Active power distribution network negative sequence impedance pilot protection method and system suitable for DG access

Abstract

The invention discloses an active power distribution network negative-sequence impedance pilot protection method and system suitable for DG access, and the method comprises the steps: carrying out the constraint of the negative-sequence active current outputted by an inversion-type DG outputting the negative-sequence current after a fault occurs, and enabling the output value of the inversion-type DG to be not greater than 0; the three-phase voltage and the three-phase current at the protection installation position are obtained in real time, and the negative sequence impedance at the protection installation position is calculated; judging a fault direction by using the negative sequence impedance amplitude and the phase characteristics, and assigning a fault direction identifier; and judging whether an internal fault or an external fault occurs by using the fault direction identifiers at the protection mounting positions on the two sides of the protected section, and performing fault protection. According to the method, protection criteria are designed by using the negative sequence impedance angle and amplitude at the same time, so that the negative sequence impedance properties in the active power distribution network are uniform, the protection problems in all scenes can be solved, and meanwhile, the reliability and sensitivity of fault recognition by using the negative sequence impedance phase are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection of power systems, and in particular to a negative-sequence impedance longitudinal protection method and system for active distribution networks suitable for DG access. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] With the development of renewable energy and the expansion of distribution network, more and more distributed generators (DG) of different types are connected to the distribution network. The distribution network has undergone great changes, resulting in the performance degradation of traditional three-stage overcurrent protection, which cannot meet the protection needs of active distribution network.

[0004] The existing technology adopts a differential protection scheme based on positive-sequence fault component current, phase current, sequence current, and q-axis current, which improves the adaptability of differential protection in active distribution networks; however, current differential protection requires data synchronization in principle, and relies on high-speed data channels when applied. The distribution network points are numerous and wide, and the configuration of synchronization equipment and high-speed channels for each protection terminal increases the construction cost. In order to reduce the demand for data synchronization for protection, the existing technology uses current amplitude, impedance, and current phase change to construct protection criteria; in order to reduce the pressure on communication caused by data transmission, the existing technology adopts a longitudinal protection scheme based on the transmission logic signal of the positive-sequence energy direction and current direction polarity. The above principle alleviates the pressure on the application cost of protection construction, but does not fully take into account the impact of DG control strategy, and there is a problem of limited application scenarios.

[0005] In addition, in order to solve the impact of T-connected load branches, the existing technology uses current amplitude, pseudo-power, and negative-sequence current phase to construct protection criteria; in order to solve the impact of T-connected DG branches, the existing technology compensates for the T-connected DG output current by using virtual current differential protection and longitudinal protection based on terminal positive-sequence current, or uses DG to inject harmonics to design harmonic current differential protection criteria.

[0006] The above methods provide new ideas for solving the problem of active distribution network protection, but there are still certain technical limitations, such as:

[0007] (1) The access locations of loads and DGs in active distribution networks are relatively flexible. There may be a situation where both T-connected loads and DGs are present. The access point is uncertain, and it is difficult to compensate by estimating the current. In addition, due to the different access locations of loads and DGs and the different types of DGs, there are multiple combinations. According to different combinations, the lines in active distribution networks are divided into multiple types. The above methods are only for a specific model in theoretical analysis, and can only deal with one or some types of lines, but not for all types of lines, and are not universal.

[0008] (2) There are currently two types of regulations for new energy grid connection: negative sequence current suppression strategy and negative sequence current output strategy. Under these two strategies, when an asymmetric fault occurs in the active distribution network, the negative sequence impedance angle will have the following situations: a. The negative sequence impedance angle cannot be calculated due to the absence of negative sequence current; b. The negative sequence impedance angle is resistive-capacitive due to the output of negative sequence reactive current; c. The negative sequence impedance angle is resistive-inductive due to the absorption of negative sequence reactive current; Due to the various situations of the negative sequence impedance angle, it is impossible to form a unified criterion for detection. Summary of the invention

[0009] In order to solve the above problems, the present invention proposes a negative-sequence impedance longitudinal protection method and system for an active distribution network with DG access, which uses the amplitude and phase characteristics of the negative-sequence impedance to determine the fault direction and assign a value to the fault direction identifier; the fault direction identifiers at the protection installations on both sides of the protected section are used to determine the faults inside and outside the zone and whether to send a trip signal.

