Electrode

1. 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).
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)
Enlarged Image
Fig. 1. Macro control loop

2. 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.
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
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•(t)+Ki•(t)+Kd d(t)/dt
When analyzing the regulation process, experience shows that the use of the
Enlarged Image
Fig. 2. Generic electrode
circuit

3. 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•(t)+Ki•(t)
(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 (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.
Built-in Protections
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

4. condition I” > Imax identifies the over-current condition, and the controller reacts by
forcing the electrode to rise at high speed.
Auxiliary Functions
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)
Regulating Valves
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.
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5. 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
Fig. 3. Regulator pane

6. 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