TM
5-811-1/AFJMAN
32-1080
crossings, or overhead-to-underground transitions.
reactance of the conductor. Xd is the reactance
Also, armless construction requires bucket trucks
which results from flux between the radius of one
for maintenance due to loss of climbing space.
foot and the equivalent conductor spacing based on
b. Insulated cable lines. Aerial insulated cables
a mean distance (D). The two values of reactance
will be of the factory-assembled, messenger-
can be found in conductor tables and added to-
supported type. The use of self-supported insulated
gether for the total alternating-current reactance.
cable or of messenger-supported insulated cable
5-5. Power Factor Correction.
with insulated spacers will not be used.
System power factor is influenced mainly by the
(1) Medium-voltage lines. Such construction is
characteristics and mechanical loads of the motors
advisable where it is necessary to avoid exposure
supplied. Such characteristics vary widely and
to open wire hazards, for example, high reliability
service in heavy storm areas. Cable will be of the
therefore the kVAR capacity cannot be correctly
factory-assembled, messenger-supported type.
estimated at the time of the distribution system
(2) Low-voltage lines. Low-voltage lines will be
design, but only after firm data is available. One
of the neutral-supported secondary and service
year of operating history is needed before the
drop type which uses a bare messenger as a
amount of fixed and switched capacitance can be
neutral conductor and as a support for insulated
selected to best meet actual operating conditions.
phase conductors. Weatherproof conductors (line
Large motors are often provided with integral
wires), which are supported on secondary racks,
capacitors.
are less attractive and more expensive to install
then neutral-supported cable. Use of secondary-
cation of line and station shunt capacitors requires
a life-cycle cost analysis using the methodology in
rack construction will be limited to minor exten-
sions of existing systems.
10 CFR 436. Capacitors are justified when the
savings to investment ratio of the installation is
5-4. Voltage Drop.
greater than one. An example of computing the
Voltage drop on the distribution system will com-
average energy savings per year is shown on
ply with the minimum voltage requirements of
figure 5-3. Where a serving utility does not have a
power factor clause, only line losses will apply.
ANSI C84.1. Voltage drop on the low-voltage
b. Capacitor equipment. Capacitors for overhead
distribution system will comply with the recom-
mendations of the NEC. Figure 5-1 shows typical
distribution systems can be pole-mounted in banks
of 300 to 1,800 kVAR for most medium-voltage
distribution of voltage drops through the supply
system. Designers will consider all the system
systems up to 34.5 kV phase-to-phase. Pad-
voltage drops in order to ensure that voltage levels
mounted capacitor equipment is available in the
same range of sizes and voltage ratings for under-
are in accordance with ANSI C84.1 and the NEC.
a. Voltage drops. An example of an aerial line
ground systems. Power capacitor equipment will
have grounded wye connections so switch tanks
voltage drop calculation is given on figure 5-2.
and frames will be at ground potential for greater
This example uses the approximate formula
method which ignores angles and which is suffi-
personnel safety. Grounded capacitors can bypass
some line surges to ground, provide a low imped-
ciently accurate for all but abnormal conditions,
ance path for harmonics, and group fusing need
such as where system power factors are extremely
not be so precise. For maximum efficiencies, capac-
low. Proximity effects, sheath currents, and geo-
itor equipment will be located as close to the load
metric construction may need to be taken into
controlled as is feasible. Surge arresters will be
account in calculations of impedance for under-
specified to limit the magnitude of voltage surges
ground circuits. Various tables and voltage drop
curves are available from manufacturers for un-
caused by capacitor switching. Applications of
surge arresters will be in accordance with the
derground circuits. For aerial circuits, impedance
IEEE C62 series of standards.
may be determined using values of resistance and
c. Capacitor control. Switched capacitors will be
reactance.
provided only when differences between full-load
and light-load power factors warrant such control.
less at 60 Hz frequencies, the skin-effects of alter-
The load and power factor profile of the system
nating current are negligible and direct-current
will determine the economics of switched control,
resistance values can be used.
c. Reactance. Normal practice is to separate
and whether there is a necessity for more than one
switching step. Time clock control is the least
inductive reactance into two components. Xa is the
costly type of control, but can only be used where
reactance which results from flux within a radius
power factor and demand vary on a firm time
of one foot of the conductor plus the internal
5-2
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