jicable’11, 19 - 23 june 2011 - versailles - france experience with joint failures in new smart-...

Jicable’11, 19 - 23 June 2011 - Versailles - France

EXPERIENCE WITH JOINT FAILURES IN NEW SMART-GRID MV CABLE CIRCUITS

Frank DE VRIES, Liandon BV, (The Netherlands), [email protected]

Jacco SMIT Liandon BV, (The Netherlands), [email protected]

Ger SEBREGTS, Liander NV, (The Netherlands) [email protected]

mailto:[email protected]



Index

• Alliander test program of MV cables• Failures in the network• Failure investigation• Additional laboratory tests• Cable load• Conclusion and further steps


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.


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 2008 2 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


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


Failure investigation


In many cases the failure occurred outside the inner joint bodyIn a few cases the failure occurred around the connector

Failure investigation


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


Ohmic Resistance of new and failed connectors

1

10

100

1000

10000

a b c 1a 1b 1c 2a 2b 2c 3a 3b 3c 4a 4b 4c

NEW FAILED

ConnectorsR

esi

sta

nce

(m

icro

oh

m)

Temp. development

• Laboratory tests show an induced current through earth wires of 0,5A.

• Field measurements showed no abnormalities in harmonic current


Harmonic current in cable circuit

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Harmonic frequency

co

ntr

ibu

tio

n (

%)

Cable load

• Cables of DG show significant difference in load pattern compared to normal MV distribution cable – Dynamics

– Maximum

– Direction


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

• I2 x R– Conductor / connector– Earth system


Cable temperature

Pw-inPw-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.


Maximum continuous cable load [A/MVA]

according to company standards according to the installation in practice

350 A / 6 MVA 261 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.


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.


Thank you for your attention


For more information:[email protected]


jicable’11, 19 - 23 june 2011 - versailles - france experience with joint failures in new smart-...

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