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  1. 1. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME TECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), pp. 01-13 © IAEME: www.iaeme.com/ijeet.asp Journal Impact Factor (2013): 5.5028 (Calculated by GISI) www.jifactor.com IJEET ©IAEME A COMPARATIVE STUDY OF DIODE AND THYRISTOR CONVERTERS USED IN THE ALUMINUM SMELTERS Prof. Sharwan Kumar Jhajharia Department of Electrical and Electronics Engineering, Manipal University Jaipur, Jaipur – 303007, India ABSTRACT This paper gives a comparative study of the two types of rectifiers used in the aluminum smelter at Hindalco, Renukoot, namely Diode and Thyristor rectifier under the parameters of harmonics, power factor, AC/DC conversion efficiency, investment and maintenance cost. Rectifiers are used widely in the AC/DC conversion in the aluminum smelters where direct current is needed. Different types of the rectifiers having different technologies with various advantages and disadvantages over each other. A well comparative analysis is done between the two types of rectifiers using the current harmonics power factor, AC/DC conversion efficiency, investment and maintenance cost data. The paper gives an overview of each of the comparative parameters so as to give which of the rectifier is better and to be preferred. Keywords: Harmonics, Power Factor, Rectifier, Converter. INTRODUCTION Alumina (Al2O3) is one of the most widespread metal compounds in nature and makes up about 8.8% of the earth’s crust. Alumina is transformed into Aluminum through electrolytic reduction. Aluminum is used across industries and across products in our daily lives due to its relative cost-effectiveness and its alloys are used extensively for adding strength and utility. Aluminum production is therefore of vital economic importance across the world. The single most important factor in Aluminum production is the cost of energy used in the process of converting Alumina to Aluminum. This energy, which accounts for nearly 35%-40% of the total cost of production of the metal, is primarily in the form of Direct Current (DC) power. With the primary sources of electricity across the world providing Alternating Current (AC) power, an efficient AC-DC converter becomes a critical component in the electrolytic reduction process of extracting non-ferrous materials in general and Aluminum in particular. With the growing 1
  2. 2. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME importance of Aluminum and other non ferrous industries, the requirement for efficient AC–DC num non-ferrous AC converters has increased multi-fold. fold. The Hall-Heroult process is the basis of all primary Aluminum smelting plants in the world. Heroult In this process, Alumina (Al2O3) is dissolved in an electrolytic bath of molten cryolite (Sodium Aluminum Fluoride) at operating temperatures ranging from 920˚C to 970˚C under the influence of 920˚C high intensity direct current. Compared to the production of other metals such as Copper and Zinc, Aluminum smelting is significantly more energy intensive. The specific energy consumption of DC g power in a typical Aluminum smelter ranges from 13000 KWH to 14500 KWH per tonne of metal produced. Across the world, more than 200 Aluminum smelters are currently producing mo molten Aluminum by consuming large amount of electrical energy from their captive power plants or grids. One such plant, which will be the main focus of research of this paper, is owned by Hindalco Industries Ltd in Renukoot, UP, India. This plant currently has an annual production capacity of 410,000 tonnes of Aluminum and 98,000 tonnes rolled products, 91,000 TPM of wire rods and 33,000 TPA extrusions. The Hindalco Aluminum smelter consists of 11 pot lines with each line containing 200 pots connected in series. As the smelting process is continuous, a smelter cannot series. easily be stopped and restarted. Any interruption in production due to failure of power supply for more than four hours causes the metal in the pots to solidify. This then leads to the need for an a expensive rebuilding process. AC-DC convertor technology was based in Mercury based converters till late 1950. In 1960, DC the first diode rectifier was used in Aluminum smelters and ten years later thyristor rectifiers were in operation. These two technologies have been competing with