CHAPTER 3 CONVERTERS AT HINDALCO

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1 CHAPTER 3 CONVERTERS AT HINDALCO 3.1 Overview of HINDALCO AC-DC Converters 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 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.

2 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 POT LINE NO. VOLTAGE CURRENT VOLTAGE CURR CONVERTE ENT R TYPE MAKE Diode English Electric Diode Westing House Diode Westing House Diode BHEL Diode BHEL Diode BHEL Thyristor ABB Thyristor BHEL Diode ABB Diode ABB 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 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 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.

4 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 60 60

5 Assumptions (1) Ideal diodes: Von=0 (2) L is very large (3) Three phase balanced input id is constant 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 Fig. 3.4: Single line diagram of pot line

6 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: 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 12pulse 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

7 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 proportionalintegral (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 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 63 63

8 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] 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 crosssection 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 64 64

9 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:? =. KA/? C?? I JA/? C?DEFGH L A. 6-+? I dwt 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 +/ 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 65 65

10 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] 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 Fig 3.6: Diode bridge connections for 12 pulse 66 66

11 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) Conduction angle Two Bridge parallel connection (12) 30o Vrms /phase secondary Vdo/2.34 Irms of secondary 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% 67 67

12 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] 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 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

13 Table 3.3: Comparison of DC measuring systems of pot line 7 and 9 S.No. Pot Line 7 Pot Line 9 CM7038 LKP ka 80kA 1 Type 2 Measuring Range 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 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 pushbuttons 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) 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 69 69

14 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 70 70

15 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 71 71

16 Fig. 3.9: Elementary Circuit diagram of a fully controlled converter 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 Thyristor Commutation Methods. The commutation methods are initially classified in to two broad categories, natural and forced commutation 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 below1) Line commutation 2) Load voltage commutation 72 72

17 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 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 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

18 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]

19 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] 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 75 75

20 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. 76