CHAPTER 3 CONVERTERS AT HINDALCO -...

CHAPTER 3

CONVERTERS AT HINDALCO

3.1 Overview of HINDALCO AC-DC Converters

3.1.1 Introduction

Aluminium electrolysis process requires electrical energy in the form of direct current

and DC power can be obtained by converting AC into DC. Earlier mercury arc

rectifiers were in use till late 1950. In 1960, the first diode converter was used in

Aluminium Smelters and after twenty years, thyristor converters were also in

operation in Aluminium industries. The semiconductor convertors are having high

efficiency being static devices. Today in HINDALCO Industries, converter units of

100 KA are in operation and Smelter plants with the process current of 360 KA for

360 pots in line are operating in HINDALCO industries Ltd. (India) as well as in

other parts of the world. At present, the two types of AC-DC converters are most

commonly used in Aluminium Smelters [1].

1) Uncontrolled converters (Diode converters) and

2) Controlled converters (Thyristor converters)

These two types of semiconductor converter technologies have been competing with

each other over the last three decades. This research is also based on two types of

converters i.e. diode converter and thyristor converter. In diode based AC/DC

converter system where on load tap changers and saturable Transductors are used for

DC output current regulation and control. While in thyristor converter a firing angle

control is used to regulate the DC output [27].

The pot line 7 is having thyristor based converter system and potline#9 is equipped

with diode based converter system. To carry out a comparative study of practical

aspects of mixed use of diode and thyristor convertor technologies in Aluminium

Smelter the case study was taken at HINDALCO Renukoot Smelters. In India,

HINDALCO Industries Ltd is the only Aluminium producer where both types of

converters are being used at HINDALCO Renukoot and HINDALCO Hirakud

Smelters.

In HINDALCO Smelters, pot lines 7 and 8 are having thyristor convertor system and

remaining 9 pot lines are operating with diode converter technology. The details of all

the convertors of 11 pot lines with their ratings are shown in Table 3.1.

Table 3.1: AC-DC convertor ratings of pot lines of HINDALCO Smelter

AC-DC CONVERTOR RATINGS of POT LINES of HINDALCO SMELTER

Rated Capacity During Emergencies/Contingencies

POTLINENO.

VOLT-AGE

CURRENT VOLT-AGE

CURRENT

CONVERTER TYPE

MAKE

1 780 61.5 825 50.0 Diode English Electric

2 825 64.0 850 50.0 Diode Westing House

3 825 64.0 850 50.0 Diode Westing House

4 850 66.0 920 50.0 Diode BHEL

5 850 66.0 920 50.0 Diode BHEL

6 850 66.0 920 50.0 Diode BHEL

7 900 70.0 950 60.0 Thyristor ABB

8 900 70.0 950 60.0 Thyristor BHEL

9 900 70.0 950 60.0 Diode ABB

10 900 70.0 950 60.0 Diode ABB

11 900 70.0 950 60.0 Diode ABB

The reason behind the selection of pot line 7 and 9 for this research was that both the

converters are having same current and voltage ratings and procured from the same

supplier i.e. M/S ABB. Pot line 7 is equipped with thyristor converter system while

pot line 9 is having diode converter system.

The pot line 7 has two Rectifier transformers (ABB make), each of 74.3MVA rating

were commissioned in 1995 along with two converter units of 900 V/70KA [28].

The pot line 9 was commissioned in 2001 has two Regulating Transformers of

81.52MVA rating, two Rectifier Transformer of 73.56MVA and two Converter units

of 900V/70KA [29].

The acceptance of these rectification technologies across Aluminium Smelters is

based on the combination of several factors that are taken into consideration. These

include – system reliability, investment cost, efficiency, current harmonic distortion,

power factor, current regulation accuracy and the maintainability of the converters.

3.1.2 Uncontrolled Converters (Diode Converters)

A diode is a device which conducts current easily in one direction, but does not

conduct in opposite direction except for a small amount of leakage current.

A Power diode has two terminals i.e. Anode and Cathode. It is a two layer “p” and

“n” junction device. Basic schematic diagram and symbol is shown in Fig. 3.1.

Fig. 3.1: Schematic diagram of diode converter and its symbol

When an AC wave is supplied to a diode it conducts the current provided the anode is

made positive with respect to cathode then power diode is said to be forward biased.

