Loredana Cristaldi1*, Daniele Clerici*, Marco Faifer* and Alessandro Ferrero*
1Corresponding author: Loredana Cristaldi
*DEIB
-Politecnico di Milano
Piazza L. Da Vinci, 32 – 20133 Milano - Italy e-mail: loredana.cristaldi@polimi.it
e-mail: daniele.clerici@polimi.it e-mail: marco.faifer@polimi.it e-mail: alessandro.ferrero@polimi.it
Abstract
The melting process in arc furnaces depends on the presence and propagation of the electric arc between the electrodes and the scrap charge. The heat produced by the arc current melts the scrap down to create the molten steel.
The characteristics of the electric arc depends mainly on two factors: the “quality” of the scrap charge and the “quality” of the electrical supply and components that contribute to the generation of the electric arc.
In this paper the correlation between melting process effectiveness and the deviations of the actual electrical quantities from the preset values for different scrap mixtures will be investigated. Furnace linings and electrodes degradation is also taken into account.
Key words: Condition Based Maintenance, Arc furnaces, Signature Analysis.
1 Introduction
Arc furnaces have seen a widespread diffusion in melting facilities in the last decades. Instead of Bessemer oxygen converters, for example, they favor a better flexibility of the process. Key factors are more accurate melting process control, less expensive melting charge material (steel scrap), small production batch capabilities, and lower energy requirements (The AISE Steel Foundation, 1998).
It is well known that melting process depends on the presence and propagation of the electric arc between electrodes and scrap charge. The heat produced by the arc current melts the scrap down to create the molten steel (Paschkis, 1945).
The characteristics of the electric arc depends mainly on two factors: the “quality” of the scrap charge and the “quality” of the electrical supply. The furnace process control is automatically set to maintain a specific energy delivery from electrodes to scrap charge; this is done by continuously raising and lowering the electrodes in order to increase or decrease the energy associated to the electric arc following the process requirements (Gandhare and Lulekar, 2007). This action
effectiveness is reduced if the feeding voltage is unstable and/or the scrap charge density is uneven or too low (presence of holes and spots inside the charge or poorly conductive pieces of scrap).
The scrap charge composition is usually classified starting from the type and origin of the steel scrap and relative percentages of each type employed; in this way each scrap charge results in a mixture of various types of materials, selected with respect to their metallurgical properties.
Scrap charge density is indirectly influenced by charge composition, so melting process performance is in a way correlated to the type of employed scrap mixture (Paschkis, 1945). How strong is this correlation depends on the various other factors involved in the melting process and scrap selection.
Moreover, due to the huge amount of produced heat and high electric energy consumption, furnace parts and its ancillaries require a frequent maintenance; in some cases this is done following a preventive maintenance policy on a fixed time interval (every two weeks), like tub and roof lining replacement, while in other cases this is done when needed, like electrodes lengthening and replacement.
In this paper we want to focus on the analysis of the correlation between scrap charge composition, deviations of actual electric quantities from preset reference values, overall degradation of furnace, and actual melting process performances. In particular, the effects of supply voltage quality and type of employed scrap charge will be analyzed and discussed.
This paper is organized as follows: section two is devoted to the analysis of the test plan architecture, section three analyzes the furnace melting process, while the experimental results are analyzed in section four. A discussion about these results is reported, with the conclusions, in section five.
2 The architecture of the analyzed system
In order to verify the applicability of a condition monitoring policy to this kind of system, a plant has been chosen as case study. In this plant, a monitoring system has been specifically designed and installed, in order to implement the analysis described in the previous section.
2.1 The electric system
Calvisano steel plant has been chosen as case study. It is supplied by a dedicated 132kV connection to the 132 kV line that interconnects the 380 kV station in Lonato to the 220 kV station in Marcaria.
Both stations are located in the district of Brescia, in Lombardia region, in the Northern part of Italy. The dedicated connection to Calvisano plant is about 10 km long, and is connected to the above 132 kV line with a T-connection placed few kilometres after the sub-station of Montichiari, 10 km from the main station in Lonato.