each other over the last multiple decades. gies At present, in Aluminum smelters the two types of AC-DC converters most commonly used are: AC DC 1) Uncontrolled rectifiers or Diode rectifiers and 2) Controlled rectifiers or Thyristor rectifi rectifiers The acceptance of these rectification technologies across Aluminum smelters is based on the combination of several factors that are taken into consideration. These include – system reliability, investment cost, efficiency, current harmonic distortion, current ripple, DC voltage, current regulation accuracy and the maintainability of the converters. Uncontrolled Rectifiers (Diode Rectifiers) Diode rectifiers are the simpler form of rectifiers and used as front end converters in DC power supplies. The circuit diagram of a diode rectifier is shown in Fig. 1 Fig 1 2
  3. 3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME In 12 pulse rectification the predominant harmonic components in the current wave form are 11th and 13th. In this case the currents are balanced and there is no neutral current problem. To mitigate the low order harmonic problems of harmonics, multipulse (such as 24 and 36) are used at large plants like Hindalco. Phase shifting transformers with the appropriate phase shift are used to achieve 24 pulse operations. The dual advantage of a higher number of pulses is to lower total harmonic distortion (THD) of AC mains current and ripple free DC current. Control of current & voltage for a Diode rectifier is achieved by changing the input voltage by following means as follows: 1) By using On Load Tap Changers (OLTCs) at the rectifier transformers on the primary side of the transformers a rough voltage control can be achieved. 2) The Saturable Control Method by introducing variable impedance into the circuit, ahead of the diode rectifier. Using this impedance, a smooth control is achieved in the range of 50 - 80 V DC. On Load Tap Changers and Saturable reactors are often used together to improve voltage control. Control rectifiers (Thyristor rectifiers) The Saturable Control Rectifiers or Thyristor rectifiers are capable of voltage regulation by means of gate control and the thyristor rectifier is controlled electronically. For a thyristor rectifiers, the fundamental component of current lags the respective phase voltage by triggering angle α with a displacement factor of cosα. Fig 2 shows the circuit diagram of a fully controlled rectifier. Fig 2 In Thyristor Rectifiers, the firing angle helps to control the voltage and current. When a thyristor rectifier is controlled through a small delay firing angle it performs like a diode rectifier using saturable reactor control. It has fast and smooth output control to the order of milliseconds. Due to the use of these semiconductors AC – DC converters in Aluminum smelters over many years, several serious issues related to the power system and efficiency of conversion have become a prominent feature of Aluminum smelters. These include the generation of current harmonics, voltage distortion in controlled rectifiers, poor power factor, voltage dips, issues relating to reliability, maintainability, efficiency and investment cost etc. Given the competing technologies available, it remains a complex task for the industry to objectively choose the appropriate converter technology for Aluminum smelters. Over the last two decades, both these technologies have competed for market share and mind share and have effectively been opted for based on supplier marketing and recommendations from maintenance and operation engineers. 3
  4. 4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME In this research a comparative study of diode and thyristor converters with an assessment of the advantages and disadvantages of each technology has been undertaken. The practical pros and cons of both technologies have been elaborated in great detail, specifically these 4 parameters, ACDC conversion efficiency, current harmonics, power factor, and AC voltage dips. This research is expected to be of immediate relevance to the Non Ferrous industry, particularly Aluminum Plants, to help them meet the challenges of objectively selecting the most efficient and suitable converter technology for their Aluminum smelters. For this research, I have selected 2 pot lines of HINDALCO smelter at Renukoot. Pot line number 7 is having Thyristor rectifier technology and pot line no. 9 is equipped with diode rectifier technology. Each parameter is discussed comprehensively to compare these two converter technologies. Current harmonics of pot line#7 and pot line9 Harmonics Ideally, voltage and current waveforms are perfect sinusoids. However, because of the increased popularity of electronic and other non-linear loads, these waveforms quite often become distorted. This deviation from a perfect sine wave can be represented by harmonics—sinusoidal components having a frequency that is an integral multiple of the fundamental frequency. Calculation of current harmonics 4
