toolbox for power system fault analysis using matlab - mohd fitry bin ismail

7/30/2019 Toolbox for Power System Fault Analysis Using Matlab - Mohd Fitry Bin Ismail

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TOOLBOX FOR POWER SYSTEM FAULT ANALYSIS USING MATLAB

MOHD FITRY BIN ISMAIL

UNIVERSITY MALAYSIA PAHANG


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ABSTRACT

Power system fault analysis is the process of determining the bus voltages and

line currents during the occurrence of various types of faults. Faults on power systems

can be divided into three-phase balanced faults and unbalanced faults. Three types of

unbalanced fault occurrence on power system transmission lines are single line to

ground faults, line to line faults, and double line to ground faults. Fault studies are used

to select and set the proper protective devices and switchgears. The determination of the

bus voltages and line currents is very important in the fault analysis of power system.

The process consists of various methods of mathematical calculation which is difficult

to perform by hand. The calculation can be easily done by computer which is generated

by a program developed using MATLAB. GUI (Graphical User Interface) will be

provided with the programs as they are the components of the toolbox. This user

friendly toolbox will assist user which among electrical engineering student or trainee

engineer to perform the fault analysis of power system.


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CHAPTER 1

INTRODUCTION

1.1 Background

This project is focusing on the development of a toolbox for power system fault

analysis using MATLAB. Power system fault analysis is the process of determining the

magnitude of voltages and line currents during the occurrence of various types of faults.

The magnitude of these currents depends on the internal impedance of the generators

plus the impedance of the intervening circuit [2]. It can be of the order of tens of

thousand of amperes [2]. Faults on power systems can be divided into three-phase

balanced faults and unbalanced faults. Three types of unbalanced fault occurrence on

power system transmission lines are single line-to-ground faults, line-to-line faults, and

double line-to-ground faults. The magnitude of the fault current must be accurately

calculated in order that mechanical and thermal stresses on equipment may be estimated

[2]. Fault studies are used to select and set the proper protective devices and

switchgears [4].

The determination of the bus voltages and line currents is very important in the

fault analysis of power system. The process consists of various methods of

mathematical calculation which includes loads of formula and matrix approach to

determine the magnitude of the voltage and current. The calculation may form a large

rows and columns of matrix depending on the number of busses. The calculation is

possible when dealing with small number of busses. However it is difficult to perform


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by hand when dealing with large number of busses. We will discuss the method of

analysis in the methodology.

Hence, the development of this project will ease user to perform the calculations

of fault analysis despite encountering large number of buses. The calculation can be

easily done by computer which is generated by a program developed using MATLAB.

The program will simulate the input data keyed in by the user. Graphical User Interface

(GUI) will be provided with the programs. The program and the GUI will be packed in

a software package performing the fault analysis study and simulation as they are the

components of the toolbox which will be developed for the training and educational of

power system fault analysis. The toolbox will be user-friendly and will assist the

consumer whom does not have any programming background.

1.2 Objectives of Project

The objective of this project is to study the common fault types which are

balance and unbalance fault of the transmission line in the power system. Secondly is to

perform the analysis and obtain the results from simulation on those types of fault using

MATLAB. Lastly is to develop a toolbox for power system fault analysis for

educational and training purposes.

1.3 Scopes of Project

The scope of the project is to build a software package to assist user to perform

the fault analysis calculations. The targeted user is among trainee engineer and power

system students which have less experience in computer programming or C language.

In order to achieve the objectives of the project, some command in MATLAB program


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should be studied and understand so that the software package would operate as desired.

Moreover, MATLAB GUIDE (GUI part in MATLAB) should be mastered so that user

friendly software can be developed.


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CHAPTER 2

LITERATURE REVIEW

2.1 Fault Analysis

Short circuit currents flow when a fault occurs in power system. The magnitude

of these current can be of the order of tens of thousands of amperes, and consequently,

the magnitude of the fault current must be accurately calculated in order that

mechanical and thermal stresses on equipment may be estimated [2]. The types of fault

occur in power system are; balanced three-phase fault and unbalanced fault which are

single line to ground, line-to-line fault, and double-line to ground fault.

