WAZIPOINT Engineering Science & Technology: Electrical Substation Equipment: Bus Bar and Its Functions

Sunday, July 16, 2023

Electrical Substation Equipment: Bus Bar and Its Functions

In a substation, a bus bar refers to a metal bar or a system of bars used to conduct and distribute electrical power within the facility. It serves as a central point for connecting and interconnecting various electrical components and devices, such as circuit breakers, transformers, disconnect switches, and other equipment present in the substation.

Why are Bus Bars Used in Electrical Substations?
Typical Bus-bar
Fig-Typical Bus Bar System

The primary purpose of a bus bar in a substation is to efficiently transfer and distribute electrical power between different parts of the substation and the power grid. It acts as a common electrical connection point, allowing for the smooth and safe transfer of electricity at various voltage levels.

Bus bars are typically made of copper or aluminum due to their excellent electrical conductivity and mechanical properties. These metals can handle the high current levels present in substations without significant losses or overheating. The bus bars are designed to handle specific current loads and are often configured in a way that minimizes the impedance and maximizes the efficiency of power transfer.

Types of Bus Bars Used in Electrical Substations

There are different types of bus bar arrangements, including:

Single Bus Bar: This configuration uses a single set of bus bars to connect all the components in the substation. While it is a simple design, it lacks redundancy, which means any fault or maintenance on the bus bar could result in a complete outage.

Double Bus Bar: This setup consists of two sets of bus bars, usually called "Main" and "Transfer" buses. The main bus is the primary connection for normal operation, while the transfer bus is used during maintenance or emergencies. It offers greater reliability and enables easy isolation and maintenance of equipment.

Ring Bus or Breaker-and-a-half: In this arrangement, multiple circuit breakers are connected in series to form a ring. The bus bars are connected at different points along the ring. It provides redundancy and flexibility in operation and maintenance.

Multi-Bus Bar: Large substations may have multiple bus bars with various configurations to cater to complex operational requirements, including backup and redundancy.

Overall, the bus bar is a critical component in a substation, facilitating the efficient and reliable transfer of electrical power, making it an essential part of the power grid infrastructure.

Typical Bus-Bar Arrangement Systems for High Voltage and Extra High Voltage up to 400kV Capacity Substations are discussed in a separate episode.

Copper and Aluminum Bus-bar Size 

The size of copper and aluminum bus bars depends on various factors such as the maximum current they need to carry, the ambient temperature, and the acceptable temperature rise. Bus bars are used to distribute electrical power in various applications, and their size is crucial to ensure they can handle the current without excessive heat generation or voltage drop.

When selecting the size of copper or aluminum bus bars, engineers often refer to standards or guidelines provided by electrical regulatory bodies or industry organizations. One such standard is the National Electrical Code (NEC) in the United States, which provides tables and formulas for determining the ampacity (current-carrying capacity) of bus bars based on their material, size, and temperature rating.

Here's a general approach to sizing copper and aluminum bus bars:

Determine the maximum current: Identify the maximum current that the bus bar needs to carry. This value is typically based on the total load or fault current of the electrical system.

Select the temperature rise: Decide on the acceptable temperature rise of the bus bar. The temperature rise is the increase in temperature of the bus bar due to the flow of current and is usually limited to a specific value (e.g., 30°C or 50°C).

Check the ampacity tables: Refer to the appropriate ampacity tables in standards like the NEC. These tables provide information on the maximum current a bus bar of a specific size and material can carry while staying within the acceptable temperature rise limits.

Account for any derating factors: Consider any derating factors that might apply to the specific installation. Derating factors account for factors like ambient temperature, proximity to other heat sources, and the number of conductors in the bus-bar system.

Size the bus bar: Once you have all the necessary information, select the appropriate size of copper or aluminum bus bar that meets the current-carrying requirements while staying within the allowed temperature rise limits.

It's important to note that bus-bar sizing is a critical task and should be performed by qualified electrical engineers or professionals familiar with electrical design and safety standards. Working with electricity can be hazardous, and ensuring the proper sizing and installation of bus bars is essential to maintain a safe and reliable electrical system.

The copper and Aluminum Bus-bar Size Selection Chart are discussed in another episode.

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