In this condition the power diode behaves as a closed switch. As the diode voltage is

increased the current will initially be zero until the voltage is less than its threshold

voltage. Threshold voltage is the minimum anode voltage beyond which the diode

current increases rapidly and diode starts conducting. The forward resistance of the

conducting diode is very small. Therefore the forward voltage drop is also very less.

When cathode of power diode is made positive with respect to anode, the diode is said

to be reversed and it behaves as an open circuit.

Diode converters are the simpler form of converters and used as front end converters

in DC power supplies. A simple elementary circuit diagram of a diode converter

system is shown in Fig. 3.2.





































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Diode Converter System

Fig. 3.2: Circuit diagram of a diode converter system

Diode Converters are the simplest of all converter technologies. There are three basic

types of converters as follows:

The Half wave Converter

The full wave Converter

The bridge Converter

But in Aluminium Smelters bridge converter’s circuits are very popular. Three phase

diode bridge rectification with wave forms of output voltage is explained below in

figure 3.3a & 3.3b.

Fig. 3.3 (a): Three phase diode bridge converter

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figure 3.3a & 3.3b.


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figure 3.3a & 3.3b.


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Assumptions

(1) Ideal diodes: Von=0

(2) L is very large id is constant

(3) Three phase balanced input

Fig 3.3 (b): Output voltage wave form of three phase diode bridge converter

Diode convertors are the simple form of the converters and used as front end

convertors in DC power supply. The circuit diagram of pot line 9 which is having

diode convertor system is shown in Fig. 3.4.

Fig. 3.4: Single line diagram of pot line 9

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Assumptions









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Assumptions









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As shown in Fig. no. 3.4, there are two number of units i.e. 9A & 9B of 70kA 900V

DC converter systems for pot line 9. Regulating and Rectified Transformers are

designed for total 24 pulse with ±7.5 degree phase shift for each unit. Normally pot

line 9 runs in 24 pulse configuration, in case of the tripping of a converter unit or

being stopped for maintenance then the remaining converter unit can feed 100% load

with 12 pulse configuration only. The harmonic filters are designed by keeping in

view all the operating conditions i.e. 12 pulse and 24 pulse. The case study has been

carried out for 12 pulse configuration keeping 70KA load on unit 9A in this

configuration the fifth and seventh harmonic currents are supressed [6,30].

In pot line #9 two units of 70KA and 900V DC considered for a converter system. At

emergency each of these converter systems can supply 950V DC at 60kA [29].

The description of the major equipment of pot line 9 (diode converter system) is as

follows:

3.1.2.1 Regulating Transformer

Regulating Transformer works on auto-transformer principle. Therefore Regulating

transformers are used variable voltage for smelting process, which requires variable

voltage to maintain a fixed current. The variation of voltage may take place in the

smelting process due to following reasons:

1) Increasing and decreasing of pots due to repair of pots in the potline.

2) Anode effects and,

3) Addition of pots in sequence during starting etc.

The voltage variation can be done in two ways:

A. By providing taps on the primary side of the Rectifier transformer but for 12-

pulse circuits where primary phase shift is done, this would be more

complicated and un-economical [31-32].

B. Feeding primary of rectifier transformer through a separate auto regulating

transformer, which is widely, accepted practice. The Rectifier Transformer

being a constant-current transformer, the output current of Regulating

Transformer also constant. Hence KVA of Regulating Transformer changes

with output voltage.

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Voltage control for a Diode converter is achieved by changing the input voltage of the

Regulating Transformer by following means as follows.

1. By using On Load Tap Changer (OLTC) on the primary side of the Regulating

Transformers, by which a rough voltage control can be achieved.

2. The saturable core Transductors control method by introducing variable

impedance into the circuit, ahead of the diode converter. The saturable core

Transductor consists of two windings i.e. DC windings on the centre leg and

AC windings on the outer legs. By varying the control current in the DC

winding the impedance of AC winding can be varied. An increase of DC

control current increases the DC flux and causes the Transductor to operate

with a high level of saturation and correspondingly lower impedance.

Similarly a decrease of control current reduces the DC saturation and increases

the impedance. In this way the effective impedance is controlled to cause

smooth control of output voltage. In pot line 9, OLTC and saturable

Transductor are being used together to have smooth voltage control.

