X International Symposium on Lightning Protection 9 th -13 th November, 2009 – Curitiba, Brazil LIGHTNING SURGES TRANSFERRED TO THE SECONDARY OF DISTRIBUTION TRANSFORMERS DUE TO DIRECT STRIKES ON MV LINES, CONSIDERING DIFFERENT LV LINE CONFIGURATIONS Paulo F. Obase 1 , Fabio Romero 1 , Jorge M. Janiszewski 2 , Alexandre Piantini 1 , Acácio Silva Neto 1 , Thais O. Carvalho 1 , Alberto Araújo Filho 3 1 Institute of Electrotechnics and Energy / University of São Paulo, Brazil – pfobase@iee.usp.br, fromero@iee.usp.br, piantini@iee.usp.br, acacio@iee.usp.br, thais@iee.usp.br 2 Polytechnic School / University of São Paulo, Brazil – jorge@lcs.poli.usp.br 3 Electrical Distribution Company of State of Tocantins, Brazil – alberto.araujo@redenergia.com Abstract - This paper aims at analyzing the behaviour of lightning surges transferred to low-voltage lines due to direct strikes on medium voltage networks considering two cases: open wire and twisted conductor low-voltage lines. The analysis considers the influences of parameters such as stroke current front time (t f), ground resistance (Rg) and ground resistivity (ρg) on the overvoltages transferred to low-voltage lines. The overvoltages are calculated employing the Alternative Transients Program (ATP) and the simulations refer to a typical rural distribution network of the Electrical Distribution Company of the State of Tocantins (CELTINS), Brazil. To evaluate the surges transferred to the low-voltage line, a high frequency model was developed for the distribution transformer. The results show that the use of twisted conductors configuration for the secondary circuit can reduce the magnitudes of the surges transferred through the distribution transformers. 1 INTRODUCTION Lightning discharges are responsible for a significant amount of unscheduled supply interruptions in electrical overhead lines, usually causing permanent damages to equipment such as distribution transformers. There are various ways by which lightning can disturb medium (MV) and low-voltage (LV) lines. Transients may be caused by direct strokes to the MV or LV line conductors, by lightning induced overvoltages, and by surges transferred through the distribution transformer. In recent years, several studies have been developed concerning transferred surges to LV networks [1-3]. In this work, the characteristics of surges transferred from the primary to the secondary side of distribution transformers due to direct strikes on MV lines are evaluated considering open wire and twisted conductor LV line configurations. In the simulations a typical rural distribution network of the feeder Nova Olinda – Arapoema, of CELTINS, is adopted. CELTINS is the company responsible for electricity distribution in the State of Tocantins, in the North of Brazil. The feeder Nova Olinda – Arapoema supplies power to a great number of consumers and was selected as the most critical with relation to transformer damages caused by lightning in the period 2006 - 2008. The rural distribution networks of CELTINS are located in regions characterized mainly by pastures with stunted vegetation so that the MV line conductors correspond, in general, to the highest points in these regions. Thus, the typical rural distribution network adopted in the simulations is highly exposed to lightning and the overvoltages in the primary and secondary circuits may be caused by direct and indirect strokes. However, a significant number of transient events in LV lines also can be due to surges transferred to the secondary circuit through distribution transformers [4, 5]. 2 SYSTEM DESCRIPTION AND MODELING For the calculations, initially a high frequency model of a single-phase transformer (15 kVA, 20.25 kV / 440 V - 220 V) was developed according to the procedure described in [6]. The model gives good results for simulations considering the transformer both in the no- load condition and for resistive and inductive loads. To illustrate the results obtained with the model, Fig. 1a presents a comparison between measured and calculated transferred voltage waveforms for the experimental set up depicted in Fig. 1b. The transformer model is presented in Fig. 2, which shows a good agreement between the two waveforms.