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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),

ISSN 0976 – 6553(Online) Volume 5, Issue 4, April (2014), pp. 61-83 © IAEME

61

A CASE STUDY AT HINDALCO ALUMINIUM SMELTERS TO CHOOSE

THE RIGHT CONVERTER TECHNOLOGY FOR THE FUTURE FROM THE

WIDELY USED DIODE CONVERTER TECHNOLOGY AND THYRISTOR

CONVERTER TECHNOLOGY IN THE ALUMINIUM SMELTERS

Prof. Sharwan Kumar Jhajharia

Department of Electrical and Electronics Engineering, Manipal University Jaipur,

Jaipur – 303007, India

ABSTRACT

In aluminium smelters two types of rectification technologies are being widely used i,e diode

rectifier and thyristor rectifier. There is a varying degree of acceptability for aluminium industries

due to power quality, stringent reliability requirement and maintainability of the converters.At

present, the selection of a particular technology is quite a complex job to the industry. In spite of

having similar DC current and DC voltage requirements, the rectification technology differs from

industry to industry.At present throughout the world,aluminium industries are using both type of

rectification technologies.In this research paper solution has been presented to select the better

converter technology for aluminium smelters by the actual measurements of the parameters of the

operational converters by maintaining the same DC kA at Hindalco industries, Renukoot and a

detailed analysis has been carried out with the results to show that the thyristor converter technology

is the most energy efficient and cost effective solution from the operation and investment point of

view in comparison to diode converter technology.

Keywords: Rectifier, Thyristor, Smelter, Reliability, Maintainability.

INTRODUCTION

Across the world more than 240 aluminium smelters are producing primary aluminium The

process of producing aluminium comprises of two major steps: First is refining of bauxite to alumina

and second is smelting of alumina to aluminium.Smelting process involves electrolysis and this

electrolysis is a DC power based process.For electrolysis the requirement of DC power is essential

INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &

TECHNOLOGY (IJEET)

ISSN 0976 – 6545(Print)

ISSN 0976 – 6553(Online)

Volume 5, Issue 4, April (2014), pp. 61-83

© IAEME: www.iaeme.com/ijeet.asp

Journal Impact Factor (2014): 6.8310 (Calculated by GISI)

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IJEET

© I A E M E



62

and it is obtained by AC-DC converters.Today worldwide two types of converter technologies are in

use for the aluminium smelting process. In Hindalco industry both type of converter technologies are

being used i,e diode rectifier and thyristor rectifier technologies.

As per the available statics from the beginning of 1960’s diode rectifiers are in use but from

1980’s thyristor rectifiers were marketed by the suppliers of converters such as M/s ABB and M/s

AREVA (Alstom), BHEL,etc. that thyristor converter technology is more energy efficient than

diode rectifiers as it eliminates the use of regulating transformer with OLTC and transductors and

current remains constant in the smelter with the use of thyristor rectifier in comparison to diode

rectifiers.However after some time many industries again started favouring the technology of diode

converters and today it is observed that in last two decades there is a mixed use of both the

technologies in the different parts of the world.In India itself the new plants such as Aditya

Aluminium and Mahan Aluminium of Hindalco industries and Vedanta Aluminium preferred diode

converters against thyristor converters.

To select the right converter technology for aluminium smelters is still a complex job for the

aluminium industry. In this research paper the detailed analysis has been done on the basis of current

harmonics, power factor, energy efficiency, reliability and maintainability of both the technologies to

decide the best converter technology for upcomingsmelters.

The case study was carried out at Hindalco industries, where both type of converters are in operation

in the smelter with the same current rating. These converters are in operation at pot lines 7 & 9 at

132 KV levels. Potlines #7 has been in operation with thyristor converter technology since 1995

whereas potline #9 has been in operation with diode converter technology since 2001.

The measurements were carried out in order to assess the level of harmonics and power factor

without filter banks and with detuned filter banks at the respective feeder’s end at 132 KV level as

well as the load end to measure the harmonics generated by the AC-DC converters and its effect on

the network with and without harmonic filters by maintaining the load conditions same for both the

pot lines during the course of measurements of parameters.

INSTRUMENT DESCRIPTION

Instrument used for field measurements is METREL Power Harmonics Analyser (MI 2092),

Slovenia, Europe make.It is one of the most sophisticated and accurate instrument for the purpose.

