C I R E D 19 th International Conference on Electricity Distribution Vienna, 21-24 May 2007 Paper 0197- CIRED2007 Session 1 Paper No 0197 Page 1 / 4 FERRORESONANCE IN THREE-PHASE POWER DISTRIBUTION TRANSFORMERS: SOURCES, CONSEQUENCES AND PREVENTION Garikoitz BUIGUES, Inmaculada ZAMORA, Victor VALVERDE, Angel Javier MAZÓN, José Ignacio SAN MARTÍN University of Basque Country (Spain) garikoitz.buigues@ehu.es, inmaculada.zamora@ehu.es ABSTRACT Ferroresonance is a phenomenon that involves high distorted levels of overvoltage and overcurrents. During the last decades, the three-phase ferroresonance research has suffered several changes due to the continuously changing equipment and network. This paper analyses every main factor that is somehow related with the origin, consequences and the preventive decisions that may lead us to avoid the three-phase ferroresonance to appear. Besides this, it gives the opportunity to get a quick identification of such a complex phenomenon. INTRODUCTION Although the word ferroresonance first appeared in 1920 to describe a complex electrical phenomenon, today it remains hard to analyze, maintaining the erratic nature of its performance and its difficulty to be predictable. Ferroresonance is a resonance situation with nonlinear inductance, in which the inductive reactance not only depends on frequency but also on the magnetic flux density of an iron core coil (e.g. transformer iron core). Theoretically, this nonlinear inductance could be represented by two inductive reactances (linear zone and saturation zone), according to the situation on the saturation curve. Nowadays, the connection of three-phase transformers through underground cables is growing fast in residential, commercial, industrial and rural applications. Due to this increasing situation, the possibilities of having a series connected capacitance and a non-linear inductance, prone to ferroresonance, become more probable. Not only is the cable capacitance (and consequently its length) an important factor to take into account in the transformers ferroresonance, but also other elements are completely necessary for ferroresonance to appear. All those factors affect the ferroresonance appearance in several ways, producing the phenomenon just as well as making some damaging consequences appear. Consequently, it is necessary to have a general idea about what would be the best preventive decisions to take in order to avoid unexpected surprises. SOURCES OF THREE-PHASE FERRORESONANCE Influential factors The possibility of a ferroresonance phenomenon appearance is mainly based on an existing series connected capacitance and a non-linear inductance. But, factors that may influence on it are not only limited to the cable capacitance consideration, but also to several constructive, design, operation and protective parameters, such as: • One or two phase fuse operation and switching with phase opening/closing delays. • Transformer primary winding connections. • Transformer core design and the use of low-loss transformers. • Transformers with low load levels or in an unloaded situation. • The capacitance provided by many elements: underground cables, overhead conductors, shunt capacitors, transformer stray capacitance and grading capacitance of circuit breakers. • Network characteristics (voltage rating, short circuit power, transformer power) • Initial conditions: remanent flux, switching instant, circuit losses and charge in the capacitances Although up to three decades ago the problem of ferroresonance was limited to certain situations, the significant evolution of the electrical networks has leaded the phenomenon to a more complex analysis. Therefore, the main elements involved in this complexity increase are the following ones: • The increasing use of insulated underground cables with higher capacitance • Single phase connection and disconnection • The extended use of MOV type surge arresters • Improved low-loss core transformers with different magnetic response