fault components of each branch on the bus; if a fault occurs in the bus, disconnect the switches of each branch on the bus;

[0056] A voltage component determination unit, connected to the bus fault discrimination unit, configured to calculate the positive-sequence voltage fault component of the bus according to the three-phase voltages of the bus before and after the occurrence of the active distribution network fault if the bus does not have a fault;

[0057] A phase relationship determination unit, respectively connected to the current component determination unit and the voltage component determination unit, configured to determine the phase relationship value between each branch on the bus and the bus according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of each branch on the bus;

[0058] A fault category determination unit, connected to the current and voltage acquisition unit, configured to determine the fault category of the area where the bus is located according to the three-phase voltages of the bus; the fault category is a light fault area or a heavy fault area;

[0059] A light fault processing unit, connected to the fault category determination unit, configured to determine whether a fault occurs in each branch according to the phase relationship value between each branch on the bus and the bus if the bus is in a light fault area, and if a fault occurs, disconnect the switch of the corresponding branch;

[0060] Heavy fault handling unit, connected to the fault category determination unit, is configured to, if the bus is in the heavy fault area, for any non-connecting branch on the bus, if the non-connecting branch is the branch where a load or a motor-type distributed power source is located, determine whether the non-connecting branch has a fault according to the phase relationship value between the non-connecting branch and the bus; if the non-connecting branch is the branch where an inverter-type distributed power source is located, determine whether the non-connecting branch has a fault according to the phase relationship value between the non-connecting branch and the bus and the three-phase current of the non-connecting branch; if a fault occurs, disconnect the switch of the corresponding non-connecting branch.

[0061] Connecting branch discrimination unit, connected to the heavy fault handling unit, is configured to determine whether each connecting branch has a fault according to the phase relationship value between each bus and each connecting branch, and if a fault occurs, disconnect the switch of the corresponding connecting branch.

[0062] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects: Determine whether an active distribution network has a fault according to the three-phase current of each branch. For any branch, calculate the positive-sequence current fault component of the branch according to the three-phase current of the branch before and after the occurrence of the active distribution network fault; for any bus, determine whether the bus has a fault according to the positive-sequence current fault components of the branches on the bus; if the bus has a fault, disconnect the switches of the branches on the bus; if the bus does not have a fault, determine whether the bus is in the light fault area or the heavy fault area. If the bus is in the light fault area, directly determine whether each branch has a fault according to the phase relationship between the positive-sequence current fault component of each branch and the positive-sequence voltage fault component of the bus. If the bus is in the heavy fault area, for the branches where loads and motor-type distributed power sources are located, directly determine whether the branches have faults according to the phase relationship between the positive-sequence current fault component and the positive-sequence voltage fault component of the bus. For the branches where inverter-type distributed power sources are located, further determine whether the branches where the inverter-type distributed power sources are located have faults according to additional criteria. Finally, determine whether the connecting branches have faults through the feeder agent, improving the reliability of the line protection of the active distribution network with unmeasurable branches. Description of the Drawings

[0063] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0064] Figure 1 It is a flowchart of the method for protecting the line of the active distribution network with unmeasurable branches according to the present invention.

[0065] Figure 2It is the overall flowchart of the line protection process for the active distribution network;

[0066] Figure 3 It is the schematic diagram of the 10kV distribution network model;

[0067] Figure 4 It is the module schematic diagram of the active distribution network line protection system with unmeasurable branches of the present invention.

[0068] Symbol description:

[0069] Current and voltage acquisition unit - 1, power grid fault discrimination unit - 2, current component determination unit - 3, bus fault discrimination unit - 4, voltage component determination unit - 5, phase relationship determination unit - 6, fault type determination unit - 7, light fault processing unit - 8, heavy fault processing unit - 9, tie branch discrimination unit - 10. Specific implementation manner

[0070] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0071] The purpose of the present invention is to provide a method and system for protecting the lines of an active distribution network with unmeasurable branches. After a fault occurs in the active distribution network and the bus fault is excluded, it is further determined whether the bus is in a light fault area or a heavy fault area. For each branch when the bus is in a light fault area or the branch where the load and motor-type distributed power sources are located when the bus is in a heavy fault area, it is directly determined whether each branch has a fault according to the phase relationship between the positive-sequence current fault component of each branch and the positive-sequence voltage fault component of the bus. For the branch where the inverter-type distributed power source is located in the heavy fault area, it is further determined whether the corresponding branch has a fault according to an additional criterion. Finally, it is determined whether the tie branch has a fault through the feeder agent, so as to improve the reliability of protecting the lines of the active distribution network with unmeasurable branches.

[0072] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners.