The instrument is designed to ascertain comprehensive real time monitoring, recording and

analysis of three phase power system with wide range of functions.

• True rms Voltage

• True rms Current

• Power

• Power Factor

• Power Scope

• Harmonic Analysis



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The instrument measures Harmonic distortion analysis up to 63rd

harmonic,both on line and

on recorded data.Both power factor and current harmonics are measured simultaneously.The

equipment measure the system parameter continuously but down loads the desired data only at pre-

specified intervals to a personal computer connected to the Harmonic Analyser.

For the measurements, the inputs to the instrument were taken from the Bus Potential

Transformer and the respective Feeder Current Transformer.

SCOPE

The scope of study includes:

Measurement of current harmonics and power factor at 132 KV bus at the same load without

filter banks and with filter banksfor both type of AC-DC converters as the DC load of pot lines is

being kept always constant i,e 70 KA.

SYSTEM CONDITIONS

At the time of measurement the system conditions were as follows:

Voltage : Fairly constant, varies between 130 KV to 135KV

Load Current :Full DC load i,e 70 KA.

Power Factor of thyristor converters

(without filter banks) :Varies from 0.82 lag to 0.84 lag

(with tuned filter banks) :Varies from 0.90 lag to 0.93 lag

Power Factorof diode converters

(without filter banks) :Varies from 0.89 lag to 0.93 lag

(with tuned filter banks) :Varies from 0.95 lag to 0.99 lag

The actual reading of Power Factor, Efficiency and THD of unit 7A & 9A of Hindalco,

Renukoot in the month of December 2013 and January 2014 were takenwith and without filter bank

in service and following analysis were carried out.

1. Variance Analysis

2. Dot Plot

3. Scatter Plot

4. Histogram

5. Regression Analysis



64

Data for Thyristors (t_) and Diode(d_) without filter bank are as follows:

THYRISTOR

CONVERTER

DIODE CONVERTER

Serial

No

t_eff t_pf t_thd d_eff d_pf d_thd

1 98.77 0.82 10.10471 97.75 0.891 9.1

2 98.86 0.821 10.10333 97.80 0.892 9.1

3 98.94 0.824 10.10058 97.84 0.893 9.09

4 99.00 0.824 10.09911 97.93 0.896 9.09

5 99.01 0.824 10.0974 97.99 0.898 9.09

6 99.04 0.824 10.09197 98.02 0.899 9.09

7 99.06 0.825 10.09157 98.04 0.904 9.09

8 99.08 0.825 10.09074 98.15 0.904 9.09

9 99.10 0.825 10.08869 98.20 0.904 9.09

10 99.10 0.826 10.07857 98.22 0.906 9.09

11 99.11 0.826 10.07411 98.28 0.906 9.09

12 99.13 0.826 10.07314 98.35 0.908 9.09

13 99.13 0.827 10.07036 98.41 0.908 9.08

14 99.15 0.827 10.06337 98.46 0.91 9.08

15 99.16 0.827 10.05431 98.62 0.911 9.08

16 99.16 0.828 10.05006 98.64 0.911 9.08

17 99.16 0.828 10.04828 98.67 0.912 9.08

18 99.17 0.828 10.04817 98.67 0.912 9.08

19 99.19 0.829 10.04396 98.67 0.913 9.08

20 99.19 0.829 10.04303 98.69 0.913 9.08

21 99.20 0.83 10.03452 98.69 0.914 9.07

22 99.23 0.83 10.03327 98.73 0.917 9.07

23 99.24 0.831 10.03293 98.75 0.918 9.07

24 99.24 0.831 10.0285 98.77 0.919 9.07

25 99.26 0.832 10.02646 98.81 0.92 9.07

26 99.26 0.832 10.02619 98.83 0.921 9.06

27 99.28 0.835 10.02162 98.89 0.924 9.06

28 99.28 0.836 10.01981 99.02 0.926 9.06

t_eff :Thyristor efficiency

t_pf :Thyristor power factor

t_thd :ThyristorTotal Harmonic Distortion

d_eff : Diode efficiency

d_pf :Diode power factor

d_thd : Diode Total Harmonic Distortion



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1. Variance Analysis

For Thyristor Results are as follows

Variable N StDev Variance

t_eff 28 0.125 0.0156

t_pf 28 0.00378 0.000014

t_thd 28 0.0288 0.000828

95% Confidence Intervals

Variable Method CI for StDev CI for Variance

t_eff Chi-Square (0.099, 0.170) (0.0097, 0.0288)

