*2.3.2. Neutral voltage transfer*

It can be seen from Figure 3 that overvoltage were observed not only on the line conductors of phase A, B, and C but also on the neutral. As will be explained in the next section, neutral voltage transfer can occure through 2 different mechanisms: electromagnetic and capacitive transfer.

In the presence of transformer core saturation, 3rd harmonic neutral voltage will be present on the windings through electromagnetic transfer as long as the neutrals of the respective windings are not grounded. In the presence of neutral voltage on any of the transformer windings, the stray capacitance among the windings and earth will result in capacitive neutral voltage transfer.

## *2.3.3. Switching surges on both EHV and MV systems*

From Fig. 2, it can be seen that there were 37 switching operations during the 48-hour period prior to the breakdown. This unusually high number of switching operation can create lots of switching surges (with magnitude of around 7 times the rated line-to-ground peak voltage in the medium voltage system) which, when propagating through the NPP local power network, can degrade the insulation level or even cause breakdown of CB's in the local power network [8,9].

It should be noted that while there were 37 switching operations on the EHV side, none of the switching surges were captured by the transient recorder in the 345kV GIS in Fig. 3(a) due to insufficient bandwidth of the transient recorder. In a follow-up field test [9] after the event, it was found that such switching often causes switching surges of around 7 times the rated line-to-ground peak voltage!
