Random energization of unloaded transformers may generate inrush currents, which easily causes protective relay mis-operation, imposes sever mechanical stresses on the transformer windings and reduces power quality of the system. Controlled switching technology is used for suppressing these inrush currents. The key to realize controlled switching is the effective estimation of the residual flux in transformer core after a transformer de-energization. Based on the analysis of the theories of controlled switching unloaded power transformer, a method of measuring the residual flux based on numerical integration is proposed. Energization of unloaded transformers is simulated with PSCAD/EMTDC, and simulation results validate the effectiveness of the controlled switching technology. Unloaded transformer controlled switching test platform is built, and the tests of random and controlled switching are conducted. The experimental results indicate that the controlled switching considering residual flux is effective in suppressing the inrush current compared with random switching, and validate the effectiveness of residual flux measurement method. Finally, the suitable controlled switching strategy of unloaded transformer with gang-operated breaker is proposed, and simulation results validate its effectiveness. This work is supported by National Natural Science Foundation of China (No. 51477142), Sichuan Provincial Department of Education Project (No. 12ZB131 and No. 14ZB0123), and Innovation Fund of Postgraduate of Xihua University (No. ycjj2017060).