The calculated demand is then compared with the kVA per square foot for other customers of that same type as a check to ensure that the value is in the expected range. Another important feature of the software is the calculation of the hours-use number for the customer. This number is defined as the total annual kWh divided by the maximum kW demand, and can also be thought of as the number of hours of annual peak demand. The magnitude of this number is an excellent indicator as to the general shape of the load duration curve for the customer. The higher the hours-use number, the flatter the load shape — closer to a constant load. The lower the number, the more “peaked” the load shape. Knowing the general shape of the load-cycle curve is essential to knowing the loading effect on the distribution transformer. Loads with a very high demand but a very low hours-use number may be served more effectively with a smaller transformer than loads of the same kVA demand with higher hours-use loads.

Southern Company is a member of the Distribution System Testing Application and Research (DSTAR) group and was heavily involved with the project to develop a transformer total owning cost software (TOCS) program. The DSTAR program uses annual 8760-hour load data and weather temperature data to predict transformer life based on the ANSI/IEEE transformer loading formula. Using the DSTAR software as a basis, transformer design parameters from Southern's transformer suppliers and the 8760-hour load research data, we could accurately calculate the size transformer needed for each account. Z Solutions took the calculations of the same ANSI/IEEE transformer loading procedures, which DSTAR had developed in spreadsheet form, and implemented them into a statistical package. This greatly improved the efficiency of data input and the output of results.

Before, an analysis would consist of only a few load models and transformer designs. Now, an analysis of multiple load shape properties and all possible transformer designs may be performed. Literally thousands of results may be organized and reported in order to reveal differences in transformers and how they may be used. These results often lead to unexpected findings.

New sizing guidelines were developed based on the load-carrying analysis reflecting both energy and demand impacts. In general, the newly sized transformers were one to two transformer sizes smaller than what was generally specified in the past. This reflected both the new analyses and greater confidence in the improved forecasting methods.

After completion of the commercial analysis, the project team turned their attention to the residential market. This task again involved the analysis of specific loads gathered from residential cost-of-service load research data. In the residential market, one transformer usually serves multiple homes, including high-density multi-family developments, where as many as 200 units may be served. The key factor in estimating the kVA demand of these applications is not only the size of the homes and the types of appliances used, but also the coincidence of the maximum demand of the homes. Again, multiple analyses were performed to estimate the impact of all these factors on sizing requirements. The work is being finalized and reviewed for implementation at this time.

The last step in this process will be to close the loop on the entire process. This will include the consideration of new transformer designs and sizes based on the findings from the previous work and the frequency of occurrence of the kVA demands on the system.

G. Bruce Shattuck began his career with Alabama Power as a student engineer in 1969 and graduated from the University of Alabama with a BSEE degree in 1971. He progressed through various positions of increasing responsibility to his present position as principal engineer in the Power Delivery — Distribution Engineering Services group. He is past chairman of the Southeastern Electric Exchange UD Committee and the DSTAR Committee. He is a member of the Association of Edison Illuminating Companies (AEIC) Cable Engineering Committee and serves as the Southern Company lead product engineer for underground cables. GBSHATTU@southernco.com

R. Gary Huff is the president of Flamingo Solutions, LLC. He is an expert on analysis of consumer and business demand behavior and was a subcontractor with Z Solutions when the work of this article was conducted. Huff was previously manager of the load research at Southern Company, where he directed load research sample design and analysis for its four operating companies. garyhuff02@earthlink.net

Transformer size selection based on the customer's kVA demand range, hours of use and transformer design