Introduction

Sizing current transformers

Current transformers (CTs) are crucial components in power systems, playing a vital role in measurement and protection applications. They're designed to step down high alternating current (AC) values to more manageable levels, making it safe for instruments and protective relays to monitor and measure electrical systems. In this post, we'll dive into the world of current transformers, exploring their equivalent electrical schematics, accuracy ratings, and performance characteristics like excitation curves and saturation.

Equivalent Electrical Schematics of a Current Transformer

A CT is a passive electrical device consisting of primary and secondary windings. It operates on the principle of magnetic induction. The equivalent electrical schematic of a current transformer includes: Primary winding (Ideal current source, IP) Secondary winding (Ideal transformer with turns ratio N1/N2) Excitation voltage (VE) Magnetizing impedance (ZE) Winding resistance (RS) Secondary voltage (VS) Load impedance (ZB)

Understanding the equivalent circuit can help analyze the performance of the CT, including its accuracy and phase displacement. The number of secondary turns is proportional to the amount of saturation a CT can withstand, with more turns resulting in a higher knee-point voltage and better saturation resistance (e.g. 2000:5 is better equiped to handle saturation than 50:5). The rated burden (B) is inversely proportional to the CT's accuracy, as a higher burden installation more stress on the secondary winding and increases the likelihood of core saturation. Additionally, the core material directly impacts the CT's saturation characteristics, with different materials offering varying saturation levels and permeability.

Excitation Current vs Excitation Voltage Curves for CTs

The excitation current vs excitation voltage curve, or the magnetization curve, is a characteristic curve that demonstrates the relationship between excitation current and excitation voltage under no-load conditions. The curve has three regions: Linear region: The core isn't saturated, and the CT operates with high accuracy. Knee-point region: The knee-point is where a small increase in excitation voltage causes a significant increase in excitation current. Saturation region: The core saturates, accuracy decreases, and phase displacement increases. To maintain accurate performance, CTs should ideally operate in the linear region, avoiding saturation. Current transformer manufacturers do not typically have the excitation plots on the data sheets as they can slightly change with updates the CT design (such as the core). The curves should be requested from the manufacturer when selecting a CT.

Reading Accuracy Ratings of CTs:

The accuracy rating of a CT is represented as a combination of the accuracy class and the rated burden. IEEE C57.13 has a defined notation. For example, 0.3B1.8: Accuracy class (0.3): Maximum error of 0.3% under specified conditions. Rated burden (B1.8): The maximum load in ohms the CT can support on its secondary winding while maintaining its specified accuracy (1.8 Ω). Includes the cable leads and the relay(s). The rating factor of a CT (defined by manufacturer) is a multiplier of the primary CT rating for which the accuracy class still applies. For example, let's consider a CT with a rated primary current of 100 A, an accuracy class of 0.3, and a rating factor of 2.0. This means that the CT can handle a continuous primary current of up to 200 A (100 A x 2) while still maintaining its 0.3% accuracy.

The Significance of 10% Error with Respect to Saturation and Secondary Voltage Ratings:

The 10% error, or composite error, is a parameter used to describe CT performance under saturated conditions. It is typically specified as a multiple of the CT's secondary voltage rating. For example, a CT with a 10% error specified at 20 times its secondary voltage rating saturates when the voltage across the secondary winding reaches 20 times the rated secondary voltage. In protection applications, it is crucial to select a CT with a high enough 10% error to ensure proper coordination between the CT and protection relay. Standard burdens (this is set): Secondary voltage rating Standard burden 10 B-0.1 20 B-0.2 50 B-0.5 100 B-1 200 B-2 400 B-4 800 B-8

Current transformers are essential in power systems, providing accurate measurement and protection capabilities. Understanding their electrical schematics, accuracy ratings, and performance characteristics is key to ensuring they function optimally in their intended applications. By considering factors like excitation curves, saturation, and accuracy ratings, engineers can select the appropriate CTs for various power system applications, ensuring safety and efficiency.