|
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
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)
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.
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 (TOCS) 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
The TOCS program 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.
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.
|