In diode converter system, the tap changer operates in accordance with the upper and

lower limit of a control current for Transductor. A coordinated approach for

Transductors 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 output measured current of

twelve pulse converter is compared with the reference current and a proportional-

integral (P-I) controller adjusts the control current of the respective Transductor,

thereby maintaining the output current at the desired value. The OLTC operates

outside the controlled current range of Transductors. The Transductor control is used

for smooth control of DC voltage between two OLTC positions so that the frequency

of operation of the tap changer is reduced. The Transductor is designed for 80V for

offering smooth control of the Current and having an overlapping Voltage of 15 V, so

that it avoids spurious changing of taps [33].

As explained by the Fig. 3.5. The voltage control is a mixture of step and linear

regulation. Whenever the regulation is done through Transductors then regulation is

linear. But it is step wise when it is done through OLTC. Some operational problem is

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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.

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

philosophy of Transductors and OLTC and it takes some time to maintain a stable

load.

Fig. 3.5: Control characteristic for a voltage controlled Transductor

Since the Smelter set point is normally fixed, saturable Transductors are used for fine

control and tap changers for coarse control to operate continuously in closed loop

mode in order to achieve automatic current control [22].

3.1.2.2 Rectifier Transformer

It is essentially a step down transformer, the secondary of which corresponds to the

output dc voltage. This is also called main transformer in the converter system, as the

output terminals of this transformer are connected to the rectifier units. For large

Aluminium Smelters, Rectifier Transformer ratings are as large as 200 MVA, taking a

supply at 220 KV and providing a low voltage output to the rectifiers between 800 V

to 1500 V. The LV currents therefore may be as high as 25 KA to 50 KA and the LV

conductors thus have a substantial cross-section. In order to bring out the large cross-

section LV leads, the LV winding mostly made the outer winding rather than

occupying its usual position next to the core and it will consist of a number of parallel

disc-wound sections arranged axially with their ends connected directly to vertical

copper bus bar risers [33].

Regardless of their rating, the feature which singles out Rectifier Transformers for

special attention is the problem of harmonic currents created by the Diode and

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Thyristor converters and fed back into the supply system. The problem is taken care

of by using appropriate rating harmonic filter banks.

The Rectifier Transformers differ from normal Power Transformers in many respects,

though they look the same outwardly. A power transformer’s basic role is to either

step up or step down the voltage and transmit power. On the other hand Rectifier

Transformer steps down the voltage and is used with rectifier cubicles to convert AC

power into DC power.

The general expression for direct current Id with the resistive load ‘R’ connected to

Rectifier Transformer with m secondary phases and phase connected to a Diode,

assuming negligible Diode drop and transformer impedance, can be written as:

? JA/? C?DEFGHB@A KA/? dwtI=C? . ? . 6-+ AI L ?

Where Em is peak phase to neutral voltage of the transformer. This is because each

Diode conducts for an interval of 360/m degree [33].

The load of a Smelter is constant in nature as electrolysis process of Aluminium

requires constant current as it helps to maintain better current efficiency of the

Smelter; therefore Rectifier Transformers are basically a constant current

transformers. As stated earlier the LV windings of Rectifier Transformers carry heavy

currents so heavy bus bars are used for the connections between windings and the

terminals. Special attention is given while selecting size and material of the bus bars

carrying currents. This is to reduce the stray losses and reactive drop also.

Primary side windings of converter transformers are star, delta or Zig-Zag. At

HINDALCO Zig-Zag windings are used to provide a phase shift of +/- 7.5 °.

Secondary windings are star and delta and these windings are special type to carry

high currents. Since these windings are connected to the parallel bus bars and carries

high currents. These heavy currents create a lot of stray losses due to both hysteresis

and eddy current losses. To minimize it wherever needed steel with least possible

magnetization is used or non-magnetic steel with low permeability and high resistivity

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is used. Interleaving of bus bars is done to allow linking of only the resultant flux with

the tank. LV bus bars are placed in such a way that its width is always perpendicular

to the Tank wall [33].

3.1.2.3 Diode Rectifier Cubicles.

The phase shifting transformers are being used to have multi-pulse system and it is

very much helpful to mitigate the harmonic currents produced by the AC-DC

converters.

