The role of the neutral line in a three-phase four-wire (380/220V) power supply system

The direct grounding of the neutral side of the secondary side of the transformer is called the working ground. Since the neutral point is directly connected to the ground zero potential, the neutral line drawn is called the neutral line, that is, the TN-C system (three-phase four-wire power supply system). PEN line in ). The main functions of the neutral line in a three-phase four-wire (380/220V) power supply system are:

1. In the case of three unbalanced loads, the neutral line is turned on, and the unbalanced current flows back to the neutral point, so that the line voltage and phase voltage of the power supply system are basically balanced;

2. When the insulation of the electrical equipment with protection and zero is broken, the short-circuit current will form a loop through the neutral line. Since the zero-line impedance is small, the short-circuit current will be large, which causes the protection device to act quickly to disconnect the power supply. , thereby playing a protective role;

3. Zero line or power circuit of single-phase 220V electrical equipment.

As shown in the figure below, in the case of three-phase load imbalance (minimum phase A load, slightly larger phase B load, and maximum phase C load), once the neutral wire is disconnected, serious consequences will occur.

analyse as below:

1. When the neutral line breaks at point a, the single-phase load connected after the breaking point is charged with the live and neutral lines. But there is no voltage, so the load does not work properly;

2. When the neutral line breaks at point b, the single-phase load of phase B (L2) and phase C (L3) after the disconnection point is equivalent to series connection to the two phases (380V) of B and C. The voltage of the C phase which causes a large load is low, and the voltage of the B phase with a small load is high. If the phase B and the phase C load are as large, the phase B and phase C loads are each subjected to a voltage of 190V;

3. When the neutral line breaks at point c, since there is no neutral line to conduct unbalanced current, in order to maintain the vector sum of the three-phase current equal to zero, the neutral point must be displaced to the C phase with large load, resulting in three phases. The voltage is unbalanced, that is, the load C phase voltage is low, and the load A phase voltage is high. The more serious the three-phase load imbalance, the greater the neutral point displacement, and the more serious the three-phase voltage imbalance.

4. Due to the three-phase voltage malformation caused by the broken line of the zero wire, the working characteristics of the electrical equipment change, the voltage is too low to work, and the excessive voltage will shorten the service life and even cause economic loss caused by burning the equipment;

5. Once the neutral line is disconnected, the electrical equipment protected by zero will lose its protection; if the equipment leaks, it will cause personal electric shock. At this time, even if the equipment does not leak electricity, the zero-line itself has a dangerous voltage to electrify the equipment casing, which may also cause personal electric shock.

In the low-voltage three-phase four-wire (380/220V) power supply system, due to the existence of single-phase load, the three-phase load is unbalanced. In order to ensure the safety and reliability of the neutral line, the regulations stipulate that the neutral current should not exceed 25% of the phase current. No switches or fuses should be installed on the main zero line. The cross section of the neutral line must not be less than 1/1 of the phase line. 2.