【正文】 1 Modelling and transient stability of large wind farms 1. Introduction Denmark has currently about 2300 MW wind power capacity in onland and few offshore settings, which corresponds to more than 20% of power consumption(in average). Further, construction of two largescale offshore wind farms of 150 MW power capacity each has been announced. The first large offshore wind farm in Denmark will be constructed at Homs Rev by the year 2021 in the area of the system operator ELTRA .This will be followed by the first in the area of the Eastern Danish system operator ,ELKRAFT System ,large offshore wind farm at Rodsand by the year 2021. The installed capacity in onland settings and in bined heatpower units(UHP)will increase as well, whilst the power production and control ability of the conventional power plants with respect to voltage and frequency are reduced . In the years to e ,the power production pattern in the Danish power system will change from the power supply from conventional power plants―as it is known today―to a power supply mix, where about 3040%of power consumption(in average) is covered by wind power. In other words, the power technology will undergo changes from a wellknown technology builtup about conventional power plants to a partly unknown technology―wind power. In the year to e it will be focusing on maintaining power system stability and voltage stability, for example at a short circuit fault, ensuring power supply safety and other important tasks as amount of wind power is drastically increasing. This situation makes it necessary to find solutions with respect to maintaining dynamic stability of the power system with large amount of wind power and its reliable operation. These solutions are based on a number of requirements that are formulated with respect to operation of the large offshore wind farms and the external power system in case of failure events in the external system. The paper contains separate subjects dealing with design of windmills for large offshore applications and their control that shall be taken into account with respect to improving the shortterm voltage stability. 2. system stability requirements In terms of shortterm voltage stability, the major goal is the voltage reestablishing 2 after failure events in the power system with large amount of wind power. The transmission system operator is responsible for maintaining power system stability and reliable power supply. As the situation is today, the majority of the Danish windmills onland are stall wind turbines equipped with conventional induction generators and acconnected to the power system. In case of a short circuit fault in the power system, those windmills are easily overspeeded and, then, automatically disconnected from the power system and stopped. Such automatic disconnections will be very fast and ordered by the windmill protection system relay settings. When the onland windmills are automatically disconnected, there is no dynamic reactive pensation demands related to them, despite their large power capacity. When the voltage is reestablished, the onland windmills will be automatically reconnected to the power system in 1015 min afterwards and continue their operation, The onland windmill relay settings are decided by the windmill manufacturers or the windmill owners and these, as usual, cannot be changed by the transmission system operator. In case of the large offshore wind farms, the power system operator has formulated the Specifications for Connecting Wind Farm to Transmission Network. In accordance with the Specifications, the voltage stability at failure events in the external power system shall be maintained without any subsequential disconnection of the large offshore wind farms. Establishing dynamic reactive pensation of the large offshore wind farms can be, therefore, necessary. The amount of dynamic reactive pensation depe