Limitation of difference voltage level in power transformers is related to some technical limitation in power transformer construction such as withstand stress voltage between LV and HV windings. Reffering to "Transformer Engineering- Design and Practice (S.V.Kulkarni , S.A.Khaparde),we can find:
The gap between low voltage (LV) and high voltage (HV) windings is subdivided into many oil ducts by means of solid insulating barriers. The insulation system of oil cooled power transformers consists of combination of oil and solid insulations (paper and pre-compressed board). The oil and solid insulations are cheaper than most other insulating materials, and as a combination they give much higher dielectric strength than individually.
In an oil-pressboard insulation system, since the maximum admissible stress in a good quality pressboard for one-minute power frequency overvoltage is about 40 kVrms/mm, the limiting design stress values are primarily due to stress in oil. In a good quality system with degassed oil and near uniform field distribution (e.g., between barriers in the HV-LV gap), typical PD inception field strength
(stress) value for a 8 mm oil duct is 10 kVrms/mm for insulated winding conductors [46,52>. Since the stress is high at winding conductor corners, the PD inception field strength of 8 mm oil duct next to windings for degassed oil is lower, i.e., about 8.4 kVrms/mm. The allowed (design) stress values can be lower than these PD inception strength values depending on the required safety margin and quality of manufacturing processes. Thus, the dielectric strength of the solid insulation is much higher than that of the oil. Since the electric stress (E) is inversely proportional to the permittivity, the stress in the oil is significantly higher than that in the solid insulation. The oil permittivity (~ 2.2)is about half that of the solid insulation, hence the oil stress is twice the solid insulation stress in uniform fields. Since the oil has to bear higher stress, it is recommended to make barriers as thin as possible and give more space for the oil ducts. In the oil-paperpressboard insulation system, the strength of the insulation arrangement ispredominantly decided by the strength of the oil ducts. Hence, the design of transformer insulation system essentially means the design of oil ducts and oilsolid interfaces. For a given total gap between two windings, there is no advantage gained in increasing solid insulation thickness just because its strength is higher.
On the contrary, higher solid insulation thickness results in more stress in oil ducts. The barriers should be as thin as possible, permitted from mechanical strength considerations. Barriers with a thickness of 1.5 to 2 mm are generally used for getting good mechanical stability. The lower the oil duct width, the higher the withstand stress (kV/mm) is. The number of barriers in the interwinding gap should be adequate so that larger oil gaps are avoided (width of oil duct more than 12 mm is usually not recommended in the inter-winding gap).
Insulating barriers have an additional function of acting as barriers against the propagation of a discharge streamer in the oil between electrodes. The barriers break the oil path into smaller ducts and prevent lining up of impurities in the oil.
The size of first duct next to windings should be properly chosen. Although a small width is desirable as it gives higher kV/mm withstand value, thermal considerations may not allow the use of duct width below 6 mm. The first duct is usually of 6 to 10 mm size. Withstand for the ducts (first duct and other ducts in HV-LV gap) can be found out by referring published curves for four types of gaps, viz. degassed oil between barriers, gas saturated oil between barriers, degassed first oil duct and gas saturated first oil duct. The maximum stress in the first duct has to be accurately determined (say, by FEM analysis). The breakdown gradients obtained for a typical configuration are given in [24>. The values of breakdown gradients for yoke-end-line-lead arrangement (line terminal at yoke end of winding) are lower than that for center-line-lead arrangement (line terminal taken at the midpoint of the winding stack with two parallel branches). This is because the field distribution in the first duct at the line terminal is quite nonuniform for the former as compared to the latter arrangement. Actual design values should be smaller than the breakdown values depending on the margin to be kept.