Transmission And Distribution Book

[Heike Fallago]

Thetransmission network is like the motorways, carrying vehicles (electricity) at high speed (high voltage) across the country. This is the network of big pylons and overhead lines you see around the country.

The distribution networks are the local roads, connecting motorways with communities to help vehicles complete their journey. These are the smaller pylons (and underground cables) carrying lower voltage lines.

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This page is a part of OSHA's commitment to provide employers and workers in the electric power generation, transmission, and distribution industry with information and assistance to help them comply with OSHA standards and ensure a safe workplace.

The core difference between transmission and distribution power lines is that transmission power lines are for long-distance, high-voltage electricity transportation, whereas distribution power lines are for shorter distances and lower voltage electricity transportation. To illustrate, transmission lines are the power lines you might see on the side of the freeway, while distribution lines can be seen on the sides of streets.

Both transmission and distribution power lines serve separate purposes in bringing electricity from the source of generation to the end consumer in homes and businesses across the country. In order to reach residential, commercial, and industrial customers, electricity goes through transmission and distribution lines on its energy grid journey. As the electricity travels from one type of power line to another, it is passed through transformers, which modify the voltage in order to minimize energy loss. The transmission and distribution of energy via the grid can be broken down into the steps below:

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Electricity is generated at power plants and moves through a complex system, sometimes called the grid. The grid includes electricity substations, transformers, and power lines that connect electricity producers and consumers. Most local grids are interconnected to maintain reliability and for commercial purposes, forming larger, more dependable networks that helps suppliers consistently produce the right amount of electricity to meet demand.

In the United States, the entire electricity grid consists thousands of miles of high-voltage power lines and millions of miles of low-voltage power lines. This network of power lines connects thousands of power plants to hundreds of millions of electricity customers across the country.

The origin of the electricity that consumers purchase varies. Some electric utilities generate all the electricity they sell using just the power plants they own. Other utilities purchase electricity from other utilities, power marketers, and independent power producers or from a wholesale market organized by a regional transmission reliability organization.

The retail structure of the electricity industry varies from region to region. The company selling you power may be:A not-for-profit municipal electric utilityAn electric cooperative owned by its membersA private, for-profit electric utility owned by stockholders (often called an investor-owned utility)

Power plants generate the electricity that is delivered to customers through transmission and distribution power lines. High-voltage transmission lines, such as those that hang between tall metal towers, carry electricity over long distances. Higher voltage electricity is more efficient and less expensive for long-distance electricity transmission. Lower voltage electricity is safer for use in homes and businesses. Transformers at substations increase (step up) or reduce (step down) voltages to adjust to the different stages of the journey from the power plant on long-distance transmission lines to distribution lines that carry electricity to homes and businesses.

At the beginning of the 20th century, more than 4,000 electric utilities operated in isolation from each other. As the demand for electricity grew, especially after World War II, utilities began to connect their transmission systems. These connections allowed utilities to share the economic benefits of building large and often jointly owned power plants to serve their combined electricity demand at the lowest possible cost. Interconnection also reduced the amount of extra generating capacity that each utility had to hold to ensure reliable service during times of high and peak demand. Over time, three large, interconnected systems evolved in the United States.

The stability of the electricity grid requires electricity supply to constantly meet electricity demand, which in turn, requires numerous entities that operate different components of the grid to coordinate with each other. Local electricity grids are interconnected to form larger networks to maintain reliability and for commercial purposes. At the highest level, the U.S. power system in the Lower 48 states is made up of three main interconnections, which operate largely independently from each other with limited transfers of electricity between them:

The Eastern and Western Interconnections in the United States are also linked with Canada's power grid. The network structure of the interconnections helps maintain the reliability of the grid by providing multiple routes for power to flow and allowing generators to supply electricity to many load centers. This redundancy helps prevent transmission line or power plant failures from causing interruptions in service to retail customers.

The three major grid interconnections describe the large-scale physical structure of the grid. The regional operation of the electric system is managed by entities called balancing authorities. They ensure that electricity supply constantly matches power demand. Most of the balancing authorities are electric utilities that have taken on the balancing responsibilities for a specific part of the power system. All of the regional transmission organizations in the United States also function as balancing authorities. ERCOT is unique because the balancing authority, interconnection, and regional transmission organization are all the same entity and physical system.

A balancing authority ensures that electricity demand and supply are finely balanced to ensure the grid is safe and reliable. If electricity demand and supply fall out of balance, local or even widespread blackouts can result. Balancing authorities maintain appropriate operating conditions for the electric system by ensuring that enough electricity is available to serve expected demand, which includes managing electricity transfers with other balancing authorities.

Electric utilities are responsible for maintaining the safety of their systems and planning for the future power needs of their customers. Initially, the electric power industry developed voluntary standards to ensure they coordinated with linked interconnections. Today, mandatory reliability standards for planning and operating power systems and for addressing security concerns at critical electrical infrastructure are in place. The North American Electric Reliability Corporation developed and enforces mandatory grid reliability standards approved by the Federal Energy Regulatory Commission (FERC). In Canada, Canadian regulators fill this role.

Construction of electricity infrastructure in the United States began in the early 1900s and investment was driven by new transmission technologies, central-station generating plants, and growing electricity demand, especially after World War II. Now, some of the older, existing transmission and distribution lines have reached the end of their useful lives and must be replaced or upgraded. New power lines are also needed to maintain the electrical system's overall reliability and to provide links to new renewable energy generation resources, such as wind and solar power, which are often located far from where electricity demand is concentrated.

We offer a wide range of services, from initial scoping and feasibility studies all the way through design, construction and ongoing maintenance. Our goal is to meet the unique needs of each project and provide flexible solutions that keep the power flowing smoothly, safely and reliably to millions of homes and business. Our adaptable resources and innovative approaches have played a pivotal role in some of the world's most ambitious transmission and distribution projects, including ElecLink, a remarkable 65km electricity cable between France and Great Britain through the Channel Tunnel. This groundbreaking interconnector significantly enhances the energy capacity and security of both nations.

We provide a comprehensive range of services covering all transmission and distribution needs. Our expertise extends from Low Voltage (LV) to Extra High Voltage (EHV) cabling, with a particular emphasis on 132kV to 400kV projects. Watch our short video to learn how we have used innovative solutions to lay a 33kv cable.

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