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Voltage Flicker of Power Supply I Arc Furnace and Its Suppression Countermeasures Weng Limin Tian Zhiping (Power Room of Wuhan Iron and Steel Design and Research Institute, Wuhan 430080) (Design Institute of Jiuquan Iron and Steel Company, Jiayuguan 735100, Gansu) changed "departure, analyze its conceptual characteristics, and Comparing the countermeasures against suppression, it has a complete understanding of voltage fluctuations and flicker in power quality and its countermeasures, which is helpful for application in practical engineering.
1Electric characteristics of electric arc furnace and the concept of voltage flicker EAF is one of the main means of modern steelmaking, it is a large load of the power supply grid, and often produces sudden, strong current shock during operation, especially in melting In the initial stage, the range of current fluctuation reaches 0/300 of the rated current of the electric furnace transformer. After analysis, the electrical characteristics are: the three-phase current is intensely changed, so the electric arc furnace is the source of voltage fluctuation and flicker; the three-phase current waveform is severely distorted. Therefore, the electric arc furnace is a source of higher harmonic generation; the three-phase current is seriously unbalanced, so the electric arc furnace is a source of three-phase voltage imbalance.
Voltage flicker is the most important interference and adverse effect of the electric arc furnace on the power supply system. Because of the strong current impact, the grid voltage fluctuates rapidly, the frequency is 0. 1~30Hz. Especially the voltage fluctuation between 1~10Hz frequency will cause the illumination of the incandescent lamp and the TV picture to flicker, making people feel annoyed. This type of interference is called "Fluctuation" or "Flicker". Strong flicker can cause unstable motor rotation, electronic devices malfunction or even damage, and the actual power of the power supply users (including the electric arc furnace itself) will be reduced. Flicker is a public hazard to the power grid. In recent years, with the popularization of precision instruments, computers, electronic biomedical devices and household appliances, the meaning of the word flicker has been broadened, including the adverse effects of changes in power supply voltage on these sensitive devices.
2 Voltage Flicker Estimation Voltage flicker is a special reflection of voltage fluctuation. The severity of flicker must be related to the voltage variation caused by load change. The voltage fluctuation follows the following rules: R, J from the power supply to the evaluation bus segment Equivalent resistance and reactance of the power supply system, "; Hn evaluates the rated voltage of the bus, kV. A statistical value, that is, a short-term flicker value.
3 Main factors affecting voltage flicker The electric arc furnace smelting process is divided into two stages according to its electrical characteristics: melting period and refining period.
Melting period. After the scrap in the furnace is loaded and the three-phase electrode is inserted at the same time, a power-frequency high-current arc is generated between the electrode and the scrap, and the scrap heat is used to start melting the scrap. Due to the existence of a direct arc between the scrap and the electrode, the melting of the scrap inevitably causes a change in the length of the arc, which in turn causes the arcing point to move, and the arc is extremely unstable.
The change in the arc current causes a voltage flicker, which is caused by the rapid change of the arc due to the movement of the arcing point. The sharp change due to the short circuit of the electrode is called very flicker. In smelting, the degree of change in arc current is related to the size of the scrap. If the molten material is light and small, the arc is relatively stable, and the flicker problem is relatively light. If the molten material is large scrap, in addition to causing arc instability, the electrode is short-circuited when falling, and the flicker problem is relatively serious.
After the middle of the melting period, there is a considerable amount of molten steel at the bottom of the furnace, the arc is relatively stable, and the degree of flicker is greatly reduced.
Refining period. After the melting is completed, iron ore and oxygen are added while raising the temperature for oxidative refining. After that, the steel slag is subjected to reductive refining, and lime is added to perform desulfurization and deoxidation. During this period, the arc is very stable, the current does not change, and the flicker problem basically disappears. The factors affecting the arc voltage flicker are: the magnitude of the impedance in the power system or the size of the system short-circuit capacity: the greater the system short-circuit capacity, the lighter the voltage fluctuation and flicker generated by the arc furnace on the system bus; the arc furnace loop impedance : The larger the impedance, the smaller the arc current change, the lighter the flicker; the fluctuation of the active and reactive power of the electric arc furnace and the frequency of its change: at the same frequency, the greater the amount of power change, the more severe the flicker; Set reactive power compensation devices such as shunt capacitors.
There are also non-electrical factors, such as furnace capacity and melting time; whether there is scrap preheating device; whether oxygen is blown; the material, shape and loading sequence of scrap.
4 suppression of voltage flicker Through analysis, external suppression measures must be taken in the case of severe disturbances. However, in many cases, the level of flicker is not very high, and can be effectively controlled by improving the operating conditions of the electric arc furnace, such as: Take measures such as preheating, oxygen blowing, and addition of combustion-supporting substances.
