Page 647 - Industrial Power Engineering and Applications Handbook

18/61 2 Industrial Power Engineering and Applications Handbook
-------+
Note
A lightning surge protective level is found to be more stringent
than the switching surge or FOW protective levels. Hence, it
Protection to be Surge arrester is sufficient to check the protective distance requirement for
near the equipment the lightning surge alone.
and not on the line In such cases, care must be taken to reduce the protective
-G distance, I, if possible, otherwise equipment with a higher BIL
may have to be selected or an arrester with a lower V,,,,
Figure 18.21 Ideal location of a surge arrester considered. Alternatively one may have to provide two arresters
in parallel, which is also an acceptable practice.

18.6.3 Required energy capability in kJkV,
This is the energy the arrester has to absorb while clearing
a switching surge. It also depends upon the distance of
the arrester from the equipment it is protecting, as
discussed above. The basic parameters that will determine
the required energy capability (I'Rt), are current amplitude,
steepness, duration and number of likely consecutive
discharges. The energy capability of an arrester depends
upon the size of the ZnO blocks. By resizing these blocks,
an arrester of a higher energy capability can be designed.
Hence, an arrester with a higher kJ/kV, capacity will be
larger than one with a lower KJ/kV,. ZnO blocks can
generally absorb much higher energies at low currents
with long durations (i.e. power frequency stresses under
normal operation) than higher currents for short durations
(i.e. surge voltage capacitive discharges). To select the
right type of arrester the energy capability of the arrester
in kJ, therefore, must be determined by what it has to
absorb on every discharge. System studies on and past
Figure 18.22 Effect of distance on the protective level experience of similar other installations can be a good
guide for the likely number of consecutive discharges an
arrester may have to perform at a time. Generally, two
consecutive discharges are considered to be adequate.
Like other equipment, a surge arrester becomes too heated
facturers and may be obtained from them for more accurate too during normal service, even when it is not conducting,
selection of an arrester. due to its continuous charging current, Zzno (Figure 18.4)
however small the loss content. When the arrester is
Example 18.4 conducting, the level of discharge current (I = kV-) is a
For the arrester of the previous example, function of the protective level of the arrester, Vres, and
the rest of the severity of the voltage surge is absorbed
V,,, = 420 kV, by the arrester.
V, = 336 kV The rating of an arrester is therefore defined in terms
V,,, = 844 kV for a lightning surge protective margin at 20 kA
discharge current, from the manufacturer (Table 18.11) of its energy capability to absorb at least the required
energy on each discharge and the number of discharges
It we consider the lightning surge with a steepness of 2000 the arrester may have to perform in quick succession. IEC
kV/ps, then for a total distance of, say, 8 m from the arrester 60099-4 has prescribed the minimum energy capability
to the equipment that an arrester must possess, at each discharge. The
- 844 ~ 2000 x 2 x 8 x graphs in Figure 18.23 indicate these levels for different
s- 0.3 classes. It may be noted that an arrester with a higher
= 844 + 107 energy capability level will mean less strain or risk of
= 951 kV failure for the arrester, but at a higher cost. A brief procedure
If we maintain a protective margin of 15% (to be more safe), to determine the energy level that the arrester may have
then the minimum BIL that the equipment under protection to absorb on each discharge is given below.
must have Of the three types of surges noted earlier (Table 18.1)
= 1.15 x 951 which the arrester may have to sustain and absorb, the
switching surge energy would be the maximum that would
= 1094 kV exist for the longest duration, compared to lightning or
which is well below the BIL of the equipment (1175 kV) as in very steep-fronted FOWs (Section 17.3) which are of
Table 13.3, list I. One can therefore safely select the arrester very short duration. Normally, therefore, a switching surge
with the next higher rated voltage, V,, at 360 kV and a V,,, at alone is considered for this purpose. During the operation
904 kV. of a surge arrester, a part of the surge, equivalent to the