**1. Introduction**

Offshore wind will play a significant role in both Ireland's and Europe's decarbonisation plans. Ireland's large offshore territory, coupled with high wind availability across each season [1,2], make it an ideal candidate for offshore wind development. In line with the National Energy and Climate plan, 5 GW of offshore wind is planned for deployment in Ireland by 2030 [3]. According to the Sustainable Energy Authority of Ireland (SEAI)'s wind energy road map, Ireland has potential to far exceed this 5 GW of offshore wind with a predicted installed capacity of 30 GW by 2050 [4]. This growth prediction coincides with a general decrease in onshore wind farm planning applications. Harper et al. have evaluated the regulatory effects of wind turbine planning and financing in the United Kingdom [5]. This study identified onshore wind as having a 44% success rate compared with 89% in the offshore wind sector.

Wind gust analysis has been extensively performed for traditional wind turbine systems. Turbulence and wake effects and extreme load predictions for horizontal axis wind turbines have been studied by Brand et al. [6–8]. The effect of wind gusts on vertical axis wind turbines using Computational Fluid Dynamics (CFD) has been examined by Onol et al. [9]. The distribution of extreme gusts has been previously investigated for traditionally interconnected wind turbines by Cheng et al. [10]. Gust detection and prediction methods using Doppler LiDAR are an area of current development for wind farms [11]. The use of LiDAR for wake management has also been explored showing a wind farm power increase of 7.552% with a reduction in downwind turbulence [12].

This rapid expansion within the sector leaves an opportunity for the development of new interconnection technology such as the Direct Interconnection Technique (DIT) which is considered in this paper. This technique is a method of integrating renewable generation first proposed by Pican et al., 2011 [13]. This technique of integration minimises

**Citation:** O'Donnell, C.W.; Ebrahimi Salari, M.; Toal, D.J. A Study on Directly Interconnected Offshore Wind Systems during Wind Gust Conditions. *Energies* **2022**, *15*, 168. https://doi.org/10.3390/ en15010168

Academic Editors: Eugen Rusu, Kostas Belibassakis and George Lavidas

Received: 16 November 2021 Accepted: 24 December 2021 Published: 27 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the utilisation of Back to Back Power Converters (B2BC) in offshore turbines by connecting each turbine to a common offshore synchronous bus which can then be transmitted back to shore by High Voltage Alternating Current (HVAC), High Voltage Direct Current (HVDC) or Low-Frequency Alternating Current (LFAC) [14,15]. The power conversion equipment can be relocated to a single offshore site allowing for better access and optimisation or where transmission constraints permit, relocated entirely onshore.

Power electronic conversion systems exhibit a high failure rate among wind turbine subassemblies [16,17]. This resultant downtime, coupled with the difficulty and cost of servicing offshore turbines [18], demonstrates the potential that DIT has for improving reliability and reducing costs associated with offshore wind. While detection and protection methods can aid in reducing power electronic converter failures [19–21], offshore maintenance of these power converters has been noted as a critical element in the levelised cost of energy [22], given the requirement for transport of parts and technicians to these offshore locations. According to a case study conducted by Su et al. failure of electrical subsystems accounted for the third highest rate of failure, accounting for 14% and 26% of total failures for the two farms studied. This accounted for 301 hours of downtime in project 1 and 693 h in project two [23].

DIT begins by spinning a pilot generator connected to the offshore bus establishing the bus reference voltage and frequency. Each subsequent generator is then spun up and connected to the bus with the pilot generator governing system frequency and voltage, and load sharing controllers optimising behaviour on subsequent generators. This high inertia system electrical bus is then transmitted onshore through the use of HVAC, LFAC or HVDC as required and grid interconnection is performed by a large scale B2BC. This method has also been extended to Airborne Wind Energy (AWE) systems by Salari et al. 2018 [24]. The difference between traditional interconnection and direct interconnection can be observed in Figures 1 and 2.

In the case of the traditional interconnection, each generator is effectively separated from the local wind farm bus by the B2BC in the wind tower. This facilitates separation of the generator, and transients caused by wind gusts for example, from the local farm bus, and the individual B2BC provides a means of dealing with transient conditions on the generator side [25]. With DIT, as multiple generators are directly interconnected to the same bus, any gust generated transient condition experienced initially by a leading-turbine-to-wind will affect the interconnected system of generators.

**Figure 1.** Traditional Interconnection.

**Figure 2.** Direct Interconnection.

This paper studies the effects of wind gusts by simulating a directly interconnected wind farm and introducing a wind gust to a leading turbine. Gusts of varying types, magnitudes and transient times are applied to a leading turbine and overall system responses are investigated as described in detail in the simulation methodology section. Gust tolerance levels are measured and discussed with comparison to real-world coastal wind data.
