EXPERIENCE WITH JOINT
FAILURES IN NEW SMARTGRID MV CABLE CIRCUITS
Frank DE VRIES, Liandon BV, (The Netherlands), frank.de.vries@alliander.com
Jacco SMIT Liandon BV, (The Netherlands), jacco.smit@alliander.com
Ger SEBREGTS, Liander NV, (The Netherlands) ger.sebregts@alliander.com
Jicable’11, 19 - 23 June 2011 - Versailles - France
Index
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Alliander test program of MV cables
Failures in the network
Failure investigation
Additional laboratory tests
Cable load
Conclusion and further steps
Jicable’11, 19 - 23 June 2011 - Versailles - France
Testprogram
• All new installed cables are
tested according NEN3620
– Voltage Withstand Test
cablesheath 5kVDC-5 min
– Voltage Withstand Test main
insulation 3xUo, 0,1Hz15min
– PD measurements (under
consideration)
• At Alliander all cable
systems with joints are
tested with PD
measurements up to 2xUo.
Jicable’11, 19 - 23 June 2011 - Versailles - France
Failures in the network
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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
Jicable’11, 19 - 23 June 2011 - Versailles - France
Results from the investigation
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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, 19 - 23 June 2011 - Versailles - France
Failure investigation
In many cases the failure occurred outside the inner joint body
In a few cases the failure occurred around the connector
Jicable’11, 19 - 23 June 2011 - Versailles - France
Failure investigation
Jicable’11, 19 - 23 June 2011 - Versailles - France
Connector resistances
– At a cable load of 300A this
leads to heat losses over 90W
per phase!!
Ohmic Resistance of new and failed connectors
10000
Resistance (micro ohm)
• Resistance of a new
connector is 15µΩ
• Connector resistances of
failed joints are over 1000µΩ
!
1000
100
10
1
a
b
c
1a
1b
1c
2a
2b
NEW
2c
3a
3b
3c
4a
4b
FAILED
Connectors
Yellow:
New installed connectors
Red:
Connectors of failed joints
Jicable’11, 19 - 23 June 2011 - Versailles - France
4c
Temp. development
• Laboratory tests show an
induced current through
earth wires of 0,5A.
Harmonic current in cable circuit
0,5
0,45
0,4
0,35
contribution (%)
• Field measurements showed
no abnormalities in harmonic
current
0,3
0,25
0,2
0,15
0,1
0,05
0
1
3
5
7
9
11
13
15
17
19
21
23
25
Harmonic frequency
Jicable’11, 19 - 23 June 2011 - Versailles - France
27
29
Cable load
• Cables of DG show
significant difference in load
pattern compared to normal
MV distribution cable
– Dynamics
– Maximum
– Direction
Jicable’11, 19 - 23 June 2011 - 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.
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Thermal resistivity of the soil = 0,75Km/W
Cable depth = 1m
Ground temperature = 15C
No other heat sources are present
Pw-out
Pw-in
Cable temperature
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I2 x R
–
–
Conductor / connector
Earth system
Jicable’11, 19 - 23 June 2011 - Versailles - France
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
Jicable’11, 19 - 23 June 2011 - Versailles - France
Conclusions
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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, 19 - 23 June 2011 - Versailles - France
Further steps / recommendations
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For the existing cable circuits in DG networks Alliander has
developed a strategy for the future based on different actions
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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
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Engineering standards should be taken under consideration for new
cable circuits in DG
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Investigate the most suitable after installation tests for new
cablecircuits to prevent thermal failures.
Jicable’11, 19 - 23 June 2011 - Versailles - France
Thank you for your attention
For more information:
frank.de.vries@alliander.com
Jicable’11, 19 - 23 June 2011 - Versailles - France