Annealing effects on single-phase E-I transformer performance under nonlinear loads

Single-phase E-I transformers are widely used in power-conversion applications, including grid-connected flyback micro-inverters for solar photovoltaic systems, yet their performance often deteriorates due to high core loss and harmonic distortion generated by nonlinear loads. This study evaluated the effect of the annealing process on core loss, voltage regulation, energy efficiency, and voltage/current harmonic distortion of a single-phase E-I transformer under nonlinear loading. Three transformer samples were tested under no-load and nonlinear-load conditions (rectifier and inverter) before and after annealing at 180, 350, and 450 °C for 60 minutes, followed by harmonic-distortion measurement using a power logger on a flexible conveyor system and a single-phase induction motor. Annealing at 350 °C produced the most significant core-loss reduction, from 150.3 W to 110.6 W, while linear-load efficiency reached 97-98% and voltage regulation remained stable near 98%. Harmonic measurements showed voltage total harmonic distortion (THDv) of 5.8% on the flexible conveyor system and 5.6% on the induction motor without a variable speed drive, both below the 8% IEEE 519 limit, while individual current harmonic distortion (IHDi), current total harmonic distortion (THDi), and total demand distortion (TDD) also remained within the standard's safe margins. These findings indicate that medium-temperature annealing can improve the magnetic performance of the transformer core while keeping power quality within standard limits under nonlinear loading. The practical implication is that this annealing guideline can be adopted by small and medium-scale transformer manufacturers to improve energy efficiency without costly equipment investment.