2. Thesis and Dissertations

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    Analysis and Design of Fixed-Frequency Controlled LCL-T Type DC-DC Soft-Switching Power Converter for Renewable Energy Applications
    (National Institute of Technology Karnataka, Surathkal, 2021) G, Vijaya Bhaskar Reddy.; Harischandrappa, Nagendrappa.
    Electrical power is one of the important requirements for sustainable development of any nation. A wider gap is being created between the power supply and the ever increasing power demand. The available conventional energy sources are either insufficient or cannot sustain for long to meet the current power demand as they are depleting in nature. Renewable energy sources (RESs) have been the most attractive alternate sources of energy for meeting the ever increasing power demand. Power generation from renewable energy sources depend on atmospheric conditions and hence the power produced is highly fluctuating in nature. To convert this fluctuating power into usable constant power, a power conditioning system is essential. DC-DC converter is one of the important components of the power conditioning system. This research is to find a suitable DCDC resonant power converter topology that can be used in solar power generation applications and investigate on its performance. Therefore, in this work, the literature survey on resonant converter topologies, power controlling methods, and analysis methods are presented. Fixed-frequency control makes the design of magnetic components and filters simple for effective filtering. Therefore, in this study, two fixedfrequency control schemes have been proposed. The first fixed-frequency control scheme is phase-shifted gating (PSG) control and the second is modified gating signal (MGS) control. The proposed PSG and MGS control schemes are experimentally validated and the choice between schemes is made by comparing the performance of the converter. It is found that both the gating schemes are effective in regulating the output voltage for variable input voltage and loading conditions. However, the efficiency of the converter is found to be higher with MGS due to the fact that only one switch loses ZVS as compared to two with the PSG when operated with maximum input voltage. Also, the variation in pulse-width angle (δ) required to regulate the output voltage is small in MGS as compared to that with PSG. The complete behavior of the resonant converter at different intervals of the operation can be predicted by analysing the circuit in steady-state iii and transient state. Two steady-state analysis methods have been proposed in this work. Firstly, fundamental harmonic approximation (FHA) method, and second, Fourier series (FS) method. The proposed steadystate analysis methods are experimentally validated. The performance of the LCL-T converter designed by using the FHA and FS analysis methods is compared. Fourier series method gives efficient results since it considers n-harmonic components of voltages and currents as compared to the fundamental harmonic approximation (FHA) method where, only fundamental component is considered. In order to understand the complete behavior of the converter for fluctuations in the input, load, and control parameters, small-signal modeling of the converter is essential. Therefore, an extended describing function (EDF) method available in the literature is used in this work for small signal modeling of the converter. It is convenient to derive all small-signal transfer functions and improve the accuracy by using the EDF method since it combines both the time-domain and frequency-domain analyses.
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    Investigations on High-Frequency Transformer Isolated CLL Resonant DC-DC Power Converter for Renewable Energy Applications
    (National Institute of Technology Karnataka, Surathkal, 2021) Patil, Uday.; Harischandrappa, Nagendrappa.
    Renewable energy sources have been the most promising substitute for conventional energy sources and are employed worldwide due to their cleanliness and sustainability. The power generated by using renewable energy sources is highly fluctuating as it depends on the environmental conditions and other operating conditions. A power conditioning unit comprising of DC-DC converter is required to convert this fluctuating power into a usable constant power. There is an increase in the demand of DC-DC power converters with compact size, higher power density and higher conversion efficiency for the applications involving wide variations in input voltage and load. These converters are also used in applications where electrical isolation is required. The intent of this research is in finding a suitable soft-switching DC-DC power converter topology and investigating on its performance for the applications in renewable energy generation. In this dissertation, literature review on various resonant power converter topologies and gating control scheme is carried out. The full bridge CLL resonant converter topology and fixed frequency gating scheme are chosen for the study. Three different converter topologies with suitable gating scheme were proposed and examined in detail. The first converter presented is a high-frequency transformer isolated CLL resonant DC-DC power converter operated with phase-shift gating scheme and modified PWM gating scheme. Modeling, analysis, design, simulation and experimental results of the converter are presented. Theoretical and experimental results are compared and performance of the converter when operated with both the gating schemes is analysed. A high-frequency isolated full bridge zero-voltage-transition (ZVT) CLL resonant DC-DC converter operated with a modified PWM gating scheme has been iv proposed. Various modes of operation of the converter are described using typical operating waveforms and equivalent circuit diagrams. Analysis, design, simulation and experimental results are presented and discussed. The proposed converter is able to provide ZVS for all the inverter switches for entire variations in input voltage and loading conditions. A three-phase interleaved full bridge CLL resonant DC-DC converter with fixed frequency modified PWM gating control has been proposed for medium to high power applications. Modeling and analysis of the converter is presented and the design procedure using a design example is explained. The designed converter is simulated using PSIM software to predict the performance of the converter for variations in input voltage and load conditions. The converter operates in ZVS for all the inverter switches with minimum input voltage and only one switch loses ZVS in each bridge/module for higher input voltages.
