With the development of power electronics technology, frequency converters are increasingly widely used in power electronics systems, industry, and many other fields. However, the high-order harmonics generated by frequency converters are also causing increasingly serious damage to the public power grid. These include: 1) Harmonics cause additional harmonic losses in power grid components, reducing the efficiency of power generation, transmission, and consumption equipment. Large amounts of third-order harmonics flowing through the neutral line can cause overheating and even fires; 2) Harmonics affect the normal operation of various electrical equipment, causing mechanical vibration, noise, and overheating in motors, severe localized overheating in transformers, and overheating in capacitors, cables, and other equipment, leading to insulation aging, shortened lifespan, and even damage; 3) Harmonics can cause localized parallel and series resonances in the power grid, amplifying the harmonics and causing serious accidents; 4) Harmonics can interfere with nearby communication systems, causing noise and reduced communication quality, or even information loss and malfunction of the communication system; 5) Harmonics can cause malfunctions in relay protection and automatic devices, and inaccurate measurements by electrical measuring instruments.
Because harmonic voltages and currents in public power grids cause significant damage to electrical equipment and the grid itself, many countries around the world have issued national standards for limiting grid harmonics, with regulations formulated by authoritative bodies. The harmonic standards established by various countries are generally quite similar. In China, the State Bureau of Technical Supervision issued the national standard (GB/T14549-93) <<Power Quality: Harmonics in Public Power Grids>> in 1993 , which came into effect on March 1, 1994.
Variable frequency drives (VFDs) are among the most widely used devices in industrial speed control and transmission. A VFD converts the mains frequency (50Hz) into AC power of various frequencies to enable variable speed operation of motors. The control circuit controls the main circuit, the rectifier circuit converts AC to DC, the DC intermediate circuit smooths and filters the output of the rectifier circuit, and the inverter circuit converts the DC back into AC. Due to the switching characteristics of the VFD's inverter circuit, it creates a typical nonlinear load on its power supply. Therefore, power electronic devices, represented by VFDs, are one of the most significant sources of harmonics in the public power grid.
Harmonics refer to the components greater than integer multiples of the fundamental frequency obtained from the Fourier series decomposition of a periodic non-sinusoidal alternating current quantity. They are often called higher harmonics, while the fundamental frequency refers to the component with the same frequency as the power frequency. The voltage generated by a three-phase AC generator in a power system can be considered essentially sinusoidal, meaning the voltage waveform has virtually no DC or harmonic components. However, various harmonic sources exist in the power system (harmonic sources are electrical devices that inject harmonic current into the power grid or generate harmonic voltage in the power grid), especially converters and similar equipment. The mechanism of harmonic generation on the input side of a frequency converter is that any circuit with a rectifier circuit on the power supply side generates harmonics due to its nonlinearity. The mechanism of harmonic generation on the output side of a frequency converter is that in an inverter circuit, for voltage-type circuits, the output voltage is a rectangular wave, and for current-type circuits, the output current is a rectangular wave. Rectangular waves contain numerous harmonics, which can adversely affect the load. Therefore, even if the voltage of the power source in a power system is sinusoidal, the presence of nonlinear components will always result in harmonic currents or voltages in the power grid. Thus, the existence of power grid harmonics is primarily due to the presence of various nonlinear components in the power system.
Currently, the following methods can be used to control harmonics:
(1) Isolation, shielding, and grounding of the frequency converter: The power supply of the frequency converter system is independent of the power supply of other equipment. Alternatively, an isolation transformer can be installed on the input side of the frequency converter and other electrical equipment. Or, the frequency converter can be placed in an iron box, and the outer shell of the iron box can be grounded. At the same time, the output power supply of the frequency converter should be laid as far away from the control cable as possible (at least 50mm apart). If it must be laid close to it, it should cross at an orthogonal angle as much as possible. If it must be laid parallel, the length of the parallel section should be shortened as much as possible (not exceeding 1mm). The output cable should be run through a steel pipe and the steel pipe should be electrically connected and reliably grounded.
