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					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)
                                                                             IJEET
Volume 4, Issue 6, November - December (2013), pp. 01-13
© IAEME: www.iaeme.com/ijeet.asp                                           ©IAEME
Journal Impact Factor (2013): 5.5028 (Calculated by GISI)
www.jifactor.com




  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


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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME

                        num              non-ferrous
importance of Aluminum and other non ferrous industries, the requirement for efficient AC–DC    AC
                                 fold.
converters has increased multi-fold.
                   Heroult
         The Hall-Heroult process is the basis of all primary Aluminum smelting plants in the world.
In this process, Alumina (Al2O3) is dissolved in an electrolytic bath of molten cryolite (Sodium
                                                               920˚C
Aluminum Fluoride) at operating temperatures ranging from 920˚C to 970˚C under the influence of
high intensity direct current. Compared to the production of other metals such as Copper and Zinc,
                     g
Aluminum smelting is significantly more energy intensive. The specific energy consumption of DC
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
                                      series.
containing 200 pots connected in series. As the smelting process is continuous, a smelter cannot
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.
             DC
         AC-DC convertor technology was based in Mercury based converters till late 1950. In 1960,
the first diode rectifier was used in Aluminum smelters and ten years later thyristor rectifiers were in
operation.
                             gies
         These two technologies have been competing with each other over the last multiple decades.
                                                       AC DC
At present, in Aluminum smelters the two types of AC-DC converters most commonly used are:
    1) Uncontrolled rectifiers or Diode rectifiers and
                                            rectifiers
    2) Controlled rectifiers or Thyristor rectifi
    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
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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.


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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, AC-
DC 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




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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:

                                 Unit 9 diode based        Unit 7 thyristor based
                  Harmonic     Current Harmonics ( %      Current Harmonics ( % of
                     No           of Fundamental)              Fundamental)
                  Dc Com.                0.25                       0.29
                     1                   100                         100
                     2                   0.42                       0.52
                     3                   0.72                       2.15
                     4                   0.59                       0.88
                     5                   0.29                         1
                     6                   0.95                       0.16
                     7                   0.66                       0.36
                     8                   0.22                       0.52
                     9                   0.22                       0.76
                     10                   0.5                       0.21
                     11                  4.97                       6.08
                     12                  1.39                       0.25
                     13                  3.89                       5.34
                     14                  0.64                       0.56
                     15                   0.4                        0.2
                     16                   0.5                       0.51


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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               0.46                       1.01
                     18               0.87                       0.43
                     19               0.22                       0.27
                     20               0.25                        0.1
                     21               0.31                       1.06
                     22               0.39                       1.22
                     23               1.56                       2.78
                     24               0.73                       1.52
                     25               1.34                       1.16
                     26               1.17                       1.21
                     27               0.93                       0.47
                     28               0.87                       0.25
                     29               0.45                       0.11
                     30               2.87                       0.53
                     31               1.62                       0.55
                     32               0.39                       0.58
                     33               0.38                       0.17
                     34               0.52                       0.11
                     35               0.55                        0.4
                     36               0.83                       0.12
                     37               1.33                       0.47
                     38               0.77                       0.23
                     39               0.79                       0.24
                     40               0.48                        0.1
                     41               0.56                       0.29
                     42               1.08                       0.02
                     43               0.72                       0.25
                     44               0.75                       0.31
                     45                0.2                        0.7
                     46               0.31                       0.77
                     47               0.57                       0.25
                     48               0.48                       0.35
                     49               2.41                       0.08
                     50               0.52                       0.71
                     51               0.54                       1.11
                     52               0.53                       1.18
                     53               0.37                       1.25
                     54                0.5                         1
                     55               0.62                       0.83
                     56               0.36                       0.27
                     57               0.11                        0.7
                     58               0.17                       0.25
                     59               0.18                       0.24
                     60               0.47                       0.11
                     61               1.02                       0.39
                     62               0.21                       0.57
                     63               0.22                       0.47
                    Thd               9.08                      10.09


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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

        3                                                                                              potline#7

        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

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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
                                                                              potline#7
           Axis Title




                        98.5                                                  potline#9

                         98                                                   avg#9
                                                                              avg#7
                        97.5

                         97

                        96.5


Power factor




                          = Is1/ Is cosØ1




         PF=DF.DPF



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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
                                                                                   additional capacitors
filter network supplied with a diode or thyristor system is usually equipped with additi
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.
                                                                               factors of pot
The data was taken on random intervals and it was found that the average power fac
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                                    For Potline#7
       KVA = 1/0.926 =1.0799                   KVA = 1/0.867 =1.1534
Hence loss = (1.0799)2 =1.166                 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
                      tries,
disturbances in industries, resulting in large economical losses. Today these problems are often
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.
               s
       As far as the two pot lines are concerned at HINDALCO, Renukoot the AC voltage dips
are no challenge for diode rectifiers but in case of thyristor rectifiers ride through possible for
several hundred milli-seconds.

Definition of voltage dips
                             oltage
       The definition of a voltage dip is not unambiguous, and often set only by two parameters,
depth/magnitude and duration. Different sources however present different alternatives how these
                                                                                         t
parameters are interpreted. In this report, the voltage dip magnitude is ranged from 10% to 90% of
nominal voltage (which corresponds to 90% to 10% remaining voltage) and with a duration from
                                                   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

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 pot-
lines 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.                  Factors                                     Contribution
       1       THD and Losses                                             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




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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                         Man
  Losses-THD & Due
 to power factor
                                           Control
 variation

 Investment Cost and                       Repairing time
 Maintenance Cost
                                                                                 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.

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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 HINDALCO-
      smelter 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.

                                             Advantages
                                                Diode                           Thyristor
 Reliability/Availability          Very high, but mechanical        Very high
                                   parts involved.
 Maintenance                       Required for OLTC                Easy
 Efficiency                        Lower than Thyristor system      Higher than a diode system
                                   as no regulating transformer
                                   reactors are required
 Investment cost                   About 15% higher (not valid if   lower
                                   Thyristor needs harmonic
                                   filters )

                                          Developments
                                              Diode                           Thyristor
 Synchronizing voltage             Not needed                       Taken from transformer
                                                                    secondary possible because of
                                                                    PLL in AC800PEC, no extra
                                                                    VT required.
 Potline trip                      No challenge                     Freewheeling implemented in
                                                                    AC800PEC.
 AC voltage dips                   No challenge                     Ride through possible for
                                                                    several hundred milli-seconds,
                                                                    not recommended for parallel
                                                                    operation of rectifiers.

                                           Disadvantages
                                               Diode                           Thyristor
 Harmonics                         Typical order of (mostly) 24-    About 30% higher than with
                                   pulse systems.                   diodes.
 Power factor                      Approximately 0.92 to 0.95 at    Approximately 0.86 to 0.89 at
                                   rated data.                      rated data.
 Capacity of PFC unit to get a     Approximately 30% of             Normally about 30% higher.
 power factor > 0.95               nominal rectifier rating.
                                                  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

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
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       (IJEET), Volume 1, Issue 1, 2010, pp. 32 - 46, ISSN Print: 0976-6545, ISSN Online: 0976-
       6553.
   12. Tanay Rastogi, Mohd. Tabish Siddiqui Prof. R.Sudha and Prof. K. Govardhan, “Analysis of
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       0976-6545, ISSN Online: 0976-6553.




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