[0010] In some embodiments, the following technical solutions are adopted:

[0011] A negative sequence impedance longitudinal protection method for an active distribution network with DG access, comprising:

[0012] After a fault occurs, for the inverter-type DG that outputs negative-sequence current, the negative-sequence active current it outputs is constrained so that its output value is not greater than 0;

[0013] Obtain the three-phase voltage and three-phase current at the protection installation in real time, and calculate the negative sequence impedance at the protection installation;

[0014] The negative sequence impedance amplitude and phase characteristics are used to determine the fault direction and assign a value to the fault direction identifier;

[0015] The fault direction markings at the protection installations on both sides of the protected section are used to determine whether an internal or external fault has occurred, and fault protection is performed.

[0016] As an optional solution, for the inverter-type DG that outputs negative sequence current, the negative sequence active current output by it is constrained so that its output value is not greater than 0. The specific method is:

[0017] Calculate the positive-sequence reactive current, negative-sequence reactive current, and negative-sequence active current of the fault current output by the inverter-type DG that outputs negative-sequence current respectively;

[0018] According to the calculation results, use the dq-abc transformation to synthesize three-phase currents, and judge whether the three-phase currents exceed the set current limit. If they exceed the limit, reduce the positive-sequence reactive current, negative-sequence reactive current, and negative-sequence active current of the output fault current in the same proportion to ensure that the output angle of the negative-sequence impedance remains unchanged; if they do not exceed the limit, output the positive-sequence active current according to the remaining capacity.

[0019] As an optional solution, calculate the positive-sequence reactive current I q + 、negative-sequence reactive current I q - and negative-sequence active current I d - output by the inverter-type DG that outputs negative-sequence current respectively. Specifically:

[0020]

[0021] I d - =λI q - ;

[0022] where, U + 、U - represent the positive-sequence voltage and negative-sequence voltage respectively, I N and U N represent the rated current and rated voltage respectively, and λ is the negative-sequence reactive current coefficient, and its value is less than or equal to 0.

[0023] As an optional solution, obtain the three-phase voltage and three-phase current at the protection installation location in real time, and calculate the negative-sequence impedance at the protection installation location. Specifically:

[0024] Based on the three-phase voltage, obtain the negative-sequence voltage component, and compare the amplitude of the negative-sequence voltage with the set threshold value. If the amplitude of the negative-sequence voltage is greater than the threshold value, calculate the negative-sequence impedance at the protection installation location; otherwise, return to continue obtaining the three-phase voltage and three-phase current at the protection installation location;

[0025] The negative-sequence impedance at the protection installation location is the ratio of the negative-sequence voltage phasor to the negative-sequence current phasor at the protection installation location.

[0026] As an optional solution, use the amplitude and phase characteristics of the negative-sequence impedance to judge the fault direction and assign a value to the fault direction identifier. Specifically:

[0027] When the amplitude of the negative-sequence impedance is greater than the threshold value, the fault direction identifier is assigned a value of 0;

[0028] When the amplitude of the negative sequence impedance is less than or equal to the threshold value, and the phase of the negative sequence impedance is in the positive direction range, the fault direction flag is assigned a value of 1;

[0029] When the amplitude of the negative sequence impedance is less than or equal to the threshold value and the phase of the negative sequence impedance is in the reverse direction range, the fault direction flag is assigned a value of -1.

[0030] As an optional solution, the fault direction marks at the protection installations on both sides of the protected section are used to determine whether an internal fault or an external fault has occurred. Specifically:

[0031] When the sum of the fault identifications on both sides of the protected section is greater than or equal to 1, it is judged as an internal fault and a tripping command is issued; otherwise, it is an external fault and the protection is reset.

