Public Power - September/October 2007 - (Page 48)

FOR ENGINEERS Distribution Transformer Loss Evaluation By Mark Feller An Edison Electric Institute survey of investor-owned utilities conducted in 1978 found that a majority of the respondents were evaluating transformer losses when purchasing transformers at that time. As the cost of energy has increased over the years, more utilities have instituted loss evaluation when purchasing transformers to the point where it is now done nearly universally. So one may question why another article on loss evaluation is needed. In my experience working with municipal utilities in Ohio, I’ve observed that many municipal utilities do evaluate losses, but some still do not. Lack of information, lack of motivation, or lack of understanding of the procedure probably explains why some municipal utilities still do not take advantage of this useful tool for determining the best transformer designs based on economic benefit when making purchasing decisions. Power distribution people naturally gravitate to working with physical things; poles, conductors, insulators, etc. If we wanted to be financial analysts, there are plenty of opportunities in banking, insurance, and on Wall St., but we have chosen this field for a reason. I was once offered the opportunity to become an analyst of high-tech companies for an investment firm catering to high net worth investors. It didn’t take me too long to recognize that, as a born and bred engineer, I would be like a fish out of water, doing something 48 SEPTEMBER-OCTOBER 2007 like that, even if a very wealthy fish. I have reluctantly concluded that while tackling engineering economics may not be as much fun as designing a substation and is not the thing I most want to do, it is certainly something I must do, if I am to be a good engineer. And it is something the distribution superintendent, or whoever is charge of purchasing transformers, needs to do. Physical machines and plant, whether used in mining, manufacturing, transportation, or in any industry, are cash flow machines as much as they are production machines or product delivery systems. An electrical distribution system is a cash flow machine. Customer dollars flow in and the system managers direct the flow of dollars out for funding of maintenance, capital investments, purchased power or fuel, personnel, fixed and variable overheads, and losses. Some of these aspects are beyond our control but those things within our control deserve our attention with regard to both operational efficiency and optimizing return on capital investment. Engineering economics looks at the impact that alternative designs of systems or equipment have on economic performance and provides guidance for reducing technical losses in the operation of the system. There is probably more information available today on the subject of transformer loss evaluation than in any other area of distribution system engineering. An Internet search on this subject will return thousands of hits, with probably hundreds of relevant documents. So the lack of information cited above as a reason why some may not be doing loss evaluation is really a lack of relevant information. Much of the literature on the topic has been written for industrial users, i.e., for taxable entities, based on load factors found in the industrial environment, and based on retail electric power tariffs. Many more articles are targeted to large vertically integrated investor-owned utilities with different business cost structures than ours and attempt to account for all sorts of nuances, such as taxes and depreciation, early retirement, residual value, and assumed loading practices that are considerably different than those used by municipal electric utilities. All of these things mostly serve to confuse the issue when working with municipal utilities. About 10 years ago, the American Public Power Association realized the need for an engineering economics guide expressly written for municipal utilities and, through the DEED program, published the Distribution System Performance Improvement Guide. I ordered a copy as soon as I learned of its existence. I had been doing transformer loss evaluations for many years based on methods found in the trade literature and adapted as best I could to the particular characteristics of the systems I was working with. The APPA guide brought a great deal of clarity and simplification to this work. Basically, economic evaluation of losses simply recognizes that transformers will have energy losses over their operating lifetime and the losses will cost money over that time. The economic analysis uses a mathematical determination of the cost of the future losses expressed in today’s dollars to enable a rational comparison of alternative products proposed by manufacturers in response to a request for quotation. The procedure covered extensively in the literature calls for adding to the purchase price of a transformer the cost of power and energy lost in its operation over its assumed lifetime expressed as “A” and “B” constants for the core and coil loss components. It is not the intent of this article to cover what has been so extensively written about elsewhere but rather to suggest that the method for determination of the A and B constants recommended by the Distribution System Performance Improvement Guide is entirely sufficient for the municipal operating environment, while at the same time very simple. Two spreadsheets accompany this article. Transformer Loss.xls calculates the A and B constants using the method presented in the guide. TransformerLoss with esc.xls adds the options of including power cost and unit loading escalation over time with a limit on maximum permitted load. These are available for download via the online version of this article on the APPA Web site. By running some test cases, one will usually find the difference in the results from the two methods to be less of a factor in the overall result than the effect of sensitivity to changes of the assumptions made in the input data. PUBLIC POWER http://www.appanet.org/files/Excel/Transformer%20Loss.xls http://www.appanet.org/files/Excel/Transformer%20Loss.xls http://www.appanet.org/files/Excel/Transformer%20Loss%20with%20esc.xls http://www.appanet.org/files/Excel/Transformer%20Loss%20with%20esc.xls

Table of Contents Feed for the Digital Edition of Public Power - September/October 2007

Contents Washington Focus 10 Questions Public Power (Every) Week Training Tomorrow’s Work Force What’s in Your Wallet? U.S. Lags in Broadband Deployment Now What? For Engineers Safety Community Broadband For Governing Boards Human Resources Hometown Connections Customer Service DEED Index to Advertisers Advertisers by Category Parting Shot

Public Power - September/October 2007

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