FRT Criteria, Protection, and Control Coordination
In the context of FRT capability, undervoltage and overvoltage protection relays must operate accurately to achieve ride through and ensure safety of the wind farm. The safety of the wind units is also important as wind farm operating on extended envelope requirements cannot afford any malfunctioning of these relays.
In addition to voltage envelope requirement for abnormal operation, grid codes also demand wind farms to operate on extended voltage and frequency limits during normal operation. These requirements have been well-defined in some of the grid codes and are to be satisfied even at the expense of active power . Robust control design of the wind farm can help achieve these requirements. Not all available technologies meet these requirements, thus requiring additional control blocks, demanding certain protection requirements.
All the above-mentioned expectations from large wind farms have a direct impact on the fault response of the wind units while riding through the faults and participating in market services at the same time. Increasing penetration of larger wind farms using different WTGs raises questions on system protection schemes. The literature review identifies that this aspect requires much more attention. Ongoing research for large-scale wind integration is around standardization of practices followed in transmission systems and exploring new options for control that will support the grid. Normal system operation with wind has been well studied with issues like forecasting, reliability, power quality, transient stability, and FRT impacts following large-scale wind integration . However, there are no standard protection schemes for wind farms, yet are available like those available for conventional generation plants. Wind generators collective response has not been discussed much especially under abnormal operations such as grid distur- bances and faults.
Reference  discusses impacts of distributed generation on protective device coordination, but focuses on distribution system rather than transmission system. In [4, 5] the author has analyzed the performance of conventional protection schemes used for a 225 MW wind farm and some issues such as disconnection of whole generation in case of fault in a single wind generator is highlighted. This leads toward investigation and design of new intrawind farm protection schemes and better coordination strategies for future integrations. Reference  discusses the earth fault protection for decentralized wind power plants. Overcurrent protection based on a particular model and testing facility for Type-1, 2, and 3 wind farms has been discussed but only from the modeling validation viewpoint . The importance and necessity of overvoltage and overvoltage lightning protection has also been identified based on a Chinese case study but not emphasizing over any particular WTG or its protection issues . Some of the potential WTG faults and their effective management through the IEC 61850 perspective and control viewpoint is discussed in [9–11]. In order to assess accurate protection settings, realistic wind farm models are required. These models should closely represent the dynamic behavior of connected wind farms. Accuracy of the machine model, ability of the model to be used to carry out balanced and unbalanced studies, type of the model based on its differential equation order, and accurate transfer functions of the control blocks are essentially required.
There is a need to carry out detailed system short-circuits and fault studies under various operating scenarios. Though short-circuit models for WTGs have been proposed in a few publications, standardization of relaying schemes is still a work in progress. These studies require wind farm modeling and all fault calculations and protection relay settings would thereby be influenced by the models.
Terminal voltage, stator, and rotor current magnitude during a fault are influenced by the model type .
The next section presents a case study developed in DIgSILENT® Power- Factory to carry out comparative analyses of dynamic fault behavior of all four mentioned WTG arrangements from a grid interface perspective. The impacts of each WTG arrangement on protection operation and performance for distance, differential, and overcurrent protection are explored and discussed in detail as the scope of this chapter.