Neutral line breakage detection method and system based on third harmonic energy variation

Abstract

The invention discloses a neutral line breakage detection method and system based on third harmonic energy variation, and the method comprises the steps: firstly, extracting the current and voltage of an A-phase equivalent load, calculating the third harmonic component through Fourier decomposition, obtaining the third harmonic energy through calculation, and finally comparing the third harmonic energy with the third harmonic energy calculated at the last sampling time, and finally, calculating a third harmonic variation, comparing the third harmonic variation with a set threshold value, and judging whether a neutral line breaking fault occurs according to a result. According to the method, the neutral line breakage fault and the load fluctuation can be correctly distinguished, and the neutral line breakage detection capability is effectively improved.

Description

Technical Field

[0001] The present invention belongs to the field of electric power systems, and in particular relates to a neutral line break detection method and system based on third harmonic energy variation. Background Art

[0002] The low-voltage distribution system is an important part of the power system. It mainly adopts a three-phase four-wire power supply method and is widely used in residential electricity, commercial and light industry. The low-voltage distribution system is usually composed of transformers, distribution lines, distribution boxes, power equipment, etc. Its core function is to transmit high-voltage power to various users in the form of low voltage after the transformer steps down the voltage. In the three-phase four-wire system, the neutral line is an indispensable component. It and the three-phase phase line together form a power supply network, which is mainly responsible for the balanced distribution of load voltage. When the system load is unbalanced, the existence of the neutral line enables the three-phase voltage to maintain a symmetrical state and ensure the normal operation of each phase load. The neutral line thus plays a key role in maintaining system stability and voltage balance. However, neutral line disconnection failures often occur in low-voltage distribution systems, especially in rural and remote areas where load imbalance is more common. The disconnection of the neutral line will cause the neutral point potential of the system to shift, which will cause problems such as overvoltage in the light-load phase and undervoltage in the heavy-load phase, posing a threat to the safety of electrical equipment. In severe cases, it may even cause fires and personal injuries. The hazards of a disconnected neutral line make the detection of this fault of great significance. Existing neutral line disconnection detection methods mainly include two categories: no signal injection type and small signal injection type. The no signal injection type method is usually based on the monitoring of voltage, zero-sequence current and harmonics, and determines whether the neutral line is disconnected by analyzing parameters such as neutral point potential and three-phase harmonic changes. However, when the three-phase load of the system is relatively balanced, the changes in these parameters are small, and the detection accuracy is limited. The small signal injection type method injects DC or small signals and uses the changes in the circuit under different disconnection conditions to locate the fault point, but this method has challenges in equipment cost and implementation complexity.

[0003] In summary, with the growth of power demand and the increase of asymmetric loads, the detection of neutral line break in low-voltage distribution systems has become an important issue to ensure the safe operation of power systems and protect the safety of user property. Researching and developing more accurate and rapid neutral line break detection technology is of great significance to improving the reliability of distribution systems. Summary of the invention

[0004] The purpose of the present invention is to provide a neutral line break detection method and system based on the third harmonic energy change to overcome the defects of the prior art. The present invention can correctly distinguish between load fluctuations, normal operation, neutral line break and other working conditions.

[0005] In order to achieve the above object, the present invention adopts the following technical scheme:

[0006] The neutral line break detection method based on the third harmonic energy variation comprises the following steps:

[0007] Step 1, measuring the voltage of the phase A equivalent load relative to the neutral line and the current flowing through the phase A equivalent load in each area of ​​the low-voltage power distribution system, extracting the third components of the voltage and current of each phase A equivalent load by Fourier decomposition, and further calculating the third harmonic energy;

[0008] Step 2: Compare the calculated third harmonic energy with the third harmonic energy obtained before the last time Δt to obtain the third harmonic energy change ΔP;

[0009] Step 3, set the setting values ​​K1 and K2, and K2>K1+100%. When at least one A-phase equivalent load satisfies |ΔP|≥K2, a neutral line break fault occurs; if at least two A-phase equivalent loads satisfy |ΔP|≥K1, it is also considered that a neutral line break fault occurs.

