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Nowadays, it is becoming difficult to secure a transmission line route when a new transmission line is constructed due to social environment and resident complaints. As existing urban areas are expanded and new cities are constructed, the necessity of high-capacity underground power transmission has been increasing because of load concentration in downtown areas. In North America, Europe and Japan, the research has been carried out for 2nd generation gas insulated transmission lines(GIL) which is more environmentally friendly and economical compared to previous GILs. The new GILs have been applied to real power systems from early 2000s. In South Korea, GIL is being considered as a possible solution for replacing 345kV high-capacity overhead transmission lines with underground transmission ones, so KEPCO is planning to develop and apply a new 362kV rated GIL underground transmission lines instead of overhead transmission lines. In this paper, the overvoltage generated at the combined transmission line adapting GIL was reviewed using EMTP.

As the expension of metropolis and increasing of housing site, GIL(Gas Insulated Line) comes to the fore as an alternative to overhead transmission line avoided by local resident. The certification and experimental test for the GIL is now in progress in under ground test lab, Go-chang. For the test, we propose a method of PQ(pre-qualification) test which is essential for development and field application in this paper. The test following this method take about 200 days or 300 days under the circumstances.

This paper deals with the method and algorithm used to find fault locations in gas insulated transmission line. The method uses UHF sensors and digital oscilloscope to detect discharge signals emitted to the outside through insulating spacer in the event of breakdown inside GIL. UHF sensors are the external type and installed at outside of insulating spacers of GIL. And we used wavelet signal processing to analyze the discharge signals and confirm the exact fault location findings in the GIL test line. This method can overcome demerit of TDR(Time Domain Reflectometer) method having been applied to detect fault location for conventional underground transmission lines, and Ground Fault Sensors used in conventional GIS systems. TDR method requires high level of specialty and experience in analyzing the measured signals. Ground fault sensors are installed inside GIL and can be destroyed by high transient voltage. This paper`s method can simplify the fault location process and minimize the damage of sensors. In addition, this method can estimate the fault location only by the time difference when discharge signals are arrived to detecting sensors at the ends of GIL sections without reasons of breakdown. To test the performance of our method, we installed sensors at the ends of test line of GIL(84m) and sensed discharge signals occurred in GIL, energized with AC voltage generator up to 700kV.