If you have more than 10% of overall power from wind then there are issues keeping power supply stable
Mr. Frost, of the Alberta system operator, said European countries such as Denmark and Germany have been able to maintain a high proportion of wind power in their electricity systems mainly because they have multiple connections to other countries' power grids. That gives them substantial flexibility to import or export power to compensate for wind fluctuation.
Germany, for example, has 39 international interconnections, he said, making variable wind conditions much easier to manage.
Underground electical grid connections cost from $500,000 to $3 million per mile. $120,000 per mile for the erection of overhead lines. Connecting countries and states is very costly.