Public Power - May 2008 - (Page 34)

R I G H T- S I Z I N G B Y B E N TA NS E Y When it comes to matchingload, it’s distribution transformer size with sometimes more art than science. Installing a transformer that is too small may cause outages and voltage drops, while going too large risks unnecessary load loss and capital expenditure. Finding a solution means figuring out logistics and crunching numbers. Enter the Fayetteville Public Works— a municipal utility in North Carolina that serves 80,000 customers with a peak approaching 480 MW. Last year, after a near complete turnover of staff engineers, the utility’s electrical engineering department—with Rick Anderson, PWC’s manager of electrical engineering, and Tommy Cooper, PWC’s standards engineer leading the project— decided to take a fresh look at its transmission and distribution system. The central focus of the examination was on the utility’s three-phase, padmounted distribution transformers. The department knew, like most municipal utilities concerned about reliability, it had a historic tendency to oversize transformers serving commercial and industrial loads. The department set out to identify oversized distribution transformers by treating them as stock inventory. When an oversized transformer could be used elsewhere, it would be moved and replaced with a smaller, more appropriately sized unit. For instance, PWC has a 1,500-kVA transformer at a site where the maximum load is 573 kVA. When the time comes, it will be replaced with a 500-kVA unit. In addition, a group of four 500-kVA transformers is serving a site with a maximum load of 1,026-kVA. They will be replaced with four 300 kVA units. Experts say electrical contractors hired by developers have multiple incentives to push oversized transformers. 34 MAY 2008 Rick Anderson, (left) manager of electrical engineering, and Tommy Cooper, (right) standards engineer, for Fayettesville Public Works in North Carolina, led a project that examined oversized transformers. The project replaced oversized transformers that could be used elsewhere with smaller, more appropriately sized units. Photo by Nicole Inman. “It’s cheaper to put in something too big than to undersize it and be sued for a system that overloads,” said Kurt Conger, a utility consultant in the Seattle area. Contractors also recommend “fat panels” because under the National Electric Code, they assume in any building, every electrical device will be on, every outlet plugged in and every circuit at maximum amperage. “They have to wire to meet connected load, so that’s the figure they give,” said Cooper, who has been in the business more than 40 years, including 11 at Fayetteville. Transformers can tolerate periods of higher-than-nameplate loads, so connected load figures requested by developers are often disregarded. “Through experience, we know to multiply connected load by 0.4 to get demand,” said Cooper. About a quarter of all distribution losses take place in the transformers. Over its life-cycle, the cost of energy losses can amount to several times the transformer’s purchase price, or “first cost.” Losses come in two forms—load winding or losses produced in the coils and no-load losses associated with the core. The former depends on how heavily the transformer is loaded, while the latter remain constant irrespective of load. While many utilities can’t control how efficient their generation is, because of a contract they are under or because they can’t force customers to install compact fluorescent bulbs, transformer sizing is something utilities can control, like transmission line losses and delivered voltage. In that regard, PWC does a good job PUBLIC POWER

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

Public Power - May 2008 Contents Perspective LEEDing the Way Ontario Moves to Mandatory Time-of-Use Pricing Journey to the Smart Grid Right-Sizing Transformers Energy Audits for Large Industries Economic Development Community Broadband Reliability Hometown Connections Parting Shot

Public Power - May 2008

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