Page 627 - Industrial Power Engineering and Applications Handbook

18/592 Industrial Power Engineering and Applications Handbook
High current non-linear region
(current predominately capacitive)
Predominately resistive

/D = Peak discharge current
or protective current
through the arrester
I" = Conducting current at
vra,
2, = Impedance of the
protected circuit
Impedance of the
arrester at Vres
Characteristic of ZnO
(m - 20 to 50)
(Rated voltage) V, Characteristic of Sic
(= - 2 to 6)
(MCO") vc
Characteristic of a linear
resistance (= = 1)

V,,,
Very low leakage I, I" = - I, = -
vi - vres
current in pA in pA ZP ZS
(0.4-1 0 mA)
(2.5-40 kA)
(peak value of
Current through the arrester .--)
8/20 ps wave)
Figure 18.4(a) Characteristics of a ZnO block
400 18.2.1 Electrical representation of a
ZnO element
200
r:: A ZnO element basically represents a capacitive leakage
circuit. In its leakage current range it may be electrically
represented as shown in Figure 18.5, where Zzno is the
leakage current, capacitive in nature, and I, and I, its
E 60
4 capacitive and loss components, respectively. The success
' of an element will depend upon its low loss-component,
3
which would mean a lower loss during continuous
40
20
Amps./cn? -
io
10-7 io-6 10-~ io4 io3 IO-^ io-' ioo 10' io2 io3

@ = 25°C @ = 100°C
0 = 50"~ @ = 125°C
Figure 18.4(b) V-l characteristics of a ZnO block

illustrate typical electrical characteristics of a ZnO arre-
ster, suggesting that in the event of a surge voltage, with
a prospective amplitude V,, its resistance will fall rapidly,
and it will absorb much of the current and energy up to
(V, - V,,,) discharged by the voltage surge. The rest, i.e.
the conducting voltage ( Vre,) depending upon the arrester's
protection level, will appear across the arrester and the Note I, would consist of resistive as well
equipment it is protecting. It has an excellent energy as 3rd harmonic component.
absorption capability. Some of its basic characteristics
according to Figures 18.4(a) and (b), are noted below. Figure 18.5 A ZnO element