Fuse blew in transformer. At 2:45am voltages were still good and above the threshold of 166v or 80%. At 3:35am the fuse finally blew completely and voltage dropped below 166v on B phase. Once fuse blew all together, generator kicked in and repowered site as it should. To save future issues I raised Voltage Dropout from 166v/80% to 185v/90%. Topped off generator because it ran for several hours. Level is at 89%.