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- Genre: Academic theses
- Creators: Ayyanar, Raja
Description
DC-DC converters are widely employed to interface one voltage level with another through step-up or step-down operation. In recent years, step-up DC-DC converters have been a key component in harnessing energy through renewable sources by providing an interface to integrate low voltage systems to DC-AC converters or microgrids. They find increasing applications in battery and fuel cell electric vehicles which can benefit from high and variable DC link voltage. It is important to optimize these converters for higher efficiency while achieving high gain and high power density. Non-isolated DC-DC converters are an attractive option due to the reduced complexity of magnetic design, smaller size, and lower cost. However, in these topologies, achieving a very high gain along with high efficiency has been a challenge. This work encompasses different non-isolated high gain DC-DC converters for electric vehicle and renewable energy applications. The converter topologies proposed in this work can easily achieve a conversion ratio above 20 with lower voltage and current stress across devices. For applications requiring wide input or output voltage range, different control schemes, as well as modified converter configurations, are proposed. Moreover, the converter performance is optimized by employing wide band-gap devices-based hardware prototypes. It enables higher switching frequency operation with lower switching losses. In recent times, multiple soft-switching techniques have been introduced which enable higher switching frequency operation by minimizing the switching loss. This work also discusses different soft-switching mechanisms for the high conversion ratio converter and the proposed mechanism improves the converter efficiency significantly while reducing the inductor size. Further, a novel electric vehicle traction architecture with low voltage battery and multi-input high gain DC-DC converter is introduced in this work. The proposed architecture with multiple 48 V battery packs and integrated, multi-input, high conversion ratio DC-DC converters, can reduce the maximum voltage in the vehicle during emergencies to 48 V, mitigate cell balancing issues in battery, and provide a wide variable DC link voltage. The implementation of high conversion ratio converter in multiple configurations for the proposed architecture has been discussed in detail and the proposed converter operation is validated experimentally through a scaled hardware prototype.
ContributorsGupta, Ankul (Author) / Ayyanar, Raja (Thesis advisor) / Lei, Qin (Committee member) / Bakkaloglu, Bertan (Committee member) / Ranjram, Mike (Committee member) / Arizona State University (Publisher)
Created2022
Wide Bandgap Semiconductor Based Electric Vehicle Charging Systems: Modeling, Magnetics, and Control
Description
Adhering to an ever-increasing demand for innovation in the field of onboard electric vehicle (EV) charging, several technical aspects pertaining to the design and performance enhancement of integrated multi-port charger topologies are discussed in this study. This study also elucidates various research challenges pertaining to each module of the topology and elucidates technically validated solutions for each.Firstly, targeting the input side totempole power factor corrector (TPFC) circuit, a novel digital filter based Active Mitigation Scheme (AMS) is proposed to curb the third harmonic component, along with a novel discretized sampling-based robust control scheme. Experimental verification of these techniques yields an enhanced Total Harmonic Distortion (THD) of 1.68%, enhanced efficiency of 98.1% and resultant power factor of 0.998 (lag).
Further, focusing on the bidirectional CLLC based DC/DC converter topology, a general harmonic approximation (GHA) based secondary side turnoff current minimization technique is discussed. Numerous fabrication and design-based constraints and correlations for parametric modelling of high frequency planar transformer (HFPT) are explained with analytical and 3D Finite Element Analysis (FEA) findings. Further, characterization of the plant transfer function of all-inclusive CLLC model is described along with hybrid Sliding Mode Control (SMC) based control scheme. The steady state experimental results at 1kW rated load show a peak efficiency of 98.49%, while the quantification of dynamic response portray a settling time reduction of 46.4% and an over/undershoot reduction of 33%.
Further, comprehensive modeling of triple active bridge (TAB) DC/DC converter topology is presented with special focus on the control scheme and decoupling capabilities to independently regulate the output bridges. With an objective to reduce the overall losses and to add a dimension of controllability, a three-loop control scheme is proposed with power flow optimization. Inculcating the benefits of multiport and resonant topologies, a comprehensive multi-variable loss optimization study of a Triple Active C^3 L^3 (TAC^3L^3) converter is discussed. The performance of eight different hybrid modulation schemes is compared with respect to the developed global loss minimization objective function. Experimental validations for various loading conditions are presented for a wide-gain bidirectional operation (400V/500-600V/24-28V), portraying a peak converter efficiency of 97.42%.
