I. Introduction
More and more renewable energy sources are being connected to power systems, often via dc/ac converters (also called inverters). The most important and basic requirement for such applications is to keep inverters synchronized with the grid before and after being connected to the grid so that 1) an inverter can be connected to the grid and 2) the inverter can feed the right amount of power to the grid even when the grid voltage changes its frequency, phase, and amplitude [1]– [8]. It has been a norm [9] to adopt a synchronization unit, e.g. a phase-locked loop (PLL) and its variants [4], [10]– [15], to make sure that the inverter is synchronized with the grid. This practically adds an outer-loop controller (the synchronization unit) to the inverter controller.