No one can get past 15% of their electricity as solar and wind on a large scale. Europeans are building coal and nuclear plants at 15% solar and wind because of the wall.
Supposedly, Iowa is an example of 36% wind power. The number has no meaning, because the grid is linked to lines going into Illinois and Wisconsin, where it is about 1% of the electrical energy, and some of it goes into ethanol production which presumably tolerates inconsistencies.
Of course, a large part of the reason is fluctuations. There is no realistic answer to the fluctuations caused by solar and wind. Existing infrastructure is taxed at 10% fluctuations, while attempts to cope allow another 5% to the outer limits.
Transmission lines are a significant element of the limitations created by the 15% wall. In general, greens are unconcerned about costs and assume society should pay any price including more transmission lines. But the reality is that transmission lines are so expensive that consumers will not tolerate the expense of more transmission line.
As a general rule, transmission lines cost about as much as the generating systems for commercial solar and wind. It means, if transmission lines are constructed, they double the cost of installation. This problem has not been highly noticeable, because the small amounts of solar and wind at this time piggyback on existing infrastructure. When new lines must be constructed, they are so expensive that they never allow expansion to the levels required for going beyond the 15% wall.
In other words, even though the activists say, throw any amount of money at the problem, it never happens. Transmission lines are too expensive. In fact, a shortage of long transmission lines is creating a national security problem, while society will not invest in long lines because of the extreme expense.