Why Is the Neutral Current Greater Than the Live Current? (2)
2. Third harmonic
Third harmonic and its hazards In the power system, the conventional electrical equipment with linear impedance variation characteristics will not cause interference to the sinusoidal waveform with a frequency of 50 Hz provided by the power system. However, with the rapid development of modern power electronic technology, more and more electrical equipment with nonlinear impedance characteristics (also known as nonlinear loads) are used. When these non-linear loads work, the power supply frequency and waveform of the power system will be distorted, and sinusoidal waveforms of different frequencies can be decomposed. The main reason for the large neutral current is the 3rd harmonic. In the power system, the normal power supply frequency is 50HZ. The so-called "third harmonic" is the 150HZ AC sine wave mixed in the 50HZ circuit. This 150HZ AC sine wave is called the third harmonic because it is three times that of 50HZ. The third harmonic mainly exists in the low-voltage distribution network, and the building system is the most serious. The main hazards to the power grid are increased power loss, shortened equipment life, malfunction of grounding protection, malfunction of remote control, and overheating of the power grid. To the distribution, station can cause malfunction of electronics, damage to capacitors, additional magnetic fields, neutral overload, and cable fires.
Solution of third harmonic：Generally speaking, the phase line and the neutral line together form the path of the 3rd harmonic current. If the 3rd harmonic current is blocked on the neutral line, then naturally there will be no 3rd harmonic current on the phase line. The installation of an electrical neutral current filter can effectively eliminate the harmonics on the neutral line and reduce the current on the neutral line by more than 90%.
So, what harm will the zero-line current cause? Let's talk about a data, the current on the neutral line will be up to 1.73 times the current on the phase line. According to the formula of P=I2R, the power consumption on the neutral wire is 1.732 times, or 3 times, the power consumption on the phase wire. Such a large power consumption will inevitably lead to the high temperature of the zero line. One result is that the zero line is blown out, and another more serious consequence is that it will cause a fire. There are mainly the following aspects:
1. The heating of the neutral cable will cause accelerated aging of the insulation, and the breakdown of the insulation will cause a short circuit, increasing the fire hazard.
2. The zero-line current circulates inside the transformer, reducing the capacity of the transformer, increasing the loss of the transformer, causing the transformer to heat up seriously, and may burn the transformer in severe cases.
3. If the zero line is overcurrent for a long time, it will be blown out, the system voltage will change from 220V to 400V, and the equipment will be burned out.
4. Cause zero drift, make the zero line generate voltage to the ground, and cause harm to equipment and personal safety.
5. The zero-line current is too large, causing the switch to trip frequently and irregularly.
6. Increase the loss, heat generation, accelerated insulation aging and shorten the service life of equipment and materials in the power supply system.
