Title:Understanding the Working Characteristics of Current Transformers and How to Check Saturation

As technology evolves, current transformers have become increasingly popular in many industries, especially the power industry. However, in recent years, incidents of incorrect differential protection due to current transformer saturation have been reported in several power plants in Guangdong Province, China. This is a serious issue that needs to be addressed to ensure reliable and accurate protection.

To understand why saturation occurs and how it affects protection, we must first look into the working principle and characteristics of a current transformer. Current transformers function similarly to transformers and can be analyzed using equivalent circuit theory. During normal operation, the magnetizing impedance is high, and the leakage impedances are low, allowing a nearly linear relationship between the primary and secondary current. However, when the magnetic flux density in the core reaches saturation, the magnetizing impedance significantly drops, resulting in a non-linear relationship between the primary and secondary current. This saturation can occur due to high primary currents, high secondary loads, or a combination of both.

To test for saturation, one of the most direct methods is to load the secondary with actual resistance and gradually increase the primary current until saturation occurs. However, for protection-grade transformers, the saturation point could be 15 to 20 times higher than the rated current, making on-site testing difficult. Another method is the voltage-current characteristic (V-I) test, which can be used to determine the saturation voltage and the corresponding saturation current. During this test, the primary is disconnected and the secondary is connected to a voltage source with a known impedance. By gradually increasing the current, the saturation point can be determined by observing the saturation voltage.

To ensure reliable protection, it is crucial to select the appropriate current transformer and perform necessary tests to check for saturation. In addition, it is recommended to increase the secondary cable section or replace the transformer with a larger capacity unit if saturation occurs during testing. By following these measures, accurate and reliable protection can be achieved.

In conclusion, understanding the working principle and characteristics of a current transformer and testing for saturation are crucial steps in ensuring reliable protection. By using appropriate methods, such as the V-I characteristic test, we can effectively check for saturation and take necessary measures to prevent incorrect protection.