Global Tracking Passivity--based PI Control of Bilinear Systems and its Application to the Boost and Modular Multilevel Converters

This paper deals with the problem of trajectory tracking of a class of bilinear systems with time--varying measurable disturbance. A set of matrices {A,B_i} has been identified, via a linear matrix inequality, for which it is possible to ensure global tracking of (admissible, differentiable) trajectories with a simple linear time--varying PI controller. Instrumental to establish the result is the construction of an output signal with respect to which the incremental model is passive. The result is applied to the boost and the modular multilevel converter for which experimental results are given.Comment: 9 pages, 10 figure

Nonlinear constrained and saturated control of power electronics and electromechanical systems

Power electronic converters are extensively adopted for the solution of timely issues, such as power quality improvement in industrial plants, energy management in hybrid electrical systems, and control of electrical generators for renewables. Beside nonlinearity, this systems are typically characterized by hard constraints on the control inputs, and sometimes the state variables. In this respect, control laws able to handle input saturation are crucial to formally characterize the systems stability and performance properties. From a practical viewpoint, a proper saturation management allows to extend the systems transient and steady-state operating ranges, improving their reliability and availability. The main topic of this thesis concern saturated control methodologies, based on modern approaches, applied to power electronics and electromechanical systems. The pursued objective is to provide formal results under any saturation scenario, overcoming the drawbacks of the classic solution commonly applied to cope with saturation of power converters, and enhancing performance. For this purpose two main approaches are exploited and extended to deal with power electronic applications: modern anti-windup strategies, providing formal results and systematic design rules for the anti-windup compensator, devoted to handle control saturation, and “one step” saturated feedback design techniques, relying on a suitable characterization of the saturation nonlinearity and less conservative extensions of standard absolute stability theory results. The first part of the thesis is devoted to present and develop a novel general anti-windup scheme, which is then specifically applied to a class of power converters adopted for power quality enhancement in industrial plants. In the second part a polytopic differential inclusion representation of saturation nonlinearity is presented and extended to deal with a class of multiple input power converters, used to manage hybrid electrical energy sources. The third part regards adaptive observers design for robust estimation of the parameters required for high performance control of power systems

Control System Design, Analysis, and Simulation of a Photovoltaic Inverter for Unbalanced Load Compensation in a Microgrid

This thesis presents a control scheme for a single-stage three-phase Photovoltaic (PV) converter with negative sequence load current compensation. In this thesis a dual virtual impedance active damping technique for an LCL filter is proposed to address the issue of LCL filter resonance. Both inverter-side current and the capacitor current are used in the feedback loop. Using both signals provides higher DC rejection than using capacitor current alone. The proposed active damping scheme results in a faster transient response and higher damping ratio than can be obtained using inverter-side current alone. The feedback gains can be calculated to achieve a specified damping level. A method of determining the gains of the Proportional and Resonant current controller based on frequency response characteristics is presented. For a specified set of gain and phase margins, the controller gains can be calculated explicitly. Furthermore, a modification is proposed to prevent windup in the resonator. A numerically compensated Half-Cycle Discrete Fourier Transform (HCDFT) method is developed to calculate the negative sequence component of the load current. The numerical compensation allows the HCDFT to accurately estimate the fundamental component of the load current under off-nominal frequency conditions. The proposed HCDFT method is shown to have a quick settling time that is comparable to that obtained with conventional sequence compensation techniques as well as immunity to harmonics in the input signal. The effect of unbalance compensation on the PV power output depending on the irradiance and the operational region on the power-voltage curve is examined. Analysis of the DC link voltage ripple shows the region of operation on the P-V curve affects the amplitude of the DC link voltage ripple during negative sequence compensation. The proposed control scheme is validated by simulation in the Matlab/Simulink® environment. The proposed control scheme is tested in the presence of excessive current imbalance, unbalanced feeder impedances, and non-linear loads. The results have shown that the proposed control scheme can improve power quality in a hybrid PV-diesel microgrid by reducing both voltage and current imbalance while simultaneously converting real power from a PV array

Flexible AC/DC Grids in Dymola/Modelica - Modeling and Simulation of Power Electronic Devices and Grids

