Modified Pre-Fluxing Method for Energization of Single-Phase Transformers

Inrush current is the high-magnitude current drawn by a power transformer upon its energization. The severity of this current depends on the transformer residual flux, power system impedance, and the voltage phase angle at the energization instant. Aimed at mitigating inrush current, which as such can negatively influence e.g. transformer protection, this paper proposes a modified pre-fluxing method (MPFM) for the energization of single-phase transformers. This is to adjust the residual flux of the core to its maximum feasible value, estimate this adjusted value, and then control the transformer’s energization. To achieve this, a simple pre-fluxing device is introduced for the flux adjustment. Throughout the flux adjustment, the residual flux is estimated using appropriate formulas. The energization instant is determined based on the estimated residual flux and circuit breaker operating characteristics. The proposed method is implemented in PSCAD/EMTDC and evaluated under a wide range of operating conditions. Extensive simulations demonstrate the MPFM’s ability to accurately estimate the core’s adjusted residual flux. The superiority of the MPFM over the conventional pre-fluxing method can be confirmed by comparing the reductions of the inrush current’s magnitude brought about by MPFM and the conventional pre-fluxing method.