Conditions affecting transformer error - News - Global IC Trade Starts Here Free Products

inductance

This article discusses the nonlinear behavior of transformer cores under protection conditions. When the primary current is large, the secondary current does not change linearly, leading to a composite error in the transformer. The concept of accuracy class and accuracy limit is introduced to help engineers better understand the performance requirements of protective current transformers. Based on an engineering example, the selection of a suitable protection transformer is demonstrated, and the results meet the required standards.

When it comes to selecting the accurate stage for a protection current transformer (TA), it's important to understand its role in the power system. The TA works alongside relay protection devices to signal them when a fault occurs, such as a short circuit or overload, allowing the system to isolate the faulty section and maintain safety. Unlike measuring transformers, which operate within a normal current range with high accuracy, protection TAs are designed to saturate quickly during fault conditions. This prevents damage to measuring instruments caused by excessive short-circuit currents.

The working principle of a protection TA differs significantly from that of a measuring TA. Under normal conditions, the primary current is within an acceptable range, and the transformer operates accurately. However, when the current increases beyond a certain threshold—often several times the rated value—the core begins to saturate. At this point, the excitation current contains significant harmonic components, causing the secondary current waveform to become distorted and non-sinusoidal. Even if the primary current is a perfect sine wave, the secondary current may not be, which affects both the magnitude and phase of the signal.

To ensure reliable operation, the error (both current and phase) must remain within the specified limits outlined in the error curve. The combined error is used as a key parameter to evaluate the overall performance of the current transformer during fault conditions. Proper selection of the accuracy class is crucial to ensure that the transformer can provide accurate signals for relay operation while also protecting the system from potential damage.