evaluation of offshore hvdc grid configuration options · evaluation of offshore hvdc grid...

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/

https://www.strath.ac.uk/staff/bellkeithprof/

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

Relatore

Note di presentazione

Despite there being a degree of certainty around the delivery of offshore HVDC grids, there are still a lot of options out there for potential developers in terms of different technology options - i.e which brand of VSC of converter to use and whether or not DC breakers are available and at what cost. There are also various topology options ranging from radial to meshed grids or configuration options monopole and bi-pole solutions and all of these decisions also feed into the protection strategy that you employ. There is a whole host of work looking at the validity of many of these different options but certainly at the time I started my PhD very few studies had considered the impact of reliability to any great extent, or in other words how some of these different options might cope with a lifetime of fault conditions. It must be said there is now some parallel work in the area, the most notable of which is a dedicated Cigre working group looking at the same topic which is due to publish results very soon. So, there are clearly some open questions remaining surrounding offshore network design and the main questions we’ve looked to address in this work are firstly: what is the true value of added redundancy in the offshore grid setting given the high capital costs involved, and secondly: Do we really need to pursue onshore style protection with DC breakers deployed throughout the system or are there other options out there given that the cost and commercial availability of DC breakers remains uncertain.

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

Relatore


A comparison can first be made of the capital expenditure required for each grid option. The radial option has extremely high cable costs which are significantly reduced when you move to the radial+ grid option with shared transmission routes. The radial+ option is therefore found to be almost half a billion pounds cheaper. As the level of interconnection is increased however in the multi-terminal and then meshed grid options, the cable costs and especially the DCCB costs rise sharply such that the meshed grid is marginally the most expensive option overall. It has been assumed for each of these studies that the cost of a single DCCB unit is 1/6th of the cost of the equivalent sized 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

Relatore


The reliability performance of each of the grid options can then be investigated by assessing the expected level of undelivered energy due to faults on the offshore network under each of the three reliability scenarios outlined previously. Results are given as a % of the available offshore wind energy and it is immediately clear that there is a wide variation in performance between the three reliability scenarios and that if offshore networks are to be delivered in reality then reliability performance would need to strive to reach the more acceptable levels found in the best and central case scenarios where undelivered energy is in the order of a few % and not the 10-15% that is found in the worst case scenarios. The two radial options have very similar and relatively poor reliability performance but the introduction of the alternative transmission path in the multi-terminal grid option immediately improves performance and the level of undelivered energy is reduced by around one third. However, it is found that the additional performance benefit of moving to the meshed grid option from the multi-terminal grid option is much more marginal with only a very slight further reduction in the expected level of 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%

Relatore


The total net present value of each grid option over an expected 25-year lifecycle can finally be evaluated by taking the total costs associated with capital expenditure and reliability performance that we’ve just discussed along with other calculated parameters such as the expected level of electrical losses and O&M costs away from the total value of deliverable wind energy. By doing this we find that the radial option provides least value for money as it has both high capital costs and poor reliability performance. The radial + option is the cheapest in terms of capital costs which makes it the most favourable option if you assume best case reliability performance. As the number and duration of failures increases however, as in the central and worst case scenarios, the poor reliability performance of the radial+ option begins to show and it’s overall value decreases compared with other options. In the central and worst case reliability scenarios the relatively strong reliability performance of the multi-terminal option make it the most favourable option despite having relatively higher up front costs than the radial+ option. The small additional benefits of moving to the meshed grid option are found to be outweighed by the relatively high extra capital cost involved. It is clear then that there are trade-offs to be made between maximising reliability performance and minimising capital costs and in this scenario the multi-terminal grid option comes out as the most favourable compromise.

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

Relatore


To investigate some of the other open questions a number of other case studies were developed based on the multi-terminal grid topology. The 1st of these look at alternative protection strategies with solution 5 investigating a so called ‘minimum breaker’ option. Such an approach has been suggested by ALSTOM amongst others and makes use of full bridge converters, in this case assumed to be of the alternative arm configuration, in conjunction with a reduced number of strategically placed DCCBs. In the event of a fault the DCCBs will act and essentially split the grid into two sub-systems, one of which remains in normal operation. The sub-system that contains the fault however makes use of the reverse current blocking capability of the full-bridge converters to bring the current to zero allowing standard isolators to act to isolate the fault before any remaining healthy connections can be brought back online. It is assumed that this whole process can be achieved within a few hundred milliseconds.

