In order to make AC power has a convenient power conversion function, industrial power uses three-phase sinusoidal alternating current and the current phase (reflecting the direction of current) is 120 degrees different from each other. Usually we call each such wire the phase line (FireWire).

The voltage is 380V, three fire lines. Three-phase transmission is much more economical than single-phase power transmission under the same conditions of transmission distance, transmission power, power factor, voltage loss and power loss. Three-phase motor performance is also better than single-phase motor performance, simple structure and easy maintenance. Three-phase system has been widely used.

Zero line reverse automatic aligner

The neutral line is the line drawn from the neutral side of the secondary side of the transformer. With the phase line constitute the power supply for the electrical equipment, under normal circumstances, the zero line in the secondary side of the transformer neutral point with the ground repeated ground, play a dual protection.

Both the live line and the neutral line are charged lines. If the two-phase power supply is connected to an electrical device, then current flows through the cable.

The general feeling is that the FireWire is charged because if a person touches (including some indirect contact) the FireWire, part of the current flows from the person's body. It is like an original water pipe and a tap in the middle. The neutral line is not energized because the other end of the power supply (neutral line) is connected to ground. When there is no position difference, no current will be formed when the neutral line touches the neutral line. Therefore, there is a feeling that the zero line is not charged. Both the neutral line and the live line are originally generated by the power supply. The positive direction of the current flows from one source to the other, through external equipment. Form a loop. The difference between the neutral line and the live line is that one of the two terminals of the power supply is connected to the ground.

If the three-phase load is balanced, the main zero line current into the line is zero; when the three-phase load is slightly unbalanced, the main zero line into the line will have a smaller current; only when a phase in the three phases is out of phase, Line main zero current may be greater than the phase current. Because the use of single-phase finishing inside the advertising light box will generate a large number of third harmonics, the normal power third harmonic content rate is about 80%, and the three current harmonics cannot be identically cancelled due to the same phase angle and vector angle. , can only be caused by the superposition of the neutral line (neutral line), resulting in zero line current greater than the phase current. The zero-sequence filter, which can be retrofitted with new Henderson filters, can filter more than 90% of the neutral current. When the three phases are unbalanced, the neutral line will bear the unbalanced current. At the same time, if there are 3rd harmonics in the three phases, they will be superposed on the center line, that is, the neutral current is the sum of the three-phase 3rd harmonic currents.

The reasons are as follows:

1, three-phase sinusoidal symmetric AC, in the three-phase symmetrical load that is balanced load, because the current vector size is equal, the direction that the phase difference of 120 degrees, and its vector is zero and its zero-sequence current is zero;

2, three-phase sinusoidal symmetric AC, in the three-phase asymmetric load that is unbalanced load, because the current vector size is not equal, the direction that the phase difference is not full 120 degrees, its vector sum is not zero, that is, the zero-sequence current is not Balance current, but less than any phase current

3, three-phase sinusoidal symmetrical AC power supply, in the three-phase load due to the presence of non-linear components such as the existence of three-phase load current in the DC component and the third harmonic multiple times higher, the zero-sequence current is the arithmetic sum, That is, zero-sequence current may be greater than the phase current, such as three-phase half-wave rectifier circuit, any phase current is 1/3 of the load current, this load current is the zero sequence current;

4. However, in the three-phase bridge rectifier circuit, since there are currents in the positive and negative half cycles of the alternating current, and the positive and negative half cycles are symmetrical in one week, the three phases are symmetrical, that is to say, the direct current component and the tertiary integer The higher harmonics of the multiplier do not exist, so the vector sum of the three-phase currents is zero, that is, the zero-sequence current is zero;

5. In the single-phase bridge rectifier circuit, since there is current in the positive and negative half cycles of the alternating current, and the positive and negative half cycles are symmetrical in one week, the high order harmonics of the direct current component and the integral multiple of the third order of the single phase current. Wave does not exist;

6, if the three-phase load is a load such as a single-phase bridge rectifier circuit, although the imbalance, the vector sum of the three-phase current is not zero, that is, the zero-sequence current is not zero, the zero line current will not be greater than the phase line Current.

Let's consider a data: the current on the zero line will reach 1.73 times of the phase current. According to the formula of P=I2R, the power consumption on the zero line is 1.732 times the phase line power consumption, which is 3 times! Such high power consumption will inevitably cause the zero line temperature to be too high. One result is that the zero line will be blown. Another more serious consequence is that it will cause a fire.

The harm of excessive neutral current:

1: Causes the aging of the insulation aging due to the heating of the zero-line cable, causing a short circuit due to breakdown of the insulation, which increases the risk of fire!

2: The zero-line current circulates inside the transformer to reduce the transformer capacity, increase the loss of the transformer, and cause the transformer to generate heat. In severe cases, the transformer may be burned.

3: The zero line overcurrent will be blown. The system voltage will change from 220V to 400V and the device will be burned.

4: Cause zero drift, causing the zero line to generate voltage to the ground, causing damage to equipment and personal safety.

5: The neutral current is too large, causing frequent tripping of the switch.

6: Increase the loss of equipment and materials in the power supply system, generate heat, and accelerate insulation aging to shorten the service life.

The “neutral current is too large” situation discussed here is that when the three-phase load is balanced, the phenomenon of excessive zero-line current still occurs, and the neutral current is more than 1.5 times of the phase current.

This situation often occurs in the following situations:

·In a computer-centric building

· Large screen display

· Large area billboard

·Stage dimming equipment

When the neutral current is too large, common problems include: zero line overheating, tripping, transformer overheating. At this time, we must try to solve the problem of excessive neutral current. Simply increasing the cross-sectional area of ​​the zero line (for example, laying a zero line) can only solve the problem of zero line overheating and cannot eliminate the phenomenon of tripping or transformer overheating.

There are two methods to solve the problem of excessive neutral current, one is to connect the NBF zero-line current blocker in the zero line, and the other is to use SPHF to supply power to the load.

The installation method of NBF neutral current blocker is shown as in Fig. 1. The NBF zero line blocker is very effective in reducing the third harmonic current on the zero line and can reduce the neutral current to less than 20% of the original. For example, an advertising light box (using an electronic ballast) has a current of 90 A per phase, three-phase balance, and a neutral current of 160 A. After NBF is installed, the neutral current is less than 10 A.

The use of NBF can effectively reduce the third harmonic current on the zero line, but it will cause some adverse effects. A typical phenomenon is that the zero-line voltage deviates from 0V. At this time, the three-phase voltage will appear unbalanced. To solve this problem, move the grounding point from the transformer to the power distribution cabinet, as shown in Figure 2. It should be noted that at this time, the neutral line should be disconnected at the ground of the transformer.

When NBF is selected, it should be selected according to the maximum value of the phase current. That is, when there is only one phase load, all phase currents flow through the neutral line, that is, through the NBF. For example, when the phase current is 100A maximum, NBF with a rated current of 100A is selected.

When a computer or other device is centralized, it is recommended to use an SPHF single-phase harmonic filter. The use of SPHF is shown in Figure 3. The method of use is similar to a patch panel. An SPHF filter can supply power to a group of devices as long as the total current does not exceed the rated current of the filter. For example, a SPHF with a rated current of 10A can supply approximately 10 computers.

The SPHF single-phase harmonic filter is an ideal configuration equipment for the information room. It not only reduces the 3rd harmonic current and eliminates fire hazards. And it can effectively suppress the surge on the power grid, which is equivalent to a surge protector. When SPHF is used as an information device, it can guarantee the safety in thunderstorms.

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