To explain the 12 pulse delta/wye parallel full wave rectifier fed by three phase

winding, six phase transformer with a delta primary along with a delta secondary and

a wye secondary (Ddoyn11). The purpose of this star delta transformer connection is

to introduce a 30° phase shift between the source and bridge [31-32]. This results in

inputs to the two bridges which are 30° apart. The two bridge outputs are similar, but

shifted by 30°. Hence each three phase secondary feeds a six pulse full wave bridge

rectifier and each rectifier is connected in parallel with the load. This configuration is

called a 12 pulse converter [18-19,34]. The purpose of multi-phase rectifiers is to

reduce the magnitude of current THD and improve the power factor.

This rectifier topology is a preferred topology in Aluminium Smelters as it is better

from the operation and maintenance point of view. The diode bridge connections for

12 pulses have been shown in Fig 3.6. And its 12 pulse output voltage waveform has

been shown in Fig no. 3.7.

Fig 3.6: Diode bridge connections for 12 pulse

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converters.















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converters.















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Fig. 3.7: 12-pulse output voltage waveform

In 12 pulses, rectifications 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 harmonics, multi pulse (such as 24 and 36)

are used in 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 pulse is to lower total harmonic distortion (THD) of AC mains

current and have ripple free DC current. The theoretical performance data of 12 pulse

converter system are given in Table 3.2.

Table 3.2: Theoretical performance data of 12 pulse converter system [33].

No. of secondary phases (m) Two Bridge parallel connection (12)

Conduction angle 30o

Vrms /phase secondary Vdo/2.34

Irms of secondary 0.408 Id

P eq of primary 1.01 Pdo

P eq of secondary 1.05 Pdo

P total 1.01 P do

PUF 0.99

SUF 0.95

Ripple value 1.4%

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From the Table 3.2, it is concluded that three phase twelve-pulse-wave bridge rectifier

is the most commonly used rectifier, because it provides a high transformer utility

factor and low ripple factor because the out pulse frequency is twelve times the supply

frequency [35].

3.1.2.4 Semiconductor Fuses

The semiconductors fuses are fast acting fuses and primarily used for short circuit

protection. I2t values of the semiconductors fuse link is always kept less than the I2t

capability of the device.

For protection, special current limiting Semiconductor fuses are being connected in

series with the semiconductor elements to ensure protection against internal short

circuits by isolating faulty element. The semiconductor fuse will open in the event

when the diode fails and will ensure no interruption of service.

Protection relays are also installed in the local control cubicle to allow safe and

reliable operation. The protective relays release the primary breaker in case of a

serious fault. The converter system is cooled by deionized water.

3.1.2.5 DCCT

DCCT is used to measure the direct current in Aluminium Smelters. DCCT works on

the principle of Hall-Effect. The Hall Effect says –“When electrical current passes

through a sample placed in a magnetic field, a potential proportional to the current

and magnetic field is developed across the material in a direction perpendicular to

both current and to the magnetic field [36]. The DCCTs installed at HINDALCO

Smelter are having high accuracy and works on the closed loop system to measure the

DC bus currents up to 100KA. The DC current measurement system used in Pot line 7

and Pot line 9 is placed on the Main DC bus bar and it consists of two piece

measuring head, a metering unit and two multi-conductor cables. The actual value of

delivered by D.C. metering system is compared with a pre-set (set point) value. The

difference between these two values is used to control an electronic regulator in a

change of the responsible actuator and processed at PHSC to perform current

regulation. The comparison of the DCCTs of pot lines 7 and 9 is shown below in

Table 3.3.

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Table 3.3: Comparison of DC measuring systems of pot line 7 and 9

S.No. Pot Line 7 Pot Line 9

1 Type CM7038 LKP80

2 Measuring Range 100 kA 80kA

3 Voltage Output Signal 1mV/kA, +/- 0.1%full scale

1mV/kA, +/- 0.1%full scale

4 Current Output signal 1A/5kA, +/- 0.2%full scale

1A/5kA, +/- 0.1%full scale

3.1.2.6 Microprocessor control of Diode converter system

Control and operation of the converter system is designed fully digital regulation and

control system. Each converter unit is capable of constant current regulation of ± 1%

accuracy of rated current. The actual value delivered by the DC metering system is

compared with pre-set (set point) value. The difference between these two values is

used to control an electronic regulator resulting in a change of the responsible

actuator. This is accomplished by utilizing a fully digital current control implemented

in the software of the high speed controller. PHSC is one of the fastest highest speed

controllers in the world.

This digital regulation offers very accurate and fast responsive control, extensive

converter monitoring, overall system supervision and a most flexible and reliable

operation [9].

