Three-phase lines are common: three-phase three wire, three-phase four wire
The three - phase three - wire system is star connection neutral point ungrounded and Angle connection method, the general high - voltage power supply uses three - phase three - wire system, three - phase four - wire is star connection neutral point grounding, low - voltage system uses three - phase four - wire system.
1. Why is the three-phase three-wire system adopted on the high voltage side
The three-phase circuit consists of three sinusoidal power sources with the same amplitude, the same frequency and the same difference between each other. If the impedance and properties of the load connected to each of the three wires are the same, the load in the three wires is symmetrical. In the condition of load symmetry, the sum of the current flowing through the midline at each time is zero.
And in daily life, commonly used electrical equipment such as lights and other household appliances, they are single-phase load. The biggest characteristic of this kind of electricity use is that the load is scattered and unbalanced, which is easy to lead to load impedance, phase current and load phase voltage asymmetry in this environment. Therefore, it is easy to lead to the phase voltage is higher or lower than the rated voltage, resulting in load can not work normally or even damage, therefore, it is necessary to use the neutral line to maintain the balance of the three-phase voltage.
In the high voltage system, the three phase load belongs to the balance load, do not need to balance the middle line.
2. What is the difference between three-phase three-wire and three-phase four-wire?
The three-phase four-wire has one more neutral power supply line than the three-phase three-wire. The three-phase three-wire can only provide 380V voltage power supply. The three-phase four-wire can provide both 380V voltage and 220V voltage power supply.
The three phase and three wire can not pass the zero phase feedback current when the load is unbalanced, easy to burn the load, but the three phase and four wire can solve this problem.
3. Why is the three-phase and four-wire system commonly used in low-voltage power grids?
Three - phase and four - wire system with star connection can provide two voltage values at the same time, that is, line voltage and phase voltage
It can be used for both three-phase power load and single-phase lighting. For example, the commonly used low voltage 380/220V can not only provide the three-phase AC motor that needs the power supply voltage 380V, but also provide the single-phase 220V lighting power supply. In addition, when the three-phase load is asymmetric, because of the constant effect of the neutral line impedance, it can also eliminate the voltage displacement of the neutral point.
4. How to choose the three-phase three-wire system or three-phase four-wire system SVG/AHF module?
Depends on whether the transformer is three - phase three - wire or three - phase four - wire.
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