Transformer Owning Costs Software

What is TOCS?

TOCS is a powerful tool for analysing and comparing the total owning cost of distribution transformers. The TOC S engine includes:

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  • Analysis of transformer bids for procurement
  • Transformer application for special application
  • Generalized loading guides
  • Optimizing the transformer rating carried in inventory

Distribution transformers represent a significant cost to electric utilities, both as a capital investment and as an operating expense. Optimizing the total owning cost of distribution transformers can help utilities reduce this investment.

The DSTAR Transformer Owning Cost Software (TOCS) is a tool for analysing 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.

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. Moderate overloads create a decrease in transformer life, and the economic penalty of shortened life can often be traded off against the cost savings of selecting a lower kVA rating.

Program Features

  • Hour-by-hour thermal modeling to determine transformer life
  • User supplied daily and seasonal load cycles and ambient temperature cycles
  • Harmonic loading effects
  • Voltage regulation and and maximum temperature limits
  • Database structure for transformer data
  • User-supplied economic data such as costs of losses, purchase, installation and disposal costs, costs of capital

Purpose of TOCS

The insulation life of a transformer is a function of the thermal loading to which the insulation is exposed. TOCS allows the user to estimate the effects of expected loading on the service lives of distribution transformers. It calculates the total owning costs that result for these estimated service lives, including the costs of losses. These results allow the user to select the set of transformers with the lowest total owning cost for a variety of loading scenarios. The total owning cost evaluation includes:

  • yearly cost of losses
  • invoice, installation, and salvage/disposal costs
  • cost of capital
  • effects of reduced transformer lifetime due to thermal aging

TOCS provides several significant features not found in other owning cost analysis programs:

  • TOCS estimates the transformer service life based on the insulation loss-of-life calculations described in ANSI C57.91-1995. The total owning cost calculated by TOCS reflects the change in estimated transformer life. This allows users to compare the economic benefits of applying distribution transformers above or below nameplate rating.
  • TOCS includes the effects of heating due to harmonic currents on insulation loss-of-life.
  • TOCS provides users with a database of transformer and loading data that can be reused. The program lets users directly compare the effects of a particular loading scenario on multiple transformer designs.

It is generally recognized that very few distribution transformer fail as a direct result of overheating; most failures are attributed to overvoltage. However, insulation degradation due to thermal ageing can in fact contribute to overvoltage susceptibility, and therefore the insulation loss of life calculated by TOCS can provide a reasonable estimate of transformer service life.

Working with the TOCS Software

The TOCS program is based on a Microsoft Access database, which allows users to build and maintain a library of data that can be used to set up analysis cases in any combination. This library contains transformer design data, daily loading cycles, harmonic spectra for load current, and yearly load growth factors.

The users defines an analysis case by choosing the desired data from the library, entering economic parameters, and selecting the transformers to which the load is applied. When the user runs the analysis, the program determines the lifetime and total owing cost for the transformer(s) under that loading scenario.

TOCS Structure

TOCS can be divided into three basic parts:

  • The TOCS program
  • The TOCS database
  • Utility files

From the user’s perspective, all three pieces are integrated into a single program. The TOCS utility files are required by the system to run TOCS in the Microsoft Windows environment and are transparent to the user, so these will not be discussed further. The database and user interface are discussed in more detail below.

Figure 1 gives an overview of the TOCS structure and of program flow between various windows (operation of these windows is described in more detail later).

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Figure 1

The TOCS Database

The TOCS database is stored in a Microsoft Access format. TOCS includes a run-time version of Access, so users are not required to have Microsoft Access to run TOCS. TOCS includes import/export functions transfer of data to and from other applications. However, users with good knowledge of Access database programming can use Access to write their own custom reports, or perform specialized import or export of TOCS data.

The TOCS database consists of a number of data tables that store all information required to set up and run a TOCS analysis case. This database is one of the strengths of TOCS. Users can add their own specific information to the database, building a data library for future analysis. The data that make up these tables are required for running a total owning cost analysis; these are described in more detail in the remainder of this section.

The TOCS Optimizer

TOCS can calculate the curves of total owning cost versus load level for a range of transformers. These loading curves can be used to choose the transformer sizes to maintain in inventory, and to optimize the load range for which each transformer range is applied. A standalone application supporting this process is the TOCS Optimizer. This application performs the following optimization functions:

Optimizing the assortment of transformer sizes a utility should stock. There are fixed and variable costs in maintaining an inventory of distribution transformers. Overall costs can often be minimized by eliminating purchase and stocking of certain ANSI standard transformer ratings. For example, one large utility found that it was economical to eliminate 750 kVA transformers from inventory, and substitute 500 kVA or 1000 kVA transformers for these applications. TOCS includes a stocked-size optimization function to perform this process automatically.

Optimizing the schedule of load ranges for which each transformer size is applied. The correct load range for which a transformer should be applied is not simply bounded by the nameplate rating of the given transformer and the nameplate rating of the next smaller size. For example, a 150 kVA transformer is not necessarily best applied to loads between 76 and 150 kVA. The optimal load range is established by the points where the transformer rating provides the least total owning costs, compared to larger and smaller units. In the example illustrated below, the 150 kVA is optimal choice for much greater peak loads: between 120 and 230 kVA.

Optimizing the selection of transformers for procurement based on a limited first-cost budget. While selecting transformers based on total owning cost should, in theory, provide the best economic choice in the long run, there are many uncertainties and also financial constraints imposed by the current utility business environment. Many utilities are balancing total owning cost with first-cost considerations. One approach is to establish a transformer procurement first-cost limit or budget. The TOCS optimizer will automatically determine the transformer to procure which provide the least total owning cost while meeting the limited first-cost budget goal.

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TOCS was created, ehanced, or modified in the following projects: P5-5, P6-4a, P6-4b, P10-4