[0073] The active distribution network includes a feeder agent, multiple regional agents, multiple buses, and multiple branches. Each regional agent corresponds to a bus, and each regional agent is used to control the disconnection of the branch switches on the corresponding bus. The feeder agent is connected to each regional agent. The feeder agent is set at the outlet of the feeder. The feeder agent and the regional agent are in a superior-subordinate relationship. The feeder agent is the superior and can communicate with the subordinate regional agents. The regional agent can only communicate with the regional agents upstream and downstream of the line.

[0074] Embodiment 1

[0075] As Figure 1 shown, the line protection method for the active distribution network with unmeasurable branches provided in this embodiment includes:

[0076] S1: Real-time collect the three-phase currents of each branch and the three-phase voltages of each bus in the active distribution network.

[0077] S2: Judge whether the active distribution network has a fault according to the three-phase currents of each branch, and determine the moment when the fault occurs when the active distribution network has a fault.

[0078] Specifically, for any branch, according to the three-phase currents of the branch collected within three cycles, judge whether the formula ||i(t)-i(t-N)|-|i(t-N)-i(t-2N)||≥0.1I ave holds. If it holds, the active distribution network has a fault, and the moment t is the fault occurrence moment. Wherein, i(t) is the three-phase current of the branch collected at the moment t, that is, the current three-phase current sampling value, i(t-N) is the three-phase current of the branch collected at the moment t-N, that is, the three-phase current sampling value one cycle before, i(t-2N) is the three-phase current of the branch collected at the moment t-2N, that is, the three-phase current sampling value two cycles before, N is the number of sampling points in one cycle, and I ave is the current amplitude of the branch when the active distribution network operates normally.

[0079] In this embodiment, when the active distribution network system operates normally, the three-phase currents of the line are collected through the regional agent. Calculate the sudden change amount of the three-phase currents through the left part of the above formula. If the above formula is satisfied, that is, the sudden change amount is greater than or equal to 0.1 times the current amplitude during normal operation, the active distribution network may have a fault, and more complex judgments need to be combined with the collected three-phase voltage data. If the above formula is not satisfied, it proves that the active distribution network has no fault, then continuously collect the three-phase currents and judge whether the above formula is satisfied.

[0080] S3: For any branch, calculate the positive-sequence current fault component of the branch according to the three-phase currents of the branch before and after the fault occurs in the active distribution network.

[0081] Further, step S3 includes:

[0082] S31: Calculate the phase current vector values of each branch before the active distribution network fault occurs based on the three-phase currents of the branch before the active distribution network fault occurs through the full-cycle Fourier algorithm.

[0083] S32: Calculate the positive-sequence current of the branch before the active distribution network fault occurs based on the phase current vector values of each branch before the active distribution network fault occurs.

[0084] S33: Calculate the phase current vector values of each branch after the active distribution network fault occurs based on the three-phase currents of the branch after the active distribution network fault occurs through the full-cycle Fourier algorithm.

[0085] S34: Calculate the positive-sequence current of the branch after the active distribution network fault occurs based on the phase current vector values of each branch after the active distribution network fault occurs.

[0086] In this embodiment, the following formula is used to calculate the positive-sequence current of branch n:

[0087]

[0088] where, I n,a is the positive-sequence current of branch n, is the phase a current vector value of branch n, is the phase b current vector value of branch n, is the phase c current vector value of branch n, j is the imaginary unit. The formulas for calculating the positive-sequence current in step S32 and step S34 are the same.

[0089] S35: Calculate the positive-sequence current fault component of the branch based on the positive-sequence currents of the branch before and after the active distribution network fault occurs. Specifically, subtract the positive-sequence current after the fault occurs from the positive-sequence current before the fault occurs to obtain the positive-sequence current fault component.

[0090] S4: For any bus, determine whether the bus has a fault based on the positive-sequence current fault components of each branch on the bus. If the bus has a fault, disconnect the switches of each branch on the bus. Specifically, sum the positive-sequence current fault components of each branch on the bus to obtain the positive-sequence current fault sum. If the positive-sequence current fault sum is greater than the bus fault current threshold, the bus has a fault; otherwise, the bus has no fault.

[0091] That is, use the formula to determine whether the bus has a fault. Where, is the positive-sequence current fault component of branch n on bus m, D is the set of branches on bus m, I busis the bus fault current threshold. When the positive-sequence current fault on the bus satisfies the above formula, the corresponding regional agent determines that the fault is located on the bus, disconnects all switches on the branch, and isolates the bus.

[0092] S5: If the bus does not have a fault, calculate the positive-sequence voltage fault component of the bus according to the three-phase voltages of the bus before and after the fault occurs in the active distribution network.

