Page 629 - Industrial Power Engineering and Applications Handbook

18/594 Industrial Power Engineering and Applications Handbook
The types of surges referred to above are defined in The protection level of an arrester, V,,,, is a function
Table 18. I. of the magnitude of arrester discharge current (I,), and
the time to peak of the surge (tl), and is influenced by
the following.
18.4 Protective margins
18.4.1 Steepness (tl) of the FOW
On the BIL discussed above a suitable protective margin The protection level of the arrester diminishes with the
is considered to provide sufficient safety to the protected steepness of the wave. As tl falls, V,,, of the arrester
equipment against unforeseen contingencies. IEC 6007 1 - rises, leaving a smaller protection margin across the
2 has recommended certain values to account for these protected equipment. Refer to the characteristics of an
and they are given in Table 18.2. arrester as shown in Figure 18.7, for a 10 kA, 8/20 ps
impulse wave. For a front time of, say, 0.5 ps, it will
BIL of the equipment have a V,,, of approximately 118% of its rated V,,, at 8
Protective margin = (1 8.2)
Impulse protection level ps, and hence will reduce the protection margin as in
of the arrester ( VreS) equation (1 8.2).

Table 18.1 Defining a surge
Predominant Maximum system Power system Voltage shape Equivalent current BIL of the
surge voltage shape at which equipment
vm the arrester is
tested'
Lightning > I kV- Secondary 1.2/50 ps 8/20 ps See Section 18.3
245 kV transmission or
primary
distribution

Lightning Mainly 1.2/50 p5 8/20 ps
> 245kV' transmission See Section 18.3
Switching 250/2500 ps 30/60 ps
(total time t2 = 2750 ps,
Figure 17.2(b)
~- .- ._~ ~.
Rise time, t, (Figure 17.3)
may be less than
FOW > 245 kV Mainly 0.1 ps but total time up to 1/20 ps Note 3
(switching) transmission 3000 ps and surge
frequency 30 kHz to < 100
MHz
Notes
A lightning strike may commence at around IO2 to 10' Volts (1000 kV) between the clouds and the ground. By the time it reaches the
ground, it loses a part of its intensity. Although it may still be around 1000 kV at ground level, it is possible that sometimes switching
surges at an EHV system above 245kV are more severe than a lightning surge, the more so because the amplitude of a switching surge
rises with the rise in system voltage, while a lightning strike remains nearly constant irrespective of the system voltage. For these
voltages, the national and international standards have prescribed separate impulse withstand levels as noted in Table 13.3 for switching
as well as lightning surges. They have also classified these severities in categories I, 2 and sometimes 3, depending upon the extent of
system exposure to lightning as noted in Section 13.4.1(3).
In a surge arrester, it is easier to assess the severity of a voltage wave through an equivalent current wave, but it is found that the
characteristic of an equivalent current wave is not exactly identical to the required voltage wave. It is noticed that the time of rise of
a voltage wave is generally shorter than its equivalent current wave, and hence more severe than the current wave. The reason is the
non-uniform distribution of the current through the cross-section of the conductor, because of skin effect and discontinuities as
discussed earlier. Refer to Figure 18.6 explaining this. To ovcrcomc this deficiency, the actual time of rise of the test current surges,
while simulating the characteristics in a laboratory, is slightly shortened (for an 8/20 ps wave, the test wave rise time will be slightly
less than 8 ps), to ensure the same severity of the test current wave as the actual voltage wave. A surge arrester is required to clear
successfully all the three types of voltage surges as prescribed. It is imperative to ensure that the selected arrester is capable of clearing
all such voltage surges with the same ease and safety. Accordingly, protective curves are established by the arrester manufacturers over
a wide range of likely surges, in terms of lightning, switching and FOWs. They provide those to the user for ease of arrester selection
(Figure 18.7).
For steep-rising waves (FOWs), no steepness or impulse withstand level is prescribed in these standards, as both the rise time and
amplitude of such waves cannot be predefined. They will depend upon various system parameters, such as grounding method, cable or
line length, other equipment installed on the system, their surge impedances, switching conditions (current chopping and restrike of
interrupting contacts etc.) and the trapped charge, such as on a fast bus transfer etc. The choice of impulse level for a particular fast-
rising wave for equipment to'be exposed to such transients is a matter of system study (such as TNA or EMTP, Section. 18.5). The user
must define these for the equipment manufacturer.