A Study Of Effect Of Impurities On Distribution Transformer Oil

ABSTRACT

Water impurity is the essential factor of reducing the insulation performance of transformer oil, which directly determines the operating safety and life of a transformer. Molecular dynamics simulations and first-principles electronic-structure calculations are employed to study the diffusion behavior of water molecules and the electrical breakdown mechanism of transformer oil containing water impurities. The molecular dynamics of an oil-water micro-system model demonstrates that the increase of aging acid concentration will exponentially expedite thermal diffusion of water molecules. Density of states (DOS) for a local region model of transformer oil containing water molecules indicates that water molecules can introduce unoccupied localized electron-states with energy levels close to the conduction band minimum of transformer oil, which makes water molecules capable of capturing electrons and transforming them into water ions during thermal diffusion. Subsequently, under a high electric field, water ions collide and impact on oil molecules to break the molecular chain of transformer oil, engendering carbonized components that introduce a conduction electronic-band in the band-gap of oil molecules as a manifestation of forming a conductive region in transformer oil. The conduction channel composed of carbonized components will be eventually formed, connecting two electrodes, with the carbonized components developing rapidly under the impact of water ions, based on which a large number of electron carriers will be produced similar to “avalanche” discharge, leading to an electrical breakdown of transformer oil insulation. The water impurity in oil, as the key factor for forming the carbonized conducting channel, initiates the electric breakdown process of transformer oil, which is dominated by thermal diffusion of water molecules. The increase of aging acid concentration will significantly promote the thermal diffusion of water impurities and the formation of an initial conducting channel, accounting for the degradation in dielectric strength of insulating oil containing water impurities after long-term operation of the transformer.

Keywords: transformer oil; electrical breakdown; water impurity; insulating oil; molecular dynamics; first-principles calculation

TABLE OF CONTENTS

 TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

TABLE OF CONTENT

CHAPTER ONE

• INTRODUCTION
• BACKGROUND OF THE PROJECT
• PROBLEM STATEMENT
• AIM AND OBJECTIVE OF THE PROJECT
• SIGNIFICANCE OF THE PROJECT
• SCOPE OF THE PROJECT

CHAPTER TWO

LITERATURE REVIEW

2.1     OVERVIEW OF THE STUDY

2.2     TRANSFORMER OIL

2.3     TRANSFORMER OIL TYPE

2.4    TRANSFORMER OIL PROPERTIES

2.5    MECHANICAL IMPURITIES IN TRANSFORMER OIL

2.6          CONTAMINANTS OF TRANSFORMER OIL

2.7      REVIEW OF RELATED STUDIES

CHAPTER THREE

3.0      METHODOLOGY

3.1      MODEL ESTABLISHMENT AND SIMULATION METHOD

3.1.1   Transformer Oil Model

• Model of Oil-Water-Acid System

3.2      SIMULATION METHODS

CHAPTER FOUR

• SIMULATION RESULTS AND ANALYSIS

4.1     DIFFUSION COEFFICIENT

• FIRST-PRINCIPLES ELECTRONIC STRUCTURE CALCULATION
• BREAKDOWN MECHANISM OF TRANSFORMER OIL WITH WATER IMPURITIES

CHAPTER FIVE

5.1      CONCLUSION

5.2      RECOMMENDATION

REFERENCES

CHAPTER ONE

1.0                                             INTRODUCTION

1.1                               BACKGROUND OF THE STUDY

Transformer is one of the main assets in the electrical power industry which needs to be maintained for guaranteed uninterrupted power transmission in order to get assured revenue benefits. Transformer’s life mainly depends on the quality of its insulating oil i.e. transformer oil. Since transformer oil serves as a dielectric material and as an effective coolant in transformer, to perform these functions effectively the transformer oil must be purified regularly. Transformer internal insulation is primarily composed of an oil-paper composite system. Transformer oil, as the insulation and cooling fundamental, directly determines the operation safety and life of a transformer (Wang et al., 2018). In the manufacturing, transportation, assembly, and operation processes of transformers, water impurities that inevitably existing in transformer oil will reduce the electrical and aging resistance of transformer oil and even lead to the breakdown infraction of transformer internal insulation (Lundgaard et al., 2004). The researches on the diffusion behavior of water impurities in transformer oil and its effect on the dielectric breakdown of the transformer can provide a theoretical basis for further exploring dielectric failure mechanisms of transformer oil to improve the safe and stable operation of transformers.

At present, most of the theoretical atomic-level researches of water impurities in transformer oil merely focus on the analyses of diffusion and distribution law, without reckoning on water impurities in the breakdown process of transformer oil. Therefore, in this paper, molecular simulation technology is used to establish the multiple-molecule model of transformer oil with water impurities to calculate mean square displacement and diffusion coefficients of water molecules in transformer oil at different temperatures, based on which the electron properties of the local region containing water molecule in transformer oil are calculated by means of a first-principles method to explore electrical breakdown mechanism of water-containing transformer oil.

1.2                                                   PROBLEM STATEMENT

To extend the service life and ensure the operability of oil-filled transformer equipment, the attention is paid to the development of methods for monitoring the state of oil-barrier insulation. When monitoring the technical condition of transformer oil, the class of liquid purity is determined depending on the rated voltage of the equipment. However, the influence of the parameters of mechanical impurities on the breakdown voltage is not taken into account, thereby lowering the requirements for the quality of oil barrier insulation. This makes it relevant to study the influence of the size distribution of impurity particles on the electric strength of the internal insulation of power transformers and determine the parameters of particles of mechanical impurities to justify the underestimation of the quality indicators of transformer oil in operation.

1.3                                   AIM AND OBJECTIVES OF THE STUDY

The main aim of this work is to study the effect of impurities on distribution transformer oil. The objectives are:

1. To study oil-barrier insulation
2. To study factors that reduces the insulation performance of transformer oil

• To carry out a molecular dynamics Simulations and first-principles calculations.

1.4                                                     SCOPE OF THE STUDY

To determine the maximum and minimum voltages, the standard values of the average breakdown voltages and the results of operational tests of transformer oil in a standard spark gap were used. The relation between the particle size of mechanical impurities and the breakdown voltage of transformer oil has been established. The particle size range that defines the maximum and minimum breakdown voltages has been determined, and the values of limit concentrations of mechanical particles have been established. The obtained parameters of impurity particles which determine the maximum and minimum breakdown voltages of the operating oils can be used to evaluate the technical condition when diagnosing the internal insulation of power transformers in order to increase their operational reliability, as well as to adjust the regulatory requirements for the quality of operational transformer oil according to the content of mechanical impurities.

1.5                                           SIGNIFICANCE OF THE STUDY

This study will go a long way helping in determining the essential electrical properties of transformer oil. Also, the study will be useful to power engineers in identifying if a certain oil is suitable for future use or not.

Finally, the study will be used in preventing untimely failures and maximize safety