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Feasibility assessment of wireless series reactive compensation of long submarine AC cables
Author: Gaspard Lugrin, Research Scientist, SINTEF Energy ResearchPresenting: Andrzej Holdyk, Research Scientist, SINTEF Energy Research
EERA DeepWind'2020, Trondheim, 16 January 2020
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• Long AC subsea cable• Connects offshore
installation with shore
• Main applications:• Offshore Wind Power
Plants (OWPPs)
• Oil and gas platforms
January 2020EERA DeepWind'20202
Background
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• Submarine cables have large capacitance
• Always generate reactive power
• Capacitive current is added to the load current
• Long distances require compensation
January 2020EERA DeepWind'20203
Long AC subsea cableSmall loadNominal loadNominal load-long cable
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• Compensation usually done using shunt reactors
• Due to costs, reactors are usually placed at:• Substation
• Platform, near the load
• Additional platform in the middle
• Could also be placed at the sea bottom
January 2020EERA DeepWind'20204
Compensation of long AC subsea cables
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• Shunt reactors must be encapsulated
• Cable must be split and connected to the structure• HV wet-mate connectors
• Might be difficult to disconnect from the system in case of failure
January 2020EERA DeepWind'20205
Compensation placed at the sea bottom
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• Magnetic coupler:• Iron core
• Primary circuit: cable
• Secondary circuit:• Coil
• Pressure tolerant power electronics converter
• Storage device
• Clamped around a cable• No need for splitting the cable
• No need for connectors
• No problems in case of failure
January 2020EERA DeepWind'20206
Initial idea: wireless compensation with magnetic coupling
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Feasibility studies
• Feasibility studies looked into:• Load flow
• Can we dynamically compensate the cable?
• Is the entire system stable?
• Do we still need shunt compensation?
• Cable design and possibilities of connection
• Coupler• Main characteristics and estimation of weight of couplers
January 2020EERA DeepWind'20207
UG
ZG
LSER
ZL
Cable0.5 length
Cable0.5 length
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Results: Load flow analysis
• Initial idea: series inductive compensation only:• At low transmitted power, full compensation requires arbitrary high voltage and causes a transmission
angle larger than 90°; small partial compensation worsens the voltage at load.
• For cables longer than a given value (depending on system parameters), full compensation causes transmission angle larger than 90°.
8 January 2020EERA DeepWind'2020
UG
ZG
LSER
ZL
Cable0.5 length
Cable0.5 length
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Results: Load flow analysis
• Proposed method: combination of shunt and series inductive compensation• Increase of power transfer capability or operative cable length in comparison with a case where no
compensation is present along the cable
• Requires variable shunt inductances
• The total installed reactive power for full compensation is larger with the proposed method than with shunt inductive compensation only.
• Transient behaviour should be checked
9 January 2020EERA DeepWind'2020
UG
ZG
LSER
LSH1 LSH2 ZL
Cable0.5 length
Cable0.5 length
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Limitations due to cable design
• Initial idea: coupling on a three-core cable• Cannot couple to a 3-phase cable directly
• Armour, semiconductive layers, sheath
January 2020EERA DeepWind'202010
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Limitations due to cable design
• Proposed method: compensation unit• Compensation unit pre-installed on a cable section
• Subsea system: no need for a platform (potential cost reduction)
• The method is not "non-intrusive"
11 January 2020EERA DeepWind'2020
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Coupler design
• Initial idea: single-turn secondary winding coupler• Very large size and weight
• Alternative: multiple-turn secondary winding• Weight is reduced in comparison with the single turn secondary winding
• Would require to coil the cable
• Not relevant if the compensation is pre-installed on the cable.
12
d d d
d
2d d5d
dd/3
d/3
5d
d d
d+d/3
d
January 2020EERA DeepWind'2020
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Conclusions
• Initial idea: non-intrusive inductive compensation
• Limitations in the practical feasibility of the initial idea
• Alternative solutions:• Combination of shunt and series inductive compensation
• Use of a compensation unit pre-installed on the cable
• Advantages• Increase power transfer capability or operative cable length in comparison with a case where no
compensation is present along the cable
• Compensation comparable (but not as good) as shunt compensation alone
• Subsea system: no need for a platform (potential cost reduction)
13 January 2020EERA DeepWind'2020
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