Electrical Power Distribution

The vast majority of electrical power is generated by local power plants using huge turbine-operated 3-phase generators fueled by fossil fuels such as coal or natural gas. These generators produce AC (Alternating Current) power at 12.47kV (15kV Voltage Class). In order to transport this generated power output to local industry and the general public, it must be able to go long distances with minimal losses. This generated power goes directly to a step-up transformer at the power plant and is increased to transmission voltage levels to travel long-distances across transmission power lines at 138kV to 500kV, 60Hz. This power then travels to substations with step-down transformers where it is reduced to 13.8kV, 60Hz. It is then sent to local distribution power lines with various types of transformers. These transformers vary in size depending on the voltage requirements for commercial or residential use.

There are advantages and disadvantages between higher transmission voltages vs lower transmission voltages, such as:

Advantages: Carries more power for a given current, less voltage drop for a given power flow, fewer line-losses, cover a much wider area, need fewer substations.

Disadvantages: Reliability (more customer interruptions), utility crews do not like working on higher-voltage systems, higher-voltage equipment costs more.

Therefore, the transmission voltage is a balance between distance, power requirements (demand), and cost.

In the US, standard Commercial power is 3-phase, 480/208Y/120 and 240/120 Delta VAC, 60HZ. Standard residential power is single-phase, 120/240 VAC, 60HZ. All voltages are nominal. Incoming commercial and residential power is fed to distribution panels where it is connected through circuit breakers to the local building wiring system. It is then ready for use by the customer.

Common Generator Voltage

In the U.S., the most common power plant generator voltage is 13.8kV, 60Hz, because it coincides with standard motor voltage ratings. In Europe, the most common generator voltage is 11kV, 50Hz. However, manufacturers of large generators can produce a wide range of voltages from 11kV to 28kV, 50Hz or 60Hz. It is cheaper to generate lower voltages and then step them up. Hence, the 13.8kV has become the standard generator voltage in the U.S.

Distribution Voltages

Most line-to-line distribution voltages are between 4 and 35kV. The four major “voltage classes” are 5, 15, 25, and 35kV. A voltage class is a term applied to a set of distribution voltages and the equipment common to them; it is not the actual system voltage.

For example, a 15kV insulator is suitable for application on any 15kV class voltage, which would include “system voltages” of 12.47kV, 13.2kV, and 13.8kV. Common power line and substation equipment such as cables, terminations, insulators, bushings, reclosers, and cutouts all have a voltage class rating. Only voltage-sensitive equipment like surge arresters, capacitors, and transformers have voltage ratings dependent on the actual system voltage.

Most utilities use the 15kV class of voltages. The most common 15kV voltage in its class is 12.47kV, which has a line-to-ground voltage of 7.2kV.

When the transmission voltage is stepped down to distribution level using a transformer with a wye secondary, the line-to-ground voltage is the line-to-line voltage divided by the square root of 3:

12.47kV / 1.73 = 7.2kV

Renewable Energy Sources

In many locations, we can see a tremendous amount of growth in renewable energy sources such as wind and solar power. This is another topic for discussion. However, these alternative energy sources are also capable of delivering the same amount of power to the electrical grid without the pollution caused by power plants and are less costly. The disadvantages are, they require a large number of generating systems and a great deal of real estate. Therefore, they are not a practical replacement for power plants. However, they can and do make a significant contribution to supplement our nation’s electrical power demand where wide open spaces are available.