Bonett (0.087, 0.192) (0.0076, 0.0368)

t_pf Chi-Square (0.00299, 0.00514) (0.000009, 0.000026)

Bonett (0.00293, 0.00523) (0.000009, 0.000027)

t_thd Chi-Square (0.0227, 0.0392) (0.000517, 0.001533)

Bonett (0.0253, 0.0351) (0.000643, 0.001233)

For Diode Results are as follows

Variable N StDev Variance

d_eff 28 0.371 0.138

d_pf 28 0.00955 0.000091

d_thd 28 0.0113 0.000128

95% Confidence Intervals

Variable Method CI for StDev CI for Variance

d_eff Chi-Square (0.294, 0.506) (0.086, 0.256)

Bonett (0.314, 0.472) (0.099, 0.223)

d_pf Chi-Square (0.00755, 0.01300) (0.000057, 0.000169)

Bonett (0.00776, 0.01264) (0.000060, 0.000160)

d_thd Chi-Square (0.0090, 0.0154) (0.000080, 0.000238)

Bonett (0.0092, 0.0151) (0.000084, 0.000227)

Observation

From above analysis it is proven that the st. deviation of Thyristor unit (Efficiency, Power

factor and Thd) is lower than Diode rectifier unit. Stability in Thyristor is more w.r.t. Diode unit.



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2. Dot Plot & Histogram

For Thyristor

For Diode



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Observations This analysis prove that the efficiency of Thyristor based convertor is better that diode

convertors. Power factor and Thd is lower in Thyristor convertor but in further study the impact of

filter bank in PF and Thd of both types of unit is taking place.

3. Scatter Plot

For Thyristor

For Diode

Observations This analysis prove that in Thyristor convertors there is relationship of efficiency with respect

to Thd and PF. But in diode convertors efficiency has direct relationship with PF but Thd has

negligible impact on efficency generated by the convertors.

4. Regression Analysis The purpose of regression analysis is to form an equation of efficiency, PF and Thd of both

type of convertors and analyse their impact and carry out the SWOT analysis to prove that which

type of convertor is better for Aluminium industry.



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For Thyristor By using multivariable regression technique for thyristor convertors Power Factor and THD

against response variable Efficiency yielded the following equation.

t_eff = 94.2 + 21.45 t_pf - 1.273 t_thd

Obs t_efficiency t_fit Resid Std

1 98.768 98.9094 -0.1414 -3.44

2 98.8587 98.9326 -0.0739 -1.71

3 98.9387 99.0004 -0.0617 -1.42

4 98.9961 99.0023 -0.0062 -0.14

5 99.0055 99.0045 0.001 0.02

6 99.041 99.0114 0.0296 0.65

7 99.0635 99.0334 0.0302 0.68

8 99.0804 99.0344 0.0459 1.03

9 99.0994 99.037 0.0623 1.38

10 99.1042 99.0713 0.0329 0.72

11 99.1139 99.077 0.0369 0.8

12 99.1255 99.0782 0.0472 1.02

13 99.1345 99.1032 0.0313 0.68

14 99.1503 99.1121 0.0382 0.83

15 99.1557 99.1237 0.032 0.71

16 99.1567 99.1505 0.0062 0.14

17 99.1573 99.1528 0.0046 0.1

18 99.1661 99.1529 0.0132 0.29

19 99.1881 99.1797 0.0083 0.18

20 99.19 99.1809 0.0091 0.2

21 99.1984 99.2132 -0.0148 -0.33

22 99.2329 99.2148 0.0181 0.41

23 99.2374 99.2367 0.0008 0.02

24 99.24 99.2423 -0.0023 -0.05

25 99.2563 99.2663 -0.01 -0.22

26 99.2629 99.2667 -0.0038 -0.09

27 99.28 99.3368 -0.0568 -1.4

28 99.2839 99.3606 -0.0767 -2.06



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For Diode Convertors Here also by using multivariable regression technique for diode convertors, Power Factor and