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    Development of Situational Awareness Platform for the Safety in Mining
    (National Institute of Technology Karnataka, Surathkal, 2021) Ramesh B.; Vittal, K Panduranga.
    Mining Industry has several safety requirements as per the regulations laid down by the government and other agencies. The environmental impact of the mining industry is one of the important aspects which needs to be monitored continuously as its impact concerns the health and safety of workers as well as residents. The gas samples from the mine area are generally drawn and checked for oxygen, methane, CO2, and CO gas. More methane gas in the absence of proper ventilation can cause severe health hazards to miners. Any deviation in the composition of the atmosphere especially in methane or CO could be sensed early and any untoward incidents like explosion or fire breakout could be prevented. The center of the study is to monitor, update, analyze and respond to a situation in and around mines. the center of the study conducted. To monitor the situation, sensor networks are utilized. The data from sensor networks helps to monitor the environmental parameters. Wireless Sensor Network (WSN)s are useful in many fields such as coal mine safety monitoring, agriculture management, healthcare, and also for vehicle monitoring. Sensor data collection using different embedded sensors, ARM7 microcontroller, and Zigbee is studied. The use of Arduino microcontroller board for monitoring is also studied. The study is mainly to monitor the parameters in the deep mining environment. The possibility of remotely monitoring updating and controlling the mining environment using Raspberry Pi is studied. The use of sensors and Thingspeak to get the sensor data on the web and to obtain its graph in realtime is explored. Then the controlling of the raspberry pi with the help of XBee communication and remotely controlling with the help of a computer is studied. This is done for the moisture level control using a relay and pump as an example. This method has also other applications. Making use of other types of sensors that are relevant for the mining environment, monitoring and control can be achieved. To analyze the situation, the data of five parameters namely Carbon Monoxide (CO), Sulfur Dioxide (SO2), Particulate Matter 10 (PM10), Particulate Matter 2.5 (PM2.5), and Ozone were analyzed for the year 2018 and 2019 for Singrauli of Madhya Pradesh state, where 10 open pit mines are operating. For Talcher of Odisha state, where deep coal mine is operational, the analysis was performed for the year 2019. The analysis is performed using different machine learning techniques like neural network curve fitting analysis and Self Organizing Maps. Graphical User Interface is developed using Matlab software to analyze the data and to display the environmental situation. This is done for both locations. The analyzed situation is tabulated for both locations.
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    Investigations on Performance Improvement of Electrical Power Distribution System with Incorporation of DSTATCOM and Distributed Energy Sources
    (National Institute of Technology Karnataka, Surathkal, 2021) Saralaya, Sanath.; Sharma, K Manjunatha.
