Method and system for detecting neutral line breakage based on third harmonic energy change amount

Abstract

The application discloses a neutral line breakage detection method and system based on third harmonic energy change amount, first, the current and voltage of the A-phase equivalent load are extracted, the third harmonic component is calculated through Fourier decomposition, and the third harmonic energy can be calculated, finally, the third harmonic energy calculated at the last sampling time is compared, finally, the third harmonic change amount is compared with the set threshold value, and whether the neutral line breakage fault occurs is determined according to the result. The method can correctly distinguish the neutral line breakage fault and load fluctuation, and effectively improves the neutral line breakage detection capability.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of power systems, and particularly relates to a neutral line disconnection detection method and system based on third harmonic energy change. BACKGROUND

[0002] Low-voltage distribution system is an important part of power system, mainly adopts three-phase four-wire power supply mode, and is widely used in residential electricity, commercial and light industry fields. The low-voltage distribution system is usually composed of transformers, distribution lines, distribution boxes, electrical equipment and the like, and its core function is to deliver high-voltage power to various users in the form of low-voltage after being reduced by transformers. In the three-phase four-wire system, the neutral line is an essential component, which together with the three-phase lines forms the power supply network, and 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, ensuring 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 faults occur frequently in low-voltage distribution systems, especially in rural and remote areas where load imbalance is more common. Neutral line disconnection can cause the neutral point potential in the system to deviate, and in turn cause problems such as overvoltage of light load phase and under-voltage of heavy load phase, posing a threat to electrical equipment safety, and in severe cases, even causing fires and personal injury incidents. The harm of neutral line disconnection makes the detection of this fault significant. Existing neutral line disconnection detection methods mainly include two categories: signal injection-free type and small signal injection type. Signal injection-free type methods are usually based on voltage, zero sequence current and harmonic monitoring, and determine whether the neutral line is disconnected by analyzing parameters such as neutral point potential and three-phase harmonic change. However, when the three-phase load of the system is balanced, the changes of these parameters are small, and the detection accuracy is limited. The small signal injection type method injects direct current or small signal, and uses the loop changes under different disconnection conditions to locate the fault point, but this method has challenges in device cost and implementation complexity.

[0003] In summary, with the growth of power demand and the increase of asymmetric loads, the detection of neutral line disconnection 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. It is of great significance to develop more accurate and rapid neutral line disconnection detection technology to improve the reliability of distribution systems. SUMMARY

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

[0005] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0006] The neutral line breakage detection method based on third harmonic energy change amount comprises the following steps:

[0007] Step 1, measure the voltage of the relative neutral line of each A-phase equivalent load of the low-voltage power distribution system and the current flowing through the A-phase equivalent load, extract the third harmonic components of the voltage and current of each A-phase equivalent load through Fourier decomposition, and further calculate the third harmonic energy;

[0008] Step 2, compare the calculated third harmonic energy with the third harmonic energy obtained at the previous time Δt to obtain the third harmonic energy change amount 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, it is considered that the neutral line has a breakage fault; if at least two A-phase equivalent loads satisfy |ΔP|≥K1, it is also considered that the neutral line has a breakage fault.

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

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

[0012]

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

[0014]

[0015] The third harmonic energy of any A-phase equivalent load before the breakage position when the breakage occurs between equivalent loads:

[0016] The third harmonic energy of any A-phase equivalent load after the breakage position when the breakage occurs between equivalent loads:

[0017] Wherein, U o is the voltage at the neutral point of the power supply; U o1 is the neutral point voltage of region 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 resistance i to the equivalent impedance of the neutral line; k i is the ratio of the impedance value of the measured resistance i to the impedance of other loads; I iThe equivalent third harmonic current emitted for the ith equivalent load; k represents k equivalent loads before the broken line position; and n represents the total number of equivalent loads in the region.

[0018] Further, the third harmonic energy variation ΔP in step 2 is specifically:

[0019] The third harmonic energy variation of the A-phase equivalent load i before the broken line position when the broken line position is before all equivalent loads:

[0020]

[0021] The third harmonic energy variation of the A-phase equivalent load i before the broken line position when the broken line position is between equivalent loads:

[0022] The third harmonic energy variation of the A-phase equivalent load i after the broken line position when the broken line position is between equivalent loads:

[0023] wherein n ∑i is the impedance ratio sum of the A-phase equivalent load i and all loads, k ∑i is the impedance ratio sum of the A-phase equivalent load i and all loads before the broken line position.