THD against response variable Efficiency yielded the following equation.

d_eff = 66.5 + 37.70 d_pf - 0.26 d_thd



70

Obs d efficiency d_fit Resid Std

1 97.753 97.7443 0.0087 0.11

2 97.8048 97.782 0.0228 0.3

3 97.8423 97.8223 0.02 0.29

4 97.9258 97.9353 -0.0095 -0.13

5 97.9943 98.0107 -0.0164 -0.21

6 98.0223 98.0484 -0.0262 -0.33

7 98.0365 98.2369 -0.2005 -2.49

8 98.1529 98.2369 -0.0841 -1.05

9 98.2016 98.2369 -0.0353 -0.44

10 98.2177 98.3123 -0.0946 -1.2

11 98.2761 98.3123 -0.0362 -0.46

12 98.3548 98.3877 -0.0329 -0.43

13 98.4135 98.3903 0.0233 0.29

14 98.4555 98.4657 -0.0102 -0.12

15 98.6183 98.5034 0.115 1.41

16 98.6393 98.5034 0.136 1.66

17 98.6681 98.5411 0.127 1.56

18 98.6693 98.5411 0.1283 1.58

19 98.6713 98.5787 0.0926 1.15

20 98.6903 98.5787 0.1116 1.39

21 98.6932 98.619 0.0742 0.94

22 98.7271 98.7321 -0.005 -0.06

23 98.7513 98.7698 -0.0185 -0.23

24 98.7665 98.8075 -0.041 -0.51

25 98.8094 98.8452 -0.0358 -0.45

26 98.8318 98.8854 -0.0537 -0.73

27 98.8887 98.9985 -0.1099 -1.45

28 99.0242 99.0739 -0.0497 -0.65



71

Actual Models of Thyristor and Diode based convertors after regression analysisare.

t_eff = 94.2 + 21.45 t_pf -

1.273 t_thd

PF THD

d_eff = 66.5 + 37.70 d_pf -

0.26 d_thd

PF THD



72

Basic Analysis with filter banks

Data with Tuned Filter banks

Serial

No

t_eff t_pf t_thd d_eff d_pf d_thd

1 98.809 0.906 2.604707 97.797 0.958 2.07

2 98.89667 0.91 2.603325 97.84084 0.96 2.07

3 98.97971 0.911 2.597397 97.87726 0.964 2.06

4 99.03313 0.912 2.591566 97.96381 0.964 2.06

5 99.04348 0.912 2.563368 98.03333 0.967 2.06

6 99.08 0.913 2.554307 98.06326 0.969 2.06

7 99.10655 0.914 2.548282 98.08145 0.971 2.05

8 99.11636 0.914 2.533275 98.19586 0.972 2.05

9 99.13635 0.914 2.526463 98.24261 0.973 2.05

10 99.14719 0.915 2.52162 98.25267 0.976 2.05

11 99.14987 0.915 2.500579 98.31407 0.976 2.05

12 99.16548 0.916 2.49911 98.38984 0.978 2.05

13 99.17352 0.916 2.491967 98.45155 0.979 2.04

14 99.19032 0.918 2.474115 98.49148 0.979 2.04

15 99.19171 0.919 2.473143 98.65733 0.979 2.04

16 99.20067 0.919 2.470364 98.68233 0.981 2.04

17 99.20233 0.92 2.44817 98.70406 0.981 2.04

18 99.21113 0.921 2.443027 98.70733 0.982 2.04

19 99.233 0.921 2.426189 98.71333 0.984 2.04

20 99.23306 0.921 2.419813 98.72733 0.985 2.04

21 99.23639 0.921 2.390735 98.73023 0.985 2.04

22 99.2749 0.921 2.388687 98.7641 0.986 2.03

23 99.282 0.921 2.378572 98.79429 0.986 2.03

24 99.28242 0.924 2.350061 98.80845 0.986 2.03

25 99.29433 0.925 2.343961 98.85035 0.992 2.02

26 99.30686 0.925 2.334516 98.87579 0.994 2.02

27 99.319 0.927 2.332926 98.92767 0.996 2.01

28 99.32187 0.928 2.328499 99.05919 0.997 2.01



73

Dot Plot and Line Plot

Comparing results for Efficiency, Power Factor and THD between Thyristor and Diode.

