Frequency converter harmonic generation and suppression - Database & Sql Blog Articles

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The motor system driven by the frequency converter has obvious advantages such as energy saving effect, easy adjustment and maintenance, and network, and is being used more and more, but the nonlinear, impactful working mode of the power system brings interference problems. Also received much attention. For a frequency converter, its input and output generate high-order harmonics, and the harmonics at the input will affect the utility grid through the input power line. First, the generation of harmonics From the structure point of view, the frequency converter can be divided into two categories of indirect frequency conversion and direct frequency conversion. The indirect frequency conversion turns the power frequency current into a direct current through the rectifier, and then transforms the direct current into a controllable frequency through the transformer. The direct inverter converts the power frequency AC into a controllable frequency communication without an intermediate DC link. Each phase is a reversible line of anti-parallel between two sets of thyristor rectifiers. The positive and negative groups are switched to each other according to a certain period, and the voltage U0 of the alternating transmission is obtained on the load. The amplitude of U0 is determined by the control angle of each rectifier, and the frequency is determined by the switching frequency of the two sets of rectifiers. At present, there are still many inverters. There are three different structural forms of indirect frequency conversion: (1) variable voltage with a controllable rectifier, frequency conversion with inverter, voltage regulation and frequency modulation are performed on two links, respectively, and the two should be coordinated on the control circuit. (2) With uncontrolled rectifier rectification chopper transformer, inverter frequency conversion, this inverter rectification link wave device, with pulse width voltage regulation (3) with uncontrolled rectifier rectification, PWM inverter simultaneous frequency conversion, This kind of frequency converter only uses a controllable shutdown full-control device (such as IGBT) output waveform to be a very realistic sine wave. No matter which kind of inverter, a large number of non-linear power electronic components such as thyristors are used. No matter which rectification method is adopted, the way in which the inverter draws energy from the power grid is not a continuous sine wave, but a pulsating break. The continuation method requests current from the grid. The pulsating current and the impedance along the path of the grid form a pulsating voltage drop superimposed on the voltage of the grid to distort the voltage. According to Fourier analysis, the asynchronous sine wave current is due to the same frequency. The fundamental wave and the frequency are greater than the harmonic composition of the fundamental frequency. Second, the harm of harmonics In general, the impact of the frequency converter on the power system with large capacity is not very obvious, but for systems with small system capacity, the interference caused by harmonics can not be ignored, it is a pollution to the public power grid, objective The existence of the hazard to the utility grid and other systems is roughly: (1) harmonics cause additional harmonic losses in the components of the utility grid, reducing the use of power generation, transmission and electrical equipment, a large number of third harmonic currents Over the center line can cause the line to overheat or even cause a fire. (2) Harmonics affect the normal operation of various electrical components. In addition to causing additional losses, the effects of harmonics on the motor can also cause mechanical vibration, noise and overcurrent, which can cause overheating of capacitors, cables and other equipment, insulation aging, shortened life and damage. (3) Harmonics can cause local parallel resonance and series resonance of the utility grid, thereby amplifying the harmonics, which greatly increases the above-mentioned hazards and even causes serious accidents. (4) Harmonics can cause interference to adjacent communication systems, resulting in reduced communication quality and even loss of information, making the communication system unable to work properly. Third, the suppression of harmonics The inverter is convenient, efficient and of great interest. At the same time, a large amount of harmonics and reactive power are injected into the power grid, so that the power quality is continuously deteriorated. On the other hand, with the popularization and application of a large number of sensitive devices represented by computers, people have higher and higher requirements for power supply quality of public power grids, and many countries and regions have established their own harmonic standards. In 1984 and 1993 respectively, China passed the "Power System Harmonic Management Regulations" and "GB/T-14549-93 standards to limit the harmonic pollution of power supply systems and electrical equipment. Harmonic suppression The basic idea of ​​the method is three. One is to install harmonic compensation device to compensate harmonics, and the other is to transform the power electronic device itself, it does not generate harmonics, and the power factor can be controlled to 1, and the third is in the city. Appropriate measures are taken to suppress harmonics in the electrical network. The specific methods are as follows: (1) Installing the appropriate reactor The input side power factor of the inverter depends on the internal AC-DC conversion circuit system, and the parallel power factor can be utilized. Teach the DC reactor and the power supply side AC reactor to reduce the THDV of the incoming current by about 30%-50%, which is about half of the harmonic current of the reactor. (2) Install the source power filter In addition to the traditional LC debug filter currently in use, an important trend in harmonic suppression is the use of active power filters. It is connected in series or in parallel to the main circuit to detect harmonics from the compensation object in real time. The flow, the compensation device generates a compensation current equal to the magnitude of the harmonic current, and the direction is intended to be reversed, so that the grid current only contains the fundamental component. This filter can track and compensate the harmonics whose frequency and amplitude are changed. Its characteristics are not affected by the system, and there is no danger of harmonic amplification. Therefore, it has received much attention and has been widely used in Japan and other countries. (3) Multi-phase pulse rectification is used when conditions permit or harmonics are required to be relatively small. In the case of multiphase rectification, the 12-phase pulse rectification THDV is about 10%-15%, and the THDV of 18-phase pulse rectification is about 3%-8%, which meets the requirements of international standards. The disadvantage is that special transformer is needed. It is not conducive to equipment modification, and the price is high. (4) Using filter module components There are many filter modules or components on the market that are specially used for anti-conduction interference. These filters have strong interference capability and are also resistant. The interference of the electricity itself is transmitted to the power supply, and some also have the function of peak voltage absorption, which has many advantages for various types of electrical equipment. (5) Development of a new type of converter, large-capacity converter reduces harmonics The main method is to use multiple technologies. The high power factor rectifiers of several kilowatts to several hundred kilowatts mainly use PWM transformers to form four-quadrant AC speed control inverters. This inverter not only outputs voltage and current is sine wave, input. The current is also a sine wave, and the power factor is 1. It can also realize the bidirectional transmission of energy, which represents the development direction of this technology. (6) Select the three-phase distribution transformer of the three-phase distribution transformer of D-YN11 wiring group The high-voltage side winding is connected in a triangle shape, the low-voltage winding is star-shaped and the neutral point is connected with "11" to ensure that the phase electromotive force is close to a sinusoidal shape, and the phase electromotive force waveform distortion is avoided. At this time, the 220V load supplied by the regional low-voltage power grid The line current will not exceed 30A, and 220V single-phase power supply can be used. Otherwise, it should be powered by 220/380V three-phase four-wire. The method of reducing or weakening the inverter is as follows: (1) Adding an AC reactor between the inverter and the motor To reduce electromagnetic radiation during transmission. (2) Using a transformer with a spacer layer, it is possible to isolate most of the conducted interference before the transformer. (3) Use a double-integral A/D converter with certain high-frequency interference cancellation, and (4) Select a meter with a switching power supply. Equal-voltage equipment (5) Signal line and power line are separately wired, try to use twisted pair to reduce common mode interference. (6) In the control system using single chip microcomputer, plc, etc., increase the detection signal when programming software. And soft filtering of the output control part to enhance the system's own anti-interference ability. IV. Conclusion The use of frequency converters brings convenience and great benefits to people, and it will be more commonly used. However, due to its unique working mode, it has caused certain damage to the public network and become one of the sources of harmonic pollution of the power grid. Therefore, analyzing and researching methods for suppressing harmonics will become a very important issue.