Download Disconnect Switches

Disconnect Switches
• There are many purposes for disconnect switches in
substations and power lines.
• They are used to isolate or deenergize equipment for
maintenance purposes, transfer load from one source to
another in planned or emergency conditions, provide visual
openings for maintenance personnel, and other reasons.
• Disconnect switches usually have low current interrupting
ratings compared to circuit breakers.
• Normally, power lines are first deenergized by circuit
breakers (due to their high current interrupting ratings),
followed by the opening of the air disconnect switches for
isolation.
Substation Switches
• There are many types of substation disconnect switches,
such as vertical break and horizontal break types. Disconnect
switches are normally gang operated.
• The term gang is used when all three phases are operated
with one operating device. Air disconnect switches are usually
opened and closed using control handles mounted at the base
of the structure.
• Sometimes, motor operator mechanisms are attached to the
control rods to remotely control their operation. A verticalbreak switch is shown in Figure 4-32 and a horizontal- break
switch is shown in Figure 4-33.
Substation Switches ……
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.
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-close when the transient
overvoltage condition was gone.
• The arrester clamped or limited the high-voltage transient
and prevented the equipment.
Lightning Arresters …….
• 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.
• 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.
Arresters fall into different energy dissipation classes:
1.
2.
3.
4.
Station class
Distribution class
Intermediate class
Secondary class
Lightning Arresters …….
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.
Electrical Bus ……
Capacitor Banks
• Capacitors are used to improve the operating efficiency of
electric power systems and help transmission system voltage
stability during disturbances.
• 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 steadystate 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……
• 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.
Substation Capacitor Banks
• Figure 4-39 shows a typical substation capacitor bank. The
vertical circuit breakers on the far right of the picture provide the
switching function of these substation capacitor banks.
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.
• 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, transmission
and generation losses are prevented.
Distribution Capacitor Bank……
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 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).
Reactors …….
Figure: Transmission line model
Reactors ……
• 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, highvoltage 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
connected during periods of low load.
Reactors ……
• 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.
Reactors ……
• 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.
Series Reactors—Distribution
• Distribution substations occasionally use series reactors to reduce
available fault current.
• Distribution lines connected to substations that have several
transmission lines or are near a generation plant might have
extremely high short-circuit fault current available if something
were to happen out on the distribution line.
• By inserting a series reactor on each phase of each distribution
line, the fault current decreases due to the fact that a magnetic field
has to be developed before high currents flow through the reactor.
• Therefore, the circuit breaker tips the series reactor before the
current has a chance to rise to full magnitude. Otherwise, the high
fault current could cause excessive damage to consumers’ electrical
equipment.
Assignment # 2
1. Collect the data of generation plants installed in
Pakistan. Also note the generation and distribution
capacity in term of voltage and power in these grid.
2. Collect data for atleast 10 generation plants. Data
includes:
• Rating of the generator unit (power and voltage)
• Types
• Cost of generation for 1 unit