A DIN transformer bushing is the main insulation device outside the transformer tank. The lead wires of the transformer winding pass through the insulating bushing of the DIN transformer bushing, providing insulation between the lead wires and between the lead wires and the transformer casing, while also securing the lead wires. Depending on the voltage level, insulation bushings can be in the form of pure porcelain bushings, oil-filled bushings, or capacitor bushings. Pure porcelain bushings are mostly used for transformers of 10kV and below. They consist of a conductive copper rod inside a porcelain bushing, with the space inside the porcelain bushing filled with air for insulation. Oil-filled bushings are mostly used for 35kV transformers. They have a copper rod inside a porcelain bushing filled with oil, with insulation paper wrapped around the copper rod. Capacitor bushings consist of a main insulation capacitor core, porcelain sections at the top and bottom for external insulation, connecting sleeves, oil reservoirs, spring assemblies, bases, grading spheres, measuring terminals, wiring terminals, rubber gaskets, insulation oil, and so on. They are used in high voltage transformers above 100kV. 

That's an introduction to the structure and function of DIN transformer bushings. Hope it helps you.