[0032] In other embodiments, the following technical solutions are adopted:

[0033] An active distribution network negative sequence impedance longitudinal protection system suitable for DG access, comprising:

[0034] The negative-sequence active current constraint module is used to constrain the negative-sequence active current output by the inverter-type DG that outputs negative-sequence current after a fault occurs, so that its output value is not greater than 0;

[0035] Negative sequence impedance calculation module, used to obtain the three-phase voltage and three-phase current at the protection installation in real time, and calculate the negative sequence impedance at the protection installation;

[0036] A fault direction judgment module is used to judge the fault direction by using the negative sequence impedance amplitude and phase characteristics, and assign a value to the fault direction identifier;

[0037] The fault protection module is used to use the fault direction identification at the protection installations on both sides of the protected section to determine whether an internal fault or an external fault has occurred and to perform fault protection.

[0038] In other embodiments, the following technical solutions are adopted:

[0039] A terminal device comprises a processor and a memory, wherein the processor is used to implement instructions; and the memory is used to store a plurality of instructions, wherein the instructions are suitable for being loaded by the processor and executing the above-mentioned active distribution network negative sequence impedance longitudinal protection method applicable to DG access.

[0040] In other embodiments, the following technical solutions are adopted:

[0041] A computer-readable storage medium stores a plurality of instructions, wherein the instructions are suitable for being loaded and executed by a processor of a terminal device, for the above-mentioned active distribution network negative sequence impedance longitudinal protection suitable for DG access.

[0042] In other embodiments, the following technical solutions are adopted:

[0043] A computer program product includes a computer program / instruction, which, when executed by a processor, implements the above-mentioned active distribution network negative sequence impedance longitudinal protection method applicable to DG access.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] (1) The present invention constrains the negative sequence current output by the inverter-type DG. When an asymmetric fault occurs in the active distribution network, the negative sequence impedance calculated at the protection installation location has only two conditions: a. The angle is resistive-inductive; b. The amplitude is extremely large. Under this feature, the negative sequence impedance amplitude is introduced to deal with the situation where there is no negative sequence current injection; the method of the present invention uses the negative sequence impedance angle and amplitude to design the protection criterion, so that the negative sequence impedance properties in the active distribution network are unified, which can deal with protection problems in all scenarios, and at the same time improves the reliability and sensitivity of fault identification using the negative sequence impedance phase.

[0046] (2) The method of the present invention constrains the inverter-type DG of the output negative sequence current strategy so that its equivalent negative sequence impedance is resistive-inductive. On this basis, the negative sequence impedance amplitude and phase are used to determine the fault direction, taking into account all possible situations that may occur in the active distribution network due to different loads and DG access. Therefore, it is not affected by the line type and has a wide range of applications. Since the negative sequence quantity design criterion is adopted, it has a strong ability to resist transition resistance. At the same time, it has the characteristics of simple implementation, low application cost and high sensitivity.

[0047] (3) The present invention adopts a longitudinal protection scheme, in which only logic signals are transmitted on both sides of the section, without the need for high-bandwidth communication and strict synchronization, and has good economic efficiency.

[0048] Other features and advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a flow chart of the negative sequence impedance longitudinal protection method of the active distribution network applicable to DG access in an embodiment of the present invention;

[0050] Figure 2 A schematic diagram of a power distribution network simulation model according to an embodiment of the present invention;

[0051] FIG. 3( a ) is a diagram of an embodiment of the present invention. 4 Fault B 1 B 4 The simulation results of the negative sequence impedance angle and negative sequence impedance amplitude of the segment;

[0052] FIG. 3( b ) is a diagram of an embodiment of the present invention. 4 Fault B 4 B 5 The simulation results of the negative sequence impedance angle and negative sequence impedance amplitude of the segment;

