Problem Analysis and Technical Improvement of DC Arc Furnace Electrode Lifting Control System

1 Overview

During the smelting process of a DC steelmaking arc furnace, the initial melting, smelting, oxidation and reduction stages have different requirements for the power supply system (i.e., the size of voltage and current) in order to save energy and reduce consumption, improve work efficiency and achieve the best technical and economic indicators.

Different from the control method of AC steelmaking arc furnace, the arc voltage (arc voltage) and arc current (arc current) of DC arc furnace are controlled by two completely independent adjustment systems. The arc voltage and arc current can be automatically adjusted linearly and independently to meet the power supply requirements of the smelting process.

The arc current regulation system uses the thyristor rectifier as the regulation object. The arc current automatic closed-loop current regulator controls the DC output current of the thyristor rectifier. Under static conditions, the automatic current stabilization system can keep the arc current constant. Within the effective control range, changes in arc voltage will not cause changes in arc current, which creates conditions for independent adjustment of arc length and arc voltage.

During the smelting process of the DC arc furnace, the electrode lifting control system takes the electrode positioning mechanism as the control object, and its main function is to adjust the length of the arc column (i.e., arc length). According to the analysis of relevant literature, there is approximately the following relationship between the arc voltage Va and the arc length la:

Va=kala

In the formula, ka=0.6～1.1V/mm is called arc voltage gradient, and the change of arc voltage gradient depends on the atmosphere in the furnace. Compared with the change speed of arc current and arc voltage, the change of ka value is considered to be very slow and can be approximately regarded as a constant. It can be seen that the arc voltage and arc length are approximately linearly related. Adjusting the length of the arc is equivalent to adjusting the arc voltage. When the arc current is constant, adjusting the arc length is also equivalent to adjusting the size of the arc power. Because the arc power Pa=IaVa. Therefore, the performance of the electrode lifting control system directly affects the input of electric power and the operating state of the electric furnace.

2 Analysis of the main problems and causes

Two DC steelmaking electric arc furnaces with a nominal capacity of 5 tons at the Special Steel Plant of Gansu Huazang Metallurgical Group began trial operation in 1997. After commissioning, it was found that the electrode lifting control system was not working properly. The main problems were as follows:

(1) Unstable operation, frequent and long-lasting electrode movement, and the electric furnace cannot continuously and stably obtain effective arc power input;

(2) The arc voltage is low. In the fifth gear with the highest AC voltage, the arc voltage (DC) value can only reach about 250V, which seriously limits the increase of arc power. The power factor on the grid side of the transformer is also relatively low.

(3) The arc voltage adjustment range is small, and the actual arc voltage value does not change linearly with the given arc voltage value. The control performance cannot meet the requirements of the smelting process, and the arc voltage cannot be accurately stabilized at the given value;

Figure 1 Basic principle diagram of the original regulator

Figure 2 Basic principle diagram of arc voltage and speed double closed loop regulator

(4) The circuit failure rate is high, and power outages are frequent for maintenance, adjustment or replacement of components, which seriously affects production.

According to the problems that occurred during the trial operation, after careful on-site observation and careful analysis of the control circuit system, it is believed that the main reasons for the above problems are as follows:

(1) The control concept of AC arc furnace is still followed, and there is a problem of unclear concepts. The difference between the arc voltage setting and the arc voltage feedback signal is used as the setting of the speed regulator, and no difference amplification link is set. The original regulator circuit diagram is shown in Figure 1, which is the basic circuit principle diagram of the original electrode lifting regulator. As can be seen from the figure, there is no arc voltage regulation link in the circuit. The feedback arc voltage signal Uf is directly compared in series with the arc voltage setting signal Ug output by the given potentiometer after the proportional coefficient is converted by the proportional amplifier composed of 4N2. The difference signal generated by the comparison result is directly used as the setting of the speed closed-loop regulator. The comparison difference signal is not amplified, and the static difference between the two is too large, making it impossible to have a good follow-up performance between the feedback and the setting. As a result, the arc voltage adjustment range is small, the arc voltage cannot be accurately stabilized at the given value, and the arc voltage increase is limited. Even in the highest voltage range (AC250V), the arc voltage (DC) can only swing back and forth around 250V. At the same time, it also leads to a low power factor on the grid side of the transformer.

(2) There are many problems in circuit design. The control power supply on the control board uses a single-phase rectifier circuit, and there is no capacitor with sufficient capacity for filtering at the input end of the voltage regulator. As a result, the ripple component on the control power supply, especially the main power supply, is quite serious, which directly interferes with the normal operation of the circuit.

