All customers connected to a public electricity network, whether generators or consumers, must comply with agreed technical requirements. Electric networks rely on generators to provide many of the control functions, and so the technical requirements for generators are unavoidably more complex than for demand customers. These technical requirements are termed ‘Grid Codes’. The technical requirements governing the relationship between generators and system operators need to be clearly defined. The introduction of renewable generation has often complicated this process significantly, as these generators have physical characteristics that are different from the directly connected synchronous generators used in large conventional power plants. In some countries, a specific grid code has been developed for wind farms, and in others the aim has been to define the requirements as far as possible in a way which is independent of the power plant technology. The technical requirements within grid codes and related documents vary between electricity systems. However, for simplicity the typical requirements for generators can be grouped as follows: • Tolerance the range of conditions on the electricity system for which wind farms must continue to operate; • Control of reactive power often this includes requirements to contribute to voltage control on the network; • Control of active power often this includes requirements to contribute to frequency control on the network; • Protective devices; and • Power quality. It is important to note that these requirements are often specified at the Point of Common Coupling (PCC) between the wind farm and the electricity network. In this case, the requirements are placed at wind farm level, and wind turbines may be adapted to meet these requirements. It is also possible for some requirements to be met by providing additional equipment, as for example for FACTS devices. One of these new connection requirements regarding wind energy is fault ride-through capability. In the past, wind generators were not allowed to remain connected to the utility when voltage at the PCC fell below 85 %, forcing their disconnection even when the fault happened far from the wind farm (Jauch et al, 2007; Rodriguez et al, 2002). That is the reason