Impact of exceeding max. rated power frequency voltage

[cuky2000]

A 345 kV switchyard was energized successfully while ago. For more than 3 months, the operating voltage of 368 kV RMS marginally exceed the system maximum rated voltage of 362 kV RMS. The system and equipment are also rated for 1300 kV crest BIL and 900 kV crest switching surge.
There is not indication of damage so far.

QUESTIONS:
a) What will be the long term impact exceeding this rating from the technical stand point?

b) There is any guideline or publication to determine if the BIL or switching surge is the dominant factor for rating the equipment and system?

c) Should the short circuit rated of 40 kA be derated for higher operating voltage?

Thanks in advance for your input.

 

[QBplanner]

We used to have 345kV 3CBs (360kV air blast breakers) at a ring bus station.
Two of them failed after decades.

Under light load conditions, the terminal station normal operated at 380kV which is high considering system normal.

I believe it was due to the insulation failures even though nobody studied the reason later.

Sorry I am not a IC or equipment guy but a system guy so I can not offer more helps for the second and third questions.

 

[RRaghunath]

The impact of low level over voltage should be slow deterioration of insulation and increased corona discharges with associated increased interference in telecommunication circuits.
Cannot see any possibility of sudden failure.
The fault currents (when the fault occurs with system voltage is high)would be higher and could be a concern if the circuit breakers are marginally rated.
Bigger concern would be with the grid transformers. These would be operating in saturated state (at least partially) drawing high magnetising currents that can lead to high operating temperatures etc. This will also cause core hotspots in the transformers and resultant gassing etc.