This article aims to clarify some terms that are often misunderstood by novice engineers, such as primary and secondary systems, their equipment, and configurations. As an example, we will examine the transmission and distribution systems of North America. Electricity is generated from various sources, including fuels (such as coal, gas, nuclear, and oil), as well as renewable sources like water and wind.
Differences Between Primary and Secondary Distribution Networks (You MUST Know!)
As we all know, electricity is produced in power plants. These power plants utilize three-phase generators with ratings ranging from 50 MVA to 1300 MVA. The terminal voltages for these generators can be as low as a few kV or as high as 20 kV, depending on their age and size. The voltage levels are constrained by the capabilities of insulating materials.
Substations in power plants house transformers that lower the current and raise the voltage to minimize energy losses during transmission. These substations are equipped with surge arresters and circuit breakers to safeguard the transformers of the Generator Step-Up (GSU) unit and the buses.
Transmission & Distribution Systems
The transmission system serves three main functions:
1. Distributing the power generated by generators.
2. Facilitating energy exchange between utilities.
3. Powering the transmission and distribution system through the generator.
The transmission system consists of a network of three-phase transmission lines and transmission substations. Transmission voltages range between 230 kV and 765 kV. The ratings of single-phase and three-phase circuits range from 400 MVA for 230 kV up to 4000 MVA for 765 kV. Some transmission systems incorporate High-Voltage Direct Current (HVDC) with solid-state converters and back-to-back AC/DC links.
To connect bulk power substations with distribution substations in the sub-transmission network, substations, step-down transformers, and sub-transmission lines are utilized.
In certain cases, a sub-transmission can directly supply a single customer’s distribution load through a circuit breaker. Sub-transmission voltages range from 69 kV to 138 kV.
In distribution substations, step-down transformers are employed to convert sub-transmission voltages to primary distribution voltages ranging from 2.2 kV to 46 kV for local distribution. These transformers are linked to substation buses via surge arrester protection and circuit breakers, which then connect to three-phase primary feeders known as distribution circuits.
Each substation bus typically serves multiple feeders. The ratings of distribution substations range from 15 MVA in older installations to 200 MVA and higher in newer setups. Distribution substations may include equipment for regulating primary voltages, such as Load Tap Changers (LTC) installed on distribution substation transformers or stand-alone voltage regulators.
Feeders rated at 4.16 kV typically have a capacity of 4 MVA, while those rated at 13.8 kV are usually rated at 12 MVA. Feeders can be segmented into three-phase sections connected by sectionalizing switches or fuses.
Fuses are used to connect several single-phase laterals to each feeder section and to connect three-phase laterals with feeders. Separate primary feeds supply power to industrial or large commercial loads. Feeders, laterals, and overhead cables traverse streets and provide distribution transformers to lower voltage to secondary distribution levels (120-480V).
Distribution transformers, commonly rated from 5 to 5000 kVA, are mounted on utility poles or platforms for overhead cables, and in vaults or at ground level for underground cables. These transformers are protected against failure and overload by circuit breakers or fuses on the secondary and/or primary sides.
These transformers supply single-phase or three-phase power to residential and commercial customers through secondary mains and service conductors. Meters are used to measure kilowatt-hours consumed for billing and other data purposes.
This post was written by Justin Tidd, Director at Becker SMC! For nearly a half a century, Becker Mining has been at the forefront of industry safety. Becker/SMC is the industry’s leader in increasingly more sophisticated electrical control systems. Most of the major innovations, design features and specialized electrical components have been developed by Becker/SMC.
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