CHAPTER - 3 PERFORMANCE ANALYSIS OF INDUCTION FURNACE

Transcription

1 CHAPTER - 3 PERFORMANCE ANALYSIS OF INDUCTION FURNACE The production of iron and steel in India is widely preferred by the induction furnaces having varying capacities. The production volume of this steel industry is around 25 million tons per year. An induction furnace is also preferred for melting as well as formation of various other scraps such as Aluminum, Copper, Steel, Brass, Bronze, Cast Iron, Silicon, Gold, and Silver etc. which are further used in many other industrial and commercial applications. The induction furnace causes an enormous disruption to the utility and adjacent customers during its operation due to its non-linear characteristics. This is a serious phenomenon responsible for power quality degradation in the power system. In Indian industry, an induction furnace plays a significant role in the steel production. Mild steel is usually produced from the scrap material using an induction furnace in the industries. Table 3.1 shows the comparison of an induction furnace and electric arc furnace. Table 3.1: Comparison of an Induction furnace and Electric arc furnace S. No Parameters Induction Furnace Approximate Range Electric Arc Furnace 1. Net Output (kwh/t) Electrodes (kg/t) Nil Dust Production (kg/t) Flux (kg/t) Nil Refractory (kg/t) Slag Production (kg/t) Noise (db) Melting loss 1-2% 7-9% 9. Alloying Exact and Simple Not Exact 10. Net Electric Supply No Flicker and Moderate load Flicker disturbances and High load 46

2 Earlier, the electric arc furnaces (EAFs) were typically used in the steel industries and rolling plants but with the evolution of an induction furnace, the use of an arc furnace is limited. Besides the parameters shown in Table 3.1, induction furnace also has low requirements on the electrical power grid. It has no electrodes, relatively low initial cost, and small space requirements. Most of the induction furnaces used for manufacturing processes now a days are suitable to run on diesel generator supply and their operation is totally automatic in nature. One main limitation of induction furnace is its sensitivity of the refractory lining, which may lead to cracks in the product formation leading to emergency shutdown. Table 3.2 shows the manufacturing cost comparison of both the furnaces during initial periods [116]. Table 3.2: Cost comparison of an Induction furnace and Electric arc furnace S. No. Parameters Induction Furnace (Rs/ton) Electric Arc Furnace (Rs/ton) 1. Alloy materials Electrical Energy Consumption Electrodes Fluxing agent Oxygen Consumption Refractory Shredded Scrap Total For the production of one ton of steel in a steel industry, an arc furnace would require around 440 kwh of energy at a temperature of around 1500 C. The oxidation elements like iron, carbon, and silicon along with the burning of natural gasses results in the chemical energy formation which when mixed with the electrical energy results in the melting of steel along with the various losses as shown in Figure 3.1. This production efficiency has 47

3 been significantly improved since last 2-3 decades and also the consumption of electrical energy has got reduced up to 50% by using an induction furnace. Fig. 3.1: Steel production process using electric arc furnace Table 3.3 shows the history of an induction furnace and its linkage with the Indian steel industry. Table 3.3: Evolution of an Induction furnace S. No. Period Induction furnace history (in years) Experiments on an induction furnace performed First induction furnace patented Steel production begins using an induction furnace in USA Entry of the induction furnace in Indian industry Medium frequency induction furnace imported Sudden growth of induction furnace started in the industry Huge production of stainless steel from scrap and mild steel production 48