All the signals coming from the converter equipment, process sensors and push-

buttons are processed at PHSC (Programmable High Speed Controller) to perform

current regulation, converter equipment control as well as alarm/event indication.

In HINDALCO Aluminium Smelters, the intelligent load management system with

SCADA has been also implemented to provide integration of switchgear, Rectifier

equipment, load management and protection systems in normal as well as in

emergency to avoid interruptions [14].

3.2 Controlled converters (Thyristor converters)

3.2.1 Introduction

Due to its simplicity, reliability and efficiency the thyristor converters are most

commonly used for higher power converter applications. The thyristors are similar to

the diode converters but controlled electronically which eliminates the need of OLTC

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and saturable core Transductors. When thyristor converter is fired at a very small

firing angle then it performs similar to a diode using saturable core Transductor

control [1].

Thyristor is basically a three junctions, four layer, p-n-p-n semiconductor switching

device. The schematic diagram and circuit symbol of thyristor is shown in Fig 3.8.

Fig. 3.8: Schematic diagram of Thyristor & its symbol

It has three terminals namely anode, cathode and gate. It has three junctions viz. J1, J2

and J3 as shown in fig 3.8. As diode, thyristor is also unidirectional device i.e. current

flows from anode to cathode and blocks the current flow from cathode to anode but

thyristor also blocks the current flow from anode to cathode until gate signal is

provided between gate and cathode terminals i.e. gate terminal is controlling terminal

of thyristor.

It is known that diode automatically turns ON at the instant when it becomes forward

biased and turns OFF when it is reverse biased but this is not the case with thyristor,

in spite of being forward biased it conducts only when a gate pulse is impressed on its

gate terminal.

For thyristor a control circuit block is used which generates and supply the gate firing

pulse to each thyristor at the right time in every cycle. The control of the DC output

voltage is obtained by adjusting the “phase” of the gate firing pulse with respect to a

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control [1].









of thyristor.




gate terminal.




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control [1].









of thyristor.




gate terminal.




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reference instant. The dc output voltage can be continuously varied from maximum

positive to zero through a delay angle from zero to 90°. At α = 90°, the average load

voltage is zero, the in phase component of input current is zero and the reactive

component of current becomes maximum [31]. This type of control is described as

“phase control.” This phase angle delay is called firing angle. In case of diodes this

firing angle is always is zero.

When anode is made positive with respect to cathode, a thyristor can be turned ON by

any of the following methods.

1. Forward Voltage Triggering.

2. Gate Triggering

3. dv/dt Triggering

4. Temperature Triggering.

5. Light Triggering.

Gate Triggering is the most reliable and efficient method therefore it is most usual

method to turn on the thyristor. In this method, a voltage is applied between gate and

cathode terminals due to which charges are injected into the inner p region. As

charges are injected into the inner p region, magnitude of forward break over voltage

is reduced because depletion layer at junction J2 is reduced. Even when the current

into the gate stops the thyristor continues to allow the current to flow from anode to

cathode.

The voltage regulation is carried out by means of gate control in thyristor converters

which is controlled electronically [7]. For a thyristor converter, the fundamental

component of current lags the respective phase voltage by triggering angle “α” with a

displacement factor of cos (α) [37, 38]. The firing angle delays the start of the

conducting of the current. This affects the active and reactive power taken from the

supply, i.e. the power factor [25]. In pot line 7 thyristor converter also having 12 pulse

converter system as in port line 9. In Fig no. 3.9, the elementary circuit diagram of

fully controlled converter has been shown

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Fig. 3.9: Elementary Circuit diagram of a fully controlled converter

3.2.2 Commutation

The gate has no control over the thyristor once it turns ON, it can be turned OFF only

by reducing its forward anode current to a level below the holding current value.

Switching from the ON-state to the OFF-state is called turn OFF and this technique is

called commutation. A conducting Thyristor gets automatically commuted when a

reverse bias voltage appears across it.

3.2.3 Thyristor Commutation Methods.

The commutation methods are initially classified in to two broad categories, natural

and forced commutation.

3.2.3.1 Natural Commutation

In natural commutation, the circuit in which thyristor is connected has a natural or

built in ability to turn OFF the thyristor [5]. The natural commutation switching is the

simplest and most efficient but suffers from low power factor due to thyristor firing

angle and commutation angle which can cause stability problem in the weak power

system.