[0053] FIG. 3( c ) is a diagram of an embodiment of the present invention. 4 Fault B 5 B 6 Simulation results of the segment negative-sequence impedance angle and negative-sequence impedance amplitude. DETAILED DESCRIPTION

[0054] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0055] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0056] Embodiment 1

[0057] Most distribution network faults are asymmetric faults, and the proportion of symmetric faults is less than 5%. Even symmetric faults are basically developed from asymmetric faults, so the reliable identification of asymmetric faults is particularly important. Negative sequence components can reflect asymmetric faults. Protection based on negative sequence components has high sensitivity and excellent performance, such as negative sequence directional elements, which play an important role in transmission networks. In addition, in order to meet the observation needs of distribution networks and reduce the cost of voltage transformers, voltage transformers are gradually configured in distribution networks, which provides a basis for the application of negative sequence directional elements.

[0058] Based on this, in one or more embodiments, a negative sequence impedance longitudinal protection method for an active distribution network suitable for DG access is disclosed, combined with Figure 1 , specifically including the following process:

[0059] S101: After a fault occurs, for the inverter type DG that outputs negative sequence current, the negative sequence active current output by it is constrained so that its output value is not greater than 0.

[0060] According to different control strategies, the inverter DG in the active distribution network is classified into two types: negative sequence current suppression strategy and negative sequence current output strategy.

[0061] This embodiment constrains the negative-sequence active current output by the inverter-type DG with the output negative-sequence current strategy to ensure that its output value is less than or equal to 0, so that the negative-sequence impedance angle calculated at the protection installation location during an asymmetric fault is resistive-inductive, which is beneficial to the design of the protection scheme.

[0062] Specifically, under the conditions of meeting the grid connection regulations and the requirements of the present invention, the inverter-type DG outputting negative sequence current adopts a positive and negative sequence separation strategy, which is as follows:

[0063] After the fault occurs, calculate I q + , I q - , I d - :

[0064]

[0065] I d - =λI q - ;

[0066] Among them, I N and U N are rated current and rated voltage respectively, U + and U - are the positive sequence voltage and negative sequence voltage respectively, λ is the negative sequence reactive current coefficient, and its value is less than or equal to 0.

[0067] According to the above calculation results, the three-phase current is synthesized through dq-abc transformation, and it is determined whether the three-phase current exceeds the set current limit; the current limit is generally 1.2-1.5 times the rated current.

[0068] If the limit is exceeded, I q + , I q - , I d - The same proportion is reduced, ensuring I q - , I d - The ratio remains unchanged, which ensures that the negative sequence impedance angle remains unchanged; if it does not exceed the limit, the output I d + (If I q + , I q - , I d - If the combined phase current is less than 1.2-1.5 times the rated current, the remaining margin is output as Id + ).

[0069] When an asymmetric fault occurs in the active distribution network, there are only two situations in which the negative sequence impedance is calculated at the protection installation location: a. The angle is resistive-inductive; b. The amplitude is extremely large. Under this feature, the protection criterion is designed using the negative sequence impedance angle and amplitude at the same time, so that the negative sequence impedance properties in the active distribution network are unified, which can cope with protection problems in all scenarios, and at the same time improve the reliability and sensitivity of fault identification using the negative sequence impedance phase.

[0070] S102: Acquire the three-phase voltage and three-phase current at the protection installation in real time, and calculate the negative sequence impedance at the protection installation.

[0071] In this embodiment, the negative sequence voltage is first used as the starting criterion, which is as follows:

[0072] |U - |>U set ;

[0073] In the formula, |U - | is the negative sequence voltage amplitude; U set is the threshold value, which can be adjusted according to the voltage level, measurement device error and reliability requirements in specific implementation.

[0074] If the negative sequence voltage amplitude is greater than the threshold value, it indicates that an asymmetric fault has occurred and the protection is activated; otherwise, no protection judgment is made.