    In recent years, the usage of nonlinear loads in the distribution system has increased drastically. Because of these nonlinear loads, power qual- ity problems result in the system. The power quality problems such as voltage sag-swell, harmonics, voltage icker and unbalance in the system voltage will appear. The current situation in the distribution system has become complicated due to the injection of power from renewable energy sources into the distribution grid. The integration of distributed gener- ation source to the distribution network has created more power quality issues such as voltage unbalance and harmonics problem. Due to these facts, in recent years, globally there is more concern for addressing power quality problems in modern distribution network. Custom power device is one of the solutions to solve power quality issues in distribution system. Distribution Static Compensator (DSTATCOM) is used in distribution system for performance improvement. This device supply the reactive power to the distribution system to ensure power factor improvement, voltage regulation along with harmonics mitigation, load balancing and neutral current compensation. Also, DSTATCOM is expected to enhance the performance of distribution system in presence of distributed genera- tion sources. Firstly, the e cient working of DSTATCOM depends on the control algo- rithm which is used to generate switching pulses for controlling devices. Most of controller, proposed in the literature, considers only positive se- quence component. To overcome these problems, there is a need of devel- oping a control strategy which has the ability to reduce negative sequence components and have advantages of both current and voltage control mode for the unbalanced load condition. In this work an improved control al- gorithm is proposed. This research work presents investigation of per- formance of DSTATCOM with proposed positive sequence controller and proposed positive and negative sequence controller in distribution system under di erent operation scenarios. The performance of proposed con- trollers are analysed for a test system and a 9 bus distribution system. Secondly, in modern power system, the penetration of renewable energy sources has increased since there are no negative impact on environment. v According to grid codes, renewable energy sources are not required to contribute to restore the voltage stability. As a result, the reliability and voltage stability of the power grid is negatively a ected due to distur- bances. The DSTATCOM can be used to improve the low voltage ride through behavior of micro grids in both grid connected and islanded mode of operation. The compensation of positive and negative sequences com- ponents are required to support voltage restoration as well as the power grid stabilization and DSTATCOM should be able to give satisfactory per- formance in improving low voltage ride through capability. In this work, the performance of DSTATCOM in di erent fault scenario and low volt- age ride through capacity of controller are studied. Lastly, to get e cient operation, the DSTATCOM should be optimally placed and size should be determined. The primary goal of this work is to lessen the active power loss of the radial distribution system with voltage pro le improvement. In this report, Ant Colony Optimization technique is utilized for the ideal position of DSTATCOM in the distri- bution framework for lessening of line losses and rise of voltage pro le. Later, the performance is analysed by incorporating the DSTATCOM at an optimised location in a practical system. When DG is incorporated into the system, the characteristics of the system will change; it will lead to more power quality issues in the power distribution system. This op- erating scenario should also be considered during performance analysis of DSTATCOM and there is a need to study impact of DSTATCOM and DG on distribution system. The MATLAB simulation is carried out and the performance of the pro- posed controllers are analysed with di erent cases by considering unbal- anced load conditions and step change in the load. Also, the voltage ride through capability of DSTATCOM with proposed controller is studied. The observed results demonstrate that the overall system performance improves with the proposed positive and negative sequence controller. The ant colony optimisation algorithm is applied on IEEE 33 and a real time practical 207 radial distribution system by using MATLAB program. Results demonstrate the reduction in line losses and rise in voltage pro le for all test frameworks. From the impact analysis of distributed generators and DSTATCOM on distribution system it is observed that the proper se- lection of DSTATCOM and distributed generator are need to be done for improving distribution systems performance.
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    Efficient Control of Power Conversion Interfaces for Solar Grid-Tie Inverter
    (National Institute of Technology Karnataka, Surathkal, 2021) Damodaran, Roopa Viswadev.; Venkatesaperumal, B.
