The gaseous and solid insulation of the GIS has to be exposed to an AC voltage withstand test. Due to the big capacitance variations of various GIS installations, it is often that a variable frequency hi-pot test unit is applied. The variable frequency high potential unit can create low frequency (30 Hz to 300 Hz) voltage applications at magnitudes and durations defined in standards. This one-minute low- frequency voltage withstand verification is done at 80% of the rated low-frequency withstand voltage done in the manufacturer's factory. A conditioning voltage application sequence, with magnitude and durations determined by the manufacturer, should precede the defined one-minute withstand test. The intention of the conditioning verification is to drive any small particles, if they exist, to low electric field intensity locations such as particle traps. The objective of these high voltage verifications is to check that the elements of the gas-insulated substation have survived transport, have been assembled correctly, that no extraneous material has been left inside the chambers, and that the GIS installation can withstand the test voltage. The conditioning voltage application sequence and the one-minute low-frequency voltage withstand verification have to be completed after the GIS installation has been installed, the gas chambers have been filled to the manufacturer's suggested nominally rated fill density and the moisture content and purity of the gas have been checked to be within defined limits
AC Voltage Withstand Arrangements and Applications
When the GIS installation being checked is connected to the GIS equipment that is already in operation, the in-service part has to be electrically isolated from the tested part. Nevertheless, it is highly possible that the test voltage could be 180 degrees out of phase with the in-service voltage, potentially exposing the open gap of a disconnect switch, being used for isolation, to voltages in excess of what can be withstood. Hence, an isolated section with suitable grounds has to be applied between the in-service GIS and the GIS to be checked. This ensures that the test voltage cannot create service disruptions to the electrical system nor can the service voltage create severe defects to the testing equipment or danger to the test staff. Due to the electrical loading restrictions of the testing equipment, it may be mandatory to isolate GIS sections using open disconnects and test each section separately. To achieve this, it may need that parts of the GIS equipment be subjected to more than one test voltage application. The parts that are not being examined have to be earthed. Isolating parts of the GIS equipment may give an extra benefit of field testing the open gap of some disconnecting switches, even though such a field test is not are requirement. In addition, it may be mandatory to isolate GIS sections to facilitate the location of a disruptive discharge or to limit the energy potentially discharged during a disruptive discharge. The test voltage source may be connected to any convenient point of the phase being examined.