Method for neutral line breakage detection and leakage protection of power distribution network

Abstract

The invention provides the method for detecting the disconnection of the neutral line of the low-voltage power distribution network by adopting a direct-current power supply injection mode, and considering that the detection mode is possibly mutually influenced with the work of an electric leakage protection device based on a current transformer, the electric leakage protection device is added while the disconnection of the neutral line is detected, so that the detection accuracy is improved. Finally, neutral line breakage and electric leakage of electrical equipment of each user of the power distribution network are detected at the same time, and protection is carried out, so that severe conditions such as burning of the electrical equipment and abnormal operation of a load can be prevented.

Description

TECHNICAL FIELD

[0001] The technical field of the patent is neutral line fault detection, in particular to neutral line break detection and leakage protection in low-voltage distribution network. BACKGROUND

[0002] At present, the power supply mode adopted by the power system is mostly three-phase four-wire system. In the three-phase four-wire low-voltage distribution network, the current flowing through the neutral line includes three-phase nonlinear load harmonic current and three-phase unbalanced current. In the case of three-phase load asymmetry, the unbalanced current flows through the neutral line to the system side to ensure that the potential of the neutral line is zero, so as to eliminate the neutral point potential change to keep the phase voltage of each phase load symmetrical, that is, the phase voltage of each phase load is equal to the phase voltage of the power supply, and is independent of load change. If one phase of the three-phase is disconnected, only the phase will be affected, and the voltage of the remaining two phases remains unchanged, and the electrical equipment connected to the two phases can still work normally.

[0003] Most of the loads in the distribution network are asymmetric, so there is unbalanced current flowing through the neutral line. If the neutral line is disconnected, it will inevitably cause the neutral point of the three-phase load to deviate, the voltage of the lighter load phase will rise above the rated value, which may cause the electrical equipment to burn out, and the voltage of the heavier load phase will decrease, so that the electrical equipment cannot work normally. Therefore, in order to ensure that the phase voltage of the three-phase load is equal to the voltage of the power supply, it is necessary to detect the disconnection of the neutral line and take protective measures. SUMMARY

[0004] The present application proposes a method for detecting the disconnection of the neutral line in the low-voltage distribution network by injecting a direct current power supply.

[0005] The theoretical scheme adopted by the present application is as follows:

[0006] (1) Neutral line break detection

[0007] The main circuit structure of the neutral point direct current source scheme at the load side is shown in Figure 1 When the neutral line is not disconnected, the voltage across the current source is small because the resistance of the distribution transformer and the line is much smaller than the resistance of the load. After the neutral line is disconnected, the voltage across the current source increases due to the increased resistance. The disconnection of the neutral line can be determined according to the change of the voltage across the current source.

[0008] Parallel resonant branch and load in parallel operation, where the neutral point in series DC current source, DC current source and smaller resistance and diode in parallel. Parallel resonant branch is to isolate the AC power, avoid the DC current source on the AC system impact. The role of resistance is the neutral line after the break, the DC current mainly through the resistance, through the load of DC is small, so the resistance voltage for the judgment of the current source voltage rise provides a reference value, that is, the neutral line after the break, the current source voltage almost rise for the resistance value and the current source set value of the product, in order to can obviously see the change of the voltage across the current source, the selection of DC resistance needs to be much larger than the resistance value of transformer and line. The role of diode is the voltage of DC current source clamping in 0.7V, to prevent damage to the power supply.

[0009] (2) Neutral line break positioning

[0010] The power supply bureau to the power unit to transport three-phase alternating current, three-phase power to different users, the existence of the neutral line is to eliminate the neutral point point change, so that the three-phase voltage symmetry, so only need to install neutral line break detection device in front of each power unit. Each power unit in parallel operation, if a power unit neutral line break, the circuit breaker of the power unit will act, so that the load and power supply are disconnected, so as to not affect the normal power supply of other power units.

[0011] The neutral line break positioning principle is shown in Figure 2 Each power unit in line end setting detection device, through the different position detection device break signal to judge the neutral line break position. For example, three power units, if the position 0 neutral line break, then the three detection devices after the break will send break signal, similarly, if the position 1 neutral line break, detection device 2 and detection device 3 will send break signal. If the neutral line branch line position 4, 5, 6 break, the corresponding detection 1, detection 2, detection 3 will send break signal.

[0012] It can be seen that when the neutral line trunk line break, the detection device after the break will send break signal, and the power unit after the break will need to disconnect the power supply. When the neutral line branch line break, the detection device of the corresponding power unit will send break signal, and the power unit will disconnect the power supply.

[0013] (3) Leakage protection principle

[0014] Leakage protector can ensure that when the low-voltage power grid is touched by human (phase and ground) and equipment (ground) leakage, it can quickly cut off the power supply, so that the person in danger is separated or the leakage equipment stops running. The leakage protection principle is shown in Figure 3As shown, the current transformer is a detection element for detecting leakage protection, and the line current (three-phase current and neutral line current) is taken as the primary side coil current. In the absence of leakage, the vector sum of the line currents is zero according to Kirchhoff's current law, and the magnetic flux generated in the core is zero, so there is no induced current in the secondary side coil. When leakage occurs, the vector sum of the primary side currents is not zero, and the alternating magnetic flux generated in the core induces a current in the secondary side coil. When the current is greater than a certain value, the circuit breaker disconnects the power supply, achieving leakage protection.

