IEEE C37.122.7-2021 pdf free download – IEEE Guide for Field Testng of Gas-Insulated Substatons Rated Above 52 kV.
.2.2 Gas leakage tests
4.2.2.1 Vacuum rise test Prior to the gas leak test, a vacuum rise test will be effective in identifying substantial leaks on field assembled flanges/joints prior to initial gas filling, but it may not be effective in correctly identifying leaks when the vessel is pressurized. A vacuum rise test consists of measuring how much vacuum (as measured by means of a vacuum gauge) is lost in a gas compartment after the vacuum pump has been disconnected, and before gas filling. The manufacturer may provide an allowable value of vacuum loss over a predetermined amount of time. If the vacuum loss is greater than specified, then a leak should be suspected. Certain factors can cause false readings with this test technique, which include leaks caused by the vacuum gauges and vacuum processing equipment, as well as vacuum lost due to interior gas compartment moisture, which may be sublimating as it is pulled from interior epoxy materials. Users should discuss vacuum processes with the manufacturer and follow the manufacturer’s recommendations before filling the equipment.
4.2.2.2 Accumulation-type gas leak test for SF 6 SF 6 gas leak testing should be performed immediately after filling to the manufacturer’s recommended temperature compensated pressure. Testing shall be performed on all field-assembled enclosure joints, field welds, field connected monitoring devices, gas valves, gas piping, etc. As described earlier, an accumulation- type leak test is recommended on field-assembled items and joints, and consists of wrapping plastic sheets to create “bags” around these areas to capture SF 6 gas molecules that intermittently escape from the leak. See Figure 1 for best bagging practice.
The bags avoid background readings from interfering with test results. Covers or caps should be placed on self- sealing fll valves to avoid trapped gasses from being measured with the test sample. Additional verifcation of factory assembled joints should also be performed in conjunction with feld leak testing if there is a suspected leak. Certain chemicals used in sealing/assembling the GIS, such as alcohol and silicone sealant, may have an effect on the equipment used to detect a leak, causing a false reading. Dust, cobwebs, water, and other contaminants are also known to cause a false reading. Prior to leak testing, always ensure the area to be tested is clean and dry.
The bag should be in place and sealed at least 12 h prior to performing a leak test on the joint. Without signifcantly disturbing the bag, a small incision should be made above the pocket (shown in Figure 1). The nozzle of the SF 6 gas detector is inserted into the small incision and fed into the pocket at the bottom of the bag. Depending on the testing equipment used, the operator should consult the manufacturer for what is an acceptable leakage rate/passing criterion. The leakage rate in ppmv or go/no-go results for all tested locations on the GIS should be documented. If a leak is detected, it is recommended that the detector be removed from the suspected leak area, re-calibrated, and returned to the leak area again to validate the presence of a leak. In the event that a leak is confrmed using the hand-held leak detector, additional investigation to pinpoint the leak is required. The operator has a number of options, some listed as follows:
a) Remove the bag and use a liquid leak detection solution or soapy water. Since this method is not as sensitive as using the gas leak detector, it may not be able to show the exact location of the leak.IEEE C37.122.7 pdf download.