The overall evaluation of small autonomous hybrid power systems (SAHPS) that contain renewable and conventional power sources depends on economic and environmental criteria, which are often conflicting objectives. The solution of this problem belongs to the field of non-linear combinatorial multiobjective optimisation. In a multiobjective optimisation problem, the target is not to find an optimal solution, but a set of non-dominated solutions called Pareto-set. The present article considers as an economic objective the minimisation of system's cost of energy (COE), whereas the environmental objective is the minimisation of the total greenhouse gas (GHG) emissions of the system during its lifetime. The main novelty of this article is that the calculation of GHG emissions is based on life cycle analysis (LCA) of each system's component. In LCA, the whole life cycle emissions of a component are taken into account, from raw materials extraction to final disposal/recycling. This article adopts the non-dominated sorting genetic algorithm (NSGA-II), which in combination with a proposed local search procedure effectively solves the multiobjective optimisation problem of SAHPS. Two main categories of SAHPS are examined with different energy storage: lead-acid batteries and hydrogen storage. The results indicate the superiority of batteries under both economic and environmental criteria.

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Multiobjective genetic algorithm solution to the optimum economic and environmental performance problem of small autonomous hybrid power systems with renewables

With the fast-paced changing technologies in the power industry, new references addressing new technologies are coming to the market. Based on this fact, there is an urgent need to keep track of international experiences and activities taking place in the field of modern transformer design. The complexity of transformer design demands reliable and rigorous solution methods. A survey of current research reveals the continued interest in application of advanced techniques for transformer design optimization. This paper conducts a literature survey and reveals general backgrounds of research and developments in the field of transformer design and optimization for the past 35 years, based on more than 420 published articles, 50 transformer books, and 65 standards.

Transformer Design and Optimization: A Literature Survey

This paper presents a novel method to estimate transformer saturation characteristics that extend to the 'deep' saturation region. The basic idea of the method is to use recorded inrush voltage and current waveforms to estimate the saturation characteristics. Accurate representation of a transformer's 'deep' saturation region is important for studying the impact of transformer energization and its inrush currents. The performance and accuracy of the proposed method have been verified by a close agreement between simulation results and recorded inrush waveforms.

Estimation of transformer saturation characteristics from inrush current waveforms

We present a novel model-based approach for parameter identification of a laminated core, such as magnetic permeability and electrical conductivity, of power transformers on the basis of frequency response analysis (FRA) measurements. The method establishes a transformer core model using the duality principle between magnetic and electrical circuits for parameter identification with genetic algorithms. We use reference input impedance frequency responses, calculated by a well-known lumped parameter model of a three-phase transformer and finite-element computations, to analyze identification accuracy of the method. The results verify the ability of the approach to accurately identify the core lamination parameters with respect to the reference values. The approach can be used for parameter identification of a demagnetized core with known geometrical parameters when the core lamination samples are unavailable for experimental tests. The approach can also be employed for transformer core modeling and FRA result interpretation at low frequencies.

Transformer Core Parameter Identification Using Frequency Response Analysis

This article presents an extensive survey of current research on the transformer loss problem, particularly from the view of practical engineering applications. It reveals that the transformer loss problem remains an active research area. This article classified the transformer loss problem into three main groups: (a) tank losses due to high-current bushings, (b) losses in transformer core joints, and (c) stray losses in the transformer tank. It is based on over 50 published works, which are all systematically classified. The methods, the size of transformers, and other relevant aspects in the different works are discussed and presented.

A Review of Transformer Losses

A new three phase transformer model is presented in this paper, taking into account air gaps, saturation, core losses, and lamination anisotropy. The flux distribution in a straight overlap joint has been investigated using a lumped circuit model. It is shown that the actual air gap varies largely between an upper and a lower limit as a function of core saturation. Therefore, mitered joints have been modeled using a new concept of an equivalent variable air gap. This consists of defining the length of an equivalent air gap as a function of the instantaneous value of the flux in the laminations. A new approach to the core loss modeling is highlighted. It is characterized by a nonlinear conductance placed in parallel with the saturated magnetizing inductance. Furthermore, transformer yokes and limbs have been divided into longitudinal elements according to the rolling direction to take into account the lamination anisotropy effects. The transient and steady state behaviors of the transformer model have been tested. A good agreement is obtained between computed and measured results.

New transformer model including joint air gaps and lamination anisotropy

In this paper, an important discussion of transformer design and modeling is given. Anisotropy effects of the transformer laminations have been stressed in the light of the investigation of a three-phase three-limb transformer T-joint. Three simple T-joint modeling schemes accounting for the lamination anisotropy are proposed. The fundamental result is that the flux lines remain almost parallel to the rolling direction (RD), except in a small area around the T-joint. Therefore, transformer yokes and limbs have been divided into longitudinal elements according to the RD. Reluctances related to these elements are connected according to the magnetic circuit coupling and yield a new transformer circuit model. A good agreement is obtained between computed and measured transient currents. Furthermore, the new transformer model enables some localized analysis. The computed localized fluxes and loci of flux density vectors near to the T-joint exhibit a satisfactory agreement with many results based on FEM or experimental investigations.

M. Elleuch

Papers

New transformer model including joint air gaps and lamination anisotropy

Anisotropy in three-phase transformer circuit model