An oil filled high power transformer for high voltages with coils. The transformer including a number of stacked layers of, in the main concentric, insulated conductors forming transformer coils. The winding layers are separated by spacers. One or more spacers are provided with at least one integrated electrical discharge barrier extending off the central body of the spacer in the vicinity of the area where the spacer is in contact with a winding. Breakdown along spacer and alongside the spacer-oil interface is reduced getting improved breakdown strength of the oil filled transformer.