[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

mounted transformers can have arresters too, especially those at the end of a radial line, and they can
be inside the tank, plugged into dead-front bushings, or at a nearby riser pole, where primary lines
transition from overhead to underground.
FIGURE 2.2.39 Cutaway showing CSP components. (By permission of ABB Inc., Jefferson City, MO.)
set the breaker on its emergency position, which allows the transformer to temporarily supply a higher
overload until the utility replaces the unit with one having a higher kVA capacity. The secondary oil
breaker is also handy to disconnect load from a transformer without touching the primary connections.
2.2.13.6 CSP ®2 Transformers
Overhead transformers that are built with the combination of secondary breaker, primary protective link,
and external lightning arrester are referred to generically as CSPs (completely self-protected transformers).
This protection package is expected to prevent failures caused by excessive loads and external voltage
surges, and to protect the system from internal faults. The breaker is furnished with a signal light and
an emergency control as described above. The protective link is often mounted inside the high-voltage
bushing insulator, as seen in Figure 2.2.39.
2.2.13.7 Protection Philosophy
CSP transformers are still in use, especially in rural areas, but the trend is away from secondary breakers
to prevent transformer burnouts. Continued growth of residential load is no longer a foregone conclusion.
Furthermore, utilities are becoming more sophisticated in their initial transformer sizing and are using
computerized billing data to detect a transformer that is being overloaded. Experience shows that modern
distribution transformers can sustain more temporary overload than a breaker would allow. Most utilities
would rather have service to their customers maintained than to trip a breaker unnecessarily.
2.2.13.8 Lightning Arresters
Overhead transformers can be supplied with primary lightning arresters mounted nearby on the pole
structure, on the transformer itself, directly adjacent to the primary bushing, or within the tank. Pad-
2
CSP ® is a registered trademark of ABB Inc., Raleigh, NC.
2.2.14 Economic Application
2.2.14.1 Historical Perspective
Serious consideration of the economics of transformer ownership did not begin until the oil embargo
of the early 1970s. With large increases in the cost of all fuels, utilities could no longer just pass along
these increases to their customers without demonstrating fiscal responsibility by controlling losses on
their distribution systems.
2.2.14.2 Evaluation Methodology
An understanding soon developed that the total cost of owning a transformer consisted of two major
parts, the purchase price and the cost of supplying thermal losses of the transformer over an assumed
life, which might be 20 to 30 years. To be consistent, the future costs of losses have to be brought back
to the present so that the two costs are both on a present-worth basis. The calculation methodologies
were published first by Edison Electric Institute and recently updated in the form of a proposed ANSI
standard (IEEE, 2001).The essential part of the evaluation method is the derivation of A and B factors,
which are the utility’s present-worth costs for supplying no-load and load losses, respectively, in the
transformer as measured in $/W.
2.2.14.3 Evaluation Formula
The proposed ANSI guide for loss evaluation expresses the present value of the total owning cost of
purchasing and operating a transformer as follows (in its simplest form):
TOC Transformer Cost A No Load Loss B Load Loss
(2.2.8)
where
A = loss-evaluation factor for no-load loss, $/W
B = loss-evaluation factor for load loss, $/W
The guide develops in detail the calculation of A and B factors from utility operating parameters as
shown in Equation 2.2.9 and Equation 2.2.10, respectively:
SC EC HPY
A
FCR 1000
SC RFEC LSF HPY PL
B
FCR 1000
where SC = GC + TD
SC = avoided cost of system capacity
GC = avoided cost of generation capacity
TD = avoided cost of transmission and distribution capacity
EC = avoided cost of energy
HPY = hours per year
FCR = levelized fixed-charge rate
RF = peak responsibility factor
LSF = transformer loss factor
PL = equivalent annual peak load
2
(2.2.9)
(2.2.10)
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC