7.4: Basic Insulation Level

7.4 Basic Insulation Level

The AC operating voltage of a transformer winding is shared equally by all the turns in the winding, and the turn-to-turn voltages are constant. Surge voltages, however, can create much higher turn-to-turn voltages at the exposed end of the winding due to capacitance distributions. Figure 7.8 shows an elementary five-section representation of these capacitances. The turn-to-turn and turn-to-ground capacitances are equal, and the inductance is large enough to be ignored. Note that nearly 62% of the incident surge voltage is taken up on the first turns. The turn-to-turn spacing and turn-to-turn insulation are sometimes increased at the HV end of windings in medium-voltage transformers, and grading shields are used to improve the surge voltage distributions.

Figure 7.8: Surge voltage distribution in a transformer winding.

The ability of a transformer to withstand these surge voltages is described by the basic insulation level (BIL), the peak voltage of a standard test pulse with a rise time to peak value of 1.2 ?s and then a decay to half value in 50 s. BIL voltages and sine wave test voltages are standardized for various voltage classes and types of transformers. These values are shown in IEEE C57.12.00 for oil-filled transformers and C57.12.51 for dry types.