2.2 Types of Faults

In the transmission line, the common types of fault occurrence are [9]:

i. Balanced three-phase faultii. Single line-to-ground fault

iii. Line-to-line faultiv. Double line-to-ground fault

Figure 2.1 shows a graphical view of fault respectively.


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3-phase

transmissionline

i ii

iii iv

Figure 2.1: Four Common Types of Fault

In theory, solving transmission lines fault problems requires a circuit analysis

approach and mathematical skills as shown in the next Sections. Terms like Thevenin

theorem, mesh analysis, nodal analysis or any other method learnt in the basic circuit

analysis should be considered while mathematical skills required for forming a Bus

Impedance Matrix (Zbus) in order to put them in matrix pattern. In general, the analysis

of any fault condition is performed in the following order [3]:

i. Represent the given power system by its positive, negative and zero-sequencenetworks (the zero-sequence network is omitted for faults without earth, and

both the negative and zero-sequence networks are omitted for the balanced

three phase fault condition). This representation requires the calculation of per

unit (p.u.) impedances for generators, transformers, lines, cables and other

elements of the power system.

ii. Reduce each of the sequence networks to its simplest form. The equivalentpositive, negative and zero-sequence networks are represented as a series and

series-parallel combinations of the p.u. impedances. These are replaced by the

single equivalent impedance for each sequence network. It may also involve

the use of the delta-star or star-delta transformations.


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iii. Use the appropriate symmetrical-component equations to find the phasesequence components of the current in fault under the particular short-circuit

condition.

iv. Determine the required p.u. phase-current values at the point of fault.v. Finally, calculate the actual values of the phase-currents by multiplying

obtained p.u. values by the base current at the point of fault.

The procedure outlined above provides a complete analysis of the given power system

for the specified fault condition and can be easily implemented in computer aided

tutorials [3].

2.2.1 Balanced Three-Phase Fault Analysis

This type of fault is defined as the simultaneous short circuit across all three

phases. It is the most infrequent fault but the most severe type of fault encountered

because the network is balanced, it is solved on per-phase basis. The two phases carry

identical currents except for the phase shift[4]. Balanced three phase fault is also called

as symmetric fault [6]. The fault network can be solved by the Thevenins method. The

procedure is shown in the example below [4]. Figure 2.2 shows one line diagram of a

simple three-bus power system and a balanced three phase fault with fault impedance Zf

occurs at bus 3.


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XG1 XG2

XT1 XT2X12

X13 X23

Figure 2.2(a): One Line Diagram of a Simple Three-Bus Power System

The fault is simulated by switching on the impedance Zf at bus 3 as shown in

Figure 2.2(a). Thevenins theorem stated that the changes in the network caused by the

added branch (the fault impedance) shown in Figure 2.2(b) is equivalent to those

caused to the added voltage V3 (0) with all other sources short-circuited as shown in

Figure 2.2(c) [4].

X12

X13 X23

X12

X13 X23

X01 X01X02

X02

Figure 2.2: (b) Impedance Network for Fault at Bus 3. (c) Thevenins Equivalent

Network


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Simplify (c) into (d):

zf

Z33

Figure 2.2(d): Thevenins Equivalent Network Simplified

From Figure 2.2(d), the fault current at bus 3 is [4]:

(1)

Another method of determining fault current is using the Zbus method [4][9]. Analyze

the basic n-bus network to obtain the Bus Admittance Matrix (Ybus), from the line

impedance. Consider impedance network from Figure 2.2 and fault at bus 3:

(2)

(3)Thus, the fault current [4],

(4)

The bus voltage during fault [4],

(5)


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2.2.2 Unbalanced Fault Analysis

In the analysis, we need to represent the given power system by its positive,

negative and zero-sequence networks as shown in Figure 2.2 (the zero-sequence

network is omitted for faults without earth). This representation requires the calculation

of per unit (p.u.) impedances for generators, transformers, lines, cables and other

elements of the power system [3].

