Electricity: From Power Plants to Consumers
Nature of electricity
Electricity is generated as it is used. Unlike other commodities, there is very little ability to store
electricity. Because of the instantaneous nature of the electric system, constant modifications must
be made to assure that the generation of power matches the consumption of power. The electric
system we’ve grown to depend on is very complex and dynamic, ever adjusting to meet changing
The amount of power on a line at any given moment depends on generation production and
dispatch, customer use, the status of other transmission lines and their associated equipment, and
even the weather. The transmission system must accommodate changing electricity supply and
demand conditions, unexpected outages, planned shutdowns of generators or transmission
equipment for maintenance, weather extremes, fuel shortages, and other challenges.
The transmission grid
The electrical transmission system is more complex and dynamic than other utility systems, such as
water or natural gas. Electricity flows from power plants, through transformers and transmission
lines, to substations, distribution lines, and then finally to the electricity consumer (Figure 1). The
electric system is highly interconnected.
The interconnectedness of the system means that the transmission grid functions as one entity.
Power entering the system flows along all available paths, not just from Point A to Point B. The
system does not recognize divisions between service areas, counties, states, or even countries.
The current transmission grid includes not only transmission lines that run from power plants to
load centers, but also from transmission line to transmission line, providing a redundant system that
helps assure the smooth flow of power. If a transmission line is taken out of service in one part of
the power grid, the power can usually be rerouted through other power lines to continue delivering
the power to the customer.
In essence, the electricity from many power plants is “pooled” in the transmission system, and each
distribution system draws from this pool. This networked system helps to achieve a high reliability
for power delivery, since any one power plant that shuts down should only constitute a fraction of
the power being delivered by the power grid.
This pooling of power means that power is provided from a diversity of sources, including coal,
nuclear, natural gas, oil, or other renewable energy sources such as hydropower, biomass, wind, or
A transmission line outage acts like a dam, forcing the electricity around the blockage onto other
lines. If adjacent transmission lines cannot handle the power that is rerouted, safety devices may
switch them off, further impeding power flows and potentially leading to cascading outages and
system failure, i.e., a blackout. This is one of the disadvantages of the interconnectedness of the
transmission grid. Failures in one location can quickly affect the entire system, producing a large
scale blackout. For reliable power transmission, a region requires backup transmission lines with
Components of the transmission system
Power plants generate three-phase alternating current (AC). This means that there are three wires
coming out of every plant.
On a transmission structure, the three large wires are called conductors and carry the electric power.
They are usually about an inch in diameter. There is also a smaller wire at the top of the structure,
called a shield wire. The shield wire is designed to protect the power line from lightning. Poles with
two sets of three wires (conductors) are called “double-circuit” poles. Sometimes a distribution line
is strung under the transmission lines reducing the need for additional power poles.
Electricity is transferred from the power plant to the users, through the electric grid. The grid
consists of two separate infrastructures: the high-voltage transmission system and the lower-voltage
distribution system. High-voltage transmission lines minimize electrical losses and are therefore
used to carry electricity hundreds of miles. Transmission lines in Wisconsin range from 69 to 345
kilovolts (kV). Higher voltage lines, such as 500 and 765 kV lines are not used in Wisconsin but are
in other states. The lower-voltage lines (distribution system) draw electricity from the transmission
lines and distribute it to individual customers. Lower voltage lines range from 12 to 24 kV. The
voltage that connects to your house is 120 to 240 volts.
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Simplified Electric System
The interface between different voltage transmission lines and the distribution system is the
electrical substation. Substations use transformers to “step down” voltages from the higher
transmission voltages to the lower distribution system voltages. Transformers located along
distribution lines further step down the line voltages for household usage.
Transmission line design
The electric lines that generate the most public interest are high-voltage transmission lines. These
are the largest and most visible electric lines. Most large cities require several transmission lines for
reliable electric service. Figure 2 shows two 345-kV double-circuited transmission structures sharing
the same right-of-way (ROW). Double-circuited means that the transmission structure is carrying
two sets of transmission lines, each with three conductors.
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Two High Voltage Double-Circuit Transmission Structures
Transmission lines are larger than the more common distribution lines that exist along rural roads
and city streets. Transmission line poles or structures are between 60 and 140 feet tall. Distribution
line structures are approximately 40 feet tall.
There are several different kinds of transmission structures. Transmission structures can be
constructed of metal or wood. They can be single-poled or double poled. They can be single-
circuited carrying one set of transmission lines or double-circuited with two sets of lines. Figure 3
shows a close up of a commonly built double-circuited, single-pole transmission structure. Figure 4
shows diagrams of different types of transmission structures.
Close-up of a Double-Circuit Transmission Structure
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Different Transmission Structures
Horizontal Line Post
Different transmission structures have different material and construction costs, and require
different right-of-way widths, distances between structures (span length), and pole height. These
issues also vary with different voltages. In the past, many transmission lines were constructed on H-
frame wood structures and metal lattice structures. New lines are most often constructed with single
pole structures because of right-of-way width limitations and environmental considerations. Current
right-of-way widths vary between 80 to 140 feet. A typical right-of-way is diagrammed in Figure 5.
Typical Span 800-1000 feet
120-foot Total Right-of-Way
Typical Right-of-Way Diagram
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Pole height and load capacity limitations control allowable span length either on the basis of ground
clearance or ability to support heavy wind and ice loads. In areas where single-pole structures are
preferred, weak or wet soils may require concrete foundations for support. Where a transmission
line must cross a street or slightly change direction, large angle structures (Figures 6 and 7) or guy
wires may be required. Poles with guy wires impact a much larger area. Angle structures are usually
more than double the diameter of other steel poles. They are made of steel, usually five to six feet in
diameter, and have a large concrete base. The base may be buried ten or more feet below the
ground surface. The diameter of the pole and the depth the base is buried depends on the condition
of the soils and the voltage of the line.
