Design and Simulation of TCRTSC in Electric Arc Furnace for Power Quality Improvement at Steel Making Plant (No1 Iron and Steel Mill, Pyin Oo Lwin, Myanmar)
Thet Mon Aye, Soe Win Naing
Dept. of Electrical Power Engineering, Mandalay Technological University, Mandalay, Myanmar
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Thet Mon Aye, Soe Win Naing. Design and Simulation of TCRTSC in Electric Arc Furnace for Power Quality Improvement at Steel Making Plant (No1 Iron and Steel Mill, Pyin Oo Lwin, Myanmar). Journal of Electrical and Electronic Engineering. Vol. 3, No. 3, 2015, pp. 3035. doi: 10.11648/j.jeee.20150303.12
Abstract: Electric Arc Furnaces (EAFs) are unbalanced nonlinear and time varying loads, which can cause many problems in the power system quality. As the use of arc furnace loads increases in industry, the important of the power quality problems also increase. So, in order to optimize the usage of electric power in EAFs, it is necessary to minimize the effect of arc furnace loads on power quality in power systems as much as possible. Therefore, in this paper, design and simulation of an electric plant supplying an arc furnace is considered. Then by considering the high changes of reactive power and voltage flicker of nonlinear furnace load, a thyristor controlled reactor compensation with thyristor switched capacitor (TCRTSC) are designed and simulated. Finally, simulation results verify the accuracy of the load modeling and show the effectiveness of the proposed TCRTSC model for reactive power compensating of the EAF. The installation site for this proposed system is No (1) Iron and Steel Mill (Pyin Oo Lwin). And data is taken from this Steel Mill. Simulation results will be provided by using MATLAB/ Simulink.
Keywords: Electric Arc Furnaces, Power Quality, Voltage Flicker, MATLAB/Simulink, TCRTSC Compensation
1. Introduction
The use of electric arc furnaces (EAFs) for steel making has grown dramatically in the last decade. Of the steel made today 36% is produced by the electric arc furnace route and this share will increase to 50% by 2030. The electric arc furnaces are used for melting and refining metals, mainly iron in the steel production. AC and DC arc furnaces represent one of the most intensive disturbing loads in the subtransmission or transmission electric power systems; they are characterized by rapid changes in absorbed powers that occur especially in the initial stage of melting, during which the critical condition of a broken arc may become a short circuit or an open circuit. In the particular case of the DC arc furnaces, the presence of the AC/DC static converters and the random motion of the electric arc, whose nonlinear and timevarying nature is well known, are responsible for dangerous perturbations such as waveform distortions and voltage fluctuations [2].
Nowadays, arc furnaces are designed for very large power input ratings and due to the nature of both, the electrical arc and the melt down process, these devices can cause large power quality problems on the electrical network, mainly harmonics, interharmonics, flicker and voltage imbalances. The EAFs are among the largest electrical loads in power systems. Regarding the fast and heavy deviations of electric power in these loads, the bus voltage of these furnaces is unbalanced and has large oscillations. Moreover, the EAFs cause deteriorating of the power quality, making voltage flicker, unbalancing in voltage and current, and occurring odd and even harmonics in power systems. In order to study in this field and improve the above mentioned factors, exact and complete design of the power system with arc furnaces should be performed [1].
2. Power Quality and Electrical Arc Furnaces
Power quality can be interpreted by the existence of two components:
• Voltage quality. It expresses the voltage deviation from the ideal one and can be interpreted as the product quality delivered by the utilities.
• Current quality. It expresses the current deviation from the ideal one and can be interpreted as the product quality received by the customers.
The main Power quality disturbances are:
• Harmonics
• Undervoltages or Overvoltages
• Flicker
• Transients
• Voltage sags
• Interruptions.
An electric arc furnace (EAF) transfers electrical energy to thermal energy in the form of an electric arc to melt the raw materials held by the furnace [3]. Typical steelmaking cycles are:
• Arc ignition period (start of power supply)
• Boring period
• Molten metal formation period
• Main melting period
• Meltdown period
• Meltdown heating period respectively.
3. Model of Power System with AC Electric Arc Furnace
The electric diagram of a source supplying an EAF is illustrated in Figure 1.
