I agree with those suggesting re-furb rather than replace. Whoever does your maintance make sure thay give a once over on a regular basis for excessive partial discharge. Trip timing test are also good for the mechanics if they can be done on this type of switch (anyone got experience?) That...
Partial Discharge testing. Could be either type testing or a production test though relying on a type test alone would not be wise. Manadatory only if the customer specifies the test! (EU law - US may be different) IEC 60270 specifies methodology but not levels. Levels are specified by purchaser...
There is much confusion on terminology in this area. For me a corona is a type of partial discharge. And a partial discharge is a discharge that does not bridge the electrodes. Whereas an arc does bridge the electrodes.
Our American cousins tend to use Corona and Partial Discharge...
I have come to this thread a bit late so I can't really add much but picture is spectacular and underlines the importance of regular condition assessment of any HV switch. The HSE publishes an excellent guide "Keeping electrical switchgear safe". Although it is aimed at larger supply switchgear...
In response to the original question, Ultrasonic detection is a good method of detecting surface discharge on MV components e.g. bushings, dry terminations.
TEV is better for discharges deep inside the insulation.
Using both techniques will ensure that most types of discharge can be detected...