[0075] If the negative sequence voltage amplitude is greater than the threshold value, the negative sequence equivalent impedance Z is set according to the current reference direction from the busbar to the line. - The definition is as follows:

[0076]

[0077] In the formula, and They are the negative sequence voltage phasor and the negative sequence current phasor respectively.

[0078] S103: Determine the fault direction by using the negative sequence impedance amplitude and phase characteristics, and assign a value to the fault direction identifier.

[0079] There may be multiple combinations of downstream protected sections. According to the different downstream access components, the sections can be divided into the following three types:

[0080] Type I: There is a motor DG connected downstream of the section;

[0081] Type II: Only inverter-type DG with negative sequence current suppression strategy is connected in the downstream of the section;

[0082] Type III: There is no motor-type DG connected to the downstream of the section, but there is an inverter-type DG connected with an output negative sequence current strategy.

[0083] For Type I and Type III lines, each element can be equivalent to a negative-sequence impedance with the same properties. The properties of each component unit remain unchanged after being connected in series and parallel. The fault direction can be determined based on the phase of the negative-sequence impedance.

[0084] For Type II lines, the negative-sequence network downstream of the fault point is open, and the calculated negative-sequence impedance is infinite. The fault direction can be determined based on the negative-sequence impedance amplitude.

[0085] It can be seen that the fault direction identification value depends on the calculated negative sequence impedance amplitude and phase characteristics, specifically:

[0086] When the amplitude of the negative sequence impedance is greater than the threshold value, the fault direction flag is assigned a value of 0;

[0087] When the amplitude of the negative sequence impedance is less than or equal to the threshold value, and the phase of the negative sequence impedance is in the positive direction range, the fault direction flag is assigned a value of 1;

[0088] When the amplitude of the negative sequence impedance is less than or equal to the threshold value and the phase of the negative sequence impedance is in the reverse direction range, the fault direction flag is assigned a value of -1.

[0089] Among them, the positive direction range and the reverse direction range are the specified change ranges of the negative sequence impedance angle; when the negative sequence impedance angle is in the positive direction range, it is judged to be in the positive direction; when it is in the reverse direction range, it is judged to be in the reverse direction.

[0090] Specifically, it can be summarized as follows:

[0091]

[0092] Among them, ∠Z - represents the phase of the negative sequence impedance, is the sensitive angle, |Z - | represents the magnitude of negative sequence impedance, Z set is the negative sequence impedance amplitude threshold; S is the fault direction identifier, S equal to 1 or 0 indicates a forward fault, and S equal to -1 indicates a reverse fault.

[0093] S104: using the fault direction markings at the protection installations on both sides of the protected section, it is determined whether an internal fault or an external fault has occurred, and fault protection is performed.

[0094] In this embodiment, when the sum of the fault direction identifiers on both sides is greater than or equal to 1, it is determined to be an internal fault and a trip command is issued; otherwise, it is determined to be an external fault and the protection is restored.

[0095] The basis for judging an in-zone fault when the sum of the fault direction identifiers on both sides is greater than or equal to 1 is as follows:

[0096] When the logic signals on both sides are both 1, it is judged as an in-zone fault. For example, in-zone faults occur in Type I and Type III lines; when the logic signals on one side are 1 and on the other side are 0, it is judged as an in-zone fault. For example, in-zone faults occur in Type II lines.

[0097] When the logic signals on one side are 1 and on the other side are -1, it is judged as an out-of-zone fault. For example, out-of-zone faults occur in Type I and Type III lines; when the logic signals on both sides are 0, it is judged as an out-of-zone fault. For example, an out-of-zone fault occurs upstream of a Type II section.

[0098] In this embodiment, a directional protection criterion is constructed based on the negative-sequence impedance characteristics of each component unit in the active distribution network. By restricting the negative-sequence current output of the inverter-type DG, the nature of the negative-sequence impedance in the active distribution network is made uniform, improving the reliability and sensitivity of fault identification using the phase of the negative-sequence impedance. The method is not affected by the line type, has excellent ability to withstand transition resistance, can handle the situation where there are T-connected loads or T-connected DG branches on the line, and has a wider application range. Only logic signals are transmitted, without the need for high-bandwidth communication and strict synchronization, which can reduce the construction cost. The method of this embodiment has strong adaptability in different operating modes and different fault scenarios.

