Reliable operation of smart power grids depends on situational awareness made possible by real time system monitoring and accurate system condition estimation. Unfortunately, the tools used for real time system monitoring, which include supervisory control and data acquisition (SCADA) systems and phasor measurement units (PMU), are vulnerable to cyber-attacks due to the necessity of network connectivity. In this paper, the primary focus is given to the kind of cyber-attack in which a malicious entity injects false data into the power system which replaces or augments measurement data. We aim to study the impact of these false data injection attacks on PMU devices to determine and estimate the risks they represent. Two different scenarios are investigated, with and without redundancy of PMU measurements. Potential operator actions after a combined cyber-physical attack on the grid are also studied.