Browsing by Author "Wen, Bo"
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- 3D Commutation-Loop Design Methodology for a SiC Based Matrix Converter run in Step-up mode with PCB Aluminum Nitride Cooling InlayBaker, Victoria Isabelle (Virginia Tech, 2021-07-22)This work investigates three-dimensional power loop layout for application to a SiC based matrix converter, providing a symmetric, low-inductance solution. The thesis presents various layout types to achieve this design target, and details the implementation of a hybrid layout to the matrix converter phase-leg. This layout is more easily achievable with a surface-mount device package, which also offers benefits such as ease in manufacturing, and a compact package. In order to implement a surface-mount device, a PCB thermal management strategy should be utilized. An evaluation of these methods is also presented in the work. The final power loop solution that implements an aluminum nitride inlay is evaluated through simulated parasitic extraction and experimental double pulse tests. The layout achieves small, symmetric loop inductances. Finally, the full power, three-phase matrix converter demonstrates the successful implementation of this power loop layout.
- Development of a Power Hardware-in-the-Loop Test Bench for Electric Machine and Drive EmulationNoon, John Patrick (Virginia Tech, 2020-12-15)This work demonstrates the capability of a power electronic based power hardware-inthe- loop (PHIL) platform to emulate electric machines for the purpose of a motor drive testbench with a particular focus on induction machine emulation. PHIL presents advantages over full-hardware testing of motor drives as the PHIL platform can save space and cost that comes from the physical construction of multiple electric machine test configurations. This thesis presents real-time models that were developed for the purpose of PHIL emulation. Additionally, real-time modeling considerations are presented as well as the modeling considerations that stem from implementing the model in a PHIL testbench. Next, the design and implementation of the PHIL testbench is detailed. This thesis describes the design of the interface inductor between the motor drive and the emulation platform. Additionally, practical implementation challenges such as common mode and ground loop noise are discussed and solutions are presented. Finally, experimental validation of the modeling and emulation of the induction machine is presented and the performance of the machine emulation testbench is discussed.
- DQ-Frame Small-Signal Stability Analysis of AC Systems with Single-Phase and Three-Phase ConvertersLin, Qing (Virginia Tech, 2024-06-21)The widespread integration of power converters in applications such as microgrids and data centers has introduced significant stability challenges. This dissertation presents a novel approach to modeling and comprehensive stability analysis for both single-phase and three-phase converters, addressing vital gaps in the existing literature. The first part of the dissertation (Chapters 2 to 4) focuses on single-phase power supply units, proposing an impedance model and a loop gain model based on dq-frame analysis. These models have been validated through extensive experimental testing, demonstrating their effectiveness in stability analysis across a range of system configurations, including single-phase, three-phase three-wire, and three-phase four-wire systems. The second part (Chapters 5 and 6) examines three-phase converters used for integrating renewable energy into microgrids. It introduces a grid-forming control, followed by a detailed investigation into its impedance modeling and stability assessment. This part specifically tackles the challenges posed by the appearance of right-half-plane poles in stability analysis, proposing a new stability margin index to address these issues. The efficacy of these research findings is further substantiated by the development and implementation of a Power-Hardware-in-the-Loop testbed, providing practical validation. Overall, this dissertation has enhanced the modeling, understanding, and management of stability issues in power electronics systems, offering valuable insights and methodologies that are likely to influence future research and development in the field.
