Justin K. Reed

Hierarchical Control of Bridge-of-Bridge Multilevel Power Converters

Multilevel converters are among the members of the family of power-converter topologies for realizing higher power levels and better waveform quality. In addition to the established topologies of neutral-point-clamped three-level and cascaded H-bridge converters, novel topologies that offer attractive features, such as ease of modularity and functionality, continue to be introduced. Among these, the bridge-of-bridge multilevel converters have the potential for realizing multimegawatt systems with ease. This paper is aimed at presenting a systematic approach to developing their dynamic and steady-state models, leading to a hierarchical-control approach that is intuitive to realize and versatile in application. This paper presents the dynamic and steady-state models and computer simulations that demonstrate the approach in dc-1φ/ac and 3φ/ac- 3φ/ac power-conversion applications. An experimental validation of the models using a dc-1φ/ac asymmetrical-half-bridge converter is presented.

Capacitive Power Transfer for Rotor Field Current in Synchronous Machines

Permanent magnet (PM) synchronous machines are utilized in a wide variety of applications due to their many desirable characteristics, including high torque density capability and high efficiency. In the near future, however, the demand for the PM rare earth materials is projected to exceed world production. As a result, electric machines that do not rely on rare earth materials, such as wound field synchronous machines (WFSMs), are receiving renewed attention for use in traction and wind energy applications. However, WFSMs require a current delivery mechanism to the rotor such as mechanical slip rings whose components require periodic replacement and generate adverse debris within the machine enclosure. Rotary transformers may replace slip rings but also introduce rotor speed dependences and magnetic coupling difficulties. This paper proposes a capacitive noncontact power transfer technique to eliminate the need for mechanical slip rings while also avoiding the pitfalls of rotating transformer technologies. The capacitive power transfer system is compared to traditional rotor power coupling techniques and its performance is validated with experimental results.

Aerodynamic Fluid Bearings for Translational and Rotating Capacitors in Noncontact Capacitive Power Transfer Systems

Wireless power transfer (WPT) is commonly accomplished with magnetic (inductive) techniques for a wide range of applications. Electrostatic or capacitive power transfer (CPT) approaches to WPT have had limited exposure primarily due to lower achievable power density when compared to inductive WPT techniques. Recently, high-frequency (in kilohertz to megahertz) power electronics have reintroduced capacitive techniques as an option for WPT over short distances ( <; 2 mm) for applications such as slip ring replacement. To further the practicality of CPT, capacitive coupling must be maximized in an effective manner, i.e., the volumetric capacitance density of rotating/translational capacitors must be significantly increased. This paper proposes the use of aerodynamic fluid bearings to maximize capacitive coupling between stationary and moving surfaces, by minimizing their separation distance, allowing for greater surface area per unit volume. The technique allows micrometers of separation distance between moving surfaces while maintaining manufacturability and mechanical robustness. Coupling capacitance is increased up to 100 times greater than rigid plate rotating and translational CPT systems. Additional benefits include the estimation of mechanical system parameters such as speed. Operational characteristics and design highlights are presented and corroborated with experimental results for general slip ring replacement applications.

Modeling power semiconductor losses in HEV powertrains using Si and SiC devices

Silicon carbide (SiC) power semiconductor devices are known to have potential benefits over conventional silicon (Si) devices, particularly in high power applications such as hybrid electric vehicles (HEVs). Recent literature studying the use of SiC JFETs in HEV inverters indicate a substantially increased gas mileage. This paper further investigates this change in inverter efficiency due to the adoption of SiC using analytical loss models and empirical loss data obtained from experimental Cree 1200V 10A DMOSFETs and Schottky diodes. A motor inverter efficiency map is developed and used in the VEHLIB simulator to evaluate fuel consumption benefits. Distribution of conduction and switching losses in both Si and SiC inverters is explored.

