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Evaluation of offshore HVDC grid configuration options
Keith Bell and Callum MacIver Dept. of Electronic and Electrical Engineering University of Strathclyde, UK
This work has benefited from support by: Centre for Doctoral Training on Wind Energy Systems (EP/G037728/1) Transformation of the Top and Tail of Energy Networks (EP/I031707/1)
Presented by Keith Bell, ScottishPower Professor of Smart Grids and a co-Director of the UK Energy Research Centre https://www.strath.ac.uk/staff/bellkeithprof/ http://www.ukerc.ac.uk/
The news yesterday…
Graphic: The Guardian
Offshore wind in Europe
http://rave-static.iwes.fraunhofer.de/en/map/
Offshore wind in Europe
• High capacity factors • Plentiful locations • Wind farm costs coming down
– e.g. excluding costs of connections to shore, €54.50/MWh for Borssele 3 and 4, €49.90/MWh for Krieger’s Flak
• Developments becoming larger and further offshore
• AC or HVDC connection? – Developers have greater confidence in AC than in HVDC…
• … but what is the ‘cross-over distance’ at which HVDC is cheaper? – If shunt compensation deployed rationally, ‘cross-over distance’ is
further than previously thought
– However, • System benefits of HVDC option? • Transient over-voltages and harmonic resonance for AC option • Incentives on developer to minimise losses? • SO-TO Code requirements at network owner interface? • Can an AC option be extended to become part of an offshore network?
Reducing £/MWh of network
Figure: Elliott et al, “A comparison of AC and HVDC options for the connection of offshore wind generation in Great Britain”, 2015
Reducing £/MWh of network
• Maximise utilisation by making connections part of a network – Between two synchronous areas – ‘Embedded’ within a single synchronous area
Is an offshore network like an onshore network, just with its feet wet? 1. Undersea cables are much more expensive than onshore overhead lines. 2. Offshore substations are very much more expensive than those onshore
since they depend on purpose built platforms and ‘marinised’ equipment. 3. For HVDC branches at a certain voltage, a connection to the AC system
must use a power converter that will incur a certain minimum cost that is very large. – Minimise the number of converters; use multi-terminal HVDC grid
4. High-voltage DC circuit breakers (DCCB) – have not yet been demonstrated in such a way as to fully establish
commercial viability – are likely to be both large and expensive relative to AC circuit breakers
at comparable voltages. 5. Aside from oil and gas production platforms, there is no demand
connected offshore → continuity of supply is not so important
The above can lead to different design conventions for offshore
The need for DC breakers
• Suppose that area 1 has a loss of infeed limit of 1.3GW • Without protection to detect and DC breakers to clear HVDC grid faults,
2GW would be lost for a fault anywhere on the HVDC grid – Add DC breakers and protection – Or limit offshore wind production
• Lost value from offshore energy and increase in the effective levelised cost
If we are not to exceed the loss of infeed limit, a fault between OSC1 and point A should be cleared by breakers opening at A
Another option?
Split the network pre-fault; re-configure post-fault to restore generation
Pre-fault split: maximum loss of infeed in area 1 is 1GW
• Fault between OSC1 and A • Fault cleared from AC side • De-energised DC grid re-configured
• Disconnectors at A opened • Disconnectors at B closed
• Re-energise offshore grid See Bell, Xu and Houghton, “Considerations in Design of an Offshore Grid”, CIGRE Science & Engineering, 2015
What form should an offshore HVDC grid take? • Several options available to developers of offshore grids
Technology – type of VSC converter? DC Breakers? Topology – radial vs meshed, monopole vs bipole Protection strategies – influenced by above choices
• Few studies have considered reliability performance of networks extensively How faults are managed under different network options Now some work in area, e.g. CIGRE WG B4.60
• Key open questions What is the value of redundancy?