To achieve natural commutation of thyristor converter there are many methods as

given below-

1) Line commutation

2) Load voltage commutation

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3.2.2 Commutation














system.


given below-

1) Line commutation


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3.2.2 Commutation














system.


given below-

1) Line commutation


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3) Load commutation

4) Self commutation.

When the line voltage has correct polarity then each thyristor gets commuted

sequentially and this does not require any special force to commutation circuit. The

incoming thyristor gets the incoming supply voltage and outgoing thyristor gets

reverse voltage and gets naturally commuted. The term natural commutation and line

commutation are the same. In industrial applications the line commutation is more

popular. Line commutation is possible when converters are connected to an AC

voltage bus as the alternating voltage is essential to serve the commutating voltage.

Secondly it is also essential that the voltage, which is being used for commutating

voltage, must have polarities that will reverse - bias the outgoing thyristor.

3.2.3.2 Forced Commutation.

Forced commutation is applied when the thyristor is forward biased and carries

current greater than the holding current at the instant when commutation is desired. In

general, with forced commutation the thyristor is turned OFF in a shorter time than in

a line commutated circuit [5].

Forced Commutation methods can be classified in to four broad categories:

1) Series voltage commutation.

2) Parallel voltage commutation.

3) Series current commutation.

4) Parallel current commutation.

The forced commutation methods are not being used in Aluminium Smelters.

3.2.4 Thyristor Converter System

As shown in figure no. 3.10, there are two number of units i.e. 7A & 7B. The rating of

each unit is 70 KA, 900V DC. The pot line 7 converter system consists of 2 units of

900V, 70 KA thyristor converters. Even if one converter unit is out of circuit then the

remaining unit has the ability to cater the full load of the pot line.

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Fig. 3.10: Single Line Diagram of pot line 7 converter system.

As shown in the figure 3.11 the secondary of the rectifier transformer has wye and

delta group windings. The reason behind this is that when a 12 pulse system is

produced operating two groups of 6 pulse converters having a 30° phase difference in

parallel, the difference of instantaneous output voltage between the 6 pulse converters

causes a cross current to flow. The cross current is determined by the DC output

voltage, magnitude of control delay angle and impedance of the circuit through which

cross current flows [6].

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Fig. 3.11: The Thyristor bridge connection for 12 pulses.

Aluminium Smelters are always rated with the full voltage and full current rating. At

the time of start-up process the numbers of pots are very less and the requirement of

DC output voltage becomes less and the value of firing angle increases as the need of

active power is less and reactive power is quite high and the cross current also

becomes fairly large due to large firing angle [31]. Due to this high cross current,

losses increases and control becomes unstable and local heating increases near the DC

conductors. To avoid this delta group and wye group transformers are separated from

each other in the common tank and in one enclosure to make the impedance large

enough to suppress the cross current. In Thyristor Converters, the firing angle helps to

control the voltage and current. When a thyristor converter is controlled through a

small delay firing angle it performs like a diode converter using saturable Transductor

control. It has fast and smooth output control to the order of milliseconds [6].

3.2.5 Microprocessor control of Thyristor converter system

In modern AC-DC converter technology, the microprocessor is used for the control of

firing circuits due to its high reliability and flexibility. Phase control of a thyristor has

three basic operations – line synchronization, delay control, and pulse distribution to

the thyristors. All these functions are being performed by the microprocessor [9, 35].

The load regulation of pot line 7 at HINDALCO Smelter is being done by AC800

PEC which is high performance controller with PI controlling and corresponding to

the feedback give firing pulse from its optical card through the different optical fiber

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

converter cubicle has four LTC cards in all and the top of them is the master card to which

the firing pulse comes. This firing pulse signal is then communicated to down the line

LTC cards. In the card there are two pulse transformers one for a thyristor and another one

for the firing pulse which are given to the thyristors through these LTC Cards. The ultimate

change in the delay angle is proportional to the control signal. This scheme has an

inherent characteristic, as the rate of change of the firing angle rather than the firing

angle itself is controlled [9]. This system is very much reliable if main controller fails then

second control is available in hot standby.

The microprocessor controller offers decreased harmonic distortion and improved

power factor due to equidistant firing technique which practically eliminates unequal

thyristor firing angles and thus reduces the generation of harmonics [37]. The cycle time

of process of this microprocessor is less than 100µ s.

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