From Power Electronics Design: A Practitioner's Guide

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.

Products & Services
Flyback Transformers
Flyback transformers (FBT) or line output transformers (LOPT) are designed to produce a very high voltage by storing energy in their magnetic windings.
Cast Coil Transformers
Cast coil transformers have windings that are solidly-cast in resin under a vacuum in a mold. This manufacturing process covers the transformer windings in a strong epoxy resin with a high dielectric strength.
Instrument Transformers
nstrument transformers are used to step-down current or voltage to measurable values. They provide standardized, useable levels of current or voltage in a variety of power monitoring and measurement applications.
Board Mount Transformers
Board mount transformers are transformers designed to be embedded in or mounted on computer boards.
Autotransformers are a special type of power transformer with only one winding.

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