Frequency optimisation for DC/DC converters in DC-connected offshore wind turbines

In all-DC wind farms, DC/DC converters connect the low voltage wind turbine output to the high voltage DC array. One potential benefit of using DC/DC converters is that they require a relatively small transformer due to their higher frequency operation. However, there is no consensus as to what this frequency should be. This paper aims to determine the optimal operating frequency for DC/DC converters in 15 MW wind turbines connected to an 80 kV DC array. A multi-objective optimisation is performed taking into account the DC/DC converter volume, weight and losses. Frequencies ranging from 500 Hz to 5 kHz were tested for unidirectional, bidirectional, single-phase and three-phase converters. The optimal frequencies for unidirectional and bidirectional converters were found to be approximately 2 kHz and 1 kHz, respectively

A systematic review of DC wind farm collector cost-effectiveness

DC collection systems have been suggested to improve the cost-effectiveness of offshore wind farms but no consensus currently exists on which configurations are the most promising. This paper aims to determine the primary DC wind farm candidates for commercialisation based on cost-effectiveness and technological risk. A systematic review was performed of the literature that formally assesses the cost, losses or reliability of DC wind farm configurations. The optimal configurations were found to be dependent on the methodology and assumptions used by each study, as well as the individual wind farm characteristics. Series and series-parallel DC designs without offshore platform performed well in terms of costs, but have challenges in operation and reliability that limit the short-term opportunity for commercialisation. The standard DC parallel topology has the lowest technological risk, but the mean cost reported in the literature is similar to that of AC topologies. Standard parallel DC wind farms are the primary candidate for the first commercial DC wind farm demonstrators, but the optimal design will likely need to be determined on a case-by-case basis. Guidelines for this assessment are provided

Review and comparison of single and dual active bridge converters for MVDC-connected wind turbines

A key component for all-DC wind farms is the DC/DC converter. The converter must have multi-megawatt power capability, a high step-up ratio, provide galvanic isolation, and operate efficiently while being able to fit in the wind turbine nacelle. The single active bridge (SAB) and dual active bridge (DAB) converters in standalone or cascaded configuration are promising topologies that have the potential to meet these requirements. This paper reviews the operation and control of these converters, and compares their volume, weight, and efficiency for a 15 MW wind turbine with 80 kV DC connection. The results show that the standalone topologies are significantly smaller and lighter than their cascaded counterparts. However, all topologies fit inside the wind turbine nacelle. The SAB designs are the most efficient and robust, as they use diodes in the output bridge. The DAB topologies have the advantage of bidirectional power flow at the cost of additional switches and losses. The standalone DAB requires series-connected switches in the output bridge, which may difficult to implement. The cascaded topologies offer higher reliability without significantly increasing losses, making them the most attractive option for future DC wind turbines

Modellierungsverfahren und Regelstrategien für wechselrichterdominierte Inselnetze

Published in print by Universitätsverlag der TU Berlin, ISBN 978-3-7983-2872-3 (ISSN 2367-3761)The character of modern power systems is changing rapidly and inverters are taking over a considerable part of the energy generation. A future purely inverter-based grid could be a viable solution, if its technical feasibility can be first validated. The focus of this work lies on inverter dominated microgrids, which are also mentioned as 'hybrid' in several instances throughout the thesis. Hybrid, as far as the energy input of each generator is concerned. Conventional fossil fuel based generators are connected in parallel to renewable energy sources as well as battery systems. The main contributions of this work comprise of: The analysis of detailed models and control structures of grid inverters, synchronous generators and battery packs and the utilization of these models to formulate control strategies for distributed generators. The developed strategies accomplish objectives in a wide time scale, from maintaining stability during faults and synchronization transients as well as optimizing load flow through communication-free distributed control.Die Struktur der modernen Energieversorgung hat sich in den letzten Jahrzehnten massiv geändert. Dezentrale Generatoren, die auf Wechselrichtern basieren, übernehmen einen großen Teil der Energieerzeugung. Ein ausschließlich wechselrichterbasiertes Netz wäre ein realistischer Ansatz, wenn seine technische Machbarkeit verifiziert werden könnte. Die wichtigste Beiträge dieser Arbeit sind: Die Analyse von Modellen und Regelstrukturen von Netzwechselrichtern, Synchrongeneratoren und Batterieanlagen. Die entwickelten Modelle werden verwendet, um Regelstrategien für dezentrale Generatoren in Mittelspannungsinselnetzen zu formulieren. Die erste Strategie ist eine Synchronisationsmethode für netzbildende Wechselrichter. Zweitens wird die Leistungsaufteilung in Mittelspannungsinselnetzen mittels Droop Regelung analysiert. Weiterhin erfolgt die Untersuchung der transienten Lastaufteilung zwischen netzbildenden Einheiten mit unterschiedlichen Zeitkonstanten. Beim Betrieb mehrerer paralleler Wechselrichter wird der Einfluss der Netzimpedanz auf die transiente Lastaufteilung analysiert. Die dritte entworfene Regelstrategie umfasst die Integration der Sekundärregelung in die Primärregelung. Der Ladezustand von Batterien wird mit der Lastaufteilung gekoppelt, um die Autonomie des Netzes zu stärken. Abschließend wird eine Kurzschlussstrategie für netzbildende und netzspeisende Wechselrichter entwickelt. Ziel der Strategie ist die Maximierung des Kurzschlussstromes. Als zusätzliche Randbedingung soll keine Kommunikation zwischen Generatoren stattfinden

