55 / 64
53
www.read-wca.comWire & Cable ASIA – May/June 2017
The triggering of the signal processing is also essential
for the measuring quality and accuracy. The simulation
and experiments confirmed that a simple edge triggering
is sufficient on DC cable systems. On AC cable systems
the operational voltage itself prevents such simple edge
trigger. A very fast breakdown detection device is therefore
implemented to release the signal processing.
The required components of the transient recorder depend
on its application for cable testing or monitoring. The
recorder for cable testing is a small plug-in device which
operates by its own PC or by that of the computerised HV
test system. It mainly contains the measurement hardware
(
Figure 4
).
The recorder for cable monitoring is a robust and small
stand-alone device. Besides the measurement hardware
it contains a PC with especially adapted software. This
PC runs continuously over years, can be restarted
and operated remotely and must be supplied by an
uninterrupted power supply (UPS) just in case the cable
fails (
Figure 4
).
Experimental Tests
Practical measurements on different cable samples were
carried out to prove the measuring principle and the
simulation results.
Thereby the AC or DC cable samples were wound on cable
drums or turntables.
AC 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, broad-
band divider (resistive-
capacitive), AC high
voltage divider (un-
damped 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 10kVrms until the spark
gap was 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
.
❍
❍
Figure 4
:
3D models of transient recorders for cable tests (left)
and cable monitoring (right)
❍
❍
Figure 5
:
AC cable with spark gap (detail)
❍
❍
Figure 6
:
AC source and HV divider
❍
❍
Figure 7
:
Measurement with broadband divider