Frequency converter harmonic hazard and solutions - Solutions - Huaqiang

Filter 18.432M

1. Introduction In the field of industrial speed control transmission, compared with the traditional mechanical speed regulation, there are many advantages in using the frequency converter to adjust the speed. The application is very extensive, but due to the switching characteristics of the inverter inverter circuit, the power supply is A typical non-linear load is formed, and the frequency converter is usually operated in parallel with other equipment in the field, such as computers and sensors, which are often installed close together, which may cause interaction. Therefore, the power electronic device represented by the frequency converter is one of the most important harmonic sources in the public power grid, and it has an important influence on the power quality in the power system.
2. The root cause of process harmonics generated by harmonics is due to nonlinear loads. When the current flows through the load, it does not have a linear relationship with the applied voltage, and a non-sinusoidal current is generated, thereby generating harmonics, as shown in the following figure.

The harmonic frequency is an integral multiple of the fundamental frequency. According to the French mathematician M. Fourier's analysis principle, any repeated waveform can be decomposed into a sine wave containing the fundamental frequency and a series of harmonics of the fundamental multiple. Component. Harmonics are sinusoidal waves, each with a different frequency, amplitude, and phase angle. Harmonics can be distinguished as even and odd. In a balanced three-phase system, even harmonics have been eliminated due to the symmetrical relationship, and only odd harmonics exist, and the odd harmonics cause more damage than even harmonics.
3. Harmonic hazards For power systems, the hazards of power harmonics are mainly manifested in the following aspects:
(1) Increase the additional loss of transmission, supply and use of equipment, overheating the equipment, reducing equipment utilization and economic benefits:
1 The impact of power harmonics on transmission lines:
Harmonic currents increase the power loss of the transmission line. When the harmonic frequency injected into the grid is located in the resonance region near the resonance point of the network, insulation breakdown occurs on the transmission line and the power cable line.
2 power harmonics on the transformer:
The presence of harmonic voltage increases the hysteresis loss, eddy current loss and the electric field strength of the transformer. The presence of harmonic current increases the copper loss. For transformers with asymmetric loads, the harmonic components of the field current are greatly increased.
3 The impact of power harmonics on power capacitors:
When a voltage containing power harmonics is applied across the capacitor, the harmonic impedance of the capacitor is small, and the harmonic current is superimposed on the fundamental wave of the capacitor, so that the capacitor current becomes larger, the temperature rises, and the life is shortened, causing the capacitor to pass. The load may even explode, and the harmonics may also cause power harmonic resonance in the grid together with the capacitor, which will aggravate the fault.
(2) Affect the reliability of relay protection and automatic devices:
Especially for electromagnetic relays, power harmonics often cause relay protection and automatic device malfunction or refusal, causing their operation to lose selectivity, reduce reliability, easily cause system accidents, and seriously threaten the safe operation of the power system.
(3) Interference with the operation of the communication system:
When the odd-frequency low-frequency harmonic current flowing through the power line is coupled by the magnetic field, the interference voltage is generated in the communication line adjacent to the power line, which interferes with the work of the communication system and affects the clarity of the communication line, even at the extreme. In this case, it also threatens the safety of communication equipment and personnel.
(4) Impact on electrical equipment:
Power harmonics can cause distortion of the graphics of TVs and computers, fluctuations in picture brightness, and overheating of components in the machine, causing errors in computers and data processing systems, and even damaging the machine.
In addition, power harmonics can also adversely affect the indications of measurement and measurement instruments and rectifiers. It has become a major public hazard affecting power quality in current power systems.
4. Harmonic control measures to control harmonic problems, suppress radiation interference and power system interference, and can adopt shielding, isolation, grounding and filtering techniques.
1 using a passive filter or an active filter;
The use of passive filters is mainly to change the impedance of the power supply at a special frequency, suitable for stable, unchanged systems. The use of active filters is mainly used to compensate for non-linear loads. The LC filter is a traditional passive harmonic suppression device. It is composed of a filter capacitor, a reactor and a resistor. It is connected in parallel with the harmonic source. In addition to filtering, it also has the function of reactive power compensation.
2 increase the capacity of the transformer, reduce the impedance of the loop and cut off the transmission line method;
The distortion current due to the non-linear load produces a distortion voltage drop across the impedance of the cable, and the resultant distortion voltage waveform is applied to other loads connected to the same line, causing harmonic currents to flow over it, thus reducing Harmonic hazard measures can also be achieved by increasing the cross-sectional area of ​​the cable and reducing the impedance of the loop. At present, more domestic use of increasing transformer capacity, increasing cable cross-sectional area, especially to increase the neutral cable cross-section, as well as the use of large-scale circuit breakers, fuses and other protective components, but this way can not Fundamentally eliminates harmonics, which in turn reduces protection features and functions, increases investment, and increases the hidden dangers of the power supply system.
3 Use a green inverter without harmonic pollution.
The quality standard of the green inverter is: the input and output currents are both sinusoidal, the input power factor is controllable, and the power factor can be set to 1 with any load, and the output frequency of the power frequency up and down can be obtained. The built-in AC reactor of the inverter can suppress harmonics well and protect the rectifier bridge from the instantaneous spike of the power supply voltage. Practice shows that the harmonic current without the reactor is significantly higher than that of the reactor. Harmonic current. In order to reduce the interference caused by harmonic pollution, a noise filter can be installed in the output loop of the inverter. And reduce the carrier frequency of the inverter if the inverter allows. In addition, in high-power inverters, 12-pulse or 18-pulse rectification is usually used, so that in the power supply, the harmonic content is reduced by eliminating the lowest harmonic. For example, 12 pulses, the lowest harmonic is 11 times, 13 times, 23 times, 25th harmonic. By analogy, for 18 pulses, the lowest harmonics are the 17th and 19th harmonics.
The low harmonic technology applied in the frequency converter can be summarized as follows:
(1) Parallel multiplexing of inverter units, using two or more inverter units in parallel, and canceling harmonic components by waveform superposition.
(2) Multipleization of the rectifier circuit, using 12-pulse, 18-pulse or 24-pulse rectification in the PWM inverter to reduce harmonics.
(3) The series multiplication of the inverter unit, using a 30-pulse serial inverter unit multiplexed line, the harmonics can be reduced to a small extent.
(4) Adopt new frequency conversion modulation methods, such as diamond modulation of voltage vector. At present, Xu, multi-frequency converter manufacturers have attached great importance to harmonic problems. At the time of design, the greening of the inverter has been ensured from the technical means, thus fundamentally solving the harmonic problem.
Conclusion In summary, the causes of harmonic generation can be clearly understood. Passive filters and active filters can be used in specific treatments to reduce loop impedance, cut off harmonic transmission paths and develop green using harmonic-free pollution. Inverter and other methods, control the harmonics generated by the inverter to a minimum range, achieve scientific and rational use of electricity, suppress grid pollution, and improve power quality.-->