1. OFFSHORE RENEWABLE PLANT HVDC POWER COLLECTOR AND DISTRIBUTOR
EWEA 2013
February, 2013, Vienna, Austria
OFFSHORE RENEWABLE PLANT HVDC POWER COLLECTOR AND DISTRIBUTOR
National Renewable Energy Centre
Chong Ng, Principal Engineer – Reliability & Validation
Paul McKeever, R&D Manager

2. Narec – Created by Government to stimulate the RE industry, A Controlled and Independent Testing Environment
Existing
50m blade test
Still water tank
Wave flume
Simulated seabed
Wind turbine training tower
Electrical and materials laboratories
New
3MW tidal turbine drive train
Offshore anemometry hub
100m blade test
15MW wind turbine drive train
99.9MW offshore wind demonstration site /14

3. Presentation Contents
Technical Paper Background
Existing Systems
HVAC transmission systems
HVDC systems
Proposed HVDC System
Selected Challenges
Conclusions
Next Steps

4. Technical Paper Background
UK requires offshore wind to meet its renewable energy generation targets (2020, 2030, 2050…) – UK Energy Bill … by 2020, 30% from Renewable Energy
Likely to involve larger turbines (10MW? 20MW?) – FP6 UpWind Project
Offshore plant would benefit from an appropriate power collection, transmission and distribution technology
HVDC potentially provides better efficiency, particularly over longer distances
Benefits from power semiconductor and copper cost trends

5. HVAC Transmission Systems
Commonly used in many offshore wind farms
Can suffer from excessive reactive current
Increases cable losses
Reduces power transfer capability
Reactive power compensation required (extra equipment)
Can suffer from high line losses and excessive voltage drops
Extra cables required
Inter-dependant characteristics need careful consideration
Transmission voltage level, cable capacitance and charging currents…

6. Existing HVDC Systems
Modern HVDC systems generally have advantages such as:
Lower transmission losses
Fully controllable power flow
No reactive power generation or absorption (‘cable only’ connections)
Reduce/eliminate AC harmonic filter with the latest multilevel converter technologies (e.g. MMC HVDC)
HVDC transmission systems can be categorised, by the converters used, into three categories:
Line-commutated Converters (LCC), Capacitor Commutated Converters (CCC) and Voltage Source Converters (VSC) as illustrated below
Point to point HVDC power transmission – Wind Farm Inter-array?
What do we want?
A dedicated high efficiency, robust, flexible and low cost power collection, transmission and distribution technology for use within the wind farm too

7. Proposed HVDC System
HVDC power transmission from the point of generation
Reduce losses and components (i.e. make use of Turbine MV converter and availability of HVDC gird)
Multi-terminal HVDC system
Increase availability
Offers flexibility and redundancy
Reduce cost
Removal of/minimise offshore substation
Reduced cable losses (HV operation)

8. Proposed HVDC System
Hybrid HVDC Transformer (figure shows simplified circuit):
Steps up MVDC to HVDC
Reduced voltage stress on primary side and current stress on secondary side allows use of “off the shelf” force commutation devices
Uses magnetic transformer to avoid high conversion ratio
Potential to require less power capability from switches (30%) when compared with conventional 2-level 3-phase HVDC converter
Many potential challenges that need full investigation (e.g. switching control, network stability, economic impact, protection and isolation…)

9. Proposed HVDC System Switching device comparison:
Proposed Hybrid HVDC Transformer vs. conventional HVDC converter (3-phase 2-level topology)
Assumptions
n = number of series connected power switching devices in half of the bridge arm
6.5kV rated switching devices
VSC-based HVDC converters use 3-phase, 2 (or multi) level converter topology
Assumes 2 devices in series is sufficient to withstand the MV voltage stress
150kVdc example
HVDC side needs n >= 30 devices in series
For conventional VSC-based HVDC systems
6n >= 180 devices
For hybrid HVDC transformer
4n + 8 >= 128 devices
29% saving in power semiconductors used

10. Selected Challenges The time to implement
Dependent on development/readiness of the offshore wind industry
Managing multi-vendor solutions
Will this be a problem?
Practical implementation (i.e. is it realistic?)
Needs further investigation; this is still a concept
Will the subsea power cable size increase with no centralised collector?
Shouldn’t increase for similar voltage levels; the overall power stays the same
Would a platform still be required as a maintenance hub?
A mobile platform could be used for this purpose
Is there an operational impact?
Turbine operation should be unaffected
System optimum operation and control needs developing

11. Conclusions Potential advantages for offshore wind farm applications
An alternative to AC and point to point HVDC transmission topologies
Suitable installation in every single power source
Increases flexibility and redundancy of the entire HVDC system
Positive impact on wind farm availability and O&M costs
Eliminates/minimises the need for a centralised offshore collection platform
Potential lower component count at converter level
Modular component sets across the system
100MW power block in centralised system vs. 20 x 5MW power blocks in hybrid HVDC transformer system
Increased component count at system level (due to de-centralisation)
Balanced by no offshore substation and fewer components, e.g. fewer power semiconductors and filters…

12. Next Steps
Investigate, in detail, the feasibility of this HVDC system concept
Detailed study of the proposed hybrid HVDC transformer
Explore the feasibility of the following advantages:
High flexibility leading to ‘independent’ turbines
Additional redundancy and high system availability (no centralised substation)
High efficiency (power collection and O&M efficiency)
Cost reduction potential
Installation in individual turbines
Optimisation of materials (copper, semiconductor devices…)
Investigate the use of SiC switching devices
Higher power density and heat tolerance

13. Thank you for listening!
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