Stability and redundancy studies on the electrical grid on Gotland with respect to 500 MW of new wind power and a VSC HVDC link to the mainland

Abstract: The electric grid of Gotland is connected to the mainland via a 90 km HVDC Classic bipole of 2 ' 130 MW. The HVDC link balances the load and production on the island to maintain the frequency within limits, the load varies between 50 and 180 MW throughout the year. The power production on the island comes mainly from wind power. Today, the installed power is about 170 MW but the wind power production will be further exploited and the plan is to add another 500 MW of wind power capacity to the existing plants. These plants will be connected to a new 130 kV transmission grid which will have a connection to the existing 70 kV grid at a new substation called Stenkumla. Along with the increased wind power production on the island comes the need of increased transmission capacity to the mainland. A VSC HVDC link of 500 MW is planned for this purpose and it will be connected to Stenkumla. As of today, it is not certain whether the two grids will be connected or not. Having connected grids is in the interest of the grid owner Gotlands Energi AB, GEAB since they then could utilize the technology of the new link and thereby ensure stableoperation during faults that today would lead to black out. In this thesis the feasibility of having connected grids was investigated and the study was divided into three main parts. •Reactive power and voltage profiles •Short circuit study •Converter trip study This study shows that under the assumptions made regarding production grid layout and proportion of WTG types there will be no need for adding reactive power compensation equipment. That is provided that demands are set on wind power plant contractors to have their equipment contributing with reactive power compensation, even during no load. A trip of the SvK VSC HVDC converter during full power production causes the most severe stress to the system. The major problem proved to be surviving the first 100 ms after converter trip without loosing angular stability and the most important measure to improve the stability was active power reduction of the wind turbines. The overall conclusion is that it is feasible to have connected grids during normal operation but demands has to be put on wind power plant contractors.