Analysis Of Power Systems Under Fault Conditions With Matlab Simulation

 ABSTRACT

The analysis of Power Systems under fault condition represents one of the most important and complex task in Power Engineering. The studies and detection of these faults is necessary to ensure that the reliability and stability of the power system do not suffer a decrement as a result of a critical event such a fault.

This project will conduct a research, analyze the behavior of a system under fault conditions and evaluate different scenarios of faults.

 CHAPTER ONE

1.0                                                        INTRODUCTION

The fault analysis of a power system is required in order to provide information for the selection of switchgear, setting of relays and stability of system operation. A power system is not static but changes during operation (switching on or off of generators and transmission lines) and during planning (addition of generators and transmission lines).

Thus fault studies need to be routinely performed by utility engineers (such as in the CEB).

Faults usually occur in a power system due to insulation failure, flashover, physical damage or human error. These faults may either be three phase in nature involving all three phases in a symmetrical manner, or may be asymmetrical where usually only one or two phases may be involved. Faults may also be caused by either short-circuits to earth or between live conductors, or may be caused by broken conductors in one or more phases.

Sometimes simultaneous faults may occur involving both short-circuit and broken-conductor faults (also known as open-circuit faults). Balanced three phase faults may be analysed using an equivalent single phase circuit.

During normal operating conditions, current will flow through all elements of the electrical power system within pre-designed values which are appropriate to these elements’ ratings. Any power system can be analyzed by calculating the system voltages and currents under normal and abnormal scenarios .Unfortunately, faults could happen as a result of natural events or accidents where the phase will establish a connection with another phase, the ground or both in some cases.  A falling tree on a transmission lines could cause a three-phase fault where all phases share a point of contact called fault location. In different occasions, fault could be a result of insulation deterioration, wind damage or human vandalism.

Faults can be defined as the flow of a massive current through an improper path which could cause enormous equipment damage which will lead to interruption of power, personal injury, or death. In addition, the voltage level will alternate which can affect the equipment insulation in case of an increase or could cause a failure of equipment start-up if the voltage is below a minimum level. As a result, the electrical potential difference of the system neutral will increase. Hence, People and equipment will be exposed to the danger of electricity which is not accepted.

In order to prevent such an event, power system fault analysis was introduced. The process of evaluating the system voltages and currents under various types of short circuits is called fault analysis which can determine the necessary safety measures and the required protection system. It is essential to guarantee the safety of public. The analysis of faults leads to appropriate protection settings which can be computed in order to select suitable fuse, circuit breaker size and type of relay.

The severity of the fault depends on the short-circuit location, the path taken by fault current, the system impedance and its voltage level. In order to maintain the continuation of power supply to all customers which is the core purpose of the power system existence, all faulted parts must be isolated from the system temporary by the protection schemes. When a fault exists within the relay protection zone at any transmission line, a signal will trip or open the circuit breaker isolating the faulted line

.To complete this task successfully, fault analysis has to be conducted in every location assuming several fault conditions. The goal is to determine the optimum protection scheme by determining the fault currents and voltages. In reality, power system can consist of thousands of buses which complicate the task of calculating these parameters without the use of computer software

such as Matlab. In 1956, L.W. Coombe and D. G. Lewis proposed the first fault analysis program.

There are two types of faults which can occur on any transmission lines; balanced faults and unbalanced faults. In addition, unbalanced faults can be classified into single line-to-ground faults, double line faults and double line-to-ground faults. The most common types taking place in reality are as follow:

Line-to-ground fault: this type of fault exists when one phase of any transmission lines establishes a connection with the ground either by ice, wind, falling tree or any other incident. 70% of all transmission lines faults are classified under this category.

Line-to-line fault: as a result of high winds, one phase could touch anther phase & line-to-line fault takes place. 15% of all transmission lines faults are considered line-to-line faults.

Double line-to-ground: two phases will be involved instead of one at the line-to-ground faults scenarios. 10% of all transmission lines faults are under this type of faults.

Three phase fault: in this case, falling tower, failure of equipment or even a line breaking and touching the remaining phases can cause three phase faults. In reality, this type of fault not often exists which can be seen from its share of 5% of all transmission lines faults.

In order to analyze any unbalanced power system, C.L. Fortescue introduced a method called symmetrical components in 1918 to solve such system using a balanced representation.