- Integration Challenges In High Power Density Wide Bandgap Based Circuits for Transportation ApplicationsHu, Jiewen (Virginia Tech, 2021-12-03)Because of the increasing emphasis on environmental concerns, there has been a growing demand for lower fuel consumption in modern transportation applications. To reduce fuel comsumption, higher efficiency, higher power density power converters are desired. The new generation of wide bandgap (WBG) power semiconductor devices pushs the switching frequency and output power of the electric system in transportation to a higher level thanks to their higher blocking voltage, higher operating frequency, and smaller parasitic elements. With benefits such as size reudcetion, costs saving, and reliability improvement, integration technologies have been widely adopted in power electronic systems, especially with the emergence of WBG semiconductor devices. These improvements will futher translate into reduced fuel consumption, extended operating range, and increased passenger compartment. Transportation applications pose a challenging environment for converter integration. The fast switching speed and the high blocking voltage of WBG semiconductor devices also put forward higher requirements for converter integration. First, the power converters used in transportation applications are often powered from the batteries that support multiple loads. During load changes, crank, or jump-start, undesired transients exist, which requires the power converters to be capable of operating under a wide-input-voltage range. This requirement results in a very limited design region of acceptance, making the converter hard to handle uncertainties. However, the integration process might bring large uncertainties, such as material property changes. This phenomenon can degrade converter performance or even cause design failures. Besides, the power converters for transporation applications often work in harsh environment, such as high ambient temperature or low air density. The former can lead to overheated and the latter degrades insulation strength, both of which hinder high power density design. Moreover, with the advent of all kinds of portable devices, converters are required to deliver more power. The introduction of universal serial bus (USB) power delivery (PD) extends the delivered power. To meet the specification, the power converters should provide a wide-output-voltage range, which brings challenges to the converter design. Furthermore, the charger is usually fed by an ac voltage of more than 100 V, which is then stepped down to 5 V – 20 V. The high step-down ratio increases the converter loss. To address the wide-input-voltage and high-temperature challenges, a dual-output, PCB-embedded transformer based active-clamp Flyback (ACF) gate-drive power supply (GDPS) for automotive applications is proposed. It has been demonstrated that the PCB-embedding technique effectively improves converter power density. The final prototype achieves a power density of 53.2 W/in3, a peak efficiency of 89.7 %, a transformer input-output capacitance of 9.7 pF, an input-voltage range of 9.9 V – 28 V, and a maximum operating temperature at low-line (LL) voltage of 105 °C and 115 °C at high-line (HL) voltage. Yet the above unit failed to meet all of the design targets due to the material property degradation in transformer. This degradation is caused by the mechanical stress induced in the integration process. To investigate its impact on wide-input-voltage converter design, several PCB-embedded magnetic boards are fabricated with different core materials and stress levels. Based on the analysis, experimentally derived correction factors are proposed and applied to the models used in the multi-objective optimization (MDO) process. The improved design successfully achieves the targeted wide-input-votlage range. When aircrafts climb during flight, air density reduces and the breakdown voltage decreases correspondingly. The insulation design becomes a challenge for the gate driver for SiC-based airborne applications. To provide sufficient insulation strength and achieve high power density simultaneously, a Paschen curve based insulation co-ordination is proposed. Electric-field control methodology is applied to the layout design. By properly designing the field control plates, the peak electric field has been shifted from the air to fr4 material that features much higher dielectric strength. The proposed gate driver attains a small size of 128.7 mm × 61.2 mm × 23.8 mm. Partial discharging tests are conducted in an altitude chamber. The experimental result shows that the proposed gate driver provides sufficient insulation strength at 50, 000 ft. To tackle the wide-output-voltage range and high-step-down ratio challenges in the USB-C PD charger in airborne applications, a LLC converter with PCB-winding based transformer with built-in leakage inductance is presented. A flying-capacitor based voltage divider (FCVD) switching bridge is proposed to replace the conventional half-bridge or full-bridge switching bridge. The propsed FCVD shows a current reduction of over 50 % than the conventional half-bridge with the same circuit elements. The prototype achieves a high efficiency of 90.3 % to 93.2 % over 5 V to 20 V outputs, and a high power density of 73.2 W/ in³, which is almost two time larger than the state-of-the-art power density. Partial discharging tests are also conducted in an altitude chamber. A partial discharing inspection voltage of 800 V is found at 10, 000 ft, which is much higher than the requirement.
- Output Impedance Modeling and Measurement of a 28 kW Synchronous GeneratorShan, Keyue (Virginia Tech, 2022-06-09)Synchronous machines (SMs) are a vital part of today's world, and precise modeling is important for studying their stability. In this thesis, a small-signal analysis is done on the d-q frame for the AC SM. Starting from formulating the SM's abc frame equations, a d-q transformation is done based on the SM rotor frame so that the inductance from the abc frame representation will be fixed. Then, the SM's fundamental parameters are obtained from standstill frequency response testing. In the interest of having the most complete model possible, a governor controller and an exciter controller have been designed, and their performance has been evaluated according to the International Standard ISO-8628. A d-q steady-state analysis has been carried out and the resulting small-signal perturbation has been added to the steady-state equations. The model has been analyzed with exciter control only, with governor control only, and with both controllers, and in the end, the small-signal d-q impedance model for the SM has been verified by simulations and experiments in the Center for Electronics Systems (CPES) at Virginia Tech. The impedance measurement unit (IMU) was built by CPES. It is designed for measuring three-phase AC power systems and DC systems. Nevertheless, even though the single-phase system can be connected to the IMU, the impedance result in the end is not correct. Modifications have been made to the IMU so that it is able to calculate the single-phase AC power system impedance. The experimental results demonstrate that implementation has been completed.
