Jicable’11, June Versailles - France EXPERIENCE WITH JOINT FAILURES IN NEW SMART- GRID MV CABLE CIRCUITS Frank DE VRIES, Liandon BV, (The Netherlands), Jacco SMIT Liandon BV, (The Netherlands), Ger SEBREGTS, Liander NV, (The Netherlands)

Index Alliander test program of MV cables Failures in the network Failure investigation Additional laboratory tests Cable load Conclusion and further steps Jicable’11, June Versailles - France

Testprogram All new installed cables are tested according NEN3620 –Voltage Withstand Test cablesheath 5kVDC-5 min –Voltage Withstand Test main insulation 3xUo, 0,1Hz-15min –PD measurements (under consideration) At Alliander all cable systems with joints are tested with PD measurements up to 2xUo. Jicable’11, June Versailles - France

Failures in the network 2 failures in 2006 in a windfarm in North-West of the Netherlands 2 more failures in the same windfarm in 2007 All cables passed the after installation tests executed in 2002 Failures were related to poor workmanship In failures occurred in a total other region of Alliander In all cases the failures were caused by thermal problems A company broad investigation was started in 2009 Jicable’11, June Versailles - France

Results from the investigation 23 thermal failures were found 21 failures in cable circuits at DG Most failures occur after 4-6 years in service All failures appeared in 3 phase cables with solid Aluminum conductors In all cases a compressed type connector was used Different joint manufacturers Failures do not appear in one specific region Jicable’11, June Versailles - France

Failure investigation Jicable’11, June Versailles - France In many cases the failure occurred outside the inner joint body In a few cases the failure occurred around the connector

Failure investigation Jicable’11, June Versailles - France

Connector resistances Resistance of a new connector is 15µΩ Connector resistances of failed joints are over 1000µΩ ! –At a cable load of 300A this leads to heat losses over 90W per phase!! Yellow: New installed connectors Red: Connectors of failed joints Jicable’11, June Versailles - France

Temp. development Laboratory tests show an induced current through earth wires of 0,5A. Field measurements showed no abnormalities in harmonic current Jicable’11, June Versailles - France

Cable load Cables of DG show significant difference in load pattern compared to normal MV distribution cable –Dynamics –Maximum –Direction Jicable’11, June Versailles - France

Cable load Maximum cable load for XLPE cables is based on a conductor temperature of 90ºC and is calculated by IEC60287 In our practical company standards laying parameters (e.g. thermal resistivity of soil, depth, other heatsources, etc..) for MV cablecircuits are estimated values and not further investigated. Thermal resistivity of the soil = 0,75Km/W Cable depth = 1m Ground temperature = 15C No other heat sources are present I 2 x R –Conductor / connector –Earth system Jicable’11, June Versailles - France Cable temperature P w-in P w-out

Laying conditions For one specific wind farm the laying conditions of several cable circuits were further investigated. It seemed that the properties of the practical company standards were not met in the practice. Jicable’11, June Versailles - France Maximum continuous cable load [A/MVA] according to company standardsaccording to the installation in practice 350 A / 6 MVA261 A / 4,5 MVA

Conclusions In the last 5 years several failures occurred in new “smart grid” networks at Distributed Generation Units (wind farms, heat couplers, etc.). These failures occur in MV joints of XLPE cable circuits after a few years in operation. The failures are caused by thermal problems coming from the compression type connectors on solid Aluminum conductors. This seems to be a sensitive connection especially when applied in dynamic and high loaded circuits. Significant increase of contact resistance was found in the compression type connectors of failed joints. In many cases the compression type connector reached temperatures far above the allowable limits causing the semicon layer to fail and subsequently the break down of the joint. In DG networks extra attention is needed for the laying conditions of the cable circuit in order to determine the maximum cable load. Jicable’11, June Versailles - France

Further steps / recommendations For the existing cable circuits in DG networks Alliander has developed a strategy for the future based on different actions –Analyses of the network to determine most suspicious cable circuits –Preventive replacement of joints –Installation of Smart Cable Guard (online PD monitoring systems with localization) on selected cable circuits –Temperature measurements on selected joints Engineering standards should be taken under consideration for new cable circuits in DG Investigate the most suitable after installation tests for new cablecircuits to prevent thermal failures. Jicable’11, June Versailles - France

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