    Amongst the available renewable sources, solar photovoltaic (PV) energy sources is becoming dominant due to its long operational life, lesser emission and decreasing installation and maintenance costs. The grid integration of PV sources eliminates the requirement of additional storage and provides support for the peak loads. With the increasing number of PV sources integrated to the grid, the standards for grid integration are continuously revised in order to ensure that only stable, safe, e cient and reliable systems are integrated with the grid. The present standards recommend the sources to stay connected to provide support to the grid especially during voltage sags. Due to this reason, developing improved topologies and control strategies for power conversion interfaces (PCI) that can perform satisfactorily under varying grid conditions. Several control techniques for PCIs exist in literature based on the pulse width modulation (PWM). Amongst these, the hysteresis control (HC) exhibits superior performance when compared to other conventional PWM techniques. The HC is one of the most simple modulation techniques especially for grid tie inverters (GTIs). It regulates the ripple current output of GTI within xed hysteresis limits. This results in a varying switching frequency which is not implicitly known prior to control implementation. Hence, HC of conventional PCI for GTI is hindered by its varying switching frequency, requirement of high precision AC current sensor and undecided switching intervals. Due to these reasons, HC is also not often used in novel PCI topologies, despite all its advantages. In this thesis, the above mentioned shortcomings are discussed along with their solutions so as to improve the HC. These contributions can provide assistance to researchers and engineers in design and implementation of applications such as GTIs using HC. The output lter design, switching device selection, switching and conduction loss calculations of GTIs are predominantly dependent on the switching frequency. Hence, estimating the minimum and maximum switching frequencies is essential for a reliable system design. The existing estimations for HC assume linear ripple current. Since the frequency variation is large in HC, this assumption is invalid for the range of low frequencies. Inaccurate estimation of switching frequency can have considerable e ect on system design. In this thesis, a more precise and generalized expression to estimate the switching frequency of multilevel GTI is obtained by time-domain analysis. The accurate estimation results in the improvement of system design which is demonstrated with an example of a second order lter. iii The e ect of changes in system parameters on the switching frequency is also analysed to determine the operating point for an accurate system design. One other limitation of HC is the requirement of high precision AC current sensors. Though no current sensorless HC can be found in the literature, the current emulation technique of eliminating current sensor may be modi ed and implemented for HC. However, the computational requirement for such a control is high. To this end, an AC current sensoreless HCC for two-level GTI is developed by formulating the switching intervals as a function of known system variables. All real time conditions such as non-linearity of ripple current, dynamic changes in operating conditions and e ects of digital sampling are considered while developing the algorithm of the proposed AC current sensorless HC. Due to the uncertain switching intervals of HC, its use in the relatively novel PCIs are not vastly explored. Hence in this work, the closed loop controls for Z-source inverter (ZSI) and microinverter ( I) using HC are presented. A single stage PCI with independent input and output side controls can be achieved using ZSI. The shoot through interval is an important parameter which decides the boost ratio of ZSI. For ZSI with HC, determining the shoot through interval is di cult due to the undecided switching intervals. Hence, in this thesis, a detailed analysis of ZSI as a single-stage PCI is discussed. A closed loop control using HC with shoot through error estimation is developed based on this analysis. AC modules and the associated PIC, commonly referred to as Is have been gaining attention due to the advantage of eliminating partial shading in series connected PV modules. The prime features of a I are high e ciency, high gain ratio with less number of switching components and preferably without using coupling inductors or transformers, simple control and reduced THD. Keeping these in mind, an e cient I with pseudo DC-link (pDC-l) for grid integration of AC module is proposed as a part of this work. A high gain buck-boost Z-source converter (ZSC) is used to obtain a pDC-l from a single PV module. An unfolding circuit follows the ZSC. It operates at fundamental frequency which reduces the switching losses without a ecting the THD. A simple closed loop control using voltage HC is developed to obtain a pseudo DC-link voltage. A current HC ensures power balance by controlling the current to the inverter bridge. Simulations and experimental validations are carried out to verify the theoretical analysis and ensure the proposed controls achieve the standards of grid integration of PV systems. The simulation studies are carried out in MATLAB while ensuring the real time operating conditions.
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    Pulse Amplitude Modulation Control of BLDC Motor using Bridgeless SEPIC with Coupled Inductors
    (National Institute of Technology Karnataka, Surathkal, 2021) Pavana; U, Vinatha
    This thesis presents a novel approach for the control of brushless direct current (BLDC) motor using pulse amplitude modulation (PAM) control of voltage source inverter (VSI). The PAM control of VSI is accomplished by using a one-cycle controlled bridgeless single ended primary inductance converter (SEPIC) with coupled inductors. The PAM control also known as DC link voltage control of VSI, reduces switching losses by allowing the operation of VSI at fundamental frequency. The adoption of the coupling between the input and output side inductors in the bridgeless SEPIC converter reduces the value of the inductance required, allows better integration of magnetic components and hence lowers the overall size compared to conventional bridgeless SEPIC. In the work presented in this thesis, the design of coupled inductors for the bridgeless SEPIC is done using the split winding scheme. The conventional method of achieving the desired coupling by adjusting the air-gap is tedious. Using the split winding scheme, a mathematical approach is proposed to obtain the closed form solution for distribution of windings over three limbs of E-core to achieve the desired coupling. The bridgeless SEPIC with coupled inductors is designed to get a wide variation of DC link voltage and is operated in discontinuous conduction mode (DCM) for the complete range of DC link voltage. The DCM operation simplifies power factor correction (PFC) control scheme to a voltage mode control, since it has inherent input current shaping feature. One-cycle control technique which is a nonlinear control technique, applied in the voltage mode scheme improves the quality of supply current by reducing the distortion compared to proportional-integral (PI) control technique. The one-cycle control also enhances the performance of DC link voltage control, with improved start-up and transient state response.The proposed BLDC motor drive is modelled. The superiority of one-cycle control technique over PI control for a wide range of speed control is validated using simulation results. Using the laboratory prototype of bridgeless SEPIC with coupled inductors, the achievement of supply current shaping and adjustable speed in the BLDC motor drive is verified experimentally using field programmable gate array (FPGA) based digital PI controller.
