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Authors: Maurya, Rakesh
Keywords: Electrical Power
DC Power
Electrochemical Processes
Issue Date: Dec-2013
Publisher: Dept. of Electrical Engineering iit Roorkee
Abstract: As bulk of electrical power is extensively utilized in the form of A.C., but contemporary, DC power has also gained significant importance due to wide applications of electronic devices. Some of the industrial processes where controlled DC is required are electroplating, electrolysis, anodizing, and metal refining, electronic welding, plasma torch applications, battery charging, electrical traction, telecommunication and computer industry. There are several applications which require high power at reduced DC voltage power because high voltage cannot be applied due to the system requirements. These applications include telecommunication, computer server system, electrochemical processes, DC arc furnaces, Large Hadran Collider (LHC), and nuclear fusion research of magnetic confinement etc. The power supply installed in aforesaid applications is named as Low Voltage High Current (LVHC) power supply. In light of the variety of the aforementioned applications, several power converters are reported in literature which could be employed for low, medium and high power applications. Keeping in view of high power applications at low output voltage, the use of transformer is essential for voltage scaling, electrical isolation and safety. Based on electrical isolation, the existing topologies are broadly classified in two groups namely line frequency transformer based converter and high frequency transformer based converter. These groups of power converters are normally compared from the viewpoint of size and weight, location of transformer and DC-DC converter in order to search for suitable power circuit for providing high current at low output voltage. The majority of high-power rectifier manufacturers around the world inevitably use line frequency transformer based power converter due to low cost and well-established technique. Invariably, these converters consist of a power conditioning system which comprises of phase shift transformers and multiple AC-DC converters that transfers power to load in single-stage conversion. The phase shifting transformer is essential to provide desired phase shift in input voltages of converters and is achieved by different connections of three-phase and single phase transformers. Additionally, phase shifting transformer provides electrical isolation and reduction in voltage to achieve high current at low voltage. Although such multipulse converters help in achieving reduction in DC voltage ripples and harmonics in input current simultaneously, but they are quite bulky and more weight due to line frequency operation. Further, the efficiency of such system is low to deliver same power at reduced voltage. To comply harmonics regulation set by IEEE Standard 519-1992, the line frequency transformer based converter requires additional filters at the input which further increases size , weight and cost. ii In light of the above limitations, the high frequency transformer based power converters are selected in which power conditioning system consists of two stages namely AC –DC and DC-DC power conversion. The most common practice for AC-DC conversion is to use 1-phase/3-phase diode bridge rectifier followed by a large capacitor to obtain ripple free DC voltage. As a result, the line current becomes non-sinusoidal and has several undesired effects on both the utility and consumer sides such as losses and overheating in transformer, shunt capacitors, power cables, reduced power factor and distortion of the line voltage due to the line impedance drop. Hence, PFC pre-regulator are used to ensure power quality issues at utility sides. The output DC voltage obtained by PFC pre-regulator from 1  , 230 V, 50Hz / 3   , 415 V, 50 Hz AC power supply varies from 400-1000V DC. In order to meet demand of large current at low output voltage, the survey of power converter topologies for the second stage of power conversion (DC-DC) is extremely important. Keeping in view the requirement of low output voltage from high input DC voltage, high conduction losses, and controller design, the performance of the following DC-DC converters are evaluated.  Isolated DC-DC Converter  Inter-Connected Converter  Three-phase DC-DC Converter Isolated DC-DC converters in its basic forms i.e. forward, fly-back, push-pull, half and full bridge for the second stage of implementation face the problem of high voltage stress on the switching devices of front end converter and it has become a major concern in designing converter. Conventionally, in order to reduce voltage/current stress, each power devices are replaced by two or more series/parallel connected switches respectively. In practice, the devices are not identical but have forward dropping characteristics within a narrow band. In addition, the parasitic elements significantly influence the unequal voltage and current sharing by the switching devices. Alternatively, the modular approach for designing LVHC DC-DC converter be adopted in which low rating DC-DC converter modules are integrated in series or parallel, both at input side as well as output side to achieve desired input and output specifications. Among four possible inter-connections namely input-parallel-output-parallel (IPOP), input-parallel–output-series (IPOS), input-series–output-parallel (ISOP), and inputseries– output-series (ISOS), the ISOP connection is preferred for low voltage high current applications due to better voltage stress at front end converter and current stresses at load end converter, increase power processing capability, improved reliability because of more even distribution of stresses, ease of expansion and repair. Keeping this in mind, the performance of two identical modules of push-pull converters is investigated. The effect of parasitic elements such as effective series resistance of iii capacitors and output inductors, leakage and magnetizing reactances of transformers and with inter-connected in series at input side and in parallel at output side are included. The converter modules of same rating cannot be identical i.e. any mismatch in circuit parameters leads to unequal sharing of power by individual converter modules i.e. the module which supplies more power suffers more stress and the probability of its failure increases. In order to share equal power by two push-pull converters, three controllers namely common output voltage controller, inner current controller and input voltage controller are employed. A stable feedback system is designed to regulate the output voltage. A systematic development of a small-signal linear dynamic model of proposed converter is carried out using State-Space Averaging (SSA) technique. The transfer functions of different control blocks of converter are obtained. Furthermore, stability analysis of control loops is carried out to