Residual current refers to the current vector of each phase (including the neutral line) in the low-voltage distribution line and the current that is not zero. According to Kirchhoff's current law: the vector sum of the currents flowing into any node in the circuit is equal to zero, ie ΣI = 0 (actually the normal leakage current of the line and the consumer). Therefore, in an ideal low-voltage distribution system, the residual current should be zero.
What is the difference between residual current and leakage current?1. Leakage current is a kind of residual current, and the meaning of residual current covers leakage current;
2. From the perspective of protection working principle, the leakage protector and the residual current protector are exactly the same. It is more reasonable to call the residual current protector, because not only the leakage causes the protector to operate, but also the three-phase unbalance and harmonic current will make Protector action
3. The residual current is translated according to the IEC standard, and there is no definition of leakage current;
4. Most domestic manufacturers have changed their name to residual current action protection devices, and some still use the name of the leakage protector.
The residual current value is not zero due to the following conditions:
1. The installation and construction of the distribution line is not standardized, the construction quality is rough, and the work is cut, resulting in the distribution line not meeting the actual use capacity requirements. When the steel pipe is threaded, the inner wall of the steel pipe is scratched and the wire insulation layer is scratched.
2. The distribution line is in disrepair for a long time, and its insulation layer is aging and damaged.
3. The distribution circuit is not properly designed, including the user to increase the load at will, causing the wire to be overloaded and heated, and the insulation of the wire insulation is invalid.
4, the user internal private pull the line, the installation is extremely irregular.
5. The line is affected by natural conditions, such as the wire hitting the tree, the wind blows the wire, and the air is wet, causing the wire insulation level to drop.
6, various man-made damage caused by broken lines and so on.
The residual current value is not zero. According to the national standard, the residual current value below 1000mA is used as the monitoring alarm range of the residual current monitoring equipment.
In the Kunming subway project, the residual current value detected in the individual power distribution links is too large (3A, 6-7A), and the reasons may be discussed from the following aspects:
1. Is the three-phase and single-load in the power distribution system wired correctly?
In particular, ensure that the N line required for the load distribution line of the load equipment is correct and that the PE line cannot be used instead of the N line (usually, after the single line load device replaces the N line with the PE line, the device can still work normally, but in this case, the detection is performed. The residual current will be too large).
In the incorrect connection shown in the figure above, the residual current will increase. If the above situation occurs, the N line of the load device must be corrected and the residual current can be restored.
2. Is the distance between the distribution box and the electrical equipment too long?
Because the residual current monitoring equipment is generally installed in the distribution box, if the distance between the distribution box and the power equipment is too long, the distribution line will have capacitance to the ground, that is, distributed capacitance! The longer the wire, the larger the distributed capacitance, which may cause a large leakage current and cause a large residual current at the distribution box end.
3. Is there too many circuits in the distribution box?
If a distribution box contains more power distribution circuits, and the residual current monitoring equipment is installed at the total incoming line of the distribution box (as shown in Figure 3), it is also possible that the residual current monitoring device is monitored. The residual current value is too large. For this case, you should choose to install a residual current monitoring device at each distribution branch (as shown in Figure 4).
4. Is it related to the type of load on the distribution line, such as whether there is a high-power inverter device on the distribution line? Causes multiple harmonics on the line.
5. Is the transformer connection of the residual current monitoring device correct?
Residual current is too large1, install residual current action protector
The use of residual current action protector grading protection is a fundamental way to reduce electric shock accidents and improve the reliability of power supply. It is also an inevitable trend in the development of residual current action protection in China. According to the power supply mode, economic conditions and production of residual current action protector in China's low-voltage power grid, it is feasible and most effective to use two-pole leakage protection in low-voltage power grids.
Firstly, the installation rate and operation rate of the last-stage residual current action protector should be increased. The final stage protection is to prevent direct contact with electric shock as the main target, and the respective protection surfaces are small and do not interfere with other users. High-sensitivity and fast residual current action protectors with rated leakage currents of not more than 30 mA should be selected for mobile household electrical equipment and power tools that are widely used in urban and rural households. The residual current action protector selected by the single-phase consumer should also have an over-voltage protection function to prevent the occurrence of a disconnection of the three-phase four-wire total neutral line and the wrong connection of the subscriber line to the 380V line voltage. Abnormal overvoltage damage to household appliances. It is best to use a protector with leakage, overvoltage, overload and short circuit protection, so that all abnormal conditions of single-phase consumers can be protected. For places where secondary injury occurs after electric shock, such as aerial work or electrical equipment used on the river bank, a fast residual current action protector with a leakage current of 10 mA can be installed.
The second level of protection is system total protection or branch protection. This level of protector can be installed at the low-voltage outlet of the distribution transformer or at the head end of each branch line. The protection range is the main line (or branch line), the lower line and the entrance line of the low-voltage power grid, and also serves as the end. Backup protection for the stage residual current action protector. The rated leakage current should be determined according to the actual leakage current of the protected line and equipment.
The two-stage residual current action protector constitutes a leakage protection protection network. The first-stage protector provides direct contact protection for some occasions with severe conditions and high risk of electric shock; the second-stage protector expands leakage protection. Coverage increases the safety level of the entire low-voltage grid. The two-level protection should be reasonably matched, and the leakage current and the operation time should be leveled. It is recommended to consider the rated leakage current of the upper residual current operated protector to be 2.5 to 3.0 times of the rated leakage current of the next stage. The operating time of the residual current operated protector of the upper stage is increased by one action level from the next stage of operation time, which is about 0.1~ About 0.2s.
2. Strengthen the operation management of residual current action protector
In order to make the installed residual current action protector play its proper protection, it is necessary to strengthen the operation management of the residual current action protector, and the power departments at all levels should pay attention to this work. The power departments at the provincial level and below, including the township (town) power supply stations, should be equipped with special (part-time) personnel to manage the grid safety and residual current action protectors. When using a product, the user should test the function and performance indicators of the product before use. Only products that have passed the functional performance test can be installed on site. The county-level power departments should be equipped with the necessary test equipment and qualified full-time personnel to ensure the implementation of this measure. At the same time, the residual current action protector maintenance point should be set up separately or in pieces to ensure that the damaged residual current action protector can be repaired in time.
3. The power supply department should improve the quality of service.
In the daily work, the power supply department should conduct diligent inspections, diligent maintenance, and diligent maintenance on the lines and facilities to find out the hidden dangers in time, eliminate the faults in time, and conscientiously fulfill the power supply service commitments.
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