Steady state modelling of non-linear power plant components

Rico Melgoza, J. Jesús. (1997) Steady state modelling of non-linear power plant components. PhD thesis, University of Glasgow.

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Abstract

This thesis studies the problem of periodic. waveform distortion in electric power systems. A
general framework is formulated in the Hilbert domain to account for any given orthogonal
basis such as complex Fourier. real Fourier. Hartley and Walsh.· Particular applications of
this generalised framework result in unified frames of reference. These domains are unified
frameworks in the sense that they accommodate all the nodes. phases and the full spectrum
of coefficients of the orthogonal basis. Linear and linearised, non-linear elements can be
combined in the same frame of reference for a unified solution.
In rigorous waveform distortion analysis. accurate representation of non-linear characteristics
for all power plant components is essential. In this thesis several analytical forms
are studied which provide accurate representations of non-linearities and which are suitable
for efficient. repetitive waveform distortion studies.
Several harmonic domain approaches are also presented. To date most frequency domain
techniques in power systems have used the Complex Fourier expansion but more efficient
solutions can be obtained when using formulations which do not require complex algebra.
With this in mind. two real harmonic domain frames of references are presented: the real
Fourier harmonic domain and the Hartley domain. The solutions exhibit quadratic rate of
convergence. Also, discrete convolutions are proposed as a means for free-aliasing harmonic
domain evaluations; a fact which aids convergence greatly.
Two new models in the harmonic domain are presented: the Three Phase Thyristor
Controlled Reactor model and the Multi-limb Three Phase Transformer model. The former
uses switching functions and discrete convolutions. It yields efficient solutions with strong
characteristics of convergence. The latter is based on the principle of duality and takes
account of the non-linear electromagnetic effects involving iron core, transformer tank and
return air paths. The algorithm exhibits quadratic convergence. Real data is used to
validate both models.
Harmonic distortion can be evaluated by using true Newton-Raphson techniques which
exhibit quadratic convergence. However, these methods can be made to produce faster solutions
by using relaxation techniques. Several alternative relaxation techniques are presented.
An algorithm which uses diagonal relaxation has shown good characteristics of convergence
plus the possibility of parallelisation.
The Walsh series are a set of orthogonal functions with rectangular waveforms. They
are used in this thesis to study switching circuits which are quite common in modern power
systems. They have switching functions which resemble Walsh functions substantially.
Accordingly, switching functions may be represented exactly by a finite number of Walsh
functions, whilst a large number of Fourier coefficients may be required to achieve the same result. Evaluation of waveform distortion of power networks is a non-linear problem
which is solved by linearisation about an operation point. In this thesis the Walsh domain
is used to study this phenomenon. It has deep theoretical strengths which helps greatly in
understanding waveform distortion and which allows its qualitative assessment.
Traditionally, the problem of finding waveform distortion levels in power networks has
been solved by the use of repetitive linearisation of the problem about an operation point.
In this thesis a step towards a true non-linear solution is made. A new approach, which uses
bi-linearisations as opposed to linearisations, is presented. Bi-linear systems are a class of
simple, non-linear systems which are amenable to analytical solutions. Also, a new method,
based on Taylor series expansions, is used to approximate generic, non-linear systems using
a bi-linear system. It is shown that when using repetitive bi-linearisations, as opposed to
linearisations, solutions show super-quadratic rate of convergence.
Finally, several power system applications using the Walsh approach are presented. A
model of a single phase TCR, a model of three phase bank of transformers and a model of
frequency dependent transmission lines are developed.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Colleges/Schools: College of Science and Engineering > School of Engineering > Electronics and Nanoscale Engineering
Supervisor's Name: Acha, Dr. Enrique
Date of Award: 1997
Depositing User: Ms Mary Anne Meyering
Unique ID: glathesis:1997-5319
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 25 Jun 2014 15:34
Last Modified: 25 Jun 2014 15:35
URI: http://theses.gla.ac.uk/id/eprint/5319

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