# Effects of Blade Extension on Power Production and Ultimate Loads of Wind Turbines
## 1 Introduction  
In the last decade, the global wind power industry developed rapidly. By the end of 2021, the accumulative installed capacity had reached 837 GW in the world. Thereinto, China's newly installed wind power capacity has ranked first in the world for 12 consecutive years, with a cumulative installed capacity of 300.15 GW [1]. Among the huge number of wind turbines, more than 60 GW of them have operated for over 10 years. Due to the conservative micro-sitting or immature technology, the power performance of some early-built wind farms is much less than satisfactory. It is also found that, in a large wind power base, the later-built upstream wind farms can significantly reduce the incoming wind speed of the downstream wind farms and decrease their power performance [2, 3]. To this end, the technical transformation of the old turbines shows a very good prospect to enhance the economic benefits of the early-built wind farms. For technical transformation, people can replace or modify the components or systems without changing the foundation and tower to upgrade the turbines' performance. Generally, many of the components including the turbine blade, pitch system, generator, etc. are involved in the technical transformation. For turbine blades, a host of technical transformation schemes were proposed, such as the blade extension [4, 5, 6, 7, 8], vortex generator [9, 10, 11, 12], winglet [13, 14, 15], etc. Therein, blade extension can directly increase the swept area, and thus increase the energy production of the turbines [4, 5, 6, 7]. Apart from the power enhancement, the load increase due to the blade extension is also extensively concerned. The extension scheme was studied by [PERSON] et al. [4] in the aspect of aerodynamic shape, aerodynamic performance, weight and steady loads. The results indicate that in terms of the technicaldifficulty and engineering cost, blade tip extension is better than blade root extension to achieve a similar power enhancement. [PERSON] et al. [5] evaluated the loads of a fixed pitch 750 kW wind turbine with blade extension. They found that the newly obtained loads after blade extension are still in the security domain due to the large safety margin of the original design. Meanwhile, a 6.16% increase of annual energy production due to the technical transformation was obtained. Furthermore, the variation of both the ultimate and fatigue loads was analyzed by [PERSON] et al. [6] for a 1.5 MW variable speed wind turbine, and they found that the loads after technical transformation complied with the design requirement and that the annual energy production of a single turbine could increase by about 400,000 kWh. Considering a specific wind farm site, [PERSON] et al. [7] evaluated the blade extension scheme resorting to the software of GH-Bladed. The results indicate that the AEP can be increased by 6.5%, and meanwhile, the loads can well meet the design requirements due to the large safety factor used in the original design. The unsteady aerodynamic loads on the blade were experimentally investigated by [PERSON] et al. [8] to confirm the feasibility of adhesive bonding technology for blade extension. Recently, some demonstration cases were performed, such as the blade extension renovation projects of Goldwind Science & Technology and China Datang corporation. As is illustrated above, blade extension can significantly enhance the power production of the turbines, and meanwhile induce additional loads for wind turbines. Although people have gained significant knowledge on blade extension of turbines, there is still a lack of systematic load evaluation for wind turbines with different extension lengths and different operating conditions.