The electrical grid in North America is a combination of various elements that provide a means of taking electrical power from where it is produced to where it used.
The electrical grid is defined as an interconnected network of power stations, transmission lines, and distribution stations that transmit electrical power from suppliers to consumers. It provides a point of connection for renewable energy systems that produce electrical power.
The long-distance movement of electricity is referred to as transmission; the transmission system moves bulk electricity from the transmission substations located at electrical power plants and renewable energy systems to distribution substations.
An electrical substation is a network of switching equipment, control equipment, and transformers that are used to convert the voltage to a different level. Substations form the basic interface to the transmission system.
After the distribution substation, the final delivery system to the consumer is called the distribution system.
Figure 1 Multiple Three-Phase High- Voltage Transmission Lines for Moving Extra Power
When larger renewable energy systems are added to the grid, high-voltage transmission lines may need to be doubled to move the extra power.
Because the transmission company owns the adjacent land, it is more cost-effective to place the additional high-voltage transmission lines in parallel with an existing set.
Control of the Grid
The grid control system provides a means of turning on and off sections of the grid to isolate one or more sections when there is a problem, and to move electrical power from one section of the grid to another when the demand changes.
In the United States, the three major grid sectors are isolated so that a problem in one sector can be contained and not affect the other two.
Control centers in sectors and subsectors monitor the grid continually to ensure that sufficient electricity is available and that blackouts are limited to a small area if they do occur.
In recent years, switching equipment on the grid has been upgraded with sophisticated computer and network controls that allow more efficient operation of the grid.
This technology is being placed in the substations at industrial users and commercial users to allow the grid controllers to determine which loads can be disconnected if there is a problem anywhere on the grid.
These more sophisticated control systems allow load sharing, which lets the grid control companies move electricity from area to area as the load shifts throughout the day.
All of these controls are being integrated into something that is now being called the smart grid.
While there is some disagreement as to the definition of the smart grid, it generally refers to the electrical distribution technology that uses computer-based remote control and automation to improve the delivery efficiency of electricity and provide information to the customer to optimize electrical power use.
Electrical Power Transmission System
The present power grid has roots in the early days of electrical generation. One of the biggest obstacles facing the early producers of electrical power was how to get the power to consumers efficiently.
Multiple power plants needed to be connected together so that their total output would be available as the demand for the electrical power changed over time from distant locations.
Another issue was how to maintain continuous power to consumers when power plants needed to be disconnected from the grid for maintenance or other conditions.
Cooperation between producers was needed to control the movement of electrical power through the transmission lines and to provide maintenance for the growing complexity of the generation and distribution system.
Figure 2 shows the North American Electric Reliability Corporation’s (NERC’s) interconnections.
You can see that the grid in the map consists of four major interconnections: Quebec, Eastern, ERCOT (Electric Reliability Council of Texas), and Western.
Each of the subsections of each interconnection is listed in the legend at the bottom of the map. The Quebec Interconnection, the Eastern Interconnection, and the Western Interconnection all include large sections of Canada.
Figure 2 North American Electrical Interconnections for the Electrical Grid
In nearly all cases, the voltage from various sources is stepped up with a three-phase transformer for transmission, and then it is stepped down in substations located near the customer.
The voltages used for the transmission system are separated into different voltage levels for long-distance transmission. In North America, one of the highest voltages used is 765,000 V. Other lines transmit voltage at 500,000 V, 345,000 V, and 230,000 V.
Typically, higher voltages are used for longer distances because they are more efficient. For large commercial and industrial customers, the three-phase voltage is usually supplied at high voltages, and the customer provides the transformers to reduce the voltage to the final value.
For residential and smaller commercial customers, the three-phase is stepped down by the electrical power company and converted to single-phase. In some areas of the world (some parts of Europe, for example), three-phase is supplied to houses.
Figure 3 illustrates the basic process from the generating sites to the transmission system (green), to the distribution system (red).
Figure 3 Basic Electric Power Generation, Transmission, and Distribution Systems. Distribution substations reduce the voltage and send it to the end users.
Figure 4 Early Electrical Grid for Electric City Trolleys
One of the earliest uses of electricity and a grid was to provide electrical power for electric trolleys in larger cities. (The photograph shows an electric trolley in current use in San Diego, California.)
With the first electrical production, transmission, and distribution, both dc and ac were sent on separate lines.
Customers who required dc received their power on one set of lines, and customers who needed ac received their power on a separate set.
The actual transmission lines, towers, transformers, and switchgear are owned by individual companies, which are called transmission companies (transcos) that manage this hardware.
Grid companies (gridcos) are companies that manage the grid function, which is the interconnecting and routing of electricity through the electrical system. Some parts of the grid and grid management may be controlled by some local or regional utility companies.
Electricity is often treated as a commodity, and it is bought and sold on short- and long-term contracts. Some of the ownership of the equipment and the sale of electricity are regulated by government agencies.