ContributorsChandwani, Ashwin Vijay (Author) / Mallik, Ayan (Thesis advisor) / Ayyanar, Raja (Thesis advisor) / Kannan, Arunanchala Mada (Committee member) / Hedman, Mojdeh (Committee member) / Arizona State University (Publisher)
Created2022
Description
In recent years, there has been an increasing need for effective voltage controls in power systems due to the growing complexity and dynamic nature of practical power grid operations. Deep reinforcement learning (DRL) techniques now have been widely explored and applied to various electric power operation analyses under different control structures. With massive data available from phasor measurement units (PMU), it is possible to explore the application of DRL to ensure that electricity is delivered reliably.For steady-state power system voltage regulation and control, this study proposed a novel deep reinforcement learning (DRL) based method to provide voltage control that can quickly remedy voltage violations under different operating conditions. Multiple types of devices, adjustable voltage ratio (AVR) and switched shunts, are considered as controlled devices. A modified deep deterministic policy gradient (DDPG) algorithm is applied to accommodate both the continuous and discrete control action spaces of different devices. A case study conducted on the WECC 240-Bus system validates the effectiveness of the proposed method.
System dynamic stability and performance after serious disturbances using DRL are further discussed in this study. A real-time voltage control method is proposed based on DRL, which continuously regulates the excitation system in response to system disturbances. Dynamic performance is considered by incorporating historical voltage data, voltage rate of change, voltage deviation, and regulation amount. A versatile transmission-level power system dynamic training and simulation platform is developed by integrating the simulation software PSS/E and a user-written DRL agent code developed in Python. The platform developed facilitates the training and testing of various power system algorithms and power grids in dynamic simulations with all the modeling capabilities available within PSS/E. The efficacy of the proposed method is evaluated based on the developed platform.
To enhance the controller's resilience in addressing communication failures, a dynamic voltage control method employing the Multi-agent DDPG algorithm is proposed. The algorithm follows the principle of centralized training and decentralized execution. Each agent has independent actor neural networks and critic neural networks. Simulation outcomes underscore the method’s efficacy, showcasing its capability in providing voltage support and handling communication failures among agents.
ContributorsWang, Yuling (Author) / Vittal, Vijay (Thesis advisor) / Ayyanar, Raja (Committee member) / Pal, Anamitra (Committee member) / Hedman, Mojdeh (Committee member) / Arizona State University (Publisher)
Created2024
Description
Electric substation physical plans are developed with consideration given to lightning protection. To develop these plans utility design engineers use various methods. This thesis focuses on developing a computer program for two methods/models for substation shielding against direct lightning strokes. The first method is being used currently in the industry to protect the substation structures. The second model is a new and more physics based approach towards lightning phenomenon. Both the methods consider only direct lightning strikes that can hit the substation equipment. Hence, the travelling waves, indirect strokes or over-voltage arriving at the substation equipment are not considered. The Electro-Geometric method (EGM) based Rolling Sphere Method (RSM) is used to develop first part of the program. The aim of the program is to design the protection system for the substation equipment quickly and error free. The protection system uses lightning masts and/or shield wires to protect the station equipment. These are grounded solidly with low impedance to earth. The MATLAB based program gives a two dimensional visual representation of the zone of protection and therefore helps utility engineers to position shielding system. As this program is converted further into an executable file, it can be used on any computer to produce the results without need of any other software. The second part of the thesis focuses on developing the MATLAB code for protection of substation equipment using the Rizk model which is not used as of now for shielding system design in industry. Using more physics based model, simulation of downward lightning leader and connecting upward leader is shown.
Finally both the methods are compared. This includes consideration of a 220 kV substation layout arrangement. The equipment are protected using shielding masts and the comparison is made in terms of number of the protective equipment needed. It is found that the classical rolling sphere model gives more conservative results than the physics based model. Hence the results shows that it is possible to use present methods and still protect the equipment sufficiently.
Finally both the methods are compared. This includes consideration of a 220 kV substation layout arrangement. The equipment are protected using shielding masts and the comparison is made in terms of number of the protective equipment needed. It is found that the classical rolling sphere model gives more conservative results than the physics based model. Hence the results shows that it is possible to use present methods and still protect the equipment sufficiently.
ContributorsMarathe, Vinit (Author) / Karady, George G. (Thesis advisor) / Ayyanar, Raja (Committee member) / Holbert, Keith E. (Committee member) / Arizona State University (Publisher)
Created2016
Description
Constraint relaxation by definition means that certain security, operational, or financial constraints are allowed to be violated in the energy market model for a predetermined penalty price. System operators utilize this mechanism in an effort to impose a price-cap on shadow prices throughout the market. In addition, constraint relaxations can serve as corrective approximations that help in reducing the occurrence of infeasible or extreme solutions in the day-ahead markets. This work aims to capture the impact constraint relaxations have on system operational security. Moreover, this analysis also provides a better understanding of the correlation between DC market models and AC real-time systems and analyzes how relaxations in market models propagate to real-time systems. This information can be used not only to assess the criticality of constraint relaxations, but also as a basis for determining penalty prices more accurately.
