Nowadays, with the rapid development of power electromagnetic compatibility technology, there are more and more types of power loads, especially the motor system driven by frequency converter has been more and more applied with its convenient process control, significant energy-saving effect, simple maintenance, digitization and other characteristics. No matter what type of converter, high harmonics will be generated at the output end of the converter, and the incoming harmonics will affect the public power grid through the input power line. The harmonic at the output end has a great adverse impact on the load such as the motor, which promotes the power grid to generate more and more harmonics under stable operation. Where harmonics come from, what harm they have, how to detect them, and today we are concerned with how to suppress them.
2. Harmonic source
Harmonics come from various harmonic sources. It is said that "harmonic source" generally refers to various specific electrical equipment, namely nonlinear electrical equipment or nonlinear power load. Among them, power electronic equipment is an important part of the power grid. There are a large number of transformer sets, iron core reactors, electric arc furnaces, variable frequency drive or silicon controlled rectifier DC drive equipment, computers, UPS power supply for critical loads, energy-saving fluorescent lamp systems, electrical products and electronic products are harmonic sources. This nonlinear load will lead to grid pollution, power quality degradation, distribution equipment failure, serious situation and even safety accidents.
3. Harmonic damage
When the volume of power electronic equipment reaches 0.3-0.0% of the short-circuit capacity of the connected power grid. When the power grid parameters caused by harmonic resonance are about 5 times or lower, it may be easy to generate harmonic resonance "harmonic instability" within a certain range. 5.7 The second harmonic content accounts for a large proportion of the total harmonic content, of which the fifth harmonic content accounts for 45% of the total harmonic content, of which 80%. Harmonic damage is mainly reflected in the following aspects:
(1) When the skin collection effect of high harmonic current on the motor transformer is serious, it is easy to increase the proportion of partial high temperature and transformer copper damage.
(2) Due to the interaction between harmonic current and fundamental magnetic field, it will cause bad vibration analysis, accelerate shaft and bearing fatigue, shorten service life and improve transformer noise.
(3) The harmonic shaft of the motor can easily penetrate the lubricating oil film of the bearing under the voltage of larger harmonics, which will cause the discharge to damage the bearing.
(4) A large number of third harmonics in the power grid lead to the rapid "belly bulge" and "shooting" of the power capacitor. During the evacuation operation, the third harmonic will cause the line to overheat and even cause fire.
(5) Under certain circumstances, the transient stab voltage generated during the operation of power electronic equipment is the main cause of cable insulation failure and damage.
(6) Harmonics may also cause misoperation of the protection device.
(7) The high frequency and large energy of harmonic signal can have the characteristics of capacitive coupling, electromagnetic induction and electrical transmission, which will inevitably affect the communication.
(8) Harmonics exceeding the standard can easily damage the electronic components or reduce the service life of electronic components.
(9) Harmonics will lead to partial parallel resonance and series resonance of the public power grid, and then increase the harmonics, which will greatly increase the above injuries, and even lead to other major accidents.
4. National standards
Gb ／ t14549-93 Power quality - Harmonics of public power grid Table 1.
Table 2 shows the specified values of harmonic current introduced into the common node.
Table 1 Power quality - harmonic standard of public power grid
Table 2 Allowable value of harmonic current injected into common connection point
5. Harmonic measurement
The harmfulness of the above harmonics makes it an important factor of power quality in the power grid and monitoring the daily operation of the power grid when necessary. Due to the diversity of harmonic problems in power grid, it is difficult to accurately reflect the specific situation of power grid according to some theories. Generally, the detected harmonic effect of the power grid is used to ensure the safe operation of the power grid and high-quality power supply.
At present, according to the purpose of instrument performance test, it can be divided into: harmonic detector alarm, harmonic analyzer and power quality comprehensive detector. Developed countries have excellent technology in the development and application of harmonic analyzer, with complete functions, wide detection range, convenient and reliable use, and high price. However, the characteristics of domestic instruments may be poor, and the data collection is not ideal, but the price is very low. For example, domestic dxj series harmonic detector, American Fluke power quality detector, Nissan data power detector, domestic gxf series harmonic analyzer, etc.
6. Harmonic control
The countermeasures to suppress harmonic damage are mainly to suppress the harmonic source itself and the communication path. Basic concepts: first, assemble harmonic compensation equipment to compensate harmonics; second, transform the power electronic equipment itself to reduce harmonics. The power factor can be controlled to 1; third, take appropriate measures to suppress harmonics in the municipal power grid. The specific methods are as follows:
6.1 Measures taken to suppress harmonics
(1) When selecting power electronic equipment, try to select converter transformer with large pulse or certain phase shift angle. The 12-phase pulse rectifier thdv is about 10% - 15%, and the 18-phase pulse rectifier thdv is about 3% - 8%. The disadvantage is that the special transformer must be used, which is not conducive to equipment transformation, and the price is high;
(2) Power supply current and line impedance. Find out the cause of unbalanced load characteristics and improve three-phase imbalance of power supply;
(3) When the equipment itself cannot be improved, the active power filter can be assembled around the harmonic source to absorb the harmonic current, but the compensation level is low and the cost is high;
(4) Devices with harmonic diversity should be centralized, otherwise the scattered or overlapping application will moderately limit the working mode of multiple harmonics;
(5) Improve power supply capacity and reduce harmonic content;
(6) Strengthen the series reactor, enhance the electrical distance with the electrical system, and reduce the interaction of harmonics;
(7) Assemble reactive power compensation equipment to suppress voltage fluctuation, flicker and three-phase imbalance;
(8) The power line and power line shall be separated, and twisted pair shall be selected as far as possible to reduce common-mode interference;
(9) Improve the performance of equipment or equipment and improve the anti-interference of electrical equipment;
(10) When using the communication electronic control system, try to increase the operation part of the detection signal and output software filter to improve the anti-interference of the system itself.
The installation method of passive power supply wave is simple and small, and the reactive power compensation effect is good, which is suitable for stable harmonic field. The harmonic current generated by the nonlinear load and the voltage loss between the filter and the system node are the same as the harmonic current generated by the load. On the contrary, the harmonic current plays the role of compensating harmonics. Good dynamic performance and short response time (15 μ s) Three-phase compensated harmonic current has low power loss (less than 3% of the maximum power of the equipment), and is suitable for temporary harmonic or sites with complex harmonic components, rapid changes and high contingency.
6.2 Application cases
Frequency conversion speed regulation is widely used in the factory. It is common to see 6-pulse start frequency converter, and the content of 5th harmonic is high. Everyone has a 980kw/690v Siemens 6-pulse converter to drive three-phase asynchronous motor. Nokia's passive power filter is connected to the input end of the converter. The key is to absorb the five harmonics generated by the converter. The distortion rate of the first five harmonics is 28.8%, 1.7% after delivery, and the compensation rate exceeds 90%. The 5th harmonic interference is basically avoided and the service life of components is extended. The capacitor in the passive filter system increases the power factor at the end of the line to 100%. According to the energy saving statistics, the electricity saving after investment is 8000 kilowatt-hours less than one month before investment.
Harmonics have damaged the public power grid to a certain extent and become one of the pollutants of the power grid. At present, various methods have certain defects. Therefore, analysis and research on how to suppress harmonics will also become a key issue.