It is significantly more difficult

It is significantly more difficult for this design to meet the mechanical requirements
at the extra-high voltage levels and the new ultra-high voltage levels (UHV: Us > 800
kV) in particular. For example, the deflection under load is no longer trivial and may
cause problems. Above a certain overall length, the arrester can only be installed in a
suspended position with a moving support. In addition, the high voltage levels often
require a number of MO resistor columns to be connected in parallel, which can be very
costly because entire arresters, together with their housing, must be arranged alongside
each other. In this case, a structure that was introduced in the late 1980s may be used, as
illustrated in Figure 18. This high-voltage arrester with polymer housing uses a composite
hollow core insulator, as used with instrument transformers and bushings. To
distinguish this design from other arresters with polymer housing, it is also known as
the "tube design". One notices immediately that, in principle, this has the same design
as the one in Figure 8. Indeed, essentially only the porcelain insulator has been replaced with a composite hollow core insulator. The composite hollow core insulator is made up
of an FRP tube on which the sheds – practically only ever made out of silicone rubber –
are directly molded on, or pushed on and vulcanized in the form of individual prepared
sheds. This design principle offers some considerable advantages for applications up to
the highest voltage levels. Since the inner structure of an FRP tube (for example, relative
content of glass fibers, or the winding angle of the fibers), its wall strength and its
diameter can, within a large range, be selected without restrictions, such a tube can be
endowed with almost any mechanical property. As a result, to name just a few, it can be
optimized with respect to tensile strength, bending strength, or internal pressure
strength. Thus, it is possible to design high-voltage arresters, which are so mechanically
strong, that they can endure the most severe earthquakes intact and at the same time be
used as a post insulator in a substation.