1、 PDF外文：http:/ Modelling and transient stability of large wind farms 1. Introduction Denmark has currently about 2300 MW wind power capacity in on-land and few offshore settings, which corresponds to more than 20% of power consumption(in average). Further, construction of two large-scale offsho
2、re 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 2002 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 Syste
3、m ,large offshore wind farm at Rodsand by the year 2003. The installed capacity in on-land settings and in combined heat-power 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
4、years to come ,the power production pattern in the Danish power system will change from the power supply from conventional power plantsas it is known todayto a power supply mix, where about 30-40%of power consumption(in average) is covered by wind power. In other words, the power technology will und
5、ergo changes from a well-known technology built-up about conventional power plants to a partly unknown technologywind power. In the year to come it will be focusing on maintaining power system stability and voltage stability, for example at a short circuit fault, ensuring power supply safety a
6、nd 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 requi
7、rements 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
8、 taken into account with respect to improving the short-term voltage stability. 2. system stability requirements In terms of short-term voltage stability, the major goal is the voltage re-establishing 2 after failure events in the power system with large amount of wind power. The transmi
9、ssion system operator is responsible for maintaining power system stability and reliable power supply. As the situation is today, the majority of the Danish windmills on-land are stall wind turbines equipped with conventional induction generators and ac-connected to the power system. In case of a sh
10、ort 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 on-land windmills are automati
11、cally disconnected, there is no dynamic reactive compensation demands related to them, despite their large power capacity. When the voltage is re-established, the on-land windmills will be automatically re-connected to the power system in 10-15 min afterwards and continue their operation, The on-lan
12、d 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 Tra
13、nsmission Network. In accordance with the Specifications, the voltage stability at failure events in the external power system shall be maintained without any sub-sequential disconnection of the large offshore wind farms. Establishing dynamic reactive compensation of the large offshore wind farms ca
14、n be, therefore, necessary. The amount of dynamic reactive compensation depends, generally, on the windmill technology and in the wind farms and is influenced by the windmill electrical and mechanical parameters. In other countries, similar specifications may be found as the result of large incorpor
15、ation of wind power into the local power system. 3.Wind farm model The windmill technology in offshore settings has to be robust, developed and known practical applications. The wind turbine concept with conventional induction generators has been in operation in on-land settings in Denmark dur
16、ing many years, which is why it may be considered that this technology will be used offshore as well. The wind turbines are equipped with blade angle control system-pitch or active stall that make it possible to 3 adjust the set-points of the wind turbines by the blade by the blade angle adjus
17、tments. The complete representation of the wind farm is chosen because the commonly asked question concerning large wind farms is whether there can be electromechanical interaction between a large number of the closely placed windmills excited by disturbances in the power system when the windmills a
18、re working at different set-points, equipped with relatively soft shafts and even having different mechanical data, and equipped with control systems, for instance pitch. The model of the offshore wind farm is implemented in the dynamic simulation tool PSS/E and consists of 80 wind turbines of 2MW p
19、ower capacity each, see Fig.1. Each wind turbine is simulated by a physical windmill model consisting of : 1. the induction generator model with representation of the stator transients, 2. the windmill shaft system model, 3. the aerodynamic model of the wind turbine, 4. the pitch control system give
20、n by the control logic and the blade servo. For computation of wind turbine aerodynamics there are used airfoil data for a 2 MW pitch windmill equipped with an induction generator. Each wind turbine is via its 0.7 KV/30KV connected to the wind farm internal network. The internal networ
21、k is organised in eight rows with 10 wind turbines in each row. Within the rows, the wind turbines are connected through the 30 KV sea cables. The distance between two wind turbines in the same row is 500 m and the distance between two rows is 850 m. The rows are through the 30 KV sea cables connect
22、ed to the offshore platform with 30 KV/132 KV transformer and, then, through the 132 KV sea/underground cable to the connection point in the transmission system on-land. There is chosen an ac-connection of the offshore wind farm to the transmission network. An irregular wind distribution over the wi
23、nd farm area there is assumed since the wind turbines are shadowing each other for incoming wind. The efficiency of the wind farm is 93%at the given wind distribution and the power production pattern is shown in Fig.1. Furthermore, the windmill induction generators have a little different short circuit capacities viewed from their terminals into the internal network and this is why the wind turbine initial set-points are different.
