A method for detecting leakage fault in low voltage distribution network

Abstract

A method for detecting leakage faults in a low-voltage distribution network belongs to the technical field of relay protection of power systems. It is characterized by comprising the following steps: step a, real-time acquisition of neutral line current and residual current; step b, obtaining components of neutral line current and components of residual current; step c, determining the residual current component i R (t) is greater than a preset threshold value γ, if so, execute step d, otherwise execute step a; in step d, the line terminal unit starts the fault judgment condition, and if the fault judgment condition is met, execute step f, otherwise execute step e; in step e, the line is disturbed by a non-grounding fault; in step f, a leakage fault occurs downstream of the line. The present low-voltage distribution network leakage fault detection method solves the problem that the existing leakage protection device of the current TT system is difficult to function due to the large inherent residual current, and overcomes the defect that the TN system cannot realize leakage fault protection and positioning due to the existence of repeated grounding points.

Description

Technical Field

[0001] The invention discloses a method for detecting leakage faults in a low-voltage distribution network, belonging to the technical field of relay protection of power systems. Background Art

[0002] In low-voltage power distribution networks, when the insulation of lines or equipment is damaged, the system will cause leakage current to the ground, and these leakage points are very fatal electric shock and fire hazards. Leakage faults often occur between the phase line and the ground. Due to the high resistance of the residual current loop, the fault current is tens to hundreds of milliamperes, and it is generally impossible to use conventional overcurrent protection to detect and remove the fault. However, when the current flowing through the human body reaches 30mA, it is enough to cause ventricular fibrillation within seconds to minutes and cause death. Therefore, when a leakage fault such as insulation damage, joint leakage or electric shock occurs in the low-voltage power distribution system, that is, an electric shock accident, the fault must be detected in time and the line or equipment must be taken out of operation.

[0003] At present, the detection and removal of leakage faults are mainly achieved through residual current protection devices (RCDs). RCDs are divided into four types according to their working principles:

[0004] (1) Amplitude comparison type, also known as modulus type, its basic principle is: when the amplitude of the residual current detected is greater than its protection setting value, the protection will be activated. The defect of this type is that it uses the amplitude of the residual current to detect leakage faults, which is mainly suitable for systems with small inherent residual currents. However, for common TT systems, although the inherent residual current of the end-user branch line is generally not greater than 30mA, due to the large power supply area, the current at the trunk outlet is the sum of the currents of each branch, and its value is generally greater than the setting value of the protection device (300-500mA). Therefore, the TT system has the defect of large inherent residual current, which makes this type of protection unable to be put into use.

[0005] (2) Current pulse type. Its basic principle is to detect the amplitude change of the residual current, and the protection will be activated when the amplitude change is greater than the protection setting value.

[0006] (3) Amplitude and phase detection type. Its basic principle is: on the basis of detecting the change in the residual current amplitude, the change in the phase angle difference between the residual current and the reference voltage is detected. When the change in amplitude and the change in phase angle are both greater than the protection setting value, the protection is activated.

[0007] The above three types also have a common defect that due to the randomness of the phase angle between the ground fault current and the inherent residual current, the change in the residual current amplitude is not obvious or even negative, resulting in protection failure.

[0008] (4) Phase mutation type. Its basic principle is to detect the phase change of the residual current and operate when the phase change exceeds the protection setting value. The defects of this type are: although the phase change is used to detect leakage faults, it cannot overcome the influence of repeated grounding points in the system, cannot be applied to TN systems, and will cause false operation when the line is switched. Summary of the invention

[0009] The technical problem to be solved by the present invention is: to overcome the deficiencies of the prior art and to provide a method for detecting leakage faults in a low-voltage distribution network which solves the problem that the existing leakage protection device is difficult to function due to the large inherent residual current in the current TT system, while overcoming the defect that the leakage fault protection and positioning cannot be realized due to the existence of repeated grounding points in the TN system.

