U.S. Department of Energy - Energy Efficiency and Renewable Energy
Tribal Energy Program
North American Power Grids
North America has a number of independent electrical grids. These grids connect power generation plants that supply energy to high-voltage lines. These high voltage lines carry electricity to substations. Substations decrease the voltage for further distribution to medium-voltage distribution lines.
The generators, power lines, substations and power distribution system are the responsibility of various utility companies working together under various regional standards to keep each grid operational.
In the eastern half of North America, the grids often have super-high voltages (as high as 765,000 volts) that connect distant large generating stations to densely populated urban centers. Texas has its own grid under Texas law and custom. Florida, because it is a peninsula, is only connected at its northern border. The various Hawaiian Islands and various parts of Alaska have their own grids.
The western half of North America, including part of Mexico, has one very large grid called the Western Interconnect. Power can flow from British Columbia to Kansas, to California, or from Mexico to Wyoming to Alberta, and to all places between (in either direction). A tree falling in Oregon has, in the past, taken out the power systems in many far away states.
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How the Electrical Grids Work
In the power grid system, energy is used in real-time as it is generated. In other words, on each grid, energy is being generated and used constantly, in the same amounts. Supply and demand on the grid system must remain in balance. There is very little ability to store electricity, and it is difficult for the grid to accommodate large, rapid changes in use and generation. Power usage is called the "load" or "demand<". Obviously, this fluctuates quite a bit — both daily and seasonally — but with many cities connected together, the aggregate, or diversified demand, behaves in a relatively predictable manner.
Variations in demand present a management challenge: System operators must generate as much power as is needed at any given time, at many different places. Transmission lines simply accommodate the flow. Power supply is controlled by ramping the power up or down in generating plants. Of course, it is also possible to "drop" the load (stop supplying power to the customer), but this is used as a last resort.
There is a substation on the grid at each location where there is a connection of one facility to another, such as between generation and transmission lines, from one transmission line to another, or between transmission lines and groups of loads. Substations can adjust the voltage levels up or down or disconnect or reconnect portions of the grid. Substations also have equipment to meter the flow, to protect the grid, and to provide information about status and traffic on the grid.
Connections are done by physically closing breakers, which are like gates that allow the flow of electricity. To disconnect, the breakers are physically opened, which prohibits the flow. Transformers step up or step down the voltage. As a general rule, higher voltages are used for moving power longer distances or for moving larger quantities of power. Low voltages are used for the local delivery of power. Small pole-top transformers change the distribution voltage to the lower voltage used in homes and most businesses.
Metering information is used by utilities for billing and accounting purposes. It is used to understand the flow of power in remote control centers and to adjust the generation to the load. Each utility manages its own part of the grid, called a "control area," according to industry standards.
How the Electrical Grids are Managed
Facilities making up the grid are individually owned. They are parts of a whole enterprise, designed not only to provide power, but also to make money (or sometimes to serve customers while breaking even). Methods for valuing each component of the grid, and for accounting for and valuing the transactions taking place on the grid. have evolved over the years. The accountings and valuations do not always have a true relation to the physical reality of the power flowing on the grid.
The right to add energy onto, or to use energy from the grid, is based on agreements. The agreements include tariffs, contracts and rates. Tariffs are electric utility rules for use of their systems. Tariffs are approved by regulators and comply with regional standards. Contracts are established between generators, transmission owners, utilities, and some large users, and create the terms for use of the systems. Contracts can be long-term or short-term. Rates are the prices paid for system use. Rates must be approved and applied consistently. They can be based on complex accounting for system costs, or in some cases can be based on market prices (that is, whatever price can be negotiated).
Once a contract is in place, under a tariff and rate structure, system users have the right to schedule power up to the terms of their contracts. Scheduling is the process of determining power needs and communicating them to control areas on, first a monthly basis, then adjusting the monthly schedules on a daily basis based on assumed power needs, then further adjusting the requirements on an hourly basis. Today, scheduling is done on an Internet-based system. Power is then "dispatched" to the user as needed.
There are special challenges in accounting for variable resources, such as some renewable energy sources. Wind and solar power are produced only when the wind is blowing or sun is shining, so steady, predictable wind and solar resources are more valuable than spotty, dynamic resources. However, the grid can currently absorb a certain amount of intermittent resources without difficulty. Advanced management and control systems are being developed to handle these dynamics in the long run.
System operators can adjust schedules under the tariff or contract, to make sure system constraints are honored. They may also purchase or call for "redispatch", which is the ramping up or down of generators to make sure that transmission line capacities and loads are balanced. They monitor and manage transmission line maintenance. This may require removing a line or substation from service. This must be carefully planned, as each such outage or change in the system configuration will impact the flow of power over other parts of the system.
System operators also deal with the frequent emergencies caused by storms, trees falling into wires, wildfires, human error, and other causes.
How the Electrical Grids are Governed
The electrical grids are governed by a variety of international, national, regional, and state bodies. International treaties govern certain electrical entitlements and operational matters between the United States, Canada, and Mexico.
The Federal Energy Regulatory Commission (FERC) was created by the Federal Power Act and has jurisdiction over interstate transactions and facilities. Even if they are within one state, the high-voltage facilities are generally deemed to be interstate facilities because of the interconnected nature of the grid. FERC approves utility tariffs, establishes regulatory policies governing the grid, and decides general policy matters through rule makings in which all interested parties may voice their opinions. FERC also approves company tariffs and agreements and rates of return on transmission transactions. FERC also resolves disputes by hearing protests and interventions of interested parties in FERC dockets. FERC also approves the rates of the federal Power Marketing Administrations, which distribute power generated from large government-owned hydropower plants.
The North American Electric Reliability Council (NERC) is a voluntary association of utilities that governs technical electrical engineering standards to make sure the grid is reliable. Regional NERC bodies, such as the Western Electrical Coordinating Council (WECC), set standards for each interconnected grid and oversee engineering studies for system changes and additions. This is because any change to the system has the potential to impact all other system operations.
States, and tribes (to the extent they choose to exercise their sovereign power under the law), can control the siting of transmission and generation projects and matters that are particular to the state or reservation. This includes the distribution of energy from the transmission system.