How Power Distribution Systems Work?
Electrical power supplied to residential, commercial, and industrial buildings is typically generated by a utility at a centralized location and then transmitted and distributed through the utility’s power transmission and distribution system to its end users.
A utility's power transmission and distribution system is responsible for controlling, protecting, transforming, and regulating electrical power to ensure safe delivery to consumers. This system begins at the point of power generation and generally ends at the metered service entrance of a building, where the building’s own electrical distribution network starts. A utility power transmission and distribution system comprises several components, including transmission substations (step-up transformers), transmission lines, distribution substations (step-down transformers), and distribution lines (see Figure 1).
Transmission Substations
Transmission substations are outdoor facilities within the utility system used to alter voltage levels, centralize system switching, monitoring, and protection, and redistribute power. Typically, these substations operate at high voltages (HV) ranging from 69 kV to 345 kV and extra-high voltages (EHV) exceeding 345 kV. They also adjust the number and size of lines that extend from the station.
Transformers
Transformers are electrical devices designed to convert voltage from one level to another using electromagnetism. In power distribution systems, they facilitate the safe and efficient stepping up or stepping down of voltage levels. While transformers adjust voltage, they do not change the overall amount of power delivered—other than minor losses due to heat, the power entering and leaving a transformer remains equal. Rated in kVA, transformers enable utilities to distribute significant amounts of power cost-effectively (see Figure 2).
Transformers help reduce costs by allowing power to be transmitted over long distances with smaller conductors, as increasing voltage lowers current. This practice results in smaller, more economical power lines, with generated voltages being stepped up for long-distance transmission and stepped down closer to the end-user. Although transformers can step both current and voltage up or down, the terms "step up" and "step down" always refer specifically to voltage.
Transmission Lines
Transmission lines are aerial conductors that carry large amounts of electricity over long distances at high voltages. To ensure safety, these lines must be spaced adequately apart. Transmission voltage levels depend on the distance and the power being transmitted, with higher voltages used for longer distances or greater power loads.
Transmission lines operate across a broad range of voltages, from a few kilovolts to hundreds of kilovolts. The purpose of stepping up voltage in transmission lines is to transport large amounts of power using smaller conductors, as conductor size is determined by the current it must safely carry without overheating. For a given power level, current decreases as voltage increases, meaning smaller conductors can be used for high-voltage transmission .
Increasing transmission voltage reduces power losses between the utility generator and the final point of delivery, with voltage doubling potentially cutting power losses by up to 75%. Higher voltages also reduce conductor size and allow for lighter poles and towers spaced farther apart, further lowering construction and material costs.
Utility generators typically produce three-phase power, with high-voltage distribution lines grouped in sets of three. A neutral/ground conductor is routed above the power lines to help dissipate lightning strikes, grounding at every power pole and substation. The power is stepped up and down multiple times before reaching the end user.
Distribution Substations
Distribution substations are outdoor facilities positioned near the point of use. They adjust voltage levels, centralize monitoring, protection, and system switching, and redistribute power. While transmission substations handle high voltages, distribution substations work at lower voltages, generally outputting 12 kV to 13.8 kV.
These substations also provide a location to test systems, adjust voltage outputs, add or disconnect lines, and reroute power during outages or other distribution problems. The output from a distribution substation often produces multiple lines with different voltage levels, further distributing power toward end users.
Distribution Lines
Distribution lines carry electricity from a substation to the building’s service entrance. These lines form an interconnected network often referred to as the "grid," allowing power to be rerouted between utilities or switched to meet varying demands.
In this grid system, multiple lines work together to maintain reliable power delivery, ensuring that users have continuous access to electricity, even during system disturbances or maintenance activities.
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