[0010] Furthermore, the formula for the third harmonic energy calculated in step 1 is as follows:

[0011] The third harmonic energy of any A-phase equivalent load under normal circumstances:

[0012]

[0013] The third harmonic energy of any A-phase equivalent load when the line is broken before all equivalent loads:

[0014]

[0015] When a line break occurs between equivalent loads, the third harmonic energy of any A-phase equivalent load before the line break position is:

[0016] When a line break occurs between equivalent loads, the third harmonic energy of any A-phase equivalent load after the line break position is:

[0017] Among them, U o is the voltage at the neutral point of the power supply; U o1 is the neutral point voltage of zone 1, Z i is the measured A-phase equivalent load i; θ i is the impedance angle of the measured A-phase equivalent load i; R i is the resistance part of the measured A-phase equivalent load i; n1 is the ratio of the impedance value of the measured impedance i to the neutral line equivalent impedance; k i is the ratio of the impedance value of the measured impedance i to other load impedances; I iis the equivalent third harmonic current emitted by the i-th equivalent load; k means there are k equivalent loads before the disconnection position; n means the total number of equivalent loads in this area.

[0018] Furthermore, the change ΔP of the third harmonic energy in step 2 is specifically:

[0019] The change of the third harmonic energy of the A-phase equivalent load i when the line is broken in front of all equivalent loads:

[0020]

[0021] When the disconnection position is between equivalent loads, the change in the third harmonic energy of the A-phase equivalent load i before the disconnection position is:

[0022] When the disconnection position is between equivalent loads, the change in the third harmonic energy of the A-phase equivalent load i after the disconnection position is:

[0023] Among them, n ∑i k is the impedance ratio of the A-phase equivalent load i and all loads, ∑i It is the ratio of the A phase equivalent load i and the load impedance before all the disconnection positions.

[0024] Furthermore, in step 3, the set values ​​K2=300%, K1=120%.

[0025] The neutral line break detection system based on the third harmonic energy variation includes:

[0026] Third harmonic energy calculation module: used to measure the voltage of the phase A equivalent load relative to the neutral line and the current flowing through the phase A equivalent load in each area of ​​the low-voltage power distribution system, extract the third components of the voltage and current of each phase A equivalent load through Fourier decomposition, and further calculate the third harmonic energy;

[0027] The third harmonic energy variation calculation module is used to compare the calculated third harmonic energy with the third harmonic energy obtained before the last time Δt to obtain the third harmonic energy variation ΔP;

[0028] Judgment module: used to set the set values ​​K1 and K2, and K2>K1+100%. When at least one A-phase equivalent load satisfies |ΔP|≥K2, a neutral line break fault occurs; if at least two A-phase equivalent loads satisfy |ΔP|≥K1, it is also considered that a neutral line break fault occurs.

[0029] Furthermore, the formula for calculating the third harmonic energy is as follows:

[0030] The third harmonic energy of any A-phase equivalent load under normal circumstances:

[0031]

[0032] The third harmonic energy of any A-phase equivalent load when the line is broken before all equivalent loads:

[0033]

[0034] When a line break occurs between equivalent loads, the third harmonic energy of any A-phase equivalent load before the line break position is:

[0035] When a line break occurs between equivalent loads, the third harmonic energy of any A-phase equivalent load after the line break position is:

[0036] Among them, U o is the voltage at the neutral point of the power supply; U o1 is the neutral point voltage of zone 1, Z i is the measured A-phase equivalent load i; θ i is the impedance angle of the measured A-phase equivalent load i; R i is the resistance part of the measured A-phase equivalent load i; n1 is the ratio of the impedance value of the measured impedance i to the neutral line equivalent impedance; k i is the ratio of the impedance value of the measured impedance i to other load impedances; I i is the equivalent third harmonic current emitted by the i-th equivalent load; k means there are k equivalent loads before the disconnection position; n means the total number of equivalent loads in this area.

[0037] Furthermore, the change in the third harmonic energy ΔP is specifically:

[0038] The change of the third harmonic energy of the A-phase equivalent load i when the line is broken in front of all equivalent loads:

[0039]

[0040] When the disconnection position is between equivalent loads, the change in the third harmonic energy of the A-phase equivalent load i before the disconnection position is:

[0041] When the disconnection position is between equivalent loads, the change in the third harmonic energy of the A-phase equivalent load i after the disconnection position is:

[0042] Among them, n ∑i k is the impedance ratio of the A-phase equivalent load i and all loads, ∑i It is the ratio of the A phase equivalent load i and the load impedance before all the disconnection positions.

[0043] Furthermore, the set value K2=300%, K1=120%.

[0044] A computer device comprises a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the neutral line break detection method based on the third harmonic energy variation when executing the computer program.

