How electrode regulation in arc furnace occurs?

Answer 1

Electronic Controller

The electronic controller is the "heart" of the electrode regulation system. It acquires the physical signals, compares them with the setpoints by computing an error function and then generates the output references for the regulating valves.

Electrode Basic Movement Rules

Being in relation with electrode currents and voltages only, the electrode regulation system follows some basic rules to regulate the movement of the arc furnace electrodes. These rules include:

• U" = 0, I" = 0: The furnace breaker is open, and the electrode is kept in its position.
• U" > 0, I" = 0: The furnace breaker is closed without arc, and the electrode is lowered at maximum speed.
• U" > 0, I" > 0: The furnace breaker is closed with arc, and the regulator is in regulation mode.
• U" = 0, I" > 0: This is the short-circuit condition, and the electrode is forced to rise at the maximum speed to reach one of the previous situations.

Control Action

Enlarge this pictureFig. 2. Generic electrode circuit

There are different possible approaches to implement the regulation control algorithm. The most used and reliable method is a classic approach using PID controllers, which is represented by the following formula:

Out(t)=Kp•e(t)+Ki•òe(t)+Kd de(t)/dt

When analyzing the regulation process, experience shows that the use of the proportional component only provides simple control of electrode position during "flat bath" operation (i.e. refining/heating phases or continuous charge). The addition of the integrative component in the control algorithm gives more opportunities to adjust the system, adding custom parameters to face all the possible situations occurring during the melting process. The derivative component is normally omitted because it introduces arc instability, increasing the complexity of the controller without providing any measureable benefit to the melting or heating process.

The simplified control algorithm commonly used is:

Out(t)=Kp•e(t)+Ki•òe(t)

e(t) is the error function computed as the difference between process and measured values of a controlled variable, which is plant-specific. Mechanical characteristics, electric line design and signal acquisition quality affect the variable selection. A common and widely applied approach is the selection of the impedance as the controlled variable. This control mode is known as control at constant impedance, which means the controller acts to minimize the error function defined as e(t)=Z"-Z"ref, where Z''=U"∕√3 I" is the measured impedance and Z''ref=U"ref∕√3 I"ref is the reference impedance. The output function, out(t), is the control function that has to be applied to the regulating valves.