Electrical Power Distribution
The final step in getting electricity to the customer is distribution.
A distribution substation is a large set of transformers and switching equipment that is located at the edge of a city or near an industrial site that steps down the voltage from the transmission site to a lower voltage for distribution.
Feeder lines transmit this lower voltage to the customers’ sites, either on overhead lines or buried lines.
Service systems near the customers include the transformers that drop the voltage level to the final delivered voltage and provide the service drop.
The last portion of the grid includes any wiring at the customers’ sites and grounding to a local earth ground.
The grounding system for electrical power systems is the earthing system used as a safety measure to ensure that certain parts of a system are at the same electrical potential as the earth’s surface.
A grounding system generally consists of one or more copper rods that have been inserted into the earth to a depth of 2.4 m (8 ft).
These copper rods are bonded or connected with heavy copper conductors to parts of the system, such as metal cabinets, to keep them at ground potential.
If more than one rod is used, a copper conductor connects each of copper ground rods together so they are all at the same potential.
The ground system is used for two reasons.
Proper grounding protects workers from electrical shock in case the frame becomes “hot” because of damage or a system failure.
The ground provides a low-resistance electrical path to the earth between any metal in the system frame and the ground rods, thus causing high current and a circuit breaker to trip.
If the system were not grounded, and the circuit breaker did not trip, anyone who came into contact with any metal parts could receive a severe electrical shock or be killed by electrocution.
If you come upon a circuit breaker that trips open, you must test the system to see if a short circuit has occurred before restoring it.
When you close the circuit breaker and it immediately opens again, you should suspect a short circuit somewhere in the system, and you must determine the cause before trying again.
The second reason a system is grounded is to help protect the system from damage in the event of a lightning strike or other fault.
If a renewable energy system is struck by lightning, a large amount of electrical energy in the lightning bolt must be dissipated quickly into the earth, where it will not cause damage.
If the electrical system or its structures are not grounded properly, the location where the lightning strike enters the system can be severely damaged.
The ground system must be checked periodically to ensure that it is connected correctly and that all connections provide a low-resistance path to the earth.
In some installations, additional lightning arrestors may need to be added to protect the system from lightning strikes.
Providing a grounding system is required by the electrical code, and it must be maintained and inspected annually to ensure that it is operational.
Ground Fault Interrupt Circuit Breakers
An important safety feature (such as protecting personnel from serious electrical shock while they work near wet locations) is the ground fault interrupt (GFI) circuit breaker.
This special circuit breaker can trip if the hot and neutral current differs by more than a few milliamps, which can occur when a person is receiving an electrical shock.
The circuit breaker consists of a small current transformer that encloses parts of the hot and neutral lines.
When the lines have the same current, the fluxes from each cancel in the core. If they differ, a sensitive circuit detects the difference and trips a breaker. This feature is important in certain renewable energy systems where personnel are working in wet conditions, such as a rooftop.
If a person is receiving an electrical shock, the current in the person returns to ground, rather than through neutral, and causes the hot and neutral lines to differ, thus tripping the GFI breaker. While a shock may still be the result, it will be very brief and less likely to be lethal.
A serious problem can occur when high-voltage lines come into contact with each other, or when one of the lines contacts a lower-voltage feeder line or grounded metal parts. Either situation can occur during a storm (a tree falling across lines, for example).
The low-voltage systems could be subject to a high voltage, which can cause a condition called flashover.
Flashover is a serious hazard and can cause a fire or other serious result. A properly grounded neutral can force most of the current through the neutral line to ground, which can trip breakers and help avoid more serious consequences.
Electrical Power Utilization
Electrical power is routed to the customer either on overhead or underground wiring. Figure 5 shows an overhead connection with all of the switchgear and metering.
When electrical power is transmitted overhead, it can be overloaded slightly without damaging the wiring, but it is exposed to storm damage.
Figure 5 Overhead Connections to a Commercial Building
Power can be installed underground by a special machine (see Figure 6) that installs a conduit underground without digging a trench. It installs sections of pipe as it drills horizontally approximately 3 ft below the surface.
When the drill and pipe reach the destination, the pipe is withdrawn one segment at a time, and it pulls the plastic electrical conduit into the hole it has created. The conduit has a pull wire installed inside it to enable pulling the electrical cable through the conduit.
Underground wiring is used in congested areas and where unsightly wiring is not wanted, but it costs much more to install. Frequently, the spare wiring is installed in case a fault develops.
Figure 6 Underground Power Installation. A specialized machine buries conduit underground for running electrical power below the surface.
- What are three segments of the grid?
- What is the interface to the transmission system?
- What is the distribution system?
- Why is the voltage level raised for long-distance transmission on the grid?
- What does GFI stand for?
- Power stations, transmission lines, and distribution stations
- Substations form the basic interface to the transmission system.
- The delivery system from the substations to consumers
- Voltage is increased so current can be lower, which reduces both power loss and allows smaller wire.
- Ground fault interrupt