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POWERHU - A PC-Based Electric Power System Analysis Software Package for Electric Power System Courses

Murat Songür and Bahri Ercan


Abstract - A software package is developed to perform electrical power system analysis on a personal computer. The software is capable of performing impedance calculations, load-flow studies, fault analysis and transient-stability analysis of electric power systems. It is intended for electric power system students, and is realized in such a manner that a problem can be solved by using alternative methods. Each step during the calculations can be visualized. The notations used in the program are mostly compatible with the formats used in electric power system textbooks. The program is developed under MATLAB 4.0 for Microsoft Windows.



I. Introduction

Electric power system analysis software ran on mainframe computers in the early years after the introduction of digital computers. Today it is possible to perform the analysis of a considerably large system on a 486 PC. Although there are many software applications on the market today that perform the analysis of electric power systems on PCs, most are intended for professionals. These programs take detailed input data about the system, use fast algorithms to perform the solutions, and then present the results obtained. Such software is most useful when only the final results are sufficient for the user.

A problem for electric power system students is the solution of problems in textbooks. In the case of a load-flow problem, most of the effort is focussed on iterative calculation, not on how the problem is solved. The same is true for stability studies. Professional software for analysis of electric power systems can help such students to prove their solutions; however, only the validity of the final results can be checked. Also, such software requires some detailed input parameters to which power system students have not yet been introduced.

POWERHU, a software package for performing electrical power systems analysis on a personal computer, is developed in such a manner that the problems can be solved in the same way as they are in most textbooks. The solutions are obtained in the same way as they are presented and displayed so that all the steps can be followed and visualized. The software is capable of performing impedance calculations of transmission lines, load-flow analysis, fault analysis and transient stability analysis.

The program has been developed as a MATLAB for Windows application. This approach enabled the use of graphical controls and use of MATLAB's built-in functions. As the m-files written for MATLAB are not compiled into binary form, students are also able to see the inner structure of the program and have the chance to see how the electric power system analyses are implemented on a computer. The source is simplified so that its structure can be visualized easily.



II. The Software Package POWERHU

POWERHU is a MATLAB for Windows (1) application for performing the analysis of electric power systems on a personal computer. MATLAB software requires at least a 386 processor with 387 co-processor or a 486DX processor with a minimum of 4 megabytes RAM to run. POWERHU software occupies only 200 kilobytes of hard disk space, after the host software is loaded on the system.

POWERHU has a modular programming structure. The program runs from a main shell that refers to required sub-programs by means of graphical menus. The sub-programs also refer to their corresponding sub-programs in a similar manner.


(a)

(b)
A block diagram of POWERHU is given in Fig. 2.

The program takes its input from data files created by the program itself. This approach enables the storage of input data on the hard disk once a system is defined. It can also be used for future studies. The outputs created by the program also are stored in disk files. All files generated by the program are in bare ASCII format so that interchange between various software can be maintained.


A. Impedance Calculations Module

The first numerical analysis module developed is for the calculation of the transmission line impedance of overhead lines. The transmission line impedance parameters introduced in the book "Elements of Power System Analysis" (2) are taken as a basis for this module. This module uses two database files for calculation of the impedance of transmission lines. Data for the conductors and the pole types are stored in two different database files. Desired items are selected from the database files and by the introduction of the total line length and the operating frequency, the results are obtained.


B. Load Flow Analysis Module

The second numerical module is the load-flow analysis module, simplified to take account of only the basic electric power system parameters. Only initial voltage and power values for the busbars, line impedance and transformer impedances, including transformer tap data, are taken into account. Busbars are assumed to be ideal and charging impedances of the lines are neglected. After defining the input data for the system, the user is asked to choose an algorithm for the load-flow analysis by means of a graphical menu.

Gauss-Seidel, Newton-Rhapson and Fast-Decoupled load-flow analysis modules are available in the software package. This feature enables the power system students to examine differences in the performance of alternative algorithms. Convergence plots of the iterations are displayed at the end of the calculations to visualize this difference. The convergence plots for a system with 5 busbars obtained by Gauss-Seidel algorithm and the Fast-Decoupled algorithm are shown in Fig.3.a. and Fig.3.b.


(a)

(b)
By turning the trace option to "On" during load-flow studies, it is possible to visualize all the iterations at once. This capability is useful for electric power system students so that manual calculations can be verified by the results obtained from the computer.

At the end of the calculations the bus voltages, line currents and power flow between branches are displayed and stored on a disk file.


C. Fault Analysis Module

The third module of the POWERHU software package deals with the fault analysis of electric power systems. This module requires the output from the load-flow module to supply the initial data for the system.

As the initial conditions are obtained, the type of the fault is to be identified. The software classifies faults into two major groups: short-circuit and open-circuit faults. Short-circuit faults are sub-classified as one line-to-ground, two line-to-ground, three line-to-ground and line-to-line faults. Open circuit faults are sub-classified as one line-open and two line-open faults. The fault type is chosen from mouse driven graphical menus.

The calculations are performed after the definition of the fault location and the fault impedance. The results calculated during fault analysis include the sequence current and voltage values, as well as the fault current, at the fault location. Branch currents and phase voltages throughout the whole system also are displayed. All the fault analysis results are stored in an associated output file for further reference.


