In the context of continuous industrial automation advancement, the development of semiconductor devices—particularly large-scale thyristors and inverters—has seen a significant rise. These nonlinear loads introduce high-order harmonic currents into the power grid, distorting voltage and current waveforms and causing severe power grid pollution.
Harmonics are essentially a series of waves whose frequencies are integer multiples of the fundamental frequency. For example, the second harmonic has a frequency of 100 Hz, the third harmonic is 150 Hz, and so on. The combination of these harmonics with the fundamental frequency leads to voltage waveform distortion in the power system. The severity of this distortion depends on the number and amplitude of the harmonics, which directly affects the extent of grid pollution and potential damage to electrical equipment.
Common sources of harmful harmonics include personal computers, silicon rectifiers, capacitive power supplies, electric arc furnaces, medium-frequency power systems, and various types of inverters and choppers. Among these, thyristor-based medium-frequency power supplies and electric arc furnaces are particularly problematic, as they cause the most severe grid pollution and pose the greatest risk to electrical devices.
The impact of higher harmonics on electrical equipment is significant. Harmonic currents from nonlinear loads can lead to overheating, increased losses, and unstable system performance. Transformers, motors, and neutral lines are especially vulnerable. In transformers, harmonics increase core losses and winding resistance, leading to excessive heat. Neutral lines in three-phase four-wire systems may carry large harmonic currents, causing voltage drops and potential overcurrent issues.
Power cables are also affected by harmonics. High-frequency components can accelerate insulation degradation, reduce service life, and even cause internal breakdowns. For instance, an oil-impregnated paper-insulated cable used in a steel furnace experienced multiple short-circuit failures due to harmonic distortion, significantly reducing its lifespan.
Capacitors are highly sensitive to harmonic distortion, which increases their losses and causes overheating. Induction motors suffer from voltage distortion, leading to overheating and reduced operational life. Switchboards may experience hotspots, especially at neutral busbars, and meters may show inaccuracies due to harmonic interference.
To mitigate these issues, several preventive measures can be implemented. One approach is to install harmonic filters near nonlinear loads, such as thyristor rectifiers or inverters, to absorb specific harmonic frequencies. Separating the power supply circuits for nonlinear loads and using Y/Δ transformer connections can also help reduce harmonic propagation. Additionally, limiting transformer load and using enhanced cooling methods can prevent overheating caused by harmonics.
Protecting induction motors involves connecting them to independent feeders and reducing line impedance through larger conductor sizes or harmonic filters. As industrial automation continues to grow, maintaining high power quality becomes increasingly important. Addressing harmonic issues effectively will ensure stable and safe operation of power systems in the future.
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