Transformer Design and IE applications

NATIONAL INSTITUTE OF INDUSTRIAL ENGINEERING

PGDIE-42

IE Assignment submitted by:
Venkata Karthik Menti
Roll No: 102

The state of the art in engineering methods for

transformer design and optimization: A survey

AUTHORS:
ELEFTHERIOS I. AMOIRALIS, MARINA A. TSILI, PAVLOS S. GEORGILAKIS

Department of Production Engineering & Management, Technical University of Crete, GR-73100, Chania, Greece

Faculty of Electrical & Computer Engineering, National Technical University of Athens, GR-15780, Athens, Greece

1. INTRODUCTION: 

Transformer design is a complex task in which engineers have to ensure that compatibility with the imposed specifications is met, while keeping manufacturing costs low.

This paper provides an overview of research, development and application of various computational methods for transformer design, based on an extensive number of published papers.

The paper is organized as follows:
Section 1 gives us the introduction to the paper.
Section 2 describes the various transformer types & main considerations during transformer design process.
Section 3 includes the survey overview of research dedicated to transformer characteristics.
Section 4 provides an overview of the research conducted on transformer design optimization.
Section 5 concludes the paper.

2. TRANSFORMER DESIGN:

A transformer is a device with two or more stationary electrical circuits that are conductively disjointed but magnetically coupled time varying magnetic field and is used for transferring power one circuit to another by means of electromagnetic induction at the same frequency.

Transformers are one of the primary components for the transmission and distribution of electrical energy.

Their design results mainly from the range of application, the construction, the rated power and the voltage level.

2.1  Transformer Types:

      

Transformers are broadly classified on the basis of two parameters namely:

1. Power and voltage ratings:

• Distribution Transformers( Low ratings)
• Power Transformers( High ratings)

     2.   Cooling method:

• Oil-immersed Transformers
• Dry-type Transformers

In addition there are special purpose transformers such as converter transformers, test transformers, instrument transformers or telecommunications transformers

2.2  Transformer design considerations

Transformer design must take into account numerous performance parameters and technical constraints. The research in the relevant literature may deal with each one of these parameters separately, or concern the overall transformer optimization. Fig. 1 presents the main categories of the literature survey, which define the structure of the survey overview presented in the next Sections.

3. RESEARCH DEDICATED TO SPECIFIC TRANSFORMER CHARACTERISTICS:

The numerous computational methods and engineering models proposed for transformer analysis and the accurate prediction of their characteristics can be roughly categorized into four main groups: 

1.       Numerical techniques that consist some of the most widely used tools for transformer simulation. Among the proposed techniques of this group, the Finite Element Method (FEM) is the most prevalent one.

2.       Stochastic methods including Artificial Intelligence (AI) techniques, such as Genetic Algorithms (GAs), which have seen increased usage in the transformer design area over the last few years. 

3.       Improved versions of the transformer equivalent circuit, in order to include semi-empirical descriptions of the core and winding characteristics that affect the accuracy of calculations.

4.       Experimental methods, combining data provided by measurements with analytic or other methods, in order to provide efficient models for the accurate representation of certain transformer characteristics.

3.1 No-load losses:

No-load losses are the continuous losses of a transformer, regardless of load, namely they exist whenever the unit is energized. No-load losses are also called iron or core losses because they are mainly a function of the core materials. The two main components of no-load losses are eddy currents loss and hysteresis loss. 

3.2 Load losses:

Load losses result from load currents flowing through the transformer. Load losses are also called copper or wire or winding losses. The two components of the load losses are the I²R losses and the stray losses. I²R losses are based on the measured DC resistance. The stray losses are a term given to the accumulation of the additional losses experienced by the transformer.

3.3 Leakage field and short circuit impedance:

The calculation of transformer leakage flux is a prerequisite to the calculation of reactance, short-circuit impedance, short-circuit forces and eddy current losses.

Stochastic methods are also employed for solving problems of this category like an exact equivalent circuit model for the estimation of all impedance parameters of three winding transformers, with the use of GA.