4 3.1 CLASSIFICATION OF INDUCTION FURNACES An induction furnace is used for the melting of various precious metals. In this, the heat is generated by the principle of induction. The high voltage supply given to primary coil is converted into a low voltage and high current in the secondary, which itself is a metal to be melted by the induction principle. An induction furnace is preferred in foundries as compared to any other form of the furnace because of the following advantages: Flexibility in operation because of controllable melting process. Better production yield Rate of production increases because of quick process and heat is developed instantaneously. Low cost the cost of operation of induction furnace is around Rs. 8500/- per ton of steel as compared to the costly operation of electric furnace. Environmentally Sound No extra harmful gasses are released during its operation. Fast and efficient melting operation. Fast start-up process - Full power supply is available at the time of starting instantaneously and the working temperature is reached readily. Natural Stirring Medium frequency induction furnace results in natural stirring resulting in the homogeneous melt. Compact Installation High melting rates can be achieved through small induction furnaces. Earlier, the induction furnaces were acting as single-phase loads and draw huge current from one phase due to which the supply system becomes unstable at the time when the furnace is running on full load. With the application of three-phase converters, the induction furnaces then started to operate at three-phase supply due to which both the rating as well as the power factor of the furnace tends to improve. The induction furnaces operating at power frequency are generally connected between the two phases of a three-phase supply. The capacitors and inductors connected in the delta are used for balanced loading of three phase circuit. The capacitor bank having taps is used to obtain controllable resonance of the 49

5 furnace coil. The balancing devices should have a reactive power equal to 57% of the active power of furnace coil. An induction furnace can be classified into two types: 1. Channel Type Induction furnace: A channel is used for heating by an induction furnace, which is present at the bottom of the main bath. It generally passes through the coil assembly and a steel core at the bottom. The molten metal is contained in a steel shell which is connected to the induction unit through a throat. This whole arrangement seems like a transformer in which the secondary component forms a molten metal loop in which the heat is generated which circulates the metal into the main wall creating a stirring action. This type of furnace is generally not preferred for the production of steel and usually, finds its applications in low melting alloys and are also preferred for manufacturing of cast iron. 2. Coreless Induction furnace: It consists of a refractory lined vessel which is also known as a crucible. It generally consists of a power supply unit, transformer, converter, capacitor bank and charging unit for the device. For maintaining the temperature of the coil, a cooling system is also provided. The furnace is normally fed from three-phase, high voltage power source. When current passes through the coil, then it results in the formation of the electromagnetic field which sets up eddy current in the material which gets heated up and results in melting of the material. For that purpose, an inverter is used to increase the frequency from its normal value to a much higher value. A capacitor bank is provided for reactive power compensation in the unit. To obtain a constant load regulation, the inverter current makes use of the resonant frequency. The coil of this type of furnace contains a heavy duty and high conductivity copper tubes wounded into a helical coil. This coil is enclosed in a steel shell and to prevent its heating, magnetic shielding is used and in most of the cases, it is water-cooled. To obtain the desired level of voltage and frequency used for heating purposes, a power circuit is used which vary these quantities in accordance with the manufacturing requirements. As the frequency increases, the power of furnace increases with less turbulence. This type of furnace is exclusively used for melting of point alloys and all grades of iron, steel, and many other non-ferrous alloys. This is also suitable for 50

6 allotting and re-melting purposes due to its vibrant temperature control and controlled melting circulation within the bath. 3.2 OPERATION OF INDUCTION FURNACE The high rating induction furnace being widely used in the industry should follow prescribed power quality standards so that the disturbance to the nearby industries should be as minimum as possible. Many of the furnace operators are using capacitor banks to improve the power factor thereby reducing their electricity bill. However, various harmonic sources in the steel unit may inter-relate with these capacitor banks if they are not suitably installed. For steel manufacturing, the scrap is melted by the furnace using high-frequency magnetic field. The poorer the quality of scrap, higher is the energy needed to give the scrap a proper shape. So, anticipatory methods and suitable configurations are vital in order to limit this harmonic distortion level. During the production of mild steel through induction furnace, certain precautionary measures in terms of chemical analysis of the input materials are required in order to furnish the end product as per our requirement. First, the input materials are being charged up to 50% and then analysis of bath sample is done for its further chemical composition by addition of certain metallics. The iron content of the charged material is enhanced if there is a high percentage of Sulphur, Carbon, and Phosphorus in the bath sample. When about 80% of the melting process is completed, bath sample is considered to be in the final stage of further analyzing the sample. The pig iron is added to the sample if there is low carbon content in the charged material. The oxidation of Silicon and Manganese is done using sponge iron, which also helps in removing Sulphur and Phosphorous in the charged material and hence the traceable elements for the production of steel can be controlled using sponge iron. The ultimate output of an induction furnace is liquid steel (also known as liquid gold) at a very high temperature whose quality depends upon the steel conformation and type of raw materials used for the process. If this temperature is not controlled, then it may cause erosion in the refractory lining and will consume more power than normal. The power consumption is also more if the liquid steel is overheated unnecessarily. The superheat temperature is 51

