Rod Gaps
A rod gap
may be used to measure the peak value of power frequency and impulse voltages.
The gap usually consists of two 1.27 cm square rod electrodes square in section
at their end and are mounted on insulating stands so that a length of rod equal
to or greater than one half of the gap spacing overhangs the inner edge of the
support. The breakdown voltages as found in American standards for different
gap lengths at 25° C, 760 mm Hg. pressure and with water vapour pressure of
15.5 mm Hg. are reproduced here.
The
breakdown voltage is a rod gap increases more or less linearly with increasing
relative air density over the normal variations in atmospheric pressure. Also,
the breakdown voltage increases with increasing relative humidity, the standard
humidity being taken as 15.5 mm Hg. Because of the large variation in breakdown
voltage for the same spacing and the uncertainties associated with the
influence of humidity, rod gaps are no longer used for measurement of a.c. or
impulse voltages. However, more recent investigations have shown that these
rods can be used for d.c measurement provided certain regulations regarding the
electrode configurations are observed. The arrangement consists of two hemi
spherically capped rods of about 20 mm diameter as shown in Fig. 4.3.1.
The
earthed electrode must be long enough to initiate positive breakdown streamers
if the high voltage rod is the cathode. With this arrangement, the breakdown
voltage will always be initiated by positive streamers for both the polarities
thus giving a very small variation and being humidity dependent. Except for low
voltages (less than 120 kV), where the accuracy is low, the breakdown voltage
can be given by the empirical relation.
Where h
is the absolute humidity in gm/m3 and varies between 4 and 20 gm/m3 in the
above relation. The breakdown voltage is linearly related with the gap spacing
and the slope of the relation B = 5.1 kV/cm and is found to be independent of
the polarity of voltage. However constant A is polarity dependent and has the
values
A= 20 Kv
for positive Polarity = 15 Kv for negative polarity
The
accuracy of the above relation is better than ±20% and, therefore, provides
better accuracy even as compared to a sphere gap.
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