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Cloud computing will play an essential role in smart power grid by providing a cost-efficient, scalable IT platform for the delivery and management of electricity. A key component in the cloud-based smart power grid is the intelligent electronic devices (IEDs), which monitor, control, and protect the power system, and collect data back into the cloud for analysis and management. To maintain proper and robust operations of the power system, the collected data must be time-stamped under a common time reference. This in turn demands that the clocks of the distributed IEDs be synchronized to the required precision. A viable approach is to equip some of the IEDs with the Global Positioning System (GPS) as the time references and use the communication links already existing between the IEDs to synchronize the clocks of those without GPS. The question is, given a set of such references nodes, how to decide whether the clocks of all the IEDs can be synchronized to the required precision. If not, how can one add fewest reference nodes to meet the requirements? This paper addresses these issues and proposes efficient algorithms to solve the problems.