[0010] The technical solution adopted by the present invention to solve the technical problem is: the low-voltage distribution network leakage fault detection method includes a line terminal unit arranged at the transformer outlet and in each branch line, each line terminal unit serves as a monitoring point, and is characterized in that it includes the following steps:

[0011] Step a, the line terminal unit collects the neutral line current and residual current of the corresponding monitoring point in real time;

[0012] Step b: After the line terminal unit collects the corresponding neutral current and residual current, it filters the collected neutral current and residual current to obtain the component i of the neutral current. N (t) and the residual current component i R (t);

[0013] Step c: The line terminal unit determines the collected residual current component i R (t) is greater than the preset threshold γ, if the residual current component i R (t) is greater than the threshold value γ, and step d is executed. If the residual current component i R (t) is less than or equal to the threshold γ, return to step a;

[0014] Step d, when the residual current component i R (t) is greater than the preset threshold value γ, the line terminal unit starts the fault judgment condition. If the neutral line current component i N (t) and the residual current component i R (t) if both the amplitude relationship criterion and the phase relationship criterion are satisfied, execute step f; otherwise, execute step e;

[0015] Step e: the neutral current component i of the current monitoring point N (t) and the residual current component i R(t) if the amplitude relationship criterion and the phase relationship criterion are not satisfied at the same time, the line terminal unit determines that the corresponding line is disturbed by a non-grounding fault and returns to step a;

[0016] Step f: When the line terminal unit determines that a leakage fault occurs downstream of its corresponding line.

[0017] Preferably, the threshold γ is set to 200-400mA for the outlet of the distribution transformer; 400-600mA for the line with a power supply radius greater than 250m; 40-60mA for each branch line; and 20-40mA for the user's incoming line.

[0018] Preferably, in the step e, the amplitude relationship criterion is:

[0019]

[0020] Among them, β is between 1.0 and 1.5.

[0021] Preferably, the value of β is 1.2.

[0022] Preferably, in the step e, the phase relationship criterion is:

[0023]

[0024] Among them, the value of α is 5°~15°.

[0025] Preferably, the value of α is 10°.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] In the present low-voltage distribution network leakage fault detection method, by setting measurement points (line terminal units) on the transformer outgoing line and branch line, and judging whether a leakage fault occurs in the line downstream of the monitoring point according to the magnitude relationship and phase relationship between the neutral line current component and the residual current component in the line collected by the monitoring point, compared with the traditional leakage fault judgment method, the problem that the existing leakage protection device of the current TT system is difficult to function due to the large inherent residual current is solved, and at the same time, the defect of the TN system that leakage fault protection and positioning cannot be realized due to the existence of repeated grounding points is overcome. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The figure is a flow chart of a method for detecting leakage faults in a low voltage distribution network. DETAILED DESCRIPTION

[0029] Figure 1 The best embodiment of the present invention is shown below in conjunction with the attached Figure 1 The present invention is further described.

[0030] like Figure 1 As shown, a method for detecting leakage faults in a low-voltage distribution network comprises the following steps:

[0031] Step 1001, start;

[0032] The low voltage distribution network leakage fault detection method is started to be implemented.

[0033] Step 1002, collecting neutral line current and residual current;

[0034] The line terminal units (LTUs) installed in the transformer outgoing lines and each branch line of the distribution line are used as monitoring points. The monitoring points set at the transformer outgoing lines collect the neutral current and residual current at the transformer line outlet in real time. The line terminal units set in each branch line collect the neutral current and residual current at the outlet of each branch line in real time.

[0035] Step 1003, respectively obtaining components of the neutral line current and the residual current;

[0036] After each line terminal unit collects the neutral current and residual current at the corresponding line outlet, it filters the collected neutral current and residual current to obtain the component i of the neutral current. N (t) and the residual current component i R (t).

[0037] Step 1004, whether the residual current component exceeds a preset threshold;

[0038] The line terminal unit determines the collected residual current component i R (t) is greater than the preset threshold γ, that is:

[0039] i R (t)>γ

[0040] If the above judgment formula is met, the fault judgment condition is started and step 1005 is executed. If the above judgment formula is met, return to step 1002.

[0041] In the judgment formula i R(t)>γ, γ is the starting criterion, and γ takes different values ​​according to the different installation positions in the three-level residual current protection of the low-voltage line: for the distribution transformer outlet, γ is set to 200-400mA, preferably 300mA. For lines with a large power supply radius (greater than 250m), γ is set to 400-600mA, preferably 500mA. For each branch line, γ is set to 40-60mA, preferably 50mA. For the user's incoming line, γ is set to 20-40mA, preferably 30mA.