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    Multiple-Terminal Grid Interconnected Offshore Wind Farms: Development of Transient Behavioural Simulation Models and Protection Schemes
    (National Institute of Technology Karnataka, Surathkal, 2021) Mohan M.; Vittal, K Panduranga.
    Offshore wind farms (OWF) are the highly penetrative energy sources in the electric power systems due to the continuous increase in energy demand. The generated offshore wind power cannot be supplied directly to the customers due to the variable generations and installed locations; instead, it can be integrated with the AC grid using high voltage alternating current (HVAC) or high voltage direct current (HVDC) link. The HVDC link is mostly preferred for the long-distance bulk power transmission due to its various advantages such as low losses, possible to control the power flow and perform an asynchronous operation, no charging current and stability issues. Voltage source converters (VSC)-based HVDC transmission system is a favorable option to interconnect the remote renewable sources with the AC grids since it has fault current blocking capability, operation with weak AC grids and maintains constant DC voltage even if power direction changes. Multi-terminal (MT) HVDC network becomes more attractive over two-terminal configurations due to the reduced number of terminals and sustains the power flow even under fault in a DC line. The research work in this thesis built transient behavioural model of the multi-terminal VSC-based HVDC link connected offshore wind farms, and also different case studies are carried out to evaluate the performance of the MT VSC-based HVDC system under power system disturbances. One of the main limitations of VSC is its vulnerability to DC faults. The protection of the DC line is more challenging due to low impedance, no natural zero crossing, low rise time and high steady-state fault currents. DC faults in a multi-terminal VSC-based HVDC transmission system gives very high peak fault current within a few milliseconds. The protection unit installed in the AC grid can address only steady-state faults in the DC grid. Protection unit has to be developed before semiconductor-based device damages due to very less overload capability. Two-end measurement gives certain time delay for long transmission lines which will slow down the protection decision speed. This thesis presents the development of a single-ended protection scheme for DC faults in multi-terminal VSC-based HVDC transmission system both without and with current limiting reactors. In this protection scheme, the protection iii starting unit uses the under-voltage criterion to detect the faults. The fault discrimination is done by using three conditions such as rate of change of DC voltage and current, and increment of transient energy. Current limiting reactors are designed and connected in series with the DC circuit breaker (CB) to maintain the DC fault current within the breaker capacity until protection unit isolates the faulty line. With the penetration of VSC-based HVDC system into the AC grid, the challenges in the distance relaying of AC transmission line has increased. When the fault occurs in a line close to the point of common coupling (PCC) in an AC grid with VSC-based HVDC transmission system, Zone-2 distance relay overestimates the fault distance due to the fast control of VSC. This makes distance relay to treat Zone-2 fault as Zone-3 fault, whereas Zone-3 fault is pushed out of the protection zones, leads to protection miscoordination. Therefore, the research work in this thesis intends to investigate the impact of VSC-based HVDC system on distance protection of AC transmission lines.
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    Performance Analysis and Improvement of Power Systems Ring-Down Electromechanical Mode Identification Algorithms
    (National Institute of Technology Karnataka, Surathkal, 2021) Rao, Krishna.; Shubhanga, K N.