7 decided based upon the steel composition and temperature loss while transferring the liquid steel to a molding machine. Thus, to minimize the power loss due to overheating, a potentiometer based control system is usually adopted in order to save the energy. In order to extract the maximum liquid steel, a control mechanism is provided which tilt the induction furnace to an angle of 90. The thermal efficiency of an induction furnace generally ranges from 58% to 76%, but in some cases where the power losses are minimum and a better control system is installed, it may reach up to 80%. For example, for the melting of iron using induction furnace, the amount of energy required is around 600 kwh practically but around 350 kwh theoretically. This large difference between the two limits is due to the following factors: 1. Refractory losses 2. Eddy current losses 3. Electrical bus-bar losses in the supply system 4. Cooling water losses 5. Operational losses during liquid steel transfer The block diagram of a typical induction furnace used in this industrial unit is shown in Figure 3.2. The frequency selector switch makes this furnace distinctive from the other devices. The frequency can be varied from 50 Hz up to several khz using this switch. This is being done in order to ease the melting process. The greater the frequency, lesser will be the melting time. The metal loading describes the type of metal scrap to be used for the manufacturing process. The frequency variation is possible by using converter circuit in the system. The metal loading describes the type of metal scrap to be used for the manufacturing process. For effective heating of the scrap in the furnace, the impedance matching network transforms the low current and high voltage of the inverter to high current and low voltage. The power selection unit is used to calculate power in terms of kwh/ton which is different for various types of metals. It is calculated based on the metal type, operating temperature, size, design specifications etc. 52

8 Frequency Selection Unit AC INPUT Metal Loading Power Unit Power Selection Unit Induction Furnace Coil Impedance Matching Fig 3.2: Layout of Induction furnace Some of the actual onsite snap-shots of a steel plant involving induction furnace are shown in Appendix B. For the production of Ingots (Cast Iron), the power consumption required is 550 kwh/ton. This value varies for different metals and the quality of scrap used for the production of Ingots. The melting time matters a lot as, during this period, induction furnace draws heavy current from the supply mains and inject harmonics into the system. The melting time is calculated as follows: Input Power available = 600 kw Power consumption for Ingots = 550 kwh/ton Thus, Melting Time = x 1 hour = 0.91 hour = 54.6 min Thus, for melting 1 ton of Ingots, it requires 54.6 min. The melting time varies from metal to metal. Table 3.4 shows the power consumption of different metals and alloys. 53

9 Table 3.4: Power consumption of different metals S. No. Scrap Type Power Consumption (kwh/ton) 1. Steel Solid Aluminium Light Aluminium Cast Iron Spheroidal Graphite Iron Mild/ Stainless Steel Depending upon the operating frequency, an induction furnace can be classified into three categories as shown in Table 3.5. Earlier low-frequency models of induction furnaces were used which were directly connected to the 50 Hz utility supply. But because of its limitations in control and lower efficiency, medium frequency induction furnace systems have seen a growth in their usage; whereas high-frequency systems have limited special purpose use only. The medium frequency systems operate at frequencies ranging from 150 to 1000 Hz. But despite its advantages, the medium frequency induction furnaces develop significant problems in the power interface. For example, both fixed and variable frequency harmonics are generated by the furnace leading to huge harmonic distortion. Table 3.5: Classification of Induction furnaces S. No. Type of induction furnace Frequency Range (in Hz) 1. Low Power frequency Less than Medium Power frequency High Power frequency More than