[0042] Step 1005, the residual current meets the fault judgment condition;

[0043] The line terminal unit determines the collected residual current component i R (t) Whether the following fault determination conditions are met:

[0044]

[0045] Wherein, β is the amplitude relationship criterion of the fault judgment condition, wherein β is 1.0 to 1.5, preferably 1.2; α is the phase relationship criterion of the fault judgment condition, wherein α is 5° to 15°, preferably 10°.

[0046] If the line terminal unit detects that the component i of the neutral current it detects N (t) and the residual current component i R (t) When both the amplitude relationship criterion and the phase relationship criterion are satisfied, step 1007 is executed. If the line terminal unit detects that the component i of the neutral line current detected by it is N (t) and the residual current component i R (t) If both the amplitude relationship criterion and the phase relationship criterion are not satisfied simultaneously, execute step 1006.

[0047] Step 1006, non-ground fault disturbance;

[0048] When the line terminal unit determines that the corresponding line satisfies both the amplitude relationship criterion and the phase relationship criterion, it determines that the corresponding line has a residual current component i caused by a non-ground fault disturbance. R (t) is greater than a preset threshold γ.

[0049] Step 1007: a leakage fault occurs at the current monitoring point;

[0050] When the line terminal unit determines that a leakage fault occurs downstream of its corresponding line.

[0051] In this low-voltage distribution network leakage fault detection method, a leakage fault detection method is established by setting measurement points (line terminal units) on the transformer outgoing line and branch line, and integrating the magnitude relationship and phase relationship between the neutral current component and the residual current component during the fault: when non-fault operations such as load switching or phase zero short circuit faults occur, the neutral current component is greater than the residual current component and the phases of the two are not equal; when a leakage fault occurs, the residual current component is greater than the neutral current component and the phase angle difference between the two is close to zero. If the current measurement point detects that the residual current component is greater than the neutral current component and the phase angle difference between the two is close to zero, it is judged that there is a leakage fault downstream of the measurement point.

[0052] Therefore, compared with the traditional leakage fault judgment method, the present low-voltage distribution network leakage fault detection method solves the problem that the existing leakage protection device of the current TT system is difficult to function due to the large inherent residual current. At the same time, it overcomes the defect of the TN system that leakage fault protection and positioning cannot be achieved due to the existence of repeated grounding points.

[0053] The above is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

Claims

1. A method for detecting leakage faults in a low-voltage distribution network, comprising a line terminal unit arranged at the transformer outlet and in each branch line, each line terminal unit being used as a monitoring point, characterized in that: The steps include: Step a, the line terminal unit collects the neutral line current and residual current of the corresponding monitoring point in real time; Step b: After the line terminal unit collects the corresponding neutral current and residual current, it filters the collected neutral current and residual current to obtain the component i of the neutral current. N (t) and the residual current component i R (t); Step c: The line terminal unit determines the collected residual current component i R (t) is greater than the preset threshold γ, if the residual current component i R (t) is greater than the threshold value γ, and step d is executed. If the residual current component i R (t) is less than or equal to the threshold γ, return to step a; Step d, when the residual current component i R (t) is greater than the preset threshold value γ, the line terminal unit starts the fault judgment condition. If the neutral line current component i N (t) and the residual current component i R (t) if both the amplitude relationship criterion and the phase relationship criterion are satisfied, execute step f; otherwise, execute step e; Step e: the neutral current component i of the current monitoring point N (t) and the residual current component i R (t) if the amplitude relationship criterion and the phase relationship criterion are not satisfied at the same time, the line terminal unit determines that the corresponding line is disturbed by a non-grounding fault and returns to step a; Step f, when the line terminal unit determines that a leakage fault occurs downstream of its corresponding line; In the step e, the amplitude relationship criterion is: Among them, β takes the value of 1.0 to 1.5; The phase relationship criterion is: Among them, the value of α is 5°~15°.

2. The method for detecting leakage faults in a low voltage distribution network according to claim 1, characterized in that: The threshold γ is set to 200-400mA for the outlet of the distribution transformer; 400-600mA for the line with a power supply radius greater than 250m; 40-60mA for each branch line; and 20-40mA for the user's incoming line.

3. The method for detecting leakage faults in a low voltage distribution network according to claim 1, characterized in that: The β value is 1.

2.

4. The method for detecting leakage faults in a low voltage distribution network according to claim 1, characterized in that: The value of α is 10°.

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