The main physical principle of an electric arc furnace is the transfer of energy from the electricity line to the furnace through heat radiation and conduction generated by an electric arc.
The control of the arc furnace energy-transfer process is achieved by regulating the electric arc power and arc length that is acting on the position of the arc furnace electrodes and ensuring the application of a scheduled power profile. A reliable, scheduled power profile is required for an efficient melting or heating operation, which makes the electrode regulation system an important component of any electric arc furnace (EAF), ladle furnace (LF) or submerged-arc furnace (SAF).
|Fig. 1. Macro control loop|
Macro Control Loop
Components of an electrode regulation system are as follows:
- Mechanical devices – to support the electrodes (i.e. electrode mast and arms) and to electrically connect them to the power transformer (flexible cables)
- Electrical instruments – to acquire the actual electrode voltage and current values (i.e. CTs and PTs)
- Electronics and control – to compare the measurements with the required working point, generating an electrical reference for the regulating valves (electronic controller)
- Hydraulics – the electrode’s mechanical supporting structure is moved using hydraulic cylinders (i.e. hydraulic circuit and regulating valves)
Electrical Signal AcquisitionThe electrode regulation system’s performance is strongly related to the quality of the signals acquired by the electronic controller. A high precision and reliability level is needed in order to allow a quick and consistent response of the regulator.
The acquisition of currents and voltages is performed on the secondary side of the power transformer through CTs (current transformers) and PTs (potential transformers). Their roles are to allow the controller to acquire the measurements, generating a proportional reduced signal.
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.
|Fig. 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:
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:
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.
Regulators, in addition to the control action, have built-in protections to prevent electrode breakage and over-currents due to short circuits. The presence of nonconducting materials in the furnace scrap charge is common in every EAF plant. If an electrode contacts a nonconductive material, the controller will react by moving the electrode down toward the scrap charge. This creates a potential electrode breakage risk.
In order to avoid this situation, the electrode regulation system controller receives as input a pressure signal for each electrode coming from a transmitter placed on the electrode hydraulic circuit. The condition P” < Palarm and I” < Ialarm identifies when the electrode has come into contact with a nonconductive material and activates the protection procedure, forcing the electrode rising movement.
Another situation that an electrode regulation system controller needs to effectively identify is the over-current. An over-current is caused by a short circuit (scrap drop). In this circumstance the current increases quickly over acceptable limits, resulting in unnecessary stress on the electrodes, transformer and medium-voltage line. The condition I” > Imax identifies the over-current condition, and the controller reacts by forcing the electrode to rise at high speed.
In addition to the standard control action typical of each regulator, the system is equipped with some auxiliary functions both for process control and analysis purposes. Some examples are:
- Management of the transformer tap changer position
- Electrode position control (high and low position limit switches)
- Acceleration control (preventing breakages)
The regulating valves are the only hydraulic circuit components interacting with the electrode regulation system. They convert the electric reference signal coming from the controller into a mechanical movement.
The device selection must follow the characteristics of the mechanical structures that have to be moved in order to ensure the speed and acceleration required by the system. In particular, the maximum electrode speed has to be achieved at 80% of the electric reference in order to ensure the full functionality of the valve.
|Fig. 3. Regulator pane|
Tenova Digital Regulation (TDR) Technology
With more than 100 working installations around the world, TDR is Tenova’s solution for EAF and LMF electrode regulation systems. Continuous improvement of the product has resulted in the achievement of operating excellence in terms of performance, reliability and operational simplicity.
The high quality of the signals that feed the controller is a key factor for the regulator performances. In order to achieve the best results, TDR acquires the electrode’s currents and voltages directly from the CTs and PTs through fast-sampling equipment. This approach allows the system to save the typical delays associated with conventional AC/DC converters.
The electronic controller is designed to operate in real time with multi-task capabilities. These features allow a 1-ms cycle time keeping separate the regulation and computation operations.
The computation unit acquires the fast-sampled data and calculates the true RMS voltage and current values together with all the derived variables such as impedance, active and reactive power, and power factor.
The regulation unit hosts the control algorithm, which is fed by the calculated true RMS values and the selected working point. The algorithm computes the error functions, which are processed by two PID controllers in cascade (one fast controller fed by a slower PID). The result function is ready to be conditioned as the reference signal for the regulating HRR servovalves.
The selection of the controlled variable follows the constant impedance approach, which is complemented by the assignment of different weights to the current or voltage components. The active power value is a valid alternative for the control variable instead of the impedance. This is useful in cases where there is a power restriction or limitation in the network.
Another control variable alternative is the constant electrode current, which is used in cases where electrode consumption minimization is the main goal. The system is complemented with electrode breakage and short-circuit over-current protection routines along with a number of auxiliary functions, including fast stabilization of the arcing current, management of the tap-changer position with automatic current reduction on tap movements and foamy-slag control
The integrated calculation and analysis of the current harmonic content is included in the regulator algorithm. The system is designed to be able to independently compute the current harmonic content. The Total Harmonic Distortion (THD) calculation and a complete suite of analysis tools help to improve furnace operations. Harmonics up to the 15th are relevant to the arc regulation for EAF-type vessels.
TDR-H (Harmonic Control)
The High Response Regulation (HRR) servo-valves are the result of Tenova’s extensive experience with EAF hydraulics. They are designed and manufactured in-house by Tenova according to the unique plant characteristics in order to ensure the speed and acceleration requirements. The specific design allows the HRR to function with either emulsified water (HFA) or glycol-water (HFC).
The main characteristics are:
- Greater stability during “flat-bath” operation (signal lower than 50% of the total) in order to obtain better electrical power transfer
- Greater speed in case of an unstable arc (signal higher than 50% of the total) by increasing the dynamic response of the system during scrap melting
- Integrated “fast-raise” function with no need of external valve for an optimal dynamic response in combination with the harmonic control
- “Fail-safe” design to obtain the automatic rise of the electrode in case of electric or oleo-dynamic failure
For more information: Contact Kyle J. Shoop, Ph.D., manager, technology and sales; Tenova Core, 100 Corporate Center Drive, Coraopolis, PA 15108; tel: 412-262-2240; fax: 412-262-6090; e-mail: firstname.lastname@example.org. Other authors are Luca Origoni, automation manager, Tenova Core and Francesco Memoli, vice president, Tenova Core Metal Making
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: arc furnace, radiation, conduction, foamy slag, scrap melting, arc length