2560 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 5, OCTOBER 2008 Influence of Resistance of Electric Arc on Transients Due to Disconnector Switching in Air-Insulated Substations Salih Carsimamovic, Zijad Bajramovic, Meludin Veledar, Miroslav Ljevak, Adnan Carsimamovic, and Predrag Osmokrovic Abstract—In this paper, switching overvoltages due to discon- nector switching in air-insulated substations (AISs) are presented. Measurements of these switching overvoltages were performed in the AIS Grabovica (Hydro Power Plant—HPP Grabovica) on River Neretva and the AIS Kakanj (Thermo Power Plant—TPP Kakanj). These power plants are important objects for operation of Bosnia and Herzegovina’s electric power system. Investigations of operating of air-disconnector-type centre break was performed in order to determine switching-overvoltage levels on primary and secondary circuits that can lead to relay tripping in AIS Grabovica and AIS Kakanj. During operations of disconnector (synchroniza- tion or disconnecting of generator from network), malfunctions of signaling devices and burning of supply units of protection relays appeared. At the same time, sparking between primary terminals of the instrument current transformer occurred. The influence of resistance of electric arc between the contacts of the disconnector is analyzed. Computer simulations by means of alternative transients program–electromagnetic transients pro- gram are performed. Differences between the measured and the calculated values were under 4.52% for overvoltage factor and under 2.56% for overvoltage wave frequency. Comparison of the transient computer simulations with the field measurements showed that calculations could be used for the assessment of the transient overvoltages caused by disconnector switching. Index Terms—Air insulation, arc discharges, overvoltage protection, transient response. I. I NTRODUCTION A S GENERATOR stations and substations have become more complex, their required standards of reliability and stability have become more demanding. These demands have been met by the widespread use of high-speed and low- power electronic systems. This progress in the technology has increased the electromagnetic interferences. Electromagnetic- Manuscript received August 2, 2007; revised January 24, 2008. Current ver- sion published November 14, 2008. The work of P. Osmokrovic was supported by the Ministry of Science and Environmental Protection of the Republic of Serbia under Contract 141046. S. Carsimamovic and Z. Bajramovic are with the Faculty of Electrical Engineering, University of Sarajevo, 71000 Sarajevo, Bosnia and Herzegovina. M. Veledar is with the ABB Representation for Bosnia and Herzegovina, 71000 Sarajevo, Bosnia and Herzegovina. M. Ljevak is with the Energoinvest Bosnia and Herzegovina, 71000 Sarajevo, Bosnia and Herzegovina. A. Carsimamovic is with the Independent System Operator Bosnia and Herzegovina, 71000 Sarajevo, Bosnia and Herzegovina. P. Osmokrovic is with the Faculty of Electrical Engineering, University of Belgrade, 11120 Belgrade, Serbia (e-mail: opredrag@verat.net). Digital Object Identifier 10.1109/TPS.2008.2004238 interference problems can manifest themselves as errors or failures during equipment operation. Each occurrence of inter- ference involves a source of disturbance, coupling mechanism and path, and a susceptible piece of equipment [2]. Discon- nector switching operations in power stations and substations cause a great deal of high-frequency overvoltages. The purpose of this paper is to investigate and interpret free-burning-arc behavior from engineering perspective and to determine arc model parameter and computer models of circuits, appropriate for simulating transients due to disconnector switching in air- insulated substations (AISs). Resistance of the free-burning electric arc, forming in air between disconnector contacts, rep- resents one part of these analyses. Electric arc is alternatively switched on and off, which causes its resistance to change from nonconducting to conducting state and vice versa in a short period of time. Previous investigations of the free-burning electric arc provide limited insight into the disconnection mech- anism. The nature of the formation and extinction of the free- burning arc in air makes the mathematical representation of the arc resistance rather complex. It is therefore customary in these analyses to represent the arc resistance through simpli- fied formulas, or to assign a constant value to it. Values of arc resistance used in calculations range from 0.5 to 10 Ω, and in some cases, up to 30 Ω. Literature relating to current interruption using air-disconnector-type centre break is quite sparse. The study of the disconnector’s current-interrupting capability has not attracted a research interest appropriate to the lack of available knowledge on the subject. Literature provides only a limited insight into the mechanism of current interruption in air. II. PROBLEM OUTLINE Switching operation of disconnector in power stations and substations give rise to electromagnetic interferences because they generate abrupt voltage collapses ΔU across the contacts of the switching equipment [3], [4]. The collapse time Δt is dependent on the distance between the contacts (several tens or some hundreds of nanoseconds) of equipment in AISs [2]. The voltage collapse ΔU (Fig. 1) applied to the circuit gives rise to damped oscillating waves of voltage and current. Disconnector’s contacts in AISs move slowly, causing nu- merous strikes and restrikes between the contacts. Strikes and 0093-3813/$25.00 © 2008 IEEE