The detected
phase modulation is proportional to the total
voltage. As the voltage is distributed on several
crystals the peak fields in the vicinity of the crystals
are reduced. As a result, the quartzes can be
housed in a rather slender and thus relatively low
cost composite insulator. The insulator is filled with
a soft polyurethane resin. The solid-state insulation
eliminates the need of SF6 gas and monitoring of
gas pressure.