The electric system of Calvisano steel plant is schematically shown in Fig. 1. The 132 kV line feeds a HV bus bar to which two transformers are connected. The main transformer has two secondary windings. One of them supplies the arc furnace (EAF) and the Static VAR Compensator (ABB FACTS, 2012) through a 32.4 kV bus, while the second one supplies the ladle furnace (LF) through a 15 kV bus. The second transformer supplies a 15 kV auxiliary.
The Static VAR Compensator is tuned to compensate, in particular, the second, third and fourth harmonic components, that are supposed to be the largest ones. The same compensator has been
designed to compensate the reactive power at fundamental frequency (Grunbaum et al., 2005) . The compensator can be controlled according to different strategies: two of them are power factor regulation at power grid connection point, and EAF bus bar voltage active regulation. The supply system is completed by an on-load tap changer installed immediately after the 32.4kV secondary winding of the main transformer. Its main purpose is to maintain the bus bar voltage inside the allowable range for the compensator.
The EAF subsystem connected to the 32.4kV bus bar is composed by the EAF itself, a supply transformer with 30kV primary voltage and secondary voltage that can be regulated from 506 to 922 V by a tap changer (Tap 1 through 21); its rated power is 80MVA.
The measurement systems have been installed on all buses and are represented by the red boxes in Fig. 1.
2.2 The monitoringsystem
The monitoring architecture is based on instruments that are connected to the current (CT) and voltage (VT) transformers already installed in the plant. The CTs and VTs output signals are scaled further on to the ±10 V range, compatible with the input dynamics of the employed Analog-to-Digital conversion (ADC) board. The transducers employed to this purpose have been specifically designed following the same specifications as those followed in (Ferrero et al., 2002a, 2002b). In particular, the voltage transducers are based on non inductive, resistive voltage dividers and the required insulation level is provided by an isolation amplifier. The current transducers are based on non inductive shunts and the required gain and insulation level is provided again by an isolation amplifier. The voltage transducers show a relative standard uncertainty on the gain of 0.04% of the full-scale value and the current transducers show a relative standard uncertainty on the gain of 0.05% of the full-scale value up to 5 kHz. The phase shift between the voltage and current channels is 6 · 10 3 rad at 50 Hz and is linear up to 5 kHz. Therefore, these contributions to uncertainty can be neglected when compared to the uncertainty contributions coming from the CTs and VTs.
Fig.1 – Calvisano steelwork plant electric system
The voltage and current signals are acquired and converted into digital by NI USB-6351 acquisition boards, featuring 16 analog input channels, 16-bit resolution and 1 MHz/channel sampling rate in multichannel acquisition mode. These ADC boards do not feature a simultaneous sampling of the
input channels. However, the time delay between two contiguous channels introduced by the non-simultaneous sampling is within 2 s, and does not add a significant contribution to the phase-angle error of the CTs and VTs. Therefore, these ADC boards have been considered an effective trade-off between cost and metrological performances.
The ADC boards are triggered by the furnace control signals, so that voltage and current samples can be acquired and stored during furnace operation and can be correctly referred to each phase of the fusion process. All samples are then post-processed and the significant electrical parameters, i.e.
voltages and currents, active and reactive powers from fundamental frequency up to the 11th order harmonic, are computed and analyzed in order to perform the monitoring activity.
Due to technical, production and safety issues, it has not been possible to connect any measuring system to the terminals of the EAF transformer or nearby the EAF area, nor interfacing it with the measuring and control system of the furnace itself; so the electrical quantities at the primary and secondary windings of the EAF transformer are derived from those measured at the 32,4kV bus bar, using technical specifications data of EAF subsystem parts provided by Calvisano steelwork staff.
