Full resolution (JPEG) - On this page / på denna sida - 1958, H. 2 - Effect of Non-slandard Surge Voltages on Insulation, by Sune Rusck
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Fig. 2. The voltage at the points A, B and C in fig. 1.
Measurements with a constant voltage
In order to determine the function r = f(U,s) a
series of measurements of the time lag of a 1 x 1 cm
rod gap has been made at Asea’s High Voltage
Laboratory. As it was essential that the applied voltage
was constant during the whole discharge a special
circuit was developed. This circuit is shown in
figure 1.
The purpose of the rod-gap is to delay the
spark-over in the tested gap until the oscillations in the
surge generator have been damped. During the time
when the discharge is completed in the rod-gap the
tested gap is insulated from the surge generator by
a sphere gap. The function of the two gaps is
demonstrated in figure 2 showing the voltages at the
points A, B and C. It was possible to obtain a surge
with a fronttime of less than 0.3 ^is, a verry long tail
and a waveform without oscillations i.e.
approximately a step voltage.
The time lags of ultraviolet radiated rod gaps
between 10 and 70 cm with an applied voltage of up to
550 kV were investigated. The results of the
measurements are shown in figure 3. Each point shown
in the figure is the mean value of ten shots. In the
same figure it is shown that the measured points
can be summarized by the formula
t - (^r (5)
where r = the time lag in ^s
s — the distance of the gap in cm
U = the voltage in kV
The sparkover voltage U0(s) is approximately a
linear function of the distance s and experiments
show that equation (5) may also be written
where the constant r0
(UoV
=To Itt)
4.7 [is.
(6)
Checking of the assumptions
By combining the equations (4) and (6) the
following differential equation is obtained
3 Uo (jxy 3 Uo(x) dx
(7)
where U0(x) = the sparkover voltage of a gap with
a distance of x cm
J7(t) = the voltage as a function of the time.
After integration of the equation (7) we obtain
t
Uo3 (s) - Uo3 (x) = ju3 (0 dt
(8)
If we put x = O in equation (8) the time lag x is
obtained from
1 =
U(i) V
Uo(s))
(9)
The time t = O in equations (8) and (9) is taken as
that moment when the leader starts i.e. that moment
when U(t) exceeds U0(s).
According to equation (5) the equation (9) may
also be written
T
"it
u (0
(20 + 10s).
dt
(10)
By using surges with different times to the
half-value equation (10) can be checked and with it also
the assumptions made. In table 1 the results of a
series of tests with steep surges having the half-value
times Th — 5, 10 and 20 ^s are given. Every value
given corresponds to the mean value of ten tests.
From the table the conclusion may be drawn that
T
the equation J U(t)3dt = constant can be used to
o
compute the time lag. The differences between the
measured values of
u (0
20 + 10s
dt and the theo-
Fig. 3. Results of the measurement with a constant voltage.
(20 + 10s\3
The curves show the function r = ^-—-J .
Positive polarity is marked with X and negative
with *.
ELTEKNIK 1958 19
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