Page 645 - Industrial Power Engineering and Applications Handbook

18/61 0 Industrial Power Engineering and Applications Handbook
by drawing its elements' I 0~ V" characteristics as shown
in Figure 18.17 and drawing a load line on it to represent
the characteristic of the prospective surge. It is drawn
with V, as the ordinate and the current through the
equipment I, (had there been no arrester) as the abscissa:
VI
Then I, = -
z,
and the protective current through the arrester on a
discharge
Voltage absorbed or the voltage that is
discharged to ground
I, =
Impedance of the line and the equipment

Discharge current - 7 where V,,, = residual surge voltage across the surge
4
V, = prospective surge voltage
/ - VI - Vre,
/"
-
p
arrester) which must be below the equipment
1. Surge arrester protective characteristics arrester (the impulse protective level of the
2. BIL of the equipment. BIL, with a sufficient protective margin,
3. Actual voltage surge. and Z, = equivalent impedance of Z, and Z2.
(the impedance of arrester, being too small,
Figure 18.17 Load diagram
is ignored)
The criteria to determine the safe protective level of
an arrester are the BIL of the equipment, as shown in
Table 1 1.6 for motors, Tables 13.2 or 14.1 for switchgears,
Table 32.1(A) for bus systems, and Tables 13.2 and 13.3
for all other systems. Motors have a comparatively lower
BIL, but they are not connected directly on an outdoor

Constant
i qe7Thes impedance a
r4
Z,
. .. r - ----L ~.
Network with
,
,
I ____.
:
~
1
linear circuit
ZI
I
r
parameters
--
......
I
;-- ---L..
I
I
,
and constant
voltage sources
m x9
May be connected
to any other (b) b
(b)
network. Thevenin's
equivalent circuit
-G TG
Z, = Source impedance
Z, = Impedance of equipment being protected
Z, = Impedance of surge arrester. At V,, it is negligible Z,
Figure 18.18 Equivalent circuit applying Thevenin's theorem
conducting at a certain voltage and carry a certain
discharge current. The voltage at which the conduction
will start is the impulse protective level of the arrester (c) (d)
and is termed the residual voltage (Vre5) of the arrester. A simple power circuit Equivalent impedance
By a manufacturer it is determined for each particular
arrester to establish its protective level. It is established Figure 18.19 Thevenin's theorem