Table 1 Common countermeasures for voltage flicker of electric arc furnace Comparison method Suppression countermeasures Suppression principle Effect Maintenance cost is separately supplied from Shenyuan special unit, dedicated transmission line power supply and conventional load, but there is still flashover on the user side of the electric furnace. The short-circuit capacity of the power supply system of the first-level voltage supply electric furnace is increased. The high-easy high-series capacitors are connected to the power supply side of the series. The increase of the reactance of the high-series series reactors suppresses the reactive power fluctuation, but the active power of the electric furnace is reduced, and the operating efficiency is generally low. Starting from the user side of the electric furnace, the saturated reactor parallel filter (SSR type) uses the saturation characteristics of the saturable reactor to automatically compensate the reactive power. The high thyristor control reactor (TCR type) automatically compensates for reactive power by thyristor phase control, but there is a high The subharmonic is larger and easier to use as the flicker suppression device. According to the inherent load characteristics of the electric arc furnace, the following requirements should be met; the load can be quickly changed in response to the half cycle and below; the three-phase unbalanced load can be dealt with; Magnetic interference effects of varying amplitudes; large capacity manufacturing.
5 Conclusion! Go to page 4 3 Installation The comb cable firewall can be installed before the cable is laid (pre-installation), or after the cable is laid or put into operation (post-installation). Now take the post-installation as an example: firstly, according to (a), (7), the comb-shaped firewall and fixed angle steel are made. After the cable is laid, the cables are arranged in order on the cable bracket according to the design requirements, using a hammer drill or The electric hammer is drilled and drilled in the trench walls on both sides of the cable trench in the cable trench. Then, the firewall is placed in the trench along the axis of the cable trench. At this time, the firewall comb teeth correspond to the gap between the cable layers, and the comb will be combed. The shape of the firewall is rotated 90. Put it in place. At this time, all layers of cables enter the corresponding firewall perforations. 8~12 M14X105mm expansion bolts and 4 fixed angles are used to clamp the comb-shaped firewall into the cable trench. . At this time, separate the cables of each layer according to the specified spacing, fill the space around the cable with the cable firewall material, seal the cover, and install the entire firewall.
For the cable channel in which the comb cable firewall is installed, when the cable needs to be added or replaced in the future, the capacity expansion scheme can be determined according to the number and difficulty of cable laying: when the cable is small or the number is small, the corresponding cable can be added. The position is out of the fire blocking material, and the gap can be blocked after laying the cable as usual; if the number of cables is large or difficult to lay, the comb-shaped firewall can be removed according to the opposite installation procedure, and the cable can be reinstalled after the cable is laid. Because the fire blocking material is not dry, it can be reused without additional investment.
4 Note The thickness of the comb cable firewall and the thickness of the material should be determined according to the section size of the cable trench and the tunnel (tunnel) and the number of cables inside. The clamped steel and fixing bolts should not be selected too small to ensure the cable is generated. There is enough fire separation effect in case of fire.
The shape of the comb cable firewall can be flexibly changed according to the actual situation, including the installation of the patrol passage door on the wall to meet the maintenance needs. When the change is made, the comb hole is not closed, and the cable can be incorporated later.
The fixing bolts of the firewall can also be pre-buried, and the position is selected near the comb hole, so as to prevent the wall from rotating when the firewall is large, which makes installation difficult.
When the comb-shaped firewall leaves a large capacity, in order to reduce the cost, the cable-free comb hole can be filled and sealed with ordinary refractory bricks, and the gaps existing around the firewall should be tightly sealed.
5 Conclusions Through the implementation of the new comb cable firewall, we have realized the following advantages: the material can be welded by ordinary steel plate, or can be formed by NGR high-efficiency flame-retardant FRP material, which can be produced according to the actual situation on site or More standardized cable trenches, firewalls in cable tunnels are sized in batches.
Generally, electrical and gas welding operations are not required during installation, and the construction is simple and easy. In addition, it can be used for new and expanded projects as well as retrofits for operating systems.
Effectively limit the range of cable fires, have reliable anti-fire and fire-proof performance, and have a small one-time investment, which is easy for general factories to accept.
Received period: 001-09 first Guo Zhongtian (1958-), male, Jilin Tonghua, graduated from industrial electrification, senior engineer, now mainly engaged in enterprise electrical management.
(Continued from page!) Thyristor Control Reactor Type (TCR) SVC (StaticVarCompensation) device is the most widely used type of interference such as shock-type load suppression voltage flicker in steelmaking electric arc furnaces at home and abroad. Thyristor control linear reactor realizes fast and continuous reactive power regulation. The fixed reactive power compensation and filtering device not only expands the SVC adjustment range to the capacitive region, but also eliminates the high harmonics; TCR device It can also realize phase separation control and has better ability to suppress asymmetric load. Many examples of localization also prove that the use of TCR type SVC device is an effective measure to suppress arc furnace voltage fluctuation and voltage flicker.
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