10 3.3 FURNACE CIRCUIT: CONSTRUCTION AND WORKING An induction furnace is similar to a transformer which consists of the metal to be melted in the form of charge inside a ceramic crucible which helps to retain the heat produced and also the electromagnetic field produced because of the alternating current in the coil as shown in Figure 3.3. Fig. 3.3: Structure and components of induction furnace The crucible is further lined with a refractory lining and a cooling jacket. The function of both these is to separate the hot metal from the primary coil. After the refractory lining, a power coil (primary coil) is there through which an alternating current is passed using a power supply source. This results in reversing magnetic field which links with the metal and produces eddy currents and circulates current in the metal by the induction principle. The eddy currents generate heat by joule s heating while in ferromagnetic materials hysteresis may also be one additional factor for heat generation. Once melted, the eddy currents cause a dynamic stirring of the melt, assuring good mixing. Also since the frequency has an effect on depth of penetration of current in metal, medium frequency power supply is used. Laminated iron forms magnetic yoke that forms the outer boundary of the furnace. 55

11 3.4 POWER SUPPLY SYSTEM OF INDUCTION FURNACE An induction furnace is a highly inefficient load since its power factor ranges from 0.10 to 0.20 which is very low. This is mainly due to high leakage inductance because of the large inductive components involved in its circuitry. Induction furnace typically operates in a wide frequency range of 100 Hz to several khz. The power supply system functions two purposes: to provide power to the primary coil and the other to control the melting of the metal. Since the depth of penetration of current in the metal depends on the frequency of supply, the supply frequency has to be kept in a certain range of khz which was found to be versatile and most efficient frequency range, so the input AC supply at 50 Hz has to be varied which is possible only by a combination of two steps of rectification plus inversion as shown in Figure Φ, 50 Hz AC Supply Step-down Transformer Rectifier Section AC-DC DC Choke Coil Parallel Plate Coupling Capacitor Inverter Section DC-AC (1-Φ) Induction Furnace Fig. 3.4: Block diagram of an Induction furnace In order to achieve the controllable resonance, the coupling capacitor is required after the inverter. Under working conditions, the furnace act with this capacitor bank in resonant frequency whose value varies with the scrap quality. The self-inductance of the furnace coil changes at regular intervals and hence inverter is used to control the frequency under resonant conditions. During initial conditions, the frequency is low and its value increases when the scrap starts melting. For efficient melting operation, controlled current and frequency supply are needed and there are two types of solid state power supplies used for medium frequency induction furnace: 56

12 DC Capacitor 1. Voltage Fed power supply 2. Current Fed power supply Voltage Fed Power Supply It is also known as series furnace resonant circuit. The voltage-fed power supply operates with full coil current but low voltage. It has good controllability and fewer losses because reactive power is stored in the capacitors unlike a current-fed power supply circuit, in which the reactive power is stored in the inductors. Rectifier Filter Inverter Three-Phase Supply Series Tuning Capacitor Induction Furnace Fig. 3.5: Voltage fed power supply circuit It makes use of an input diode rectifier and a parallel connected DC capacitor to produce and store DC, respectively. This DC capacitor delivers or engrosses surplus energy for starting and stopping the inverter, which in turn controls melting power by its commutation frequency. The output inverter regulates the current to the furnace and the series tuning capacitor. Such type of supply is capable of handling high currents and is shown in Figure Current Fed Power Supply It is also known as parallel furnace resonant circuit. This supply involves a phasecontrolled rectifier and filter circuit along with an inverter to obtain desired frequency and voltage output to control the melting power. Rectifier provides controlled voltage to the DC link and two series inductors provide energy storage as well as filtering. Also, a starter 57