In this project, literature review section will be provided to summarize the methods used to analyze such cases. Then, a description of the symmetrical components methods will be discussed in detail. Its mathematical model will be presented. After that, a 6-bus system will be under fault for analysis. This analysis will take place using the manual calculations.

1.2                                              OBJECTIVES OF THE STUDY

To be able to perform analysis on power systems with regard to load flow, faults and system stability. This paper proposes a MATLAB based Graphical User Interface (GUI) tool which can serves as a user friendly visual tool for power system fault analysis. This GUI calculates fault level voltages and currents for all the different types of faults and displays them along with their waveforms accordingly. The GUI will serve as an educational tool to help the students understand the intricacies involved in fault analysis. Different Artificial Neural Network (ANN) architectures are proposed for designing an appropriate classifier for classification of the different types of faults that occurs in a real system. Finally, in order to test the classifier in test system conditions it is integrated with the developed GUI.

1.3                                                 SCOPE OF THE PROJECT

Electrical powers system is growing in size and complexity in all sectors such as generation, transmission, distribution and load systems. Types of faults like short circuit condition in power system network results in severe economic losses and reduces the reliability of the electrical system.

Electrical fault is an abnormal condition, caused by equipment failures such as transformers and rotating machines, human errors and environmental conditions. Theses faults cause interruption to electric flows, equipment damages and even cause death of humans, birds and animals. all these issues on fault are analyzed in this work using MATLAB simulation.

1.4                                         CAUSES OF ELECTRICAL FAULTS

• Weather conditions: It includes lighting strikes, heavy rains, heavy winds, salt deposition on overhead lines and conductors, snow and ice accumulation on transmission lines, etc. These environmental conditions interrupt the power supply and also damage electrical installations.
• Equipment failures: Various electrical equipments like generators, motors, transformers, reactors, switching devices, etc causes short circuit faults due to malfunctioning, ageing, insulation failure of cables and winding. These failures result in high current to flow through the devices or equipment which further damages it.
• Human errors: Electrical faults are also caused due to human errors such as selecting improper rating of equipment or devices, forgetting metallic or electrical conducting parts after servicing or maintenance, switching the circuit while it is under servicing, etc.
• Smoke of fires: Ionization of air, due to smoke particles, surrounding the overhead lines results in spark between the lines or between conductors to insulator. This flashover causes insulators to lose their insulting capacity due to high voltages.

1.5                                         EFFECTS OF ELECTRICAL FAULTS

• Over current flow: When fault occurs it creates a very low impedance path for the current flow. This results in a very high current being drawn from the supply, causing tripping of relays, damaging insulation and components of the equipments.
• Danger to operating personnel: Fault occurrence can also cause shocks to individuals. Severity of the shock depends on the current and voltage at fault location and even may lead to death.
• Loss of equipment: Heavy current due to short circuit faults result in the components being burnt completely which leads to improper working of equipment or device. Sometimes heavy fire causes complete burnout of the equipments.
• Disturbs interconnected active circuits: Faults not only affect the location at which they occur but also disturbs the active interconnected circuits to the faulted line.
• Electrical fires: Short circuit causes flashovers and sparks due to the ionization of air between two conducting paths which further leads to  fire as we often observe in news such as building and shopping complex fires.

1.6                                               FAULT LIMITING DEVICES

It is possible to minimize causes like human errors, but not environmental changes. Fault clearing is a crucial task in power system network. If we manage to disrupt or break the circuit when fault arises, it reduces the considerable damage to the equipments and also property.

Some of these fault limiting devices include fuses, circuit breakers, relays, etc. and are discussed below.

• Fuse: It is the primary protecting device. It is a thin wire enclosed in a casing or glass which connects two metal parts. This wire melts when excessive current flows in circuit. Type of fuse depends on the voltage at which it is to operate. Manual replacement of wire is necessary once it blowout.
• Circuit breaker: It makes the circuit at normal as well as breaks at abnormal conditions. It causes automatic tripping of the circuit when fault occurs. It can be electromechanical circuit breaker like vacuum / oil circuit breakers etc, or ultrafast electronic circuit breaker.
• Relay: It is condition based operating switch. It consists of magnetic coil and normally open and closed contacts. Fault occurrence raises the current which energizes relay coil, resulting in the contacts to operate so the circuit is interrupted from flowing of current. Protective relays are of different types like impedance relays, mho relays, etc.
• Lighting power protection devices: These include lighting arrestors and grounding devices to protect the system against lighting and surge voltages.