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

Relatore


A further option is to do away with DCCBs altogether and rely solely on AC side protection as we do with existing point to point HVDC projects. The main argument against this is that the whole DC grid would need to be shut-down, at least temporarily, in the event of a DC side fault which could lead to the loss to the AC system of unacceptably large levels of power infeed. Two mitigate this impact it is proposed that the grid could be split into different electrical sub-systems pre-fault, each of which would carry no more power than the allowed loss of infeed risk limit on the connected AC system. In this case study, this is achieved by switching out the link between WF2 and WF3 under normal operation. In the event of a fault the AC side protection would isolate the entire affected grid section allowing for the specific fault to be located and isolated while the system is offline. If appropriate the link and any remaining healthy grid sections could then be switched on and power infeed restored via the remaining healthy grid sections. It is assumed that such a process could be undertaken within a few tens of minutes at most.

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

Relatore


A final grid option investigates an alternative bipole converter configuration strategy on the two main transmission links. This requires an additional low voltage return conductor to be used in addition to the two HV cables but allows for the links to operate at half capacity for single pole converter and cable faults and thus adds another layer of redundancy into the system.

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

Relatore


So again the capital costs of each grid option can be investigated and compared to the standard multi-terminal grid option. It is found that the reduction in DCCB costs has a significant impact making the minimum breaker and AC protected grid options more financially favourable in terms of up front costs. In contrast, the extra cable costs associated with the bipole grid option make it relatively expensive compared with the other options.

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

Relatore


Looking at the reliability performance of each of the grid options it can be shown that there is no expected penalty in terms of undelivered energy from moving to the alternative protection strategies of the minimum breaker and AC protected grid options and in fact there is even a marginal improvement in performance which is essentially due to the removal of a layer complexity from the grids meaning there are less components that can fail. The bipole grid option, despite it’s high capital costs, is however found to offer another step-change improvement in reliability performance which significantly reduces the level of undelivered energy under each of the reliability scenarios.

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%

Relatore


Once again these results can be combined with electrical loss and O&M cost calculations to determine the overall value of each of the grid options in terms of their 25 year net present value. The low cost and comparatively good reliability performance of the minimum breaker and AC protected option mean they both compare favourably in terms of overall value compared with the DCCB based mutli-terminal grid option. The full bridge converters used in the minimum breaker solution are assumed to suffer from increased electrical losses however which means that the AC protected grid option offers the best value for money under both the best case and central case reliability scenarios. The bipole grid option suffers from high capital costs but offers excellent reliability performance which make it the most favourable option if the worst case reliability scenario is assumed. One significant advantage of the bipole grid option is that although other options offer better value under good general reliability performance there is less risk associated with the bipole option if you were to experience uncharacteristically high failure rates for example. It could therefore be viewed as a ‘least regret’ option.

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.

Relatore


So to conclude I think I have shown that there is clear value in the introduction of redundant transmission paths into offshore grids as it can significantly reduce the level of undelivered energy. A clear step change in reliability performance is observed when you move between radial and multi-terminal grid options and again when you introduce the bipole converter configuration. The case for highly meshed grids is however less apparent given the increasingly high capital costs involved. I’ve also shown that alternative methods to using DC breakers are potentially viable with no reliability penalty and with reduced costs, so long as you are capable of managing the associated different operating conditions and for example, temporary DC grid shut-downs. I think it is clear that reliability performance should be a key parameter when deciding upon the design of an offshore grid and that there are a number of trade offs to be made between the redundancy you build into the system and the associated capital costs which should be assessed on a case by case basis to find the appropriate compromise in any particular scenario The results however have been shown to be highly sensitive to the input assumptions used so unless more accurate data becomes available to improve certainty of these results then developers may wish to consider a least regret approach to their decision making.

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

http://www.bestpaths-project.eu/




https://www.promotion-offshore.net/




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


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


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


evaluation of offshore hvdc grid configuration options · evaluation of offshore hvdc grid...

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