Power Transformer Design Practices

Fang Zhu, Baitun Yang

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000360110
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

The book presents basic theories of transformer operation, design principles and methods used in power transformer designing work, and includes limitation criteria, effective utilization of material, and calculation examples to enhance readers' techniques of transformer design and testing.

It includes:

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• Core and winding commonly used, and their performances

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• Insulation structures and materials, methods for improvements on dielectric strengths on partial discharge, breakdown and electrical creepage

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• Losses and impedance calculations, major influential factors, and methods to minimize load loss

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• Cooling design and the method to obtain effective cooling

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• Short-circuit forces calculations, the ways to reduce the short-circuit forces, and measures to raise withstand abilities

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• No-load and load-sound levels, the influential factors and trends, and abatement techniques

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• In-depth discussion of an autotransformer's special features, its stabilizing winding function, and its adequate size

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• Tests and diagnostics

The ways to optimize design are also discussed throughout the book as a goal to achieve best performances on economic design.

The book contains great reference material for engineers, students, teachers, researchers and anyone in the field associated with power transformer design, manufacture, testing, application and service maintenance. It also provides a high level of detail to help future research and development maintain electrical power as a reliable and economical energy resource.

1Introduction

Power transformers are an important component in electrical power transmission networks. Several types of power transformers exist, such as core form and shell form regarding the relation between winding and core. Under the core form specialty, there are three-leg core, five-leg core and single-leg core transformers. This book deals with three-leg core form transformers, which are widely used.

1.1Basic Theory

The basic function of the power transformer is changing the system voltage from one level to another through electromagnetic interaction, so the Law of Electromagnetic Induction and some parameters which are important in transformer design are introduced here.

1.1.1Voltage and Current of Windings

A winding having N turns is placed in a magnetic field, assuming each of its turns is linked with the same magnetic flux, ϕ, which varies with time, t. By the Law of Electromagnetic Induction, the voltage induced in the winding, v(t), is

(1.1)$$v\left(t\right)=-N\left(d\mathrm{\varphi }/dt\right)$$

If ϕ is sinusoidally varying flux with amplitude ϕ_m and frequency f

(1.2)$$\mathrm{\varphi }={\mathrm{\varphi }}_{m}cos\left(2\mathrm{\pi }ft\right)$$

Equation (1.1) is changed to

(1.3)$$v\left(t\right)=2\mathrm{\pi }fN{\mathrm{\varphi }}_{m}sin\left(2\mathrm{\pi }ft\right)=\sqrt{2}\mathit{Vsin}\left(2\mathrm{\pi }ft\right)$$

Where V is the effective value of the induced voltage, or voltage as called therein after. It is deducted from Equation (1.3).

(1.4)$$\left(V/N\right)=\sqrt{2}\mathrm{\pi }f{\mathrm{\varphi }}_m=4.44f{B}_m{A}_{Fe};B_{\text{max}}=\frac{\left(V/N\right)}{4.44f{A}_{Fe}}$$

Where B_max is maximum flux density in the core, Tesla; A_Fe is the net core cross-sectional area the flux going through, m², A_Fe = F_stack × A, where F_stack is stacking factor considering lamination coat thickness, it is in the range of 0.96~0.97, A is the core cross-sectional area, m². f is frequency, Hz. (V/N) is called volts per turn. Equation (1.4) gives the relation between magnetic performance and electric performance of a transformer. In a practical transformer, the primary and secondary windings are wound around a steel core in which the mutual flux goes through, and the great magnetic properties of core steel greatly increases the mutual flux density compared to air core. As results, (V/N) can be increased to handle high voltage or large power rating units, or in another way, with the same (V/N) the core cross-sectional area can be reduced dramatically, resulting in size reduction of the core, the windings, and overall size of the unit.

Understanding a real transformer starts usually from understanding an ideal transformer. An ideal two-winding transformer is a transformer which meets the following assumptions. First, both primary and secondary windings are linked only to m...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• Authors
• Chapter 1: Introduction
• Chapter 2: Core
• Chapter 3: Windings
• Chapter 4: Insulation
• Chapter 5: Impedances
• Chapter 6: Load Loss
• Chapter 7: Cooling
• Chapter 8: Short-Circuit Obligation
• Chapter 9: Sound Levels
• Chapter 10: Autotransformers
• Chapter 11: Testing
• Index