Three factors must be considered in the
evaluation of the dielectric capability of an insulation structures
—the voltage distribution must be calculated between different
parts of the winding, the dielectric stresses are then calculated
knowing the voltages and the geometry, and finally the actual
stresses can be compared with breakdown or design stresses to
determine the design margin.
Voltage distributions are linear when
the flux in the core is established. This occurs during all power
frequency test and operating conditions and to a great extent under
switching impulse conditions (Switching impulse waves have front
times in the order of tens to hundreds of microseconds and tails in
excess of 1000 μs.)
These conditions tend to stress the
major insulation and not inside of the winding. For shorter-duration
impulses, such as full-wave, chopped-wave, or front-wave, the voltage
does not divide linearly within the winding and must be determined by
calculation or low voltage measurement. The initial distribution is
determined by the capacitative network of the winding.
For disk and helical windings, the
capacitance to ground is usually much greater than the series
capacitance through the winding. Under impulse conditions, most of
the capacitive current flows through the capacitance to ground near
the end of the winding, creating a large voltage drop across the line
end portion of the coil.
The capacitance network for shell form
and layer-wound core form results in a more uniform initial
distribution because they use electrostatic shields on both terminals
of the coil to increase the ratio between the series and to ground
capacitances.
Static shields are commonly used in
disk windings to prevent excessive concentrations of voltages on the
line-end turns by increasing the effective series capacitance within
the coil, especially in the line end sections.
Interleaving turns and introducing
floating metal shields are two other techniques that are commonly
used to increase the series capacitance of the coil.
Following the initial period,
electrical oscillations occur within the windings. These oscillations
impose greater stresses from the middle parts of the windings to
ground for long-duration waves than for short-duration waves.
Very fast impulses, such as steep
chopped waves, impose the greatest stresses between turns and coil
portions. Note that switching impulse transient voltages are two
types— asperiodic and oscillatory. Unlike the asperiodic waves
discussed earlier, the oscillatory waves can excite winding natural
frequencies and produce stresses of concern in the internal winding
insulation.
Transformer windings that have low
natural frequencies are the most vulnerable because internal damping
is more effective at high frequencies. Dielectric stresses existing
within the insulation structure are determined using direct
calculation (for basic geometries), analog modeling, or most
recently, sophisticated finite-element computer programs.
Allowable stresses are determined from
experience, model tests, or published data. For liquidinsulated
transformers, insulation strength is greatly affected by
contamination and moisture. The relatively porous and hygroscopic
paper-based insulation must be carefully dried and vacuum impregnated
with oil to remove moisture and gas to obtain the required high
dielectric strength and to resist deterioration at operating
temperatures.
Gas pockets or bubbles in the
insulation are particularly destructive to the insulation because the
gas (usually air) not only has a low dielectric constant (about 1.0),
which means that it will be stressed more highly than the other
insulation, but also air has a low dielectric strength.
High-voltage dc stresses may be imposed
on certain transformers used in terminal equipment for dc
transmission lines. Direct-current voltage applied to a composite
insulation structure divides between individual components in
proportion to the resistivities of the material.
In general the resistivity of an
insulating material is not a constant but varies over a range of
100:1 or more, depending on temperature, dryness, contamination, and
stress. Insulation design of high-voltage dc transformers in
particular require extreme care.
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