The basic function of modern steelmaking electric arc furnaces is to input as much electrical power as possible into the molten pool to achieve high productivity, low material and energy consumption, and good environmental indicators. Electric arc furnaces for steelmaking are classified into three types based on their average transformer rated capacity per ton of steel or per unit furnace area: ordinary power (RP), high power (HP), and ultra-high power (UHP). The concept of ultra-high power electric arc furnace was proposed in the 1970s, with the goal of greatly improving the productivity and reducing costs of electric arc furnace steelmaking, ushering in a new era of development in electric arc furnace steelmaking technology. However, due to the multifaceted impact and interference on the power grid during production, all aspects related to power quality have also been found in practice. Due to the high power level and capacity of transformers in ultra-high power electric arc furnaces, which can reach tens of megavolt amperes, they cause serious impact and interference to the power grid during the steelmaking process. These "public hazards" must be controlled and treated.

1. Interference with the power grid

1.1 Low power factor

Electric arc furnaces obtain electrical energy from the power grid, with a portion converted into useful thermal energy and another portion being reactive energy. In order to ensure stable combustion of the arc, the power factor of the arc furnace cannot be too high. Due to the high inductance of the electric arc furnace load, the connection of the electric arc furnace deteriorates the power factor of the power supply grid. During the melting period of ultra-high power electric arc furnaces, the power factor can even be as low as 0.1, causing a severe decrease in bus voltage. The decrease in voltage correspondingly reduces the active power of the electric arc furnace, prolonging the melting period and decreasing productivity.

1.2 Voltage flicker and fluctuation

Ultra high power electric arc furnaces are a significant load on the power grid and often generate sudden and strong voltage surges during operation, resulting in rapid fluctuations in grid voltage with frequencies ranging from 0.1 to 30Hz. Voltage fluctuations with frequencies between 1 and 10Hz can cause flickering in incandescent lamps and television screens, causing people to feel irritated. This type of interference is called "flickering" or "flicker". Intense flickering can cause unstable motor rotation, electronic device misoperation or even damage, and can also reduce the actual power of users (including arc furnaces themselves) supplied by the power grid. Flickering is a public hazard to the power grid.

For actual, limited capacity power grids, the percentage of voltage fluctuations caused by electric arc furnace loads is

ΔU%=ΔQ/Sk×100% (1)

In the formula, Δ Q represents the impact of reactive power, Mvar;

Sk - Small short-circuit capacity of the power supply busbar for this electric arc furnace, MVA.

Considering that the reactive power variation between the normal operating state and short-circuit state of the electric arc furnace is roughly equivalent to the rated capacity of the furnace transformer, i.e. Δ Qmax ≈ SF, the following equation can be used to estimate the percentage of large fluctuations on the power grid

ΔUmax%=SF/Sk×100% (2)

The national standard "Electricity Quality - Allowable Voltage Fluctuations and Flicker" (GB12326-2000) specifies the allowable values of voltage fluctuations and flicker at the common connection points of the power system. The fluctuations and flicker generated during the operation of ultra-high power electric arc furnaces often exceed this specified value and must be suppressed.

1.3 Asymmetric three-phase voltage and current

Another important issue causing the deterioration of power supply quality in the power grid is the asymmetry of three-phase voltage and current caused by the asymmetry of three-phase loads in electric arc furnaces. The consequence is a decrease in the economy and productivity of all users of the relevant power grid, including electric arc furnaces. Analysis shows that when the short-circuit capacity Sk of the power grid at the public connection is more than 50 times higher than the rated capacity SF of the arc furnace transformer, the asymmetry of the power grid voltage caused by the three-phase load asymmetry of the arc furnace does not exceed the allowable value specified in the national standard "Electric Energy Quality - Allowable Unbalance of Three phase Voltage" (GB/T15543-1995).

Due to the current large-scale and higher power generation of electric arc furnaces, the capacity of the power grid is often only 20-40 times or lower than that of the arc furnace transformer, so compensation must be adopted.

