Power grid harmonics come from three aspects:
First, the quality of the power supply is not high to produce harmonics: because the generator three-phase winding is difficult to achieve absolute symmetry in the production, the core is difficult to achieve absolute uniformity and some other reasons, the power supply will produce some harmonics, but in general, very few.
The second is the generation of harmonics in the transmission and distribution system: the power transformer is mainly the generation of harmonics in the transmission and distribution system, due to the saturation of the transformer core, the nonlinear magnetization curve, and the economic design of the transformer, the working magnetic density is selected on the near saturation section of the magnetization curve, so that the magnetization current is a peaked waveform, and thus contains odd harmonics.
Its size is related to the structural form of the magnetic circuit and the degree of saturation of the iron core.
The higher the saturation degree of the core, the farther the transformer operating point deviates from the linear, the greater the harmonic current, of which the 3 harmonic current can reach 0.5% of the rated current.
The third is the harmonics generated by electrical equipment: thyristor rectifier equipment. Because thyristor rectifier has been widely used in electric locomotive, aluminum electrolytic cell, charging device, switching power supply and many other aspects, it causes a lot of harmonics to the power grid. We know that the thyristor rectifier device uses phase shift control to absorb the sine wave of the missing Angle from the power grid, which leaves another part of the missing Angle sine wave to the power grid, and thus leaves another part of the missing Angle sine wave to the power grid, obviously contains a large number of harmonics in the remaining part.
If the rectifier device is a single-phase rectifier circuit, it contains odd harmonic current when connected to inductive load, and the content of the third harmonic can reach 30% of the fundamental wave. When the capacitive load is connected, it contains odd harmonic voltage, and its harmonic content increases with the increase of the capacitance value.
If the rectifier device is a three-phase fully controlled bridge 6-pulse rectifier, the primary side of the transformer and the power supply line contain 5 or more odd harmonic currents; If it is a 12-pulse rectifier, there are 11 or more odd harmonic currents.
Statistics show that the harmonics generated by the rectifier device account for nearly 40% of all harmonics, which is the largest harmonic source. Frequency conversion device.
Frequency conversion devices are often used in fans, pumps, elevators and other equipment, due to the use of phase control, harmonic components are very complex, in addition to containing integer harmonics, but also contains fractional harmonics, the power of such devices is generally larger, with the development of frequency conversion speed regulation, the harmonic caused by the power grid is also more and more.
Electric arc furnace, calcium carbide furnace.
Because it is difficult for the three-phase electrode of the electric furnace to contact the uneven charge at the same time when heating the raw material, the combustion is unstable, causing the three-phase load imbalance and generating harmonic current, which is injected into the power grid through the triangular connection coil of the transformer.
Among them, the main harmonics of 27 times can reach 8% 20% of the basic wave on average, and the maximum can reach 45%.
Gas discharge electric light source.
Fluorescent lamps, high-pressure mercury lamps, high-pressure sodium lamps and metal halide lamps belong to gas discharge electric light sources.
Analyzing and measuring the volt-ampere characteristics of this kind of electric light source, we can see that its nonlinearity is very serious, and some also contain negative volt-ampere characteristics, which will cause odd harmonic current to the power grid. Household appliances.
Television, video recorders, computers, dimming lamps, temperature control cookware, etc., because of the voltage regulation and rectification device, will produce deep odd harmonics.
In washing machines, electric fans, air conditioners and other winding equipment, due to the change of unbalanced current can also make the waveform change.
Although these household appliances are smaller in power, they are huge in number and are also one of the main sources of harmonics.
On this issue, harmonic distortion can occur in a system for a variety of reasons, including:
1, the existence of nonlinear components: for example, amplifiers, filters, mixers, etc., due to nonlinear characteristics, will make the input signal harmonic multiplication, resulting in harmonic distortion.
2. The existence of non-ideal amplifiers: In actual circuits, amplifiers often have some non-ideal characteristics, such as gain changes with frequency, noise, cross distortion, etc., which can also lead to harmonic distortion.
3. The presence of signal transmission lines: if the characteristics of the signal transmission line are uneven or there is loss, it will lead to signal distortion, including harmonic distortion.
4. The influence of power supply noise: Power supply noise refers to the high-frequency interference in the power supply, if these disturbances pass through the circuit, it will cause harmonic distortion.
5. Unreasonable system design: If the system design is unreasonable, for example, the signal amplitude is too large, the frequency is too high, it will lead to harmonic distortion.