In the three-phase four-wire low-voltage power supply system, the function of the neutral line is to ensure that the impedance on the zero line is zero when the three-phase load is asymmetrical, so as to eliminate the neutral point displacement, so that the voltages of the phases remain symmetrical, that is, each The phase voltage of the phase load is always equal to the phase voltage of the power supply and is independent of load changes. Once one phase of the three phases breaks, it only affects the phase, and the other two phases remain unchanged, ensuring that the electrical equipment connected to the two phases can still work normally.
However, if the neutral line in the three-phase four-wire is disconnected for some reason, when the three-phase load is asymmetrical, the neutral point displacement of the transformer will occur, causing the three-phase voltage to be unbalanced, that is, some phase voltages are too high and may burn out. Electrical equipment, some phase voltage is too low, electrical equipment can not be used normally.
Judgment of the situation of zero line disconnection
(1) A zero-line break occurs in the single-phase power supply range, the electric light in the fault range is not bright, and other electrical appliances cannot be used. At this time, the xenon lamp electroscope is used for electroscope, and the phase line and the neutral line are bright; the digital electroscope is used to check The phase voltage is displayed on the electricity, phase and neutral lines; however, there is no voltage indication when measured with a voltmeter. According to the above situation, it can be determined that the neutral line is disconnected in the single-phase power supply range.
(2) A zero-line open circuit fault occurs in one branch of the three-phase four-wire system. The specific performance is that within the power supply range of this branch line, some users have insufficient brightness of the lamp, the fluorescent lamp cannot be activated, the brightness of the TV is reduced, the image is reduced, and there is a lack of Voltage-protected appliances cannot be turned on or automatically turned off. However, some of the user's voltage is obviously increased, the electric light is particularly bright, the electric fan speed is increased, and the situation is serious, and the electric lamp or other electrical appliances are quickly burned. If the above occurs, it can be determined that the branch zero line is broken.
(3) The zero-line open circuit fault occurs in the power supply range of the three-phase distribution transformer, that is, the zero-line bus line is broken. The specific performance is the same as the zero-line open-circuit fault of the three-phase four-wire branch, but the scope is larger, the damage is more serious, and the loss is more serious. More huge.
The main reason for the zero line disconnection
(1) The three-phase load is seriously unbalanced, the zero line current is too large or the cross-sectional area of ​​the neutral wire is too small, and the zero line is blown.
(2) Poor contact at the zero wire joint causes sparking, which takes a long time and causes the zero line to open.
(3) The neutral link of the distribution transformer is in poor contact with the wire, and the maintenance is not in place, causing the zero line to open.
(4) The internal zero line lead-out line of the distribution transformer is open.
(5) A three-phase four-wire line zero line is equipped with a fuse or a separate switch, and the fuse is blown or pulled open, causing a zero line break.
(6) When disconnecting the three-phase four-wire system, first disconnect the neutral line.
(7) Zero line breakage caused by other faults, such as strong wind blown off the zero line, the vehicle collides with the pole, and the pull line causes the zero line to break.
Precaution
(1) Three-phase four-wire system power supply, single-phase load should be distributed as evenly as possible, maintain three-phase load balance, strengthen the monitoring of three-phase current, and find that the imbalance is adjusted in time.
(2) The neutral current cannot be greater than one quarter of the phase current, and the neutral conductor cross section cannot be less than one-half of the phase cross section.
(3) The connection of the neutral line should be firm and reliable. For the introduction and lead-out of distribution transformers and power distribution panels, if aluminum conductors are used, copper-aluminum transition clamps should be used, and inspection and maintenance should be strengthened, especially for night inspections. Sparks in the joints are processed in a timely manner.
(4) For the zero line of the three-phase four-wire system, it is strictly forbidden to install a fuse or a separate switch device.
(5) When disconnecting the three-phase four-wire system, the phase line should be disconnected first, then the neutral line should be disconnected, and the wiring sequence is reversed.
(6) In the event of a zero line breakage, the three-phase power supply should be shut down as soon as possible to reduce the hazard of the accident.
Due to the randomness of the opening of electrical equipment, the three-phase load imbalance is unavoidable. Due to the damage caused by the zero-line disconnection, the event requiring compensation is constantly occurring. Therefore, only the inspection and maintenance of the neutral line should be strengthened to prevent the problem. In addition, we can ensure the safe operation of the zero line and reduce disputes and economic losses.
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