The impedance of the series resonant inductor and the capacitor to the fundamental wave is equal and the current is the same, so the fundamental wave operating voltage of the inductor and the capacito

The impedance of the series resonant inductor and the capacitor to the fundamental wave is equal and the current is the same, so the fundamental wave operating voltage of the inductor and the capacitor is the same. As explained earlier, the cost of the capacitor is lowest when the actual operating voltage of the capacitor is equal to its rated voltage, so the actual operating voltage of the inductor should be equal to the rated voltage of the capacitor. The rated voltage level of the capacitor is mostly equivalent to the grid voltage. If the actual working voltage of the inductor is equal to the rated voltage of the capacitor, it is equivalent to the inductor impedance being equal to the load impedance, and the best performance-price ratio can be achieved. On this basis, if the inductive reactance of the inductor is increased, although the filtering effect can be improved, the improvement is not much. The cost of the inductor increases, and the capacitors need to be connected in series. The cost increases sharply, and the performance-price ratio decreases. Therefore, the fundamental wave inductive reactance of the inductor is greater than the load, etc. The effective fundamental wave impedance of 200 has no practical significance. If the inductive reactance of the inductor is reduced, the filtering effect will decrease, the cost of the inductor will decrease, the capacity of the capacitor will increase, so the cost will increase, and the performance-price ratio will also decrease. In order to obtain sufficient reliability, the actual operating voltage of the inductor and capacitor should be slightly lower than the rated voltage of the capacitor. When harmonic currents flow in from the external network and affect the normal operation of the load equipment in the internal network, connecting a series filter between the power supply and the load equipment can block the harmonics and ensure the normal operation of the load equipment. When harmonics are generated by intranet equipment and affect the system, the equipment that generates harmonics is the harmonic source. Connecting a series filter between the harmonic source and the power supply can greatly reduce the harmonic current generated by the harmonic source. Small. It should be noted here: the series filter reduces the harmonic current generated by the harmonic source itself, which is equivalent to reducing the pollution generated by the pollution source, and is a permanent solution. Parallel filters cannot reduce the harmonics generated by harmonic sources, but provide a low-impedance channel for harmonic currents to prevent harmonic currents from polluting the system. This is equivalent to polluting first and then treating, and is a temporary solution. Not only that, because the impedance of the parallel filter to harmonics is very low, it usually causes the harmonic source to produce larger harmonic currents. When a series filter is connected between the power supply and the harmonic source, the input voltage waveform of the harmonic source will be severely distorted. This distortion of the voltage waveform will make the current of the harmonic source approach a sine wave. This input voltage waveform distortion may affect the normal operation of the harmonic source control circuit. If the control circuit cannot operate normally, the power supply of the control circuit should be changed to the front end of the series filter. 6.2. Active harmonic filtering device Active harmonic filtering device is developed on the basis of passive filtering device. 6.2.1. Advantages of active filtering device Active filtering device can achieve timely compensation without adding capacitive components to the power grid. It has good filtering effect. Within its rated reactive power range, the filtering effect is 100%. 6.2.2. Disadvantages of active filter devices. Active filter devices are limited by the development of power electronic components withstand voltage and rated current. The instantaneous current is sometimes extremely large. Active filter devices cannot solve the problem of electrical and electronic components being damaged if the instantaneous current is slightly larger. , and the cost is extremely high, and its production is much more complicated than a passive filter device, so the cost is much higher. For a single active filter device, the cost is extremely high, and users cannot accept it. Generally, they are unwilling to use active filtering. Regarding the harmonic content, it does not need to be filtered too cleanly, as long as it does not harm other electrical appliances. 6.2.3. Principle of active filter device Active filter device is mainly composed of power electronic components to produce a harmonic current with the same frequency and amplitude as the harmonics of the system, but with the opposite phase. Wave current cancels. 6.2.4. Applicable occasions of active filter device The main application scope of active filter is the power supply system of computer control system, especially the power supply system of office buildings and the computer control power supply system of factory. 