In the future power systems, a large number of offshore wind farms will be connected to the AC grids through high voltage DC (HVDC) and multi-terminal DC (MTDC) grids. As wind power penetration level increases, complex grid codes and regulations will be imposed on wind turbines for frequency support. To follow any grid code and requirement for frequency support, two important features should be included in the wind turbine frequency support: i) It should be able to adjust the maximum additional power that the wind turbine temporarily provides for frequency support; ii) It should be capable of adjusting the time interval in which the wind turbine provides additional temporary power. The first feature is mainly important for reducing rate of change of frequency (RoCoF) and improving the frequency nadir while the second one is mainly important for fast frequency recovery from its nadir and improving the second frequency drop. This paper indicates that the conventional method cannot offer both of the two aforementioned features. To address this issue, two approaches are proposed for frequency support by wind turbines. The first one uses $P-\omega _{r}$ and $P-f_{WF}$ droops in each wind turbine controller, where $P$ , $\omega _{r}$ , and $f_{WF}$ represent the wind turbine power, rotor speed, and wind farm frequency. The second method employs $P-\dot{\omega }_{r}$ and $P-f_{WF}$ droops in each wind turbine controller. Performance and effectiveness of the proposed methods are evaluated by time-domain simulation studies on an MTDC grid in the PSCAD/EMTDC software environment.

中文翻译：

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通过多终端HVDC电网提供可调节的风电场频率支持

在未来的电力系统中，大量的海上风电场将通过高压直流（HVDC）和多端子直流（MTDC）电网连接到交流电网。随着风力渗透水平的提高，将对风力涡轮机施加复杂的电网法规和规章以提供频率支持。为了遵循任何电网法规和频率支持要求，风力涡轮机频率支持中应包括两个重要功能：i）它应该能够调整风力涡轮机临时提供频率支持的最大附加功率；ii）它应该能够调整风力涡轮机提供额外临时功率的时间间隔。第一个特征对于降低频率变化率（RoCoF）和改善频率最低点非常重要，而第二个特征对于从其最低点快速恢复频率并改善第二个频率下降非常重要。本文指出，常规方法不能同时提供上述两个功能。为了解决这个问题，提出了两种方法来用于风力涡轮机的频率支持。第一个用途$ P- \ omega _ {r} $ 和 $ P-f_ {WF} $ 每个风力涡轮机控制器的下垂位置 $ P $ ， $ \ omega _ {r} $ ， 和 $ f_ {WF} $代表风力发电机的功率，转子转速和风电场频率。第二种方法采用$ P- \ dot {\ omega} _ {r} $ 和 $ P-f_ {WF} $每个风力涡轮机控制器的下垂。通过在PSCAD / EMTDC软件环境中的MTDC网格上进行时域仿真研究，评估了所提出方法的性能和有效性。