Modeling And Analysis Of Protective Relays For Components Switching And Frequency Deviation

ABSTRACT

This work is on power system protection. Power system protection concerns the detection of faults in the system from measured voltages and currents. However, non-fault voltage and current disturbances may lead to the inadvertent detection of a fault in the same way as voltage disturbances may lead to the tripping of end-user equipment.

The aim of the research presented in this thesis is to quantify non-fault voltage and current disturbances at the terminals of protective relays; in other words: to define power quality from the viewpoint of a protective relay.

Starting from the principles (algorithms) of the relays, a quantification approach is developed to assess the disturbance impact on the relays. The quantification is based on the relay response after processing the disturbance signals by relay filters. This makes it possible to quantify the disturbance impact by the consequences caused by the disturbances, other than by the signal waveforms of the disturbances.

For component switching disturbances, the “impact severity curve” is developed to quantify the disturbance impact on a relay. It is a measure of the potential risks that a relay faces in case of a particular power system disturbance. It also facilitates the relative comparison among different disturbance cases. For frequency deviation disturbances, the resulting errors in relay outputs are estimated, which is helpful in determining whether an accurate frequency measurement is needed.

The proposed approach in the project can be applied in testing the disturbance immunity of any future fast relay algorithm. The test results, which are in the form of impact severity curves, can be adopted as the basis of disturbance classification in the viewpoint of protective relays and further lead to the setup of immunity test database.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

CHAPTER ONE

1.0      INTRODUCTION

• BACKGROUND OF THE PROJECT
• OBJECTIVE OF THE PROJECT
• SCOPE OF THE PROJECT
• SIGNIFICANCE OF THE PROJECT
• PROJECT ORGANISATION

CHAPTER TWO

LITERATURE REVIEW

• INTRODUCTION
• CLASSIFICATION OF POWER SYSTEM DISTURBANCES
• EFFECT OF DISTURBANCES ON RELAY OPERATION
• EFFECT OF HARMONICS ON PROTECTIVE RELAYS
• EFFECT OF FREQUENCY DEVIATION ON PROTECTIVE RELAYS
• IMPACT OF COMPONENT SWITCHING ON PROTECTIVE RELAYS
• REVIEW OF DIFFERENT TYPES OF RELAY
• REVIEW OF PROTECTION TYPES

CHAPTER THREE

METHODOLOGY

• INTRODUCTION
• PRINCIPLES OF DIGITAL RELAYS IN POWER SYSTEMS
• CLASSIFICATION OF COMPONENT SWITCHING TRANSIENTS
• QUANTIFYING COMPONENT SWITCHING
• HOW TO TEST RELAYS UNDER COMPONENT SWITCHINGS
• IMPACT OF FREQUENCY DEVIATION ON RMS-BASED RELAYS
• IMPACT OF FREQUENCY DEVIATION ON DFT/DEA-BASED RELAYS
• RESPONSE OF DEA FOR IMPEDANCE RELAYS IN CASE OF FREQUENCY DEVIATION

CHAPTER FOUR

RESULT

• COMPONENT SWITCHING ANALYSIS
• DETECTION OF SWITCHING SOURCES
• QUANTIFICATION OF COMPONENT SWITCHINGS
• COMPONENT SWITCHING ON OVERCURRENT RELAYS
• OPERATION PRINCIPLES OF OVERCURRENT RELAYS

CHAPTER FIVE

• CONTRIBUTIONS IN THIS WORK
• CONCLUSION
• RECOMMENDATION
• REFERENCES

CHAPTER ONE

1.0                                        INTRODUCTION

   1.1                                                            BACKGROUND OF THE PROJECT

The term power quality was first introduced in late 1960’s [1][2][3], almost a century after the first power system was put in service. It became in common use twenty years later. Before that time, it did not attract too much attention mainly because of two reasons. One is that in the past there were much fewer power system components that could generate power system disturbances. The other is that in the past there were much fewer power system  components that were sensitive to power system disturbances. Obviously these two factors are mutually linked [4].

With the development in electronics technology, more and more power system components are equipped with electronic devices. The introduction of electronic devices enables much more flexibility for system and component operation, regulation and protection. While the improvement on the functions is enormous, its side effect is also inevitable: the presence of electronic device in the systems, together with the flexible component operation mode brought by electronic device, will lead to both transient and steady-state problems in power systems. Such power quality disturbances not only exislocally, but also propagate to other parts of the power system. On the other hand the electronic device itself is much more vulnerable to such power quality disturbances than the classical power system components. In a word, the main source of power quality disturbances today is the application of electronic technology as well as the change of operation mode benefiting from the application of such techniques.

