22-09-2012, 02:20 PM
Design Guide for Rural Substations
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INTRODUCTION
PREFACE
This bulletin provides design guidance for the increasing numbers of substations necessary to meet the
increasing electrical demands in areas served by Rural Utilities Service borrowers (here and after called
cooperatives). This guide bulletin is intended for the benefit of cooperatives, their consulting and staff
engineers, and others interested in rural substation design and construction concerns and considerations.
Substations should be designed, constructed, and operated to meet customers’ needs at the lowest possible
cost commensurate with the quality of service desired. The typical system may include substations for
voltage transformation, sectionalizing, distribution, and metering a number of times between generation
and utilization.
PURPOSE AND SCOPE
This bulletin covers rural transmission and distribution with air-insulated, outdoor substations 345 kV
(phase-to-phase) and below.
Possible design responsibilities of the engineer are covered, including preparation of construction
drawings, material, equipment and labor specifications, and any other engineering design services that
may be required.
The engineering function is generally more than furnishing of design and specifications. Recognition of
this function becomes especially important when a cooperative employs an engineering firm to
supplement its staff. (See U.S. Code of Federal Regulations, Title 7, Part 1724 (7 CFR 1724), “Electric
Engineering, Architectural Services and Design Policies and Procedures.”) The contract between a
cooperative and an engineering firm should be clear in its definition of the engineering functions to be
performed. Within this bulletin, it should be understood that the term “engineer” could mean either a
cooperative’s staff engineer(s) or a consultant’s engineer(s).
The engineer needs to use these guidelines together with experience and knowledge. A list of references
at the end of most chapters will aid in the search for more detailed information. It is recommended that,
where other resources are referenced, such as those of ANSI, IEEE, RUS, and ASTM, the substation
designer obtain and become familiar with the appropriate document.
IMPORTANCE OF ADEQUATE SUBSTATION PLANNING AND ENGINEERING
(See Bulletins 1724D-101A, “Electric System Long-Range Planning Guide,” and 1724D-101B, “System
Planning Guide, Construction Work Plans.”)
Substation planning considers the location, size, voltage, sources, loads, and ultimate function of a
substation. If adequate planning is not followed, a substation may require unnecessary and costly
modification.
The engineer’s detailed work requires use of valid requirements and criteria, appropriate guidelines, and
engineer’s own expertise in order to provide construction drawings and associated documents appropriate
for needed system improvements. The engineer’s ability to meld the diverse constraints into an
acceptable design is essential.
During the design phase, the engineer should avoid personal preferences in solving technical problems
that diverge from the use of nationally accepted standards, Rural Utilities Service (RUS) standards, or the
concept of the cooperative’s standard designs.
Adequate engineering design provides direction for construction, procurement of material and equipment,
and future maintenance requirements while taking into account environmental, safety, and reliability
considerations.
Distribution Substations
A distribution substation is a combination of
switching, controlling, and voltage step-down
equipment arranged to reduce subtransmission voltage
to primary distribution voltage for residential, farm,
commercial, and industrial loads.
Rural distribution substation capacities vary.
Substations generally include one l.5 MVA to three
5 MVA transformers. These substations may be
supplied radially, tapped from a subtransmission line,
or may have two sources of supply. Most
cooperatives’ substations have 12,470Y/7200-volt or
24,490Y/14,400-volt distribution circuits.
A special class of distribution substation would
include a dedicated customer substation. This
substation would be similar to a distribution substation
except that all of its capacity would be reserved for the
service of one customer. The secondary voltages of a
dedicated substation would also be modified to match
special requirements of the customer. Coordination
with the customer is of primary importance in
determining the technical requirements.
Transmission Substations
A transmission substation is a combination of switching, controlling, and voltage step-down equipment
arranged to reduce transmission voltage to subtransmission voltage for distribution of electrical energy to
distribution substations. Transmission substations frequently have two or more large transformers.
Transmission substations function as bulk power distribution centers, and their importance in the system
often justifies bus and switching arrangements that are much more elaborate than distribution substations.
Switching Substations
A switching substation is a combination of switching and controlling equipment arranged to provide
circuit protection and system switching flexibility.
Switching stations are becoming common on cooperatives’ transmission systems. Flexible switching
arrangements in a transmission network can aid in maintaining reliable service under certain abnormal or
maintenance conditions.