Transformers are generally Y-connected (star-connected). There are the three winding coils of the transformer. The three forks of Y are connected to three-phase wires (live wires) and the center point of Y is connected to. a thread, called the midline. The neutral line and any phase line form a single-phase power line.
On the secondary side of the transformer, from the "with and without power" point of view, the live wire and the neutral wire are the same, both are powered. But at this point we have the midpoint of the transformer (also called the neutral point) connected to ground. We assume that the earth potential is zero as a reference and the neutral line becomes zero potential, called the neutral line. When a person stands on the ground, his potential is also zero. When you touch the neutral line, there is no potential difference, so there is no electricity. If you touch the wireunder voltage, the electricity from the live wire will flow through your body, through the earth and back to the center of the transformer, and you will be shocked! It's God.
The direction of alternating current is constantly changing. Is it necessary to separate the neutral line? How to determine if the two lines output from the alternator of a power plant are live wires or neutral wires?
Domestic 220V is alternating current. Just like the definition of alternating current, the current direction of household electricity also changes periodically. But why does domestic electricity offer the concept of zero live wire? In physics, it has little to do with electricity. You can refer to electrical engineering. Let me explain briefly:
Three-phase generator:
Home electricity consists of a live wire (phase wire) and a neutral wire (neutral wire of electrity, power plants now). use three-phase alternators to generate electricity (it is recommended to search on Baidu to better understand the difference between daily electricity and experimental electricity). The neutral line is the line drawn from the neutral point of the three-phase generator and the potential is 0. The potential of the three live wires leading to the node will change periodically with u=220√2sin(ωt+φ).
Taking 50Hz electricity as an example, ω=2π/50, but the initial phases of the three live wire output points are different, φ=-2π/3, 0, 2π /3 (120°).
Neutral line:
The potential 0 of the neutral point is caused by the three initial phases φ of the generator being -2π/3, 0, 2π/3 respectively. is always dead Electricity is relatively difficult to understand based on physics. The neutral line was grounded at the generator and substation before entering intothe house, and its potential is the same as that of the earth. If a person touches the neutral line (the person standing on the ground has the same potential as the earth), they are there. there will be no danger, unless the neutral line breaks down and the potential is not the same.
Live wire:
The live wire is a line drawn from a node where the generator potential changes periodically (you can think of it as a sinusoidal alternating current , this u-t diagram). Due to the three nodes, the point is independent, so it will not be stable at potential 0 like the zero line, and will also have an amplitude of +-220√2V.
How to operate electrical appliances:
One end of the household appliance is connected to the live wire and the other end is connected to the neutral wire, then it is connected to a phase. of the generator (this depends on the phase in which your live wire is off) forman AC loop. The magnitude and direction of the current in the loop will change periodically, but the effective value (operating current) remains unchanged and the electrical appliance can operate normally.
Electric shock from a live wire:
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The potential of the live wire will change periodically (50, 60 Hz), and the potential of a person is the same as that of the earth. Once you touch the live wire, there will be a periodically changing potential difference between your. point of contact and your feet, and you will become With the "appliances" in 4, the current will flow back and forth in your body The direction of the current: 220V From the live wire to the ground, -220V comes from. earth to the live wire.
Why do high voltage transmission lines only have live wires but no neutral wires?
Both answers above are one-sided. I work in a power plantique. I can explain to you: First of all, the electricity produced by general power plants is currently three-phase (divided into three phases: A, B and C). Each phase is 120 degrees (one). circle) different from each other in space. 360 degrees, one phase occupies 120 degrees of space), and the three phases are live wires.
We all know that to have water flow, there must be water pressure, so if there is current, there must be voltage. Only if there is a live wire and a neutral wire will there be voltage. If there is only one live wire, who makes the difference? As the name suggests, the potential of the neutral wire is zero, that is, it is connected to earth. When the potential of the live wire is positive, current flows out of the live wire through the load and back to the supply. power supply via neutral wire; when the potential of the live wire is negative (i.e. thepotential is currently less than zero), current flows from the neutral wire through the load and back to the power source via the live wire. It can be seen that even though the zero line potential is still zero, this does not mean that it is useless.
Because whether it is a thermal power plant or a hydroelectric power plant, the electricity produced is three-phase, three-wire electricity. For long distance transportation, the voltage must be increased via a booster station. The goal is to transport the electricity to the destination. This does not take into account civil electricity and industrial electricity during the process, so there is no need to use a neutral line.
On the surface it is about saving materials, but in reality it is about reducing costs as much as possible. Once the high voltage electricity reaches its destination, a step-down transformer is needed to reduceremove the voltage. At this stage, not only civil and industrial electricity is taken into account, but also electrical safety.
Since the neutral wire and ground wire are only considered in the 380V/220V low-voltage distribution network, the general power supply method in the low-voltage distribution network is the three-phase four-wire power supply method.
Therefore, the neutral point of the secondary side star connection of the step-down transformer in the low voltage distribution network leads to one line, i.e. the neutral line. At present, the neutral line and one of the three-phase power lines have a phase voltage of 220V, and the line voltage of the two three-phase power lines is 380V.
Detailed information
If we consider from the point of view of electrical safety and consider the wind deviation of high voltage lines caused bys strong winds, according to the fifth “Detailed rules of application for the protection of electrical installations” The article states: “The protection zone of overhead power lines is a safety zone which must be arranged to ensure safe operation overhead power lines constructed and to ensure normal power supply. supply for people's lives.
In factories, mines, towns and villages. In densely populated areas, the protection zone of overhead power lines is the area inside the two parallel lines formed by the sum of the horizontal distance. of the conductor's edge after the calculated maximum wind deflection and the horizontal safety distance of the building after the wind deflection
About the distance between high-voltage lines and residential buildings.China n there is no clear regulation on safety distancesé between buildings, but there is a relevant standard that can be converted: if the magnetic induction intensity at the location of a residential building is <100 microtesla, it meets the building standards. After calculation:
The safety distance for high voltage lines below 1 kV is 4 meters; the safety distance for high voltage lines 1-10 kV is 6 meters.
Baidu Encyclopedia - High Voltage Transmission