[0099] Next, an active distribution network model is built using the electromagnetic transient simulation software PSCAD / EMTDC to simulate and verify a negative-sequence impedance pilot protection method for an active distribution network proposed in this embodiment.

[0100] The active distribution network model is as Figure 2 shown. The system adopts a small-resistance grounding method, and the grounding resistance is 10 Ω. Figure 2 In it, L is the load, B is the bus, R is the protection device, and TS is the tie switch. TS is in the open state during normal operation. The system transformer is 115 kV / 10.5 kV, with a rated capacity of 50 MVA. The impedance of the power source and the transformer is reduced to the low-voltage side, approximately 0.22j Ω. IBDG-P is an inverter-type DG that adopts a strategy to suppress negative-sequence current. IBDG-N is an inverter-type DG that adopts a strategy to output negative-sequence current, and the maximum output current is 1.2 times the rated current. MTDG is a motor-type DG, and the capacity of all DGs is 2 MW. L 1 、L 2 Both have a capacity of 6 MWA, and L T1 、L 3 Have capacities of 3 MVA and 9 MVA respectively, and the power factors are both 0.9. Lines B 1 B 2 ,B 2 B3 , B 1 B 4 , B 5 B 6 The length of T is 2 km. 2 T 3 is 1 km, B 4 T 1 ,T 1 T 2 ,T 3 B 5 The line positive sequence impedance is 0.125+0.095jΩ / km. Fault point f 1 、f 2 、f 3 、f 4 、f 5 Located in B 1 B 2 , B 2 B 3 , B 1 B 4 , T 2 T 3 , B 5 B 6 According to the above definition, B 1 B 4 For Type I line, B 2 B 3 For Type II circuit, B 1 B 2 , B 4 B 5 and B 5 B 6 It is a Type III line.

[0101] Considering the values ​​of the system negative sequence equivalent impedance and the line negative sequence equivalent impedance, the sensitivity angle in this example is Take 45°, Z set Take 200Ω.

[0102] The specific simulation analysis process is as follows:

[0103] (a) Impact of different segment types

[0104] In order to verify the effectiveness of the proposed scheme under different types of lines, 3 , f 2 , f 5 AG, AB, and BCG faults are set in sequence. The fault identification results of each section under internal and external faults are shown in Table 1.

[0105] Table 1f 3 , f 2 , f5 Simulation results of each section during fault

[0106]

[0107] (b) T-connection branch influence

[0108] Figure 2 Middle, B 4 B 5 The section contains three types of components. In order to verify the effectiveness of the proposed scheme in the line with T-connected branches, the f 4 Set BC fault at the position, the transition resistance is 5Ω, section B 1 B 4 , B 4 B 5 , B 5 B 6 The simulation results are shown in Figure 3(a)-Figure 3(c) .

[0109] f 4 For B 4 B 5 For an internal fault, Figure 3(a)-Figure 3(c) It can be seen that B 4 B 5 The calculated negative sequence impedance phases on both sides are in the positive direction, and there are IBDG-N and load access downstream of the section, R 8 The negative sequence impedance amplitude is much smaller than 200Ω, so the protection trips correctly. 3 、f 5 When the fault occurs 4 B 5 It can be seen from the protection judgment results that under the fault inside and outside the zone, even if the section contains multiple different types of T-connected branches, the proposed method can correctly judge. 4 For B 4 B 5 , B 5 B 6 For an out-of-zone fault, the negative sequence impedance calculated on both sides is also only related to the back-side access component unit, Figure 3(a)-Figure 3(c) It can be seen that segment B 4 B 5 , B 5 B 6 Therefore, the proposed scheme can be applied regardless of whether the protected section contains T-connected branches or not.

