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ELECTRIC POWER DISTRIBUTION EQUIMPENT AND SYSTEMS
1
Fundament
als of Distribution Systems
Electrification in the early 20th century dramatically improved productivity
and increased the well-being of the industrialized world. No longer a luxury
— now a necessity — electricity powers the machinery, the computers, the
health-care systems, and the entertainment of modern society. Given its
benefits, electricity is inexpensive, and its price continues to slowly decline
(after adjusting for inflation — see
Figure 1.1
)
.
Electric power distribution is the portion of the power delivery infrastruc-
ture that takes the electricity from the highly meshed, high-voltage trans-
mission circuits and delivers it to customers. Primary distribution lines are
“medium-voltage” circuits, normally thought of as 600 V to 35 kV. At a
distribution substation, a substation transformer takes the incoming trans-
mission-level voltage (35 to 230 kV) and steps it down to several distribution
primary circuits, which fan out from the substation. Close to each end user,
a distribution transformer takes the primary-distribution voltage and steps
it down to a low-voltage secondary circuit (commonly 120/240 V; other
utilization voltages are used as well). From the distribution transformer, the
secondary distribution circuits connect to the end user where the connection
is made at the service entrance.
Figure 1.2
shows an overview of the power
generation and delivery infrastructure and where distribution fits in. Func-
tionally, distribution circuits are those that feed customers (this is how the
term is used in this book, regardless of voltage or configuration). Some also
think of distribution as anything that is radial or anything that is below 35 kV.
The distribution infrastructure is extensive; after all, electricity has to be
delivered to customers concentrated in cities, customers in the suburbs, and
customers in very remote regions; few places in the industrialized world do
not have electricity from a distribution system readily available. Distribution
circuits are found along most secondary roads and streets. Urban construc-
tion is mainly underground; rural construction is mainly overhead. Subur-
ban structures are a mix, with a good deal of new construction going
underground.
A mainly urban utility may have less than 50 ft of distribution circuit for each
customer. A rural utility can have over 300 ft of primary circuit per customer.
Several entities may own distribution systems: municipal governments,
state agencies, federal agencies, rural cooperatives, or investor-owned utili-
1
Copyright © 2006 Taylor & Francis Group, LLC
2
Electric Power Distribution Equipment and Systems
40
20
0
1920 1940 1960 1980 2000
FIGURE 1.1
Cost of U.S. electricity adjusted for inflation to year 2000 U.S. dollars. (Data from U.S. city
average electricity costs from the U.S. Bureau of Labor Statistics.)
ties. In addition, large industrial facilities often need their own distribution
systems. While there are some differences in approaches by each of these
types of entities, the engineering issues are similar for all.
For all of the action regarding deregulation, the distribution infrastructure
remains a natural monopoly. As with water delivery or sewers or other
utilities, it is difficult to imagine duplicating systems to provide true com-
petition, so it will likely remain highly regulated.
Because of the extensive infrastructure, distribution systems are capital-
intensive businesses. An Electric Power Research Institute (EPRI) survey
found that the distribution plant asset carrying cost averages 49.5% of the
total distribution resource (EPRI TR-109178, 1998). The next largest compo-
nent is labor at 21.8%, followed by materials at 12.9%. Utility annual distri-
bution budgets average about 10% of the capital investment in the
distribution system. On a kilowatt-hour basis, utility distribution budgets
average 0.89 cents per kilowatt-hour (see
Table 1.1
for budgets shown relative
to other benchmarks).
Low cost, simplification, and standardization are all important design
characteristics of distribution systems. Few components and/or installations
are individually engineered on a distribution circuit. Standardized equip-
ment and standardized designs are used wherever possible. “Cookbook”
engineering methods are used for much of distribution planning, design,
and operations.
Distribution planning is the study of future power delivery needs. Plan-
ning goals are to provide service at low cost and high reliability. Planning
requires a mix of geographic, engineering, and economic analysis skills.