[0024] Further, the setting value K2 = 300% and K1 = 120% in step 3.

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

[0026] A third harmonic energy calculation module: for measuring the voltage of the A-phase equivalent load of each region of the low-voltage power distribution system relative to the neutral line and the current flowing through the A-phase equivalent load, extracting the third component of the voltage and current of each A-phase equivalent load through Fourier decomposition, and further calculating the third harmonic energy;

[0027] A third harmonic energy variation calculation module: for comparing 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] A judgment module: for setting the setting values K1 and K2, and K2 > K1 + 100%, when at least one A-phase equivalent load satisfies |ΔP| ≥ K2, it is considered that the neutral line has a broken line fault; if at least two A-phase equivalent loads satisfy |ΔP| ≥ K1, it is also considered that the neutral line has a broken line fault.

[0029] Further, the formula of the calculated third harmonic energy is as follows:

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

[0031]

[0032] The third harmonic energy of any phase A equivalent load before the break occurs when the break occurs before all equivalent loads:

[0033]

[0034] The third harmonic energy of any phase A equivalent load before the break when the break occurs between equivalent loads:

[0035] The third harmonic energy of any phase A equivalent load after the break when the break occurs between equivalent loads:

[0036] Where U o is the voltage at the neutral of the power supply; U o1 is the neutral voltage of zone 1, Z i is the measured phase A equivalent load i; θ i is the impedance angle of the measured phase A equivalent load i; R i is the resistance portion of the measured phase A equivalent load i; n1 is the ratio of the impedance of the measured resistance i and the neutral equivalent impedance; k i is the ratio of the impedance of the measured resistance i and the other load impedances; I i is the equivalent third harmonic current from the i equivalent load; k represents the number of equivalent loads before the break; n represents the total number of equivalent loads in the zone.

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

[0038] The change in third harmonic energy of phase A equivalent load i when the break is before all equivalent loads:

[0039]

[0040] The change in third harmonic energy of phase A equivalent load i when the break is between equivalent loads before the break:

[0041] The change in third harmonic energy of phase A equivalent load i when the break is between equivalent loads after the break:

[0042] Where n ∑i is the ratio of the impedance of the phase A equivalent load i and all loads, and k ∑i is the ratio of the impedance of the phase A equivalent load i and all loads before the break.

[0043] Further, the setting 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, and the processor implements the steps of the neutral line breakage detection method based on the third harmonic energy change amount when executing the computer program.

[0045] A computer readable storage medium stores a computer program, and the computer program implements the steps of the neutral line breakage detection method based on the third harmonic energy change amount when executed by a processor.

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

[0047] The method of the present application needs to extract current and voltage information under A-phase equivalent load, and obtain the third harmonic component through Fourier decomposition, and further calculate the third harmonic energy. The determination method utilizes that when the neutral line is normally operated, the harmonics generated by each three-phase equivalent load flow into the ground through the neutral line, and when the neutral line is broken, the load after the broken position loses the effect of the neutral line, and the generated harmonics can only flow among the loads. In order to distinguish normal load fluctuation, the threshold value is set to be increased, and in order to avoid the case that the broken position is before the last load, a larger threshold value is proposed to ensure that this working condition can be identified. Therefore, the method can effectively identify the neutral line breakage, and can identify the fault when the load unbalance degree is very low, thereby effectively improving the performance of the protection. BRIEF DESCRIPTION OF DRAWINGS

[0048] The accompanying drawings are included to provide a further understanding of the present application, and are incorporated in and constitute a part of the present application. The schematic embodiments of the present application and the description thereof serve to explain the present application, and do not constitute an improper limitation on the present application.

