Line Switches
Line disconnect switches are normally used to isolate sections of line or to
transfer load from one circuit to another. The picture in Figure 4-35 is an example
of a typical subtransmission line switch. This particular switch incorporates
vacuum bottles to help extinguish arcs from interrupting light-load
currents.
LIGHTNING ARRESTERS
Lightning arresters are designed to limit the line-to-ground voltage in the
event of lightning or other excessive transient voltage conditions. Some of
the older gap-type lightning arresters actually short-circuited the line or
equipment, causing the circuit breaker to trip. The breaker would then re-
Figure 4-35. Line switch.
LIGHTNING ARRESTERS 91
close when the transient overvoltage condition was gone. The lightning arrester
protects the equipment near the lightning arrester from experiencing
high-voltage transient conditions.
For example, suppose an 11 kV lightning arrester is installed on a 7.2 kV
line to neutral system. The lightning arrester will conduct if the line-to-neutral
voltage exceeds approximately 11 kV. Equipment connected to this distribution
system might have a flashover rating of 90 kV. Therefore, the arrester
clamped or limited the high-voltage transient and prevented the
equipment from experiencing a flashover or insulation failure.
The newer lightning arresters use gapless metal oxide semiconductor materials
to clamp or limit the voltage. These newer designs offer better voltage
control and have higher energy dissipation characteristics.
Aside from the voltage rating for which the arrester is applied, arresters
fall into different energy dissipation classes. An arrester might have to dissipate
energy up until the circuit breaker clears the line. Station class arresters
(see Figure 4-36) are the largest types and can dissipate the greatest amount
of energy. They are usually located adjacent to large substation power transformers.
Distribution class arresters (see Figure 4-37) are generously distrib-
Figure 4-36. Station class lightning arrester. Courtesy of Alliant Energy.
92 SUBSTATIONS
Figure 4-37. Distribution class. Courtesy of Alliant Energy.
uted throughout the distribution system in areas known to have high lightning
activity. They can be found near distribution transformers, overhead of underground
transition structures, and along long distribution lines. Intermediate
class arresters are normally used in substations that do not have excessive
short-circuit current. Residential and small commercial customers
may use secondary class arresters to protect large motors, sensitive electronic
equipment, and other voltage-surge-sensitive devices connected to their
service panel.
ELECTRICAL BUS
The purpose of the electrical bus in substations is to connect equipment together.
A bus is a conductor, or group of conductors, that serves as a common
connection between two or more circuits. The bus is supported by station
post insulators. These insulators are mounted on the bus structures. The
bus can be constructed of 3–6 inch rigid aluminum tubing or wires with insulators
on both ends, called a “strain” bus.
The buswork consists of structural steel that supports the insulators that
support the energized conductors. The buswork might also include air disconnect
switches. Special bus configurations allow for transferring load
from one feeder to another and to bypass equipment for maintenance.
Figure 4-38 is an example of typical buswork found in substations.
CAPACITOR BANKS
Capacitors are used to improve the operating efficiency of electric power
systems and help transmission system voltage stability during disturbances.
CAPACITOR BANKS 93
Figure 4-38. Examples of a typical electrical bus.
Capacitors are used to cancel out the lagging current effects from motors
and transformers. Capacitors can reduce system losses and help provide
voltage support. Another benefit of capacitors is that they can reduce the total
current flowing through a wire, thus leaving capacity in the conductors
for additional load.
Capacitor banks can be left online continuously to meet steady-state reactive
power requirements or they can be turned on or off to meet dynamic reactive
requirements. Some capacitor banks are switched seasonally (i.e., to
accommodate air conditioning load in the summer) and others are switched
daily to accommodate industrial loads.
Capacitor banks can be switched manually, automatically, locally or remotely.
For example, system control center operators commonly switch
substation capacitor banks on and off to meet load requirements or system
stability reactive demand requirements. Providing capacitive support maintains
good system voltage and reduces system losses.
Substation Capacitor Banks
Figure 4-39 shows a typical substation capacitor bank. Actually, this picture
shows two three-phase capacitor banks (one in the foreground and one in
94 SUBSTATIONS
Figure 4-39. Substation capacitor bank.
the background). The vertical circuit breakers on the far right of the picture
provide the switching function of these substation capacitor banks.
Distribution Capacitor Bank
Capacitor banks are installed on distribution lines to reduce losses, improve
voltage support, and provide additional capacity on the distribution system
(See Figure 4-40). Actually, reducing distribution system losses with capacitors
is very effective since that also reduces transmission losses.
The closer a capacitor is installed to the actual inductive load itself, the
more beneficial it is. For example, if capacitors are installed right at the motor
terminals of an industrial load, losses are prevented in the lines feeding
the motor, distribution losses are prevented, and transmission and generation
losses are prevented.
REACTORS
Reactor is another name for a high-voltage inductor. They are essentially
one-winding transformers. Reactors are used in electric power systems for
two main reasons. First, reactors are used in a shunt configuration (i.e., line
REACTORS 95
Figure 4-40. Distribution capacitor bank.
to ground connections), to help regulate transmission system voltage by absorbing
surplus reactive power (VARs) from generation or line charging.
Line charging is the term used to describe the capacitance effects of long
transmission lines since they are essentially long skinny capacitors (i.e., two
conductors separated by a dielectric—the air). Second, they are connected
in series to reduce fault current in distribution lines.
Reactors can be open-air coils or coils submerged in oil. Reactors are
available in either single-phase or three-phase units.
Shunt Reactors—Transmission
The electrical characteristics and performance of long, high-voltage transmission
lines can be improved through the use of shunt reactors. Shunt reactors
are used on transmission lines to help regulate or balance reactive power
flowing in the system. They can be used to absorb excess reactive power.
Reactors are normally disconnected during heavy load conditions and are
96 SUBSTATIONS
connected during periods of low load. Reactors are switched online during
light load conditions (i.e., late at night or early morning) when the transmission
line voltage tends to creep upward. Conversely, shunt capacitors are
added to transmission lines during high-load conditions to raise the system
voltage.
Another application of shunt reactors is to help lower transmission line
voltage when energizing a long transmission line. For example, suppose a
200 mile, 345 kV transmission line is to be energized. The line-charging effect
of long transmission lines can cause the far-end voltage to be on the order
of 385 kV. Switching on a shunt reactor at the far end of the line can reduce
the far-end voltage to approximately 355 kV. This reduced far-end
voltage will result in a lower transient voltage condition when the far-end
circuit breaker is closed, connecting the transmission line to the system and
allowing current to flow. Once load is flowing in the line, the shunt reactor
can be disconnected and the load will then hold the voltage in balance.
Figure 4-41 shows a 345 kV, 35 MVAR three-phase shunt reactor used to
help regulate transmission voltage during light load conditions and during
the energization of long transmission lines.
STATIC VAR COMPENSATORS 97
Series Reactors—Distribution