Capacitive voltage transformer - Oil level indicator

Haven't worked with 500kv but 80kv sight glass definitely shows oil level.

By definition, the site glass/watch glass is meant for checking the oil level. Hence, oil shall be there, if it is oil filled.

There should be oil showing and if not you should replace it ASAP. Been quite awhile but I don't think you can add oil like a transformer. Of course a Doble test will definitely tell the health of that.

Low oil level in a CCVT could lead to equipment failure, outage or other wanted situations. At least the manufacturer and the testing department should be involved ASAP.
In a 500KV bulk power station this could be a serious matter. Why take a chance?
Below are a few troubleshooting recommendations suggested by CCVT manufacturers:

Yes there should be oil visible. Sometimes reading the oil level can be slightly tricky. Just to check the obvious, hopefully you physically looked through the sight glass of both CVTs for comparison rather than just seeing a photo of one of them.

If there is any oil leakage, there would be some trace of it like spillage. If not check the other phase VT, whether it has identical indication. If not refer the user manual for diagnostics.

If you are not able to diagnose, contact the equipment supplier (vendor) or call some experts.

Please be aware that the oil drained VT poses an explosion hazard. In my view, frequent movement in that area itself could be dangerous. If it explodes, it is not only self destructive, but also damage the neighboring equipments, thus causing a prolonged outage !!!

In my view you should not be wasting anymore time, though my advice sounds nit harsh!!! Sometime you have to swallow bitter pill !!!

As noted above, an oil sight glass is by definition meant to show an oil level, typically in my experience about 1/2 way up from the bottom at an ambient temperature of 25 Celsius.

But just because there should in normal circumstances be oil visible in the sight glass doesn't mean there always is...

Over years of operating numerous substations I almost routinely found there was generally at least one CVT that [a] showed no oil in its sight glass, the operators had submitted a defect report about, requesting a top-up, so that [c] the Electrical Maintenance Department knew about it, but that [d] somehow never seemed to get topped up, purportedly because [e] something of a higher priority always seemed to pop up.

The reason this occurs is because [as noted above] the CVT simply keeps on working properly...until it either doesn't anymore or fails explosively.

This is more of a human nature thing than a technical one...

Two common English sayings used in Canada come to mind: one is "The squeaky wheel gets the grease," and the other is "It's all fun and games until somebody loses an eye."

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]