1.4 High order harmonics

During the steelmaking process, AC electric arc furnaces generate non sinusoidal distortion and various harmonics in their current, causing interference to the power grid. The main reasons are:

(1) The resistance value of the arc is not constant, and the arc resistance also varies during the half cycle of the AC arc, which causes non sinusoidal distortion of the arc current.

(2) The positive and negative half cycles of alternating current are reversed, with graphite electrodes and steel alternating as cathodes and anodes. Due to the varying electron emission capabilities of different materials, the waveforms of the positive and negative half cycles of the current are asymmetric, resulting in even harmonics.

(3) The three-phase arc is unbalanced, resulting in third harmonic.

(4) The various harmonic sources connected to the power supply system lead to the formation of various harmonics, such as rectifiers in static compensation devices.

The harmonic current components of electric arc furnaces are mainly 2-7 times, with the 2nd and 3rd times being larger, with an average value of 5% -10% of the fundamental component. Harmonic currents flowing into the power grid cause voltage waveform distortion, leading to electrical equipment heating, vibration, and protection misoperation. The national standard "Electric Energy Quality - Harmonics in Public Power Grids" (GB/T14549-93) provides specific regulations on the comprehensive voltage distortion rate and harmonic current injection amount, providing a basis and standard for suppressing harmonics generated by electric arc furnaces.

Ways to suppress the impact of electric arc furnaces on the power grid and themselves

There are generally two ways to suppress interference from ultra-high power electric arc furnaces: firstly, to increase the voltage level of the power supply to improve the short-circuit capacity of the common connection point with the power grid, so that its impact on the power grid and itself is within the allowable range; The second is to use SVC devices to limit multiple indicators within the allowable range. Compared with the two approaches, the first approach is a temporary solution, because the various values of the impact of electric furnaces on the power grid and themselves have not been eliminated, but are sent to higher voltage level power grids for diffusion. With the continuous construction and development of electric furnaces, these values accumulate in the power grid and become rampant, which will form an unacceptable level for the power grid and increase the impact on the majority of users. Therefore, the scope of use is becoming smaller and smaller. The second approach is a fundamental solution, which eliminates most of the various values of the impact of electric furnaces on the power grid and themselves on site. Therefore, their use is becoming increasingly widespread and their prospects are broad.

2.1SVC device

The SVC device developed in recent years is a device that quickly adjusts reactive power and has been successfully used in compensating for impact loads such as electricity, metallurgy, mining, and electrified railways. It can randomly adjust the required reactive power to maintain a constant system level at the connection point of impact loads such as electric arc furnaces

Qi=QD QL-QC (3)

In equation (3), Qi, QD, QL, and QC represent the reactive power at the system's common connection point, the reactive power required by the load, the reactive power absorbed by the adjustable (controllable) reactor, and the reactive power generated by the capacitor compensation device, all in kvar.

When the load generates impact reactive power Δ QD, it will cause

ΔQi=ΔQD ΔQL-ΔQC (4)

In the equation, Δ QC=0. To keep Qi constant, i.e. Δ Qi=0, then Δ QD=- Δ QL, i.e. the inductive reactive power in the SVC device is randomly adjusted with the reactive power of the impulse load, and the voltage level can remain constant.

SVC is composed of controllable branches and fixed (or variable) capacitor branches connected in parallel, and there are mainly four types.

(1) The thyristor valve controlled air core reactor type (TCR type) uses a thyristor valve to control a linear reactor to achieve fast and continuous reactive power regulation. It has the advantages of fast response time (5-20ms), reliable operation, stepless compensation, phase separation regulation, ability to balance active power, wide applicability, and low price. TCR devices can also achieve phase separation control and have good ability to suppress asymmetric loads, so they are widely used in electric arc furnace systems. However, this device adopts advanced electronic and optical fiber technology, and maintenance personnel need to be specially trained to improve their maintenance level.

