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ELECTRIC POWER DISTRIBUTION EQUIMPENT AND SYSTEMS
3
Undergroun
d Distribution
Much new distribution is underground. Underground distribution is much
more hidden from view than overhead circuits, and is more reliable. Cables,
connectors, and installation equipment have advanced considerably in the
last quarter of the 20th century, making underground distribution installa-
tions faster and less expensive.
3.1 Applications
One of the main applications of underground circuits is for underground
residential distribution (URD), underground branches or loops supplying
residential neighborhoods. Utilities also use underground construction for
substation exits and drops to padmounted transformers serving industrial
or commercial customers. Other uses are crossings: river crossings, highway
crossings, or transmission line crossings. All-underground construction —
widely used for decades in cities — now appears in more places.
Underground construction is expensive, and costs vary widely.
Table 3.1
shows extracts from one survey of costs done by the CEA; the two utilities
highlighted differ by a factor of ten. The main factors that influence under-
ground costs are:
•
Roads, driveways, sidewalks, and water pipes
— these and other obstacles slow construction and increase costs.
•
Soil condition —
Rocks and frozen ground increase overtime pay for
cable crews.
•
Urban construction is more difficult not
only because of concrete, but also because of traffic. Rural construc-
tion is generally the least expensive per length, but lengths are long.
•
Concrete-encased ducts cost more than direct-buried con-
duits, which cost more than preassembled flexible conduit, which
cost more than directly buried cable with no conduits.
91
Copyright © 2006 Taylor & Francis Group, LLC
Degree of development —
Urban, suburban, or rural —
Conduit —
92
Electric Power Distribution Equipment and Systems
TABLE 3.1
Comparison of Costs of Different Underground Constructions at
Different Utilities
Utility
Construction
$/ft
a
TAU
Rural or urban, 1 phase, #2 Al, 25 kV, trenched, direct buried
6.7
Rural, 3 phase, #2 Al, 25 kV, trenched, direct buried
13.4
Urban commercial, 3 phase, #2 Al, 25 kV, trenched, direct buried
13.4
Urban express, 3 phase, 500-kcmil Al, 25 kV, trenched, direct buried
23.5
WH
Urban, 1 phase, 1/0 Al, 12.5 kV, trenched, conduit
84.1
Urban commercial, 3 phase, 1/0 Al, 12.5 kV, trenched, conduit
117.7
Urban express, 3 phase, 500-kcmil Cu, 12.5 kV, trenched, conduit
277.4
a
Converted assuming that one 1991 Canadian dollar equals 1.1 U.S. dollars in 2000.
Source:
CEA 274 D 723,
Underground Versus Overhead Distribution Systems
, Canadian
Electrical Association, 1992.
•
The actual cable cost is a relatively small
part of many underground applications. A 1/0 aluminum full-neu-
tral 220-mil TR-XLPE cable costs just under $2 per ft; with a 500-
kcmil conductor and a one-third neutral, the cable costs just under
$4 per ft.
•
Bigger machines and machines more appro-
priate for the surface and soil conditions ease installations.
3.1.1 Underground Residential Distribution (URD)
A classic underground residential distribution circuit is an underground
circuit in a loop arrangement fed at each end from an overhead circuit (see
Figure 3.1
)
. The loop arrangement allows utilities to restore customers
more quickly; after crews find the faulted section, they can reconfigure the
loop and isolate any failed section of cable. This returns power to all
customers. Crews can delay replacing or fixing the cable until a more
convenient time or when suitable equipment arrives. Not all URD is con-
figured in a loop. Utilities sometimes use purely radial circuits or circuits
with radial taps or branches.
Padmounted transformers step voltage down for delivery to customers
and provide a sectionalizing point. The elbow connectors on the cables (pistol
grips) attach to bushings on the transformer to maintain a dead-front — no
exposed, energized conductors. To open a section of cable, crews can simply
pull an elbow off of the transformer bushing and place it on a parking stand,
which is an elbow bushing meant for holding an energized elbow connector.
Elbows and other terminations are available with continuous-current rat-
ings of 200 or 600 A (IEEE Std. 386-1995). Load-break elbows are designed
to break load; these are only available in 200-A ratings. Without load-break
capability, crews should of course only disconnect the elbow if the cable is
deenergized. Elbows normally have a test point where crews can check if
Copyright © 2006 Taylor & Francis Group, LLC
Cable size and materials —
Installation equipment —
Underground Distribution
93
Riser
poles
Open
point
Padmounted transformer
Primary
Secondary
FIGURE 3.1
An example front-lot underground residential distribution (URD) system.
the cable is live. Elbows are also tested to withstand ten cycles of fault
current, with 200-A elbows tested at 10 kA and 600-A elbows tested at 25
kA (IEEE Std. 386-1995).
The interface between the overhead circuit and the URD circuit is the riser
pole. At the riser pole (or a dip pole or simply a dip), cable terminations
provide the interface between the insulated cable and the bare overhead
conductors. These pothead terminations grade the insulation to prevent
excessive electrical stress on the insulation. Potheads also keep water from
entering the cable, which is critical for cable reliability. Also at the riser pole
are expulsion fuses, normally in cutouts. Areas with high short-circuit cur-
rent may also have current-limiting fuses. To keep lightning surges from
damaging the cable, the riser pole should have arresters right across the
pothead with as little lead length as possible.
