12
Theoritically the wave form should be steep square wave, but refer
to the incomplete reverse and circuit comsuption the real wave
becomes smaller and smoother.
V. High Voltage Impulse Flashover Method
When fault point reduces the resistance and forms constant
resistance tunnal, we have to choose the voltage impulse method
for we can not add more high voltage current by the limit of
instrument capacity. High voltage current through sphere gap can
charge the cable until spark-over, and we still use the reverse wave
produced by the flashover electirc arch. At the input terminal of
cable we detect the inductance L to read the reverse wave. The
circuit principle graph is as below. Electric wave reverse at the fault
point and reverse by L at the input terminal, so it forms multiple
reverse. Because of the self-inductance phenomenon, it shows as
open reverse at frist for the resistant effort of L, and shows as short
circuit reverse by the acumulation of current. And the whole circuit
is made of process of discharge by electric capacity C and
inductance L. So at the output terminal the curve shows as a
reducing cosine wave added by several quick impulse reverse
waves. As the graph below. We can get distance of fault from the
internal of reverse wave.
Ideal wave
Real wave
Figure 3
