Voltage dip caused by the sequential energization of wind turbine transformers

Iván Arana Aristi, Joachim Holbøll, T. Sørensen, Arne Hejde Nielsen, O. Holmstrøm, Poul Ejnar Sørensen

Research output: Chapter in Book/Report/Conference proceedingArticle in proceedingsResearchpeer-review


THIS paper presents the results from electromagnetic transient (emt) simulations using PSCAD, where the model of Nysted Offshore Wind Farm (NOWF) was created to simulate the voltage dips due to wind turbine transformer energization. Four different sequences energizing different numbers of wind turbine transformers simultaneously and two phase angles were defined and compared. NOWF consists of 72 wind turbines with a rated power of 2.3 MW. The turbines are connected in 8 radials of 36 kV submarine cables. Each radial is then connected by one “root” cable to a three winding park transformer (180/90/90 MVA, 132/33/33 kV) on the offshore platform next to the wind farm. The connection to the 132 kV grid is by a 10.5 km submarine cable and a 18.3 km land cable. Each of the 8 radials has 9 turbines, interconnected with 36 kV cables of 505 m length. The submarine cable is connected at the bottom of each turbine. The step up transformer (2,5 MVA, 33/0.69 kV) in every turbine is connected via a switch-disconnector and fuse on the MV-side. The grid codes are defined by the Transmission System Operator of each country, and some of them are not updated to regulate correctly the operation of large wind power plants. According to the British Engineering Recommendations P28, the voltage fluctuation at the point of connection with the grid may be required to be as low as 3% during the energization of a motor; and since the energization of a transformer is similar to the direct-on-line starting of induction motors, this recommendation automatically applies to the wind turbine transformers in a wind farm, regardless of the fact that wind turbine transformers are rarely energized and the investment of timed relays might be significant. At the design stage of the collection grid of large offshore wind farms, it is important to estimate the voltage dip due to the energization of wind turbine transformers; since it depends on the layout of the internal power grid, the phase angle when the switching occurs, the possible residual flux in the transformer, the magnetic properties of the transformer and the external grid properties. It was found that there is no large difference in the voltage dip for switching at peak or zero voltage. It was found as well, that the smallest voltage dips occur if the wind turbine transformers are switched-in independently So far, there have been successful applications of large offshore wind farms in Denmark and the UK. However, there is still work to be done to optimize the investment and protection system of the internal grid of the wind power plants.
Original languageEnglish
Title of host publicationEuropean Wind Energy Association 2009 Conference Proceeding
Publication date2009
Publication statusPublished - 2009
Event2009 European Wind Energy Conference and Exhibition - Marseille, France
Duration: 16 Mar 200919 Mar 2009


Conference2009 European Wind Energy Conference and Exhibition
Internet address


  • Wind power control and integration

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