Neither the feedback signal of the input regulator nor the given signal circuit has the necessary filtering link to filter the signal. The transient interference signal superimposed on the signal can easily lead to unstable circuit operation.

There are too many connecting wires from the regulator to the operating table. These external leads will introduce electromagnetic interference signals from the environment into the control circuit while transmitting signals. As a long-distance weak signal transmission line, some necessary isolation, filtering and protection measures should be considered during design, but they were not given enough attention in the original circuit design. The speed regulator output signal does not have a limiting circuit, resulting in the upper and lower limits of the trigger phase shift range being unfixed and prone to drift, causing the voltage sent to the torque motor to be asymmetric.

The main circuit uses half-controlled thyristors and diode modules, which can reduce some costs and reduce some trigger units, but it is not easy to ensure that the voltage waveform output by the AC voltage regulation circuit is completely symmetrical. The impact of asymmetric voltage waveform on the running performance of the motor cannot be ignored.

The selection of component parameters in the control circuit is not reasonable, and the parameters do not match each other. Due to the mismatch of parameters, the performance of the circuit cannot be guaranteed, and even the working properties are changed, resulting in serious damage to the components.

(3) The quality problems of components and production are also prominent, and there are many hidden faults. The parameters of some components on the control board have been repeatedly adjusted and frequently replaced, resulting in many desoldering, cold soldering, hidden short circuits and poor contact of plug points on the circuit board, affecting the reliable and stable operation of the circuit board.

3. Technical improvement measures and operation results

In view of the above problems and reasons, the electrode lifting control system has been partially improved based on the original conditions. The specific improvement measures and operating results are as follows:

(1) Improvement of the regulator

The original system only has one speed regulator, which is a single closed-loop system. The arc voltage feedback signal only acts as a disturbance signal in the system. Considering that the ultimate control goal of the electrode lifting control system is to stabilize the arc voltage, the arc voltage should be closed-loop regulated and controlled. Set an automatic closed-loop arc voltage regulator as the outer loop and the speed regulator as the inner loop. The basic schematic diagram of the improved inner and outer loop regulators is shown in Figure 2. As can be seen from Figure 2, the improved regulator is a typical dual closed-loop regulator system consisting of an arc voltage regulator and a speed regulator. Change the arc voltage setting and arc voltage feedback signals from series comparison to parallel comparison, and connect the common points of the two groups of +15V and -15V power supplies that are not connected at the common points.

(2) Replace or adjust some components and their parameters

In order to adapt to the overall requirements of the control circuit, in addition to adjusting the circuit structure of the control circuit, some components in the circuit were replaced or their parameters were adjusted to meet the basic requirements of the system.

The arc voltage regulator sets component parameters according to the large inertia regulator, and its static amplification factor is set between 3 and 6 times to improve the tracking performance between the actual arc voltage and the given arc voltage, while reducing the static difference between the two.

The speed regulator is matched with the parameters of components according to the small inertia regulator to improve the sensitivity of the torque motor action. Its static magnification is controlled within the range of 3 to 5 times.

Under existing conditions, the circuit boards are repaired to minimize the potential failures caused by manufacturing quality issues.

(3) Operational results

After the above two improvements and repairs, the electrode lifting control system was put into use in October 1998. From the use situation, the basic purpose of the transformation has been fully achieved, which is mainly manifested in:

① The electrode movement phenomenon is significantly reduced. Except for some movement caused by individual thin and light charges during melting, there is generally no obvious movement phenomenon. The power supply can be maintained continuously and stably, which has a significant effect on accelerating the melting speed.

② The arc voltage (DC) of the highest voltage level has been significantly improved from about 250V to about 300V, and the arc voltage can be stabilized at a given value more accurately, so that the arc power can be continuously and stably increased from 3500kW to 4200kW, an increase of about 20%. The increase in arc power has a significant effect on accelerating the smelting speed and shortening the smelting time.

③ The arc voltage can follow the given arc voltage relatively well. Its effective adjustment range and linearity can meet the requirements of the smelting process, creating conditions for obtaining the best technical and economic indicators. The number of torque motor damages is significantly reduced.

Although the above three effects have been achieved, due to the inherent deficiencies in the design and manufacture of the system, the stability of the circuit operation is still not ideal and component damage still occurs from time to time.

4 Conclusion

The DC arc furnace electrode lifting control method is different from the AC arc furnace electrode control method. It should be designed as a double closed-loop system with speed regulation as the inner loop and arc voltage regulation as the outer loop. The selection of component parameters in the circuit is extremely important, and necessary protection measures are also indispensable. The quality of production directly affects the reliability and normal operation of the equipment. As a complete set supplier, you should choose a professional manufacturer to customize such key equipment.