[0093] Further, when the bus does not have a fault, it is necessary to determine whether the fault is on the branch, and the positive direction is defined as the direction from the bus to the protected device. Step S5 specifically includes:

[0094] S51: According to the three-phase voltages of the bus before the fault occurs in the active distribution network, calculate the voltage vector values of each phase of the bus before the fault occurs in the active distribution network by using the full-cycle Fourier algorithm.

[0095] S52: Calculate the positive-sequence voltage of the bus before the fault occurs in the active distribution network according to the voltage vector values of each phase of the bus before the fault occurs in the active distribution network.

[0096] S53: According to the three-phase voltages of the bus after the fault occurs in the active distribution network, calculate the voltage vector values of each phase of the bus after the fault occurs in the active distribution network by using the full-cycle Fourier algorithm.

[0097] S54: Calculate the positive-sequence voltage of the bus after the fault occurs in the active distribution network according to the voltage vector values of each phase of the bus after the fault occurs in the active distribution network.

[0098] S55: Calculate the positive-sequence voltage fault component of the bus according to the positive-sequence voltages of the bus before and after the fault occurs in the active distribution network.

[0099] In this embodiment, the calculation method of the positive-sequence voltage fault component of the bus is the same as that of the positive-sequence current fault component of the branch, and the specific calculation formula is not elaborated here.

[0100] S6: Determine the phase relationship values between each branch on the bus and the bus according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of each branch on the bus.

[0101] Specifically, according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of each branch on the bus, calculate the fault component phase angles between each branch on the bus and the bus. According to the fault component phase angles between each branch on the bus and the bus, determine the phase relationship values between each branch on the bus and the bus.

[0102] In this embodiment, the following formula is used to determine the fault component phase angle between branch n on bus m and bus m:

[0103]

[0104] where, θ mn is the fault component phase angle between branch n and bus m, is the positive - sequence voltage fault component of bus m, is the positive - sequence current fault component of branch n.

[0105] Use the following formula to determine the phase relationship value between branch n on bus m and bus m:

[0106]

[0107] where, k mn is the phase relationship value between branch n and bus m. In this embodiment, use vector K to describe the phase relationship between the positive - sequence voltage fault component of the bus controlled by the regional agent and the positive - sequence current fault components of each branch: K m =[k m1 , k m2 ,..., k mn .

[0108] S7: Determine the fault category of the area where the bus is located according to the three - phase voltages of the bus. The fault category is a light - fault area or a heavy - fault area. Specifically, according to the different degrees of fault severity, the area after the fault is divided into a light - fault area and a heavy - fault area. If the per - unit value of the positive - sequence voltage of the bus is less than the minimum voltage value (0.2 in this embodiment), or the phase mutation angle of the positive - sequence voltage of the bus is greater than the set angle (38° in this embodiment), then the bus is in the heavy - fault area; otherwise, the bus is in the light - fault area. Among them, the phase mutation angle of the positive - sequence voltage is the phase difference of the positive - sequence voltage after the fault.

[0109] S8: If the bus is in the light - fault area, then according to the phase relationship values between each branch on the bus and the bus, determine whether each branch has a fault. If a fault occurs, disconnect the switch of the corresponding branch.

[0110] Further, in step S8, determining whether each branch has a fault according to the phase relationship values between each branch on the bus and the bus specifically includes:

[0111] (1) For any tie - line branch on the bus, if the phase relationship values between the tie - line branch and the two - side buses are both 0, then disconnect the switch of the tie - line branch. The tie - line branch is the branch between two buses.

[0112] Specifically, the area agents on both sides of the connection branch establish communication with the area agent on the opposite side through communication methods such as optical fiber or 5G communication, send the phase relationship value between this branch and the corresponding bus to the area agent on the opposite side, and receive the phase relationship value between this branch and the bus on the opposite side sent by the area agent on the opposite side. If the phase relationship value between the branch received by the area agent and the bus on the opposite side and its phase relationship value with the local bus are both 0, it is determined that the fault is located in this connection branch and the branch is disconnected. If the area agent receives a heavy status flag, it does not operate. If the phase relationship value between the branch and the bus is -1, the fault condition of this branch cannot be determined, and the corresponding circuit breaker is kept locked.

[0113] (2) For any non-connection branch on the bus, if the phase relationship value between the non-connection branch and the bus is 0, the switch of the non-connection branch is disconnected.

[0114] S9: If the bus is in a heavy fault area, for any non-connection branch on the bus, if the non-connection branch is the branch where the load or motor-type distributed power source is located, determine whether the non-connection branch has a fault according to the phase relationship value between the non-connection branch and the bus. If the non-connection branch is the branch where the inverter-type distributed power source is located, determine whether the non-connection branch has a fault according to the phase relationship value between the non-connection branch and the bus and the three-phase current of the non-connection branch. If a fault occurs, the switch of the corresponding non-connection branch is disconnected.