[0045] A computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of the neutral line break detection method based on the third harmonic energy variation are implemented.

[0046] Compared with the prior art, the present invention has the following beneficial technical effects:

[0047] The method of the present invention needs to extract current and voltage information from the A-phase equivalent load, and obtain the third harmonic component through Fourier decomposition, and further calculate the third harmonic energy. This judgment method utilizes that when the neutral line is operating normally, the harmonics generated by each three-phase equivalent load flow into the earth through the neutral line, and when a neutral line break occurs, the load after the break position loses the function of the neutral line, and the generated harmonics can only flow between loads. In order to distinguish normal load fluctuations, the threshold setting is increased. In order to avoid the situation where the break position is before the last load, a larger threshold is proposed to ensure that this working condition can be identified. Therefore, this method can effectively identify neutral line breaks, and can identify faults when the load imbalance is very low, effectively improving the performance of protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The drawings in the specification are used to provide further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0049] Figure 1 This is the PSCAD simulation system diagram;

[0050] Figure 2 The relationship between the third harmonic change and the threshold value in each simulation case, where (a) is the simulation result of case 1, (b) is the simulation result of case 2, and (c) is the simulation result of case 3;

[0051] Figure 3 The figure is a logic block diagram of a neutral line break detection method based on the third harmonic energy variation characteristics. DETAILED DESCRIPTION

[0052] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0053] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0054] Embodiment 1

[0055] like Figure 3 As shown, the present invention is a neutral line break detection method based on the third harmonic energy variation characteristic, comprising the following steps:

[0056] Step 1: Measure the voltage of the phase A equivalent load relative to the neutral line and the current flowing through the load in each area, and extract the third-order components of the voltage and current through Fourier decomposition, and further calculate the energy of the third-order harmonic. The formula is as follows:

[0057] The third harmonic energy of the equivalent load of phase A under normal circumstances:

[0058]

[0059] The third harmonic energy of the A-phase equivalent load when the line is broken before all equivalent loads:

[0060]

[0061] When a line break occurs between equivalent loads, the third harmonic energy of the A-phase equivalent load before the line break position is:

[0062] When a line break occurs between equivalent loads, the third harmonic energy of the A-phase equivalent load after the line break position is:

[0063] Among them, Uo is the voltage at the neutral point of the power supply; U o1 is the neutral point voltage of zone 1, Z i is the measured A-phase equivalent load i; θ i is the impedance angle of the measured A-phase equivalent load i; R i is the resistance part of the measured A-phase equivalent load i; n1 is the ratio of the impedance value of the measured impedance i to the neutral line equivalent impedance; k i is the ratio of the impedance value of the measured impedance i to other load impedances; I i is the equivalent third harmonic current emitted by the i-th equivalent load; k means there are k equivalent loads before the disconnection position; n means the total number of equivalent loads in this area.

[0064] Step 2: Compare the calculated third harmonic energy with the third harmonic energy obtained before the last time Δt to obtain the third harmonic energy change ΔP. The calculation formula is:

[0065] The change of the third harmonic energy of the A-phase equivalent load i when the line is broken in front of all equivalent loads:

[0066]

[0067] When the disconnection position is between equivalent loads, the change in the third harmonic energy of the A-phase equivalent load i before the disconnection position is:

[0068] When the disconnection position is between equivalent loads, the change in the third harmonic energy of the A-phase equivalent load i after the disconnection position is:

[0069] Among them, n ∑i k is the impedance ratio of the A-phase equivalent load i and all loads, ∑i It is the ratio of the A phase equivalent load i and the load impedance before all the disconnection positions.

[0070] Step 3: Based on experience: n ∑i The value range is 5 to 20, and the neutral line impedance is approximately 0.05 to 0.1 times the load impedance, resulting in n1 generally falling between 10 and 20. In order to minimize the impact of load fluctuations, the load fluctuation range is 5% to 20%, and the threshold is increased. Therefore, by dividing the low-voltage distribution network into multiple three-phase loads, thresholds K1 and K2 are established, where K1 is set to 120% and K2 is set to 300%.

[0071] Embodiment 2

[0072] like Figure 1 As shown, the protection scheme of the present invention is explained in the PSCAD simulation environment.

