GE RPV311, Technical Manual

Page: 151 / 295

RPV311 Chapter 9
– TW Fault Locator
RPV311-TM-EN-7 151
𝑉
𝑤𝑎𝑣𝑒
𝑜𝑣𝑒𝑟ℎ𝑒𝑎𝑑
=
𝐾𝐶
√𝜇
𝑟
𝑎𝑖𝑟
𝜖
𝑟
𝑎𝑖𝑟
≅ 1.0𝐶
In overhead lines, the velocity of the fault wave is very close the speed of light,
between 98% and 99.5%, it means that the factor K ranges from 0.985 to 0.995.
Calculating the K precisely is a very complex task, because it depends on unknown
constants of permeability and permittivity of the overhead cable dielectric, which in
this case is the air. That means that these constants may vary with humidity,
atmospheric pressure and distance between the conductors of the transmission line
in question.
Therefore, during commissioning, the K is usually set to 0.99 and improved later
based on the data from subsequent fault location distances.
6.2 K Factor Calculation – Underground Section
In this type of table the insulation material have permeability and permittivity well
defined for the entire cable, which makes it easier to calculate the k factor more
accurately.
A common material used in the insulation of these cables is the XLPE. The example
below shows the calculation of the K parameter using XLPE cable and its
permeability and permittivity constants, where the result is K = 0.66.
𝜇
𝑟
𝑥𝑙𝑝𝑒
≅ 1 𝜖
𝑟
𝑥𝑙𝑝𝑒
≅ 2.3
𝑉
𝑤𝑎𝑣𝑒
𝑜𝑣𝑒𝑟ℎ𝑒𝑎𝑑
=
𝐾𝐶
√𝜇
𝑟
𝑎𝑖𝑟
𝜖
𝑟
𝑎𝑖𝑟
≅ 0.66𝐶