  5. 5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME Current harmonics Voltage Harmonics where φI1, φIk, φV1, φVk are the phase angles (against a reference time point) of the fundamental and harmonics, respectively, and k = 2,3,…w. TOTAL HARMONIC DISTORTION THD is the ratio of the RMS value of the total harmonic currents (no fundamental part of the waveform) and the RMS value of the fundamental portion, I1, of the waveform. This value is usually expressed as a percentage of the fundamental current. This is the theoretical way to measure the current harmonics. For the project the current harmonics were measured from the rectifier station of each current harmonics and THD (total harmonic distortion) is calculated. The data collected is formulated as: Harmonic No Dc Com. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Unit 9 diode based Current Harmonics ( % of Fundamental) 0.25 100 0.42 0.72 0.59 0.29 0.95 0.66 0.22 0.22 0.5 4.97 1.39 3.89 0.64 0.4 0.5 5 Unit 7 thyristor based Current Harmonics ( % of Fundamental) 0.29 100 0.52 2.15 0.88 1 0.16 0.36 0.52 0.76 0.21 6.08 0.25 5.34 0.56 0.2 0.51
  6. 6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 Thd 0.46 0.87 0.22 0.25 0.31 0.39 1.56 0.73 1.34 1.17 0.93 0.87 0.45 2.87 1.62 0.39 0.38 0.52 0.55 0.83 1.33 0.77 0.79 0.48 0.56 1.08 0.72 0.75 0.2 0.31 0.57 0.48 2.41 0.52 0.54 0.53 0.37 0.5 0.62 0.36 0.11 0.17 0.18 0.47 1.02 0.21 0.22 9.08 1.01 0.43 0.27 0.1 1.06 1.22 2.78 1.52 1.16 1.21 0.47 0.25 0.11 0.53 0.55 0.58 0.17 0.11 0.4 0.12 0.47 0.23 0.24 0.1 0.29 0.02 0.25 0.31 0.7 0.77 0.25 0.35 0.08 0.71 1.11 1.18 1.25 1 0.83 0.27 0.7 0.25 0.24 0.11 0.39 0.57 0.47 10.09 6
  7. 7. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME 6 potline#9 5 4 3 potline#9 2 1 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 potline#7 7 6 5 4 potline#7 3 2 1 0 2 4 6 8 101214161820222426283032343638404244464850525456586062 The data collected is represented on the graph. The maximum value in case of potline#9 is 4.97 and that in case of potline#7 is 6.08. Measurement of AC-DC Conversion Efficiency Rectification ratio Rectification ratio, also called efficiency of a converter is defined as the ratio of the dc output power Pdc to ac output power Pac. n = Pdc / Pac In this case Rd = forward rectifier resistance, then n = Pdc / Pac + I2or x Rd 7
  8. 8. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME Source: The data of efficiency from 1/6/2013 to 25/6/2013 was taken from the rectifier station control. The average efficiency of the two pot lines was calculated as nav( pot line#7) =99.2228 nav( pot line#9) =98.2144 The standard variation for the pot lines are Potline#7= 0.10521882 Potline#9= 0.14350029 The standard deviation is a mathematical value that shows how much the individual data vary from their average value. The above standard deviation value shows that the efficiency of pot line#7 vary less from the their average value and hence more of a constant value of efficiency than the pot line#9 Chart Title 100 99.5 99 Axis Title potline#7 98.5 potline#9 avg#9 98 avg#7 97.5 97 96.5 Power factor = Is1/ Is cosØ1 PF=DF.DPF 8