[0115] Specifically, when the area agent detects that the bus is in a heavy fault area, it sends a heavy status flag to the upstream and downstream area agents.

[0116] Further, in step S9, for the non-connection branch where the load or motor-type distributed power source is located, determine whether the phase relationship value between the non-connection branch where the load or motor-type distributed power source is located and the bus is 0. If so, the corresponding non-connection branch has a fault. Otherwise, the corresponding non-connection branch has no fault.

[0117] For the non-connection branch where the inverter-type distributed power source is located, determine whether the phase relationship value between the non-connection branch where the inverter-type distributed power source is located and the bus is 0. If so, calculate the positive-sequence current of the non-connection branch where the inverter-type distributed power source is located according to the three-phase current of the non-connection branch where the inverter-type distributed power source is located, and determine whether the formula I n.IIDG >k IIDG I n.IIDG.ave holds. If it holds, the corresponding non-connection branch has a fault. Wherein, I n.IIDG is the positive-sequence current of the branch where the inverter-type distributed power source is located, I n.IIDG.ave is the effective value of the average current of the branch where the inverter-type distributed power source is located within 15 minutes during normal operation of the active distribution network, and k IIDGis the current coefficient of the inverter-type distributed power source. In this embodiment, k IIDG = 2.

[0118] S10: According to the phase relationship values between each bus and each connection branch, determine whether a fault occurs in each connection branch. If a fault occurs, disconnect the switch of the corresponding connection branch. Specifically, step S10 is a determination and processing process for whether a fault occurs in the connection branch when the bus is in a severe fault area.

[0119] Step S10 specifically includes: Each regional agent sends the phase relationship values between the corresponding bus and each connection branch to the feeder agent. The feeder agent determines whether a fault occurs in each connection branch according to the phase relationship values sent by each regional agent. If a fault occurs in the connection branch, a fault isolation instruction is sent to the corresponding regional agent. The regional agent disconnects the switch of the corresponding connection branch according to the fault isolation instruction.

[0120] Specifically, if the regional agent detects that the bus is located in a severe fault area, after determining whether a fault occurs in the load branch and the branch where the distributed power source is located, the phase relationship values between the upstream and downstream connection branches of the bus and the bus are uploaded to the feeder agent.

[0121] After the feeder agent receives the phase relationship values uploaded by multiple regional agents from the severe fault area, the phase relationship values are sorted according to the upper and lower levels of the branches to obtain a phase relationship sequence M:

[0122] M = [0, k 支路(n)下游 ,k 支路(n+1)上游 ,k 支路(n+1)下游 ,k 支路(n+2)上游 ,...];

[0123] The feeder agent traverses the phase relationship sequence M, and the connection branch where both the upstream and downstream are 0 for the first time is the fault branch. The feeder agent sends a fault isolation command to the corresponding regional agent, and the regional agent cuts off the corresponding connection branch.

[0124] Taking M = [0, 0, 1, 0, 0] as an example, the feeder agent determines that the fault occurs in branch n. Taking M = [0, 1, 0, 0, 0] as an example, the feeder agent determines that the fault occurs in branch n + 1. The overall process of the active distribution network line protection method provided in this embodiment is as Figure 2 shown.

[0125] The effectiveness of the active distribution network line protection of the present invention will be described below in combination with a specific simulation verification process.

[0126] 1) Model building

[0127] Use PSCAD / EMTDC to establish as Figure 3The shown 10 kV distribution network model has a positive sequence impedance per unit length of the line as (0.64 + j0.12) Ω / km. All DG and load branches are connected to the bus through a 1-km line. The distances of the tie branches SA, AB, BC, CD, and AE are 1 km, 4 km, 4 km, 2 km, and 3 km respectively. The capacities of Load 1 to Load 6 are 1.5 MVA, 2.0 MVA, 1.2 MVA, 0.2 MVA, 0.5 MVA, and 1.2 MVA respectively, and the power factor of the loads is taken as 0.9. DG7 is an MTDG (motor type distributed generation), with a capacity of 2 MW. DG1 - DG6 are IIDGs (inverter type distributed generation), and all adopt the PQ control method. The parameters are shown in Table 1. Where k max represents the maximum limit multiple of the output current of the inverter type distributed generation, k inc represents the reactive current increase coefficient during low voltage ride through of the inverter type power source.

