Utility Power Transmission and Distribution Systems
Electrical power used in residential, commercial, and industrial buildings is typically generated by a utility at a central point and transmitted and distributed to where it is required through the utility power transmission and distribution system.
A utility power transmission and distribution system controls, protects, transforms, and regulates electrical power so it can be safely delivered to the user. The utility power transmission and distribution system begins at the point of power production and normally ends at a building metered service entrance point, which is where the building distribution system begins. A utility power transmission and distribution system consists of transmission substations (step-up transformers), transmission lines, distribution substations (step-down transformers), and distribution lines (see Figure 1).
Figure 1. A utility power transmission and distribution system consists of transmission substations (step-up trans-formers), transmission lines, distribution substations (step-down transformers), and distribution lines.
Transmission Substations
A transmission substation is an outdoor facility located along with a utility system that is used to change voltage levels, provide a central place for system switching, monitoring, protection, and redistribute power. Transmission substations normally operate at high voltage (HV), 69 kV to 345 kV, and extra-high voltage (EHV), the voltage over 345 kV. Transmission substations are also used to make changes in the size and number of lines sent out from the station.
Transformers
The transformer is an electrical device that changes the voltage from one level to another by using electromagnetism. In electrical distribution systems, transformers are used to safely and efficiently increase or decrease voltage. Although a transformer can be used to increase or decrease voltage, transformers cannot be used to increase or decrease the amount of power available. Except for some minor power loss caused primarily by heat loss, the amount of power entering a transformer is the same amount of power leaving the transformer. Transformers allow utilities to distribute large amounts of power at a reasonable cost (see Figure 2). Transformers are rated in kVA, which specifies their power output capability.
Figure 2. Transformers are used in electrical distribution systems to safely and efficiently increase or decrease voltage to allow utilities to distribute large amounts of power at a reasonable cost. Image Courtesy of GE
The main advantage of increasing voltage and reducing current is that power may be transmitted through small gauge conductors, which reduces the cost of power lines. For this reason, the generated voltages are stepped up to high levels for distribution across large distances and then stepped down to meet user requirements. Though both current and voltage can be stepped down or up, when it comes to transformers, the terms "step up" and "step down" always refer to voltage.
Transmission Lines
A transmission line is an aerial conductor that carries large amounts of electrical power at high voltages over long distances. To be safe, transmission lines must be positioned far enough apart. The transmission voltage level is determined by the required transmission distance as well as the amount of power carried. A larger transmission voltage is chosen when dealing with longer distances or larger transmitted power levels (see Figure 3).
Figure 3. Transmission voltage increases with distance or transmitted power
There is a wide variety of transmission line voltages, ranging from a few kilovolts to hundreds of kilovolts. Transmission-line voltage is stepped up to allow large amounts of power to be transmitted using smaller conductors. Since conductor sizes are based on the amount of current they can safely carry without overheating, low current levels can be carried over small size conductors. The amount of current changes inversely with the amount of voltage for a given power level (see Table 1).
| Power (W) | Voltage (V) | Current (A) |
| 10,000 | 20,000 | 0.5 |
| 10,000 | 10,000 | 1 |
| 10,000 | 5,000 | 2 |
| 10,000 | 2,500 | 4 |
| 10,000 | 1,250 | 8 |
| 10,000 | 480 | 20.83 |
| 10,000 | 240 | 41.66 |
| 10,000 | 120 | 83.33 |
Table1. The amount of current changes inversely with the amount of voltage for a given power level.
In addition, increasing the transmitted voltage lowers the power losses between the utility generator and the final delivery point. Doubling the transmitted voltage can reduce the power loss by up to 75%. Because transmitting power at high voltages reduces the required size and weight of the conductors, the poles and towers that support the conductors can be smaller and spaced farther apart. Therefore, greater transmitted voltages allow for smaller conductor sizes, higher power transmission, and lower construction and material costs.
Utility generators that output three-phase power have high-voltage distribution lines arranged in groups of three. In addition to the power lines, a neutral/ground conductor is also routed with the power lines. The neutral/ground conductor is routed on top of power lines and used as a grounding wire to help dissipate lightning strikes. The neutral/ground conductor is grounded at every power pole and at the transmission and distribution substations. The voltage on the power lines is stepped up and down many times before it reaches the end-user.
Distribution Substations
A distribution substation is fundamentally an outdoor facility that is located near the point of electrical service use and is used to adjust voltage levels, provide a central place for system monitoring, switching, and protection, and redistribute power. Distribution substations take high transmitted voltages and reduce the voltage for further distribution. Transmission substations operate at higher voltages, whereas distribution substations operate at lower voltages. The output voltages of distribution substations typically range from 12 kV to 13.8 kV.
Distribution substations provide a location along the distribution system near the end-user to easily test the system, adjust voltage output, add new lines, disconnect lines, and redirect power during distribution system problems such as power outages caused by lightning strikes. See Figure 5. Distribution substations take the incoming power and, after changing the voltage level, produce multiple outputs with different voltages on each line.
Figure 4. Distribution substations provide a convenient place along with the distribution system for maintenance, checks, and line adjustments. Image courtesy of OSHA
Distribution Lines
Distribution lines are used to carry electrical power from a distribution substation to the building service entrance. Distribution lines connect parts of the system together and are often run in multiple lines so that electrical power can be switched to meet changing power requirements and switched between different utilities. The term "grid" is used to describe the network of interconnected transmission and distribution lines.
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