Abstract—Worldwide many electrical equipment insulation failures have been reported caused by switching operations, while those equipments had previously passed all the standard tests and complied with all quality requirements. The problem is mostly associated with high-frequency overvoltages generated during opening or closing of a switching device. The transients generated during switching operations in a Gas Insulated Substation (GIS) are associated with high frequency components in the order of few tens of MHz. The frequency spectrum of the VFTO generated in the 220/66 kV Wadi-Hoff GIS is analyzed using Fast Fourier Transform technique. The main frequency with high voltage amplitude due to the operation of disconnector (DS5) is 5 to 10 MHz, with the highest amplitude at 9 MHz. The main frequency with high voltage amplitude due to the operation of circuit breaker (CB5) is 1 to 25 MHz, with the highest amplitude at 2 MHz. Mitigating techniques damped the oscillating frequencies effectively. The using of cable terminal reduced the frequency oscillation effectively than that of OHTL terminal. The using of a shunt capacitance results in vanishing the high frequency components. Ferrite rings reduces the high frequency components effectively especially in the range 2 to 7 MHz. The using of RC and RL filters results in vanishing the high frequency components. Keywords—GIS, VFTO, Mitigation Techniques, Frequency spectrum, FFT, EMTP/ATP. I. INTRODUCTION AS Insulated Substations are widely used in power systems because of their high reliability, easy maintenance and small ground space requirements. The unique problem in GIS is that the generation of very fast transient overvoltages, VFTO, during the operation of disconnector and circuit breakers. The tip of which is very steep accompanied with high frequency oscillation. They have very short rise times, in the range of 4 to 100ns, and are normally followed by oscillations having frequencies in the range of 100 kHz to 50 MHz [1], [2].VFTO not only influent the operating reliability of GIS, but also causes great threat to the insulation of the high-voltage equipment, especially to windings of the transformer. This can referred to the fact that the instant of contacts breakdown can generate VFTO, whose tip is very steep, which make the voltage distribution on the windings uneven or the frequency of the oscillating wave can M. A. Abd-Allah is with the Faculty of Engineering at Shoubra, Benha University, Egypt. A. Said is with the Faculty of Engineering at Shoubra, Benha University, Egypt (e-mail: abdo_eng1987@yahoo.com). Ebrahim A. Badran is with the Faculty of Engineering, Mansoura University, Egypt. matched to the natural frequency of the equipment in the VFTO, which causes an overvoltage with a very high amplitude, which can damage the insulation of the equipment [1]-[6]. The frequency spectrum of the VFTO generated in the 220/66kV Wadi-Hoff GIS is analyzed using Fast Fourier Transform, FFT [6]. The analysis is done for the worst case for disconnector and circuit breaker switching which generated high values of VFTO at the transformer terminals. Also, the frequency spectrums with using different mitigating techniques are presented and compared with those without using the mitigating techniques. II. MODELING OF THE 220 KV WADI-HOFF GIS Due to the traveling nature of the transients, the different components can be modeled by distributed parameter lines, surge impedances and traveling times. Each GIS section is simulated by its equivalent capacitance and inductance, which can be determined as follows [2], [3], [7]. m F o d D / ) ( ln 2 C ε ε πε ≈ = (1) m H d D L / ln 2π μ = (2) Ω ≈ = = d D d D C L ln 60 ln 2 / Z 0 π εμ (3) s m LC v / 1 = (4) where, C and L are the capacitance and the inductance of the GIS busbar, respectively. d is the outside diameter of the GIS busbar and D is the inner diameter of the GIS enclosure. Z 0 is the surge impedance and ν is the propagation velocity. The single line diagram of the substation under study is illustrated in Fig. 1. The 220/66/11 kV Wadi-Hoff substation under study consists of four incoming feeders, two feeders each of 30km length, and the other two feeders each of 3km length. The substation includes three 125 MVA, 220/66/11 kV, power transformers. The feeders are connected in a two busbar arrangement with a bus coupler. The equivalent circuits of the different GIS components and the values of the different M. A. Abd-Allah, A. Said, and Ebrahim A. Badran High-Frequency Spectrum Analysis of VFTO Generated inside Gas Insulated Substations G World Academy of Science, Engineering and Technology International Journal of Electrical and Computer Engineering Vol:7, No:7, 2013 904 International Scholarly and Scientific Research & Innovation 7(7) 2013 scholar.waset.org/1307-6892/16442 International Science Index, Electrical and Computer Engineering Vol:7, No:7, 2013 waset.org/Publication/16442