[0049] Figure 1 It is a PSCAD simulation system diagram;

[0050] Figure 2 It is a relationship diagram of the third harmonic change amount and the threshold value under each simulation condition, wherein (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 It is a logic block diagram of the neutral line breakage detection method based on the third harmonic energy change amount characteristic. DETAILED DESCRIPTION

[0052] In order to make the person skilled in the art better understand the technical scheme of the present application, the technical scheme in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work should belong to the scope of protection of the present application.

[0053] It should be noted that the terms "first", "second" and the like in the specification and claims of the present application and the above-described drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or a chronological sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0054] Embodiment one

[0055] As shown in Figure 3 The present application is a neutral line disconnection detection method based on the characteristic of third harmonic energy change amount, including the following steps:

[0056] Step 1: Measure the voltage of the relative neutral line of each area A equivalent load and the current flowing through the load, and extract the third component of the voltage and current by Fourier decomposition, and further calculate the third harmonic energy calculation, the formula is as follows:

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

[0058]

[0059] The third harmonic energy of the A-phase equivalent load when the disconnection occurs before all equivalent loads:

[0060]

[0061] The third harmonic energy of the A-phase equivalent load before the disconnection position when the disconnection occurs between the equivalent loads:

[0062] The third harmonic energy of the A-phase equivalent load after the disconnection position when the disconnection occurs between the equivalent loads:

[0063] Wherein, Uo U is the voltage at the neutral point of the power supply. o1 Z is the neutral point voltage of region 1. i The measured equivalent load i of phase A; θ i R is the impedance angle of the equivalent load i of phase A as measured; i n1 represents the resistance of the equivalent load i of phase A being measured; n1 is the ratio of the measured impedance i to the equivalent impedance of the neutral line; k i I is the ratio of the measured impedance i to the other load impedance; i Let be the equivalent third harmonic current emitted by the i-th equivalent load; k indicates that there are k equivalent loads before the disconnection point; n indicates 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 previous time Δt to obtain the change in third harmonic energy ΔP. The calculation formula is as follows:

[0065] The change in third harmonic energy of the equivalent load i in phase A when the break point is before all equivalent loads:

[0066]

[0067] The change in third harmonic energy of the equivalent load i of phase A before the break point when the break point is within the equivalent load area:

[0068] The change in third harmonic energy of the equivalent load i in phase A after the disconnection point is within the equivalent load range:

[0069] Where, n ∑i Let k be the sum of the impedance ratios of the equivalent load i in phase A and all loads. ∑i It is the sum of the ratios of the equivalent load i of phase A and the load impedance before all disconnection locations.

[0070] Step 3: Based on experience: n ∑i The value ranges from 5 to 20, and the neutral line impedance is approximately 0.05 to 0.1 times the load impedance, resulting in n1 typically falling between 10 and 20. To minimize the impact of load fluctuations, which range from 5% to 20%, 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] Example 2

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

[0073] The protection scheme in the application is illustrated in the PSCAD simulation environment. A low-voltage distribution system is established in PSCAD, and the simulation schematic diagram is shown as follows. k1 to k5 represent different broken line fault points, G1 is a 10kV equivalent power supply, T1 is a 10kV / 380V transformer, Zline is a line impedance, Zload is an equivalent load of each region, and Zn is an 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 region is 1km, the distance between region 1 and the transformer is 1km, and the power factor of all loads is 0.9. The resistance part parameters of the equivalent load of each region are shown in Table 1. The simulation results are shown as follows. Figure 2 Figure 2 In the table, F1 to F5 are the change amounts of the third harmonic energy of the A-phase equivalent load of each region when k1 to k5 have a broken line fault, respectively, (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 the three sets of parameters, the occurrence of the broken line fault can be judged no matter where the broken line position is. By using the above neutral line broken line fault judgment scheme, the occurrence of the broken line fault can be reliably identified when the neutral line is broken, which lays a foundation for subsequent protection and maintenance.