(2) The advantages of thyristor controlled high impedance transformer type (TCT type) are similar to TCR type, but the manufacturing of high impedance transformers is complex and the harmonic components are slightly larger. Due to the presence of oil, it is required to set fire to the first level and should only be installed on the first floor or outdoors. When the capacity is above 30Mvar, the price is relatively expensive and cannot be widely adopted.

(3) The thyristor switch controlled capacitor type (TSC type) has phase separation regulation and direct compensation. The device itself does not generate harmonics and has low losses. However, it has stage regulation and a relatively high overall price.

(4) The self saturating reactor type (SSR type) has simple maintenance, reliable operation, strong overload capacity, fast response speed, and good flicker reduction effect. However, it has high noise, high raw material consumption, and compensates for the large harmonic current generated by asymmetric electric furnace loads, without the ability to balance active loads.

2.2 Passive filtering device

The device is composed of passive components such as capacitors, reactors, and sometimes resistors to form a low impedance path for a certain harmonic or higher harmonics, in order to suppress high-order harmonics. Due to the expansion of the regulation range of SVC from the inductive region to the capacitive region, the filter is connected in parallel with the dynamically controlled reactor, which not only satisfies reactive power compensation and improves power factor, but also eliminates the influence of high-order harmonics.

The types of filters used internationally for large-scale steelmaking electric arc furnaces include single tuned filters of various orders, double tuned filters, second-order wideband and third-order wideband high pass filters, etc.

(1) The advantages of a first-order single tuned filter are good filtering effect and simple structure. The disadvantage is that the power loss is relatively large, but it decreases with the improvement of quality factor and increases with the decrease of harmonic order. The electric furnace happens to be a low order harmonic, mainly 2-7th order, so the fundamental loss is relatively large. When the quality factor of the second-order single tuned filter is below 50, the fundamental loss can be reduced by 20% to 50%, which is energy-saving and has equivalent filtering effect. A third-order single tuned filter is a filter with low loss, but its composition is more complex and the investment is higher. It is better to use a third-order filter for secondary filters and a second-order single tuned filter for other filters in electric arc furnace systems.

(2) High pass (wideband) filters are generally used for harmonic suppression of one or more harmonics. When used in an electric arc furnace system, the filter circuit can form a low impedance path for harmonics of the 5th order and above by adjusting the parameters.

There are two basic types of filter combinations used for large electric furnaces: one is composed of 3-5 sets of single tuned filters, and the other is composed of 2-4 sets of single tuned filters and one set of second-order wideband filters. The first type of combination has poorer filtering effect on high-order harmonics, but lower power loss; The second type of combination has better filtering effect for high order filtering, clear division of labor, and simpler and easier design. The combination of the two has its own advantages and disadvantages, and the overall development trend is to reduce the number of groups while achieving good filtering effects, in order to save land and investment, while optimizing the combination as much as possible to save energy loss.

3 Active Filters

Although passive filters have the advantages of low investment, high efficiency, simple structure, and easy maintenance, and are widely used in distribution networks at present, due to the significant influence of system parameters on filtering characteristics, they can only eliminate specific harmonics, and may have amplification effects or even resonance phenomena on certain harmonics. With the development of power electronics technology, people have gradually shifted their research direction towards active filters (APF).

APF uses controllable power semiconductor devices to inject current into the power grid that is equal in amplitude and opposite in phase to the harmonic source current, so that the total harmonic current of the power supply is zero, achieving the goal of real-time compensation of harmonic current. Compared with passive filters, it has the following characteristics:

a. Not only can it compensate for various harmonics, but it can also suppress flicker, compensate for reactive power, and has the characteristic of multi energy in one machine, which is relatively reasonable in terms of cost-effectiveness;

b. The filtering characteristics are not affected by system impedance and can eliminate the risk of resonance with system impedance;

c. It has adaptive function and can automatically track and compensate for changing harmonics, with high controllability and fast response characteristics.