in recent years, with the continuous development of power electronic technology, all kinds of power electronic equipment have been widely used in industries, civil buildings, energy systems, electrified transportation and other fields, and people have paid more and more attention to the problem of power grid harmonics. The evaluation of power grid quality in various countries is no longer limited to the stability of frequency and voltage, and international and national standards have been formulated to limit harmonics accordingly. According to statistics, about 4.6% of the harmonics generated by various industries come from civil buildings. In order to have a deeper understanding of harmonic pollution in the distribution system of civil buildings and its suppression methods, our reporter specially interviewed Mr. Yao Chibiao, deputy chief engineer of Beijing Architectural Design and Research Institute. Grasp the direction and study deeply: hello, Mr. Yao! Thank you for being interviewed by Electrical Application in your busy schedule. Please tell us what is the main research direction of harmonic problems at home and abroad at present? Yao Chibiao: The international and domestic research directions of harmonic problems are mainly in the following aspects: 1) Conduct extensive and in-depth research on harmonic sources of converters. 2) In the measurement technology, the method to improve the accuracy of harmonic measurement under different harmonic conditions is put forward; Multi-channel harmonic analyzer and power quality measuring instrument are developed. 3) In the aspect of analysis and measurement technology, the influence of power grid parameter change and the accuracy of model and component parameters on harmonic calculation is analyzed, and a new mathematical method is sought for unsteady waveform distortion. 4) In terms of filtering technology, a parameter design and correction method combining time domain and frequency domain is proposed; A hybrid structure of passive and active circuits is proposed, and a new power line regulator with comprehensive performance is developed. Reporter: According to a survey report on harmonic sources published by Japan Electric Association, 9% of the users have the largest harmonic source from rectifiers, and 4.6% of the harmonics generated by civil buildings in various industries. Then, what are the causes of harmonics in the distribution system of civil buildings and the types of harmonic sources? Yao Chibiao: In the distribution system of civil buildings, when the power supply of the power grid is theoretically sinusoidal fundamental voltage, when it is applied to a nonlinear load, the current absorbed by the load is different from the waveform applied, and the distorted current will produce a voltage drop on the impedance of the power supply system, thus producing a distorted voltage. The distorted voltage will have an impact on all electrical loads (linear and nonlinear) and will produce more distorted current. In fact, the grid power supply often contains a certain proportion of distorted voltage, so the above situation may be more serious. When the grid voltage contains harmonic voltage, harmonic current is induced in the rotor of asynchronous motor, and the asynchronous speed of the rotor of asynchronous motor in turn produces subharmonic and interharmonic in the stator with frequency lower than that of the grid harmonic voltage. In the distribution system, when the reactance-to-capacitance ratio of the capacitor bank with a certain ratio of reactance or the system itself is appropriate, the harmonic current of a certain number of times will be resonated and amplified, sometimes reaching more than 1 times of the harmonic source current. Harmonic sources in civil buildings can be divided into the following two categories: 1) Power electronic devices such as harmonic source UPS power supply with semiconductor nonlinear components, DC screen, variable frequency governor, soft starter, gas discharge lamp, electronic ballast, DC power supply for household appliances and office appliances, thyristor dimmer and AC voltage regulator. The harmonic current generated by these devices depends on its own characteristics and working conditions and the voltage applied to it, but has little to do with the parameters of the power system, and can be regarded as a harmonic constant current source. The harmonic current generated by them is mainly odd harmonics, and it is also the main harmonic source in the distribution system of civil buildings. 2) Harmonic sources including arc and ferromagnetic nonlinear equipment, such as AC motors, transformers, special light sources, circuit breakers and fuses, etc., in general, odd harmonics generated by synchronous motors and interharmonics and subharmonics generated by asynchronous motors are not serious and can be ignored. The magnitude of harmonic current generated by transformer is related to the saturation degree of its core. In normal operation, the voltage is rated, and the iron core works in the range of slight saturation, at this time it is basically a linear circuit. In some special cases, such as the building has just been put into use or light load operation at night, the operating voltage of the transformer is high, the transformer is switched on and off, the load changes drastically, the voltage of the external network fluctuates, the excitation current of the transformer is too large under the condition of high overload, the DC component is increased due to serious three-phase imbalance, and the induced current generated by geomagnetic storm caused by solar flare explosion in the power grid, etc., at this time, the saturation degree of the iron core becomes deeper, and the harmonic current content is increased, mainly the third harmonic. Reasonable selection of measured data: in the distribution system of civil buildings, although D/Y transformer can be used to limit part of the third harmonic back to the power grid, the main method to suppress the harmonic is to use filters. Please introduce the ways of harmonic suppression. Yao Chibiao: