IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 2, APRIL 2009 711 A Multi-Purpose Balanced Transformer for Railway Traction Applications Zhiwen Zhang, Bin Wu, Fellow, IEEE, Jinsong Kang, and Longfu Luo Abstract—A multi-purpose balanced (MPB) transformer is proposed in this paper for electric railway applications. The MPB transformer is developed from impedance-matching (IM) trans- formers widely used in China’s railway systems. The proposed MPB transformer has three main functions: 1) it provides a sym- metrical two-phase voltage of 27.5 kV for railway overhead lines to power electric trains; 2) it supplies a three-phase balanced voltage of 10.5 kV for railway substations; and 3) it can accommodate a three-phase harmonic filter with improved power factor and re- duced manufacturing cost. In this paper, the operating principles of the MPB transformer are elaborated, the design of the har- monic filters is discussed, and the results of computer simulations are provided. A 16-MVA MPB transformer was designed and commissioned, and real-time field measurements are provided. Index Terms—Balanced transformers, harmonic filters, railway substations, reactive power compensation, traction drives. I. INTRODUCTION W ITH THE advancements of power converter and control technologies for variable-speed traction drives [1], elec- tric railways have been increasingly used in many countries. To reduce the construction cost, a railway system usually employs a single-phase supply, from which an electric train obtains its power through a single overhead line. The single-phase system generates high negative-sequence currents to the railway grid [2], [3]. The traction drives also generate a substantial amount of current harmonics into the grid, deteriorating its power quality and system reliability [4], [5]. The poor power quality may in- fluence the safety of the railway system and also the operation of other electric apparatus on the same grid [6]. Therefore, consid- erable efforts have been made to reduce the negative-sequence and harmonic currents in the railway supply systems. An important method of reducing or eliminating negative- and zero-sequence currents in the railway systems is to use bal- anced transformers such as Scott, Le Blanc, and Woodbridge transformers [7]–[9]. There are certain disadvantages associ- ated with these transformers. For instance, the material (iron and copper) utilization factor is only 81.6%, 84.5%, and 82.6% for Scott, Le Blanc, and Woodbridge transformers, respectively. Manuscript received November 28, 2007; revised August 28, 2008. First published March 04, 2009; current version published March 25, 2009. This work was supported by the Ministry of Education of P.R. China under Grant 102174 and by the Key Science and Technology project of the Tenth and the Eleventh Five-Year Plan of Hunan Province, P.R. China, under 05GK1002-1 and 06GK1003-1, respectively. Paper no. TPWRD-00770-2007. Z. Zhang and L. Luo are with College of Electrical and Information Engineering, Hunan University, Changsha, Hunan, China 410082 (e-mail: hnuzzw@hotmail.com). B. Wu is with Department of Electrical and Computer Engineering, Ryerson University, Toronto, ON M5B 2K3 Canada. J. Kang is with Department of Electrical Engineering, Tongji University, Shanghai, China 200092. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRD.2008.2008491 Fig. 1. Circuit diagram of an IM transformer. Furthermore, there is no neutral point in the primary winding of the Scott and Le Blanc transformers, which requires higher voltage insulation level for the primary winding. The Wood- bridge transformer needs two additional auto-transformers for the two-phase railway traction power supply. Research on other types of balanced transformers is in progress, including three- phase to two-phase balanced transformers and three-phase to four-phase balanced transformers [10]–[12]. Among these balanced transformers, an impedance-matching (IM) transformer technology has been widely accepted in the railway systems in China [10]. The material utilization factor for the IM transformer is 91.95%, much higher than that of the Scott, Le Blanc, or Woodbridge transformers. The circuit diagram of the IM transformer is shown in Fig. 1. The trans- former is typically rated 110 kV with a wye-connected primary winding, allowing the neutral point to be grounded. The sec- ondary winding of the transformer provides a symmetrical two- phase voltage of 27.5 kV. The primary winding has the same number of turns for each phase, i.e., , while the number of turns for the secondary windings is given by and . Such an arrangement can produce a symmetrical two-phase voltage, and , with a 90 phase displacement between phases and . In practice, the neutral point of the primary winding is nor- mally grounded. For a three-phase balanced operation, the neu- tral current of the primary winding should be zero. This can be partially achieved by matching the leakage impedances of the secondary windings [10]. Therefore, this type of transformers is known as impedance-matching transformers. In order to suppress the harmonic currents produced by the traction drives and to improve the power factor of the system, 0885-8977/$25.00 © 2009 IEEE