ON THE PROCESSING OF THE RECORDED DATA FOR THE SF6 CIRCUIT BREAKERS FROM THE TRANSFORMATION SUBSTATION 110/20/6kV CRAIOVA SOUTH Maria Brojboiu Andrei Savescu Virginia Ivanov S.C. RELOC S.A. University of Craiova 109 Decebal Blv., 200746, Craiova, Romania Facultaty of Electrical Engineering andreisavescu@hotmail.com 107 Decebal Blv., 200440, Craiova, Romania mbrojboiu@elth.ucv.ro, vivanov@elth.ucv.ro KEYWORDS Circuit breakers, maintenance, SF6, ablation. ABSTRACT The assurance of the reliability for the transformation substations is one of primary goals of the manufacturers and distributors of electricity. Therefore, a proper monitoring and maintenance program is required. The circuit breaker is a complex device, subject to the thermal and mechanical stresses during the normal or fault currents switching. The substations are frequently equipped with SF6 circuit breaker. The circuit breaker components subjected to the thermal stresses are the main contacts which suffer electrical erosion and the nozzle which is subjected to the ablation process. The ablation process appears because of the energy radiation which is transferred from the electric arc. As a result of the ablation, the nozzle geometry, the gas pressure and the electric withstand are changed. Based on the recorded data from the transformation substation 110/20/6kV Craiova South, the mass loss from nozzle, the admissible number of disconnections and the throat nozzle diameter are computed. The graphical representations highlight the impact of the interrupted current, of the arcing time and of the integral I2t over the mentioned ones. Consequently, if the switching process and the time arc value are controlled, the thermal wear can be limited and the equipments users may provide a maintenance program with minimal costs and an increasing of the lifetime. INTRODUCTION The circuit breaker is one of the most important and complex equipment from the medium and high voltage electric substations having the switching functions of the electrical circuit in the normal or fault conditions. Depending on the thermal and dynamic stability of the electrical equipment from stations, the commutation must be carried out in a prescribed period of time. Consequently, the failure or decommissioning of circuit breaker has undesirable effects on the operation of the power station, thereby providing a program of monitoring, diagnosis and maintenance of the circuit breaker it is absolutely necessary. The program of monitoring, diagnosis and maintenance has the purpose to increase the lifetime of the equipment and to reduce the operation and maintenance costs. From the maintenance costs of the electrical stations, a 40% are dedicated to the circuit breaker maintenance, meanwhile a 60% are dedicated to the general revisions ( Milthon S. et all, 2005). Therefore, the predictive maintenance systems based on the continuous monitoring of the circuit breaker lead to the significantly reducing of the costs. The predictive maintenance system has the advantage to be carried out during the operation of the equipment. A large number of references in the field are dedicated to the analysis of the functioning and monitoring of the circuit breakers (Milthon S. et all, 2003), (Richard, T., 2004), (Thanapong, S. 2006). The power stations from Craiova South are equipped with oil circuit breaker or, becoming frequently after upgrading, with SF6 circuit breakers. The use of this gas has reduced the frame sizes and increase performance s of switching. The medium voltage SF6 circuit breakers are designed of the self blast principle. This type of circuit breaker generates a gas flow by means of a piston and cylinder attached to the moving contact. When the circuit breaker is in close position the gas pressure from the puffer cylinder is equal to the pressure of filling gas. During the disconnecting operation, the SF6 gas is compressed in the cavity between puffer cylinder and the piston. The switching arc occurs between the stationary contact and the moving contact and it is develops inside of the blowing convergent- divergent nozzle from PTFE with a lower thermal conductivity. A successful current disconnecting depends on the interaction between the switching arc, the radiated energy from arc, the ablation of the nozzle material and the pressure of the gas flow. During the period of the current disconnection, in normal or fault regime, occurs the thermal wear of the circuit breaker components which are in contact with the switching arc (Richard, T., 2004), (Bang, H, 2012), (Bogatyreva, N, 2013),( Muratovic, M, 2013), (Weizong W.I., 2013). The components which are directly exposed to the radiative or conductive energy transferred from the switching arc are the electrical contacts and the blowing nozzle. As a result of the thermal wear, after one operation time or Proceedings 28th European Conference on Modelling and Simulation ©ECMS Flaminio Squazzoni, Fabio Baronio, Claudia Archetti, Marco Castellani (Editors) ISBN: 978-0-9564944-8-1 / ISBN: 978-0-9564944-9-8 (CD)