ensure closed loop operation. The Simulink model of proposed converter is developed using MATLAB/ SimPowerSystemTM and the simulation results are presented. For high power applications, a large number of small rating modules are connected in ISOP connection and to ensure equal power sharing, several controllers are to be employed. Therefore, on account of more number of components, controllers, system reliability decreases and furthermore design of controllers becomes complex. A three-phase DC-DC converter is proposed as an alternative for high power applications. It has several advantages over the single-phase DC-DC converter such as ease of power device (MOSFETs) selection due to reduced current rating, reduction of size and weight of passive components due to increased effective switching frequency by a factor of 3 compared to single-phase DC-DC converter and reduction in transformer size due to better transformer core utilization. This work presents mathematical modelling, analysis, design, simulation and experimental results of three-phase, high frequency isolation transformer based DC-DC converters suitable for low voltage and high current applications. A high-frequency isolated three-phase LLC resonant DC-DC converter with centre-tapped secondary windings of trasnformer is proposed which consists of three units of single-phase half bridge LLC DC-DC converter and operated in interleaved manner to feed high power load. The proposed topology with interleaved control technique increases the effective frequency of operation without increase in switching frequency of switches and hence reduces the size and weight of passive components. However, operation with higher frequency leads to increase in switching losses and EMI problems. The performance of proposed topology is further improved by implementation of Zero Voltage Switching (ZVS) making use of snubber capacitance, leakage and magnetizing inductances of transformer (LLC resonant tank) and hence reduced switching losses in front end converter, and better current sharing at load end converter can be achieved. The mathematical modeling of LLC resonant tank network is iv developed and its design curves are plotted against variation of normalized frequency for different values of load. Based on the design curves, the Simulink model of converter is developed using MATLAB/SimPowerSystemsTM and simulation study is carried out to investigate its performances for low voltage high current applications. To validate simulation results, a prototype model of 75W, 1.5V/50A is built and its performance is investigated under various operating conditions. For high-current applications, the rectifier configuration of current-doubler converter is preferred over the centre-tapped converter for a number of reasons. First, in the current-doubler converter the inductor currents and the transformer secondary current are lower than the corresponding currents in the centre-tapped converter. As a result, the current-doubler converter exhibits lower conduction losses than the centre-tapped converter. Second, the current-doubler converter minimizes the number of high-current interconnections that further simplify the secondary layout and reduces the layout-related loss. On the basis of secondary side load sharing, transformer design and thermal heat dissipation, the three-phase high frequency isolated DC-DC converter with three-phase rectification is proposed. It consists of three main parts: front end converter, high frequency transformer and load end converter. The front end converter comprises of N legs and each leg consists of two power switches. The midpoints of each leg are connected to one end of the primary winding of single-phase high frequency transformers and other ends of the primary winding of transformers are connected at one point. The load end converter of three-phase rectification is similar to three-phase full diode bridge in which upper diodes are replaced by inductors. The performance of proposed converter is investigated under symmetrical and asymmetrical phase shifted PWM control methods with fixed frequency operation. The steady state operation of the converter is discussed in detailed using operating waveforms and its equivalent circuits during different modes. A Simulink model of proposed converter is developed using MATLAB/ SimPowerSystemsTM and its simulation results are presented under various operating conditions. To validate the simulation results of proposed topology, a scaled prototype model is developed and experimentally tested under different operating conditions. The comparative evaluation of proposed converter under symmetrical and asymmetrical control method is carried out with respect to various parameters such as duty cycle control, voltage gain, transformer secondary winding current, voltage and current of rectifier diodes, thermal stress and ZVS and ZCS implementation. It is concluded that under symmetrical control method, all the power switches conduct uniformly unlike to asymmetrical control and hence uniform heat distribution is achieved with symmetrical control. Implementation of asymmetrical control technique requires dead band circuits as compared to symmetrical control. Furthermore, reduction in secondary side losses can be done by employing self driven synchronous rectifier replacing rectifier power diodes. v Many industrial applications such as welding, plasma cutting, and surface hardening require large DC current at low voltage. In such applications, the rating of power supply varies from several kilowatts to hundreds of kilowatts. The power supply employs in such applications particularly in arc welding process is expected to operate from open-circuit (no-load) to short-circuit (when the electrode sticks to the workpiece for a short span of time) quickly and also the transients occur during the striking of the arc, rapid arc length changes and metal transfer across the arc. The power supply must respond to these changes rapidly. In the present work, a multi-phase high frequency isolated DC-DC converter is proposed which is well suited for aforementioned applications. The proposed converter comprises of front end converter, two units of three-phase high frequency transformers and multi-phase rectifier stage at load end converter. In proposed topology, one three-phase transformer is configured in Yd1 and other transformer is configured inYd11 to provide phase shift for interleaved operation. The performance of proposed converter has been investigated under symmetrical and asymmetrical phase shifted PWM control methods with fixed frequency operation. In comparison with conventional welding machine employed in many industries, the size and weight, efficiency and dynamic response of proposed converter is improved significantly. The simulation and experimental results are obtained under different operating conditions and presented.
Appears in Collections:DOCTORAL THESES (Electrical Engg)

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