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ELECTRIC POWER DISTRIBUTION EQUIMPENT AND SYSTEMS
6
Capacitor A
pplication
Capacitors provide tremendous benefits to distribution system performance.
Most noticeably, capacitors reduce losses, free up capacity, and reduce volt-
age drop:
•
By canceling the reactive power to motors and
other loads with low power factor, capacitors decrease the line cur-
rent. Reduced current frees up capacity; the same circuit can serve
more load. Reduced current also significantly lowers the
—
I
2
R
line
losses.
•
Capacitors provide a voltage boost, which cancels
part of the drop caused by system loads. Switched capacitors can
regulate voltage on a circuit.
—
If applied properly and controlled, capacitors can significantly improve
the performance of distribution circuits. But if not properly applied or con-
trolled, the reactive power from capacitor banks can create losses and high
voltages. The greatest danger of overvoltages occurs under light load. Good
planning helps ensure that capacitors are sited properly. More sophisticated
controllers (like two-way radios with monitoring) reduce the risk of improp-
erly controlling capacitors, compared to simple controllers (like a time clock).
Capacitors work their magic by storing energy. Capacitors are simple
devices: two metal plates sandwiched around an insulating dielectric. When
charged to a given voltage, opposing charges fill the plates on either side of
the dielectric. The strong attraction of the charges across the very short
distance separating them makes a tank of energy. Capacitors oppose changes
in voltage; it takes time to fill up the plates with charge, and once charged,
it takes time to discharge the voltage.
On ac power systems, capacitors do not store their energy very long —
just one-half cycle. Each half cycle, a capacitor charges up and then dis-
charges its stored energy back into the system. The net real power transfer
is zero. Capacitors provide power just when reactive loads need it. Just when
a motor with low power factor needs power from the system, the capacitor
is there to provide it. Then in the next half cycle, the motor releases its excess
energy, and the capacitor is there to absorb it. Capacitors and reactive loads
269
Copyright © 2006 Taylor & Francis Group, LLC
Losses; Capacity
Voltage drop
270
Electric Power Distribution Equipment and Systems
Bushing
Discharge resistor
Capacitor elements
FIGURE 6.1
Capacitor components. (From General Electric Company. With permission.)
exchange this reactive power back and forth. This benefits the system
because that reactive power (and extra current) does not have to be trans-
mitted from the generators all the way through many transformers and many
miles of lines; the capacitors can provide the reactive power locally. This
frees up the lines to carry real power, power that actually does work.
Capacitor units are made of series and parallel combinations of capacitor
packs or elements put together as shown in Figure 6.1. Capacitor elements
have sheets of polypropylene film, less than one mil thick, sandwiched
between aluminum foil sheets. Capacitor dielectrics must withstand on the
order of 2000 V/mil (78 kV/mm). No other medium-voltage equipment has
such high voltage stress. An underground cable for a 12.47-kV system has
insulation that is at least 0.175 in. (4.4 mm) thick. A capacitor on the same
system has an insulation separation of only 0.004 in. (0.1 mm).
Utilities often install substation capacitors and capacitors at points on
distribution feeders. Most feeder capacitor banks are pole mounted, the least
expensive way to install distribution capacitors. Pole-mounted capacitors
normally provide 300 to 3600 kvar at each installation. Many capacitors are
switched, either based on a local controller or from a centralized controller
through a communication medium. A line capacitor installation has the
capacitor units as well as other components, possibly including arresters,
fuses, a control power transformer, switches, and a controller (see
Figure 6.2
for an example).
Copyright © 2006 Taylor & Francis Group, LLC
Capacitor Application
271
Control power
transformer
Vacuum
switch
Capacitor unit
FIGURE 6.2
Overhead line capacitor installation. (From Cooper Power Systems, Inc. With permission.)
While most capacitors are pole mounted, some manufacturers provide
padmounted capacitors. As more circuits are put underground, the need for
padmounted capacitors will grow. Padmounted capacitors contain capacitor
cans, switches, and fusing in a deadfront package following standard pad-
mounted-enclosure integrity requirements (ANSI C57.12.28-1998). These
units are much larger than padmounted transformers, so they must be sited
more carefully to avoid complaints due to aesthetics. The biggest obstacles
are cost and aesthetics. The main complaint is that padmounted capacitors
are large. Customers complain about the intrusion and the aesthetics of such
a large structure (see Figure 6.3).
FIGURE 6.3
Example padmounted capacitor. (From Northeast Power Systems, Inc. With permission.)
