Transformer Loading And Economical Evaluation

Distribution transformers constitute a large portion of a typical utility's power delivery investment, but the transformer owning costs can be minimized if the correct transformer is selected for each application. Application of oversized transformers, which experience has shown to be most typical with conventional application practices, results in wasted investment and excessive core losses. Selection of a transformer with a too-small rating leads to shortened transformer life (due to overheating) and excessive winding (Ioad) losses.

Topics related to distribution transformer evaluation and application have been a focus area for DSTAR projects, including estimation of loading, proper derating for harmonic-rich loads, and software supporting optimized transformer selection and application.

DSTAR Projects Addressing Transformer Loading and Evaluation

Load Demand Algorithm

While estimation of a new load's demand must be based on factors such as square footage, appliances, and HVAC system characteristics, the peak load of an existing load can be estimated based on the metered kWh recordings. Based on statistical reduction of a large amount of load data where hourly demand was recorded, algorithms were developed by which a utility can monitor transformer loading. Overloaded or underloaded transformers can be efficiently scheduled for replacement for a right-sized unit. The results of this project also include coincidence factors based on today's residential usage patterns


Load Demand Estimation Curves (from DSTAR Research)

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Distribution Transformer Harmonic Derating

Distribution transformers are increasingly required to supply harmonic-rich load currents. This is particularly true when supplying commercial and small industrial loads having a large portion of the load in the form of solid-state devices. This includes computer loads and adjustable speed drives. Harmonic currents cause significantly greater transformer heating than the same amount of 60 Hz current. Thus, it is necessary to derate the transformer capacity when supplying these "dirty" loads.

ANSI C57.110 provides guidelines for transformer derating due to harmonic loading. These guidelines require data generally unavailable for distribution transformers. Also, the ANSI standard tends to be quite conservative. Based on measurements made on a wide range of distribution transformers, a DSTAR project has developed comprehensive, specific guidelines for distribution transformer derating for harmonic loads.

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Transformer Owning Cost Software

Distribution transformers represent a significant cost to electric utilities, both as a capital investment and as an operating expense. Competitive pressures have encouraged utilities to refocus their efforts on transformer loading practices and total owning cost evaluation. Optimizing the total owning cost of distribution transformers can help utilities reduce this investment.

In addition to selecting between different designs of a given kVA rating, the distribution engineer also has considerable latitude in selecting the kVA rating to be used for a given load. Distribution transformers have substantial tolerance to short-term or infrequent overloads. It is often economically advantageous to intentionally overload distribution transformers. Although overloads increase the transformer temperature, shortening insulation life, the economic penalty of shortened life is often outweighed by the cost savings of selecting a smaller kVA rating.

The DSTAR Transformer Owning Cost Software is an important DSTAR software tool for analyzing and comparing the total owning cost of distribution transformers. TOCS provides utility users with a convenient way to determine the most economical distribution transformers for their system applications. Using load curves and ambient temperature patterns, TOCS performs hour-by-hour transformer thermal modeling to evaluate insulation aging. Transformer owning costs, adjusted for the expected insulation life, and including loss costs according to user-supplied economic factors, are calculated.

In addition to selecting transformer bids for procurement, and selecting transformers for a specific application, TOCS supports advanced optimization processes such as:

  • Optimizing the assortment of transformer sizes a utility should stock.
  • Optimizing the schedule of load ranges for which each transformer size is applied. (See figure below)
  • Optimizing the selection of transformers for procurement based on a limited first-cost budget.

TOCS is a very powerful tool for aiding the procurement and application of distribution transformers. Click here for a more detailed description of this program.


Example Load Range Optimization

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Transformer Loading Analyzer

The TOCS program (described above) is a powerful tool for detailed economic analysis and comparison, having a number of advanced features. Field designers and engineers, however, often need a simpler tool for calculating the effects of transformer loading on transformer aging. A very easy-to-use Transformer Loading Analyzer (TLA) program has been developed as part of the DSTAR Distribution Engineering Toolbox to address this need. The user simply provides the load cycles and selects the ambient climate for the transformer installation location and the transformers to be analyzed. The TLA quickly calculates expected transformer insulation life, which can be compared with target insulation lifes.

As with all of the DSTAR Distribution Engineering Toolbox tools, user convenience is maximized by providing company customization of parameters and preferences. For example, ambient temperature cycles for different regions of a utility's service territory can be set up by the utility's program administrator for the Toolbox. The end user need only select one of these "climates", greatly simplifying program usage. Click here for a more detailed description of the engineering toolbox, which includes the program.

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Distribution Transformer Scrap/Repair Decision Software

Utilities often take down distribution transformers for a variety of reasons, including replacement with a different size, feeder relocation or voltage upgrade, and transformer failure. In many cases, these transformers can be rehabilitated and returned to future use. The SCRAP program performs economic analysis of existing, in-service transformers, to determine the maximum expense for rehabilitation which is economically justified. Relative costs of losses, remaining life, replacement costs, and installation costs are included in the analysis. The program calculates listings of the maximum justifiable rehabilitation expense, by transformer, for use by a utility's shop to determine which returned units to repair, and which to scrap. Click here for a more detailed description of this program.

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Transformer DOE Standards Evaluation

The U.S. Department of Energy (DoE) has proposed mandatory efficiency standards for a wide range of consumer and industrial “appliances”. The proposed standards will require a very substantial increase in efficiency relative to the average distribution transformer produced today.

The DoE is acting under the authority of the Energy Policy and Conservation Act, in which Congress mandates DoE to establish minimum efficiency standards for distribution transformer efficiency. It should be noted that the law requires DoE to establish standards which are “…technologically feasible and economically justified and would result in significant energy savings…”. To support development of efficiency standards compliant with this mandate, DoE commissioned a very elaborate and complex analysis.

This white paper reviews DoE’s analysis process, and the resulting efficiency standards. The standards are compared to conventional utility total owning cost evaluation (TOC) methodology. Click here for a more detailed description of this project.

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