99999999....3333333399999999....2222666699999999....1111999999999999....1111222299999999....0000555599998888....9999888899998888....9999111199998888....88884444

99999999....0000000099998888....8888222299998888....6666444499998888....4444666699998888....2222888899998888....1111000099997777....99992222

tttt____eeeeffffffff

dddd____eeeeffffffff

Dotplot of t_eff, d_eff



74

Reliability of Convertors

Thyristor based converters are more reliable than diode based converters

The aluminum smelting plant demands for very high DC power. The multi-pulse rectifier

configuration is commonly deployed in such cases, which consists of a number of series and parallel

connected diode/thyristor bridges to supply series of electrolytic cells. Both kind of rectifier

converters i.e Thyristor based converters and diode based converters are prevalent in the market .

Still the question regarding which one is the most reliable is of utmost interest in the industry and

with the increasing current ranges and production capacities and competition on a high the reliability

factor is of prime importance.

Reliability of Rectifier Transformers is a function of various parameters viz.

Major and auxiliary equipment healthiness, System voltage reliability, Environmental causes

(lightning), generation and load stability, smelter condition (anode effects)

From the above parameters the most important of all is the major and auxiliary equipment

healthiness as all other parameters would affect the transformer loading for transient period but any

abnormality of equipment may lead to the outage of the rectifier unit for a considerable amount of

time affecting heavy production losses and monetary losses consequently.

The table below shows the equipments of Thyristor based converters and diode based

converters

Thyristor based converters Diode based converters

Main Rectifier Transformer Main Rectifier Transformer

Rectifier Cubicle Rectifier Cubicle

Cooling equipment for rectifier

transformer viz. pumps and fans

Cooling equipment for rectifier transformer viz. pumps and

fans

Cooling equipment for rectifier

Cubicle viz. pumps and fans Cooling equipment for rectifier Cubicle viz. pumps and fans

Digital control system PEC 800 for

current regulation

Programmable High Speed Controller for current regulation.

DCCT DCCT

Regulating Transformer with OLTC and offload changer

Cooling equipment for regulating transformer viz. pumps

and fans

Transductors for fine control

Transductors for control assembly

As can be seen from the above table that the more number of equipmentare associated with

the diode based converters as compared to the Thyristor based converters.

Problems related to these extra various equipmentin the diode converters which are normally

faced during life of its operation.



75

OLTC The major drawback is the wear and tear on the mechanical on load tap changer,

Contacts ‘carbonization, heat losses and breakdown of its linked accessories viz.

MCBs, Operating mechanism faults etc. that results in a need for regular preventing

maintenance.

Transductor heat losses and breakdown of its linked accessories viz. MCBs, its reactors,

auxiliary transformers etc.

Cooling

Equipment

More Motors and Pumps for diode converters used are susceptible to wear and tear

and consequent breakdowns

Thus with every increase in the equipment the possibility of its breakdown increases and

probability of diode converters outage increases.

For comparison of the reliability of Thyristor based converters and diode based converters we

chose two equipment having same work scenario i.e. same type of system disturbances same kind of

maintenance schedules and same work environment. The two Rectifier Units of M/S Hindalco

Industries Limited were selected

Unit 7A of Potline 7 which is a Thyristorbased Rectifier Unit and Unit 9A of Potline 9 which

is Diode based converter.Both the converters are of M/s ABB make.

The units are connected to 132000 Volts lines at primary and supplies 900 Volts 35 kA DC to

Potlines

Single line diagram of thyristor converter of potline #7

132 KV NORMAL BUS

132 KV STANDBY BUS

1250A, 145KV OFF LOAD ISOLATOR

STAR DELTA

REVERSE

CURRENT

RELAY

REVERSE

CURRENT

RELAY

OFF LOAD DC ISOLATORS

(+)

ve (-)

ve

STA

R

REVERSE

CURRENT

RELAY

OFF LOAD DC ISOLATORS

(+)

ve (-)

ve

7A 7B



76

Single line diagram of diode converter of potline 9

Breakdowns analysis The Data of Unit 7A and 9A breakdowns owing to equipment failure or auxiliary failure for

six years were collected and the result showed that the Diode based rectifier system had more down

time than the Thyristor based system.