Capacitive power transfer for slip ring replacement in wound field synchronous machines

Permanent magnet synchronous machines are utilized in a wide variety of applications due to their many desirable characteristics, including high torque density capability and high efficiency. In the near future, however, the demand for the permanent magnet rare earth materials is projected to exceed world production. As a result, electric machines which do not rely on rare earth materials, such as wound field synchronous machines (WFSMs), are receiving renewed attention for use in traction and wind energy applications. However, WFSMs require a current delivery mechanism to the rotor such as mechanical slip rings whose components require periodic replacement and generate adverse debris within the machine enclosure. Rotary transformers may replace slip rings but also introduce rotor speed dependencies and magnetic coupling difficulties. This paper proposes a capacitive non-contact power transfer technique to eliminate the need for mechanical slip rings while also avoiding the pitfalls of rotating transformer technologies. The capacitive power transfer system is compared to traditional rotor power coupling techniques and its performance is validated with experimental results.

Complex phasor modeling and control of modular multilevel inverters

Modular multilevel power converters in a ‘bridge of bridge’ configuration have been introduced recently for realizing ac-dc, dc-ac and ac-ac power conversion functions with voltage step-up and step-down capability, bidirectional power flow and transformerless operation. While different modulation and control approaches have been introduced in an ad-hoc manner in the introductory literature, definitive models that may be used to characterize the steady state and dynamic performance are just beginning to be uncovered. It is the objective of this paper to introduce a canonical phasor or complex vector model to represent the ac variables along with the dc variables that are unique to this family of converters. The model is used to develop a suitable control approach using established techniques. The complex vector model is represented in the synchronously rotating dq reference frame, and illustrated using an application of a dc-ac step-up inverter for residential photovoltaic applications.

Modeling of battery charging wind turbines

Small scale wind turbines are typically used in battery charging systems in off-grid and remote applications to provide electric power to small load locations such as homes, cabins and instrumentation stations. This paper is devoted to presenting the results from the modeling case-study of such a turbine. Analytical models, computer simulations and experimental results are presented.

Brushless Mitigation of Bearing Currents in Electric Machines Via Capacitively Coupled Shunting

The overwhelming trend within the industry today is to pair electric machines with variable-frequency drives. It is well known that the use of these drives can introduce high-frequency bearing currents during semiconductor switching events. This paper focuses on bearing discharge currents that gradually degrade the bearings by pitting their surfaces and can lead to premature failure. Common solutions include insulating the bearings, installing common-mode filters, or using a brush to ground the rotor shaft. These solutions are not universally scalable and may require periodic maintenance. This paper presents a new approach using noncontact (brushless) capacitive coupling. Here, a rotating capacitor is placed electrically in parallel with the bearing. At high frequencies, the impedance of the rotating capacitor is far lower than the bearing impedance, thereby shunting current around the bearings and reducing shaft voltage. The technique is experimentally demonstrated on a 3-hp (2.2-kW) induction machine. The experiment demonstrates a bearing current reduction factor of at least 8x.

Aerodynamic fluid bearings for capacitive power transfer and rotating machinery

Wireless power transfer (WPT) and electromechanical power conversion are traditionally accomplished with magnetic (inductive) techniques for a wide range of applications. Electrostatic (capacitive) approaches to WPT for electric machinery have had limited success over the history of power conversion due to the low power density, low mechanical robustness and cost that could not compete with magnetic steel. Recently, high frequency (kHz-MHz) power electronics have reintroduced capacitive techniques as a viable option for WPT over short distances (<;2mm) and in niche rotating machinery designs. To further the practicality of capacitive techniques, capacitive coupling must be maximized. This paper proposes the use of aerodynamic fluid bearings to maximize capacitive coupling between stationary and rotating surfaces by minimizing their separation distance. This technique also allows for the estimation of mechanical system parameters such as speed. Operational characteristics and design techniques are presented and corroborated with experimental results.

Low-Cost Light-Weight Quick-Manufacturable Blades for Human-Scale Wind Turbines

A novel blade design for small wind turbines intended for residential use in rural locations and/or developing nations has been developed. Traditionally, blades for these turbines are carved from weather resistant woods which may be costly and difficult to manufacture. This work seeks to reduce cost and construction time by developing blades from readily available PVC drainage pipe. The blade design underwent airfoil optimization regarding the blade twist along its length and drop, via computational fluid dynamic (CFD) simulations with open source software. Blades were built of PVC using the new CFD design and tested on field operation, the manufacturing process as well as their performance was monitored and recorded in terms of comparison with classical wood-based design.