Of course the benefits from applying electronic techniques are huge when compared with its side effect. What one can do is to reduce the side effect. With the trend of deregulation of power market, more attention has been paid on the power system disturbances because:

• In a deregulated market, anything that might affect the normal operation of power systems or their components can be, in one way or the other, directly expressed as economic losses. So power system disturbance becomes an important
• In a deregulated market, the agreement between the utilities and customers is to ensure normal operation and reduce the effects of power system disturbances. Such an agreement depends on the definition of the responsibilities for each party, as each party can be the source of power system disturbances. Also the effects can be due to ill- designed equipment on either

There are two ways to deal with power system disturbances, from the viewpoints of utilities and from the viewpoint of customers. What the utilities can do is to locate the source of the disturbances and take necessary measures to limit the severity and occurrence frequency of such disturbances, so as to ensure the stable and safe operation of power systems. For the customers, it is necessary to study the impact of power system disturbances on the equipment. Applying a particular device or control mode may migrate such disturbance effect. Generally speaking, system stability and component performance are the main issues to be studied whenever there are power system disturbances.

Whatever the sources are, power system disturbances are, according to the “observation” by the system and by end-user equipment, the transient and steady state distortion in voltage and current waveforms. Therefore study of the disturbance impact should start from analysis of voltage and current waveforms during power system disturbances.

Power system disturbances are of various types related to their different sources. Disturbances can be due to steady-state load currents, dynamic operations as well as faults. The components that suffer from such disturbances are also of various types. Among the affected components, the protective relay is an important device for the safe and reliable operation of power systems.

Protective relays play no role when the operation of power systems is stable and normal. They are designed to operate when there are faults involved in the systems. The direct consequence of relay operation is system component cutoff, which definitely affect the operation of the system. Therefore either a fail-to-trip or a mal-trip can lead to considerable economic losses. In the former case, a fail-to-trip makes the fault-involved components remain at critical conditions too long, damaging the components either in short-term or in long-term. In the latter case, a mal-trip cuts off components that are under healthy operation, possibly causing direct economic losses to customers.

Therefore two characteristics are important for protective relays: sensibility (or dependability in a more formal way) when there is a fault, and immunity (or security in a more formal way) when there is anything other than faults. The former tells how reliable a relay is when it should operate, while the latter tells how robust a relay is when it should not operate.

In this sense, protective relays should not react to any power system disturbance. However, most protective relays are designed to deal with the situation when the voltage and current signals are steady-state sinusoidal waveforms. In case of power system disturbances, whether protective relays will experience a mal-trip is dependent on the type of the disturbance as well as the structure of the relay. Although some new generation relays possess ability to be adaptive to some of the external changes, this cannot guarantee complete immunity from the disturbances because the disturbance conditions are far more than one can expect. Also with the possible higher requirement on fault-clearing time in the future, faster relays will be needed. A faster relay means a shorter decision-making time, which implies a higher possibility of mal-trip due to inadequate information obtained by the relay within such short time.

To study the impact of power system disturbances on protective relays, it is necessary to apply some typical disturbances in protective relay testing. The applied disturbances can be either obtained from practical measurements, or from reasonable simulation of practical power system structures and events.

   1.3                                                           OBJECTIVE OF THE PROJECT

The aim of this project is the creation of a set of general procedures for the testing of protection relays under non-fault disturbances. Such procedures should be applicable to both the relay algorithms that are currently in use as well as possible new algorithms developed in the future. By applying these procedures, a disturbance database can be set up. Such a database will consist mainly of voltages and currents recorded in the power system during actual disturbances.

1.4                                  SCOPE OF THE PROJECT

The quantification of non-fault voltage and current disturbances at the terminals of protective relays was discussed. It was found that for component switching disturbances, the ‘impact severity curve’ was developed to quantify the disturbance impact on a relay. The analysis suggested the application of the proposed approach in testing the disturbance immunity of fast relay algorithms.

In this work, the disturbance data are obtained at the user-terminal in the viewpoint of relays, and then ranked and designated for different applications in the future.

The project is divided into two stages. In the first stage, the power system disturbances are classified based on their characteristic values such as peak value and duration. The first stage includes discussions on the possible way of disturbance quantification in the viewpoints of both relay manufacturers and customers. The study results are presented in author’s thesis for Licentiate of engineering degree [5]. The second stage focuses on the analysis of relay algorithms as well as quantification of typical power system disturbances for different relay algorithms. The results of the second stage are the main contents in this thesis.

1.5                          SIGNIFICANCE OF THE PROJECT

This study discusses critical roles protective relays play in the operation of the electrical power system. The protective relays are designed to take action when abnormal conditions occur on the power system. Elaborate protection schemes have been developed to detect these various conditions using current and voltage measurements through current transformers and potential transformers. In this work, digital relays advantages over conventional schemes was discussed in this work.

1.6                                   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.