With the development of modern science and technology, on the one hand, the factors causing power quality problems are increasing, such as the use of non-linear loads represented by power electronic devices, the start and stop of various large-scale electrical equipment, etc.; A variety of sophisticated, sophisticated, power-sensitive electrical equipment is becoming more popular, and people are increasingly demanding power quality and reliability.
The contradictions of the above problems are becoming more and more prominent. This makes the direct damage to the power grid and power distribution system caused by the power quality problem and the losses that may be caused to human life and production. The power quality is directly related to the overall benefit of the national economy. . The increasing requirements for power quality and reliability are linked to the development of the user's process level. Modern technological advancement has promoted the automation and intelligence of the production process, and has put forward higher requirements for power quality. Losing power for 2s in a computing center can damage tens of hours of data processing results and cause millions of dollars in economic losses. In large machine manufacturers, a voltage dip of 0.1 s can cause abnormal production conditions and quality damage. Today's automated equipment-controlled continuous finishing lines are extremely sensitive to interference in power distribution systems. Unusual power supply for a fraction of a second can cause confusion inside the plant, and its losses are difficult to estimate.
These users' tolerance for unqualified power can be strictly limited to only 1~2 cycles.
The same is true of modern business centers, banks, and hospitals. The serious harms and losses caused by harmonics are often mentioned, and the sudden crash of computer systems in unattended substations is mostly a power quality problem. In China, although the overall economic and technological level is still relatively backward, the impact of power quality problems in some economically developed regions has become more prominent. Moreover, due to various reasons, China's situation is particularly backward in terms of reliability of power supply and stable level of grid voltage amplitude. How to improve and ensure the quality of power has become one of the important issues that need to be solved urgently in the field of electrical engineering at home and abroad. The systematic comprehensive compensation technology proposed in the recent period is the "root cause" way to solve the power quality problem. There are many mature measures to solve the voltage quality problem in steady state; but for the dynamic power quality problem, relying on traditional reactive power compensation and conventional filtering devices can not be effectively solved because of such things as voltage sags, surges. Power quality problems such as surges, impulses, and transient power outages are short and variable, and some power quality problems are accompanied by some or all of the loss of power.
As an extension of the facts (flexible AC transmission system based on power electronics technology) and distribution system applications - dfacts technology has become a powerful tool to improve power quality, the core device of this technology, igbt, which has a faster switching frequency than gto. And the shutdown capacity has reached a certain size, so the dfacts device has a faster response characteristics. At present, dfacts devices mainly include: dynamic voltage restorer (dvr), static synchronous compensator (d-statcom) for power distribution system, and solid state switch (ssts). Based on the svc device, statcom overcomes the constant impedance characteristics, so that when the voltage is low, it can not provide the required reactive power support, and the ability to cope with emergencies is weak; and the floor space is large, too much svc The disadvantage of causing system oscillation, statcom's reactive current output can be constant over a wide range of voltage variations, still provide strong reactive support at low voltages, and can be continuously adjusted from inductive to capacitive. Its reactive output is equivalent to 1.4~2 times of the same capacity svc; due to the flexible voltage regulation of statcom, the number of switching of transformer taps can be greatly reduced, thereby reducing the number of tap failures. In addition, statcom can also suppress voltage flicker and improve the system transient stability level. Combined with China's national conditions and existing technologies, the development of statcom should be an effective means to solve the voltage stability problem in China, and it is also the main direction of dfacts technology development. At present, there are low power factor and harmonic pollution problems in urban and rural distribution networks.
The flow of a large amount of reactive current in the power grid will lead to an increase in line loss, a decrease in transformer utilization, and a serious drop in user voltage. Harmonic pollution can cause environmental degradation in the use of electrical equipment, as well as damage to surrounding communications systems and equipment outside the utility grid. The hazards of harmonics to the utility grid and other systems are roughly due to the following aspects:
1. Harmonics cause additional harmonic losses in components in the utility grid, reducing the efficiency of power generation, transmission, and electrical equipment. A large number of third harmonics can cause overheating or even fire when flowing through the neutral line.
2. Affect the normal operation of various electrical equipment. In addition to causing additional losses, the effects of harmonics on the motor can also cause mechanical vibration, noise, and overvoltage, causing severe overheating of the transformer. Harmonics cause overheating of capacitors, cables, etc., insulation aging, and shortened life, resulting in damage.
3. Harmonics can cause local parallel resonance and series resonance in the public power grid, which will amplify the harmonics, greatly increase the harmonic hazard and even cause serious accidents.
4. Harmonics can cause relay protection and malfunction of the automatic device, and make the measurement of the electrical measuring instrument inaccurate.
5. Harmonics cause interference to adjacent communication systems, lighter noises, and lower communication quality; heavy ones cause information loss, making the communication system unable to work properly.
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