Multi-objective optimization and comparison of DC/DC converters for offshore wind turbines

A key enabling technology for DC collection systems in offshore wind farms is a suitable wind turbine DC/DC converter. However, there is no consensus regarding the topology, design, or operating frequency of this converter. This paper presents an optimization and comparison of four DC/DC converter topologies, including 1-phase, 3-phase, unidirectional, and bidirectional converters. The converters are compared in terms of their reliability, volume, weight and losses at switching frequencies ranging from 500 Hz to 5 kHz. The medium frequency transformer for each converter is designed using multi-objective optimization, and the overall converter volume calculation takes into account the insulation requirements and physical configuration of the components. The results show that if only unidirectional operation is required, the 1-phase single active bridge is the preferred option due to its high reliability, small size and low losses with an optimal operating frequency of up to 2.5 kHz. For bidirectional systems, the 1-phase and 3-phase dual active bridge topologies have a similar efficiency and optimal operating frequency of 1 kHz. Despite its higher volume, the 3-phase version is the preferred option due to its higher reliability and lower device stresses, provided there is enough available space

Control and power balancing of an off-grid wind turbine with co-located electrolyzer

Co-locating electrolyzers and offshore wind can significantly reduce the cost of green hydrogen. However, without a grid connection, a new control paradigm is required for the electrolyzer to follow the variable power supplied by the wind turbine. Commercial electrolyzers have power ramp rate limitations, which can result in a mismatch between the wind turbine and electrolyzer power, leading to frequent shutdown and potentially unstable operation. This paper is the first to develop a control system for this off-grid operation with three mechanisms to dynamically balance the power, including energy storage, rotor inertia, and enhanced pitch control. The results show that a $6.8M supercapacitor is required with a power rating and capacity of approximately 6.7 MW and 8.5 kWh to enable the system to operate through 99% of the annual wind variation. If the electrolyzer ramp rates can be doubled, the same operating hours can be achieved using only control-based power balancing methods at the cost of a marginal reduction in energy production. If commercial electrolyzer ramp rates can be tripled, the system is able to operate without the need for any power balancing

All-DC offshore wind farms : when are they more cost-effective than AC designs?

The use of MVDC collector systems has been proposed as a way to reduce the levelised cost of energy (LCOE) of offshore wind farms. This study provides a quantitative assessment of the conditions required for such all-DC wind farms to be cost-effective. A comprehensive LCOE analysis of two AC and two all-DC wind farm designs is performed, with sensitivity studies on wind farm size, distance from shore, collector voltage, and component costs. The results show that for MVDC-based wind farms to be more cost-effective than equivalent HVDC wind farms, the DC/DC converter cost must be less than 90% of the cost of an equivalent MMC, with a cost reduction of 25% for the DC platform. Alternatively, if cost reductions of 30% can be achieved for the DC platform, then the DC/DC converter can be the same cost as an equivalent MMC. For all-MVDC wind farms without HVDC conversion stage to have the lowest LCOE, the collector voltage must be increased, preferably to ±100 kV or above. The all-MVDC configuration can also become cost-effective if a reduction of more than 50% in the cable installation cost can be achieved, for example through the simultaneous burial of multiple cables