Constraint relaxations practice was replicated in this work using a test case and a real-life large-scale system, while capturing both energy market aspects and AC real-time system performance. System performance investigation included static and dynamic security analysis for base-case and post-contingency operating conditions. PJM peak hour loads were dynamically modeled in order to capture delayed voltage recovery and sustained depressed voltage profiles as a result of reactive power deficiency caused by constraint relaxations. Moreover, impacts of constraint relaxations on operational system security were investigated when risk based penalty prices are used. Transmission lines in the PJM system were categorized according to their risk index and each category was as-signed a different penalty price accordingly in order to avoid real-time overloads on high risk lines.
This work also extends the investigation of constraint relaxations to post-contingency relaxations, where emergency limits are allowed to be relaxed in energy market models. Various scenarios were investigated to capture and compare between the impacts of base-case and post-contingency relaxations on real-time system performance, including the presence of both relaxations simultaneously. The effect of penalty prices on the number and magnitude of relaxations was investigated as well.
Constraint relaxations practice was replicated in this work using a test case and a real-life large-scale system, while capturing both energy market aspects and AC real-time system performance. System performance investigation included static and dynamic security analysis for base-case and post-contingency operating conditions. PJM peak hour loads were dynamically modeled in order to capture delayed voltage recovery and sustained depressed voltage profiles as a result of reactive power deficiency caused by constraint relaxations. Moreover, impacts of constraint relaxations on operational system security were investigated when risk based penalty prices are used. Transmission lines in the PJM system were categorized according to their risk index and each category was as-signed a different penalty price accordingly in order to avoid real-time overloads on high risk lines.
This work also extends the investigation of constraint relaxations to post-contingency relaxations, where emergency limits are allowed to be relaxed in energy market models. Various scenarios were investigated to capture and compare between the impacts of base-case and post-contingency relaxations on real-time system performance, including the presence of both relaxations simultaneously. The effect of penalty prices on the number and magnitude of relaxations was investigated as well.
ContributorsSalloum, Ahmed (Author) / Vittal, Vijay (Thesis advisor) / Hedman, Kory (Thesis advisor) / Heydt, Gerald (Committee member) / Ayyanar, Raja (Committee member) / Arizona State University (Publisher)
Created2016
Description
As the world becomes more electronic, power electronics designers have continuously designed more efficient converters. However, with the rising number of nonlinear loads (i.e. electronics) attached to the grid, power quality concerns, and emerging legislation, converters that intake alternating current (AC) and output direct current (DC) known as rectifiers are increasingly implementing power factor correction (PFC) by controlling the input current. For a properly designed PFC-stage inductor, the major design goals include exceeding minimum inductance, remaining below the saturation flux density, high power density, and high efficiency. In meeting these goals, loss calculation is critical in evaluating designs. This input current from PFC circuitry leads to a DC bias through the filter inductor that makes accurate core loss estimation exceedingly difficult as most modern loss estimation techniques neglect the effects of a DC bias. This thesis explores prior loss estimation and design methods, investigates finite element analysis (FEA) design tools, and builds a magnetics test bed setup to empirically determine a magnetic core’s loss under any electrical excitation. In the end, the magnetics test bed hardware results are compared and future work needed to improve the test bed is outlined.
ContributorsMeyers, Tobin (Author) / Ayyanar, Raja (Thesis advisor) / Qin, Jiangchao (Committee member) / Lei, Qin (Committee member) / Arizona State University (Publisher)
Created2019
Description
Modular multilevel converters (MMCs) have become an attractive technology for high power applications. One of the main challenges associated with control and operation of the MMC-based systems is to smoothly precharge submodule (SM) capacitors to the nominal voltage during the startup process. The existing closed-loop methods require additional effort to analyze the small-signal model of MMC and tune control parameters. The existing open-loop methods require auxiliary voltage sources to charge SM capacitors, which add to the system complexity and cost. A generalized precharging strategy is proposed in this thesis.
For large-scale MMC-embedded power systems, it is required to investigate dynamic performance, fault characteristics, and stability. Modeling of the MMC is one of the challenges associated with the study of large-scale MMC-based power systems. The existing models of MMC did not consider the various configurations of SMs and different operating conditions. An improved equivalent circuit model is proposed in this thesis.