In this figure, bus 1 is the point of common coupling (PCC) which is the supplying bus of the EAF transformer. In order to change the input active power of the EAF, transformer T_{F}, (MV/LV) is used. This transformer is equipped with a tap changer located at the secondary winding to have the ability of changing the voltage of the furnace. The arc furnace is also connected to the PCC through the transformer T_{S}, (HV/MV). X_{C} and R_{C} are the reactance and resistance of the connecting cable line to the furnace electrodes, respectively. Also, X_{Lsc} is the short circuit reactance at bus PCC.
In this work, the steel mill No.1, (Pyin Oo Lwin, Mandalay, Myanmar) is composed of the various departments, namely, Electric Arc Furnace (EAF), Ladle Refining Furnace (LRF), Continuous Casting Machine (CCM), Water Treatment Plant (WTP), Fumes Treatment Plant (FTP), Air Separation Plant (ASP), Oxygen Plant, Wire Production Plant, Precipitated Calcium Carbonate (PCC) CaCO3, Lime, Air Compressor, Shredder and Bolt Nut. Among these departments, EAF is studied as research area.
4. Design of TCRTSC for Electric Arc Furnace
In order to compensate reactive power and voltage flicker improvement in power system including the EAF and on the basis of Figure 1, in this section, an optimal design of TCR compensator along with a parallel capacitor will be presented. The oneline diagram of this structure is illustrated in Figure 4.
As it can be seen in Figure 4, the effective inductance of the TCR compensator can be varied by changing the firing angle of the thyristors. If the thyristors are fired exactly at the voltage peak of the supply, then they will conduct perfectly. The TCR current is essentially reactive with almost 90 degrees lag phase.
The operation of EAF Plant, steelmaking cycle is represented in Table 1.
Stage of Melting  Time (min)  Voltage (kV)  Reactive Power (MVAR)  Real Power (MW) 
Bucket 1  0  11  0  0 
2.91  11  0  0  
Initial Melting  2.92  9.84  16.49  21.99 
3.68  9.84  16.49  21.99  
Bore Down  3.69  10.87  13.81  18.41 
4.32  10.87  13.81  18.41  
Melting  4.33  9.71  22.635  30.18 
8.51  9.71  22.635  30.18  
Melting  8.52  8.81  20.87  27.83 
9.09  8.81  20.87  27.83  
Melting  9.10  9.88  18.795  25.06 
10.52  9.88  18.795  25.06  
Melting  10.53  9.17  25.29  33.72 
16.27  9.17  25.29  33.72  
End Melting  16.28  10.10  24.06  32.09 
17.93  10.10  24.06  32.09  
Finish Bucket 1  17.94  10.97  0  0 
19.22  10.97  0  0  
Initial Melting/Boring  19.23  10.68  15.42  20.57 
19.94  10.68  15.42  20.57  
Melting  19.95  9.73  25.18  33.57 
20.53  9.73  25.18  33.57  
Melting  20.54  9.00  21.27  28.36 
24.03  9.00  21.27  28.36  
Melting  24.04  8.56  24.74  32.99 
29.69  8.56  24.74  32.99  
Finish Bucket 2  29.70  10.97  0  0 
31.89  10.97  0  0  
Initial Melting/Boring  31.90  7.81  16.03  21.38 
32.51  7.81  16.03  21.38  
Bore Down  32.52  10.17  24.07  32.09 
33.40  10.17  24.07  32.09  
Melting  33.41  8.87  22.38  29.84 
36.13  8.87  22.38  29.84  
Melting  36.14  8.99  25.39  33.86 
41.97  8.99  25.39  33.86  
Melting  41.98  10.99  0  0 
42.23  10.99  0  0  
Melting  42.24  10.26  24.50  32.67 
48.80  10.26  24.50  32.67  
End Melting  48.81  9.96  24.315  32.42 
50.64  9.96  24.315  32.42  
End Melting/ Superheating  50.65  9.81  23.46  31.28 
51.84  9.81  23.46  31.28  
Finish Bucket 3  51.85  10.98  0  0 
53.56  10.98  0  0  
End Melting  53.57  11  0  0 
57.17  11  0  0  
57.18  11  0  0 
The compensated values for the capacitance and the compensated TCR inductance are calculated based on this setting.
(1)
(2)
(3)
(4)
(5)
No  Load (EAF)  Compensating Inductance  Compensating Capacitance 
1  2.0 ton  23.759  213.224 
2  3.0 ton  23.579  214.848 
3  4.0 ton  23.351  216.954 
4  5.0 ton  22.843  221.780 
5  7.0 ton  22.753  222.650 
6  0.5 ton  41.832  121.104 
7  1.0 ton  37.464  135.223 
8  1.5 ton  36.039  140.573 
9  1.8 ton  35.030  144.686 
10  1.9 ton  30.796  164.500 
(From Table 1)
(From Table 2)
The maximum reactive capacity of the TCRTSC is set at
From Equation 4 and Equation 5, considering a fundamental frequency is of 50Hz. The capacitance and inductances are and.
According to the inductive and capacitive reactance, each SVC has its own firing angle reactive power characteristic, which is a function of the inductive and capacitive reactance.
It is necessary to obtain the effective reactance as a function of the firing angle, using the fundamental frequency TCR equivalent reactance.
(6)
(7)
Where is the reactance of the linear inductor and and are the thyristors’ conduction angle and firing angle repectively.
At the TCR conducts fully and its equivalent reactance becomes.
At the TCR is blocked and its equivalent reactance becomes extremely large. It is also expressed in Table 3.
Table 3. Variation of Value of with Firing Angle.
Firing Angle (Degree)  Conduction Angle (Degree)  Susceptance (S)  Perunit of 

