http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

This book, written by two major figures in adaptive control, provides a wealth of material for researchers, practitioners, and students. While some researchers in adaptive control may note the absence of a particular topic, the book‘s scope represents a high-gain instrument. It can be used by designers of control systems to enhance their work through the information on many new theoretical developments, and can be used by mathematical control theory specialists to adapt their research to practical needs. The book is strongly recommended to anyone interested in adaptive control.

Adaptive Control

Grid-connected inverters are known to become unstable when the grid impedance is high. Existing approaches to analyzing such instability are based on inverter control models that account for the grid impedance and the coupling with other grid-connected inverters. A new method to determine inverter-grid system stability using only the inverter output impedance and the grid impedance is developed in this paper. It will be shown that a grid-connected inverter will remain stable if the ratio between the grid impedance and the inverter output impedance satisfies the Nyquist stability criterion. This new impedance-based stability criterion is a generalization to the existing stability criterion for voltage-source systems, and can be applied to all current-source systems. A single-phase solar inverter is studied to demonstrate the application of the proposed method.

Impedance-Based Stability Criterion for Grid-Connected Inverters

This paper presents a new multiresonant frequency-adaptive synchronization method for grid-connected power converters that allows estimating not only the positive- and negative-sequence components of the power signal at the fundamental frequency but also other sequence components at other harmonic frequencies. The proposed system is called MSOGI-FLL since it is based on both a harmonic decoupling network consisting of multiple second-order generalized integrators (MSOGIs) and a frequency-locked loop (FLL), which makes the system frequency adaptive. In this paper, the MSOGI-FLL is analyzed for single- and three-phase applications, deducing some key expressions regarding its stability and tuning. Moreover, the performance of the MSOGI-FLL is evaluated by both simulations and experiments to show its capability for detecting different harmonic components in a highly polluted grid scenario.

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Multiresonant Frequency-Locked Loop for Grid Synchronization of Power Converters Under Distorted Grid Conditions

In this paper, a novel control method of grid-connected voltage-source converters (VSCs) is proposed. The method can be generally applied for all grid-connected VSCs but may be of most importance in high-voltage dc (HVDC) applications. Different from the previous control methods, the proposed method utilizes the internal synchronization mechanism in ac systems, in principle, similar to the operation of a synchronous machine. By using this type of power-synchronization control, the VSC avoids the instability caused by a standard phase-locked loop in a weak AC-system connection. Moreover, a VSC terminal can give the weak ac system strong voltage support, just like a normal synchronous machine does. The control method is verified by both analytical models and time simulations.

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Power-Synchronization Control of Grid-Connected Voltage-Source Converters

A controlled power electronic converter can cause local instabilities when interacting with other dynamic subsystems in a power system. Oscillations at a certain frequency cannot, however, build up if the converter differential input admittance has a positive conductance (real part) at that frequency, since power is then dissipated. In this paper, input-admittance expressions for a voltage-source converter are derived. It is seen how the admittance can be shaped in order to get a positive real part in the desired frequency regions by adjusting the controller parameters.

Input-Admittance Calculation and Shaping for Controlled Voltage-Source Converters

In this paper, the use of a method for online detection of positive- and negative-sequence components of three-phase quantities, named the delayed signal cancellation (DSC) method, is investigated. Problems that arise in practical implementation of the DSC method in computer-controlled systems are investigated. Expressions of the detection error due to nonideal discretization are derived and calculations are verified experimentally. Two methods for reducing the detection error are presented and verified. It is also shown that the given expressions and proposed methods for reducing the detection errors can also be applied to the case of grid frequency variations

Practical Implementation of Delayed Signal Cancellation Method for Phase-Sequence Separation

Current controller design for three-phase voltage-source converters, by which a passive input admittance is ideally obtained, is considered. Two design alternatives, with and without voltage feedforward, respectively, are shown to give similar performance. It is demonstrated how additional parts can be added to the controller while preserving the passivity property. This is applied particularly to resonant parts. The design method is finally applied to converters with an LCL input filter, where it is shown in many cases to compare favourably with a scheme where an inner loop is added for improved damping of the LCL resonance.

Frequency-domain passivity-based current controller design

A modeling approach is presented in this paper in order to investigate instabilities originating from dc-network resonances in voltage-source converter HVDC (VSC-HVDC) systems. Through frequency-domain modeling, it is shown that the VSC-HVDC system can be represented as a feedback interconnection of two subsystems. These subsystems are defined in this paper as a dc network impedance connected to a VSC dc-side power-dependent admittance. The conditions in which this admittance has a negative conductance are investigated since they can amplify dc network resonances. It is found that when the VSC that controls the direct voltage injects power into the dc network, the mentioned conductance becomes negative above certain frequencies. In addition, the impact of weak ac grids, control system delays, and other direct voltage controllers on the studied conductance is investigated. The theoretical findings are verified through time-domain simulations.

Modeling and Analysis of VSC-Based HVDC Systems for DC Network Stability Studies

A photovoltaic (PV) plant model is presented. It is based on a detailed electrothermal description of the panels forming strings that, in turn, form the power plant. It accounts for environmental working conditions, such as temperature and wind speed, and specific plant configuration, such as plant topology and power losses due to interconnections. The input variables of the model are the ambient temperature, irradiance, and wind speed. The model derives the working temperature of the panel taking into account also the power conversion performed by the panel; the electrical operating point is determined by simulating the actions done by the maximum power point tracker that operates at plant level. This model has been tested using a large database of experimental data from industrial PV plants characterized by power levels ranging from 250 kW to 1 MW. As shown, the model is capable to predict power production when “fed” by forecast irradiance, ambient temperature, and wind speed data.

Massimo Bongiorno

Papers

Input-Admittance Calculation and Shaping for Controlled Voltage-Source Converters

Practical Implementation of Delayed Signal Cancellation Method for Phase-Sequence Separation

Frequency-domain passivity-based current controller design

Modeling and Analysis of VSC-Based HVDC Systems for DC Network Stability Studies

Model of Photovoltaic Power Plants for Performance Analysis and Production Forecast