In power systems, harmonics are fundamentally caused by non-linear loads. When current flows through a load and does not maintain a linear relationship with the applied voltage, a non-sinusoidal current is produced, resulting in the generation of harmonics within the circuit. Harmonic frequencies are integer multiples of the fundamental frequency; as demonstrated by the analytical principles of the French mathematician Fourier, any periodic waveform can be decomposed into sinusoidal components comprising the fundamental frequency and a series of harmonics at integer multiples of that frequency. Harmonics are sinusoidal waves, each characterized by a distinct frequency, amplitude, and phase angle. Harmonics are classified as either odd or even: the 3rd, 5th, and 7th harmonics are odd, while the 2nd, 4th, 6th, and 8th are even. For instance, if the fundamental frequency is 50 Hz, the 2nd harmonic is 100 Hz and the 3rd harmonic is 150 Hz.

Generally speaking, odd-order harmonics cause more significant harm than even-order harmonics. In a balanced three-phase system, even-order harmonics are eliminated due to symmetry, leaving only odd-order harmonics. For three-phase rectifier loads, the harmonic currents that appear are of the order 6n ± 1 (e.g., 5th, 7th, 11th, 13th, 17th, 19th, etc.); variable frequency drives primarily generate 5th and 7th harmonics.

There are two fundamental approaches to addressing the issue of harmonic pollution caused by power electronic devices and other harmonic sources. The first involves installing harmonic compensation devices to mitigate harmonics—a strategy applicable to a wide range of harmonic sources. The second entails modifying the power electronic devices themselves to prevent harmonic generation and achieve a controllable power factor of unity; naturally, this approach applies specifically to power electronic devices that act as primary harmonic sources.

The traditional method for harmonic compensation involves the use of LC tuned filters. Widely adopted due to their simple structure, these filters are capable of compensating for both harmonics and reactive power. However, they suffer from significant drawbacks: their compensation performance is sensitive to grid impedance and operating conditions, and they are prone to parallel resonance with the system. This resonance can lead to harmonic amplification, potentially overloading or even destroying the LC filter. Furthermore, they are limited to compensating for harmonics at fixed frequencies, and the overall compensation effectiveness is often suboptimal.

The Harmful Effects of Harmonics

Ideally, the voltage supplied by a public power grid should maintain a single, fixed frequency and a specified amplitude. The presence of harmonic currents and voltages constitutes a form of pollution within the public power grid, degrading the operating environment for electrical equipment. Although research into harmonics and their hazards existed prior to the widespread adoption of power electronic devices, the issue did not initially receive sufficient attention. Over the past three or four decades, the rapid proliferation of power electronic devices has exacerbated harmonic pollution in public power grids. As malfunctions and accidents caused by harmonics have become increasingly frequent, the severity of the issue has drawn significant concern. The harmful effects of harmonics on public power grids and other systems generally fall into the following categories:
(1) Harmonics cause additional losses in grid components, reducing the efficiency of power generation, transmission, and end-use equipment; specifically, significant third-harmonic currents flowing through the neutral wire can cause line overheating or even fires.
(2) Harmonics interfere with the normal operation of various electrical devices. In addition to causing additional losses in electric motors, harmonics induce mechanical vibration, noise, and overvoltage, and can lead to severe localized overheating in transformers. They also cause overheating, accelerated insulation aging, and reduced service life—or even outright failure—in equipment such as capacitors and cables.
(3) Harmonics can trigger localized parallel or series resonance within the grid, amplifying harmonic levels; this significantly intensifies the hazards described in points (1) and (2), potentially leading to serious accidents.
(4) Harmonics can cause the malfunction of protective relays and automatic control devices, as well as inaccuracies in electrical metering instruments.
(5) Harmonics interfere with nearby communication systems; minor interference results in noise and reduced communication quality, while severe interference can lead to data loss and render communication systems inoperable.

Harmonic Suppression

There are two fundamental approaches to addressing the issue of harmonic pollution caused by power electronic devices and other harmonic sources. One approach is to install active harmonic filter to mitigate harmonics—a method applicable to all types of harmonic sources. The other approach involves modifying the power electronic devices themselves so that they do not generate harmonics and can maintain a controllable power factor of unity; naturally, this method applies only to power electronic devices that serve as major sources of harmonics.