Delta is an American company based in Texas, USA, and they have been the mainstay of affordable surge arrestors for the renewable energy world.
The basic design of the DELTA silicon oxide varistor (SOV) arrestor is a pair of metal electrodes separated by a silicon oxide compound. Under normal conditions, the silicon oxide is a good insulator so that no line current flows between the electrodes to ground. When an excessively high voltage occurs on the electrodes, the high energy electrical field ionizes the silicon oxide, changing it from a compound to silicon separate from oxygen. Since the silicon ion is a good conductor, the high energy current is conducted to ground. When the voltage falls toward normal, the silicon and oxygen recombine, forming silicon oxide and shutting off the conduction.
Solacity stocks a number of models of Delta surge arrestors. Be sure to read our article about lightning protection and surge arrestors as well, Delta is mentioned in there.