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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.

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)

Fig.l. Typical display of the program menu (a) and a sub-menu (b). 

Fig.2. Block diagram of the program POWERHU 

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.

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(b)

Fig.3. Converge plot of a sample system obtained from a load-flow study using the (a) Gauss-Seidel algorithm and (b) Fast-Decoupled algorithm.

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.

Fig.4. Graphs of rotor angle and shaft speed vs. time obtained for one generator during transient stability analysis with Modified-Euler method.

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.

In MATLAB command window, the program is invoked by typing

>> powerhu

Fig.5. Main program menu

Fig.6. Sub menu for data file selection. 

Fig.7. List of load-flow data files on a computer. 

Fig.8. System data for a sample system stored on the file hutest.lfi

Fig.9. Sub menu to select load-flow algorithm.

(a)

(b)

Fig.10. Converge plot of a system obtained from a load-flow study using Gauss-Seidel algorithm (a) and load-flow results (b).

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.

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

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.