The research of the thesis was aimed towards investigating the possibility of implementing different control strategies for power electronic converters in a simulation environment. The different control modes were fitted into flexible models that were interconnected in various grid topologies. The software used in order to develop the simulation environment is called Dymola and presently does not include any form of control of power electronic units. The library used is the Modelica Electric Power Library (EPL) where some power electronic converters were already implemented. The grid was controlled and kept stable for various scenarios using the developed controlled converter models. The converter models were tested separately in order to verify that the models acted in the desired manner. The models where then interconnected into a grid and simulated for different scenarios in order to get grid models that could be fitted into multiple grid applications. To further prove this, models from external Modelica libraries were used in the grid setups. The results of the simulations clearly show that constructed models support the implementation of scalable and controllable grids in Dymola

Development of Grid-Connected and Front-End Converters for Renewable Energy Systems and Electric Mobility

The spread of renewable energy sources and electric vehicles is increasing thanks to the greater awareness of the climate problems due to the large and long-lasting use of the non-renewable energy sources. The integration of renewable energy sources to the power grid, however, poses significant technical challenges, since it drastically changes its topology and nature. In fact, while the traditional power generation system is centralized, the renewable energy is distributed and intermittent. In this scenario, power converters play a central role. Power converters are the technology that enables the interconnection of different players to the electric power system. In this work, a control system for grid-connected converters has been developed. The main focus is on the current control. The most renowned current controllers, such resonant and repetitive regulators, have been studied and tested in laboratory in order to compare the performance in terms of harmonic compensation and burden of the processor. The problem of the saturation of a multi-frequency current controller has been investigated and different saturation algorithms have been proposed. The power converters have, however, wide use and the same of the method, developed for grid-connected converters can be applied to electrical motor drives with open-end windings. If a floating capacitor bridge is connected to the secondary side of the open-end stator windings, it can supply the reactive power needed by the motor and completely exploit its current capability of the power source. This feature allows the drive to obtain higher torque at higher speed, increasing therefore the output power over all the flux-weakening speed range. The floating bridge, operating as harmonic compensator, allows the inverter connected to the primary energy source to work in overmodulation and even six-step modulation, in order to further boost the performance of the drive, without compromising the quality of the phase current

Control strategy for a grid-connected inverter under unbalanced network conditions—a disturbance observer-based decoupled current approach

Abstract: This paper proposes a new approach on the novel current control strategy for grid-tied voltage-source inverters (VSIs) with circumstances of asymmetrical voltage conditions. A standard grid-connected inverter (GCI) allows the degree of freedom to integrate the renewable energy system to enhance the penetration of total utility power. However, restrictive grid codes require that renewable sources connected to the grid must support stability of the grid under grid faults. Conventional synchronously rotating frame dq current controllers are insufficient under grid faults due to the low bandwidth of proportional-integral (PI) controllers. Hence, this work proposes a proportional current controller with a first-order low-pass filter disturbance observer (DOb). The proposed controller establishes independent control on positive, as well as negative, sequence current components under asymmetrical grid voltage conditions. The approach is independent of parametric component values, as it estimates nonlinear feed-forward terms with the low-pass filter DOb. A numerical simulation model of the overall power system was implemented in aMATLAB/Simulink (2014B, MathWorks, Natick, MA, USA). Further, particular results show that double-frequency active power oscillations are suppressed by injecting appropriate negative-sequence currents. Moreover, a set of simulation results provided in the article matches the developed theoretical background for its feasibility

A design for an RGB LED driver with independent PWM control and fast settling time

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (leaves 76-77).A small sized and efficient method to power RGB LEDs for use as backlights in flat panel displays is explored in this thesis. The proposed method is to drive a parallel switched connection of LEDs with a single Average Mode Controlled buck regulator.Specifications for the switching regulator and control circuitry are described. The application circuit demonstrates current settling times between 7[mu]s and 30[mu]s at a switching frequency of 290kHz. Current settling is improved at higher switching frequencies, with settling times approaching a 2[mu]s to 4[mu]s range at 1MHz switching.by Awo Dede O. Ashiabor.M.Eng