Thyristor based converter unit 7A

UNIT # 7A BETWEEN 01/04/2008 AND 01/04/14

From To

Duration

(in hrs) REASONS

8:49 8:50 0:01 for attending the fuse failure of Thyristor (F-27 was replaced)

8:35 8:40 0:05

Between 8.35 and 8.40 hrs DMW and Rect Cubicle Fan got tripped due to

hangup of PLC

6:24 7:35 1:11 Tripped from 6.24 to 7.35 hrs DMW flow failure .

18:38 18:39 0:01

Tripped from 18.38 hrs to 18.39 hrs on due to high temperature of rectifier

cubicle.

8:48 8:51 0:03 tripped from 8.48 hrs to 8.51 hrs on operation of reverse current relay of unit 7B

11:55 11:56 0:01 Unit # 7A tripped on OVP alarm.

15:18 15:40 0:22 Malfunctioning of Transformer Mains MCB.

132 KV NORMAL BUS

132 KV STANDBY BUS

1250A, 145KV OFF LOAD

ISOLATOR

2000A, 145KV SF6 CIRCUIT BREAKER

120KV, 10KA LIGHTNING ARRESTOR

RECTIFIER

TRANSFORMER

OFF LOAD DC

ISOLATORS

(+) ve

(-) ve

TRANSDUCTOR

Q5

0

Q5

1

Q6

0

Q6

0

DIODE RECTIFIER

REGULATING TRANSFORMER

120KV, 10KA LIGHTNING ARRESTOR

REVERSE

CURRENT

RELAY

OFF LOAD DC

ISOLATORS

(+)

ve

(-) ve

TRANSDUCTO

R

Q5

0

Q51

Q60

Q60

RECTIFIER

TRANSFORMER

DIODE RECTIFIER

9A 9B



77


15:00 15:45 0:45 DMWater Pump-2 having abnormal sound.

7:28 7:30 0:02 Tripped without alarm

5:22 5:24 0:02 DMWater flow failure .

6:31 7:02 0:31 DMWater Pump Main power 2NOS fuse blown.

18:17 18:18 0:01 Due to failure of rectifier synchronising voltage.

17:55 17:56 0:01 Due to rectifier synchronising voltage failure.

3:06 Total time lost

Diode Based Converter Unit9A


From To

Duration

(in hrs) REASONS

13:48 15:06 1:18

Tripped from 13.48 hrs to 15.06 hrs on Rect Txf aux

failure

22:36 23:08 0:32

tripped from 22.36 hrs to 23.08 hrs on transductor

supply failue

3:38 4:14 0:36

Failure from 3.38 hrs to 4.14 hrs due to tripping of 110

v, MCB in ACDCDB panel

11:08 11:15 0:07

tripped from 11.08 hrs to 11.15 hrs on DM water flow

failure.

17:12 20:02 2:50

415 volt aux supply failed due to overheating of power

cable between ACDCDB panel and rect marshling box..

18:13 18:59 0:46 415 volt Aux. Failure of the control panel.

17:48 17:58 0:10 UPS supply MCB tripped.

9:16 9:20 0:04

during maintenance work in transductor panel supply

failed.

21:10 21:18 0:08 Tripped without alarm.(reason not known)

16:50 17:20 0:30

Tripped From 16.50 hrs to 17.20 hrs due to the defect in

Card U546.

8:27 17:04 8:37

S/d was taken for PM to replace 400A defective switch

at ACDC DB

19:15 19:37 0:22

Tripped from 19.15 hrs to 19.37 hrs due to Aux Failure

caused by Tripping of Txf T-24L LT Bkr due to

malfunctioning of Relay SR21

5:55 6:00 0:05

Tripped from 5.55 hrs to 6.00 hrs on failure of Aux

power due to Tripping of T-24 LV Bkr on Short -time

O/C

8:35 8:36 0:01

Due to aux failure while its 415v load transfer from

Normal to Std By Feeder at ACDC DB

8:30 9:06 0:36 Tripped on rectifier low oil level.

