I. Introduction
Most OF THE modern power-electronic apparatus need accurate information on the grid voltage. The voltage amplitude as well as its phase are both essential in order to function properly. Examples of such apparatus are power factor control, thyristor rectifiers for electrical drives, and grid-connected converters such as the static series compensator (SSC), commercially known as the dynamic voltage restorer. The dynamic performance of the SSC is important since even a dip of 20- to 50-ms duration can already lead to serious disruption of production processes. Thus, vector voltage and current control have been proposed to control the output voltage of the SSC [1], [17]. Vector-controlled SSC has put a more demanding application that requires accurate synchronization with the grid. The phase angle of the grid voltage is exploited to calculate the reference of the injected voltage in the synchronous reference frame. An error in the phase-angle detection may lead to significant errors in the injected voltage and the load may be disturbed.