[0074] Table 1: Impedance part parameters of the load of each region

[0075]

[0076] Example Three

[0077] The application provides a neutral line broken line detection system based on a change amount of third harmonic energy, comprising:

[0078] A third harmonic energy calculation module is configured to measure the voltage of the A-phase equivalent load of each region of a low-voltage distribution system relative to the neutral line and the current flowing through the A-phase equivalent load, extract the third component of the voltage and the current of each A-phase equivalent load through Fourier decomposition, and further calculate the third harmonic energy;

[0079] A third harmonic energy change amount calculation module is configured to compare the calculated third harmonic energy with the third harmonic energy obtained at the previous time Δt, and obtain the third harmonic energy change amount ΔP;

[0080] ​The judging module is used for setting the setting values K1 and K2, and K2>K1+100%, when at least one A-phase equivalent load satisfies |ΔP|≥K2, it is considered that the neutral line is broken; if at least two A-phase equivalent loads satisfy |ΔP|≥K1, it is also considered that the neutral line is broken.

[0081] Embodiment four

[0082] The application provides a computer device, including 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 change amount are realized.

[0083] Embodiment five

[0084] The application provides a computer readable storage medium, the computer readable storage medium stores a computer program, when the processor executes the computer program, the steps of the neutral line break detection method based on the third harmonic energy change amount are realized.

[0085] Those skilled in the art should understand that the embodiments of the application can be provided as a method, a system or a computer program product. Therefore, the application can adopt a completely hardware embodiment, a completely software embodiment or an embodiment combining software and hardware aspects. Moreover, the application can adopt 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 codes.

[0086] The application is described with reference to flowcharts and / or block diagrams of the method, device (system) and computer program product according to the embodiments of the application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams and the combination of the flows and / or blocks can be realized by computer program instructions. These computer program instructions can be provided to a general-purpose computer, a special-purpose computer, an embedded processor or other programmable data processing devices to produce a machine, so that the instructions executed by the computer or other programmable data processing devices produce a device for realizing the functions specified in the flowcharts and / or block diagrams. Figure 1 The functions specified in one flow or multiple flows and / or blocks Figure 1 The devices for realizing the functions specified in one flow or multiple flows and / or blocks.

[0087] These computer program instructions can also be stored in a computer readable memory capable of guiding the computer or other programmable data processing devices to work in a specific way, so that the instructions stored in the computer readable memory produce a product including instruction devices, which realize the functions specified in the flowcharts and / or block diagrams. Figure 1 The functions specified in one flow or multiple flows and / or blocksFigure 1 the function specified in the one or more blocks.

[0088] These computer program instructions can also be loaded into computer or other programmable data processing devices, so that a series of operation steps are performed on the computer or other programmable data processing devices to generate computer-implemented processes, so that the instructions executed on the computer or other programmable data processing devices provide processes for implementing the flows Figure 1 the flows or the plurality of flows and / or blocks Figure 1 the steps of the function specified in the one or more blocks.

[0089] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit the scope of protection, although the present application has been described in detail with reference to the above examples, those skilled in the art should understand: the person skilled in the art can make various changes, modifications or equivalent replacements to the specific embodiments of the present application after reading the present application, but these changes, modifications or equivalent replacements are all within the scope of protection of the claims of the present application.

Claims

1. A neutral line breakage detection method based on a third harmonic energy change amount, characterized by, It comprises the following steps: Step 1, measuring the voltage of the relative neutral line of each A-phase equivalent load in the low-voltage power distribution system and the current flowing through the A-phase equivalent load, extracting the third harmonic components of the voltage and current of each A-phase equivalent load through Fourier decomposition, and further calculating the third harmonic energy; The formula of the calculated third harmonic energy is as follows: The third harmonic energy of any A phase equivalent load under normal conditions: ; The third harmonic energy of any A phase equivalent load when the break occurs in front of all equivalent loads: ; The third harmonic energy of any A phase equivalent load before the break location when the break occurs between equivalent loads: ; The third harmonic energy of any A phase equivalent load after the break location when the break occurs between equivalent loads: ; wherein, U o V is the voltage at the neutral point of the power supply; U o1 V is the neutral point voltage of region 1, Z i i is the measured A-phase equivalent load; 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; n 1 is the ratio of the impedance value of the measured resistance i and the neutral equivalent impedance; k i is the ratio of the impedance value of the measured resistance i and the other load impedance; I i is the equivalent third harmonic current emitted by the i-th equivalent load; k represents the number of equivalent loads before the broken line position; n represents the total number of equivalent loads in the region; Step 2, comparing the calculated third harmonic energy with the third harmonic energy obtained at the previous time t to obtain the third harmonic energy change amount P ; The amount of change in third harmonic energy P Specifically: The amount of change in the third harmonic energy of the A-phase equivalent load i when the disconnection position is in front of all equivalent loads: ; The amount of change in the third harmonic energy of the A-phase equivalent load i before the broken line position when the broken line position is between the equivalent loads: ; The amount of change in the third harmonic energy of the A-phase equivalent load i after the broken line position when the broken line position is between the equivalent loads: ; wherein, is the ratio sum of the equivalent load i and the impedance of all loads for phase A, is the ratio sum of the equivalent load i and the impedance of all loads before the open circuit for phase A. Step 3, set the setting value K1 and K2, and K2>K1+100%, when at least one A-phase equivalent load satisfies P| ≥K2, the neutral line is considered to have a broken line fault; if at least two A-phase equivalent loads satisfy P |≥K1, the neutral line is also considered to have a broken line fault.

