The fundamental reason for the generation of high-order harmonics is due to the nonlinear characteristics of certain equipment and loads in the power system, namely the waveform distortion caused by the non-linear (proportional) relationship between the applied voltage and the generated current. When the power system supplies power to nonlinear equipment and loads, these equipment or loads not only report (such as transformers), transform (such as AC/DC converters), and absorb (such as electric arc furnaces) the fundamental energy supplied by the system generator, but also convert some of the fundamental energy into harmonic energy, sending a large number of high-order harmonics back to the system, causing distortion of the sine waveform of the power system and reducing the quality of electrical energy.
In the future, there are three important categories of harmonic sources in the power system.
1) Ferromagnetic saturation type: Various types of iron core equipment, such as transformers, reactors, etc., exhibit nonlinear ferromagnetic saturation characteristics.
2) Electronic switch type: It is widely used in various types of AC/DC converter assemblies (rectifiers, inverters) and bidirectional thyristor controllable switch equipment, and is flourishing in a large number of industrial and mining enterprises and household appliances such as chemical, metallurgical, mining, and electric railways; Within the system, such as rectifier valves and inverter valves in DC transmission.
3) Arc type: In the melting era of various smelting arc furnaces and the welding era of exchange arc welding machines, the highly nonlinear ignition and drastic changes of the arc cause irregular fluctuations in the current. Its nonlinearity exhibits irregular and randomly varying volt ampere characteristics between arc voltage and arc current. For three-phase power supply in the power system, there are nonlinear characteristics of three-phase balance and three-phase imbalance. The latter, such as electric railways, electric arc furnaces, and single-phase household appliances powered by high voltage, while electric railways are typical three-phase unbalanced harmonic sources in future medium voltage power supply systems.
The impact of harmonics on the power grid:
1. The important persecution of harmonics on torsional equipment and transformers is the additional consumption and fever growth caused by harmonics. Other harmonics can also cause torsional equipment and transformers to vibrate and withdraw noise, and the vibration between permanent and permanent will cause metal fatigue and machine damage.
2. The important persecution of harmonics on the route is the generation of additional consumption.
3. Harmonics can cause resonance in the inductance and capacitance of the system, reducing the harmonics. When harmonics cause resonance in the system, the harmonic voltage decreases, the harmonic current increases, causing relay protection and automatic assembly misoperation, damaging system equipment (such as power capacitors, cables, motors, etc.), triggering system chaos, and threatening the safe operation of the power system.
4. Harmonics can disturb communication equipment, increase power consumption of the power system (such as line loss), prevent reactive power compensation equipment from operating abnormally, and bring persecution to the system and users. The important steps to limit harmonics in the power grid include: increasing the number of pulsations in the converter assembly; Install exchange filters and active power filters; Enhance harmonic control
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