6.3. Reactive power compensation It is very easy for people to understand active power, but it is not easy to have a deep understanding of reactive power. In sinusoidal circuits, the concept of reactive power is clear, but when harmonics are included, there is still no generally accepted definition of reactive power. However, the understanding of the importance of the concept of reactive power and the importance of reactive power compensation is consistent. Reactive power compensation should include compensation for fundamental reactive power and compensation for harmonic reactive power.6.3.1. Generation of harmonics and reactive power Resistive-inductive loads account for a large proportion of industrial and domestic electrical loads. Asynchronous motors, transformers, fluorescent lamps, etc. are all typical resistive-inductive loads. The reactive power consumed by asynchronous motors and transformers accounts for a high proportion of the reactive power provided by the power system. Reactors and overhead lines in the power system also consume some reactive power. Resistive-inductive loads must absorb reactive power to function normally, which is determined by their own properties. Nonlinear devices such as power electronic devices also consume reactive power, especially various phase control devices. For example, phase-controlled rectifiers, phase-controlled AC power adjustment circuits and cycle converters consume a large amount of reactive power because the fundamental current lags behind the grid voltage during operation. In addition, these devices will also produce a large amount of harmonic currents, and harmonic sources consume reactive power. The fundamental current phase of the diode rectifier circuit is roughly the same as the grid voltage phase, so basically no fundamental reactive power is consumed. But it also generates a large amount of harmonic current and therefore consumes a certain amount of reactive power. In the past 30 years, the application of power electronic devices has become increasingly widespread, making power electronic devices the largest source of harmonics. Among various power electronic devices, rectifier devices account for the largest proportion. At present, almost all commonly used rectifier circuits use thyristor phase-controlled rectifier circuits or diode rectifier circuits, among which three-phase bridge rectifier circuits and single-phase bridge rectifier circuits are the most common. Harmonic pollution and power factor lag produced by rectifier circuits with resistive inductive loads are already familiar. The diode rectifier circuit using capacitor filtering on the DC side is also a serious source of harmonic pollution. The phase of the fundamental component of the input current of this circuit is roughly the same as the phase of the power supply voltage, so the fundamental power factor is close to 1. However, the harmonic component of its input current is very large, causing serious pollution to the power grid and making the overall power factor very low. In addition, power electronic devices such as phase-controlled AC power regulating circuits and cycle converters will also generate a large amount of harmonic currents on the input side. 6.3.2. Overview of reactive power compensation Reactive power is very important to the operation of the power supply system and the load. The impedance of power system network elements is primarily inductive. Therefore, roughly speaking, in order to transmit active power, the voltage at the sending end and the receiving end is required to have a phase difference, which can be achieved within a fairly wide range; in order to transmit reactive power, the voltage at both ends is required to have a amplitude. Poor, this can only be achieved within a very narrow range. Not only do most network elements consume reactive power, most loads also consume reactive power. The reactive power required by network elements and loads must be obtained from somewhere in the network. Obviously, it is unreasonable and usually impossible for all these reactive powers to be provided by generators and transmitted over long distances. A reasonable method should be to generate reactive power where reactive power needs to be consumed, which is reactive power compensation. 6.3.3. Influence of reactive power 6.3.3.1. The increase of reactive power will lead to an increase in current and apparent power, thereby increasing the capacity of generators, transformers and other electrical equipment and conductor capacity. At the same time, the size and specifications of power users' starting and control equipment and measuring instruments must also be increased. 6.3.3.2. The increase in reactive power increases the total current, thus increasing the losses of equipment and lines. This is obvious. 6.3.3.3. Increase the voltage drop of lines and transformers. If there is an impact reactive power load, the voltage will fluctuate violently, seriously reducing the quality of power supply. 6.3.4. The functions of reactive power compensation The main functions of reactive power compensation are as follows: 6.3.4.1. Improve the power factor of the power supply system and load, reduce equipment capacity, and reduce power loss. 6.3.4.2. Stabilize the voltage of the power receiving end and the power grid to improve the quality of power supply. Setting up dynamic reactive power compensation devices at appropriate locations in long-distance transmission lines can also improve the stability of the transmission system and increase transmission capacity. 6.3.4.3. In situations such as electrified railways where the three-phase load is unbalanced, the active and reactive loads of the three phases can be balanced through appropriate reactive power compensation.