3 The Furnace Melting Process
The steel production process from scrap is based on the overheating and consequently melting of the scrap charge by means of the arc current passing through it. There are three main steps in this process:
the scrap melting,
the steel refining in the Ladle Furnace the casting.
This paper will focus only on the first step, performed by the EAF (Paschkis, 1945).
The key factors of scrap melting are the composition of the scrap charge - which depends on the origin, form and metallurgical and chemical properties of the metal scrap -, the thermal energy needed and the way it has to be delivered to the scrap, and the control of chemical reaction and chemical properties of the molten steel bath, during the melting stage and before the tapping stage.
The metal scrap comes from car wrecks, steel frameworks of reinforced concrete, machining scrap, sheet metal forming scrap, etc.. A scrap charge is a mixture of two or more of these source materials. In Calvisano plant a scrap charge batch, based on the percentage of certain chemical elements, including copper, nickel and others, is adopted; each batch is marked with an alphabetical letter, ranging from B to K. The more frequently used mixtures are H and K: the first one is composed for the most part of machining scrap and small pieces of framework scrap, the second one contains a high percentage of heavy and bigger framework scrap pieces. By the way, big pieces, and car wrecks also, are cut in smaller parts. This classification is based only upon the supposed quantities of chemical elements contained in the scrap, not on the density of the scrap charge, nor on the thermal or electrical characteristics of the materials composing the scrap.
To help the melting process, the furnace tub is loaded in successive steps; in Calvisano plant four steps are the standard, though sometimes only three steps are adopted. So, the loading process, and consequently the melting process, is divided into four stages, corresponding to four baskets of scrap charge. The whole charge weighs approximately 85-90 tons and is usually divided into the baskets following these approximate quantities: 45% of charge in the first basket, 30% in the second basket, 18% in the third basket, and the remaining 7% in the fourth basket. Usually the first basket is loaded
with more fine materials in order to help the formation of the steel bath and to speed up the furnace heating (Paschkis, 1945), the roughest materials are added to the second and third baskets, and fine materials are used again in the fourth basket to facilitate the refinement process, which starts immediately after the end of the fourth basket melting.
The energy required by the melting process is delivered to the scrap by graphite electrodes, which are electrically connected in delta configuration, and are connected to the secondary windings of the EAF transformer via water cooled bars, flexible conductors and hydraulically operated holders. The amount of energy transferred to the scrap, and also the arc characteristics, have to be constantly changed during the process, to adapt the actual melting conditions to the required ones set by the control system. Both these tasks are performed varying the arc voltage and current following a preset series of steps, which are named furnace gears. A given arc voltage value and three arc current values, one per phase/electrode correspond to each step. The arc voltage is regulated using the tap changer installed on the primary winding of the EAF transformer; the arc currents are independently regulated on each phase by raising or lowering the electrodes with the hydraulic actuators.
The transition between two consecutive steps is based on energy measurement: the control system switches to the next step only when a preset energy threshold is reached. These threshold values are set in form of a specific energy upon weight, so they vary depending on the weight of each scrap charge, i.e. basket. A specific furnace gear is selected for each melting process depending on the characteristics of the employed scrap charge and the desired steel specifications.
The refinement stage starts immediately after the end of the fourth basket melting; it is performed in two steps, the first one energy controlled - the weight of the whole scrap charge is now considered - and the second one time controlled and, very often, manually overridden. For some gears refining is performed in one step, time controlled. The refinement stage is often used to correct the melting process and to add more energy if needed, this is why manual control override is so frequent.
At the end of the refinement stage, chemical analysis are performed and some elements, like lime (CaO) and ferroalloys (FeSi, etc.) are added to attain desired chemical specifications of the molten steel (The AISE Steel Foundation, 1998). Lime helps to form the slab which is very important to reduce heat transfer from bath to air. The chemical reaction evolution during melting depends strongly on the bath temperature (on which depends the formation of some unwanted by-products or lining chemical aggression from elements like phosphorous or fluorum). Bath temperature during melting is controlled, apart from electric energy delivering, also by oxygen insufflation; oxidation reactions take place in the bath and raise the temperature. Oxygen is used also to reduce the quantity of carbon, through carbon dioxide formation. The amount of the elements added after the melting process, together with its duration, specific power applied and energy delivery during the refinement stage, can provide useful information about the effectiveness of the previous stages of the melting process.
