The Importance of Selecting the Correct CT Size for Fault Detection and Protection

Current transformers (CTs) hold a crucial role in electrical power systems, accurately measuring and monitoring current levels. The accurate sizing of CTs is paramount to ensure their suitability and reliable operation within specified limits. This article focuses on the calculation of CT size for dual-power overcurrent receptacles, aiming to provide effective protection and fault identification within power systems.

Significance of CT Size Calculation:

Accurate current measurements serve as the foundation for several critical functions within a power system, including fault detection, protection coordination, and equipment monitoring. The primary purpose of a CT is to transform high primary currents into secondary currents proportionate to the primaries. These secondary currents are then utilized by meters, protective relays, and other devices to measure the primary current.

Selecting the appropriate CT size is imperative to ensure the precision and dependability of these measurements. Undersized CTs are susceptible to saturation, resulting in inaccurate measurements due to their insufficient resolution.

The calculation of CT size is therefore essential to strike a balance between accuracy and operational integrity.

Exploring CT Sizing:

The subsequent sections delve deeper into the concept of CT sizing, shedding light on the Accuracy Limit Factor (ALF) and CT selection criteria.

This comprehensive exploration is tailored to provide practical insights into CT sizing guidelines and principles for dual-power overcurrent receptacles.

Section 1: Factors Affecting CT Sizing Calculations:

This comprehensive guide delves into the factors influencing CT sizing calculations and the considerations for optimizing CT selection. The discussion emphasizes the significance of selecting the appropriate CT ratio to ensure precise current measurements and reliable protection functionality.

The article also examines the factors contributing to the determination of the correct CT secondary current.

Additionally, the article explores the significance of the short-circuit level in CT design and sizing, which is determined by fault current and duration. This parameter plays a pivotal role in CT sizing calculations. The impact of the short-circuit level on CT design is discussed, highlighting the necessity to consider thermal withstand capability and time ratings.

Furthermore, the article delves into the determination of accuracy classes for CTs in accordance with the IEC 61869-2 Standard. Various accuracy class designations are explored, elucidating how accuracy classes dictate permissible error limits. The article also addresses CT ratings and burden considerations. The CT rating, expressed in VA, determines a CT’s capacity to handle a specific load.

The article discusses the importance of matching the CT load with the CT nameplate rating to prevent saturation and ensure accurate performance. The role of saturation voltage (V Sat) in CT performance is also examined.

Understanding the relationship between apparent voltage and saturation voltage of the CT secondary winding is crucial in selecting the appropriate CT size and maintaining accurate current measurements.

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 safety, producing the best ground fault relays in the industry. 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.