Defense Intelligence Reference Document Pulsed High-Power Microwave Source Technology

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LAMINATED
Laminated insulation has been used in some very demanding applications in which size
is of primary importance and very small repeating structures are required, including
high-energy-density capacitors, high-voltage transformers, and high-voltage
transmission lines or pulse-forming lines. Laminated insulation schemes make use of
the nonlinearity of electrical breakdown strength with thickness mentioned earlier for
plastics. Such schemes offer the possibility of significant improvements in state-of-the-
art insulation, including a reduction in the size and an increase in the energy density of
high-voltage pulsed systems. As new and improved materials become available, the
possibilities for such improvements will become more substantial.
PLASTIC-PAPER-OIL
With appropriate attention to process details, very high dielectric strength is routinely
achieved using this lamination scheme. This is in fact the insulation method used in
most high-voltage and high-energy-density capacitors, with the addition of foil layers
on either side of the plastic (usually biaxially oriented polypropylene) to form the
capacitor. Use of corona-processed oil dramatically improves the utility of this insulating
scheme. The plastic is frosted on at least one side and, together with the very thin (1
mil or less) paper layer, allows the oil to penetrate throughout the volume during the
impregnation process. Without the oil, the tightly wound plastic layers can become
sealed around small volumes of air that will not be filled with oil, and breakdowns will
occur. Once the paper is impregnated with the oil, tests have shown that it attains
essentially the same dielectric strength as the oil. Often, vacuum and pressure are
alternately applied to ensure full penetration of the oil into the full volume. It is vitally
important that no bubbles or voids be left in the insulation volume. For this reason,
once the insulating volume is ready for impregnation, it should be left under vacuum at
slightly elevated temperature for at least 24 hours. This not only ensures air pockets
are removed but also allows the removal of surface moisture from the plastic and
paper, which will also contribute to voltage breakdown. The paper not only aids
impregnation but also serves as a path for residual charge to dissipate between voltage
applications. The plastic has a very high surface resistivity, and some residual charge
can become trapped on the surface after each discharge, resulting in charged regions of
different magnitudes and even polarities, which can eventually lead to dielectric failure.
Using this insulation scheme with biaxially oriented polypropylene as the plastic and
Shell Diala AX as the impregnating oil, average dielectric strength of more than 2.1
kV/mil and operating voltages higher than 1.3 MV have been attained in large volumes.
PLASTIC-PAPER-EPOXY
Since the oil is the weakest dielectric medium in the preceding insulation scheme, it is
reasonable to assume that replacing it with a stronger dielectric medium can improve
the overall dielectric strength. Another advantage of this scheme is that in the end we
would have a solid insulated volume with the advantages mentioned earlier. Thus far,
only smaller volumes (1-2 gallons) have been successfully insulated with this scheme.
The problem is that the increase in viscosity over the oil, although small, makes it more
difficult to ensure that full impregnation is achieved. Meanwhile, the programs for which
this scheme is desired insist on nearly 100-percent certainty of success. The most
successful process to date involves using quarter-inch sections of 1-mil paper followed
by quarter-inch open sections for each layer. This is a tedious task in large volumes but
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