13:40 13:44 0:04

Tripped due to the failure of auxiliary supply of the

cooler of Regulating Transformer

16:46 Total time Lost



78

Also in Diode based converters the controlling of current involves operation of OLTC and

controlling of Transductor current which uses cabling and wiring and has heat loss and loose

connection problems. But in the Thyristor converters optical fibre connections have been used to

transfer signals to pulse transformers for providing firing pulses to thyristors, which does not have

data loss, and other problems which may lead to breakdowns in Rectifier transformers.

The above mentioned factors clearly shows that the thyristor based converters are more reliable than the diode based converters.

MAINTAINABILITY REQUIREMENTS OF CONVERTERS

Maintenance of Converters plays a key roll in its overall performance. Since a rectifier

system is made up of many parts, such as transformers, insulating oil, the cooling equipment,

bushings, tap changers and rectifier cubicles etc. must be checked periodically.

All these activities require man power, money and material which adds tothe cost of operation and

maintenance.

Since the diode based converters have extra equipment like regulating transformers,

transductors, OLTCs, etc. The maintenance of these equipment increases the maintenance cost, risks

of more breakdowns and decreases the efficiency of potlines.

The maintenance schedule for both type of converter systems has been shown in the table.

It is visible from the Hindalco check sheets that the amount of maintenance needed is more in

Diode based converters than the Thyristor based converters.

Also for Diode based converters, following spare parts (additional to the thyristor based

converters units) are essential for efficient operation of the equipment

1) OLTC

2) Filter pads

3) Cooling motors

4) Cooling pumps

5) HV & LV Bushings of Regulating Transformer

Thus the inventory cost increases considerably, which is a critical component in today’s

competitive scenario.

Investment Cost The investment cost of diode rectifier system is more due to the additional requirement of

Regulating transformer. It is clear that the price of thyristors is more by 25% than diodes of a

rectifier unit of the same. It is also known that the price of tuned filters is more for thyristor

converters. However overall price of rectifier converter system is more by 20% in comparison to

diode rectifier system.

REGULATION OF CONVERTERS

Regulation of Unit 7A Unit 7A being a Thyristor controlled rectifier has fine regulation from zero to 100 % of the

rated load. The load is controlled by the firing angle and the firing angle can be varied smoothly and

linearly to meet the load regulation as the output voltage of the rectifier is Vo= 1.35 Vrms cos α

(where α is the firing angle) and Vrms is the secondary voltage of the rectifier transformer.



79

With the present digital control system PEC 800 installed at Hindalco, the firing angle can be

adjusted to fourth decimal places thereby allowing the operators to maintain the current accuracy of

100 amps in the range of set value of 70kA.

Basic Control Philosophy of current regulation through PEC Controller

The load regulation is being done by PEC controller which has PI controlling and

corresponding to the feedback give firing pulses from its optical card through the different optical

fiber cable designated for each arm to the LTC cards connected on each arm. Each arm of rectifier

cubicle has Four LTC cards in all and the top of them is the master to which the firing pulses comes.

This firing pulse signal is then communicated to down the line LTC cards. In the card are two pulse

transformer one each for a thyristor and the firing pulses are given to the thyristors through these

LTC Cards.

Regulation of Unit 9A For Unit 9A, the tap changer operates in accordance with the upper and lower limit of a

control current for transductor. A coordinated approach for reactor and tap changer is used to control

rectiformer output current.

The tap changing transformer has multiple taps in order to adjust the DC side output voltage.

Each rectiformer has a unit reference current, which is compared with the measured current in a

closed loop current control. The current control loop for a rectiformer is shown in Fig.below. The

output measured current of twelve pulse rectifier is compared with the reference current and a

proportional-integral (P-I) controller adjusts the control current of the respective reactor, thereby

maintaining the output current at the desired value. The OLTC operates outside the controlled

current range of reactors.



80

The control is a mixture of step and linear regulation. Whenever the regulation is through

transductor then the regulation is linear.

Figure 3: Control Characteristic for a Voltage Controlled Reactor

When it is through OLTC it is Step wise. Some operational problem is observed when

transductor reaches its saturation point.Here the linearity of the load control is not achieved due to

this there are current variations and anode effects in the pots of the smelter.

Whenever there is some disturbance in Smelter the load varies because of the combined

philosophy of transductor and OLTC and it takes some time to maintain a stable load.



