EuroWire March 2017

Technical article

As before, the fault can be located with the well-known propagation velocity: the result of the calculation is 758m.

DC cable (PE (for DC), > 100kV) The test configuration consisted of one cable on a turntable. The cable was connected to an adjustable DC source. The breakdown test was performed by using a spark gap at the far end of the cable ( Figure 10 ). The voltage was increased until the spark gap got fired. The resulting travelling waves were recorded.

▲ ▲ Figure 10 : DC cable, detail spark gap and attenuator

▲ ▲ Figure 11 : Measurement equipment

with known v 0

Voltage kV

Cable length l 1

[m] Velocity l 1

, with known l 0

[m/µs]

+ 6.5 - 6.5

778 776 780 777

171.4 171.7 170.9 171.7

+ 11.5 - 11.5

▲ ▲ Table 3 : Calculated cable lengths and propagation velocity

They also have shown that damping and dispersion of the measured signal depend strongly on the monitored cable. Nevertheless, the experiments have been limited to a relatively low voltage and to a short cable length. There has been no further knowledge about the behaviour of cables which are laid in the soil or in the sea. It is assumed that the much higher voltage during test or operation will have a positive effect on the measured signal. It is also presumed that the dispersion and damping on a laid cable is lower than on the drum or turntable. Furthermore, the reflection losses as seen in the measurements should not play a big role in a real situation. All of these assumptions are not proven so far. Therefore, the results of the described tests can be taken as a first step, which has to be continued with field tests on laid cables. The proposed method might be helpful as a monitoring tool during commissioning or routine tests on long cables, but also as an always-online tool to monitor the a cable under service conditions. In case of a fatal breakdown the monitored signal shall help to find the location of the fault in a very short time and without further investigations. n

[4] CIGRÉ 297. Practical aspects of the detection and location of partial discharges in power cables [5] Leißner, Sebastian, Untersuchungen zur Fehlerortung an langen HVDC-Kabeln, Diplomarbeit, 2013 [6] Highvolt data sheet 1.31/4, AC Capacitor, Type WC

Parameters: • Cable: 779m • Capacity: 310nF/km • Inductivity: 110µH/km • Voltage:

up to 12 kV, DC, both polarities

• Measurement equipment: transient recorder for fault location, broadband divider (resistive-capacitive attenuator) ( Figure 10 , Figure 11 ) The same measurements as with the AC cable were performed.

▲ ▲ Figure

12 :

Measurement with broadband

attenuator and negative DC voltage

From Equation 1 the propagation velocity v 0 can be calculated as 171.25m/µs. With that information the cable length l 1 can be determined. As a cross check the propagation velocity v 0 was calculated from the measurement with the known cable length l 0 . The maximum deviation from the reference values is < 0.4 per cent. Field Tests, Conclusions The experimental tests have shown the practical feasibility of the proposed method for fault location on AC and DC cables.

References

Highvolt Prüftechnik Dresden GmbH Marie-Curie-Straße 10 D-01139 Dresden Germany Tel : +49 351 8425 700

[1] CIGRÉ 490. Recommendations for Testing of Long AC Submarine Cables with Extruded Insulation for System Voltage above 30 (36) to 500 (550)kV [2] CIGRÉ 496. Recommendations for Testing DC Extruded Cable Systems for Power Transmission at a Rated Voltage up to 500kV [3] IEC 62067. Power cables with extruded insulation and their accessories for rated voltages above 150kV (Um = 170kV) up to 500kV (Um = 550kV) – Test methods and requirements

Email : sales@highvolt.de Website : www.highvolt.de

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March 2017