4 Experimental results
4.1 Analysis methodology
The Calvisano plant activities, has been monitored and recorded for several months, starting from November 2011in terms of electrical signal recording. The present work focuses on the analysis of a four months interval ranging from November 2012 to March 2013 (during the whole December 2012 until second week of January 2013 the plant was not operating because of periodic
maintenance tasks).All data taken from the monitoring system presented in Section 2.1 have been recorded and post processed in order to evaluate the electrical quantities of interest.
The analysis has firstly focused on the load conditions, reactive and harmonic power consumption and supply voltage analysis for almost all main loads and busbars of the steel plant electric system (see Fig.1). This activity was performed in order to build a solid knowledge about the normal operating conditions of the plant. During this initial observation activity, a lot of attention was dedicated to the actual electric supply conditions, in particular to voltage levels and transients at the coupling point to the electric grid (Marconato, 2002). An harmonic power analysis has also been performed (White et al., 2010;Clerici et al., 2012).
The main objective of this analysis was to discover specific electric supply conditions that could be correlated with poor melting performances of the EAF. For this task, production records and statistics provided by the steel plant staff have also been analyzed and investigated in order to classify the melting processes on the basis of the amount of chemical elements added at the end of the melting process and the weight deviation between the steel produced and the metal scrap utilized.
This study has revealed that sudden variations of the supply voltage at the point of common coupling (PCC) can strongly affect the performance of the melting process, primarily when scrap
“quality” and metallurgical conditions are not good. These voltage transients are typical of HV grids and are originated by the insertion and disconnection of capacitor banks used for grid voltage regulation (Miller, 1982; Hofmann et. al., 2012). The analysis of the collected data (Clerici et al., 2012) shows that these voltage transients are present only at sometimes during the day and the week, depending on the grid load conditions. Most, though not all, of melting processes performed during those periods showed problems or poor performances.
A further investigation showed that voltage variations and transients have different impact on the melting performances depending on when they occur during the process; as described in the previous section, each melting stage has its own requirements in terms of energy, voltage and current settings. In few cases higher grid voltage set-point led to better melting conditions and good EAF performances.
In order to attain a better knowledge about the influence of voltage, and currents, on the melting performances, electrical quantities at the secondary windings of the EAF transformer have been estimated, by computing them from the measured quantities at the EAF/SVC bus bar, using equivalent models of cable line, reactor and EAF transformer based on data and specifications given by plant staff. The EAF control system measures voltage and current directly on the secondary windings of EAF transformer, so a direct comparison between preset values and actual values can be done.
More attention has been given to scrap mixtures, as well as to the amount of added chemical elements, in order to create a better classification of the melting processes from the metallurgical point of view.
Refinement stage duration, along with the delivered energy amount and the specific power applied, was also taken into account in order to verify if corrections were made by furnace operators and how much they affected the overall result of the melting process.
Finally, data obtained from maintenance records were considered to investigate correlations between melting performances and the health conditions of the furnace and its ancillary parts.
4.2 Analysis results
During the monitoring period, some critical electrical events were reported by the plant technical staff. First, sometimes during January, a very unstable voltage supply was observed, which led to poor and unpredictable performances of the furnace; secondly, the SVC control criteria has been switched from power factor control mode to bus bar voltage control mode (Hingorani and Gyugyi, 2000) and the consequent set-up tests run in February led to very poor voltage regulation on the EAF subsystem for at least fifteen melting processes.
The analysis described in Section 4.1 has been applied to all melting processes performed during
The analysis described in Section 4.1 has been applied to all melting processes performed during