Symmetrical components

Summary

In electrical engineering, the method of symmetrical components simplifies analysis of unbalanced three-phase power systems under both normal and abnormal conditions. The basic idea is that an asymmetrical set of N phasors can be expressed as a linear combination of N symmetrical sets of phasors by means of a complex linear transformation. Fortescue's theorem (symmetrical components) is based on superposition principle, so it is applicable to linear power systems only, or to linear approximations of non-linear power systems. In the most common case of three-phase systems, the resulting "symmetrical" components are referred to as direct (or positive), inverse (or negative) and zero (or homopolar). The analysis of power system is much simpler in the domain of symmetrical components, because the resulting equations are mutually linearly independent if the circuit itself is balanced. Description In 1918 Charles Legeyt Fortescue presented a paper which demonstrated that any se

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In this paper, we present an exact (i.e. non-approximated) and linear measurement model for hybrid AC/DC micro-grids for recursive state estimation (SE). More specifically, an exact linear model of a voltage source converter (VSC) is proposed. It relies on the complex VSC modulation index to relate the quantities at the converters DC side to the phasors at the AC side. The VSC model is derived from a transformer-like representation and accounts for the VSC conduction and switching losses. In the case of three-phase unbalanced grids, the measurement model is extended using the symmetrical component decomposition where each sequence individually affects the DC quantities. Synchronized measurements are provided by phasor measurement units and DC measurement units in the DC system. To make the SE more resilient to vive step changes in the grid states, an adaptive Kalman Filter that uses an approximation of the prediction-error covariance estimation method is proposed. This approximation reduces the computational speed significantly with only a limited reduction in the SE performance. The hybrid SE is validated in an EMTP-RV time-domain simulation of the CIGRE AC benchmark micro-grid that is connected to a DC grid using 4 VSCs. Bad data detection and identification using the largest normalised residual is assessed with respect to such a system. Furthermore, the proposed method is compared with a non-linear weighted least squares SE in terms of accuracy and computational time.

2022

Audio Steganography using Convex Demixing

Yann Mikaël Schoenenberger

In the first part, we first introduce steganography (in chapter 1) not in the usual context of information security, but as a method to piggyback data on top of some content. We then focus on audio steganography and propose a new steganographic scheme in chapter 2 as well as a model for the noisy, analog communication channel we are considering (in section 3.2). The method we use is based on signal mixing, the science of constructing vectors in a way that their sum can then be split back into the original components. This is presented in chapter 4. The data recovery, or demixing, relies on convex optimization (presented in chapter 5) and some further signal processing detailed in chapter 6. In the second part we present our proof of concept implementation and show the results of simulation runs that have been made in order to study the properties of the overall system (chapter 7). We study how the different parameters of the system and the communication channel affect the performance of the steganographic scheme.Finally, we draw conclusions (chapter 8) about the findings and suggest (in chapter 9) what next steps could be taken in order to further study this steganographic scheme.

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Advanced Model for Multiphase Generators

György Atilla Bànyai

The current development of generators and power electric drives is characterized by increased power electronic integration. This evolution concerns particularly the variable speed power units allowing both a higher performance and substantial savings on cost but nevertheless, it implies new constraints and difficulties in term of interaction between the various components: generator, converter and network. The design and optimization of such generators is no longer possible with the same approach and same tools as for conventional machines directly connected to a symmetrical three-phase network. This Ph.D. study is related to an industrial project which was developed by ALSTOM in the same time frame in which this thesis work was prepared. Since the project relies on a new high power synchronous generator topology (a multiphase turbo-generator connected to a three phase network via a power electronic converter), not many studies were done especially because of the enormous financial resources required by such studies and limitations in respect of the maximum power that a power electronic device can commute. The goal of this study is the development of an advanced multiphase machine model which can be used in a complex system comprising power electronic elements. The model has to accurately consider the physical phenomena which are taking place in a machine while functioning in such conditions. The selected approach for the development of the machine model is a combined numerical-analytical approach. This solution was preferred since it can take benefit from the precision, a property which is characteristic to the numerical Finite Element Methods (FEM), but also from the fast computation times which is a property of the analytical models. The model presented in this thesis is based on the differential inductance parameter. The differential inductances are calculated analyzing the results of FEM simulations and are used afterwards in analytically expressed circuit equations. The machine circuit equations, having as parameters the differential inductances, are afterwards solved numerically. In order to take advantage of the existing elements necessary for the analysis of the electrical power networks (including power electronic converters), the developed method was integrated into a network simulation software package. This simulation software package was designed for industrial use where a short computation time is desired; the module with the integrated machine model is respecting this principle.

EPFL2011

Advanced Model for Multiphase Generators

György Atilla Bànyai

EPFL2011