The solid state transformer (SST) has been investigated for the distribution systems to reduce the volume and weight of power transformer. Recently, the MMC is employed into the SST due to its salient features. For design and control of the MMC-based SST, its operational principles are comprehensively analyzed. Based on the analysis, its mathematical model is developed for evaluating steady-state performances. For optimal design of the MMC-based SST, the mathematical model is modified by considering circuit parameters.
One of the challenges of the MMC-based SST is the balancing of capacitor voltages. The performances of various voltage balancing algorithms and different modulation methods have not been comprehensively evaluated. In this thesis, the performances of different voltage-balancing algorithms and modulation methods are analyzed and evaluated. Based on the analysis, two improved voltage-balancing algorithms are proposed in this thesis.
For design of the MMC-based SST, existing references only focus on optimal design of medium-frequency transformer (MFT). In this thesis, an optimal design procedure is developed for the MMC under medium-frequency operation based on the mathematical model of the MMC-based SST. The design performance of MMC is comprehensively evaluated based on free system parameters.
For large-scale MMC-embedded power systems, it is required to investigate dynamic performance, fault characteristics, and stability. Modeling of the MMC is one of the challenges associated with the study of large-scale MMC-based power systems. The existing models of MMC did not consider the various configurations of SMs and different operating conditions. An improved equivalent circuit model is proposed in this thesis.
The solid state transformer (SST) has been investigated for the distribution systems to reduce the volume and weight of power transformer. Recently, the MMC is employed into the SST due to its salient features. For design and control of the MMC-based SST, its operational principles are comprehensively analyzed. Based on the analysis, its mathematical model is developed for evaluating steady-state performances. For optimal design of the MMC-based SST, the mathematical model is modified by considering circuit parameters.
One of the challenges of the MMC-based SST is the balancing of capacitor voltages. The performances of various voltage balancing algorithms and different modulation methods have not been comprehensively evaluated. In this thesis, the performances of different voltage-balancing algorithms and modulation methods are analyzed and evaluated. Based on the analysis, two improved voltage-balancing algorithms are proposed in this thesis.
For design of the MMC-based SST, existing references only focus on optimal design of medium-frequency transformer (MFT). In this thesis, an optimal design procedure is developed for the MMC under medium-frequency operation based on the mathematical model of the MMC-based SST. The design performance of MMC is comprehensively evaluated based on free system parameters.
ContributorsZhang, Lei (Author) / Qin, Jiangchao (Thesis advisor) / Ayyanar, Raja (Committee member) / Weng, Yang (Committee member) / Wu, Meng (Committee member) / Arizona State University (Publisher)
Created2020
Description
Power systems are undergoing a significant transformation as a result of the retirements of conventional coal-fired generation units and the increasing integration of converter interfaced renewable resources. The instantaneous renewable generation penetration as a percentage of the load served in megawatt (MW), in some areas of the United States (U.S.) sometimes approaches over 50 percent. These changes have introduced new challenges for reliability studies considering the two functional reliability aspects, i.e., adequacy and the dynamic security or operating reliability.
Adequacy assessment becomes more complex due to the variability introduced by renewable energy generation. The traditionally used reserve margin only considers projected peak demand and would be inadequate since it does not consider an evaluation of off-peak conditions that could also be critical due to the variable renewable generation. Therefore, in order to address the impact of variable renewable generation, a probabilistic evaluation that studies all hours of a year based on statistical characteristics is a necessity to identify the adequacy risks. On the other hand, the system dynamic behavior is also changing. Converter interfaced generation resources have different dynamic characteristics from the conventional synchronous units and inherently do not participate in grid regulation functions such as frequency control and voltage control that are vital to maintaining operating reliability. In order to evaluate these evolving grid characteristics, comprehensive reliability evaluation approaches that consider system stochasticity and evaluate both adequacy and dynamic security are important to identify potential system risks in this transforming environment.
Adequacy assessment becomes more complex due to the variability introduced by renewable energy generation. The traditionally used reserve margin only considers projected peak demand and would be inadequate since it does not consider an evaluation of off-peak conditions that could also be critical due to the variable renewable generation. Therefore, in order to address the impact of variable renewable generation, a probabilistic evaluation that studies all hours of a year based on statistical characteristics is a necessity to identify the adequacy risks. On the other hand, the system dynamic behavior is also changing. Converter interfaced generation resources have different dynamic characteristics from the conventional synchronous units and inherently do not participate in grid regulation functions such as frequency control and voltage control that are vital to maintaining operating reliability. In order to evaluate these evolving grid characteristics, comprehensive reliability evaluation approaches that consider system stochasticity and evaluate both adequacy and dynamic security are important to identify potential system risks in this transforming environment.