- Power Electronics- based Photovoltaics Panel Fault Detection using Online Impedance Measurement TechniquePanchal, Jeet (Virginia Tech, 2022-12)Photovoltaics panel (PV) integration with the utility grid has been installed throughout the globe. The fault-monitoring technology for photovoltaics (PV) panels is a method to save energy production losses and become a key contributor to overall cost reduction in variable operating costs for photovoltaics systems. PV researchers today explore factors such as reducing utility energy bills and CO2 emissions, grid voltage stability, peak demand shaving, supply of electric power off-grid areas, and many more. The technology discussed is easy to incorporate, requires no additional hardware, doesn't alter the system’s stability, is implemented at a steady state point, and is helpful to record changes in PV cell operation from forward bias to reverse bias state. PV panel AC impedance can be used as an early-stage fault indicator. Also, comparing AC impedance magnitude and phase at maximum power point (MPP) or near MPP can help identify the nature of the fault in a PV system. The focus of the thesis is proposing the fault detection of 300 W PV panels using online AC impedance measurement, utilizing existing panel-level power optimizers and microinverters in a PV system to actively perturb small signals into the PV panel and compute its small signal impedance. The technology is incorporated in a power optimizer with C2000 MCU and helps identify hot spot faults and short circuit faults in a 300 W rooftop PV panel. Multiple PV panel faults scenarios such as hot spot faults, short circuit faults, junction box faults, and capacitor faults are investigated to deduct further the effectiveness of the online impedance measurement using a small signal. This thesis’s focus areas are, first, modeling the PV panel and power converter and incorporating fault scenarios to identify the fault indicators. Secondly, measuring PV panel impedance under normal and faulty conditions using an equipment-based offline technique. Lastly, measuring PV panel impedance under normal and faulty conditions using a power optimizer.
- Power-cell switching-cycle capacitor voltage control for modular multi-level converters(United States Patent and Trademark Office, 2018-05-08)In a modular multi-level power converter, additional switching states are interleaved between main switching states that control output voltage or waveform. The additional switching states provide current from a DC-link to charge capacitors in respective modules or cells to an offset voltage from which the capacitor voltages are controlled toward a reference voltage during each switching cycle rather than being allowed to build up over a period of an output waveform of variable line frequency, possibly including zero frequency. Since the switching cycle is much shorter than the duration of a line frequency cycle and the capacitor voltages are balanced during each switching cycle, output voltage ripple can be limited as desired with a capacitor of much smaller value and size than would otherwise be required.
- Small-Signal Modeling and Stability Specification of a Hybrid Propulsion System for AircraftsLin, Qing (Virginia Tech, 2021-05-17)This work utilizes the small-signal impedance-based stability analysis method to develop stability assessment criteria for a single-aisle turboelectric aircraft with aft boundary-layer propulsion (STARC-ABL) system. The impedance-based stability analysis method outperforms other stability analysis methods because it does not require detailed information of individual components for system integration, therefore, a system integrator can just require the vendors to make the individual components meet the impedance specifications to ensure whole system stability. This thesis presents models of a generator, motor, housekeeping loads, and battery all with power electronics interface which form an onboard electrical system and analyzes the relationship between the impedance shape of each component and their physical design and control loop design. Based on the developed small-signal model of the turbine-generator-rectifier subsystem and load subsystem, this thesis analyzes the impact of electromechanical dynamics of the turbofan passed through the generator on the dc distribution system, concluding that the rectifier can mitigate the impact. Finally, to ensure the studied system stable operation during the whole flying profile, the thesis provides impedance specifications of the dc distribution system and verifies the specifications with several cases in time-domain simulations.