    With the commissioning of Wide Area Measurement Systems (WAMS) in large power grids, measurement-based mode identification is finding wide application. From power system stability viewpoint, mode identification from ring-down signals is important. Although ring-down identification algorithms have been studied for a few decades, these still have a scope for improvement. For example, Signal-to-Estimation-error Ratio (SER), which is the recommended fitness metric to compare original and estimated signals in iterative Prony method, sometimes performs suboptimally. So a superior metric is proposed here by combining SER withMean Absolute Percentage Error (MAPE). Another popular ring-down algorithm is matrix pencil, which is normally presented in a non-iterative formulation. It is shown here that iterative formulation of matrix pencil is feasible and is slightly faster than iterative Prony. From the viewpoint of mode identification of noisy signals, Singular Value Decomposition (SVD)-based non-iterative algorithms are reported to be superior. Hence three such algorithms, namely, Total Least Squares matrix pencil (TLS matrix pencil), Hankel Total Least Squares (HTLS) and Eigensystem Realization Algorithm (ERA) are evaluated comparatively. In the process, it is shown that TLS matrix pencil and HTLS algorithms are equivalent. Evident improvement in matrix pencil algorithm performance by incorporation of SVD suggests the same possibility in Prony algorithm. So a customized formulation of Structured Total Least Squares-Prony (STLS-Prony) algorithm is developed for application to power systems. This is compared with two known formulations of SVD-augmented Prony algorithm, namely, Principal Eigenvector-Prony (PE-Prony) and Total Least Squares-Prony (TLS-Prony). A Taylor series-augmented Fourier transform called Digital Taylor-Fourier Transform (DTFT) is examined for its ability to handle exponentially varying sinusoids and a novel concept termed neper response is put forth to characterize the same. It is shown that the computational efficiency of DTFT-based mode identification can be improved greatly by raising the Taylor series order.
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    Investigations on Low Switching Frequency Pulse Width Modulation Techniques for Lower Order Harmonic Elimination and Switching Loss Reduction in Voltage Source Inverter Fed Induction Motor Drive Applications
    (National Institute of Technology Karnataka, Surathkal, 2021) Arumalla, Raviteja.; Harischandrappa, Nagendrappa.; Figarado, Sheron.
    Obtaining a better harmonic profile at relatively lower switching frequencies is one of the key tasks in the present day power electronics world. The pulse width modulation (PWM) techniques used in the inverter influence the harmonic content of the inverter output waveform and its switching power loss. Several PWM techniques have been developed earlier to improve the harmonic profile of the inverter output waveform. Dodecagonal space vector generation technique is one of them, which gives a better harmonic profile by eliminating the most dominant lower order harmonics, that is, fifth and seventh from the inverter output waveform. The elimination of fifth and seventh harmonics results in improvement in the quality of the output waveform and eliminates the sixth harmonic torque pulsations in the motor drives. By considering these advantages, two groups of dodecagonal space vector-based clamping PWM (DSVCPWM) techniques are developed in this report. The switching sequences in the first group use the zero vector once in every sub-cycle period. On the other hand, the second group of voltage vector sequences switch an active vector twice in every sub-cycle period. The proposed DSVCPWM techniques facilitate lowering of inverter switching power loss for lagging power factor loads and the better harmonic profile at higher modulation indices by clamping each phase to either positive or negative DC bus during different intervals in a fundamental cycle. Both groups have the same clamping durations in a fundamental cycle. However, due to the difference in their switching locations, the switching power loss will vary. Moreover, the variable clamping PWM techniques in the DSVCPWM group control the clamping location for a specific duration, which results in a lower switching power loss than C12SVPWM and other DSVCPWM techniques. In this report, a generalized expression for harmonic distortion of various dodecagonal PWM schemes is derived. Using this expression, one can determine the harmonic distortion magnitude for any dodecagonal PWM technique at a specified modulation index (MI) and boundary ani gle. The proposed DSV based PWM techniques are simulated in MATLAB Simulink environment and validated experimentally on a laboratory prototype. In the proposed DSVCPWM techniques, the dodecagonal space vector-based busclamping PWM technique shows a better harmonic profile than C12SVPWM and other DSVCPWM techniques at higher modulation indices. Similarly, based on the switching loss analysis, the proposed variable clamping PWM techniques have lower switching power loss than other dodecagonal PWM techniques. Better harmonic profile at higher MI and significant reduction in switching losses for lagging power factors make the variable clamping PWM technique as a good choice for the high-speed region of the motor drive applications. In addition to the development of dodecagonal space vector-based clamping PWM techniques, a novel space vector-based approach is introduced in this report, which selectively eliminates the lower order harmonics from the inverter output waveform based on voltage vector dwell time rearrangement. This technique uses a volt-second balance for control of fundamental voltage while using the dwell time rearrangement of active or zero vector in a sub-cycle to eliminate selected harmonic. The dwell time rearrangement concept is demonstrated for the hexagonal space vector to eliminate the fifth harmonic or seventh harmonic from the inverter output waveform. Further, this concept is implemented in a dodecagonal space vector structure to eliminate the eleventh harmonic or thirteenth harmonic. The proposed techniques are validated through the MATLAB Simulink environment, and their performance characteristics are compared with other space vector based PWM techniques in terms of harmonic magnitudes and voltage weighted harmonic distortion.
