Bidirectional Grid Tied Inverter Based on Multilevel Platform (2015)
The objective of this project is to develop a multilevel converter platform and deploy it as a bidirectional grid-tied inverter. The platform is based on the 3-level Active Neutral Point Clamped (ANPC) or Diode Clamped Topology. This is a flexible platform that allows the desired bidirectional operation and may be combined with other platforms or converters to further expand its capabilities. Combining the ANPC with a DSP and signal sensing allows the experimentation of different control techniques and modulation schemes with the goal of creating a 3-phase bidirectional grid-tied inverter for varied applications. The platform is constantly revised and utilized in different projects.
PWM-Geometric Control for Power Electronics: PWM-Geometric Control of Power Converters (2015)
The concept of dual-loop geometric based control is introduced in this work by combining geometric state-plane analysis for the outer voltage loop with traditional current control techniques for the inner loop. The voltage loop PI compensator is replaced by a geometric (GI) alternative that tightly controls the time-domain evolution of the state variables, providing a reliable transient response by following a desired geometrical path to reach the target steady state operating point. In this way, dynamic requirements can be successfully addressed through shaping the state variables’ time-evolution by employing a simple geometric equation to define the voltage compensator. Straight line and circular trajectories are implemented using simple parametric equations, resulting in remarkably well defined, reliable transient behaviour. The circular (non-linear) GI compensators must be implemented digitally, either by solving the geometric equations on the fly, or by pre-loading the compensators in look-up tables. Experimental results of dual-loop geometric-based controlled 50W platform validate the proposed control concept and highlight the strong contribution to the applied field made by this innovative controller.
Wind Energy Harvesting: Ripple Current Reduction in Variable Speed Wind Turbines (
Driven by the demand for environmentally friendly power sources, small-scale renewable power generation is increasing dramatically. This project consists of the characterization and mitigation of the effect inverter type loads have on small-scale wind turbines in grid-tie applications. Inverter loads consist of rectified sinusoidal currents at twice the line frequency which tend to cause ripple currents and torque in the wind turbine, leading to excessive bearing wear, higher losses, and eventually premature failure. Through the use of clever control techniques and digital filtering the negative impact of the inverter on the wind turbine can be removed and system size and cost can be reduced while increasing reliability.
Centric-Based Control for Power Electronics: Average Natural Trajectories and Centric-Based Control (
A novel control scheme that combines the advantages of fixed frequency PWM with state-plane geometric analysis is introduced to obtain fast and reliable large-signal response. The natural evolution of the average state variables is described by a large-signal model, which provides the basis to develop a reliable non-linear control scheme. The proposed technique is suitable for implementation in low cost DSPs, using low bandwidth sensing stages, and it features fast, sleek and consistent dynamic response with constant switching frequency. The contributions made to the theoretical and applied field are valid for any combination of reactive components due to the normalized approach adopted. The theoretical concepts are supported by detailed mathematical procedures. The proposed theory and controller are validated by experimental results.
Grid-Tied Applications: Multifunctional inverter for renewable energy generation (2014)
In this project, a 3-phase inverter is designed to provide independent provision of active, reactive, and harmonic currents to the electrical grid. This multifunctional inverter, thus, is able to provide active filtering of individual harmonics (up to the 17th harmonic) while injecting active power to the grid. The multifunctional inverter is controlled by a digital signal controller that executes in real time the measuring of 6 signals, the active and reactive power control, the individual calculation and control of each harmonic on a 100 microsecond time interval. The project attempts to maximize the functionality of the inverter by implementing an algorithm that continuously selects the maximum filtering capabilities of the inverter based on its power rating and the incoming power from the renewable source.
Electric Vehicle Technologies: Resonant Power Converter as a Battery Charger for Electric Vehicle (
Recently, the technology of rechargeable battery packs (Li-Ion, Lead-Acid, NiMH etc.) has been improved for deeply discharged conditions in order to provide more energy for powering electric motors in electric vehicles (EVs). In this project, higher order softly switched resonant converters and new modulation techniques are employed in the fabrication of a new industrial-grade product. The proposed charger structures can regulate the output voltage in a wide range and deliver power from the main to the battery with high efficiency, ripple-free current that fully supports battery charging profiles.
Photovoltaic Power Generation: Photovoltaic Maximum Power Point Tracking (
In this project, novel MPPT algorithms and devices are implemented in order to improve power transfer, tackling key factors of the MPPT algorithms. Such factors include steady state behaviour (oscillations and perturbation in the operating point), environmental change tracking (errors during transient conditions and accurate tracking during the events) and local maxima and mismatch (partial shading). This project produced several algorithms and control techniques oriented to PV panels that tackle the aforementioned problems. Simulations and experimental work were carried out to validate the algorithm and illustrate different operating conditions.
Wireless Power Transfer: Wireless Power Transfer Planar Spiral Winding Design Applying Track-Width-Ratio (
This work established a new method to improve and analyze the quality factor (Q) of wireless power transfer planar spiral windings using a non-unity Track-Width-Ratio (TWR) geometrical arrangement. The results made full use of the ability to change the width of the traces to improve Q for wireless powered applications. A unified dimensional system framework was proposed, covering the racetrack geometry and its derivatives - circular, rectangular, generalized octagonal, and traditional racetrack windings. The resulting dimensional system provided an accurate geometrical description of the windings to obtain high Q and a simple set of manufacturing specifications. Details and derivations of this new method to calculate inductance, DC resistance, and AC resistance estimation to obtain improved Q were presented, along with experimental verification.
HVDC Converters for Off-Shore Wind Turbines: Hi-Power VSC-HVDC converter stability and control (
In this project, a new solution for dealing with the stability of high power VSC-HVDC converters is tested and validated. The solution is named RHP zero shifthing+damping and is used to improve the stability of VSC-HVDC converters affected by the adverse effects of the non-minimum phase behaviour. Thanks to RHP zero shifthing+damping the HVDC system is able to perform faster control over its direct voltage dynamics without losing stability due to the non-minimum phase behaviour of the plant: This results in savings in implementation cost of HVDC systems and improved support of the HVDC system to the ac network dynamics. The RHP zero shifthing+damping solution was tested and validated in a scaled-down 1kW VSC prototype.
Grid-Tied Applications: LCL-filter for grid-tie converters (
LCL-filters allow the use of smaller inductance values for the connection filters of grid-tie converters. However, the LCL-filter resonance can cause stability problems and must be damped. Passive damping (resistors in series with the LCL-filter capacitors) results in additional losses. Therefore, it is most advantageous to use active damping by using adequate control techniques. In the first part of the research project, the resonance frequency is measured and a properly tuned notch filter is located at the output of voltage reference to avoid the resonance excitation. In the second part, the robust design of the LCL-filter against large grid inductance variations, typical of weak grids, is considered. This research project was jointly done with Aalborg University, Denmark.
Power Converters Controllers Characterization: Dynamic Physical Limits in Power Converters (
This work presents accurate and practical dynamic performance benchmarking rules for power converters that result from establishing the dynamic physical limits of the system. The theoretical limits of performance during transients are found in a normalized form, providing valuable insight into the behaviour of the topologies for any combination of LC parameters and input/output voltages, facilitating a benchmarking tool for power electronics control designers/researchers. The proposed dynamic performance benchmarking procedures are illustrated by comparing the response of several recently proposed controllers with the time-optimal response. Significant contributions are made to both theoretical and applied fields by providing insightful derivations and characterizations of the theoretical optimum dynamic response as well as a simple and powerful benchmarking tool to aid the control design.