2. The neutral line breakage detection method based on the amount of change in third harmonic energy according to claim 1, characterized by, The setting value K2=300%, K1=120% in step 3.

3. The neutral line breakage detection system based on the third harmonic energy variation amount, characterized by It comprises: A third harmonic energy calculation module: for measuring the voltage of the relative neutral line of each A-phase equivalent load in the low-voltage power distribution system and the current flowing through the A-phase equivalent load, extracting the third harmonic components of the voltage and current of each A-phase equivalent load through Fourier decomposition, and further calculating the third harmonic energy; The formula of the calculated third harmonic energy is as follows: The third harmonic energy of any A phase equivalent load under normal conditions: ; The third harmonic energy of any A phase equivalent load when the break occurs in front of all equivalent loads: ; The third harmonic energy of any A phase equivalent load before the break location when the break occurs between equivalent loads: ; The third harmonic energy of any A phase equivalent load after the break location when the break occurs between equivalent loads: ; where, U o V is the voltage at the neutral point of the power supply; U o1 V is the neutral point voltage of region 1, Z i i is the measured A-phase equivalent load; 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; n 1 is the ratio of the impedance value of the measured impedance i and the neutral equivalent impedance; k i is the ratio of the impedance value of the measured impedance i and the other load impedance; I i is the equivalent third harmonic current emitted by the i-th equivalent load; k represents the number of equivalent loads before the broken line position; n represents the total number of equivalent loads in the region; a third harmonic energy change amount calculation module: for comparing the calculated third harmonic energy with the third harmonic energy obtained at the last time t-1 to obtain a third harmonic energy change amount P ;​ The amount of change in third harmonic energy P Specifically: The amount of change in the third harmonic energy of the A-phase equivalent load i when the disconnection position is in front of all equivalent loads: ; The amount of change in the third harmonic energy of the A-phase equivalent load i before the broken line position when the broken line position is between the equivalent loads: ; The amount of change in the third harmonic energy of the A-phase equivalent load i after the broken line position when the broken line position is between the equivalent loads: ; wherein, is the ratio sum of the equivalent load i and the impedance of all loads for phase A, is the ratio sum of the equivalent load i and the impedance of all loads before the open circuit for phase A. Judgment module: for setting the setting value K1 and K2, and K2>K1+100%, when at least one A phase equivalent load satisfies P | ≥K2, then the neutral line occurs broken line fault; if at least two A phase equivalent loads satisfy P| ≥K1, also consider that the neutral line occurs broken line fault.

4. The neutral line breakage detection system based on the amount of change in the third harmonic energy according to claim 3, characterized by, The setting value K2=300%, K1=120%.

5. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, The processor executes the computer program to realize the steps of the neutral line breakage detection method based on the third harmonic energy change amount as claimed in claim 1 or 2.

6. A computer-readable storage medium storing a computer program, the computer program comprising instructions that, when executed by a computer, cause the computer to perform the method of any one of claims 1 to 5. The computer program is executed by the processor to realize the steps of the neutral line breakage detection method based on the third harmonic energy change amount as claimed in claim 1 or 2.

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