During the operation of transformer, iron core and fixed iron core that winding metal structure, parts, components are all in the strong electric field under the action of the electric field. They have a high potential to the ground. If iron core would not ground which create potential difference between grounded clamp and fuel tank. Under the function of potential difference would produce continued electric discharge phenomena.
During the transformer operation, the surround of winding exist strong magnetic field and the iron core, metal structure, parts are all in heterogeneous magnetic field. The distance between them are not same that exist potential difference although is small which would breakdown tiny insulation gap. So it would cause continued Microdischarge phenomenon. To ground the iron core, fixed iron core, winding metal structure, parts, component, so that they are equivalent to the oil tank at ground potential.
Transformer core grounding can only be one-point. Because the silicon steel sheets of the iron core are insulated from each other to prevent large eddy currents. It is not possible to ground all the silicon steel sheets or multi-point grounding. Otherwise it will cause a large eddy current and make the core The transformer core grounding that the iron core of any piece of silicon steel grounding. Although there is insulation between the silicon steel sheets, the insulation resistance value is very small. The non-uniform strong electric field and strong magnetic field can make the induced high voltage charge flow to the earth through the silicon steel sheet. But it prevents the eddy from flowing from one side to the other. So just one piece of grounding is the same as the whole core grounding.