Modelling and control of an electrode system for a three-phase Electric Arc Furnace

This dissertation investigates the control of the electrical energy input to a three-phase electric arc furnace (EAF). Graphite electrodes are used to convert electrical energy into heat via three-phase electric arcs. Constant arc length is desirable as it implies steady energy transfer from the graphite electrodes to the metallic charge in the furnace bath. With the charge level constantly changing, the electrodes must be able to adjust for the arc length to remain constant. In this dissertation electric arc current is used as the control variable. This is the most often used control variable in the electric arc furnace industry and implies fast adjustments of short circuits between the electrode tips and the metallic charge. The motivation behind the modelling of the electrode system for a three-phase electric arc furnace is to extend an existing EAF model developed at the University of Pretoria. The existing model investigates the control of the electric arc furnace process itself and it is assumed that the applied electrical energy input is constant. Proportional-Integral-Derivative (PID) control as well as Model-Predictive-Control (MPC) is applied to the electrode system. Time delays on the outputs of the hydraulic actuators makes it necessary to include approximations of time delays on the outputs of the linear model, which is needed for controller design. A well known general control problem is followed in this dissertation. All models are derived from first principles, and complete controller design is carried out. Most available literature lack in at least one of these fields.Dissertation (MEng (Electronic Engineering))--University of Pretoria, 2007.Electrical, Electronic and Computer Engineeringunrestricte

Modelling and analysis of electric arc loads using harmonic domain techniques

Abstract It has been reported that as much as 12% of global electricity production goes into producing artificial light using arc discharge lamps and that global annual production of these lamps may be as much as 1.2 billion units. In the liquid steel production industry, one metric tone of steel demands, on average, 400 kW-hr and in the year 2007, the crude steel output reached 1,343.5 million metric tons. In both instances, engineered electric arcs are present and represent major loads in electrical power systems which require the utmost attention. They observe a highly non-linear behaviour with the capacity to export harmonic distortion and flicker into the power system. Electric arc furnace installations, in particular, are well-known to be sources of dynamic disturbances affecting neighbouring loads. Arc discharge lamps, on aggregate, may exhibit the same perturbing effect. Over the years, the non-linear nature of these loads and their ubiquitous nature have caught the interest of researchers in all corners of the world and from different backgrounds, including this author. The research work reported in this thesis advances current knowledge in the modelling and simulation of electric arcs with particular reference to arc discharge lamps with electromagnetic ballasts and electric arc furnaces with particular reference to operational unbalances and the impact in the installation of ancillary power electronics equipment. In these two quite distinct applications, linked by the presence of engineered electric arcs, the fundamental modelling item is a non-linear differential equation which encapsulates the physic of the electric arc by applying power balance principles. The non-linear differential equation uses the arc conductance as state variable and adapts well to model a wide range of characteristics for which a set of experimental coefficients are available. A fact of perhaps equal relevance is that the non-linear differential equation is amenable to algebraic representations using operational matrices and suitable for carrying out periodic steady-state solutions of electric circuits and systems. The modelling and numerical solution takes place in the harmonic space where all harmonics and cross-couplings between harmonics are explicitly represented. Good application examples are the harmonic domain solution of arc discharge lamps with electromagnetic ballasts and the harmonic domain solution of electric arc furnaces with ancillary power electronics equipment. Building on the experience gained with the representation of the arc discharge lamps with electromagnetic ballasts, the research turns to the representation of the electric arc furnace installation with provisions for reactive power compensation using power electronic control and harmonic filters. This is a three-phase application which comprises several nodes, giving rise a large-scale model of a non-linear system which is solved in the direct frequency domain using a blend of the Newton-Raphson method and the Gauss-Seidel method, achieving robust iterative solution to a very tight tolerance. Both algorithms are implemented in MATLAB code and the raw simulation results which are the harmonic complex conjugated vectors of nodal voltages are used to assess in a rather comprehensive manner the harmonic interactions involved in both kinds of applications

On the long-term correlations and multifractal properties of electric arc furnace time series