- Stability Analysis of Three-Phase AC Power Systems Based on Measured D-Q Frame ImpedancesWen, Bo (Virginia Tech, 2015-01-20)Small-signal stability is of great concern for distributed power systems with a large number of regulated power converters. These converters are constant-power loads (CPLs) exhibit a negative incremental input resistance within the output voltage regulation bandwidth. In the case of dc systems, design requirements for impedances that guarantee stability have been previously developed and are used in the design and specification of these systems. In terms of three-phase ac systems, a mathematical framework based on the generalized Nyquist stability criterion (GNC), reference frame theory, and multivariable control is set forth for stability assessment. However, this approach relies on the actual measurement of these impedances, which up to now has severely hindered its applicability. Addressing this shortcoming, this research investigates the small-signal stability of three-phase ac systems using measured d-q frame impedances. Prior to this research, negative incremental resistance is only found in CPLs as a results of output voltage regulation. In this research, negative incremental resistance is discovered in grid-tied inverters as a consequence of grid synchronization and current injection, where the bandwidth of the phase-locked loop determines the frequency range of the negative incremental resistance behavior, and the power rating of inverter determines the magnitude of the resistance. Prior to this research, grid synchronization stability issue and sub-synchronous oscillations between grid-tied inverter and its nearby rectifier under weak grid condition are reported and analyzed using characteristic equation of the system. This research proposes a more design oriented analysis approach based on the negative incremental resistance concept of grid-tied inverters. Grid synchronization stability issues are well explained under the framework of GNC. Although stability and its margin of ac system can be addressed using source and load impedances in d-q frame, method to specify the shape of load impedances to assure system stability is not reported. This research finds out that under unity power factor condition, three-phase ac system is decoupled. It can be simplified to two dc systems. Load impedances can be then specified to guarantee system stability and less conservative design.
- Understanding Chinese Internet Users' Perceptions of, and Online Platforms' Compliance with, the Personal Information Protection Law (PIPL)Zhou, Morgana Mo; Qu, Zhiyan; Wan, Jinhan; Wen, Bo; Yao, Yaxing; Lu, Zhicong (ACM, 2024-04-23)The Personal Information Protection Law (PIPL) was implemented in November 2021 to safeguard the personal information rights and interests of Internet users in China. However, the impact and existing shortcomings of the PIPL remain unclear, carrying significant implications for policymakers. This study examined privacy policies on 13 online platforms before and after the PIPL. Concurrently, it conducted semi-structured interviews with 30 Chinese Internet users to assess their perceptions of the PIPL. Users were also given tasks to identify non-compliance within the platforms, assessing their ability to address related privacy concerns effectively. The research revealed various instances of non-compliance in post-PIPL privacy policies, especially concerning inadequate risk assessments for sensitive data. Although users identified some non-compliant activities like app eavesdropping, issues related to individual consent proved challenging. Surprisingly, over half of the interviewees believed that the government could access their personal data without explicit consent. Our findings and implications can be valuable for lawmakers, online platforms, users, and future researchers seeking to enhance personal privacy practices both in China and globally.
- Weight Estimation of Electronic Power Conversion SystemsWen, Bo (Virginia Tech, 2011-05-02)Electronic power conversion systems with large number of power converters have a variety of applications, such as data center, electric vehicles and future smart "nanogrid" in residential home. Those systems could have very different architectures. For example, one system could be based on ac, dc or hybrid power distribution bus, and the bus voltage could be different. Also those systems have great need to develop low-cost architectures which reduce weight, increase efficiency and improve reliability of the system. However, how to evaluate different architectures and select a better one is still not clear. This thesis presents a procedure to estimate weight of electronic power conversion systems, which provides an angle to evaluate different system architectures. This procedure has three steps. Step I, according to application of the system and system structure, determines the electrical and environmental specifications for each converter in the system. Step II studies the design procedures for each converter in the system and determines parameters such as the wire gauge and length of cable; the parameters of the passive components, such as inductance and capacitance; the parameters of the power switch, such as the voltage rating, current rating and loss; and parameters of the cooling system, such as the thermal resistance of the heat sink. Step III, according to the converters' parameters, carry out the physical design and selection of sub-components such as the inductor and heat sink to get the components' weight; the sum of those components' weight is the estimated system weight. This procedure has also been implemented in the form of software – system weight estimation tool. Using this software, weight of sample systems with ac dc bus and two different bus voltages have been estimated and compared.