STRUCTURE OF THE POWER SYSTEM
An interconnected power system is a complex enterprise that may be
subdivided into the following major subsystems:
• Generation Subsystem
• Transmission and Subtransmission Subsystem
• Distribution Subsystem
• Utilization Subsystem
subdivided into the following major subsystems:
• Generation Subsystem
• Transmission and Subtransmission Subsystem
• Distribution Subsystem
• Utilization Subsystem
Generation Subsystem
This includes generators and transformers.
Generators – An essential component of power systems is the threephase
ac generator known as synchronous generator or alternator. Synchronous
Generators – An essential component of power systems is the threephase
ac generator known as synchronous generator or alternator. Synchronous
generators have two synchronously rotating fields: One field is produced by the
rotor driven at synchronous speed and excited by dc current. The other field is
produced in the stator windings by the three-phase armature currents. The dc
current for the rotor windings is provided by excitation systems. In the older
units, the exciters are dc generators mounted on the same shaft, providing
excitation through slip rings. Current systems use ac generators with rotating
rectifiers, known as brushless excitation systems. The excitation system
maintains generator voltage and controls the reactive power flow. Because they
lack the commutator, ac generators can generate high power at high voltage,
typically 30 kV.
The source of the mechanical power, commonly known as the prime
mover, may be hydraulic turbines, steam turbines whose energy comes from the
burning of coal, gas and nuclear fuel, gas turbines, or occasionally internal
combustion engines burning oil.
Steam turbines operate at relatively high speeds of 3600 or 1800 rpm.
The generators to which they are coupled are cylindrical rotor, two-pole for
3600 rpm, or four-pole for 1800 rpm operation. Hydraulic turbines, particularly
those operating with a low pressure, operate at low speed. Their generators are
usually a salient type rotor with many poles. In a power station, several
generators are operated in parallel in the power grid to provide the total power
needed. They are connected at a common point called a bus.
With concerns for the environment and conservation of fossil fuels,
many alternate sources are considered for employing the untapped energy
sources of the sun and the earth for generation of power. Some alternate sources
used are solar power, geothermal power, wind power, tidal power, and biomass.
The motivation for bulk generation of power in the future is the nuclear fusion.
If nuclear fusion is harnessed economically, it would provide clean energy from
an abundant source of fuel, namely water.
Transformers – The transformer transfers power with very high
efficiency from one level of voltage to another level. The power transferred to
the secondary is almost the same as the primary, except for losses in the
transformer. Using a step-up transformer will reduce losses in the line, which
makes the transmission of power over long distances possible.
Insulation requirements and other practical design problems limit the
generated voltage to low values, usually 30 kV. Thus, step-up transformers are
used for transmission of power. At the receiving end of the transmission lines
step-down transformers are used to reduce the voltage to suitable values for
distribution or utilization. The electricity in an electric power system may
undergo four or five transformations between generator and consumers.
rotor driven at synchronous speed and excited by dc current. The other field is
produced in the stator windings by the three-phase armature currents. The dc
current for the rotor windings is provided by excitation systems. In the older
units, the exciters are dc generators mounted on the same shaft, providing
excitation through slip rings. Current systems use ac generators with rotating
rectifiers, known as brushless excitation systems. The excitation system
maintains generator voltage and controls the reactive power flow. Because they
lack the commutator, ac generators can generate high power at high voltage,
typically 30 kV.
The source of the mechanical power, commonly known as the prime
mover, may be hydraulic turbines, steam turbines whose energy comes from the
burning of coal, gas and nuclear fuel, gas turbines, or occasionally internal
combustion engines burning oil.
Steam turbines operate at relatively high speeds of 3600 or 1800 rpm.
The generators to which they are coupled are cylindrical rotor, two-pole for
3600 rpm, or four-pole for 1800 rpm operation. Hydraulic turbines, particularly
those operating with a low pressure, operate at low speed. Their generators are
usually a salient type rotor with many poles. In a power station, several
generators are operated in parallel in the power grid to provide the total power
needed. They are connected at a common point called a bus.
With concerns for the environment and conservation of fossil fuels,
many alternate sources are considered for employing the untapped energy
sources of the sun and the earth for generation of power. Some alternate sources
used are solar power, geothermal power, wind power, tidal power, and biomass.
The motivation for bulk generation of power in the future is the nuclear fusion.
If nuclear fusion is harnessed economically, it would provide clean energy from
an abundant source of fuel, namely water.
Transformers – The transformer transfers power with very high
efficiency from one level of voltage to another level. The power transferred to
the secondary is almost the same as the primary, except for losses in the
transformer. Using a step-up transformer will reduce losses in the line, which
makes the transmission of power over long distances possible.
Insulation requirements and other practical design problems limit the
generated voltage to low values, usually 30 kV. Thus, step-up transformers are
used for transmission of power. At the receiving end of the transmission lines
step-down transformers are used to reduce the voltage to suitable values for
distribution or utilization. The electricity in an electric power system may
undergo four or five transformations between generator and consumers.