In this paper, we study long-term correlations and multifractal properties elaborated from time series of three-phase current signals coming from an industrial electric arc furnace plant. Implicit sinusoidal trends are suitably detected by considering the scaling of the fluctuation functions. Time series are then filtered via a Fourier-based analysis, removing hence such strong periodicities. In the filtered time series we detected long-term, positive correlations. The presence of positive correlations is in agreement with the typical V--I characteristic (hysteresis) of the electric arc furnace, providing thus a sound physical justification for the memory effects found in the current time series. The multifractal signature is strong enough in the filtered time series to be effectively classified as multifractal

Comparison between unipolar and bipolar single phase grid-connected inverters for PV applications

An inverter is essential for the interfacing of photovoltaic panels with the AC network. There are many possible inverter topologies and inverter switching schemes and each one will have its own relative advantages and disadvantages. Efficiency and output current distortion are two important factors governing the choice of inverter system. In this paper, it is argued that current controlled inverters offer significant advantages from the point of view of minimisation of current distortion. Two inverter switching strategies are explored in detail. These are the unipolar current controlled inverter and the bipolar current controlled inverter. With respect to low frequency distortion, previously published works provide theoretical arguments in favour of bipolar switching. On the other hand it has also been argued that the unipolar switched inverter offers reduced switching losses and generates less EMI. On efficiency grounds, it appears that the unipolar switched inverter has an advantage. However, experimental results presented in this paper show that the level of low frequency current distortion in the unipolar switched inverter is such that it can only comply with Australian Standard 4777.2 above a minimum output current. On the other hand it is shown that at the same current levels bipolar switching results in reduced low frequency harmonics

Comparison between unipolar and bipolar single phase grid-connected inverters for PV applications

An inverter is essential for the interfacing of photovoltaic panels with the AC network. There are many possible inverter topologies and inverter switching schemes and each one will have its own relative advantages and disadvantages. Efficiency and output current distortion are two important factors governing the choice of inverter system. In this paper, it is argued that current controlled inverters offer significant advantages from the point of view of minimisation of current distortion. Two inverter switching strategies are explored in detail. These are the unipolar current controlled inverter and the bipolar current controlled inverter. With respect to low frequency distortion, previously published works provide theoretical arguments in favour of bipolar switching. On the other hand it has also been argued that the unipolar switched inverter offers reduced switching losses and generates less EMI. On efficiency grounds, it appears that the unipolar switched inverter has an advantage. However, experimental results presented in this paper show that the level of low frequency current distortion in the unipolar switched inverter is such that it can only comply with Australian Standard 4777.2 above a minimum output current. On the other hand it is shown that at the same current levels bipolar switching results in reduced low frequency harmonics