  9. 9. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME The distortion power factor describes how the harmonic distortion of a load current decreases the average power transferred to the load. Diode rectifiers have a power factor approximately between 0.92 and 0.95(mainly depending on the transformer impedance). The power factor of a thyristor rectifier further depends on the firing angle of the system. When operating at rated data, it is in the range of 0.86 to 0.89 at rated data. The power factor is further reduced. It is also common to lower the AC voltage to the rectifier by using a transformer tap changer. This improves the power factor in if a thyristor installation. Therefore, any filter network supplied with a diode or thyristor system is usually equipped with additi additional capacitors to achieve the desired power factor of the entire system. Source: The power factor was measured from control room for pot line#7 and potline#9 respectively. The data was taken on random intervals and it was found that the average power fac factors of pot lines are: Potline#7: p.f= 0.867 Potline#9: p.f= 0.926 If we assume that the power factors of both pot lines are unity then we can compare the approximate loses of the two pot lines. Now I α KVA (proportional) Loss α I2 For Potline#9 KVA = 1/0.926 =1.0799 Hence loss = (1.0799)2 =1.166 For Potline#7 KVA = 1/0.867 =1.1534 Hence loss = (1.1534)2 =1.330 We observe that if we keep the unity power factors for both the rectifiers then loses for thyristor is found to be more than the diode. Voltage dips and their effects One of the largest power quality problems today is voltage dips. Each year voltage dips cause disturbances in industries, resulting in large economical losses. Today these problems are often tries, solved as they are discovered, and very often the equipment selected for improved immunity are based on experience and opinions rather than a traceable technical analysis. As far as the two pot lines are concerned at HINDALCO, Renukoot the AC voltage dips s are no challenge for diode rectifiers but in case of thyristor rectifiers ride through possible for several hundred milli-seconds. Definition of voltage dips The definition of a voltage dip is not unambiguous, and often set only by two parameters, oltage depth/magnitude and duration. Different sources however present different alternatives how these parameters are interpreted. In this report, the voltage dip magnitude is ranged from 10% to 90% of t nominal voltage (which corresponds to 90% to 10% remaining voltage) and with a duration from 9
  10. 10. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME half a cycle to 1 min. The majority of voltage dips are 4-10 cycles long and with a remaining voltage of 85-90% of the nominal voltage. Investment and maintenance cost The diode and thyristor rectifier used in pot line #9 and 7 respectively have difference in investment cost and maintenance cost which depends on other factors . In the potline#9 that is diode based rectifier need regulating transformer and saturable reactors; now the cost of regulating transformer depends on the KVA rating and maintenance for OLTC is required. Whereas in the potline#7 that is thyristor based rectifier needs much of capacitor filters which add up to the cost. Then there are other losses which add up to the cost like cable losses, cost penalty for harmonic related losses. As the setups of the two pot lines are concerned they have been installed with different technologies which have different cost investment and maintenance. A rectifier system is not a mass produced product. Each system is designed for the specific requirement of its application, Different requirements and specifications per project makes a cost comparison difficult. One method to arrive at a representative cost comparison is achievable by averaging the costs for typical projects with typical industry ratings. The summary result is as shown below Technology Diode rectifier Thyristor rectifier Investment cost 115% (*) 100% (*) Not valid if thyristor needs harmonic filters of higher capacity and if not operated at rated data. Researches show that the thyristor technology is the most cost-effective solution from an investment point of view. The chart will change somewhat if energy losses are included. IDENTIFICATION OF MAJOR CAUSES AND EFFECTS Brainstorming and Fishbone Analysis A few brainstorming session was held to discuss and layout all the probable factors that can help in identification of a better suited rectifier system for the conversion for AC to DC for the potlines 7 and 9. All the factors have been identified and classified in the table below in the categories of Man, Material, and Machine & Method etc. S.N. 1 Factors THD and Losses Contribution Material 2 Control Man 3 Investment & Maintenance Cost Material 4 AC-DC Conversion Efficiency Machine 5 Voltage Dips Machine/Man/Method 6 Restart Time Machine 10
