Summary of Power Electronics News (seventeen) an example is introduced to introduce the concept of high order harmonic pollution in power grid. It briefly introduces its occurrence, harm and treatment methods. On the basis of the last discussion, this article brings together some essentials of typical reports, and then focuses on it.
Harmonic control is just like environmental pollution control. For example, a large number of harmful chemical substances are produced in chemical enterprises, which are discharged into the river without treatment, causing serious pollution of river water. The chemical industry has no choice but to use the water in the river, but without treatment, the water in the river can not be used. This is a vicious circle of objective existence.
Advanced electronic equipment, such as power electronic frequency conversion, silicon controlled rectifier equipment, digital communication equipment, electrical railway equipment, various kinds of UPS, high-power semiconductor switching devices, switching power supply, as well as electric smelting equipment, electrochemical equipment, mine lifting equipment, electric locomotive and so on are nonlinear power load. These devices make the harmonic current continuously injected into the power grid, and the waveform of power system is seriously distorted.
Electricity companies are helpless to use contaminated power. This is also a vicious circle, so there are various policies that must be self governed by pollution.
(1) what are the policies to harness the harmonics of the power grid?
It is an established policy of the country to control harmonic. Developing products in this area is a pressing matter of the moment and an important way for the development of enterprises. This is a win-win policy that can benefit both countries and enterprises.
1, in order to ensure the quality of qualified electricity, a series of standards, regulations and regulations have been issued by the state and relevant authorities in order to ensure that power grids and equipment are protected from the harm of harmonics.
2, the power quality standard of the public power grid:
The power grid is a unified whole. All the power supply, transmission and power supply equipment connected to the grid must accept the technical supervision and management of power network harmonics. The purpose of implementing the national standard of the public grid harmonics is to control the harmonics in the public power grid within the allowable range, to ensure the quality of the power supply, to prevent the harmonics from endangering all kinds of electrical equipment and appliances of the power grid and users, and to maintain a safe and economical operation. Harmonics in public power grids should be treated like pollution control of public power grids. Harmonics management should be carried out according to law. This "law" is the national standard of People´s Republic of China electric power law and power quality - public power grid harmonics, and any unit and department must be conscientiously carried out.
However, in the public power grid harmonics, domestic and international countries only have corresponding standards for high order harmonics, the highest is only 21 harmonic, and high frequency and high harmonic is not the corresponding standard.
(two) the harm of harmonics
The harmonics increase the additional loss of the transmission and distribution system and the electrical equipment - iron loss (including hysteresis loss and eddy current loss) and copper loss, overheating the equipment, reducing the efficiency and utilization of the equipment. Because the frequency of the harmonic current is the integer times of the fundamental frequency, high frequency current flows through the conductor, the effect of skin effect leads to the increase of the effective reactance of the conductor on the harmonic current, which increases the power loss and the loss of the electric energy, so that the heat of the conductor is serious. In particular, the above phenomena exist in the following links: load motor, transformer, transmission line, power electronic equipment, power capacitor (including compensation capacitance).
(three) harnessing harmonics
In order to solve harmonic problems, restrain radiation interference and power supply system interference, shielding, isolation, grounding and filtering can be adopted. The filtering device is also divided into a passive filter device or an active filter device.
(four) passive filtering and active filtering
The equipment for eliminating harmonics requires wide spectrum, large capacity, fast response time, convenient operation, reliability, long life and so on.
1, passive filter
Passive filter is the earliest, because of its simple structure, less investment, higher operation reliability and low operating cost, it is still the main means of harmonic suppression. The LC filter is composed of a filter capacitor, a reactor and a resistor, which consists of three basic forms: Series filtering, parallel filtering and low pass filtering (mixing and mixing). The series filter is mainly used for the control of the three harmonic; the low pass filter is mainly applied to the control of the high order harmonics; parallel filtering is parallel to the harmonic source, besides the function of the reactive power compensation. The main disadvantage of this method is that the compensation characteristic is affected by the impedance and running state of the power grid, and the passive filter can not be controlled. It can only compensate the harmonic of the fixed frequency. If different harmonic frequencies are filtered, different filters should be used separately. With the passage of time, the resonance frequency will change with the aging of the parts or the changes in the load of the power grid. The filtering effect is reduced, and the parallel resonance is easy to occur with the system, which causes the harmonic amplification and even the LC filter is overloaded and even burned. At present, the passive filtering device is still a better solution to deal with the harmonics with the most practical application, better effect and lower price.
2, active power filter
The active power filter is a new type of power electronic device used to dynamically suppress harmonics and compensate reactive power. This filter can track and compensate the harmonics with varying frequency and amplitude, and the compensation characteristics are not affected by the impedance of the power grid.
According to the principle of dynamic compensation harmonic, active filter uses power electronics technology and computer technology to quickly track harmonic frequency, phase and amplitude produced by harmonic source, and inject the electric current with the same amplitude, the same phase and opposite direction from the device to the power grid, so as to make the total harmonics into the power supply.