11 Power Quality Problems Due to Transformer Inrush Current A. Tokić * Faculty of Electrical Engineering, Tuzla Bosnia and Herzegovina I. Uglešić Faculty of Electrical Engineering and Computing, Zagreb Croatia SUMMARY Transformer energization can produce a large nonsinuoidal inrush current which contains both odd and higher order harmonic components that can put transformer winding under mechanical stress. Additionally, they can cause irregular tripping of harmonic protection relays. Furthermore, in relatively weak power systems, such as is the Bosnian system, the superposition of harmonic components with system resonance frequencies may produce temporary overvoltages (TOV). Transformer winding failures and metal oxide surge arrester (MOA) energy stresses can occur due to TOV. The paper demonstrates a case study of an energization of a 220/110 kV transformer and power quality problems that can appear due to higher harmonics. Energy stresses of MOA provoked by transformer energization are considered in the paper. KEY WORDS Power quality, transformer, inrush current, temporary overvoltage, surge arrester 1. INTRODUCTION Transformer energization is a regular operation in an electric power system which can lead to large transformer inrush current. The basic characteristic of inrush current is relatively slow decay time to reach its steady state value, determined by transformer and power system parameters. The magnitude and duration of inrush current have a strong dependence from transformer saturation curve. Since the magnetizing inductance in unsaturated region of this curve is high, the inrush current can take a long time to reach its steady state. On the other way, the magnetizing inductance in saturated region is dominant parameter in determination of inrush current magnitude. In addition, inrush current characteristics are depended on the switching breaker time, magnitude and polarity of residual flux [1]. These currents can provoke false operation of protective relays and fuses [2] and mechanical damage to the transformer windings due to magnetic forces [3], cause voltage sags [4], establish temporary harmonic overvoltages [5] and generally reduce power quality on the system. This paper is focused on the analysis and consequences of temporary overvoltages that result from transformer inrush current. * e-mail: amir.tokic@untz.ba A. Tokić, I. Uglešić, Power quality problems due to transformer inrush current, Journal of Energy, vol. 59 Number 1–4 (2010) Special Issue, p. 11–18 Journal of Energy journal homepage: http://journalofenergy.com/ VOLUME 59 Number 1–4 | 2010 Special Issue