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    Optimized Design of Collector System for Offshore Wind Farms and Development of A Hybrid Controller for Single VSC-HVDC and Multi-Terminal VSC-HVDC System
    (National Institute of Technology Karnataka, Surathkal, 2020) Srikakulapu, Ramu.; U, Vinatha.
    This thesis deals with the optimal design of the electrical collector system of offshore wind farms (OSWFs) and the design of a robust controller for the grid-integrated OSWF with voltage source converter (VSC)- high voltage direct current (HVDC) transmission system. The worldwide installation of offshore wind farms consists of hundreds of higher rated wind turbines, which have been significantly increased in number due to their economic benefits. First part of the work in this report describes an efficient approach for improving the wind farm power production by appropriate placement of wind turbines in OSWF using the larsen and jensen wake models. A new optimization approach based on (a) elitist ant colony optimization for travelling salesman problem and multiple travelling salesmen problem and (b) firefly algorithm for travelling salesman problem and multiple travelling salesmen problem are applied to design an optimal electrical collector system for OSWF with the objective of minimizing inter-array cable length and there by reducing the cost of power production. The objective function of the electrical collector system design is expressed based on the levelized production cost and aims to minimize the levelized production cost, minimize the length of the inter-array cable between the wind turbines, achieve wake loss reduction, and optimize the power production of OSWF. The proposed approach is tested using North Hoyle and Horns Rev OSWFs with 30 and 80 wind turbines, respectively and the results obtained is observed as a valid optimal electrical collector system design. The thesis further proposes a new hybrid controller for AC grid integrated offshore wind farm with VSC-HVDC transmission system and AC grid integrated offshore wind farms with multi-terminal VSC-HVDC transmission system. It is combination of proportional{integral (PI) based inner and sliding mode control based outer controller. With the hybrid controller, the VSCs of the HVDC transmission system are connected for control of the AC voltage, DC-link voltage, reactive power and effective power transfer between the OSWFs and an onshore AC grid. An evolutionary algorithm and proportional{integral{derivative (PID) tool are iiiutilized to realize the tuned gain parameters for hybrid and conventional controllers. The FRT capability, small signal analysis, and controller stability of the VSC-HVDC systems are analyzed. To check the stability of the system, small signal stability analysis is carried out with the hybrid controller and performance is compared with conventional PI controller. To examine the fault ride through (FRT) capability, a symmetrical fault and unsymmetrical fault are applied at an onshore AC grid side and the performances of the system based on the hybrid and PI controllers are analyzed. Dynamic model and linerized state-space model of the VSC-HVDC systems with hybrid and conventional controllers are developed. The analysis of the VSC-HVDC systems with hybrid and conventional controllers is conducted in the software environment of the MATLAB/Simulink. The simulation results show that the proposed control scheme provides effective active power transmission, AC voltage control, minimum reactive power transfer among the VSCs, and DC-link voltage regulation in the presence of system uncertainties and faulty condition. The controller stability is observed with the help of the Nyquist plot and eigenvalue analysis. The effect of parameter uncertainty on total system stability is examined with the help of eigenmatrix of the VSC-HVDC system.