This article introduces harmonic sources and types, the hazards of harmonics, and explains the differences between harmonic mitigation and passive and active filtering.
Harmonic sources and types
If a sinusoidal alternating voltage is applied to a device, but the resulting current is not a sinusoidal alternating current, such a device is called a nonlinear impedance device, or simply a nonlinear device. Since non-sinusoidal alternating current contains harmonic components, nonlinear devices are also called harmonic sources.
There are three main types of harmonic sources in industrial power grids: Three-phase bridge rectifier circuits produce waveform changes on both the positive and negative waveforms of each phase, resulting in six non-sinusoidal waveforms in one cycle, hence the name "six-pulse wave device." The harmonics from six-pulse wave devices are very regular, generating harmonics of multiples of six plus or minus one, i.e., 5th, 7th, 11th, 13th, 17th, 19th… harmonics. Furthermore, the harmonic amplitude gradually decreases as the harmonic order increases, so typically only the 5th, 7th, 11th, and 13th harmonics need to be addressed.
This type of equipment includes all devices with three-phase bridge rectifiers, such as DC drives, frequency converters, soft starters, UPS power supplies, etc., and is currently the most common type of harmonic source in industrial electrical equipment.
When equipment like industrial electric arc furnaces is in operation, the current waveform changes frequently, which decomposes into harmonics with constantly changing frequency and amplitude.
Nonlinear single-phase devices, such as electronic instruments with single-phase rectification, will generate the third zero-sequence harmonic on the neutral line due to three-phase asymmetry.
Harmful effects of harmonics on power systems and various electrical equipment
Harmonics can easily cause parallel or series resonance between the power grid and the compensation capacitor, amplifying the harmonic current by several times or even tens of times, resulting in overcurrent, damage to the capacitor, the reactor and resistor connected to the capacitor, and even serious accidents.
Harmonics can cause malfunctions in relay protection and automatic devices, abnormal fuse blowing, and inaccurate measurement by electrical measuring instruments.
Harmonics can cause mechanical vibrations in transformers, motors, and other equipment, significantly increasing noise and temperature rise, and shortening insulation life.
Harmonics can cause additional heat generation in equipment and lines, increase losses, accelerate insulation aging, and reduce service life.
Harmonics can delay the extinction of an electric arc, causing the accident to escalate.
Harmonics cause a significant increase in the neutral current in a three-phase four-wire system, leading to system failures or even accidents.
Harmonics can interfere with nearby communication systems through electromagnetic induction and conductive coupling. In mild cases, they introduce noise and reduce communication quality; in severe cases, they can cause information loss and render the communication system unable to function properly.
Harmonics can cause instability in the operation of production equipment, leading to an increase in product defect rates and reduced corporate profits.
Harmonics affect the normal operation of various electrical equipment. According to statistics, among the electrical equipment damaged by harmonics, parallel capacitors account for about 70%, and series reactors account for about 30%.
Harmonic currents cause additional harmonic losses to components in the power system, reducing the efficiency of power generation, transmission, and consumption.
2. Harmonic mitigation and the differences between passive and active filtering.
Currently, there are two main technologies for harmonic mitigation: passive filtering and active filtering.
Passive filtering technology is currently the most widely used harmonic suppression method. It is specifically designed to suppress the desired harmonic order and uses the tuning principle of inductors and capacitors to trap harmonics within the filter, thus reducing their injection into the power grid. Passive filtering devices have a simple structure, low cost, and relatively mature technology, but they also have some insurmountable drawbacks: 1. The filtering characteristics are greatly affected by system parameters, making them highly susceptible to series and parallel resonance with the system or other filtering branches;
2. It can only eliminate certain harmonics, while amplifying other harmonics.
3. It is sometimes difficult to coordinate the requirements of filtering, reactive power compensation, voltage regulation, etc.
4. When the harmonic current increases, the filter's burden increases accordingly, which may cause filter overload or even damage the equipment.
5. High consumption of effective materials and large volume.
Active filtering technology, as a new type of harmonic control method, is an effective tool for eliminating harmonic pollution and improving power quality. Compared with passive filtering technology, it has unparalleled advantages, mainly in the following aspects.
1. It achieves dynamic compensation, enabling compensation for reactive power with varying frequency and magnitude, and exhibits extremely fast response speed to changes in the compensation target;
2. An active filter is a high-impedance current source. Its connection does not affect the system impedance. Therefore, this type of device is suitable for serialized and mass production.
3. When the power grid structure changes, the device is not significantly affected by the power grid impedance, and there is no risk of resonance with the power grid impedance. Simultaneously, it can suppress series and parallel resonance.
4. No energy storage element is needed when compensating for reactive power, and the energy storage element required when compensating for harmonics is small.
5. A single device can simultaneously compensate for multiple harmonic currents and non-integer multiples of harmonic currents. It can compensate for a single harmonic and reactive power source individually, or it can provide centralized compensation for multiple harmonics and reactive power sources.
6. When the harmonic current in the line suddenly increases, the active filter will not overload and can function normally without needing to be disconnected from the system.
7. The device can output only the high-order harmonic current that needs to be compensated, without outputting the fundamental reactive power, which not only reduces the total capacity of the active filter, but also avoids reactive power backfeeding under light load.
3. Conclusion
With the increasing types and numbers of nonlinear loads and harmonic sources in the power grid, the resulting harmonics are becoming increasingly severe, necessitating urgent mitigation. Low-cost passive filtering technology is currently the most commonly used method for harmonic mitigation; this method is simple but has certain drawbacks. Active filtering technology can mitigate harmonics with varying frequencies and amplitudes, but it also suffers from high initial investment and low operating efficiency. Therefore, combining active and passive filtering technologies can reduce the cost of active filters while compensating for the shortcomings of passive filters, representing a promising development direction.