  11. 11. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME FISH BONE DIAGRAM Material Losses-THD & Due to power factor variation Investment Cost and Maintenance Cost Man Control Repairing time THYRISTOR/DIODE Voltage Dips & Restart Time Rectifiers AC-DC conversion Efficiency Machine Method Rectification Now we will discuss how each 4M factors are having their own share of : Material 1. THD : Total Harmonic distortions 2. AC-DC conversion efficiency 3. Power factor 4. Investment & Maintenance Cost- This is the factor that plays the most important role in the selection of the rectifiers, as diode based rectifiers have a high investment cost as well as maintenance cost. Man 1. Control – As experienced by the employees, the controlling of thyristor unit is much better than diode based rectifier Unit Machine 1. Voltage Dips: This factor contributes towards thyristor rectifier. 2. Restart Time: During a shutdown for a pot line there is a limit of 4 hrs before the electrolyte starts freezing. With the thyristor unit it is easy to restart as it just requires an impulse to start whereas with diode rectifiers, there is a need to increase the voltage to a specific level, which takes time. 3. Ac- Dc conversion efficiency: Thyristor has a better efficiency. Method 1. Rectification: Different types of rectification techniques like 12-pulse bridge rectifier are used in both pot lines. 11
  12. 12. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME Research methodology & Data Collection 1. Area of Study: Thyristor and Diode based rectifier units and control rooms of HINDALCOsmelter at Renukoot. 2. Sources of Data Collection: Measuring the values of parameters like power-factor, AC-DC conversion efficiency, and THDs at HINDALCO Aluminum smelter at Renukoot. 3. Method of Data analyses: In the given research, method of Data analyses is through comparing it with the values obtained from various E –Books and practical application of the principle of Fishbone and Brainstorming. CONCLUSION The purpose of this research was to understand nature and characteristics of the two types of rectifiers used in pot-line #7&9 at the Aluminum Plant. Through the study we had the inference as Thyristor based rectifiers have better efficiency than that of diode based rectifiers. But Thyristor rectifier systems’ key advantages, developments and disadvantages may be summarized as follows. Reliability/Availability Maintenance Efficiency Investment cost Advantages Diode Very high, but mechanical parts involved. Required for OLTC Lower than Thyristor system as no regulating transformer reactors are required About 15% higher (not valid if Thyristor needs harmonic filters ) Synchronizing voltage Developments Diode Not needed Potline trip No challenge AC voltage dips No challenge Harmonics Power factor Capacity of PFC unit to get a power factor > 0.95 Disadvantages Diode Typical order of (mostly) 24pulse systems. Approximately 0.92 to 0.95 at rated data. Approximately 30% of nominal rectifier rating. 12 Thyristor Very high Easy Higher than a diode system lower Thyristor Taken from transformer secondary possible because of PLL in AC800PEC, no extra VT required. Freewheeling implemented in AC800PEC. Ride through possible for several hundred milli-seconds, not recommended for parallel operation of rectifiers. Thyristor About 30% higher than with diodes. Approximately 0.86 to 0.89 at rated data. Normally about 30% higher.
  13. 13. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME BIBLIOGRAPHY 1. electrical-info.com 2. vlab.ee.nus.edu.sg 3. www.researchgate.net 4. www.engr.siu.edu 5. www.allaboutcircuits.com 6. www.yokogawa.com 7. www.copperinfo.co.uk 8. www.nptel.iitm.ac.in 9. www.power-mag.com 10. Anuradha Tomar and Dr. Yog Raj Sood, “All About Harmonics in Non-Linear PWM AC Drives”, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 1, 2012, pp. 138 - 144, ISSN Print: 0976-6545, ISSN Online: 0976-6553. 11. Lopamudra Mitra and Dr C.K.Pangrahi, “Power Factor Improvement using Active Power Factor Correction Methods”, International Journal of Electrical Engineering & Technology (IJEET), Volume 1, Issue 1, 2010, pp. 32 - 46, ISSN Print: 0976-6545, ISSN Online: 09766553. 12. Tanay Rastogi, Mohd. Tabish Siddiqui Prof. R.Sudha and Prof. K. Govardhan, “Analysis of THYRISTOR Based HVDC Transmission System”, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 2, 2012, pp. 29 - 38, ISSN Print: 0976-6545, ISSN Online: 0976-6553. 13