Very fast transient over voltages are generated during the switching of disconnectors in gas insulated substation (Gas Insulated Substation). Such over voltages can cause mal functioning of protection circuits and control circuits and also initiates faults and influence on other components such as transformers. The suppression of VFTO (Very fast transient over voltage) is very important in GIS systems. There are some deficiencies in the existing suppressing methods. In this paper a new idea for the suppressing VFTO by ferrite is put forward, and the computer simulations were conducted on 245kV GIS models with and without ferrite rings and the results are verified with the experimental results. The simulation test results are closely matched with the experimental results so the use of magnetic rings to suppress VFTO is distinctly efficient.

Based on 245kV GIS substation, very fast transient over voltages caused by disconnecting switch operation of GIS is simulated and calculated in this paper with B2SPICE program. The different protection measures are compared and analyzed, such as adding ferrite rings, adding opening and closing resistor, metal oxide arrester and R-C absorber. A low voltage experiment was conducted to estimate transient overvoltage's during switching operation. These transient voltages are recorded by using surge sensor and digital storage oscilloscope with sample rate 1GS/s and 500MHz frequency range. Suppression techniques are employed practically to verify the validation of simulation results. The simulation results are closely matched with the experimental results in the case of ferrite rings application in reduction of maximum peak of VFTO generated during switching operation.

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The transient high frequency electric and magnetic fields are generated in Gas insulated substations(GIS) systems during switching operations by both radiation and conduction mechanisms. In this paper the radiated transient electric and magnetic fields are measured by using high frequency field sensors at important locations in the 245kV GIS were presented. The measurements are carried out at three important locations including at bus link, cable termination and C.T position respectively. The results of measured values are compared with the calculated values. The results shows that transient electric & magnetic field values are correlate with the measured values at bus link, near current transformer(CT)and at cable termination , the calculated values are slightly deviated from the measured values when moves along with XLPE cable. This may be due to a fast attenuation rate of electromagnetic fields with distance away from the DS3. These results are very useful in order to prepare some precautionary protection measure to avoid malfunctioning of secondary equipment and metering equipment in GIS systems.I.

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