13 Parallel Tuning Capacitor Induction Furnace circuit and a crowbar circuit are needed to energize the inductors and to discharge them on completion of melting process, respectively. Rectifier Filter Inverter Three-Phase Supply Fig. 3.6: Current fed power supply circuit The inverter feeds a square wave current to the parallel resonant circuit (capacitor bank and heating coil). In the current-fed inverter, both the parallel tuning capacitor and the furnace coil are connected as diagonal to the inverter configuration which allows the reactive part of the furnace coil current to sidestep the solid state switching components of the inverter. However, the full voltage is applied to the inverter which may be less than or more than the rectifier DC voltage. Hence, both the rectifier and inverter sections are decoupled by the reactors who acts as a filter circuit and by means of which a constant DC voltage is applied to the inverter section. Both the furnace power and frequency is controlled by inverter section. The main advantage of the current-fed inverter is that it controls the furnace coil current smoothly and there is no need to control the DC voltage on rectifier side. Thus, the input power factor on the supply side is somehow constant and no line filters are required on the supply side. The whole configuration is shown in Figure 3.6. The current fed power supply has a less control over the furnace current than the voltage-fed supply because its inverter gets only 10% of the furnace resonant current. Rest of the reactive component of the furnace current is bypassed via parallel tuning capacitor. 58

14 Due to the use of phase-controlled rectifier in the current-fed power supply, voltage notching problem arises. Here, each phase rectifier device turns on before the other phase device has commutated off causing momentarily L-L fault leading to line voltage notching. Notching can lead to operating problems in equipment like tripping of other power supplies. Table 3.6 shows the comparison of both series and parallel resonant circuits. Table 3.6: Comparison of series and parallel resonant circuits S. No. Parameter Series Resonant Circuit Parallel Resonant Circuit 1. Generation of harmonics Less High 2. Power factor (Depends upon phase control) 3. Inter-harmonics generation No Yes 4. Line Voltage Notching No Yes (Due to phase control) 3.5 VARIOUS POWER QUALITY PROBLEMS DUE TO INDUCTION FURNACE Although induction furnace offer a lot of advantages and yield higher production without emission of harmful gases, the three-phase power supplies used for the furnace incorporate power electronic devices like phase controlled rectifiers and inverters which results in distortion and fluctuation of the supply side current because of generation of harmonics both for voltage fed and current fed power supplies. These power quality problems are a cause of worry because it affects the quality of current and voltage at the PCC where other consumers are also connected Current Harmonics Generation Most of the furnaces employ multi-bridge rectifiers together with phase-shift transformers in their power supply circuit. Depending on the number of three-phase bridge rectifiers used, the number of output pulses changes. As the number of rectifier bridges 59

15 increases, it adds a number of steps in the line current waveform, thereby making it more sinusoidal. For example, six pulse rectifiers consist of one three phase bridge which is made up of 3 arms each consisting of two thyristors. This results in six output pulses. The twelve pulse rectifiers consist of two three-phase bridges to which three phase supply is given using phase shift transformers. The current harmonics produced by different pulse rectifiers are shown in Table 3.7. Individual harmonic order (as a % of fundamental) Table 3.7: Current harmonic generation by different rectifiers Rectifier 6-pulse (Ideal) Rectifier 6-pulse (Practical) Rectifier 12-pulse (Ideal) Rectifier 12-pulse (Practical) 5 th th th th th th rd th TOTAL 29% 21.5% 15.5% 10.5% More is the number of output pulses, lesser is the number of ripples in the output current and more the waveform is towards DC. Also, it adds a number of steps to input source current making it more sinusoidal and thus reducing THD. For an ideal square waveform, the rectifier should have harmonics that are integral multiples of a number of output pulses of ±1. For example, ideally 6 pulse rectifier is ought to have harmonics of order 5, 7, 11, 13 and so on, but due to distortion, it consists of all other harmonics in a practical scenario. 60

