This section discusses the nonlinear behavior of transformer cores under protective conditions. When a large current flows through the primary winding, the secondary current does not respond linearly, leading to an overall error in the transformer's performance. The concept of accuracy class and accuracy limit is introduced to help understand how these errors affect the system. Based on real-world engineering examples, appropriate protection current transformers are selected to meet the required standards and ensure reliable operation.
Selecting the correct accuracy class for a protection current transformer (TA) is crucial. The TA works in conjunction with relay protection systems to send signals that trigger circuit breakers during short circuits or overloads, ensuring the safety of the power system. Unlike measuring TAs, which operate within a normal current range with acceptable accuracy, protection TAs are designed to saturate quickly when exposed to high fault currents. This saturation protects connected instruments from damage. These TAs typically operate at several times the normal current level, and their errors—both current and phase—must remain within specified limits. The combined error is often used to evaluate both current and phase differences.
When the primary current (i1) is small, the secondary current (i2) changes in a linear fashion. However, as i1 increases beyond a certain threshold, the magnetic core becomes highly saturated. Due to the nonlinear properties of ferromagnetic materials, the excitation current (i0) contains significant harmonic components, resulting in a distorted waveform that is far from a pure sine wave. Even if the primary current is ideal, the secondary current may not be sinusoidal, further contributing to measurement inaccuracies.
Understanding the behavior of current transformers under various load conditions is essential for proper system design. Engineers must carefully select the right type and accuracy class based on expected fault currents and system requirements. Proper selection ensures that the transformer performs reliably during critical events, maintaining the integrity and stability of the electrical network.
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