[0128] Table 1 IIDG control parameters

[0129] Number Rated power / (MW) <![CDATA[k max > <![CDATA[k inc > IIDG1 0.5 1.2 1.5 IIDG2 0.5 2 1.5 IIDG3 0.7 1.2 1.5 IIDG4 0.8 1.2 1.5 IIDG5 1 1.2 3 IIDG6 1 2 3

[0130] 2) Simulation verification

[0131] ① A fault occurs at F1

[0132] Since the phase mutation angle of the positive sequence voltage is relatively large during a three-phase short circuit, the discussion is carried out under the condition of a three-phase short circuit. A three-phase short circuit with a transition resistance of 0.7 Ω and 35 Ω is set at F1, and the simulation results are shown in Table 2. In the case of a three-phase ground fault with a 35-Ω transition resistance, the buses are all in the light fault area. The regional agent can complete the fault removal without communicating with the feeder agent. The phase relationship value k between all DG and load branches and the bus is 1. For the tie branches SA and AB, the phase relationship value k with the two-side buses is [0, 1]. For the tie branches CD and AE, the phase relationship value k with the two-side buses is [1, 0]. For the tie branch BC, the phase relationship value k with the two-side buses is [0, 0]. Therefore, after the regional agents B and C on both sides of the tie branch BC communicate, they disconnect the corresponding branches B4 and C1 respectively to complete the fault isolation.

[0133] When a three-phase short circuit occurs through a 0.7Ω transition resistance at F1, regional agents B, C, and D detect that the bus is in a heavy fault area and send the phase relationship value k between the upstream and downstream connection branches of the bus and the bus to the feeder agent. After receiving multiple phase relationship values k, the feeder agent generates a sequence M in the order of upstream and downstream of the feeder: M = [0, 1, 0, 0, -1, -1]. M corresponds to: a fixed value of 0, the phase relationship value k between branch AB and the downstream bus, the phase relationship value k between branch BC and the upstream bus, the phase relationship value k between branch BC and the downstream bus, the phase relationship value k between branch CD and the upstream bus, and the phase relationship value k between branch CD and the downstream bus. Starting from the first digit of M, the first occurrence of [0, 0] is for branch BC. The feeder agent determines that the fault is located in connection branch BC and sends a tripping command to regional agents B and C to achieve fault isolation.

[0134] Table 2 Calculation results of each agent during F1 fault

[0135]

[0136]

[0137] ② Fault occurs at F2

[0138] A three-phase short circuit with 0.7Ω and 35Ω transition resistances is set at F2, and the simulation results are shown in Table 3. In the case of a three-phase ground fault through a 35Ω transition resistance, the buses are all in a light fault area. Regional agent B detects that the phase relationship value k between branch B2 and the bus is 0 and determines that the fault is located in branch B2. The phase relationship values k between the other DG branches and load branches and the bus are all 1. The phase relationship values k between the connection feeders SA, AB and the two-side buses are [0, 1], and the phase relationship values k between the connection feeders BC, CD, AE and the two-side buses are [1, 0], and there will be no misoperation.

[0139] In the case of a three-phase ground fault through a 0.7Ω transition resistance, regional agents B, C, and D detect that the corresponding buses are in a heavy fault area. Regional agent B detects that the phase relationship value k between branch B2 and the bus is 0, and additional auxiliary judgment is required. During normal operation, the positive sequence current of branch B2 is 0.027kA, and during the fault, the positive sequence current is 0.129kA, which satisfies the corresponding fault judgment formula. Therefore, it is determined that the fault is located in branch B2. The phase relationship values k between the connection branch SA and the two-side buses are [0, 1], and the phase relationship values k between the connection branch AE and the two-side buses are [1, 0], and there will be no misoperation. The feeder agent receives the phase relationship values k between the connection branches and the bus sent by regional agents B, C, and D in the heavy fault area and generates a sequence M: M = [0, 1, 1, 0, -1, -1]. There is no [0, 0] in M. Therefore, there is no fault in branches AB, BC, and CD, and regional agents C and D will not misoperate. Therefore, when regional agent B trips branch B2, other regional agents will not misoperate.

[0140] Calculation results of each agent when F2 fails in Table 3

[0141]

[0142] ③F5 fails

[0143] A three-phase short-circuit fault with a transition resistance of 0.7Ω and 35Ω is set at F5, and the simulation results are shown in Table 4. In these two fault cases, all buses are in the light fault area. For all DG branches and load branches k is 1, and for the tie branches SA, AB, BC, CD, the phase relationship value k with the two-side buses is [1,0], and for the tie branch AE, the phase relationship value k with the two-side buses is [0,0]. Therefore, the regional agents on both sides of the tie branch AE disconnect the corresponding branches A4 and E1 respectively after communication to achieve fault isolation.