16 3.5.2 Voltage Fluctuations The induction furnace operates at a frequency of around 150 to 3000 Hz. At such high frequencies, a different type of distortion arises because of harmonics generated at a frequency which fluctuates with the furnace resonant circuit. This problem was given a new name of inter-harmonics. These harmonics pose a difficult situation since these harmonics are produced at a frequency which is not an integral multiple of the frequency of the supply making it hard to filter out these frequencies. The impacts of inter-harmonics as seen on a power system are flickering lights and computer screens, tripping of power electronic equipment and heating in the power system. The inter-harmonics in an induction furnace are generated when a power supply in the inverter is working at a frequency of let s say f1, the frequency that is reflected back to the rectifier is 2f1 and this frequency adds up with the line frequency resulting in interharmonics in the line currents Reduction in Power Factor The ideal power factor for any system is a unity which signifies lesser losses and maximum utilization of input power by the load. As the power factor deviates from unity, the quality of the system is said to deteriorate because of the harmonics and distortions arising out of power electronics devices (rectifiers and inverters) used in the power supply of an induction furnace. The power factor improves as the number of output pulses increases. Hence, the power factor for 12 pulse converter is better than 6 pulse converter. Since the current fed converter employs phase controlled converters, hence they have lower power factor as compared to voltage fed converters because power factor decreases with the increase in delay angle. The power factor of the three types is shown in Table 3.8. Table 3.8: Power factor for different converter levels S.No. Type of Converter Power factor 1. 6-Pulse Pulse Pulse

17 3.6 MEASUREMENT OF INDUCTION FURNACE PARAMETERS USING POWER QUALITY ANALYZER In distribution power systems, an in-depth analysis of various power quality issues is of utmost importance. Based upon this study, various solutions to these issues are then further investigated. These solutions are only effective if accurate and relevant data of a designated location is measured in a suitable time frame. Many of the unidentified power quality problems are due to lack of monitoring of power quality problems using a suitable equipment. For this, exact location of the measurement along with its suitable time frame is of paramount importance. Then, further steps are taken to extract meaningful data at particular instances out of the total time frame. After this, analysis and prioritization of this meaningful data are done to obtain various solutions to this particular problem. It is desirable to have more than one solution for this problem so that a comparison can be done between these to obtain a best possible solution. Power quality parameters are measured by various measurement and recording equipment. The most commonly used and advanced equipment out of them is power quality analyzer. It is used to measure real-time observations and store them in its memory. This data can be later transferred to a computer for analysis using a compatible software. Some of the power quality analyzers are installed forever in the distribution systems, whereas some are only required during any fault or troubleshooting. These analyzers are having the capability to store data for a short time or even for some days also. Various types of power quality issues i.e. voltage interruption (both short and long period), voltage sag, voltage swell, THD of both voltage and current, energy efficiency etc. are common parameters which are measured by this instrument. Here, it is used for measuring the current profile and THD of an induction furnace over the period of around two hours which also includes the furnace loading and unloading of the scrap material. The average current variation over this whole period of two hours is shown in Figure

18 Fig. 3.7: Current variation for different events When the furnace is fully loaded, it draws maximum current from the supply mains and also THD is maximum. During the initial scrap melting stage, current drawn by the furnace is less and it keeps on increasing as scrap loading occurs. Variation of THD over a period of two hours is shown in Figure 3.8. Fig. 3.8: THD variation during an Induction furnace operation The THD value is maximum when the quality of scrap is very poor due to which a large disturbance to the supply network occurs. Figure 3.9 shows the variation in induction 63

19 furnace current on maximum, minimum, and average basis during its operation for two hours. Fig. 3.9: Current variation during furnace operation for two hours The upper curve represents the maximum value of current drawn by the furnace during its operation, middle curve represents the average value of the current and lower curve represents the minimum value of the current drawn by the furnace. When the scrap is of poor quality, then the current reaches up to 1200 A, the lower value of the current drawn by the furnace is around 250 A and average value of the current drawn by the furnace is around 500 A. A huge disturbance to the grid occurs when the current is at its maximum peak level. All these results of the power quality analyzer are analyzed and based on these results, Simulink circuits are designed. The distortions of voltage and current THD are measured using FFT analysis. These THD distortions are mitigated by means of various custom power devices as discussed in Chapters