EuroWire March 2017

Technical article

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

▲ ▲ Figure 7 : Measurement with broadband divider

▲ ▲ Figure 8 : Measurement with divider type WCF, undamped T partial length [µs] 8.77 8.79 8.81

• 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.

T

[µs]

v [m/µs]

calculated length [m]

full length

16.8

170.5 172.5 174.5

748 756 765

749 758 767

751 760 769

17

17.2

▲ ▲ Table 2 : Calculated cable lengths for different signal propagation times

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.

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.

▼ ▼ Figure 5 : AC cable with spark gap (detail)

▼ ▼ Figure 6 : AC source and HV divider

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 9 : Measurement with divider type WCF, damped with 150Ω

After

this

initial

test

the

same

measurements

with

an

undamped

capacitive divider were carried out.

91

www.read-eurowire.com

March 2017