(2) Installation of AC and DC reactors: When the frequency converter is used on a distribution transformer with a capacity greater than 500KVA, and the transformer capacity is more than 10 times the frequency converter capacity, an AC reactor should be installed on the input side of the frequency converter. When the three-phase output voltage of the distribution transformer is unbalanced, and the imbalance rate is greater than 3%, the peak value of the frequency converter input current is very large, which will cause the conductors to overheat. In this case, an AC reactor needs to be installed. In severe cases, a DC reactor needs to be installed.
(3) Install a passive filter: Install the passive filter on the AC side of the frequency converter. The passive filter consists of L, C, and R components forming a harmonic resonance circuit. When the harmonic frequency of the LC circuit is the same as the frequency of a certain higher harmonic current, it can prevent the higher harmonic from flowing into the power grid. The characteristics of a passive filter are low investment, high frequency, simple structure, reliable operation, and convenient maintenance. The disadvantages of a passive filter are that the filtering is easily affected by system parameters, it may amplify certain harmonics, it consumes more resources, and it is bulky.
(4) Installing an active filter: As early as the early 1970s, Japanese scholars proposed the concept of an active filter. An active filter detects high-order harmonics in the current and, based on the detection results, inputs a current with the opposite phase to the high-order harmonic components, achieving real-time compensation for harmonic currents. Compared to passive filters, it offers high controllability and fast response, and is multi-functional. It can also eliminate the risk of resonance with the system impedance and automatically track and compensate for changing harmonics. However, it has drawbacks such as large capacity and high price.
(5) Installing Static Var Compensators: For large, impulsive loads, static var compensators can be installed to compensate for rapidly changing reactive power demands, improve the power factor, filter system harmonics, reduce harmonic current injection into the system, stabilize bus voltage, reduce three-phase voltage imbalance, and improve the power supply system's ability to withstand harmonics. Among these, the self-saturating reactor type (SR type) is the most effective, as it has fewer electronic components, high reliability, fast response speed, convenient and economical maintenance, and can be manufactured by most transformer manufacturers in my country.
(6) Separate lines: Because there is impedance in the power supply system, the harmonic load current will cause harmonic voltage distortion in the voltage waveform. Separate the power supply lines of the loads that generate harmonics from the power supply lines of the loads that are sensitive to harmonics. Linear loads and nonlinear loads are fed by different circuits starting from the same power interface point PCC, so that the distorted voltage generated by the nonlinear load will not be conducted to the linear load.
(7) Circuit multiplexing and diversification: Parallel diversification of inverter units involves connecting two or more inverter units in parallel and canceling harmonic components through waveform shifting and superposition; rectifier circuit multiplexing involves using 12-pulse, 18-pulse, or 24-pulse rectification to reduce harmonic components; series diversification of power units involves using multiple pulses (such as 30-pulse series), and power unit multiplexing circuits can also reduce harmonic components. In addition, there are new frequency conversion modulation methods, such as voltage vector deformation modulation.
(8) Improvement of inverter control methods: With the development of high-tech technologies such as power electronics, microelectronics, and computer networks, inverter control methods have developed as follows: digital control inverters, inverters are digitized using single-chip microcomputers such as MCS51 or 80C196MC , and are assisted by SLE4520 or EPLD LCD displays to achieve more complete control performance; multiple control methods are combined. Each single control method has its own disadvantages. If these single control methods are combined, their advantages can be taken into account to make up for their disadvantages, thereby achieving the effect of reducing harmonics and improving efficiency.
(9) Use idealized green frequency converters without harmonic pollution: The quality standard of green frequency converters is: both input and output currents are sinusoidal, the input power factor is controllable, the power factor can be 1 with any load, and the output power can be arbitrarily controllable above and below the power frequency.
In summary, this article provides an understanding of frequency converters and the mechanisms of harmonic generation, the harmful effects of frequency converter harmonics, and methods such as frequency converter isolation, grounding, passive filters, active filters, reactive power compensation devices, and green frequency converters. With the rapid development of power electronics and microelectronics technologies, significant progress will be made in harmonic mitigation, minimizing harmonics generated by frequency converters to suppress grid pollution and improve power quality.