D. Transient Stability Analysis Module

The last computational module of the POWERHU software package deals with transient stability analysis of electric power systems. A simplified model for transient stability is used in the program. Governor action is not taken into account. The transient stability analysis module takes the system data from the load-flow module. Previously obtained load-flow results can also be used.

After defining the fault duration, fault clearance time and total analysis time, the user is prompted for a method to solve the swing equation. Modified-Euler and 4th order Runge-Kutta methods are included in the software package. Thus, comparisons of integration methods can be visualized. If the trace mode is set to "On", the results calculated during each step of integration can be displayed. Just as in load-flow analysis, this option provides verification for manual calculations.

After the calculation of the generator parameters throughout the system, the results are displayed and written to corresponding output files. The graphs for rotor angle vs. time and shaft speed vs. time for each generator in the system are plotted on graphs. An example of such plots is given in Fig.4.


E. System Utilities Module

In order to manage the input and output data files, a system utilities module is included in POWERHU. This module is used to edit the contents of the data files or to print hard copies of either the input or result files. Viewing the results of previously run examples and previously entered system data are also supported by this module.



III. Solution Of A Load-Flow Problem Using The Program

Step-by-step solution of a load-flow problem using POWERHU is presented in the following paragraphs.

In MATLAB command window, the program is invoked by typing

which results in the main program menu.

Choosing the load-flow option, a sub menu is displayed. This menu provides the choice of creating a new system data file or choosing a previously created data file.

When the button for using an existing data file is clicked with mouse, the names of existing data files are displayed.

At this stage, the user is expected to enter the name of the system to be used in load-flow analysis. The system named "hutest" is chosen as an example. When the name of the desired system is typed, data for that system is displayed.

Choice of the load-flow algorithm from a sub menu is the next step.
When the desired algorithm is selected, the calculations are performed. Gauss-Seidel algorithm is chosen in this example. Convergence plots for the iterations are plotted and the resulting load-flow output data are displayed on the monitor.


(a)


(b)

When the load-flow study is completed, the program returns to the main menu.


IV.Conclusions

User friendly electric power system analysis software, intended for use in power system education, is developed. As the software is intended for educational purposes, problems from textbooks (2), (4), (7) are taken as a reference and solutions matching those in textbooks are obtained. The software offers alternative solution methods for load-flow and transient stability analysis, as well as an option for displaying intermediate steps during the calculation of load-flow and transient stability parameters. The fault analysis module provides the sequence current and voltage values for verification of the solutions to fault analysis problems. A database application module for calculation of the transmission line impedances is introduced.

As the software is presented as MATLAB source code, it is also possible to examine the structure of the package to see a typical electric power system computer application. Due to its modular structure, the software easily can be modified to provide alternative solution techniques and new analysis modules.

The program is currently used as a supplement to electric power system courses in the Department of Electrical and Electronics Engineering, Hacettepe University, Ankara, Turkey.



References


1. MATLAB for Windows, Version 4.0, The Mathworks Inc., Massachusetts, 1993.
2. Stevenson, W.D., Elements of Power System Analysis, 4th ed. New York: McGraw-Hill Book Co., 1988. 
3. Heydt, G.T., Computer Analysis Methods for Power Systems, New York: Macmillan Publishing Co:, 1986. 
4. Pai, M. A., Computer Techniques in Power System Analysis, New Delhi: Tata-McGraw-Hill, 1984. 
5. Arriaga, J. and Arnold, C. P., Computer Analysis of Power Systems, London: John Wiley and Sons., 1990. 
6. Anderson, P. M., Analysis of Faulted Power Systems, Ames: Iowa State University Press, 1973.
7. Stagg, G. W. and El-Abiad, A. H., Computer Methods in Power System Analysis, New York: McGraw-Hill Books Co., 1968.


Contact Information

Murat Songür
BILTAM A.S.
Ahmet Mithat Efendi Sokak
No:28/5, Cankaya, 06550
Ankara
TURKEY
Phone: 90-312-440 7757
Fax: 90-312-440 5256
E-mail: msongur@mail.btam.com.tr

Bahri Ercan
Department of Electrical and Electronics Engineering
Hacettepe University
Beytepe, 06532
Ankara
TURKEY
Phone: 90-312-299 2125
Fax: 90-312-235 4314
E-mail: bercan@eti.cc.hun.edu.tr



Biographies


Murat Songür received his BSEE degree from Hacettepe University, Ankara, Turkey in 1991. He worked at the same university from 1991 to 1997 as a research assistant responsible from the electric power systems and electric machinery laboratories. In 1995 he received his MSEE from Hacettepe University, where he developed the program POWERHU. He is currently working on his PhD studies, which he started in 1995. His topics of interests include Computer Analysis of Power Systems, Lightning Induced Voltages on Electric Transmission Lines and ZnO Arresters. Murat Songür is now employed in Biltam A.S., Turkey.
Bahri Ercan graduated from METU, Department of Electrical Engineering, in 1970. He received the MEng degree in 1974 and the PhD degree in 1979 from University of Wales, Cardiff. He worked for the Turkish Electricity Authority from 1970 to 1972 and for BOEING Services International and SISAG Ltd. from 1974 to 1976. He has been working at Hacettepe University, Department of Electrical and Electronics Engineering, since 1979. He is the Head of Electric Power Systems Section and Head of Electrical Machines Section at Hacettepe University. His fields of interest are High Voltage Techniques, Power System Analysis and Electrical Machines.