Transmission and Subtransmission Subsystem
An overhead transmission network transfers electric power fromgenerating units to the distribution system which ultimately supplies the load.
Transmission lines also interconnect neighboring utilities which allow the
economic dispatch of power within regions during normal conditions, and the
transfer of power between regions during emergencies.
Standard transmission voltages are established in the United States by the
American National Standards Institute (ANSI). Transmission voltage lines
operating at more than 60 kV are standardized at 69 kV, 115 kV, 138 kV, 161
kV, 230 kV, 345 kV, 500 kV, and 765 kV line-to-line. Transmission voltages
above 230 kV are usually referred to as extra-high voltage (EHV).
High voltage transmission lines are terminated in substations, which are
called high-voltage substations, receiving substations, or primary substations.
The function of some substations is switching circuits in and out of service;
they are referred to as switching stations. At the primary substations, the
voltage is stepped down to a value more suitable for the next part of the trip
toward the load. Very large industrial customers may be served from the
transmission system.
The portion of the transmission system that connects the high-voltage
substations through step-down transformers to the distribution substations is
called the subtransmission network. There is no clear distinction between
transmission and subtransmission voltage levels. Typically, the subtransmission
voltage level ranges from 69 to 138 kV. Some large industrial customers may
be served from the subtransmission system. Capacitor banks and reactor banks
are usually installed in the substations for maintaining the transmission line
voltage.
Transmission lines also interconnect neighboring utilities which allow the
economic dispatch of power within regions during normal conditions, and the
transfer of power between regions during emergencies.
Standard transmission voltages are established in the United States by the
American National Standards Institute (ANSI). Transmission voltage lines
operating at more than 60 kV are standardized at 69 kV, 115 kV, 138 kV, 161
kV, 230 kV, 345 kV, 500 kV, and 765 kV line-to-line. Transmission voltages
above 230 kV are usually referred to as extra-high voltage (EHV).
High voltage transmission lines are terminated in substations, which are
called high-voltage substations, receiving substations, or primary substations.
The function of some substations is switching circuits in and out of service;
they are referred to as switching stations. At the primary substations, the
voltage is stepped down to a value more suitable for the next part of the trip
toward the load. Very large industrial customers may be served from the
transmission system.
The portion of the transmission system that connects the high-voltage
substations through step-down transformers to the distribution substations is
called the subtransmission network. There is no clear distinction between
transmission and subtransmission voltage levels. Typically, the subtransmission
voltage level ranges from 69 to 138 kV. Some large industrial customers may
be served from the subtransmission system. Capacitor banks and reactor banks
are usually installed in the substations for maintaining the transmission line
voltage.
Distribution Subsystem
The distribution system connects the distribution substations to the
consumers’ service-entrance equipment. The primary distribution lines from 4
to 34.5 kV and supply the load in a well-defined geographical area. Some small
industrial customers are served directly by the primary feeders.
The secondary distribution network reduces the voltage for utilization
by commercial and residential consumers. Lines and cables not exceeding a few
hundred feet in length then deliver power to the individual consumers. The
secondary distribution serves most of the customers at levels of 240/120 V,
single-phase, three-wire; 208Y/120 V, three-phase, four-wire; or 480Y/277 V,
three-phase, four-wire. The power for a typical home is derived from a
transformer that reduces the primary feeder voltage to 240/120 V using a threewire
line.
Distribution systems are both overhead and underground. The growth
of underground distribution has been extremely rapid and as much as 70 percent
of new residential construction is via underground systems.
consumers’ service-entrance equipment. The primary distribution lines from 4
to 34.5 kV and supply the load in a well-defined geographical area. Some small
industrial customers are served directly by the primary feeders.
The secondary distribution network reduces the voltage for utilization
by commercial and residential consumers. Lines and cables not exceeding a few
hundred feet in length then deliver power to the individual consumers. The
secondary distribution serves most of the customers at levels of 240/120 V,
single-phase, three-wire; 208Y/120 V, three-phase, four-wire; or 480Y/277 V,
three-phase, four-wire. The power for a typical home is derived from a
transformer that reduces the primary feeder voltage to 240/120 V using a threewire
line.
Distribution systems are both overhead and underground. The growth
of underground distribution has been extremely rapid and as much as 70 percent
of new residential construction is via underground systems.
Load Subsystems
Power systems loads are divided into industrial, commercial, and
residential. Industrial loads are composite loads, and induction motors form a
high proportion of these loads. These composite loads are functions of voltage
and frequency and form a major part of the system load. Commercial and
residential loads consist largely of lighting, heating, and cooking. These loads
are independent of frequency and consume negligibly small reactive power.
residential. Industrial loads are composite loads, and induction motors form a
high proportion of these loads. These composite loads are functions of voltage
and frequency and form a major part of the system load. Commercial and
residential loads consist largely of lighting, heating, and cooking. These loads
are independent of frequency and consume negligibly small reactive power.
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