Page 655 - Industrial Power Engineering and Applications Handbook

18/620 Industrial Power Engineering and Applications Handbook
Example 18.5 (A) Checking for the protective level
Step-by-step selection of a surge arrester for the protection
of equipment mounted on a 400 kV transmission line. For this rating, the arrester protection levels are:
Considering conditions (i) and (ii) occurring at the same instant,
:. total TOV = 1.4 x 1.1 = 1.54 for 3 seconds Vres(max) for a switching impulse wave of 1 kA
for the purpose of energy capability = 654 kV.
TOV factor as in Figure 18.16(b), K= 1.13
For switching impulse protective margin at 2 kA
= 676 kV.
Vres(max) for a 20 kA lightning impulse (8/20 ps)
For this voltage, we have selected a Class 4 arrester, which = 844 kV.
has the nearest next higher rating as in Table 18.11 as Vre,(max) for a 0.6 ps FOW from Figure 18.20
V, = 336 kV = 1.12 x 844 = 945 kV.
BIL of the equipment to be protected, from Table 13.3
As regards the primary selection of the arrester, it is complete list I (considering the lower side to save on the cost of
at this stage. But this selection must be checked for its adequate equipment)
protective level and the energy absorption capability
For switching surges = 950 kV
For lightning surges = 1175 kV
Considerations Data
For very fast-rising surges (FOWs) = 1175 kV (assumed).
Nominal system voltage (4) 400 kV(r.m.s.) This value must be obtained from the manufacturer of
Maximum system voltage (V,) 420 kV(r.m.s.) the equipment to be protected.
&
In per unit (P.u.) 1 p.U. = 420 x - To check the protective margins:
6
= 343 kV
Maximum continuous operating
voltage (MCOV), 7 vrn = vc 420
6
~ = 243 kV
(max.)
Type of Margins 1 Margin Suitability
surge recommended available of the
Temporary overvoltages (TOVs) OV tripping as in Table arrester
as in Table 18.5 factor time 18.2
(i) Due to a ground fault for a
solidly grounded system with
a GFF of 1.4 TOVl 1.4 3 sec (i) For switching 1.15 950 = 1.40 OK
676
(ii) TOV due to load rejection surges
TOV2 1.1 1 sec (ii) For lightning 1.05 -- 1175 - 1.39 OK
844
(iii) TOV due to a short-circuit surges -- 1175 - 1.24 OK
fault TOV3 1.1 3 sec (iii) ForFOWs 1.15 945
Table 18.11 Protective characteristics of a gapless station class surge arrester for, (1) Line discharge class 4, and (2) Single
impulse energy capability = 7 k.J/k.V,

System Rated Max. MCOV TOV capability Maximum residual voltage with current wave
voltage voltap cont. as in for
operating ANSI
voltage tests Switching surgesa Lightning surges 8/20 ps
(CO V)
1 kA 2 kA 3 kA 5 kA 10 kA 20 kA 40 kA
kVpeak kVpeak kvpeak kVDeak kVDeak kVDeak kvpeak
420 330 264 267 383 363 642 664 680 71 8 759 829 91 1
336 267 272 390 370 654 676 693 73 1 773 844 928
360 267 291 418 396 701 724 742 783 828 904 994
~
372 267 301 432 409 724 748 767 81 0 856 934 1027
378 267 306 438 416 736 760 779 823 870 949 1044
381 267 308 442 419 74 1 766 785 829 877 957 1052
~
390 267 315 452 429 759 784 804 849 897 979 1077
396 267 318 459 436 771 796 816 862 911 994 1093
420 267 336 487 462 817 845 866 914 966 1055 1160
aAny impulse with a front time longer than 30 ps.
Courtesy: ABB