The session focuses on conversion systems for wind turbines based on power electronics, mainly for off-shore applications. The trend of higher power brings several challenges related to compliance with grid requirements. Harmonics and resonances become crucial issues that require proper analysis and innovative yet practical solutions. This session covers aspects related to power converter architectures and the modulation and control of the converter. We will look at the interaction with the electrical grid as well as practical aspects about how to optimise the primary control in different wind conditions and how to effectively test the control of large-scale wind systems in emulated conditions.
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Learning objectives
- To explain a number of ways that the different power converter systems (including their filter) impact the real operation of an offshore wind park;
- To identify the value of grid emulator test benches;
- To list possible advantages of frequency converters in wind energy;
- To explore methods that lead to reducing the cost reductions of integrating offshore wind energy.
Presenter
Co-authors:
Michael Smailes (1) F Chong Ng (1) Paul McKeever (1)
(1) ORE Catapult, Blyth, United Kingdom
Abstract
High definition MMC for platform-less HVDC offshore wind power collection systems
Introduction
The trend for offshore wind farms to move further offshore, combined with the falling cost of power electronics has resulted in an increase in the number of wind farms considering HVDC transmission systems. However, today’s HVDC systems are not optimised for the offshore wind industry, offering little in terms of system redundancy and accounting for more than 16% of the capital costs. A modular, high power (5–10MW), medium frequency, hybrid HVDC transformer located within the wind turbine itself will therefore been proposed by Chong Ng et. al. to address some of these issues. Due to the limited space and high power density required in the Hybrid HVDC transformer, a novel High Definition (HD) MMC control algorithm has been developed. This work then, will present the Hybrid HVDC transformer, its potential for cutting the LCoE of offshore wind and introduce the HD-MMC concept.
Approach
Detailed Simulink models of the Hybrid HVDC Transformer were created using different converter topologies. The models were then run over a range of frequencies (500 – 2000 Hz) to determine the converter losses. The generated waveforms were then used in a Matlab script to optimise the magnetic design of the transformer and calculate the total Hybrid HVDC Transformer’s losses and volume over the frequency range.
The results showed high primary side losses in both the converter and windings compared to the secondary due to the increased current. This prompted further research to improve the magnetic design and critically to develop the HD-MMC concept.
An economic analysis of the Hybrid HVDC Transformer was then performed and compared to a conventional HVDC wind farm to determine its potential for cutting the cost of offshore wind. This analysis used manufacturers data sheets, published papers and statements from the national grid companies to achieve accurate results.
Main body of abstract
The large traditional, HVDC converter has been segmented into small hybrid HVDC transformer modules located within each wind turbine. These modules step-up the internal turbine DC bus voltage to HV or MVDC for direct shunt connection to the DC transmission line or to a mini wind turbine cluster. Connection to the transmission cable is achieved through a suitable subsea hub or in-turbine HV junction box, reducing or completely eliminating, the HVAC collection and HVDC converter platforms. This offers large (15 %) cost savings, increased system redundancy and hence system availability and minimises inter-array cable losses.
By analysing the simulation results it was discovered that optimal operating frequency was 1.4 kHz using the Modular Multilevel Converter due to its modular design and low harmonic distortion. Losses were found to be too high on the primary side though due to the large current and low bus voltage. The HD-MMC algorithm was therefore developed to create additional voltage levels without any additional hardware. The converter losses and power density can therefore be improved without increasing losses or introducing large filters to improve the AC harmonics.
Conclusion
The proposed offshore wind power collection scheme together with the HD MMC control algorithm will further enable and stimulate deployment of offshore grids to improve offshore wind power network security, lower offshore wind LCoE and further enhance offshore wind development to cut carbon emissions. Detailed analysis and simulation results from several applied research projects, demonstrating the functionality of the proposed solutions will be presented in this conference to stimulate constructive discussion.
Learning objectives
Through this work, delegates will learn the current status of offshore HVDC wind farms and the challenges and opportunities facing the industry as it tries to reduce costs. They will further discover the potential in developing a system to eliminate the offshore HVDC substation as well as design optimisation considerations required in developing a high power density HVDC transformer.
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