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Talk about the harm of harmonics and the main suppression measures

1. Introduction

Nowadays, with the rapid development of power electronics technology, there are more and more types of power loads, especially the motor system driven by frequency converter, which is used more and more because of its convenient process control, obvious energy-saving effect, simple maintenance, networking and other advantages. No matter what kind of converter it is, it will produce higher harmonics at the input and output ends during the frequency conversion process, and the harmonics at the input end will affect the public power grid through the input power line. The harmonics at the output end have a great negative impact on loads such as motors, which makes the power grid generate more and more harmonics under stable operation. Where the harmonics come from, what influence they have, how to detect them, and how to suppress them are our concerns today.


2. Harmonic source

Harmonics come from various harmonic sources. The so-called "harmonic source" usually refers to various specific electrical equipment, namely nonlinear electrical equipment or nonlinear power load. Among them, power electronic equipment is the main component of the power grid, with a large number of transformer groups. Iron core reactor, electric arc furnace, variable frequency drive or thyristor rectifier DC drive equipment, computer, uninterruptible power supply (ups) for important loads, energy-saving fluorescent lamp system, household appliances and electronic equipment are harmonic sources. These nonlinear loads will lead to grid pollution, power quality degradation, power supply equipment failure, serious conditions and even fire accidents.


3. Harmonic hazard

When the capacity of power electronic equipment reaches 0. 3 to 0. 5 times of the short-circuit capacity of the connected power grid, or the power grid parameters caused by harmonic resonance are low, it may easily cause 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 up to 80%. Harmonic hazards are mainly shown in the following aspects:

(1) When the skin collection effect of high harmonic current on the motor transformer is serious, it is easy to cause local high temperature and increase the proportion of transformer copper damage.

(2) Due to the interaction between harmonic current and fundamental magnetic field, it will produce bad mechanical vibration, accelerate shaft and bearing fatigue, shorten service life, and increase transformer noise.

(3) The harmonic shaft of the motor can easily break through the lubricating oil film of the bearing when it bears the voltage of larger harmonics, which will cause discharge damage to the bearing.

(4) When a large number of third harmonics in the power grid cause the power capacitor to "bulge" and "blast" quickly and exit the operation, the third harmonic will cause the line to overheat and even fire when it flows through the neutral line.

(5) Under certain conditions, the instantaneous stab voltage generated during the operation of power electronic equipment is an important reason that affects the partial discharge and service life of power cables, leading to cable insulation failure damage.

(6) Harmonics may also cause misoperation of protection and automatic devices.

(7) The high frequency and large energy of harmonic signal can have characteristics such as capacitive coupling, electromagnetic induction and electrical conduction, which will inevitably interfere with communication.

(8) Harmonics exceeding the standard are easy to damage electronic original devices or shorten the service life of electronic original devices.

(9) Harmonics will lead to local parallel resonance and series resonance in the public power grid, thus amplifying the harmonics, thus greatly increasing the above hazards, and even causing other serious accidents.


4. National standards

Gb / t14549-93 Power quality - public grid harmonics are shown in Table 1.

Table 2 is the allowable value of harmonic current, which is injected into the common connection point.


Table 1 Power quality - harmonic standard of public power grid

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Table 2 Allowable value of harmonic current injected into common connection point

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5. Harmonic measurement

The harmfulness of the above harmonics makes it an important indicator to detect and monitor the power quality in the daily operation of the power grid when necessary. Due to the complexity of the harmonic problem in the power grid, it is difficult to accurately reflect the actual situation of the power grid through certain theoretical calculations. The measured harmonic interference of the power grid is usually used to ensure the safe operation of the power grid and high-quality power supply.

At present, from the perspective of instrument performance and test purpose, it can be divided into: harmonic detector alarm, harmonic analyzer and power quality comprehensive analyzer. Developed countries are relatively advanced in the development and use of harmonic analyzers with complete functions, wide measuring range, convenient and reliable use and high price. However, the performance of domestic instruments is still poor, and data acquisition is not ideal, but the price is relatively low. For example, domestic dxj series harmonic detector, American Fluke power quality analyzer, Nissan digital power analyzer, domestic gxf series harmonic analyzer, etc.


6. Harmonic control

The measures to suppress the harm of harmonics are mainly to suppress the harmonic source itself and its propagation path. Basic idea: first, install harmonic compensation device to compensate harmonics; second, transform the power electronic equipment itself to reduce harmonics, and 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 pulsation or certain phase shift angle. 12-phase pulse rectification

Thdv is about 10% - 15%, and 18-phase pulse rectifier thdv is about 3% - 8%. The disadvantage is that special transformer is required, which is not conducive to equipment transformation, and the price is high;

(2) When the equipment body cannot be improved, an active power filter can be installed near the harmonic source to absorb harmonic current, but the compensation capacity is small and the cost is high;

(3) Devices with harmonic complementarities should be centralized, otherwise they should be used in a decentralized or staggered manner, and the working mode with large harmonic content should be appropriately limited;

(4) Add a series reactor at the user's incoming line to increase the electrical distance from the electrical system and reduce the interaction of harmonics;

(5) Power supply voltage and line impedance. Find out the cause of unbalanced load characteristics and improve three-phase imbalance of power supply;

(6) Install reactive power compensation device to suppress voltage fluctuation, flicker and three-phase imbalance;

(7) Increase power supply capacity and reduce harmonic content;

(8) Improve the performance of equipment or equipment and improve the anti-interference ability of electrical equipment;

(9) The signal line and power line shall be wired separately, and twisted pair shall be used as far as possible to reduce common-mode interference;

(10) When using the communication electronic control system, the software filter of the detection signal and output control part should be appropriately added to improve the anti-interference ability of the system itself.

The installation of passive power supply wave is simple and small, and the reactive power compensation effect is good, which is suitable for steady-state harmonic places. The harmonic current generated by the nonlinear load and the voltage distortion between the filter and the system connection point are equal to the harmonic current generated by the output and load. The opposite harmonic current plays a role in compensating the harmonic. Good dynamic performance and short response time (15 μ s) The three-phase compensation harmonic current has low power loss (less than 3% of the rated power of the equipment), which is suitable for temporary harmonic or places with complex, fast change and strong randomness of harmonic components.


6.2 Application examples

Frequency conversion and speed regulation are widely used in factories. Most of them are 6-pulse trigger frequency converters, with high 5th harmonic content. We have a 980kw/690v Siemens 6-pulse inverter to drive three-phase asynchronous motor. A passive power filter of Nokia is installed in parallel at the input end of the converter, mainly absorbing 5 harmonics generated by the converter. The distortion rate of the first five harmonics is 28.8%, which becomes 1.7% after being put into use, and the compensation rate exceeds 90%. The 5th harmonic interference of the system is basically eliminated, and the service life of components is extended. The capacitor in the passive filter structure improves the power factor at the end of the line to 100%. The results of energy saving statistics show that after the investment, more than 8000 kilowatt-hours of electricity is saved than the month before the investment, and the economic benefits are good.


7. Conclusion

Harmonics have brought some damage to the public power grid and become one of the pollution sources of the power grid. At present, various methods have certain disadvantages. Therefore, analysis and research on how to suppress harmonics will also become a very important topic.



keywords: harmonic wave
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