3.4 Inrush Current

Transformer inrush currents are high-magnitude, harmonic-rich currents generated when transformer cores are driven into saturation during energization. These currents have undesirable effects, including potential damage or loss-of-life to the transformer, protective relay mis-operation, and reduced power quality on the system.

3.5 Stresses and dynamic behavior under short circuits

The short-circuit current in a transformer creates enormous forces on the turns of the windings.
The forces on the windings due to the short-circuit current vary as the square of the current. These mechanical and thermal stresses on the windings must be taken into consideration during the design of the transformer.

3.6 Transformer Noise

Transformers located near a residential area should have sound level as low as possible. The design and the manufacture of a transformer with low sound level require in-depth analysis of noise sources. Core, windings and cooling equipment are three main factors of noise which much concentrated upon during the design of the transformers.

3.7 Transformer Insulation

The insulation of a transformer is linked to its ability to withstand surge phenomena and over-voltages likely to occur during its operation. For this purpose, the related work may deal with the analysis of such phenomena, so as to design an adequate transformer insulation system. Other factors that affect transformer insulation life are vibration or mechanical stress, repetitive expansion and contraction, exposure to moisture and other contaminants.

3.8 Transformer Cooling

Transformer cooling is one of the most important parameters governing a transformer’s life expectancy. The total temperature is the sum of the ambient and the temperature rise. The temperature rise in a transformer is intrinsic to that transformer at a fixed load. The design of the cooling system is based on the hot-spot temperature value, and different methods for its prediction are proposed in the literature, along with the overall temperature distribution prediction, according to the transformer cooling method. 

3.9 Transformer DC Bias

Direct Current can flow in Alternating Current power lines if a DC potential difference exists between the various grounding points. Such a difference can be caused by a geomagnetic storm or the injection of DC current by one of the ground electrodes of a DC link. Direct current flowing through the earthed neutrals of transformer winding causes a DC component in the magnetising current. Owing to non-linearity, the waveform of this current is strongly distorted. The prediction and impact of this phenomenon has been studied with finite element method and equivalent magnetic circuits. 

3.10 Transformer Monitoring and Diagnostics

Despite the fact that monitoring and diagnostics are not part of the transformer design process, they are relevant to the main design considerations. As discussed earlier they constitute numerous computational methods and engineering models proposed for transformer analysis and the accurate prediction of their characteristics

4. TRANSFORMER DESIGN OPTIMIZATION

The difficulty in achieving the optimum balance between the transformer cost and performance is a complicated task, and the techniques that are employed for its solution must be able to deal with the design considerations of Section 3, so as to provide a design optimum, while remaining cost-effective and flexible. The research associated with design optimization is therefore more restricted involving different mathematical optimization methods. 

Techniques that include mathematical   models employing analytical formulas, based on design constants and approximations for the calculation of the transformer parameters are often the base of the design process adopted by transformer manufacturers. Neural network techniques are also employed as a means of design. Deterministic methods may also provide robust solutions to the transformer design optimization problem. The overall manufacturing cost minimization is scarcely addressed in the technical literature, and the main approaches deal with the cost minimization of various components.

Apart from the transformer manufacturing cost, another criterion used for transformer evaluation and optimization is the Total Owing Cost (TOC) taking into account the cost of purchase as well as the cost of energy losses throughout the transformer lifetime. Another aspect of transformer design optimization consists in providing design solutions in order to maintain certain aspects of transformer performance within the limits imposed by the technical specifications. 

5. CONCLUSION

In the present paper, an overview of the literature concerning transformer design has been undertaken, focusing on the progress realized in the past two decades.

Relevant publications from international journals have been selected, covering a broad range of engineering methods and design considerations. The difficulties to include and categorize the majority of the research in such a vast field were overcome by a convenient survey structure, taking into account various design considerations. This survey provides important information on the main directions of the considered research and the future trends in the field of transformer design.

Source:JOURNAL OF OPTO-ELECTRONICS AND ADVANCED MATERIALS
 Vol. 10, No. 5, May 2008, p. 1149 - 1158

http://users.ntua.gr/mtsili/amoiralis_files/The%20state%20of%20the%20art%20in%20engineering%20methods%20for%20transformer%20design%20and%20optimization_%20a%20survey.pdf