[0073] The protection scheme of the present invention is explained in the PSCAD simulation environment. A low-voltage power distribution system is established in PSCAD, and the simulation schematic diagram is shown below. k1 to k5 represent different line break fault points, G1 is a 10kV equivalent power source, T1 is a 10kV / 380V transformer, Zline is the line impedance, Zload is the equivalent load of each area, and Zn is the equivalent impedance on the line. The main parameters of the model are as follows: line positive sequence resistance: 0.022Ω / km; line positive sequence reactance: 0.001889Ω / km; line positive sequence capacitive reactance: 0.25477MΩ / m; line zero sequence resistance: 0.341Ω / km; line zero sequence reactance: 0.002434102Ω / km; line zero sequence capacitive reactance: 1.098177MΩ / m; the distance between each area is 1 km, area 1 is 1 km away from the transformer, and the power factor of all loads is 0.9. The resistance parameters of the equivalent load in each area are shown in Table 1. The simulation results are shown in Figure 2 As shown, Figure 2 In the figure, F1~F5 are the third harmonic energy changes of the equivalent load of phase A in each area when k1~k5 has a line break fault. (a) is the simulation result of the load parameters of case 1; (b) is the simulation result of the load parameters of case 2; (c) is the simulation result of the load parameters of case 3. It can be seen that under these three sets of parameters, the occurrence of the line break fault can be judged no matter where the line break occurs. The above-mentioned neutral line break fault judgment scheme can reliably identify the occurrence of the line break fault when the neutral line breaks, paving the way for subsequent protection and maintenance.

[0074] Table 1 Impedance parameters of loads in various regions

[0075]

[0076] Embodiment 3

[0077] The present invention provides a neutral line break detection system based on third harmonic energy variation, comprising:

[0078] Third harmonic energy calculation module: used to measure the voltage of the phase A equivalent load relative to the neutral line and the current flowing through the phase A equivalent load in each area of ​​the low-voltage power distribution system, extract the third components of the voltage and current of each phase A equivalent load through Fourier decomposition, and further calculate the third harmonic energy;

[0079] The third harmonic energy variation calculation module is used to compare the calculated third harmonic energy with the third harmonic energy obtained before the last time Δt to obtain the third harmonic energy variation ΔP;

[0080] Judgment module: used to set the set values ​​K1 and K2, and K2>K1+100%. When at least one A-phase equivalent load satisfies |ΔP|≥K2, a neutral line break fault occurs; if at least two A-phase equivalent loads satisfy |ΔP|≥K1, it is also considered that a neutral line break fault occurs.

[0081] Embodiment 4

[0082] The present invention provides a computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the steps of the neutral line break detection method based on the third harmonic energy variation are implemented.

[0083] Embodiment 5

[0084] The present invention provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of the neutral line break detection method based on the third harmonic energy variation are implemented.

[0085] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0086] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0087] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0088] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process in the computer or other programmable device. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0089] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit its protection scope. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that after reading the present invention, those skilled in the art can still make various changes, modifications or equivalent substitutions to the specific implementation methods of the invention, but these changes, modifications or equivalent substitutions are all within the protection scope of the pending claims of the invention.

Claims

1. A neutral line break detection method based on third harmonic energy variation, characterized in that: The steps include: Step 1, measuring the voltage of the phase A equivalent load relative to the neutral line and the current flowing through the phase A equivalent load in each area of ​​the low-voltage power distribution system, extracting the third components of the voltage and current of each phase A equivalent load by Fourier decomposition, and further calculating the third harmonic energy; Step 2: Compare the calculated third harmonic energy with the third harmonic energy obtained before the last time Δt to obtain the third harmonic energy change ΔP; Step 3, set the setting values ​​K1 and K2, and K2>K1+100%. When at least one A-phase equivalent load satisfies |ΔP|≥K2, a neutral line break fault occurs; if at least two A-phase equivalent loads satisfy |ΔP|≥K1, it is also considered that a neutral line break fault occurs.

2. The neutral line break detection method based on the third harmonic energy variation according to claim 1 is characterized in that: The formula for the third harmonic energy calculated in step 1 is as follows: The third harmonic energy of any A-phase equivalent load under normal circumstances: The third harmonic energy of any A-phase equivalent load when the line is broken before all equivalent loads: When a line break occurs between equivalent loads, the third harmonic energy of any A-phase equivalent load before the line break position is: When a line break occurs between equivalent loads, the third harmonic energy of any A-phase equivalent load after the line break position is: Among them, U o is the voltage at the neutral point of the power supply; U o1 is the neutral point voltage of zone 1, Z i is the measured A-phase equivalent load i; θ i is the impedance angle of the measured A-phase equivalent load i; R i is the resistance part of the measured A-phase equivalent load i; n1 is the ratio of the impedance value of the measured impedance i to the neutral line equivalent impedance; k i is the ratio of the impedance value of the measured impedance i to other load impedances; I i is the equivalent third harmonic current emitted by the i-th equivalent load; k means there are k equivalent loads before the disconnection position; n means the total number of equivalent loads in this area.