[0144] Table 4 Calculation results of each agent when F5 fails

[0145]

[0146] Embodiment 2

[0147] In order to execute the method corresponding to the above Embodiment 1 to achieve the corresponding functions and technical effects, a line protection system for an active distribution network with unmeasurable branches is provided below.

[0148] As Figure 4 shown, the line protection system for an active distribution network with unmeasurable branches provided in this embodiment includes: a current and voltage acquisition unit 1, a power grid fault discrimination unit 2, a current component determination unit 3, a bus fault discrimination unit 4, a voltage component determination unit 5, a phase relationship determination unit 6, a fault type determination unit 7, a light fault processing unit 8, a heavy fault processing unit 9, and a tie branch discrimination unit 10.

[0149] Among them, the current and voltage acquisition unit 1 is used to collect the three-phase current of each branch and the three-phase voltage of each bus in the active distribution network in real time, and convert the three-phase current of each branch and the three-phase voltage of each bus into corresponding current vector values and voltage vector values by using the full-cycle Fourier algorithm.

[0150] The power grid fault discrimination unit 2 is connected to the current and voltage acquisition unit 1. The power grid fault discrimination unit 2 is used to judge whether the active distribution network has a fault according to the three-phase current of each branch, and determine the moment when the fault occurs when the active distribution network has a fault.

[0151] The current component determination unit 3 is connected to the power grid fault discrimination unit 2. The current component determination unit 3 is configured to calculate the positive-sequence current fault component of any branch according to the three-phase currents of the branch before and after the occurrence of the active distribution network fault for any branch.

[0152] The bus fault discrimination unit 4 is connected to the current component determination unit 3. The bus fault discrimination unit 4 is configured to determine whether a bus has a fault for any bus according to the positive-sequence current fault components of the branches on the bus; if the bus has a fault, the switches of the branches on the bus are disconnected.

[0153] The voltage component determination unit 5 is connected to the bus fault discrimination unit 4. The voltage component determination unit 5 is configured to calculate the positive-sequence voltage fault component of the bus according to the three-phase voltages of the bus before and after the occurrence of the active distribution network fault if the bus does not have a fault.

[0154] The phase relationship determination unit 6 is respectively connected to the current component determination unit 3 and the voltage component determination unit 5. The phase relationship determination unit 6 is configured to determine the phase relationship value between each branch on the bus and the bus according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of the branches on the bus.

[0155] The fault type determination unit 7 is connected to the current and voltage acquisition unit 1. The fault type determination unit 7 is configured to determine the fault type of the area where the bus is located according to the three-phase voltages of the bus. The fault type is a light fault area or a heavy fault area.

[0156] The light fault processing unit 8 is connected to the fault type determination unit 7. The light fault processing unit 8 is configured to determine whether each branch has a fault according to the phase relationship value between each branch on the bus and the bus if the bus is in the light fault area; if a fault occurs, the switch of the corresponding branch is disconnected.

[0157] The heavy fault processing unit 9 is connected to the fault type determination unit 7. The heavy fault processing unit 9 is configured to, for any non-link branch on the bus if the bus is in the heavy fault area, if the non-link branch is a branch where a load or a motor-type distributed power source is located, determine whether the non-link branch has a fault according to the phase relationship value between the non-link branch and the bus; if the non-link branch is a branch where an inverter-type distributed power source is located, determine whether the non-link branch has a fault according to the phase relationship value between the non-link branch and the bus and the three-phase currents of the non-link branch; if a fault occurs, the switch of the corresponding non-link branch is disconnected.

[0158] The connection branch discrimination unit 10 is connected to the heavy fault processing unit 9. The connection branch discrimination unit 10 is configured to determine whether a fault occurs in each connection branch according to the phase relationship values between each bus and each connection branch. If a fault occurs, the switch of the corresponding connection branch is disconnected.

[0159] Compared with the prior art, the beneficial effects of the active distribution network line protection system with unmeasurable branches provided in this embodiment are the same as those of the active distribution network line protection method with unmeasurable branches provided in Embodiment 1, and will not be elaborated here.

[0160] In this specification, each embodiment is described in a progressive manner. The key point of each embodiment is the difference from other embodiments. The same or similar parts among the embodiments can be referred to each other.