Figure 2.3: Positive, Negative & Zero Sequence Network

2.2.2.1 Single Line-To-Ground Fault Analysis

Figure 2.4: 3-Phase Equivalent Circuit


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Suppose a line-to-ground fault occurs on phase a through Zf as shown in

Figure 2.4. Assuming the generator is initially on no-load, the boundary conditions at

the fault point are:

(6)

(7)

Substituting for , the symmetrical components of currents from equation

(6) and (7) are:

(8)

From (8) we find that:

(9)

Phase avoltage in terms of symmetrical components is:(10)

Substituting for from and noting , we get:

(11)

Where . Substituting for from and noting , we

get:

(12)

Thus, the fault current:

(13)

Equation (13) can be expressed in the sequence in series as shown in Figure 2.5.


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Figure 2.5: Single Line-To-Ground Fault Sequence

2.2.2.2 Line-To-Line Fault Analysis

Figure 2.6: Three Phase Generators with Fault between Phase b and c

Figure 2.6 shows a three phase generator with fault through impedance Zf

between phase b and c. By assuming the generator is initially on no-load, the boundary

conditions at the fault point are:


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(14)

(15)

(16)

Substituting for , and , the symmetrical components of currents

are:

(17)

From the equation (17), we find that:

(18)

(19)

(20)

From (19) and (20), we note that:

(21)

We know that,

(22)

(23)

Substituting for and from (23) and noting , we get:

(24)

Substituting for from (19), we get:

(25)

Since , solving for results in:


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(26)

Thus, the phase currents are

(27)

The fault current is

(28)

Equation (21) and (27) can be represented by connecting the positive and negative-

sequence networks as shown in Figure 2.7.

Figure 2.7: Line-To-Line Fault Equivalent Network

2.2.2.3 Double Line-To-Ground Fault Analysis


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Figure 2.8: Three Phase Generators with Fault on Phase B and C through Impedance Zfto Ground

Figure 2.8 shows a three-phase generator with a fault on phase b and c

through impedance Zf to ground. Assuming the generator is initially on no-load, the

boundary conditions at the fault point are:

(29)

(30)

From (22), the phase voltages and are:

(31)

(32)

Since = , we note that:

(33)

Substituting for the symmetrical components of currents in (29), we get:


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(34)

Substituting for from (34) and for from (33) into (31), we get:

(35)

Substituting for the symmetrical components of voltage from (23) into (35) and solving

for , we get:

(36)

Also, substituting for the symmetrical components of voltage in (33), we obtain:

(37)

Substituting for and into (30) and solving for , we get:

(38)

Finally, the fault current:

(39)

Equation (36) and (38) can be represented by connecting the positive sequence

impedance in series with the parallel combination of the negative sequence and zero

sequence networks as shown in Figure 2.9


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Figure 2.9: Double Line-To-Ground Equivalent Circuit

2.3 GUI (Graphical User Interface)

A graphical user interface (GUI) is a pictorial interface to a program. A good

GUI can make programs easier to use by providing them with a consistent appearance

and with intuitive controls like pushbuttons, list boxes, sliders, menus, and so forth. The

GUI should behave in an understandable and predictable manner, so that a user knows

what to expect when he or she performs an action. For example, when a mouse click

occurs on a pushbutton, the GUI should initiate the action described on the label of the

button. This chapter introduces the basic elements of the MATLAB GUIs. The chapter

does not contain a complete description of components or GUI features, but it does

provide the basics required to create functional GUIs for your programs [7].

A graphical user interface provides the user with a familiar environment in

which to work. This environment contains pushbuttons, toggle buttons, lists, menus,

text boxes, and so forth, all of which are already familiar to the user, so that he or she

can concentrate on using the application rather than on the mechanics involved in doing

things. However, GUIs are harder for the programmer because a GUI-based program

must be prepared for mouse clicks (or possibly keyboard input) for any GUI element at


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any time. Such inputs are known as events, and a program that responds to events is

said to be event driven [7].

Graphics objects are the basic drawing elements used by MATLAB to display

data. Each instance of an object is associated with a unique identifier called a handle.

