Li Hongliang
Electrical Teaching and Research Section, Anhui Energy Technology School
Abstract: This paper elaborates on the principle of harmonic generation, the hazards of harmonics, points out the common methods of harmonic suppression technology, and puts forward the significance of research on active power filters. It also discusses the research status of active power filters worldwide and their future development trends.
Keywords: harmonic governance; reactive power compensation; passive filter; active filter
Harmonic wave, precisely speaking, refers to the electric quantity in the power grid current whose frequency is an integer multiple of the fundamental wave. Generally, it refers to the electric quantity generated by currents with frequencies higher than the fundamental wave after decomposing periodic non-sinusoidal electric quantities using Fourier series. In a broad sense, the component in the non-sinusoidal wave of the AC power grid with the same frequency as the power frequency is called the fundamental wave, and any component different from the power frequency can be called a harmonic.
Due to the wide application of power electronic equipment in various industries, the types, quantities, and electricity consumption of non-linear load electrical equipment are increasing rapidly. For example, in sectors such as steel mills, power plants, mines, and transportation, a large number of equipment such as electric arc furnaces, power transformers, rectifiers, frequency converters, power converters, and electric locomotives are used. In daily life and work, the use of various electrical equipment such as computers, induction cookers, microwave ovens, energy-saving lamps, and air conditioners has also increased significantly. These non-linear loads that generate a lot of harmonic currents are injected into the power grid, significantly reducing the power quality. Therefore, harmonics bring more and more hazards and impacts to the power grid and power users.
Among the impacts and hazards caused by various factors on the power system, harmonics account for an increasing proportion. The specific hazards of harmonics are as follows: in the power grid of the power system, harmonics shorten the service life of power equipment, increase the power consumption of the power grid, cause overheating of lines and power equipment, and make the protection functions of lines and equipment ineffective. They can also cause local parallel or series resonance in substations, resulting in damage to equipment such as voltage transformers. For power transformers, harmonic currents increase the copper consumption of power transformers, cause equipment vibration, increase noise, generate local overheating, and additional harmonics in the windings can also cause heating, leading to abnormal electric energy metering and misoperation of relay protection and automatic devices. Outside the power system, harmonics can also interfere with carrier communication. For example, in voice communication, faults such as noise, data transmission distortion, unreliable real-time data of EMS and DAS, and incorrect data in meter reading systems may occur.
I. Measures for Harmonic Governance
In view of the serious hazards of harmonics in the power system, people have begun to study harmonic suppression technologies. There are mainly two methods for harmonic governance in the power system. One is active governance: starting from the source of harmonics in the power grid to reduce or even eliminate harmonic generation. The other is passive governance: adding various types of filters outside the power grid to prevent harmonics from flowing into the power grid. The passive governance methods are as follows.
(1) Using Passive Power Filter (PPF)
PPF is a filtering device composed of R, L, and C components according to a certain design structure, also known as an LC filter. PPF is the earliest passive method for harmonic governance and is still widely used today. It has advantages such as a simple structure, low initial investment cost, high reliability, and low maintenance costs. However, PPF has some unsolvable problems: first, its filtering effect is easily affected by various parameters in the system, resulting in poor stability. Second, the filtering rate of PPF is not very high, generally not exceeding 85%. Third, it is impossible to simultaneously coordinate the requirements of filtering, voltage regulation, and reactive power compensation, and the reactive power compensation for the fundamental wave is also limited. Fourth, when the system impedance is small, it cannot achieve a satisfactory compensation effect.
(2) Using Active Power Filter (APF)
APF is a new type of filtering device that relies on power electronic devices to filter harmonics with dynamically changing magnitudes and frequencies and compensate for reactive power. It can overcome the shortcomings of passive power filters, such as low filtering rate and insufficient reactive power compensation.
APF and Its Working Principle: Due to the above-mentioned unsolvable shortcomings of PPF, and with the increasing development of science and technology, especially power electronic technology, people have shifted the research direction of filtering technology from passive filters to active power filters. Compared with PPF, APF has the characteristics of sensitive response, high controllability, high harmonic filtering rate, and delicate reactive power compensation. The specific characteristics are as follows: first, APF has good isolation characteristics because it has high input impedance and low output impedance. Second, APF has convenient frequency adjustment, easy adjustment, and simple manufacturing. Third, compared with PPF, APF is more flexible in design and can obtain the amplification capability of the amplifier circuit for input signals.
II. Research Status and Development Trends of APF at Home and Abroad
The idea of using APF as a feasible solution for harmonic governance in the power system was first proposed in a paper published by B.M.Bird and J.F.Marsh in 1969. In their paper, they pointed out that in the power system, injecting third harmonic current into the power grid can reduce harmonic components and improve the current waveform, which can be regarded as the origin of the active filter idea.
In 1976, E.C.Strycula and L.Gyugui from Westinghouse Electric Corporation in the United States proposed using pulse width modulation technology and proved in principle that APF can be used as an ideal harmonic current generator, and also put forward the control principle and implementation method. However, it was not until well-known Japanese scholars Hirofumi Akagi and Akira Nabac proposed relevant detection methods based on the "instantaneous reactive power theory of three-phase circuits" in the 1980s, which were successfully applied in APF, that the development of APF was greatly promoted.
From the mid-1990s to the present, research on APF worldwide has mainly focused on the improvement of algorithms such as adaptive algorithms, neural network algorithms, and genetic algorithms, as well as the selection of parallel, series, and hybrid topologies and the optimization of control methods.
Compared with other countries, research on APF in China started relatively late. It was not until around the 1990s that papers and documents related to APF appeared in China, and some universities and research institutes began to engage in this research. However, even so, research on APF in China is still not in-depth, and the application of APF has not been widely promoted. Nevertheless, with the continuous development of power electronic technology, the application of APF in China has increasingly promising prospects.
Since its origin, APF has a history of more than 40 years, and its development and application have reached a certain level. Based on current domestic and foreign research, the future research hotspots and development directions can be predicted as follows.
(1) Hybrid Structure of APF
The active part is connected in parallel with the passive filter PPF to the power grid. The active circuit operates together with the PPF filter, where PPF filters high-order harmonics and APF filters low-order harmonics, thereby enhancing the filtering effect, reducing costs, and improving efficiency.
(2) Series-Parallel Structure of APF
APF connected to the power grid can be divided into parallel APF and series APF. Parallel APF mainly controls current input, and series APF controls voltage distribution, each with its own advantages. Therefore, the combination of series and parallel APF can integrate the advantages of both types and greatly improve filtering performance.
(3) Low Loss and Low Power Consumption of APF
Currently, energy conservation and efficiency improvement are advocated worldwide. Obviously, APF with low loss, low power consumption, and high efficiency has more development space and market competitiveness.
(4) Safety and Reliability
People use electrical equipment more frequently, and the probability of personal accidents caused by electrical equipment is increasing. Therefore, APF with good fault elimination technology and safe and reliable performance will be the general trend.
III. Conclusion
This paper briefly introduces the two main measures for harmonic governance, focuses on the characteristics and working principle of active power filters in passive governance, and discusses the research status of active power filters in various countries, including China, as well as their future development trends.
References:
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