ContributorsWang, Yingying (Author) / Vittal, Vijay (Thesis advisor) / Khorsand, Mojdeh (Thesis advisor) / Heydt, Gerald (Committee member) / Ayyanar, Raja (Committee member) / Arizona State University (Publisher)
Created2020
Description
Energy is one of the wheels on which the modern world runs. Therefore, standards and limits have been devised to maintain the stability and reliability of the power grid. This research shows a simple methodology for increasing the amount of Inverter-based Renewable Generation (IRG), which is also known as Inverter-based Resources (IBR), for that considers the voltage and frequency limits specified by the Western Electricity Coordinating Council (WECC) Transmission Planning (TPL) criteria, and the tie line power flow limits between the area-under-study and its neighbors under contingency conditions. A WECC power flow and dynamic file is analyzed and modified in this research to demonstrate the performance of the methodology. GE's Positive Sequence Load Flow (PSLF) software is used to conduct this research and Python was used to analyze the output data.
The thesis explains in detail how the system with 11% of IRG operated before conducting any adjustments (addition of IRG) and what procedures were modified to make the system run correctly. The adjustments made to the dynamic models are also explained in depth to give a clearer picture of how each adjustment affects the system performance. A list of proposed IRG units along with their locations were provided by SRP, a power utility in Arizona, which were to be integrated into the power flow and dynamic files. In the process of finding the maximum IRG penetration threshold, three sensitivities were also considered, namely, momentary cessation due to low voltages, transmission vs. distribution connected solar generation, and stalling of induction motors. Finally, the thesis discusses how the system reacts to the aforementioned modifications, and how IRG penetration threshold gets adjusted with regards to the different sensitivities applied to the system.
The thesis explains in detail how the system with 11% of IRG operated before conducting any adjustments (addition of IRG) and what procedures were modified to make the system run correctly. The adjustments made to the dynamic models are also explained in depth to give a clearer picture of how each adjustment affects the system performance. A list of proposed IRG units along with their locations were provided by SRP, a power utility in Arizona, which were to be integrated into the power flow and dynamic files. In the process of finding the maximum IRG penetration threshold, three sensitivities were also considered, namely, momentary cessation due to low voltages, transmission vs. distribution connected solar generation, and stalling of induction motors. Finally, the thesis discusses how the system reacts to the aforementioned modifications, and how IRG penetration threshold gets adjusted with regards to the different sensitivities applied to the system.
ContributorsAlbhrani, Hashem A M H S (Author) / Pal, Anamitra (Thesis advisor) / Holbert, Keith E. (Committee member) / Ayyanar, Raja (Committee member) / Arizona State University (Publisher)
Created2020
Description
This dissertation covers three primary topics and relates them in context. High frequency transformer design, microgrid modeling and control, and converter design as it pertains to the other topics are each investigated, establishing a summary of the state-of-the-art at the intersection of the three as a baseline. The culminating work produced by the confluence of these topics is a novel modular solid-state transformer (SST) design, featuring an array of dual active bridge (DAB) converters, each of which contains an optimized high-frequency transformer, and an array of grid-forming inverters (GFI) suitable for centralized control in a microgrid environment. While no hardware was produced for this design, detailed modeling and simulation has been completed, and results are contextualized by rigorous analysis and comparison with results from published literature. The main contributions to each topic are best presented by topic area. For transformers, contributions include collation and presentation of the best-known methods of minimum loss high-frequency transformer design and analysis, descriptions of the implementation of these methods into a unified design script as well as access to an example of such a script, and the derivation and presentation of novel tools for analysis of multi-winding and multi-frequency transformers. For microgrid modeling and control, contributions include the modeling and simulation validation of the GFI and SST designs via state space modeling in a multi-scale simulation framework, as well as demonstration of stable and effective participation of these models in a centralized control scheme under phase imbalance. For converters, the SST design, analysis, and simulation are the primary contributions, though several novel derivations and analysis tools are also presented for the asymmetric half bridge and DAB.
ContributorsMongrain, Robert Scott (Author) / Ayyanar, Raja (Thesis advisor) / Pan, George (Committee member) / Qin, Jiangchao (Committee member) / Lei, Qin (Committee member) / Arizona State University (Publisher)
Created2019