Using this handle, you can manipulate the characteristics (called object properties) of an

existing graphics object. You can also specify values for properties when you create a

graphics object. These objects are organized into a hierarchy, as shown in Figure 2.10.

Figure 2.10: Hierarchical Nature of Handle Graphics

The hierarchical nature of Handle Graphics is based on the interdependencies of

the various graphics objects. For example, to draw a line object, MATLAB needs an

axes object to orient and provide a frame of reference to the line. The axes, in turn, need

a figure window to display the axes and its child objects [12].

2.4 MATLAB GUIDE

GUIDE, the MATLAB Graphical User Interface development environment,

provides a set of tools for creating graphical user interfaces (GUIs). These tools greatly

simplify the process of designing and building GUIs. GUIDE tools are used to Layout

the GUI.


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Using the GUIDE Layout Editor, a GUI can easily lay out by clicking and

dragging GUI components -- such as panels, buttons, text fields, sliders, menus, and etc

into the layout area.

GUIDE automatically generates an M-file that controls how the GUI operates.

The M-file initializes the GUI and contains a framework for all the GUI callbacks - the

commands that are executed when a user clicks a GUI component. Using the M-file

editor, the callbacks to the code can be added to perform the desired functions of the

particular GUI [11].

2.5 Fault Analysis Software

Power system fault analysis software package that has already been developed

by engineers and programmers are discussed in this section.

A software package to perform power system fault analysis using the Ybus and

Zbus

method along with the symmetrical method. Provision is also provided for various

types of connection of transformers and grounding of generators [5]. The author used

MATLAB to build the software package to perform the fault analysis.

Another software package is developed by CYME group to perform the power

system analysis. The package is a complete set of power system analysis performing the

power flow analysis, optimal dispatch, transient stability, and fault analysis.

CYMFAULT [1] is the Power System Analysis Framework analysis module

dedicated to simulating fault conditions in three-phase electric power systems. User-

friendly data entry, a multitude of reports and flexibility in applying all industry-

accepted standards are features that makes CYMFAULT an Indispensable tool for these

very common and important system studies [1]. The window overview of the software


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package is shown in Figure 2.11 below. CYMFAULT is one of the components which

perform the fault analysis developed by CYME group.

Figure 2.11: Window Overview of CYMFAULT [1]

Leonardo [3] is a based tutoring system used to support the education of power

engineering students [3]. It provides a functionally interacting set of theory and

problems, and supports student progress through monitoring and assessment [3]. Figure

2.12 shows the Leonardo Expert System Shell where it divides between user and

developer. Figure 2.13 shows introduction layout of the program and Figure 2.14a,

2.14b and 2.14c shows the fault analysis example and a step by step solution

respectively.


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Figure 2.12: Leonardo Expert System Shell

Figure 2.13: Leonardos Introduction Layout


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Figure 2.14(a): Examples of Fault Analysis

Figure 2.14(b): Step By Step Solution


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Figure 2.14(c): Step By Step Solution

Artificial Neural Network (ANN) [8] is a comprehensive multi-paradigm

prototyping and development that can be used to solve complex problems [8]. It is an

approach for predicting fault in a large interconnected transmission system [8]. The

balanced and unbalanced data will be used as the inputs and outputs of ANN. Figure

2.15 show the design methodology of the software.

Figure 2.15: ANN Design Methodology


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CHAPTER 3

METHODOLOGY

3.1 Introductions

This Chapter presents the methodology of this project. The methodology is

divided to two parts, which is the simulation and analysis of fault in MATLAB

(Engineering Project 1) and the development of the Fault Analysis program using

MATLAB GUIDE (Engineering Project 2). The work flow is shown in Figure 3.1.


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START

Case Stud

Building MATLABProgram

Study & LearnMATLAB

IdentifyAppropriateCommand

Figure 3.1: Work Flow of the Project

Testing

OK?

Start Building GUI

TestingOK?

Study & Learn GUI

Propose toSupervisor

NO

NO

YES

YES

Simulation &Analysis

AnalysisOK?

YES

NO

ReportSubmission &Presentation

toolbox for power system fault analysis using matlab - mohd fitry bin ismail

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