Electromigration (EM) continues to be a serious concern for large chip design. We are focused on EM in the on-chip power grid, because grid lines carry mostly unidirectional currents and because of the very large sizes of modern grids. In the last few years, the capability to simulate EM has become available by simulating the stress in metal lines, which is the main cause of EM-induced failures. In this work, we have improved on the state of the art by developing a new EM simulator that is both faster and has better features than previous work. The work builds on recent results on the equivalence between stress and voltage, and introduces both a model reduction technique that provides up to 4.2X speedup, and a very efficient method for updating the grid currents during the void growth phase.