Protection Of Distribution Transformer Arising From Overvoltage

ABSTRACT

Increase in voltage for the very short time in power system is called as the over voltage. The voltage stress caused by over voltage can damage the lines and equipment’s connected to the system. There are two types of causes of over voltage in power system – over voltage due to external causes and over voltage due to internal causes. Over voltages can be generated at high frequency (load switching and lightning), medium frequency (capacitor energizing), or low frequency. Over-voltage due to external causes:

This cause of over voltage in power system is the lightning strokes in the cloud. By considering direct lightning strokes to a connected overhead distribution line, the effectiveness of the common practice transformer protection scheme and of an alternative one utilizing shorter surge arrester connection conductors in suppressing fast-front over-voltages was evaluated. A shorter length of the surge arrester connection conductors results in a reduction in the amplitude of the over-voltages arising at the medium-voltage terminals of the transformer and in a slower rate of increase of the overvoltage amplitude with lightning return-stroke current. The over-voltages transferred to the low-voltage terminals of the transformer are practically not affected by the length of the surge arrester connection conductors. Protection against transferred over-voltages was provided by surge protective devices installed at the low-voltage terminals of the transformer. By utilizing shorter surge arrester connection conductors the transformer failure rate, estimated through risk assessment, is reduced by approximately 11%.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

CHAPTER ONE

1.0      INTRODUCTION

• BACKGROUND OF THE PROJECT
• PROBLEM STATEMENT
• AIM OF THE PROJECT
• OBJECTIVE OF THE PROJECT
• SIGNIFICANCE OF THE PROJECT
• SCOPE OF THE PROJECT
• PROJECT ORGANISATION

CHAPTER TWO

LITERATURE REVIEW

• OVERVIEW OF DISTRIBUTION TRANSFORMER
• TYPES OF DESTRIBUTION TRANSFORMER
• USE OF DESTRIBUTION TRANSFORMER
• STRESS FACTORS AND PROTECTION METHODS
• CAUSES OF OVER VOLTAGE IN POWER SYSTEM
• EFFECTS OF OVER VOLTAGES ON POWER SYSTEMS
• REVIEW OF OVERVOLTAGE PROTECTION
• METHODS OF PROTECTION AGAINST LIGHTNING
• INSULATION COORDINATION

CHAPTER THREE

METHODOLOGY

• MODELLING OF THE EVALUATED SYSTEM

CHAPTER FOUR

4.0      RESULT ANALYSIS

• SIMULATION RESULTS AND DISCUSSION
• RISK ASSESSMENT

CHAPTER FIVE

• CONCLUSION

REFERENCES

CHAPTER ONE

1.0                                                              INTRODUCTION

1.1                                                 BACKGROUND OF THE STUDY

The fast-front over-voltages arising at distribution equipment utilizing non-self-restoring insulation, such as transformers or cables, may cause permanent failure resulting in system outages and economic losses. Therefore, distribution equipment is most commonly protected against impinging lightning surges by surge arresters and surge protective devices. It is well known that the efficiency of the afforded protection is affected by the length of the connection conductors; the latter should be as short as possible in order to achieve optimum protection [1-6].

According to common practice, distribution transformers are protected against impinging lightning surges by surge arresters installed close to their medium-voltage bushings. It can be shown theoretically that the fast-front over-voltages arising at a protected transformer increase in amplitude with the steepness of the incoming lightning surge and that the greater the separation distance between surge arresters and transformer the less is the effectiveness of the provided protection. Actually, analytical methods for the estimation of the lightning over-voltages arising at a protected transformer can be found in [7, 8]. However, as theoretical analysis is based on several simplifications, analytical results should be considered as conservative yet acceptable estimates for the protective distance and the safety margin provided by surge arresters. A more accurate evaluation of the protection afforded by surge arresters to the transformer can be made with the aid of detailed simulations using an electromagnetic transient analysis program.

In this study the effects of the length of the surge arrester connection conductors on the fast-front over-voltages arising at a typical pole-mounted 50 kVA, 20/0.4 kV transformer of the Hellenic distribution system are investigated through detailed ATP-EMTP [9] simulations. By considering both first and subsequent direct lightning strokes to the connected medium-voltage overhead line, the common practice transformer protection scheme and an alternative one utilizing shorter surge arrester connection conductors were evaluated. A better lightning performance of the transformer is achieved by implementing the alternative than the common practice protection scheme; the transformer failure rate, estimated through risk assessment, is reduced which is the aim of this study.

1.2                                                       PROBLEM STATEMENT

The most cause of damages in the lines and equipments connected to the system is caused by over-voltage coming from the distribution transformer. In other to solve these problem researchers came out the solution of protection to the distribution transformer. This protection against transferred over-voltages was provided by surge protective devices installed at the transformer. It is done by utilizing shorter surge arrester connection conductors the transformer failure rate, estimated through risk assessment, is reduced.

1.3                                                          AIM OF THE PROJECT

The main aim of this work is to use a shorter length of the surge arrester connection conductors results in a reduction in the amplitude of the over-voltages arising at the medium-voltage terminals of the transformer and in a slower rate of increase of the overvoltage amplitude with lightning return-stroke current.

1.4                                                   OBJECTIVES OF THE PROJECT

At the end of the study, the effectiveness of the common practice transformer protection scheme and of an alternative one utilizing shorter surge arrester connection conductors in suppressing fast-front over-voltages shall be evaluated.

Protection against transferred over-voltages was provided by surge protective devices installed at the low-voltage terminals of the transformer. By utilizing shorter surge arrester connection conductors the transformer failure rate, estimated through risk assessment, is reduced by approximately 11%.

1.5                                               SIGNIFICANCE OF THE PPROJECT

Many kinds of transformers can be found in the market which includes two winding power transformers, three windings power transformers, auto transformers, regulating transformers, earthing transformers.

Depending on the use of the transformer, winding connections, earthing methods, mode of operation and other various factors, decision on the kind of protection that should be used for the transformer is made. Considering the type of protection will help the transformer to last long and be safe to use.

1.6                                                        SCOPE OF THE PROJECT

In this study, the effects of the length of the surge arrester connection conductors on the lightning surges impinging on a typical wood pole-mounted 50 kVA, 20/0.4 kV transformer of the Hellenic distribution system are investigated through detailed ATP-EMTP simulations. By considering both first and subsequent direct lightning strokes to a connected overhead distribution line, the effectiveness of the common practice transformer protection scheme and of an alternative one utilizing shorter surge arrester connection conductors in suppressing fast-front over-voltages was evaluated. A shorter length of the surge arrester connection conductors results in a reduction in the amplitude of the over-voltages arising at the medium-voltage terminals of the transformer and in a slower rate of increase of the overvoltage amplitude with lightning return-stroke current.

1.7                                                         PROJECT ORGANISATION

The work is organized as follows: chapter one discuses the introductory part of the work,   chapter two presents the literature review of the study,  chapter three describes the methods applied, chapter four discusses the results of the work, chapter five summarizes the research outcomes and the recommendations.