Ampacity calculations form the
heart of underground transmission and distribution cable system design.
Accepted ampacity calculation procedures have existed for many decades - the
classic Neher-McGrath procedure was published in 1957. However there have
been significant recent improvements in portions of the calculations, in
cable materials such as laminated paper-polypropylene insulation and cross
linked polyethylene cables and, most importantly, in backfill thermal
quality and in the accuracy of characterizing the effects of the cable's
external environment (i.e. soil). Published papers report a 34% improvement
in ampacity by modifying the cable thermal environment, and a 10 - 15%
improvement is considered common.
Although many engineers routinely perform cable
ampacity calculations with PC-based programs, there is still a need for a
good understanding of the basic principles and calculation procedures. This
is especially true as higher ampacities are required, and as it becomes more
difficult to install new circuits, so that existing circuits must be
operated more efficiently. Trench optimization is becoming more common as
utilities attempt to obtain the maximum amperes from a cable. More attention
is being paid to proper modeling of daily, weekly, and even monthly loss
factors to take into account cooldown periods during load cycling, which
permits higher loading during peak periods.
In an underground cable thermal circuit, the earth
portion provides the largest single thermal resistance and thermal
capacitance, and it also has by far the greatest variability - both with
distance along the route and with time. Earth thermal resistance (a function
of soil thermal resistivity and cable burial depth) can easily vary
threefold on a specific circuit, and the cable must be rated for the
worst-case condition.
This most important thermal resistance is,
unfortunately, also the least well understood by cable engineers. The recent
improvements in thermal measurement instruments and techniques, coupled with
a better understanding of soil mechanics, and innovations in corrective
thermal backfills - such as Fluidized Thermal Backfill™ (FTB™) - permit much
better representation and control of the earth thermal circuit, resulting in
significant ampacity increases.
Topics of particular importance to underground
cable users include:
- How can I install the lowest cost system
today, and meet the required loadings in twenty years?
- Ampacity effects of different XLPE cable
shield constructions and dielectric losses in distribution cables.
- Effects of higher cable temperature operation.
- Ampacity implications of open-cut vs.
directional drilling: burial depth effect, grout material in the casing,
soil thermal resistivities, temperature and load monitoring.
- Installing extruded-dielectric cables in steel
pipe.
- Do measures to reduce magnetic field always
reduce ampacity as well?
- Uprating existing cables in order to defer new
installations.
- Soil thermal stability: interface temperature
vs. heat flux density.
- Effects of hourly load profiles - daily vs.
weekly.
- Monitoring cables and using results
effectively.
Particular topics pertaining to the earth thermal
circuit include:
- Soils: classification and geotechnical
testing.
- Factors affecting soil thermal resistivity.
- Thermal resistivity measurement and test
equipment.
- Soil thermal diffusivity.
- Thermal stability ("thermal runaway").
- Elements of a cable route thermal survey.
- Effect of soil moisture and soil compaction on
thermal resistivity.
- Corrective thermal backfills - design,
installation, and quality control.
- Special situations: submarine cables,
directional drilling.
These topics can be explained and discussed in
seminars presented by Geotherm
