The Transmission System and Its Function

Many people are surprised to discover that electricity, due to its inherent properties, cannot be economically stored in significant quantities (apart from what is contained in a battery).

This implies that electricity must be generated and supplied precisely when it is required. The Transmission System, which transports electricity to you at the remarkable speed of 186,000 miles per hour (nearly the speed of light), enables this process.

Transmission System

As presented in Understanding Transmission, the electric system encompasses generation, transmission, and distribution. The need for bulk transmission emerged as the demand for electricity increased, and small power plants, which could only serve their local areas, became insufficient. Larger, newer power plants were established, but they were often far from their load centers. Transmission lines became the essential means to deliver power to the required locations.

Connecting remote generation plants with customers posed a minor challenge: electricity must be transmitted through wires, which create resistance to energy flow, resulting in small energy losses. While not significant over short distances, the longer the wire, the greater the resistance and the higher the energy losses.

The solution to this resistance issue is to increase the voltage (or "pressure") at which electricity is transmitted through the wires. Higher voltage enhances the system's ability to overcome resistance and minimize losses. Thus, today, as energy travels hundreds or thousands of miles from its generation point, high-voltage lines of 230, 500, or 765 kilovolts ensure electricity is delivered quickly and with minimal energy loss.

Why Towers?

While electricity can sometimes be transmitted underground, “bulk” transmission systems often utilize overhead wires. A common question during the planning process is why such large steel towers are necessary. The two primary reasons are safety and reliability.

Due to the high voltages involved, local, state, and federal regulations impose specific requirements on the construction of transmission lines, primarily for safety reasons. One key requirement is the minimum height off the ground that the wires must be at their lowest point, known as “clearance.” Clearance requirements can vary widely, but a range of 60-150 feet is typical.

Height requirements also necessitate stability. Transmission lines and towers must withstand various environmental challenges, from high winds to freezing temperatures, where ice and snow deposits could otherwise cause a line or tower to collapse. Consequently, high-voltage towers are usually designed to endure so-called 50 or 100-year storms to ensure that weather conditions do not disrupt the flow of electric service.

Inside the Wires

Power is transmitted through the wires via alternating current or direct current. Both methods have their advantages; however, “three-phase alternating current” is the most common method used globally.

In alternating current (AC) transmission, the movement of the electric charge periodically reverses direction. In a three-phase AC system, the wires carry three alternating currents that reach their peak values at different times.

Three-phase systems can be classified as single or double circuit systems. A double circuit means that the transmission structure carries two sets of transmission lines, each with three conductors (wires).

In direct current (DC) systems, the flow of electric charge is only in one direction. The system operates at a constant maximum voltage, which allows existing transmission line corridors with equally sized conductors to carry 100% more power into a high-consumption area than AC.

Three-phase AC systems are generally considered less costly than DC systems for shorter distances (fewer than 400 miles). AC also offers advantages in stepping up and stepping down (see below), making it a better alternative when there are several intermediate connections along the route to serve communities.

For longer distances, and even for shorter distances where there are no intermediate taps, DC systems offer two main advantages in addition to their ability to deliver substantially more power. First, they are less costly to build because they don’t require as many wires as three-phase systems. Second, they are more efficient at preventing electrical losses due to resistance in the lines. Third, DC systems provide reliability benefits. For example, changes in load that could cause portions of an AC network to become unsynchronized and lead to cascading failures in the grid would not have the same effect on a DC system. In such scenarios, the DC link could stabilize the AC network.

However, DC systems have disadvantages, particularly concerning cost and the equipment needed to step up and step down the voltage. Despite these drawbacks, many power system operators are considering the broader use of DC systems due to their overall benefits.

Transmission System Requirements

The transmission system must meet the following requirements:

Enable the engine to remain disconnected from the road wheels until engagement is desired.
Allow the engine to smoothly and gradually connect to the road wheels without jerking when running.
Provide variable leverage between the engine and the road wheels to accommodate different conditions such as starting from rest, maintaining a constant speed, or climbing hills.
Enable reduction in engine speed as needed.
Facilitate a 90-degree change in drive direction.
Allow inner and outer road wheels to run at different speeds when the vehicle moves on a curved path.
Provide relative motion between the engine and the road wheels to compensate for uneven road surfaces.

Stepping Up and Stepping Down

While electricity traveling through high-voltage wires may have a force of 230, 500, or 765 kilovolts, that’s not how the flow begins at the generation source, nor how it ends when it reaches your house. In fact, it wouldn’t be safe in either case if that were true.

Within the transmission system, substations and transformers play crucial roles in stepping up the voltage from the generator to the bulk transmission lines, and stepping it down from the transmission lines to the local lines that distribute power to your home.

Electricity initially leaves the power plant source at around 20 kilovolts. Transformers then raise the voltage to the appropriate level for transmission, similar to how a pump increases water pressure in a pipe.

As electricity reaches a load center, the local utility distributes it to neighborhoods and businesses by reducing the voltage through substations and sending it along a network of feeder (or distribution) lines. Primary distribution lines typically operate between 2.4 and 34.5 kilovolts. The voltage is further reduced through distribution transformers to residential levels of 120 and 240 volts.

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