There are mainly the following ways to suppress harmonics: (1) Passive filters Traditional LC filters are composed of capacitors, reactors and resistors, which are divided into single-tuned filters, double-tuned filters and high-pass filters (amplitude reduction filters). Single-tuned filter only aims at a specific design frequency, such as 3, 5 and 7 times; Double tuned filter is equivalent to two parallel single tuned filters, which can absorb harmonics of two frequencies at the same time; High-pass filters (also known as amplitude reduction filters) can be divided into first-order, second-order, third-order and C-type. It presents a very low impedance in a wide frequency range, forming a low impedance path for all harmonics above a certain frequency. In practical application, several groups of single-tuned filters and a group of second-order high-pass filters are often used to form a filtering device. Because LC filter contains a certain amount of capacitance, it can improve the power factor to some extent, but it is generally far below the required compensation value. At this time, it is advisable to install parallel capacitor banks with series reactors separately, and it is not advisable to blindly increase the filter capacity, so that when switching capacitor banks step by step, the impact on the filtering effect is small. LC filter is generally connected to the distribution system in parallel with the resonant source, and the three-phase connection can be Y-type or D-type. However, the 3rd harmonic filter is a little special, because the 3rd harmonic is mainly zero-order harmonic, and most of it flows through the N line, so some 3rd harmonic filters are connected in series on the N line. For example, THF of ABB Company, its working principle is contrary to that of parallel LC filter, which produces high impedance at the resonant frequency of 15 Hz, while the impedance at other frequencies other than 15 Hz is very small. As a result, most of the third harmonic current is blocked. In addition, there is a kind of auto-tuning single-tuning filter, which changes the inductance or capacitance parameters in a certain range through a set of measurement control devices to ensure that the LC filter resonates at the actual frequency. (2) Active Power Filter (APF) The basic principle of the active power filter is to detect the voltage and current of the compensated object, and get the command signal of the compensation current through calculation by the operation circuit. The amplified compensation current cancels out the harmonic current to be compensated in the load, that is, the frequency amplitude of the compensation current is equal to that of the harmonic current, but the phase difference is 18, and finally the expected power supply current is obtained. In practical application, most active filters are installed in parallel, and series active filters are rarely used alone, and there is a lack of in-depth research. Compared with the parallel active filter, the series active filter works as a voltage source, and the active device has small capacity and high operation efficiency, which is suitable for counteracting the voltage pollution of harmonic sources with the nature of voltage source. The disadvantage is that it is difficult to switch and protect against faults, and it is impossible to dynamically compensate reactive power. The latter works as a current source with mature technology, and its compensation characteristics are not affected by the impedance of the power grid. It is suitable for offsetting the current pollution of harmonic sources with the characteristics of current source, but its disadvantages are low operation efficiency and high cost. (3) Hybrid Filter In order to solve the shortcomings of passive filter and active filter and form an ideal harmonic compensation system, it is an ideal method to mix passive filter and active filter-hybrid filter. The mixed use of parallel active power filter and passive LC filter can be divided into two ways: 1) One is that the active power filter and passive LC are connected in parallel, both of which undertake the task of harmonic compensation, and the LC filter is a high-pass filter, which mainly compensates higher harmonics, thus reducing the frequency requirements of switching elements in the active power filter and reducing the cost of the active power filter. There are also groups of LC filters to undertake most of the harmonic suppression tasks, and the active power filter only plays a role in improving the system performance, thus greatly reducing the capacity of the active power filter, which is the most common use method. 2) The other is that the active power filter is connected in series with the LC filter. In this method, the passive LC filter undertakes most of the harmonic suppression tasks, and the active power filter is connected to the LC loop through the single-phase transformer, which plays a role in improving and adjusting the filtering characteristics of the passive LC filter, which is equivalent to the automatic tuning passive filter. At this time, the active power filter does not bear the fundamental voltage of AC power supply, so its capacity is small. Series active power filter and passive LC filter are used in parallel, and their characteristics are similar to the above 2). The series active power filter is connected in series between the power supply and the passive LC filter through three single-phase transformers. For the harmonic current flowing through the system, the series active power filter shows pure impedance, and for the fundamental current flowing through the system, it shows zero impedance. Therefore, the series active power filter acts as a harmonic isolator, preventing harmonic current from flowing into the power grid and forcing it to flow into the passive LC filter, thus eliminating the possible resonance between the power grid and the LC filter. When the system voltage distortion is not serious, the capacity of series active power filter is small, which can reduce the cost.
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