Radial vs Multi-terminal vs Meshed
Should we emulate onshore style protection? Requires DC breakers throughout offshore grid? Cost, availability? 10
What is the value of different network designs? • How much of the available offshore energy cannot be sold because of
network faults? – How long does an unplanned outage last?
• Depends on the repair time which depends on vessel availability and the next sufficiently long weather window
• Weather window depends on the season – How much offshore wind energy was undelivered?
• Depends on weather conditions which depends on season • Repair times calculated with reference to wave heights
– Wave heights and wind speeds are correlated Major Offshore
(long repair) Minor Offshore (short repair) Major Onshore Minor Onshore
Components cable, transformer
converter, DC breaker transformer converter
Weather window continuous non - continuous - -
Procurement delay fixed - fixed -
Reliability assessment
• Sequential Monte Carlo simulation with correct temporal correlations – Sample faults and next sufficiently long weather window
• Offshore grid component reliability data is fairly sparse • Three data cases developed based on the spread of data that is
available and industry expert opinion – Best, Central and Worst case set of failure rate assumptions
Central Reliability Scenario Inputs
Components
Mean time to failure (Hours*)
* Transmission cable - Hours/100km
Minimum time to repair (Hours)
Fixed Delay Repair time
Onshore Converter 6480 (10 months) - 6
Offshore Converter 6480 (10 months) - 6
Onshore Transformer 438300 (50 years) 2160 (3 months) 72
Offshore Transformer 350640 (40 years) 2880 (4 months) 120
HVDC Transmission Cable 219150 (25 years) 2160 (3 months) 144
DC Circuit Breaker 219150 (25 years) - 6
Case study
• Assume 4 × 700MW clusters of wind farms connected somewhere like Dogger Bank
• What is the value of network redundancy and DC breakers? – What is the undelivered wind energy for different network
designs? – How much income is lost?
http://www.forewind.co.uk/projects/projects-overview.html
Initial HVDC connection options • Each of 4 initial options uses
– half bridge MMC VSC converters – symmetrical monopole configuration: 2 × cables at ±320 kV
Option 1: Radial Option 2: Radial+
Option 3: Multi-terminal Option 4: Meshed
Fewer converters Reduced cable km
Redundant paths added
Capital expenditure
Radial option has very high cable costs
Number and cost of DCCBs rises sharply as grid options become more interconnected
Cost of meshed grid highest despite cable savings compared with radial option
Assumes DC Breaker costs = 1/6th of VSC converter station
Reliability performance
~1/3rd reduction in undelivered energy with alternative transmission paths
Additional benefit of meshed grid relatively small
Big discrepancy between best case/worst case reliability scenarios
Expected annual undelivered energy
Overall value of grid options
Radial: worst option in all scenarios • high CAPEX • poor reliability performance Radial+: favourable when component reliability is good • low CAPEX Multi-terminal: favourable under central + worst case reliability performance Meshed: unfavourable due to high CAPEX with only marginally improved reliability performance
25 year NPV of grid options Value of Energy: £150/MWh Discount Rate: 6%
Further options
Option 5: Minimum Breaker
Full bridge AA-MMC converters used to block reverse fault current into DC Grid alongside reduced number of DC breakers DC Breakers used to isolate healthy grid section from faulted section allowing continued operation
200km
200km
15km
15km
1400MW
1400MW
700MW
700MW
1400MW 20km
Further options
Option 6: AC Protected
Half bridge MMC converters used Link between two radial+ grid sections switched out under normal conditions AC side protection used in event of fault and, if required, link switched in after delay and power re-routed
200km
200km
15km
15km
1400MW
1400MW
700MW
700MW
1400MW 20km
Further options
Option 7: Bipole transmission links
Half bridge MMC converters used with DC Breakers Main transmission links use bipole configuration with extra LV return conductor allowing partial power transfer under certain cable/converter fault conditions
200km
200km
15km
15km
1400MW
1400MW
700MW
700MW
1400MW 20km
Min breaker and AC protected options show significant savings compared with multi-terminal solution