Malzeme Üretiminde Siber Güvenlik

Industry 4.0, a new industry revolution, is happening now and several developed countries are leading the path. Internet of things (IoT) is also encompassed by Industry 4.0. In the future, more devices in factories are to be connected to Ethernet or Internet. However, this makes the companies, devices and researchers vulnerable to cyber-attacks. Recently, some cyber-attacks which have happened to some companies or countries verify the danger. Sintering systems and furnaces are used for research by universities and for series manufacturing by factories. Arc furnaces and induction furnaces are also commonly used devices in metal factories. A sintering system, an arc furnace or an induction furnace which is connected to Internet or Ethernet may also be under cyber-attack threat. The danger may be prevented by taking necessary precautions. In this study, these threeproduction systems are first briefly introduced and then inspected assuming that they have been connected to internet and examined with considering cyber-attack point of view. Some basic solutions against cyber-attacks to the aforementioned devices are suggested.Yeni bir endüstri devrimi olan Endüstri 4.0 günümüzde yaşanmakta ve özellikle bazı gelişmiş ülkeler bu_x000D_ alanda öncü çalışmalar ortaya koymaktadır. Nesnelerin İnterneti (IoT) Endüstri 4.0 tarafından kapsanan önemli_x000D_ bir altyapıdır. Gelecekte, fabrikalardaki daha fazla cihazın Ethernet veya İnternet üzerinden çalışmalarını_x000D_ sürdürmesi öngörülmektedir. Bunun kurum ve fabrika ortamındaki cihazları ve verileri siber saldırılara karşı_x000D_ savunmasız bırakabileceği açıktır. Son zamanlarda, farklı ülkelerden farklı kurumlarda yapılan bazı siber_x000D_ saldırılar tehlikeyi doğrulamaktadır. Çeşitli malzeme sinterleme sistemleri ve fırınlar üniversiteler tarafından_x000D_ araştırma yapmak ve fabrikalar tarafından seri imalat yapmak için kullanılırlar. Ark fırınları ve endüksiyon_x000D_ fırınları da metal fabrikalarında yaygın olarak kullanılan cihazlardır. Internet veya ethernet'e bağlı bir sinterleme_x000D_ sistemi, bir ark ocağı veya bir indüksiyon ocağı da siber saldırı tehdidi altında olabilir. Bu noktada gerekli_x000D_ önlemler alınarak tehlike önlenebilir. Bu çalışmada, bu üç üretim sistemi ilk öncelikle kısaca tanıtılmış ve daha_x000D_ sonra internete bağlı oldukları dikkate alınarak siber saldırı bakış açıları ile incelenmişlerdir. Çalışmada,_x000D_ belirtilen cihazlara karşı olası siber saldırılara yönelik çözümler önerilmektedir.Yeni bir endüstri devrimi olan Endüstri 4.0 günümüzde yaşanmakta ve özellikle bazı gelişmiş ülkeler bu alanda öncü çalışmalar ortaya koymaktadır. Nesnelerin İnterneti (IoT) Endüstri 4.0 tarafından kapsanan önemli bir altyapıdır. Gelecekte, fabrikalardaki daha fazla cihazın Ethernet veya İnternet üzerinden çalışmalarını sürdürmesi öngörülmektedir. Bunun kurum ve fabrika ortamındaki cihazları ve verileri siber saldırılara karşı savunmasız bırakabileceği açıktır. Son zamanlarda, farklı ülkelerden farklı kurumlarda yapılan bazı siber saldırılar tehlikeyi doğrulamaktadır. Çeşitli malzeme sinterleme sistemleri ve fırınlar üniversiteler tarafından araştırma yapmak ve fabrikalar tarafından seri imalat yapmak için kullanılırlar. Ark fırınları ve endüksiyon fırınları da metal fabrikalarında yaygın olarak kullanılan cihazlardır. Internet veya ethernet'e bağlı bir sinterleme sistemi, bir ark ocağı veya bir indüksiyon ocağı da siber saldırı tehdidi altında olabilir. Bu noktada gerekli önlemler alınarak tehlike önlenebilir. Bu çalışmada, bu üç üretim sistemi ilk öncelikle kısaca tanıtılmış ve daha sonra internete bağlı oldukları dikkate alınarak siber saldırı bakış açıları ile incelenmişlerdir. Çalışmada, belirtilen cihazlara karşı olası siber saldırılara yönelik çözümler önerilmektedir

A Study on Reduction of Harmonic Distortion caused by AC Arc Furnaces

In AC arc furnaces, arc ignition is stabilized by inductors which are connected across its supply lines. They cause power factor degradation in the system. When furnaces are modelled as linear RL load, the arc nonlinearity reduces the impact of this degradation to a significant extent. Furnace Power factor is improved by the DC voltage on the arc. In this research study, power factor of AC furnaces is estimated on the assumption that DC voltage on the arc is constant and furnace operates in steady state. Primary objective of this research was to determine the power factor of the furnace in steady state. However, other states of operation such as bidirectional arc and unidirectional arc ignition were also taken into consideration while computing the power factor of the furnace. Secondary objective of this research was to understand the effects of harmonics on arc furnace and design filter parameters to mitigate the impact of harmonics caused by the transient operation of the furnaces. Random extinction and ignition of individual arc furnaces help reduce current distortion of ultra-high-power ac arc furnaces using resonant harmonic filters. This operation can be conducted in condition of fast varying parameters of the furnace. On the other hand, this resonant harmonic filter can be built practically only as a constant-parameters device. Filters built on fixed parameters cannot be well coordinated to the furnace. This discrepancy can cause a decline in the filter’s efficacy of harmonics reduction. A reference model using MATLAB was developed to simulate different conditions on the circuit. Also, use of different resonant filters to reduce these harmonics was performed to find the most efficient filter that will increase the efficacy of this process. This efficiency was investigated, specifically, in a melting mode of an arc furnace operation. A reference arc furnace was used in this study which enabled to draw some qualitative and quantitative conclusions on possibilities of current harmonic reduction at a supply of ultra-high-power furnaces by resonant harmonic filters