 0.028  1 

 0.022  0.8 

 0.016  0.6 

 0.011  0.4 


 0.2 


 0.1 


 0.06 


 0.02 


 0.002 




The total effective reactance of the SVC, including the TCR and capacitive reactance, is determined by the parallel combination of both components and which as a function of the conduction angle becomes
(8)
And finally as a function of the firing angle becomes
(9)
As expected, the effective reactance of the SVC is a function of the firing angle. Equation 9 may be used to determine the firing angle, using the fundamental relationship.
(10)
At initial melting conduction, the reactive power of the load is set 16.49MVAR and bus voltage is 9.84kV respectively. The calculation of firing angle value is and the next stages are also calculated as before.
By using the iteration method, the next step to go on calculate until the voltage level is stable.
(11)
By iteratively, the voltage level is stable with 10.2kV at the step (18). In this way, another condition will also calculate till stable.
The new total melting time has been reduced from 51.85 min down to 47.16 min which is 9.05% reduction. The power on time has been reduced from 48.36 min down to 43.68 min which is 9.7% reduction.
The new melting times have been calculated assuming that furnace is able to transfer increased power into melting process by the similar efficiency than in the system without TCRTSC compensator. The comparison of times without and with TCRTSC is presented in Table 4 and Table 5.
Time (min) Without TCRTSC  Time (min) With TCRTSC 
51.85  47.16 
Reduction in time  9.05% 
Time (min) Without TCRTSC  Time (min) With TCRTSC  
Bucket 1  17.94  15.94 
Bucket 2  10.47  9.05 
Bucket 3  19.95  18.69 
Total  48.36  43.68 
Reduction in time  9.7% 
5. Simulation Result for Voltage and Current
The power quality means voltage quality for this work and to enhance the power quality i.e., to keep the voltage stability is the aim. Voltage stability problems usually encounter in heavily loaded systems.
EAF is operating to melt down the scrap in a moment. During melting cases, generally, the system voltage decreased from the constant level. If the running condition stopped, the voltage will suddenly rise.
The simulation diagram and simulation result without and TCRTSC are described in the following Figure 8, Figure 9 and Figure 10.
The inserted values of inductance and resistance of EAF model are 0.148mH and. Figure 8 shows Simulink block diagram of EAF without TCRTSC. The result without TCRTSC was found in Figure 9 for voltage and in Figure 10 for current. The TCRTSC compensator operates on single phase basis and thus it is balancing the voltage.
To obtain the desire results, a TCRTSC is added to the EAF. The Simulink block diagram of EAF with TCRTSC is shown in Figure 11. The system voltage of bore down process with 25.29 MVAR, 33.72 MW and 9.17 kV is stable as can be seen in Figure 12. The current flow is expressed in Figure 13.
The TCRTSC will stabilize the voltage. This is advantageous not only for the furnace itself but also for the control and protection system of the steel making plant because modern electronics and process instruments are very sensitive to voltage fluctuations. The TCRTSC can reduce the flicker down to the value requested by the power utility.
6. Conclusion
This paper provides a detailed description of a modern, TCRTSC type Static Var Compensator installed in steel making plant. The Electric Arc Furnace (EAF) for steel production is analyzed. The reactive power compensation requirements are quantitatively analyzed. The TCRTSC is a controller for voltage regulation and for maintaining constant voltage at a bus. This can result saving in operational costs, reduced line losses, reduction operating time, rise the steel tons, etc.
Acknowledgements
The author would like to express grateful thanks to her supervisor Dr. Soe Win Naing, Associate Professor, Department of Electrical Power Engineering, Mandalay Technological University, for his help, guidance and advice. The author wishes to thank to all of her teachers from Mandalay Technological University. The author’s special thanks are sent to her father, mother and sister for their support and encouragement.
References
Biography
Ms. Thet Mon Aye She received the BE degee in Electrical Power Engineeringfrom Technological University (Pakokku), Myanmar in 2007. Since June 2009, she became a Demonstrator at Technological University (Kalay) Under Ministry of Science and Technology, Myanmar. And then, she obtained ME degree in Electrical Power Engineering from Mandalay Technological University, Myanmar, in 2011 by serving her duty. Now, her mother unit is Technological University (Myitkyinar) and she is attending Ph.D degree in Electrical Power Engineering at Mandalay Technological University, Mandalay, Myanmar 