New circuits (or other solutions) must be integrated into the existing distri-
bution system within a variety of economic, political, environmental, elec-
trical, and geographic constraints. The planner needs estimates of load
Copyright © 2006 Taylor & Francis Group, LLC
Fundamentals of Distribution Systems
3
Large Generation
Stations
G
G
G
Bulk Transmission
230-750 kV
Subtransmission
69-169 kV
Primary Distribution
4-35 kV
Secondary Distribution
120/240 V
FIGURE 1.2
Overview of the electricity infrastructure.
TABLE 1.1
Surveyed Annual Utility Distribution Budgets in
U.S. Dollars
Average
Range
Per dollar of distribution asset
0.098
0.0916–0.15
Per customer
195
147–237
Per thousand kWH
8.9
3.9–14.1
Per mile of circuit
9,400
4,800–15,200
Per substation
880,000
620,000–1,250,000
Source:
EPRI TR-109178,
Distribution Cost Structure — Methodol-
ogy and Generic Data
, Electric Power Research Institute, Palo Alto,
CA, 1998.
Copyright © 2006 Taylor & Francis Group, LLC
4
Electric Power Distribution Equipment and Systems
growth, knowledge of when and where development is occurring, and local
development regulations and procedures. While this book has some material
that should help distribution planners, many of the tasks of a planner, like
load forecasting, are not discussed. For more information on distribution
planning, see Willis’s
Power Distribution Planning Reference Book
(1997),
IEEE’s
Power Distribution Planning
tutorial (1992), and the
CEA Distribution
Planner’s Manual
(1982).
1.1 Primary Distribution Configurations
Distribution circuits come in many different configurations and circuit
lengths. Most share many common characteristics.
Figure 1.3
shows a “typ-
ical” distribution circuit, and
Table 1.2
shows typical parameters of a distri-
bution circuit. A
feeder
. The mainline is normally a modestly large conductor such
as a 500- or 750-kcmil aluminum conductor. Utilities often design the main
feeder for 400 A and often allow an emergency rating of 600 A. Branching
from the mains are one or more
or
mainline
, which are also called taps, lateral
taps, branches, or branch lines. These laterals may be single-phase, two-
phase, or three-phase. The laterals normally have fuses to separate them
from the mainline if they are faulted.
The most common distribution primaries are four-wire, multigrounded
systems: three-phase conductors plus a multigrounded neutral. Single-phase
loads are served by transformers connected between one phase and the
neutral. The neutral acts as a return conductor and as an equipment safety
ground (it is grounded periodically and at all equipment). A single-phase
line has one phase conductor and the neutral, and a two-phase line has two
phases and the neutral. Some distribution primaries are three-wire systems
(with no neutral). On these, single-phase loads are connected phase to phase,
and single-phase lines have two of the three phases.
There are several configurations of distribution systems. Most distribution
circuits are radial (both primary and secondary). Radial circuits have many
advantages over networked circuits including
laterals
• Easier fault current protection
• Lower fault currents over most of the circuit
• Easier voltage control
• Easier prediction and control of power flows
• Lower cost
Distribution primary systems come in a variety of shapes and sizes (
Figure
1.4
)
. Arrangements depend on street layouts, the shape of the area covered
Copyright © 2006 Taylor & Francis Group, LLC
is one of the circuits out of the substation. The main
feeder is the three-phase backbone of the circuit, which is often called the
mains
Fundamentals of Distribution Systems
5
138 kV
Normally open
b
u
s tie
21/28/35 MVA
Z=9%
Load Tap Changing (LTC)
transformer
12.47 kV
Circuit breaker
or recloser
400-A peak
600-A emergency
feeder rating
Single-phase lateral
3-phase, 4-wire
multigrounded
circuit
Three-phase lateral
65 K
fuse
100 K
fuse
R
Recloser
Three-phase
mains
Normally
open tie
FIGURE 1.3
Typical distribution substation with one of several feeders shown (many lateral taps are left
off). (Copyright © 2000. Electric Power Research Institute. 1000419.
Reprinted with
permission.)
Copyright © 2006 Taylor & Francis Group, LLC
Engineering Guide for Inte-
gration of Distributed Generation and Storage Into Power Distribution Systems.
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