Underground designs for residential developments expanded dramati-
cally in the 1970s. Political pressure coupled with technology improvements
were the driving forces behind underground distribution. The main devel-
opments — direct-buried cables and padmounted transformers having load-
break elbows — dramatically reduced the cost of underground distribution
to close to that of overhead construction. In addition to improving the visual
landscape, underground construction improves reliability. Underground res-
Copyright © 2006 Taylor & Francis Group, LLC
94
Electric Power Distribution Equipment and Systems
idential distribution has had difficulties, especially high cable failure rates.
In the late 1960s and early 1970s, given the durability of plastics, the poly-
ethylene cables installed at that time were thought to have a life of at least
50 years. In practice, cables failed at a much higher rate than expected,
enough so that many utilities had to replace large amounts of this cable.
According to Boucher (1991), 72% of utilities use front-lot designs for URD.
With easier access and fewer trees and brush to clear, crews can more easily
install cables along streets in the front of yards. Customers prefer rear-lot
service, which hides padmounted transformers from view. Back-lot place-
ment can ease siting issues and may be more economical if lots share rear
property lines. But with rear-lot design, utility crews have more difficulty
accessing cables and transformers for fault location, sectionalizing, and repair.
Of those utilities surveyed by Boucher (1991), 85% charge for underground
residential service, ranging from $200 to $1200 per lot (1991 dollars). Some
utilities charge by length, which ranges from $5.80 to $35.00 per ft.
3.1.2 Main Feeders
Whether urban, suburban, or even rural, all parts of a distribution circuit
can be underground, including the main feeder. For reliability, utilities often
configure an underground main feeder as a looped system with one or more
tie points to other sources. Switching cabinets or junction boxes serve as tie
points for tapping off lateral taps or branches to customers. These can be in
handholes, padmounted enclosures, or pedestals above ground. Three-phase
circuits can also be arranged much like URD with sections of cable run
between three-phase padmounted transformers. As with URD, the pad-
mounted transformers serve as switching stations.
Although short, many feeders have an important underground section
— the substation exit. Underground substation exits make substations eas-
ier to design and improve the aesthetics of the substation. Because they are
at the substation, the source of a radial circuit, substation exits are critical
for reliability. In addition, the loading on the circuit is higher at the sub-
station exit than anywhere else; the substation exit may limit the entire
circuit’s ampacity. Substation exits are not the place to cut corners. Some
strategies to reduce the risks of failures or to speed recovery are: concrete-
enclosed ducts to help protect cables, spare cables, overrated cables, and
good surge protection.
While not as critical as substation exits, utilities use similar three-phase
underground dips to cross large highways or rivers or other obstacles. These
are designed in much the same way as substation exits.
3.1.3 Urban Systems
Underground distribution has reliably supplied urban systems since the
early 1900s. Cables are normally installed in concrete-encased duct banks
Copyright © 2006 Taylor & Francis Group, LLC
Underground Distribution
95
beneath streets, sidewalks, or alleys. A duct bank is a group of parallel ducts,
usually with four to nine ducts but often many more. Ducts may be precast
concrete sections or PVC encased in concrete. Duct banks carry both primary
and secondary cables. Manholes every few hundred feet provide access to
cables. Transformers are in vaults or in the basements of large buildings.
Paper-insulated lead-covered (PILC) cables dominated urban applications
until the late 20th century. Although a few utilities still install PILC, most
use extruded cable for underground applications. In urban applications,
copper is more widely used than in suburban applications. Whether feeding
secondary networks or other distribution configurations, urban circuits may
be subjected to heavy loads.
“Vertical” distribution systems are necessary in very tall buildings.
Medium-voltage cable strung up many floors feed transformers within a
building. Submarine cables are good for this application since their protec-
tive armor wire provides support when a cable is suspended for hundreds
of feet.
3.1.4 Overhead vs. Underground
Overhead or underground? The debate continues. Both designs have advan-
tages (see Table 3.2). The major advantage of overhead circuits is cost; an
underground circuit typically costs anywhere from 1 to 2.5 times the equiv-
alent overhead circuit (see
Table 3.3
)
. But the cost differences vary wildly,
and it’s often difficult to define “equivalent” systems in terms of perfor-
mance. Under the right conditions, some estimates of cost report that cable
installations can be less expensive than overhead lines. If the soil is easy to
dig, if the soil has few rocks, if the ground has no other obstacles like water
pipes or telephone wires, then crews may be able to plow in cable faster and
for less cost than an overhead circuit. In urban areas, underground is almost
the only choice; too many circuits are needed, and above-ground space is
too expensive or just not available. But urban duct-bank construction is
expensive on a per-length basis (fortunately, circuits are short in urban appli-
TABLE 3.2
Overhead vs. Underground: Advantages of Each
Overhead
Underground
— Overhead’s number one advantage.
Significantly less cost, especially initial cost.
— Underground’s number one
advantage. Much less visual clutter.
— 30 to 50 years vs. 20 to 40 for new
underground works.
Safety
— Less chance for public contact.
— Significantly fewer short and
long-duration interruptions.
— Shorter outage durations because
of faster fault finding and faster repair.
— Notably lower maintenance costs (no
tree trimming).
— Overhead circuits can more readily
withstand overloads.
— Less voltage drop because
reactance is lower.
Copyright © 2006 Taylor & Francis Group, LLC
Cost
Aesthetics
Longer life
Reliability
Reliability
O&M
Loading
Longer reach
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