[0110] Embodiment 2

[0111] In one or more embodiments, an active distribution network negative sequence impedance pilot protection system suitable for DG access is disclosed, comprising:

[0112] The negative-sequence active current constraint module is used to constrain the negative-sequence active current output by the inverter-type DG that outputs negative-sequence current after a fault occurs, so that its output value is not greater than 0;

[0113] Negative sequence impedance calculation module, used to obtain the three-phase voltage and three-phase current at the protection installation in real time, and calculate the negative sequence impedance at the protection installation;

[0114] A fault direction judgment module is used to judge the fault direction by using the negative sequence impedance amplitude and phase characteristics, and assign a value to the fault direction identifier;

[0115] The fault protection module is used to use the fault direction identification at the protection installations on both sides of the protected section to determine whether an internal fault or an external fault has occurred and to perform fault protection.

[0116] The specific implementation method of the above modules is the same as that in Example 1 and will not be described in detail.

[0117] Embodiment 3

[0118] In one or more embodiments, a terminal device is disclosed, which includes a processor and a memory, the processor is used to implement instructions; the memory is used to store multiple instructions, and the instructions are suitable for being loaded and executed by the processor. The method for negative-sequence impedance longitudinal protection of an active distribution network suitable for DG access described in Example 1.

[0119] It should be understood that in this embodiment, the processor may be a central processing unit CPU, and the processor may also be other general-purpose processors, digital signal processors DSP, application-specific integrated circuits ASIC, off-the-shelf programmable gate arrays FPGA or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or the processor may also be any conventional processor, etc.

[0120] The memory may include a read-only memory and a random access memory, and provide instructions and data to the processor. A portion of the memory may also include a non-volatile random access memory. For example, the memory may also store information about the device type.

[0121] In the implementation process, each step of the above method can be completed by an integrated logic circuit of hardware in a processor or an instruction in the form of software.

[0122] Embodiment 4

[0123] In one or more embodiments, a computer-readable storage medium is disclosed, in which a plurality of instructions are stored, wherein the instructions are suitable for being loaded by a processor of a terminal device and executing the negative-sequence impedance longitudinal protection method for an active distribution network applicable to DG access described in Example 1.

[0124] Embodiment 5

[0125] In one or more embodiments, a computer program product is disclosed, including a computer program / instruction, characterized in that when the computer program / instruction is executed by a processor, the negative-sequence impedance longitudinal protection method for an active distribution network suitable for DG access described in Example 1 is implemented.

[0126] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A negative sequence impedance pilot protection method for active distribution network suitable for DG access, characterized in that: include: After a fault occurs, for the inverter-type DG that outputs negative-sequence current, the negative-sequence active current it outputs is constrained so that its output value is not greater than 0; Obtain the three-phase voltage and three-phase current at the protection installation in real time, and calculate the negative sequence impedance at the protection installation; The negative sequence impedance amplitude and phase characteristics are used to determine the fault direction and assign a value to the fault direction identifier; The fault direction markings at the protection installations on both sides of the protected section are used to determine whether an internal or external fault has occurred, and fault protection is performed.

2. The method for negative sequence impedance pilot protection of an active distribution network suitable for DG access as claimed in claim 1, characterized in that: For the inverter type DG that outputs negative sequence current, the negative sequence active current output is constrained so that its output value is not greater than 0. The specific method is: Calculate the fault current positive-sequence reactive current, negative-sequence reactive current and negative-sequence active current output by the inverter-type DG that outputs negative-sequence current respectively; According to the calculation results, the three-phase current is synthesized by dq-abc transformation to determine whether the three-phase current exceeds the set current limit. If it exceeds the limit, the output fault current positive sequence reactive current, negative sequence reactive current and negative sequence active current are reduced in the same proportion to ensure that the negative sequence impedance output angle remains unchanged; If the limit is not exceeded, the positive sequence active current is output according to the remaining capacity.