Bipole grid option has high capital costs due to extra cabling requirement
Capital expenditure
Assumes DC Breaker costs = 1/6th of VSC converter station
Reliability performance
Min Breaker and AC protected options suffer no reliability penalty and actually marginally improve performance vs multi-terminal option
Bipole grid option has significantly improved reliability performance
Expected annual undelivered energy
Overall value of grid options
Min Breaker: Higher losses in AA-MMC but lower CAPEX and undelivered energy so better value than multi-terminal AC Protected: Lowest CAPEX and good reliability performance make it best value option in best and central case scenarios Bipole: High CAPEX but very good reliability performance mean most favourable in worst case scenario
25 year NPV of grid options Value of Energy: £150/MWh Discount Rate: 6%
Conclusions
• Redundant transmission paths reduce undelivered energy Reliability significantly improves: Radial → Multi-terminal → Bipole Case for meshed grids less apparent
• Alternative methods to HVDC circuit breakers viable No reliability penalty and financially favourable New operating conditions must be managed
• Reliability performance a key design choice alongside capital cost Trade-off between reliability and capital cost, compromises required
• Overall reliability highly sensitive to fail/repair rate assumptions More certainty required or least regret approach may be required
See MacIver, Bell, and Nedic, “A Reliability Evaluation of Offshore HVDC Grid Configuration Options”, IEEE Trans on Power Delivery, April 2016.
Major relevant European projects
• TWENTIES Work Package 5, 2009 - 2013 • BestPaths, October 2014 – September 2018 • PROMOTioN, 2015-2019
See http://www.bestpaths-project.eu/ https://www.promotion-offshore.net/
Further work on • Fault detection • Fault clearance • Converter and wind
turbine inter-operability • Diode rectifier units
System planning & grid topologies
Main Findings • A range of studies have identified different roadmaps – often different
assumptions and methodologies so hard to compare directly but: multiple regional grids more likely than a single European super grid Case for complex grids dependant on level of offshore wind
deployment Case for complex grids depends on detailed optimisation
Main Gaps Need to identify how where when and why complex offshore
topologies might develop Need to understand chronological evolutions towards multi-terminal
or meshed grids
PROMOTioN WP1 review of past projects
Operation of Converters in DC Grids
Main Findings • A range of studies have investigated steady state operation & control as
well as dynamic stability in DC grids: Solutions identified and tested to various degrees that suggest DC
grids can be operated effectively Various droop control schemes proposed (V-I, P-V etc.)
Main Gaps Need to develop more extensive modelling (steady state, RMS, EMT)
and testing of control strategies based on MMC and Diode Rectifier Unit (DRU) converter technology
Need to test interoperability of different converter technology/vendors (especially MMC and DRU) and between converters and wind turbines
Relevant Grid Codes still under development
PROMOTioN WP1 review of past projects
DC Grid Protection Systems
Main Findings Numerous protection schemes proposed by academia/industry Non-Unit (local measurements): fast Unit (data communication between 2 ends): greater selectivity Protection philosophy dependent on grid size, cost of protection, size
of acceptable loss
Main Gaps No single solution fulfils all requirements so no final consensus Combination of basic principles required Need to identify and develop solutions to be deployment ready Multi-vendor compatibility required
PROMOTioN WP1 review of past projects
DC Circuit Breakers
Main Findings 3 main design options:
Resonant/mechanical breakers (low loss, low cost, 5-10ms) Solid state breakers (high loss, high cost, <1ms) Hybrid breakers (low loss, high cost, <5ms)
Several large scale test on hybrid and resonant designs
Main Gaps Development of sufficient models for detailed and system level
studies Development of sufficient test/demonstration procedures to mimic full
stresses experienced in DC grid application Continued development of design concepts to enhance speed,
losses and cost performance
PROMOTioN WP1 review of past projects
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