DC reactor type transformer inrush current limiter M. Tarafdar Hagh and M. Abapour Abstract: A new inrush current limiter (ICL) is presented to limit the inrush current of transfor- mers. The proposed ICL consists of three similar sets. Each set includes a diode-bridge and a single DC reactor and is connected in series with the individual phases of transformer. The ICL has almost no effect on normal operation of transformer. It needs no control, measurement and gate driving system and has a simple power circuit topology. The equivalent instantaneous induc- tance of transformer is used for analysis of circuit operation. The theoretical analysis, design features, power losses and voltage distortion because of using ICL are presented. The proposed method has been tested by simulation and laboratory experiments. Both results show that the pro- posed ICL successfully limits the inrush current. List of Symbols L 1 d primary leakage inductance r 1 primary effective resistance u 1 primary terminal voltage i 1 primary current i M magnetising current w mutual flux linkage L 0 2d secondary leakage inductance referred to primary r 0 2 secondary resistance referred to primary u 0 2 secondary terminal voltage referred to primary i 0 2 secondary current referred to primary L M instantaneous magnetising inductance of the transformer Z pr primary impedances of transformer Z 0 sec secondary impedances of transformer referred to primary Z 0 load load impedance referred to primary side L NS magnetising inductance when the iron core is not saturated L Sa magnetising inductance when the iron core is saturated Z s equivalent impedance of source and transmission line Z e equivalent impedance of transformer and load r e resistance of Z e L e inductance of Z e r d resistance of DC reactor L d inductance of DC reactor V DF forward voltage drop across rectifier diodes i d DC reactor current r S equivalent resistance of source and transmission line L S equivalent inductance of source and transmission line X m magnetising reactance of transformer R p transformer iron core resistance R sc resistance in the primary and secondary winding X sc the leakage impedance in the primary and sec- ondary winding i r ripple current in DC reactor I DC average current in DC reactor I max maximum of utility current P DC DC reactor power loss P Bridge average power loss of diode-bridge P Total total power loss of ICL 1 Introduction The transformer inrush current is a transient current that occurs in an electric circuit when a transformer has been energised. This current depends on different parameters such as the magnitude of voltage, the switching-on angle, the remanent flux, the hysteresis characteristics of core, the resistance in the primary circuit and others [1]. The magnitude of the inrush current may be several times higher than the transformer rated current. This could result in high mechanical and thermal stresses. If the inrush current is suppressed by some methods, these pro- blems may be avoided. There are two basic methods for lim- iting the inrush current: (a) Interior improvement methods [2, 3], (b) Additional control circuits [4–6]. Interior improvement methods usually make use of mag- netising characteristics of transformer core. One idea is using a virtual air-gap which its equivalent thickness is con- trollable [3]. It needs an auxiliary winding inside the mag- netic core. A DC current is injected in auxiliary winding to make a local magnetic saturation with the permeability closed to m 0 . So the saturated zone is similar to an # The Institution of Engineering and Technology 2007 doi:10.1049/iet-epa:20060511 Paper first received 26th December 2006 and in revised form 12th May 2007 The authors are with the Power Electronics and FACTS Laboratory, Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran E-mail: tarafdar@tabrizu.ac.ir IET Electr. Power Appl., 2007, 1, (5), pp. 808–814 808