Primary side star connected transformers may have a graded insulation on their neutral point in order to optimize the design of the bushing & winding configuration. Due to the electrical characteristic of the neutral point, the Lightning Impulse withstand voltage levels can be graded compared to the primary phases. Thus, we can choose a lower insulated bushing and can design the winding accordingly. This will optimize the overall transformer design. However, this point needs to be agreed with the customer in any case.