stručni rad
The integration of wind farms had a serious impact on power systems of many of the European countries in the past decade. Of course, it has lead to many different problems in the power grids of those countries, and through UCTE connection, to problems of Europe as a whole. In Croatia, first wind farm is Ravna 1, on the Pag Island, on the eastern part of Adriatic Sea. It was put into operation in 2005, and is connected to the 10 kV distribution grid of the Croatian national utility HEP. Power flow, short-circuit and power quality calculations were made in order to check whether the connection of the wind farm satisfies the regulations for connection of this kind of energy source to the power system. All the calculations were made for cases with and without the wind farm connected in order to get the influence of the wind farm on the observed parameters. The wind farm consists of 7 wind turbines, each with nominal active power of 850 kW with doubly-fed induction generators, totalling installed power to 5, 95 MW. Produced power is injected into 10 kV grid which maximum consumption is approximately 4 MW. The distribution grid is connected to the rest of the power system over the Pag 110/10 kV substation with one 10 MVA transformer. This in whole means that during high production levels of the wind farm one part of the produced power of the wind farm is transferred to the 110 kV grid. Due to the small amount of power injected to 110 kV grid, influence to the wind farm on the 110 kV grid is not significant, but it is of major concern for 10 kV grid and its consumers. The power flow calculations show that voltage levels in all nodes in the 10 kV grid are within the permitted values and that no element in the grid (cable, overhead line etc.) is overloaded at any production level of the wind farm. Short-circuit calculations were made to check if the existing equipment in the grid meets the new stresses produced with the additional short-circuit power caused by the wind farm connection. In addition, they are to be used in flicker emission levels calculations both in case of continuous operation of the wind farm and in cases of switching operations of the wind farm at the point of common coupling (PCC). Three phase faults were taken into concern and the values of initial symmetrical short-circuit currents (Ik” ) were calculated according to [1]. The calculations have shown that connection of the wind farm Ravna 1 to the 10 kV grid has added to the short-circuit current, but no technical limits were exceeded. Flicker emission levels are below levels allowed for continuous operation and for switching operations. The calculations have shown that connection of the wind farm Ravna 1 influences the local grid to which it is connected, but all the regulations for connection of a wind farm are satisfied.
Doubly-fed induction generator (DFIG); distribution grid; load flow; short circuit; Flicker
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