A combination of routing the power elsewhere in the grid when possible and banks of gigantic (shipping container sized) dummy loads. As other people have noted,

A combination of routing the power elsewhere in the grid when possible and banks of gigantic (shipping container sized) dummy loads. As other people have noted, you don't need to dissipate the full amount of power all at once, you test the generators individually. http://resistive-loadbank.sell.everychina.com/p-109417602-3546kva-inductive-power-banks-for-generator-testing-dummy-load-bank.html http://resistive-loadbank.sell.everychina.com/p-109417602-35... is the first example I pulled up, and there are bigger ones.