The connection of a power transformer to the grid is associated with magnetizing inrush currents that may result in power quality issues as well as faulty relay tripping. In distributed generation, the transformer may instead be premagnetized from the source to avoid this. In this paper, a VSI is directly coupled to a transformer. Three different strategies of premagnetization are implemented into the control system, and the inrush currents are measured for various values of the remanent flux in the core. The results show good reduction in the peak magnetizing inrush currents without using any external circuitry. 1. Introduction When energizing a power transformer, the magnetizing inrush currents that follow may be several times higher than the rated currents for the transformer. The magnitude of the inrush currents may cause voltage dips in the local grid, resulting in poor power quality [1]. Also, the magnetizing inrush currents are rich in harmonic content, usually have a high direct current component, and may erroneously trigger transformer overcurrent protections [2]. Much work has been conducted in making the protective relays recognize the difference between overcurrents at a fault and a magnetizing inrush current [3]. The magnitude of the inrush currents depends on the circuit reactance, the phase angle of the voltage source, and the transformer remanent magnetic flux . The remanent flux in the transformer is the flux that exists in the transformer after it is powered down. This phenomenon has long been studied, and various methods and models can be found in the literature. To measure the remanent flux is not easy but may be approximated with measurements of the voltage integral during deenergization. Several ways to reduce or eliminate the inrush currents have been presented. Ways to handle the issues are through controlled switching and removal of remanent flux [4, 5], sequential phase energization [6, 7], insertion of damping resistors [8], capacitive loading, or thyristor switching control [9]. Work that uses PWM inverters to inject a cancelling-out current via the inverter during powerup of the transformer has also been presented [10] along with methods to detect the remanent flux by analysing the DC component during switching [11]. Many renewable energy sources are nonsynchronous with the grid, which will require a power electronics conversion stage for grid connection. A commonly used conversion is the voltage-source inverter (VSI), followed by an LCL-filter and a step-up transformer. The full control of the VSI allows the transformer
References
[1]
M. Nagpal, T. G. 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, pp. 890–896, 2006.
[2]
L.-C. Wu, C.-W. Liu, S.-E. Chien, and C.-S. Chen, “The effect of inrush current on transformer protection,” in Proceedings of the 38th Annual North American Power Symposium (NAPS '06), pp. 449–456, Carbondale, Ill, USA, September 2006.
[3]
F. Mekic, R. Girgis, Z. Gajic, and E. Nyenhuis, “Power transformer characteristics and their effect on protective relays,” in Proceedings of the 33rd Western Protective Relay Conference, October 2006.
[4]
J. H. Brunke and K. J. Fr？hlich, “Elimination of transformer inrush currents by controlled switching. I: theoretical considerations,” IEEE Transactions on Power Delivery, vol. 16, no. 2, pp. 276–280, 2001.
[5]
J. H. Brunke and K. J. Fr？hlich, “Elimination of transformer inrush currents by controlled switching. II: application and performance considerations,” IEEE Transactions on Power Delivery, vol. 16, no. 2, pp. 281–285, 2001.
[6]
Y. Cui, S. G. Abdulsalam, S. Chen, and W. Xu, “A sequential phase energization technique for transformer inrush current reduction. I: simulation and experimental results,” IEEE Transactions on Power Delivery, vol. 20, no. 2, pp. 943–949, 2005.
[7]
W. Xu, S. G. Abdulsalam, Y. Cui, and X. Liu, “A sequential phase energization technique for transformer inrush current reduction. II: theoretical analysis and design guide,” IEEE Transactions on Power Delivery, vol. 20, no. 2, pp. 950–957, 2005.
[8]
L. Cipcigan, W. Xu, and V. Dinavahi, “A new technique to mitigate inrush current caused by transformer energization,” in Proceedings of the IEEE Power Engineering Society Summer Meeting, vol. 1, pp. 570–574, Chicago, ILL, USA, July 2002.
[9]
M. S. J. Asghar, “Elimination of inrush current of transformers and distribution lines,” in Proceedings of the 1996 International Conference on Power Electronics, Drives & Energy Systems for Industrial Growth (PEDES '96), vol. 2, pp. 976–980, New Delhi, India, January 1996.
[10]
K. Shinohara, K. Yamamoto, K. Iimori, Y. Minari, O. Sakata, and M. Miyake, “Compensation for magnetizing inrush currents in transformers using a PWM inverter,” Electrical Engineering in Japan, vol. 140, no. 2, pp. 53–64, 2002.
[11]
A. Rasic, G. Herold, and U. Krebs, “Fast connection /reconnection of the VSC to the power network,” in Proceedings of the European Conference on Power Electronics and Applications (EPE '07), Aalborg, Denmark, September 2007.
[12]
R. Ekstrom, S. Apelfrojd, and M. Leijon, “Transformer magnetization losses using a non-filtered voltage-source inverter,” Advances in Power Electronics, vol. 2013, Article ID 261959, 7 pages, 2013.
[13]
D. Povh and W. Schultz, “Analysis of overvoltages caused by transformer magnetizing inrush current,” IEEE Transactions on Power Apparatus and Systems, vol. 97, no. 4, pp. 1355–1365, 1978.
