Ashfaq Ahmed
Purdue University Calumet
Hammond, Indiana
John M. Bee
Commonwealth Edison Company
Chicago, Illinois
The software package is used as a supplement to an undergraduate course in electrical power system. This has increased students' ability to understand the way a power system responds to various loads and generation schedules. Examples of simulations are included.
Students taking a first course in power systems often find it difficult to grasp a concept due to lack of hands on experimentation. There are many topics that are usually taught in a strictly theoretical way. Most of the times, students have difficulty envisioning how changes in the source, the load and the transmission line affect the total systems performance. The basic definitions are understood but the total picture of how each component of a system interacts with the other is not obvious. There is a lack of power system software package [1] that allows the student to easily understand these basic ideas.
EPSS (Electric Power System Simulation) is an easy-to-use software developed with the purpose of helping students in their understanding of some basic ideas in power system using IBM or compatible personal computers. The use of such a tool has the potential to improve the students understanding of the subject. In the rest of this paper, we will describe the software and go through a typical session. We also present some suggestions on future enhancement to this first version.
EPSS is used as an exercising tool intended for students taking a first course in power systems. This easy-to-use program gives the user a means to simulate and analyze the power system under various configuration and system conditions. Through an interactive graphical environment written in Microsoft Visual Basic [2], the software helps the students to study power system more than any amount of theory could show. By experimenting with various parameters the student can immediately see the effect on the system.
The Electric Power System Simulator models the small power system shown in Figure 1. The system consists of two 500 MW generating stations, two transmission substations (TSS's), two essential service substations (ESS's), which represent large industrial customers, and six 138 kV transmission lines. A user can simulate various events that occur on a power system and analyze the effects on the entire system by monitoring bus voltages, active power, reactive power, and apparent power that will result at each transmission line terminal.
Upon execution of the program, the main screen displays a one line diagram of the power system. We can put generating units on line via the mouse by positioning the cursor on the unit number and clicking. A Generating Unit Window as shown in Figure 2 will appear for the selected unit. The GEN KV OUTPUT textbox represents the voltage on the low side of the main power transformer at the generating station. The BUS KV textbox represents the voltage at the high side of the main power transformer connected to the 138KV bus. The generator voltage can be increased or decreased by clicking on the up or down arrows. This, in turn varies the bus voltage. Clicking on the RUN control button performs a Load Flow Analysis [3]. The values for the bus voltages, MW, MVARS, and MVA will appear at the appropriate locations on the one line display.
We could take a transmission line out of service by positioning the cursor on the control button of the desired line and clicking the mouse button. Figure 3 shows the Transmission Line Window that will appear for the selected line. The values of impedance, reactance, resistance, line length, and the control buttons ON, OFF, and RUN appear in the window. To take a line out of service, click the OFF button. The line will change from black to green indicating that it is out of service. Click the RUN button and the values for the entire system are calculated and updated. To return the line to service, click on its control button. We can then click on the ON button in the transmission line window. The line will change from green to black indicating that it is back in service. Click the RUN button. Again the load flow analysis for the power system is calculated and displayed on the one-line display.
To simulate the loss of a generating unit, click on the control button for the transmission line connected to the generator you want to trip. Click on the OFF button and then the RUN button. The generator is now off the system.
Microsoft Visual Basic was chosen to write the code because it is geared toward creating graphical applications that make the simulation easy for the user to perform and analyze. The idea behind Visual Basic is that you create windows, called Forms, on these forms you place objects called controls. You can then write code that will respond to the users actions. Figure 4 shows the flow chart of the software.
EPSS was first introduced at Purdue University Calumet in the Spring of 1995 in EET 331 Generation and Transmission of Electric Power. The software provides a means for illustrating behavior of power system without the need of programming, a feature that the students generally appreciate.
The learning approach is applied in three steps [4]. The first step begins with a problem assignment provided by the instructor. In the lecture the theory for the analysis is explained. Then a simulation is performed using EPSS. This is followed by a discussion about the behavior of the system derived in the first step and then studying the data obtained from the simulation to interrelate and explain how the power system behaves.
A preliminary evaluation of EPS was done by obtaining opinion from the fourteen students who used the software. The results are summarized in Table 1.
Based on class participation, question raised in the lecture and evaluation by the students, it appears that their interest in the course has increased. The general feeling of the students was that the simulation provides them with a welcome change of pace during lectures and showed many concepts of power system behavior in very specific manner.
Although the preliminary results seem promising. A further extensive evaluation should not only check students' approval, but could also try to find if the software is really helping the student understanding of the basic ideas. It could also generate suggestion for additional refinement.
In this paper an easy-to-use simulation software designed to be used as an educational aid is presented. Power system simulation using the software has proven to be an effective tool for learning the behavior of power system under various operating conditions and configuration. Its scope is limited, but this introduction gives the students a better understanding and motivates them to learn more about power system.
Future improvements could consist of modeling capacitor banks that the user could put on and take off the system. Allowing the load to be adjustable at the ESSs, and incorporating tie lines with other power systems are other possibilities. Modeling a transformer with tap changers that would allow the user to adjust the voltage on the low side would be an interesting addition.
