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Technical article
March 2017
91
www.read-eurowire.comAC cable (XLPE, 20 kV)
The test configuration consisted of two MV
cables connected together in series and of
slightly different lengths (
Figure 5
).
Parameters:
• Cable 1:
758m
• Cable 2:
708m
• Further parameters:
unknown
• AC voltage:
up to 10kV, 50Hz, connected to near
end of cable 1 (see
Figure 1
,
Figure 6
)
• Measurement equipment:
transient recorder for fault location,
broadband
divider
(resistive-
capacitive), AC high voltage divider
(undamped capacitive)
The artificial breakdown was generated
by using a spark gap (
Figure 5
) which
was installed either at the far end of
the complete cable length or at the
connection point between the two cables.
The voltage was increased up to 10kV
rms
and until the spark gap got fired. The
resulting signals of the travelling waves
were recorded.
The signals were taken from the HV circuit
using a resistive-capacitive broadband
divider (for reference measurements) or
an undamped capacitive HV AC divider of
type WCF
[6]
(
Figure 6
).
The HV divider output was connected
with the transient recorder by a coaxial
measuring cable.
The reference measurement with the
broadband divider is shown in
Figure 7
.
Thereby channel 1 (Ch1, blue) shows
the signal reflections when the spark
gap is connected at the far end of both
cables and channel 2 (Ch2, red) shows
the signal reflections when the spark gap
is connected to the connection point
between the cables.
The upper diagram is the complete signal
recording over about 300µs. In the middle
diagram the first and the second reflection
are zoomed out. In the lower diagram the
differentiated curves are shown with Ch11
related to Ch1 and Ch12 related to Ch2.
From this measurement the propagation
velocity is determined to
v
= 172.5m/µs
based on T = 17.0µs of Ch1 and according
to
Equation 2
. Now the
T
x
= 8.79µs of Ch2
indicates exactly the length of the cable
sample of 758m.
Assuming an uncertainty of ±0.2µs of the
time evaluation for both full length and
partial length, the following cable lengths
to failure can be estimated. Based on the
determined cable length of 758m the
maximum deviation is 11m, which is 0.75
per cent of the full cable length.
Furthermore, the measured signal shows
a significant decline. This comes from the
damping of the cable itself and from its
dispersion. Comparison of the waveforms
in Ch1 and Ch2 show that the reflection
losses are also a substantial part of the
cable losses, because the decrease of the
voltage as a function of the number of
reflections is more or less constant.
After
this
initial
test
the
same
measurements
with
an
undamped
capacitive divider were carried out.
The goal was to find out if it is possible
to get usable results of fault location
even with a voltage divider with a lower
bandwidth (
Figure 6
).
Figure 8
shows the results of a
measurement with a divider type WCF
normally used in resonant test systems
for cable tests. It is clear to see that such
a divider is actually not suitable for such
fast transient measurements. Nevertheless,
there is still a possibility to evaluate a fault
position. In the lower diagram of
Figure 8
the curves are filtered with a numerical
low-pass Bessel filter to find the transition
points of the reflection.
Assuming a well-known propagation speed
(172.5m/µs) the fault can be located at
759m. But it is clear that the uncertainty of
determination is much higher than before.
A second test with the same divider was
performed, but this time the divider type
WCF was damped with a resistor of 150Ω.
It is shown that the damping resistor
eliminates the majority of the oscillations
after the transition in the waveform.
Therefore, a further filtering is not
necessary for the evaluation.
▼
▼
Figure 5
:
AC cable with spark gap (detail)
▼
▼
Figure 6
:
AC source and HV divider
▲
▲
Figure 7
:
Measurement with broadband divider
▲
▲
Figure 8
:
Measurement with divider type WCF,
undamped
T
partial length
[µs]
8.77
8.79
8.81
T
full length
[µs]
v [m/µs]
calculated length [m]
16.8
170.5
748
749
751
17
172.5
756
758
760
17.2
174.5
765
767
769
▲
▲
Table 2
:
Calculated cable lengths for different signal propagation times
▲
▲
Figure 9
:
Measurement with divider type WCF,
damped with 150Ω