Page 625 - Industrial Power Engineering and Applications Handbook

18/590 Industrial Power Engineering and Applications Handbook
R Y B resistance used in the manufacture of an efficient surge
protection device must offer the least impedance during
a discharge. This is to provide a free flow to the excessive
discharge current to the ground, on the one hand, and to
sw
draw a negligible current under normal system conditions,
to make it a low-loss device, on the other. The alternative
was found in ZnO, ZnO is a semi-conductor device and
fl -3 is a ceramic resistor material constituting ZnO and oxides
of other metals, such as bismuth, cobalt, antimony and
manganese. Since the content of ZnO is substantial (around
90%) it is popularly known as a ZnO or metal oxide
surge arrester. It has no conventional spark gap and has
excellent energy absorption capability. It consists of a
stack of small ZnO disks (Figure 18.2(a)) in varying
C sizes and cross-sections, enough to carry the discharge

OCR

Figure 18.l(a) A typical power circuit of a non-linear resistor-
type surge arrester

V, profile becomes flat (constant) and less
than impulse voltage withstand capability (BIL)
of the equipment under protection Figure 18.2(a) ZnO blocks and their small sizes

A
-V,

t t

U 5-

Figure 18.1 (b) Characteristic of a non-linear resistor

a near-constant voltage at around the switching surge
or lightning surge spark-over voltages during the flow
of surge currents while clearing a surge.
18.1.2 Gapless surge arresters

From the above it is evident that material for non-linear Figure 18.2(b) Distribution class surge arrester