3. The method for negative sequence impedance pilot protection of an active distribution network applicable to DG access as claimed in claim 2, characterized in that: Calculate the output negative sequence current, the fault current, and the positive sequence reactive current I of the inverter DG output respectively. q + , negative sequence reactive current I q - and negative sequence active current I d - , specifically: I d - =λI q - ; Among them, U + , U - Represent the positive sequence voltage and negative sequence voltage respectively, I N and U N They represent the rated current and rated voltage respectively, λ is the negative sequence reactive current coefficient, and its value is less than or equal to 0.

4. The method for negative sequence impedance pilot protection of an active distribution network applicable to DG access as claimed in claim 1, characterized in that: Obtain the three-phase voltage and three-phase current at the protection installation in real time, and calculate the negative sequence impedance at the protection installation, specifically: A negative sequence voltage component is obtained based on the three-phase voltage, and the amplitude of the negative sequence voltage is compared with a set threshold value. If the amplitude of the negative sequence voltage is greater than the threshold value, the negative sequence impedance at the protection installation is calculated; otherwise, the three-phase voltage and three-phase current at the protection installation are returned to continue to be obtained; The negative-sequence impedance at the protection installation is the ratio of the negative-sequence voltage phasor to the negative-sequence current phasor at the protection installation.

5. The method for negative sequence impedance pilot protection of an active distribution network applicable to DG access as claimed in claim 1, characterized in that: The negative sequence impedance amplitude and phase characteristics are used to determine the fault direction, and the fault direction identifier is assigned a value, specifically: When the amplitude of the negative sequence impedance is greater than the threshold value, the fault direction flag is assigned a value of 0; When the amplitude of the negative sequence impedance is less than or equal to the threshold value, and the phase of the negative sequence impedance is in the positive direction range, the fault direction flag is assigned a value of 1; When the amplitude of the negative sequence impedance is less than or equal to the threshold value and the phase of the negative sequence impedance is in the reverse direction range, the fault direction flag is assigned a value of -1.

6. The method for negative sequence impedance pilot protection of an active distribution network applicable to DG access as claimed in claim 5, characterized in that: The fault direction marks at the protection installations on both sides of the protected section are used to determine whether an internal fault or an external fault has occurred. Specifically: When the sum of the fault identifications on both sides of the protected section is greater than or equal to 1, it is judged as an internal fault and a tripping command is issued; otherwise, it is an external fault and the protection is reset.

7. An active distribution network negative sequence impedance pilot protection system suitable for DG access, characterized in that: include: The negative-sequence active current constraint module is used to constrain the negative-sequence active current output by the inverter-type DG that outputs negative-sequence current after a fault occurs, so that its output value is not greater than 0; Negative sequence impedance calculation module, used to obtain the three-phase voltage and three-phase current at the protection installation in real time, and calculate the negative sequence impedance at the protection installation; A fault direction judgment module is used to judge the fault direction by using the negative sequence impedance amplitude and phase characteristics, and assign a value to the fault direction identifier; The fault protection module is used to use the fault direction identification at the protection installations on both sides of the protected section to determine whether an internal fault or an external fault has occurred and to perform fault protection.

8. A terminal device, comprising a processor and a memory, wherein the processor is used to implement instructions; and the memory is used to store multiple instructions, characterized in that: The instructions are suitable for being loaded by a processor and executing the active distribution network negative sequence impedance longitudinal protection method applicable to DG access as described in any one of claims 1-6.

9. A computer-readable storage medium storing a plurality of instructions, characterized in that: The instruction is suitable for being loaded by a processor of a terminal device and executing the active distribution network negative sequence impedance longitudinal protection suitable for DG access as described in any one of claims 1-6.

10. A computer program product comprising a computer program / instructions, characterized in that When the computer program / instruction is executed by the processor, the negative-sequence impedance longitudinal protection method for active distribution network applicable to DG access as described in any one of claims 1 to 6 is implemented.

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