The energizing of large power transformers has long been considered a critical event in the operation of an electric power system. When a transformer is energized by the utility, a typical inrush current could be as high as ten times its rated current. This could cause many problems from mechanical stress on transformer windings to harmonics injection, and system protection malfunction. There have been numerous researches focusing on calculation and mitigation of the transformer inrush current. With the development of smart grid, distributed generation from independent power producers (IPPs) is growing rapidly. This paper investigates the inrush current due to black start of an IPP system with several parallel transformers, through a simulation model in DIgSILENT Power Factory software. The study demonstrates that a single genset is capable of energizing a group of transformers since the overall inrush current is slightly above the inrush of the transformer directly connected to the generator. In addition, a simple method is proposed to mitigate the inrush current of the transformers using an auxiliary transformer.
 M. Nagpal, T. Martinich, A. Moshref, K. Morison and P. Kundur, “Assessing and Limiting Impact of Transformer Inrush Current on Power Quality,” IEEE Transactions on Power Delivery, Vol. 21, No. 2, 2006, pp. 890-896. doi:10.1109/TPWRD.2005.858782
 M. Steurer and K. Fr?hlich, “The Impact of Inrush Currents on the Mechanical Stress of High Voltage Power Transformer Coils,” IEEE Transactions on Power Delivery, Vol. 17, No. 1, 2002, pp. 155-160. doi:10.1109/61.974203
 J. G. Slootweg and R. A. C. de Groot, “Calculation of Transformer Inrush Currents Occurring during the Energizing of the Public Grid after a Major Black Out,” IEEE Conferences of Russia Power Technology, Arnhem, 27-30 June 2005, pp. 1-7.
 V. Molcrette, J. L. Kotny, J. P. Swan and J. F. Brundny, “Reduction of Inrush Current in Single-Phase Transformer Using Virtual Air Gap Technique,” IEEE Transactions on Magnetics, Vol. 34, No. 4, 1998, pp. 1192-1194. doi:10.1109/20.706479
 J. L. Shyu, “A Novel Control Strategy to Reduce Transformer Inrush Currents by Series Compensator,” International Conference on Power Electronics and Drive Systems, Vol. 2, 2005, pp. 1283-1288. doi:10.1109/PEDS.2005.1619885
 P. Arboleya, D. Diaz, C. Gonzalez-Moran, J. Coto and J. Gomez-Aleixandre, “An Inrush Current Limiter as a Solution of Injection Transformer Oversizing in Dynamic Voltage Restores,” International Conference on Electrical Machines and Systems, Gijon, 15-18 November 2009, pp. 1-6. doi:10.1109/ICEMS.2009.5382735
 D. P. Balachandran, R. S. Kumar and V. P. Shimnamol, “Transformer Inrush Current Reduction by Power Frequency Low Voltage Signal Injection to the Tertiary Winding,” IEEE Lausanne on Power Technology, Trivandrum, 1-5 July 2007, pp. 1953-1958. doi:10.1109/PCT.2007.4538616
 B. Kovan, F. de Leon, D. Czarkowski, Z. Zabar and L. Birenbaum, “Mitigation of Inrush Currents in Network Transformers by Reducing the Residual Flux With an Ultra-Low-Frequency Power Source,” IEEE Transactions on Power Delivery, Vol. 26, No. 3, 2011, pp. 1563-1570. doi:10.1109/TPWRD.2010.2102778
 W. Xu, S. G. Abdulsalam, Y. Cui and X. Liu, “A Sequential Phase Energization Technique for Transformer Inrush Current Reduction—Part II: Theoretical Analysis and Design Guide,” IEEE Transactions on Power Delivery, Vol. 20, No. 2, 2005, pp. 950-957. doi:10.1109/TPWRD.2004.843465
 F. Ali Asghar and K. P. Basu, “Reduction of Three-Phase Transformer Magnetizing Inrush Current by Use of Point on Wave Switching,” Student Conference on Research and Development, Cyberjaya, 16-18 November 2009, pp. 368-370.
 K. P. Basu and S. Morris, “Reduction of Magnetizing Inrush Current in Traction Transformer,” Third International Conference on Electric Utility Deregulation and Restructuring and Power Technologies, Cyberjaya, 6-9 April 2008, pp. 2302-2305. doi:10.1109/DRPT.2008.4523795
 K. P. Basu, A. Asghar and S. Morris, “Effect of Sequential Phase Energization on the Inrush Current of a Delta Connected Transformer,” International Conference on Power Electronics, Drives and Energy Systems, Cyberjaya, 12-15 December 2006, pp. 1-4.
 J. H. Brunke and K. J. Frohlich, “Elimination of Transformer Inrush Currents by Controlled Switching Part II: Application and Performance Considerations,” IEEE Transactions on Power Delivery, Vol. 16, No. 2, 2001, pp. 281- 285. doi:10.1109/61.915496
 Y. Wang, S. G. Abdulsalam and W. Xu, “Analytical Formula to Estimate the Maximum Inrush Current,” IEEE Transactions on Power Delivery, Vol. 23, No. 2, 2008, pp. 1266-1268. doi:10.1109/TPWRD.2008.919153