Two Angle Structures and a Transmission Line Crossing a Road
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Close-up of the Base of an Angle Structure
The Wisconsin transmission system
There are almost 11,500 miles of transmission line currently in Wisconsin. High-voltage
transmission lines deliver large amounts of power on a regional basis. The higher the voltage, the
more power the line can carry. The Wisconsin transmission system has a general electric flow from
northwest to southeast through the state. The western part of Wisconsin is connected by high-
voltage lines (161 and 345 kV) primarily from Minnesota. The southeastern part of Wisconsin is
connected to northern Illinois by 345 kV high voltage lines. However, there are few connections
between these two parts of the state and a lack of redundant connections. Redundant connects are
needed to ensure reliable service. The Wisconsin transmission system is currently congested under
normal power flow conditions. In addition, there are many transmission lines in Wisconsin that are
more than 60 years old, requiring upgrades or replacement. Some areas, due to the age of the lines
and the lack of redundant lines, often experience disruptions in electrical power.
Restructuring the electric industry and Wisconsin
In the recent past, individual utilities owned power plants, substations, transmission lines, and
distribution lines that generated and provided electricity to their customers. This system was known
as vertically-integrated. Restructuring the electric industry in Wisconsin resulted in the transfer of
much of the state’s transmission infrastructure to a separate company. In 1999, the eastern
Wisconsin transmission facilities were consolidated under a new company, the American
Transmission Company, LLC (ATC). ATC commenced operation in January 2001.
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Transmission Companies of Wisconsin
Transmission lines in the western portion of the state are owned by Xcel Energy Services Inc (Xcel)
and Dairyland Power Cooperative (DPC). Figure 8 shows the approximate territories of the three
companies that own and operate transmission systems. Both Xcel and Dairyland have facilities in
other states and still own transmission and generation facilities.
Wisconsin transmission needs
There are several drivers for new transmission construction.
Growth in an area’s electricity use, which often requires new distribution substations and
new lines to connect them to the existing transmission system, or increased capacity on
existing transmission lines. Wisconsin has a growth in electrical demand of about 2 percent
Older transmission lines that are in poor condition and no longer reliable might need to be
replaced by new lines. Often new lines will have a greater electricity carrying capacity than
those they replace.
The existing transmission system must be reinforced with new lines to prevent equipment
overloads and low voltages.
New power plants need new transmission lines to connect them to the existing transmission
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Transmission line projects approved by the Public Service Commission of Wisconsin (PSC) are
required by statute (Wis. Stat. §§ 196.49(3) and 196.491(3)(d)) to have costs that are in proportion
with their benefits. They must satisfy the reasonable needs of the public for adequate electric energy
supply and must also not be overbuilt or be designed in excess of probable future electric needs.
Regulation of the Electric Industry
The PSC regulates the siting and construction of new transmission lines. The Electric Reliability
Organization (ERO) sets and enforces reliability standards. ERO replaced the organization formerly
known as NERC, North American Electric Reliability Council, a voluntary utility-managed reliability
organization. The FERC oversees the ERO. Wisconsin utilities belong to two different
organizations under the ERO umbrella: Reliability First and the Midwest Reliability Organization.
Regional Transmission Organizations (RTOs)
In 1992, the Energy Policy Act (EPAct) opened access to the transmission grid for non-utility power
generators. As ordered by the EPAct, FERC passed orders that required owners of transmission
lines to provide open, non-discriminatory access to power generators engaged in interstate
commerce. These policies allowed for wholesale competition—that is, competition among power
generators for contracts with utilities, as well as contracts with large industrial firms.
During the years 1996 to 2000, FERC ordered utilities to offer other energy providers fair and open
access to their transmission lines. FERC created RTOs to provide independent oversight over the
nation’s power grid. Wisconsin is part of the Midwest Independent System Operator (MISO) (See
Wisconsin state law requires that utilities participate in MISO or another RTO. DPC is a
cooperative and not regulated by FERC, but is a conditional member of MISO. MISO began
operation in December 2001.
MISO is specifically structured to comply with FERC’s concept of an independent organization that
will ensure the smooth regional flow of electricity in a competitive wholesale marketplace. MISO
administers the use of the transmission system in its service area and will direct the physical
operation of the system by the individual transmission owners. One of the primary functions of
each RTO is transmission planning and expansion within its region that will enable it to provide
efficient, reliable, and non-discriminatory service. This is a shift from planning conducted by
individual utilities to meet their customer’s needs to RTOs planning to meet the needs of regional
electricity markets. MISO’s most recent regional transmission plan is called MTEP06 and can be
accessed on MISO’s Web site at: http://www.midwestiso.org/page/Expansion+Planning.
As part of its core mission, MISO also operates a wholesale energy market which is used to manage
transmission system congestion. FERC regulates MISO, not the PSC. Costs and benefits of MISO
continue to be the subject of debate and scrutiny.
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In prior decades, electric transmission lines were constructed from Point A to Point B in the most
direct manner possible with limited concern for communities, crops, natural resources, or private
property issues. As these older lines require improvements, they may be rerouted to share corridors
with roads, and to avoid, where practicable, community and natural resource impacts. At the same
time, continued growth in energy usage will require new electric substations and transmission lines
to be sited and constructed. New and upgraded electric facilities may impact many communities and
many property owners.
To meet future growth, communities often draft plans for sewers, roads, and development districts,
but few cities, towns, or counties include transmission lines in their plans. Transmission lines are
costly to build and difficult to site. Cities, towns, and counties can help reduce land use conflicts by:
Dedicating a strip of land along existing transmission corridors for potential future right-of-
Identifying future potential transmission corridors and substation sites in new developments,
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