The University of Strathclyde, the Offshore Renewable Energy (ORE) Catapult, and SuperNode have completed an analysis assessing the feasibility of Medium Voltage Direct Current (MVDC) technology with High Temperature Superconductor (HTS) cables for offshore wind power transmission. It shows that MVDC transmission cables based on superconductors have significantly lower lifecycle costs than conventional grid technology based on High Voltage Direct Current technology.
The results of the model are clear in that the MVDC HTS system has significantly lower lifecycle costs than the HVDC system.
The HTS system has lower costs in each of the three cost categories. The electrical losses from HTS cables are negligible due to the nature of superconductivity and are lower than HVDC. The HTS system also has significantly lower unavailability costs than HVDC systems. The HVDC system has significant unavailability associated with the onshore and offshore transformers as well as the offshore converter station and switchgear, none of which apply at the same scale for the HTS system. However, the HTS system does have some unavailability associated with the cooling system O&M. Nevertheless, the modular nature of the HTS cooling system facilitates a redundancy approach to operation that limits unavailability.
In terms of capital costs, HTS cables are more costly than HVDC cables. However, HVDC systems require large offshore platforms to facilitate the high voltage equipment needed such as the offshore converter station and switchgear, whereas a HTS system requires a smaller footprint platform due to a lower volume of electrical equipment required (no converter station, smaller switchgear). Ultimately, despite the high cost of HTS cables, the analysis has found the overall capital cost of the HTS system is significantly lower than the HVDC system.
It must be mentioned here that consideration was also given to MVDC schemes using copper cables, but these schemes are limited in their power transmission capacity due to the constraints on the cables and as such are more suited to small to medium capacity wind farms.
This CBA model shows the cost competitiveness of MVDC HTS cable technology for long distance offshore wind power transmission. This work highlights the cost effectiveness of the HTS system and emphasises the importance of researching this further.
Figure 1: 525kV HVDC Connection Scheme
Figure 2: 100kV S-MVDC Connection System
Model Assumptions
The hypothesis is that increased offshore wind generation capacity will be optimally integrated into the energy system of beyond 2030 by higher current, medium voltage connections due to three factors;
The ability to reduce AC-DC conversion steps and associated equipment and controls by aligning voltages more closely from turbine generator to onshore grid,
Several DC turbine and array cable topologies can provide upstream DC power to collector stations (although this was not considered in detail in this analysis).
The ability to reduce costly and suboptimal HVDC and transformer equipment offshore,
Platforms for 2GW+ collector stations vs bespoke, costly, supply constrained.
The ability to reduce environmental impact, improve public acceptance, and overall project cost reductions by transmitting higher volumes of power (2GW+) in single cable onshore landing points,
Particularly vs MVDC XLPE which is current-limited.
The project completed a cost benefit analysis (CBA) comparing the lifecycle costs of 100kV MVDC HTS cables with current alternatives; most notably 525kV High Voltage Direct Current (HVDC) with conventional copper cables.
The analysis considered lifecycle costs to be made up of three cost categories: capital costs (platforms, cables, installation, transformers, converters, etc.); the cost of electrical losses; and costs due to unavailability (system failure and repair).
The key assumptions of the model are:
A total power rating of 2GW.
An equal turbine spacing of 2km and water depth of 40m.
The offshore platform is 100km away from the onshore connection point.
The projects have a lifecycle of 30 years.
The S-MVDC system assumes the use of DC wind turbines and arrays to remove the need for a central offshore converter station.
A full report will be published in the near future. This report was a follow-on work package from a Power Systems analysis conducted by University of Strathclyde in conjunction with SuperNode and ORE Catapult. An IEEE paper is under peer review. Supporting analysis, assumptions, and conclusions are available upon reasonable request.
This project was co-funded by SuperNode, the Electrical Infrastructure Research Hub, a collaboration between the University of Strathclyde, University of Manchester and ORE Catapult.
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