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Transformer Owning Costs Software (TOCS)
What is TOCS?
TOCS applications include:
•
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 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.
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.
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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
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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 aging 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.
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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.
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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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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.
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Figure 1
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 illistrated 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.
Example Load Range
Optimization
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