Copyright © 2006 Taylor & Francis Group, LLC
272
Electric Power Distribution Equipment and Systems
TABLE 6.1
Substation vs. Feeder Capacitors
Advantages
Disadvantages
Feeder Capacitors
Reduces line losses
Reduces voltage drop along the feeder
Frees up feeder capacity
Lower cost
More difficult to control reliably
Size and placement important
Substation Capacitors
Better control
Best placement if leading vars are needed for
system voltage support
No reduction in line losses
No reduction in feeder voltage drop
Higher cost
Substation capacitors are normally offered as open-air racks. Normally
elevated to reduce the hazard, individual capacitor units are stacked in rows
to provide large quantities of reactive power. All equipment is exposed. Stack
racks require a large substation footprint and are normally engineered for
the given substation. Manufacturers also offer metal-enclosed capacitors,
where capacitors, switches, and fuses (normally current-limiting) are all
enclosed in a metal housing.
Substation capacitors and feeder capacitors both have their uses. Feeder
capacitors are closer to the loads — capacitors closer to loads more effectively
release capacity, improve voltage profiles, and reduce line losses. This is
especially true on long feeders that have considerable line losses and voltage
drop. Table 6.1 highlights some of the differences between feeder and station
capacitors. Substation capacitors are better when more precise control is
needed. System operators can easily control substation capacitors wired into
a SCADA system to dispatch vars as needed. Modern communication and
control technologies applied to feeder capacitors have reduced this advan-
tage. Operators can control feeder banks with communications just like
station banks, although some utilities have found the reliability of switched
feeder banks to be less than desired, and the best times for switching in vars
needed by the system may not correspond to the best time to switch the
capacitor in for the circuit it is located on.
Substation capacitors may also be desirable if a leading power factor is
needed for voltage support. If the power factor is leading, moving this capac-
itor out on the feeder increases losses. Substation capacitors cost more than
feeder capacitors. This may seem surprising, but we must individually engi-
neer station capacitors, and the space they take up in a station is often valuable
real estate. Pole-mounted capacitor installations are more standardized.
Utilities normally apply capacitors on three-phase sections. Applications
on single-phase lines are done but less common. Application of three-phase
banks downstream of single-phase protectors is normally not done because
Copyright © 2006 Taylor & Francis Group, LLC
Capacitor Application
273
of ferroresonance concerns. Most three-phase banks are connected
grounded-wye on four-wire multigrounded circuits. Some are connected in
floating wye. On three-wire circuits, banks are normally connected as a
floating wye.
Most utilities also include arresters and fuses on capacitor installations.
Arresters protect capacitor banks from lightning-overvoltages. Fuses isolate
failed capacitor units from the system and clear the fault before the capacitor
fails violently. In high fault-current areas, utilities may use current-limiting
fuses. Switched capacitor units normally have oil or vacuum switches in
addition to a controller. Depending on the type of control, the installation
may include a control power transformer for power and voltage sensing and
possibly a current sensor. Because a capacitor bank has a number of compo-
nents, capacitors normally are not applied on poles with other equipment.
Properly applied capacitors return their investment very quickly. Capaci-
tors save significant amounts of money in reduced losses. In some cases,
reduced loadings and extra capacity can also delay building more distribu-
tion infrastructure.
6.1 Capacitor Ratings
Capacitor units rated from 50 to over 500 kvar are available;
Table 6.2
s
hows
common capacitor unit ratings. A capacitor’s rated kvar is the kvar at rated
voltage. Three-phase capacitor banks are normally referred to by the total
kvar on all three phases. Distribution feeder banks normally have one or
two or (more rarely) three units per phase. Many common size banks only
have one capacitor unit per phase.
IEEE Std. 18 defines standards for capacitors and provides application
guidelines. Capacitors should not be applied when any of the following
limits are exceeded (IEEE Std. 18-2002):
• 135% of nameplate kvar
• 110% of rated rms voltage, and crest voltage not exceeding 1.2
of rated rms voltage, including harmonics but excluding transients
• 135% of nominal rms current based on rated kvar and rated voltage
2
Capacitor dielectrics must withstand high voltage stresses during normal
operation — on the order of 2000 V/mil. Capacitors are designed to with-
stand overvoltages for short periods of time. IEEE Std. 18-1992 allows up to
300 power-frequency overvoltages within the time durations in
Table 6.3
(without transients or harmonic content). New capacitors are tested with at
least a 10-sec overvoltage, either a dc-test voltage of 4.3 times rated rms or
an ac voltage of twice the rated rms voltage (IEEE Std. 18-2002).
Copyright © 2006 Taylor & Francis Group, LLC
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