الفهرس 
AbstractOnly 14 pages are availabe for public view
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Abstract
Due to the rapid increase in renewable energy integration to power systems across the world, the grid operators and other stakeholders, including renewable energy power plant manufacturers and operators, are experiencing several challenges in secure and stable grid operation. Offshore wind farms (OWFs) have been lately getting a significant focus in many wind-rich countries as the deep ocean sites offer high wind speed regions and thus high plant load/capacity factor for wind power plants. There is a strong policy push for integrating more OWFs in many countries, particularly in Europe and the USA. While the integration of OWFs connected through point-to-point HVDC (PP-HVDC) and multi-terminal HVDC (MTDC) systems introduces a new complex set of challenges to the grid operation, they also provide an opportunity to support the grid in terms of frequency and voltage stability. Owing to the rapid increase in the number of OWFs connected to the electrical grids through PP-HVDC/MTDC systems, new and updated grid code regulations (GCR) are being imposed on the HVDC-connected OWFs. The grid frequency support and low voltage ride through (LVRT) capabilities are the two such requirements of utmost importance that have been imposed by the regulators/government agencies on the OWFs. During frequency disturbance events in the main grid, the OWFs should be capable of providing inertial and primary frequency support to the connected grid. On the other hand, the OWFs should remain grid-connected during fault-induced voltage dips and contribute to the grid voltage support during the voltage dip events. The main issue with the HVDC-connected OWFs is the decoupling between OWFs and the main grid, and therefore OWFs cannot inherently detect a frequency or voltage disturbance in the main onshore grid. Hence, the OWFs cannot support the onshore grid without indirectly detecting the onshore grid disturbances. Therefore, to enable the OWFs in detecting the main grid disturbances, the onshore grid signals (voltage and frequency) should be detected by the offshore WTGs either directly through a communication medium or indirectly through a communication-independent mode. Due to time delay, low reliability and cost implications of communication-dependent schemes, only communication-independent strategies, which do not require physical communication channels between the onshore and the offshore sites have been considered in this study. To comply with the frequency support requirements, novel control strategies for PP-HVDC/MTDC-connected OWF have been proposed to address the limitations of the existing control strategies. The proposed control strategies have been compared with the conventional communication-independent strategies to demonstrate the robustness, effectiveness and reliability of the proposed frequency support strategies. During frequency support from OWFs, the proposed strategies complies adequately with GCR requirements while adhering to the set limits for main grid frequency deviation and the maximum permissible HVDC voltage deviations. On the other hand, the main issues with the reported conventional LVRT strategies are the voltage dip induced frequency (VDIF) excursions in the main grid, and high electrical stresses in the offshore grid and WTGs. Therefore, novel communication-independent LVRT control strategies have been proposed for the PP-HVDC/MTDC-connected OWFs based systems. Under the proposed LVRT control strategies, the OWFs can support the grid while complying with LVRT requirements and active power recovery (APR) ramp limits at the WTG output with improved VDIF performance. Moreover, high electrical and mechanical stresses in the offshore grids and WTGs are avoided under the proposed strategy. However, all the studies reported in the literature that dealt with the capability of MTDC-connected OWFs to comply with GCR requirements, including LVRT and frequency support capability, have ignored multiple grid disturbances that may be experienced simultaneously in different onshore grids in the MTDC connected multi-onshore grids. In addition, the communication-independent control strategies that have been employed cannot differentiate between the LVRT-triggered events and frequency deviation events on the onshore side. Therefore, a new coordinated control strategy is proposed to distinguish between the fault-induced voltage dip events and the onshore frequency excursion events based only on the local measurements, without any physical communication layer among the onshore and offshore grids, and MTDC system. The proposed coordinated control strategy shows ample benefits and robustness in complying with GCR during individual and simultaneous/back-to-back grid disturbances.