[0015] For single-phase electrical equipment, a current transformer is also used to detect leakage current. The primary side current of the transformer is the single-phase current and the neutral line current. When the electrical equipment has a leakage fault, the vector sum of the primary side currents is not zero, and a current is induced in the secondary side of the current transformer. When its value is greater than the action value, the circuit breaker acts to disconnect the electrical equipment.

[0016] The internal structure of a three-phase electrical unit is shown in Figure 4 Each electrical unit contains multiple users, and the users use single-phase electricity in three-phase electricity. The neutral line break detection is used for three-phase electricity used by the entire electrical unit. When the voltage across the DC current source is greater than the action value, it is considered that the system has a neutral line break fault, and a break signal is sent out, and the circuit breaker S disconnects the line. Three-phase leakage protection is also used for the entire electrical unit. The current transformer detects the vector sum of the three-phase line and neutral current. When the effective value of the current vector sum is greater than the action value, it is considered that the system has a leakage, and the circuit breaker disconnects the power supply. Considering that the three-phase leakage protection causes the total circuit breaker to act when a leakage fault occurs, resulting in power failure of the entire electrical unit, a one-way leakage protection is used for each user. When a leakage fault occurs, only the user with leakage is disconnected, reducing the power failure area.

[0017] The advantage of the present application is to provide a method for detecting the break of the neutral line of the low-voltage distribution network by injecting a DC power supply, and to add a leakage protection device, which ultimately realizes the simultaneous detection of the break of the neutral line of each user in the distribution network and the leakage of electrical equipment, and provides protection, which is of great significance for the normal operation of the power grid and equipment. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The DC signal injection detection principle diagram includes a three-phase voltage source, a step-down transformer, a three-phase load, a parallel resonance branch, and a DC current injection module.

[0019] Figure 2 The neutral line break positioning principle diagram includes three electrical units and three detection units.

[0020] Figure 3 The leakage protection principle diagram.

[0021] Figure 4 The internal structure of the three-phase power unit. DETAILED DESCRIPTION

[0022] The specific implementation method of the broken line detection and leakage protection of the neutral line in the low-voltage distribution network is as follows:

[0023] When the neutral line broken line detection is performed, the voltage change across the current source needs to be measured mainly, if the neutral line is not broken, since the resistance of the distribution transformer and the line is much smaller than the load resistance, the voltage across the current source is small, after the neutral line is broken, since the resistance is increased, the voltage across the current source is increased. Therefore, the broken line condition of the neutral line can be judged according to the voltage change across the current source.

[0024] When the neutral line broken line positioning detection is performed, the neutral line broken line detection device needs to be installed in front of each power unit. Each power unit operates in parallel, if the neutral line of a certain power unit is broken, the circuit breaker of the power unit acts to disconnect the load from the power supply, so as not to affect the normal power supply of other power units. The detection device is arranged at the incoming line end of each power unit, and the broken line signal of the detection device at different positions is used to judge the broken line position of the neutral line. Taking three power units as an example, if the neutral line at position 0 is broken, the three detection devices after the broken line all send the broken line signal, and the detection device 2 and the detection device 3 send the broken line signal when the neutral line at position 1 is broken. If the neutral line branch lines at positions 4, 5 and 6 are broken, the corresponding detection 1, detection 2 and detection 3 send the broken line signal.

[0025] When the leakage protection is performed, for the single-phase power equipment, the current transformer is used to detect the leakage current, the primary side current of the current transformer is the single-phase current and the neutral line current, when the power equipment occurs the leakage fault, the primary side current vector sum is not zero, the current at the secondary side of the current transformer is induced, when the value is greater than the action value, the circuit breaker acts to cut off the power equipment. For the three-phase power used by the entire power unit, when the voltage across the detection device of the direct current source is greater than the action value, it is considered that the system occurs the neutral line broken line fault, the broken line signal is sent, and the circuit breaker S cuts off the line. The three-phase leakage protection is also for the entire power unit, the current transformer detects the vector sum of the three-phase line and the neutral line current, when the effective value of the current vector sum is greater than the action value, it is considered that the system occurs the leakage, and the circuit breaker cuts off the power supply.

Claims

1. A method for detecting neutral line break using DC current injection, characterized in that Parallel resonant branch and load parallel operation, in which the neutral point series DC current source, DC current source and smaller resistance and diode parallel, by measuring the voltage across the DC current source to determine whether the neutral line is broken.

2. A method employing a plurality of detection units, characterized by The neutral line broken detection device is installed before each power unit. Each power unit is in parallel operation. If the neutral line of a power unit is broken, the circuit breaker of the power unit will act, so that the load is disconnected from the power supply, thereby not affecting the normal power supply of other power units.

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