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COMPARISON

Thyristor rectifier is more suitable for current control in comparison to diode rectifier because

of the following reasons:

1) Control is linear in case of Thyristor controlled converters

2) In case of Thyristor controlled converters Control is only dependent on alpha(firing angle)

whereas in diode it is only linear when it is in the range of transductor

3) When in OLTC range, the control is stepwise and the load can be increased in the steps as the

voltage increases in steps by OLTC. Fine control is not available in this range

4) The controlling is fast and smooth in Thyristors as compared to the sluggish control of Diode

based converters

5) The most positive point of thyristor rectifier is that the potline can be started from zero

current and the potline can be switched on with bare minimum power availability.

6) The above feature is not possible with diode based converters which can be switched on

when the required minimum power is available.

7) Since the load on the thyristor rectifiers can be finely adjusted and regulated to match the grid

exchange limit in contingency.

8) The above case is also applicable for maintaining stable frequency when the smelter power

network is isolated from grid.

All the above operational features are practiced at Hindalco Smelter, Renukoot which is

having both type of rectifier units. Below are the graphs depicting the performance of unit 9A and

7A connected with their respective potlines Pl 9 and PL 7 respectively

Pl 9 current graph plot



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SWOT Analysis was carried out to reach at the conclusion

Strength

1. Thyristor convertors are more efficient than diode convertors

2. Highter reliability in thyristor convertor because of minimum mechenical parts.

3. Easy maintenance in thyristor system.

4.Thyristor convertors are most economical from the investment point of view also.

5. Regulation is better in thyristor convertor.

6.In thyristor convertors there is fast and smooth output control within milli seconds.

Weakness

1.Thyristor convertors has Low PF w.r.t. diode

convertors and with larger firing angle the pf is

further reduced.

2. Diode based system has non linear

regulation.

3. Losses in diode convertor is more because of

the Regulating transformer and transductors.

4. Thd is higher in Thyristor convertors without

filter banks.

5.Thyristors are more costly than diodes.

Opportunity

1. Thyristor convertors gives better performance usually equipped with pf and harmonic filter bank.

2. PF and THd both improve drastically by installing pf and harmonic filter bank with thyristor convertors

3. Installation and commissioning time is less for the thyrisor convertors

Threats

1. Voltage dip has major impact on thyrisor convertors

2. Firing control in thyristor convertor system needs reliability

3. Failure of digital control system PEC 800(regulation system) may lead to stoppage of complete unit

SWOT

Pl 7 current graph plot



83

CONCLUSION

This paper reviews the qualities of the better converter technology. It presents actual

parameters of THD, p.f and energy efficiency for both type of converter technologies without

compensation and with compensation. A basic description of converter technologies are given

together with their quantification through different parameters.These actual field results give a better

insight in the benefits of thyristor converter technology that may be gained by adopting this better

technology in the future for aluminium smelters.

ACKNOWLEDGEMENT

I would like to thank the management of Hindalco industries Renukoot for the support which

was provided to me during my research. I would also like to express my sincere thanks to Mr.

Kailash Pandey, Vice President (Electricals & Automation) of Hindalco industries for his excellent

support and help for this research paper.

REFERENCES

[1] “ABB” Operation and maintenance manuals of Hindalco industries for portline no. 7 & 9.

[2] 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.

[3] Prof. Sharwan Kumar Jhajharia, “A Comparative Study of Diode and Thyristor Converters

used in the Aluminum Smelters”, International Journal of Electrical Engineering

& Technology (IJEET), Volume 4, Issue 6, 2013, pp. 1 - 13, ISSN Print: 0976-6545,

ISSN Online: 0976-6553.

[4] Prof. Sharwan Kumar Jhajharia, “A Comparative Study of Harmonic Currents on Power

Supply System Due to 24 Pulse Ac-Dc Diode Converters and Thyristor Converters in

Aluminum Smelters”, International Journal of Electrical Engineering & Technology (IJEET),

Volume 5, Issue 2, 2014, pp. 60 - 67, ISSN Print: 0976-6545, ISSN Online: 0976-6553.

[5] Sharwan Kumar Jhajharia, “Implementation of Integrated Load Management System with

Scada at Hindalco, Renukoot”, International Journal of Electrical Engineering & Technology

(IJEET), Volume 4, Issue 2, 2013, pp. 187 - 201, ISSN Print: 0976-6545, ISSN Online:

0976-6553.

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