What is necessary for laminated cores in heavy transformers?

https://en.wikipedia.org/wiki/Transformer#Cores

Your question is very vague. I'm therefore unsure what you wish to know about transformer core laminations.

One should notice that the lamination alignment is orthogonal to the electrical winding orientation. This minimizes the crossectional area of conductive ferrous core material that makes an undesired secondary loop that makes eddy currents.

Having rebuilt many transformers both large and small there are a number of details concerning the lamination. Normally they were made of grain oriented steel and one side of the lamination was insulated with varnish to isolate each lamination from its neighbour.

The number of laminations in a group effects the amount of effort to reinstall them in the core. If the winding coils are circular then the lamination widths are varied to obtain a core of approximate circular shape.

For small cores, the laminations are in single layer but for larger cores the laminations can be in groups to suit the core shape. All the laminations need to be free of burrs so the laminations do not short to each other and set up eddy currents.

The hold down bolts and cheekplates are insulated from the core with particular attention being paid to the insulation from the lamination through holes. When assembling the cores the laminations are tapped down with copper drifts so the joins between laminations and core laminations are minimal.

The more air gaps in the joins the more the iron loss of the finished transformer will be. The actual lamination design is such that the groups overlap so there is better magnetic coupling.

Some small cores up to 10KVA single phase are wound with a continuous length of iron lamination.

From a different angle, what would be the consequence of no lamination at all. ie, using a solid lump of steel?

Unfortunately the use of a solid lump of steel would create so many eddy currents that the iron losses would be enormous and the steel would heat up to a high temperature. Think about the bearing heaters where the bearing is slipped onto a laminated bar on c section transformer, though there the bearing acts as a shorted turn. They get hot very quickly unless the temperature is controlled by the thermocouple pickup.

So much energy would be used as to make the transformer a scrap metal project or a space heater. You could always use a cast ferrite core but they are brittle and work best at higher frequencies not supply frequencies.

On one larger transformer for medium voltage we even had to modify the transformer oil tank around the LV bushings by incorporating a brass bar in the tank to reduce the circulating currents around the bushing stem. Was it needed, well an engineer decided it was needed, so it was modified to his specifications for the 2MVA 415V transformer.

One main reason for the use of laminations in the core (as mentioned earlier) is that eddy currents are limited. Essentially, the eddy currents will not pass from one lamination to the next.

By having the laminations perpendicular to the magnetic field means that that eddy currents are interrupted at each core boundary and remain small relative other losses in the transformer.