3. The neutral line break detection method based on the third harmonic energy variation according to claim 2 is characterized in that: The change ΔP of the third harmonic energy in step 2 is specifically: The change of the third harmonic energy of the A-phase equivalent load i when the line is broken in front of all equivalent loads: When the disconnection position is between equivalent loads, the change in the third harmonic energy of the A-phase equivalent load i before the disconnection position is: When the disconnection position is between equivalent loads, the change in the third harmonic energy of the A-phase equivalent load i after the disconnection position is: Among them, n ∑i k is the impedance ratio of the A-phase equivalent load i and all loads, ∑i It is the ratio of the A phase equivalent load i and the load impedance before all the disconnection positions.

4. The neutral line break detection method based on the third harmonic energy variation according to claim 1, characterized in that: In step 3, the set values ​​K2=300%, K1=120%.

5. A neutral line break detection system based on the third harmonic energy variation, characterized in that: include: Third harmonic energy calculation module: used to measure the voltage of the phase A equivalent load relative to the neutral line and the current flowing through the phase A equivalent load in each area of ​​the low-voltage power distribution system, extract the third components of the voltage and current of each phase A equivalent load through Fourier decomposition, and further calculate the third harmonic energy; The third harmonic energy variation calculation module is used to compare the calculated third harmonic energy with the third harmonic energy obtained before the last time Δt to obtain the third harmonic energy variation ΔP; Judgment module: used to set the set values ​​K1 and K2, and K2>K1+100%. When at least one A-phase equivalent load satisfies |ΔP|≥K2, a neutral line break fault occurs; if at least two A-phase equivalent loads satisfy |ΔP|≥K1, it is also considered that a neutral line break fault occurs.

6. The neutral line break detection system based on the third harmonic energy variation according to claim 5, characterized in that: The formula for calculating the third harmonic energy is as follows: The third harmonic energy of any A-phase equivalent load under normal circumstances: The third harmonic energy of any A-phase equivalent load when the line is broken before all equivalent loads: When a line break occurs between equivalent loads, the third harmonic energy of any A-phase equivalent load before the line break position is: When a line break occurs between equivalent loads, the third harmonic energy of any A-phase equivalent load after the line break position is: Among them, U o is the voltage at the neutral point of the power supply; U o1 is the neutral point voltage of zone 1, Z i is the measured A-phase equivalent load i; θ i is the impedance angle of the measured A-phase equivalent load i; R i is the resistance part of the measured A-phase equivalent load i; n1 is the ratio of the impedance value of the measured impedance i to the neutral line equivalent impedance; k i is the ratio of the impedance value of the measured impedance i to other load impedances; I i is the equivalent third harmonic current emitted by the i-th equivalent load; k means there are k equivalent loads before the disconnection position; n means the total number of equivalent loads in this area.

7. The neutral line break detection system based on the third harmonic energy variation according to claim 5, characterized in that: The change in third harmonic energy ΔP is specifically: The change of the third harmonic energy of the A-phase equivalent load i when the line is broken in front of all equivalent loads: When the disconnection position is between equivalent loads, the change in the third harmonic energy of the A-phase equivalent load i before the disconnection position is: When the disconnection position is between equivalent loads, the change in the third harmonic energy of the A-phase equivalent load i after the disconnection position is: Among them, n ∑i k is the impedance ratio of the A-phase equivalent load i and all loads, ∑i It is the ratio of the A phase equivalent load i and the load impedance before all the disconnection positions.

8. The neutral line break detection system based on the third harmonic energy variation according to claim 5, characterized in that: Set value K2=300%, K1=120%.

9. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the steps of the neutral line break detection method based on the third harmonic energy variation amount as claimed in any one of claims 1 to 4 are implemented.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the steps of the neutral line break detection method based on the third harmonic energy variation amount as claimed in any one of claims 1 to 4 are implemented.

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