[0161] In this article, specific examples are used to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. An active distribution network line protection method with unmeasurable branches, characterized in that, The line protection method for an active distribution network with unmeasurable branches includes: Real-time collect the three-phase currents of each branch and the three-phase voltages of each bus in the active distribution network; Judge whether a fault occurs in the active distribution network according to the three-phase currents of each branch, and determine the moment when the fault occurs when a fault occurs in the active distribution network; For any branch, calculate the positive-sequence current fault component of the branch according to the three-phase currents of the branch before and after the occurrence of the fault in the active distribution network; For any bus, judge whether the bus has a fault according to the positive-sequence current fault components of the branches on the bus; if the bus has a fault, disconnect the switches of the branches on the bus; If the bus does not have a fault, calculate the positive-sequence voltage fault component of the bus according to the three-phase voltages of the bus before and after the occurrence of the fault in the active distribution network; Determine the phase relationship values between each branch on the bus and the bus according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of the branches on the bus; Determine the fault category of the area where the bus is located according to the three-phase voltage of the bus; the fault category is a light-fault area or a heavy-fault area; If the bus is in the light-fault area, judge whether each branch has a fault according to the phase relationship values between each branch on the bus and the bus. If a fault occurs, disconnect the switch of the corresponding branch; If the bus is in the heavy-fault area, for any non-connected branch on the bus, if the non-connected branch is the branch where a load or a motor-type distributed power source is located, judge whether the non-connected branch has a fault according to the phase relationship value between the non-connected branch and the bus; if the non-connected branch is the branch where an inverter-type distributed power source is located, judge whether the non-connected branch has a fault according to the phase relationship value between the non-connected branch and the bus and the three-phase current of the non-connected branch; if a fault occurs, disconnect the switch of the corresponding non-connected branch; judge whether each connected branch has a fault according to the phase relationship values between each bus and each connected branch. If a fault occurs, disconnect the switch of the corresponding connected branch.

2. The active distribution network line protection method with an unmeasurable branch according to claim 1, characterized in that The step of judging whether a fault occurs in the active distribution network according to the three-phase currents of each branch and determining the moment when the fault occurs when a fault occurs in the active distribution network specifically includes: For any branch, according to the three-phase currents of the branch collected within three cycles, determine whether the judgment formula ||i(t) - i(t - N)| - |i(t - N) - i(t - 2N)|| ≥ 0.1I ave holds. If it holds, a fault occurs in the active distribution network, and the fault occurrence time is the moment t. Where, i(t) is the three-phase current of the branch collected at time t, i(t-N) is the three-phase current of the branch collected at time t-N, i(t-2N) is the three-phase current of the branch collected at time t-2N, N is the number of sampling points in one cycle, and I ave is the current amplitude of the branch during the normal operation of the active distribution network.

3. The active distribution network line protection method with an unmeasurable branch according to claim 1, characterized in that, Calculating the positive-sequence current fault component of the branch according to the three-phase currents of the branch before and after the occurrence of the fault in the active distribution network specifically includes: According to the three-phase currents of the branch before the occurrence of the fault in the active distribution network, calculate the vector values of the phase currents of the branch before the occurrence of the fault in the active distribution network through the full-cycle Fourier algorithm; Calculate the positive-sequence current of the branch before the occurrence of the fault in the active distribution network according to the vector values of the phase currents of the branch before the occurrence of the fault in the active distribution network; According to the three-phase currents of the branch after the occurrence of the fault in the active distribution network, calculate the vector values of the phase currents of the branch after the occurrence of the fault in the active distribution network through the full-cycle Fourier algorithm; Calculate the positive-sequence current of the branch after the occurrence of the fault in the active distribution network according to the vector values of the phase currents of the branch after the occurrence of the fault in the active distribution network; Calculate the positive-sequence current fault component of the branch according to the positive-sequence current of the branch before and after the occurrence of the active distribution network fault.

4. The active distribution network line protection method with an unmeasurable branch according to claim 3, characterized in that Use the following formula to calculate the positive-sequence current of branch n: Among them, I n,a is the positive-sequence current of branch n, is the vector value of the a-phase current of branch n, is the vector value of the b-phase current of branch n, is the vector value of the c-phase current of branch n, and j is the imaginary unit.

5. The active distribution network line protection method with an unmeasurable branch according to claim 1, characterized in that, Determine the phase relationship value between each branch on the bus and the bus according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of each branch on the bus, specifically including: Calculate the fault component phase angle between each branch on the bus and the bus according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of each branch on the bus; Determine the phase relationship value between each branch on the bus and the bus according to the fault component phase angles between each branch on the bus and the bus.

6. The active distribution network line protection method with an unmeasurable branch according to claim 5, characterized in that Define the positive direction as the direction from the bus to the protected device; the protected device is the branch and the devices connected to the branch; use the following formula to determine the fault component phase angle between branch n on bus m and bus m: Among them, θ mn is the fault component phase angle of branch n and bus m, is the positive sequence voltage fault component of bus m, is the positive sequence current fault component of branch n; Use the following formula to determine the phase relationship value between branch n on bus m and bus m: where k mn is the phase relationship value between branch n and bus m.

