Page 752 - Industrial Power Engineering and Applications Handbook

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22.14 Designing a grounding grid humid environment, ground electrodes, being one example.
It is therefore mandatory that certain corrosion margins
are considered when choosing the size of ground electrodes
22.14.1 Minimum size of grid conductors to account for this in the long run, particularly during the
considered life span of the generating station, switchyard
This can be calculated from the formula derived by Sverak or sub-stations. Handbooks on corrosion suggest likely
and recommended by IEEE-80:
corrosion depths. For steel this is considered to be around
2.2% per year. For GI it will be much less. Considering
A= , I, (22.1 6) economics of steel over GI it is a common practice to use
only steel for such extensive and elaborate grounding
stations. For steel grids, the depth of corrosion in 12 years
is estimated to be around 3.48 mm in a soil having a pH
where value of 7.4. This figure can be used to determine the
A = cross-sectional area of ground conductor depth of corrosion for any number of years. Consider-ing
in mm2 the lifespan of a power generating station as 40 years, the
I, = ground fault current in kA (r.ni.s.) depth of corrosion during this period would be
This may be substituted with the estimated
maximum ground grid current, IG (Section
22.9.6), that may occur during the life of
the grounding station 1 1 1
-+-+-+...-
Tcap = thermal capacity factor from Table 22.5, i 2 3 12
1
in J/cm3/"C. This is derived from formula
(4.184) ph . ps in Ws/cm'/ "C (for details - 3.48 x 4.2785
-
refer to IEEE-80) 3.1032
ph = specific heat of ground conductor in Cal/ = 4.8 mm
gram/ "C
ps = specific weight in g/cm' This amount of corrosion will occur on each side of the
t = duration of fault in seconds electrode,
yo = thermal coefficient of resistivity at a :. total corrosion during a span of 40 years of operation
reference temperature of 20°C
p2,, = resistivity of ground conductor at a refer- = 2 x 4.8
ence temperature of 20°C in ,U R/cm
= 9.6 mm
KO (at 0°C) = reciprocal of w0
- I 20 This is an average value of metal erosion during the
Dc length of service and may vary with soil conditions. In
20
tmax = maximum allowable temperature in "C coastal areas, for instance, where the subsoil water is
tamb = ambient temperature in "C saline, erosion of metal would be much more rapid and
a further safety factor must be considered. Field experience
Typical values of the above constants for the most widely will be a better guide to assess this.
used metals are given in Table 22.5, based on IEEE-80.
22.14.3 Maximum touch and step voltages of a
22.14.2 Corrosion factor grounding station
Corrosion takes place in all metal conductors located in 3 In Section 22.8 we discussed the tolerablc step and touch

Table 22.5 Material constants
[ -:,, 1
Description Conducrivit> K,, = - 20 t,,,. Fusing PZO Tcup
-
8 a20 temperatiire "C pWcm J/tm-'/"C
Standard annealed soft copper wire 100 0 000393 234 1083 17241 3422
Commercial hard drawn copper wire 97 0 000381 242 1084 17774 3422
Copper-clad steel core wire 40 0 000778 245 1084/1300 4397 3 846
Copper-clad steel core wire 30.0 0.00378 245 I084/1300 5.862 3.846
Commercial EC aluminium wire 61.0 0.00403 228 657 2.862 2.556
- ~~~ ~~ ~~ ~ -
Aluminium alloy wire 5005 53.5 0.00353 263 660 3.2226 2.598
Aluminium alloy wire 6201 52.5 0.00347 268 660 3.2840 2.598
Aluminium-clad steel core wire 20.3 0.00360 258 660/1300 8.4805 2.670
Zinc-coated steel core wire 8.5 0.00320 293 41 9/1300 20.1 3.931
Stainless steel No. 304 2.4 0.00130 149 1400 72.0 4.032
Refer to Figure 22.18 giving the nomograms for more widely used metals to determine the size of conductor A in terms of duration of fault.