Public Power - November 2008 - (Page 46)

FOR ENGINEERS Analyzing Transients By Mark Feller time to time the power system engineer is faced with solving problems involving lightning or switching surges, harmonics, or voltage disturbances due to other causes. These problems involve looking at the behavior of the system over short time intervals to determine the response of the circuit to the stimulus. Rigorous analysis of the transient circuit can be a challenging undertaking so, often, simplified hand calculations are used for the more basic problems. Excellent computer software tools are available for more advanced work and the cost of admission is only one’s willingness to dig in to the details. Today’s personal computers and modern circuit analysis programs, are interactive and have powerful graphical input and output capabilities. Excellent commercial software is available for both the electronics and power fields and the open source community has also developed excellent programs. Before tackling a transient problem, spend some time with a good text on the subject. Electrical Transients in Power Systems by Allan Greenwood, published by Wiley-Interscience is a classic in the field. I have the rather dated first edition, which has about the same number of pages covering the analog computer and the Transient Network Analyzer, or TNA, as it spends on the digital computer as an analysis tool. But the real value in this text is that the author presents an effective combination of both the mathematics and practical problem46 NOVEMBER-DECEMBER 2008 From solving advice. This book will help the reader understand how to assess the dominant features of a given circuit from a transients perspective and how to simplify the circuit by eliminating elements that have little influence on the results. Greenwood also provides typical values for the parasitic capacitances and inductances of power system apparatus needed for transient studies. For power system transient problems, the leading program is the Alternative Transients Program, or ATP. It has the same roots as the commercial Electromagnetic Transients Program or EMTP and was originally developed by the Bonneville Power Administration in the early 1980s and ported to PC operating systems. Since ATP was designed specifically for transient analysis of power systems, the solution algorithms properly handle distributed circuit elements such as transmission lines and cables. The component library includes transformers, non-linear devices, rotating machines, switches, various sources and TACS devices. Although there is no cost for ATP program itself, a user license is required, and you must not be involved in “EMTP commerce” to obtain a license. Once licensed, the program and supporting materials can be downloaded from password-protected servers. There are a number of related Web sites, but the best place to start is http://www.emtp.org/. ATP has a graphical front-end processor called ATPDraw. It is used to build a circuit graphically by selecting components from a library or by building custom models, if needed. The ATP program can be invoked directly from ATPDraw. Various post processing tools are available to format the output graphs and data. ATP requires more knowledge about the workings of your computer than is typical for most software today. The heritage of the program is evident in much of the documentation, with its references to cards, decks and so on. There is definitely a “learning curve’ in front of the new user, but the wealth of solid power system analysis information available from the ATP community makes the effort more than worthwhile. There is a user group available to licensed users if there are questions after reviewing all of the available supporting documentation and literature. Occasionally for quick analysis of simple problems, such as capacitor switching, I use the LTspice SwitcherCAD III program from Linear Technology, www.linear.com. This excellent program is based on the SPICE public domain circuit simulator. It is extremely easy to use. This program is electronics-oriented, but targeted toward power electronics and power system input and output quantities are readily accommodated. Linear Technology provides this program at no cost to support the application of its power control products but also permits its use for general circuit analysis. In general, use of this program for transmission lines, cables, or rotating machines requires building involved models of pi equivalents for lines and d-q reference frame equivalent circuits for machines. It seems easier to work with ATP for these problems since this work has already been done for the user. A user group for this program is available at http://tech.groups. yahoo.com/group/LTspice/. A word of caution is in order. Significant output errors can result from a seemingly small mistake in the details of the circuit model. Problems with non-linear circuit elements can be especially troubling. Such work should be undertaken only after considerable study of the supporting documentation, including the technical papers and newsletters published by the ATP user groups. The user needs to be cognizant of the limitations in both the modeling of circuit elements and in the numerical routines used in the software. Some problems are best handled by experts with significant background in the field, especially if significant dollars are involved in the project. There are a number of consulting firms with considerable expertise in the analysis of transients in power systems. Using outside capabilities often results in both cost savings and superior system design, but such outside help is most effectively used by persons with knowledge of the task at hand, so I recommend taking advantage of the resources offered by these programs and their user communities. ❚ Mark Feller (mfeller@ cityofwadsworth.com) is communications and substation superintendent for the city of Wadsworth, Ohio. He welcomes topic suggestions for this column. PUBLIC POWER http://tech.groups.yahoo.com/group/LTspice/ http://tech.groups.yahoo.com/group/LTspice/ http://www.linear.com http://www.emtp.org/

Table of Contents Feed for the Digital Edition of Public Power - November 2008

Public Power - November 2008 Contents Perspective 10 Questions Capturing Knowledge Before It Retires Jackson’s GIS Search Keeping a Job Journal Japan Tackles the Kyoto Protocol Getting to 20 by 10 Damless Hydro Power Earthquake: The Hidden Disaster For Engineers Safety For Governing Boards DEED Hometown Connections Parting Shot

Public Power - November 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.