We've completed the AC and DC insulation tests, excitation current measurements, and TTR measurements for most of our GIS transformers. However, GIS transformer testing requires switching operations. First, ground the GIS primary side so you can safely access the primary terminals. Once done, remove the grounding bars.
On the power transformer, open the cover on the 36 kV (MV) side. On the 170 kV side, disconnect the neutral conductor,this neutral point is already the star point, so you can treat it as if the phases are shorted together(Vector group :YNyn0). On the MV side, you can either use the neutral or short all windings, they're already accessible anyway.
This setup allows you to perform AC and DC insulation tests. Just be mindful of one thing: during AC insulation testing, disconnect and short the 170 kV cable shields, and connect them to the guard terminal to keep them out of the measurement circuit. Otherwise, due to the high capacitance, you'll get very low power factor readings, which would not reflect the actual condition.
Use the cable shields for excitation current and TTR measurements. I realize this may have come out a bit long or confusing, feel free to ask if anything’s unclear.
All of these were carried out without any oil draining.