7. The active distribution network line protection method with an unmeasurable branch according to claim 1, characterized in that, The phase relationship value is 0, 1 or -1; Judge whether each branch has a fault according to the phase relationship value between each branch on the bus and the bus, specifically including: For any tie branch on the bus, if the phase relationship values between the tie branch and the two adjacent buses are both 0, then disconnect the switch of the tie branch; the tie branch is the branch between two buses; For any non-tie branch on the bus, if the phase relationship value between the non-tie branch and the bus is 0, then disconnect the switch of the non-tie branch.

8. The active distribution network line protection method with an unmeasurable branch according to claim 1, characterized in that The phase relationship value is 0, 1 or -1; Judge whether the non-tie branch has a fault according to the phase relationship value between the non-tie branch and the bus and the three-phase current of the non-tie branch, specifically including: Determine whether the phase relationship value between the non-connected branch and the bus is 0. If so, calculate the positive sequence current of the non-connected branch according to the three-phase current of the non-connected branch, and determine whether the formula I n.IIDG >k IIDG I n.IIDG.ave holds. If it holds, a fault occurs in the non-connected branch; where I n.IIDG is the positive sequence current of the branch where the inverter-type distributed power source is located, I n.IIDG.ave is the effective value of the average current of the branch where the inverter-type distributed power source is located within 15 minutes during the normal operation of the active distribution network, and k IIDG is the current coefficient of the inverter-type distributed power source.

9. The active distribution network line protection method with an unmeasurable branch according to claim 1, characterized in that The active distribution network includes a feeder agent and multiple regional agents. Each regional agent corresponds to a bus, and each regional agent is used to control the disconnection of the switches of each branch on the corresponding bus; Judge whether each tie branch has a fault according to the phase relationship values between each bus and each tie branch, specifically including: Send the phase relationship values between the corresponding bus and each tie branch to the feeder agent through each regional agent; The feeder agent judges whether each tie branch has a fault according to the phase relationship values sent by each regional agent. If a tie branch has a fault, then send a fault isolation instruction to the corresponding regional agent; The regional agent disconnects the switch of the corresponding tie branch according to the fault isolation instruction.

10. An active distribution network line protection system with unmeasurable branches, characterized in that, The active distribution network line protection system with unmeasurable branches includes: A current and voltage acquisition unit for real-time acquisition of the three-phase current of each branch and the three-phase voltage of each bus in the active distribution network; A power grid fault discrimination unit connected to the current and voltage acquisition unit for judging whether the active distribution network has a fault according to the three-phase current of each branch and determining the moment when the fault occurs when the active distribution network has a fault; A current component determination unit connected to the power grid fault discrimination unit for, for any branch, calculating the positive-sequence current fault component of the branch according to the three-phase current of the branch before and after the occurrence of the active distribution network fault; The bus fault discrimination unit, connected to the current component determination unit, is used to determine, for any bus, whether the bus has a fault according to the positive-sequence current fault components of each branch on the bus; if the bus has a fault, the switches of each branch on the bus are disconnected; The voltage component determination unit, connected to the bus fault discrimination unit, is used to calculate the positive-sequence voltage fault component of the bus according to the three-phase voltages of the bus before and after the fault occurs in the active distribution network if the bus does not have a fault; The phase relationship determination unit, respectively connected to the current component determination unit and the voltage component determination unit, is used to determine the phase relationship values between each branch on the bus and the bus according to the positive-sequence voltage fault component of the bus and the positive-sequence current fault components of each branch on the bus; The fault type determination unit, connected to the current and voltage acquisition unit, is used to determine the fault type of the area where the bus is located according to the three-phase voltages of the bus; the fault type is a light fault area or a heavy fault area; The light fault processing unit, connected to the fault type determination unit, is used to determine whether each branch has a fault according to the phase relationship values between each branch on the bus and the bus if the bus is in the light fault area, and if a fault occurs, the switch of the corresponding branch is disconnected; The heavy fault processing unit, connected to the fault type determination unit, is used to determine, for any non-connected branch on the bus, whether the non-connected branch has a fault according to the phase relationship value between the non-connected branch and the bus if the bus is in the heavy fault area; if the non-connected branch is the branch where a load or a motor-type distributed power source is located, and if the non-connected branch is the branch where an inverter-type distributed power source is located, determine whether the non-connected branch has a fault according to the phase relationship value between the non-connected branch and the bus and the three-phase current of the non-connected branch; if a fault occurs, the switch of the corresponding non-connected branch is disconnected; The connected branch discrimination unit, connected to the heavy fault processing unit, is used to determine whether each connected branch has a fault according to the phase relationship values between each bus and each connected branch, and if a fault occurs, the switch of the corresponding connected branch is disconnected.

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