Question

Magnetizing inrush current is rich in:

• A. \( 3^{\text{rd}} \) harmonics
• B. \( 5^{\text{th}} \) harmonics
• C. \( 7^{\text{th}} \) harmonics
• D. \( 2^{\text{nd}} \) harmonics

Hint:

The 2nd harmonic is significant in transformer inrush currents and is used in differential protection schemes to differentiate between inrush currents and internal faults.

Answer 1

The Correct Option is D

Step 1: Magnetizing inrush current occurs in transformers when they are switched on, leading to a sudden surge of current due to core saturation. 
Step 2: The inrush current is highly non-sinusoidal and contains a significant amount of harmonics, especially even harmonics, with the 2nd harmonic being dominant. 
Step 3: The presence of the 2nd harmonic can be explained by: 
- Flux asymmetry: When a transformer is energized at any random point in the AC cycle, residual flux in the core causes an asymmetrical magnetizing current. 
- Nonlinear magnetization curve: The transformer core operates in a nonlinear region during energization, which leads to a strong presence of the 2nd harmonic. 
Step 4: While higher-order harmonics like the 3rd, 5th, and 7th are also present, they are of lower magnitude compared to the 2nd harmonic.