A widearea backup protection algorithm based on the limited phasor measurement units (PMUs) is studied. With onebus spaced placement strategy for distributed PMUs

the bus voltage values of regional nodes are estimated by the least squares estimation method

and then the estimated line current values are calculated by estimated voltage values. After sorting the residuals of measured and estimated current values

the two lines with largest estimated values are taken as the suspected fault lines. The fault lines are determined by calculating multiple fault locations of suspected fault lines with the multiple groups of voltage fault components. Using the characteristics of fault locations lying between [0

1]

the three kinds of line fault criterions are given. The simulation results on an IEEE 14bus testing system illustrate that the proposed algorithm can accurately determine the fault lines and is not influenced by fault location and fault types. This work is supported by National Natural Science Foundation of China (No. 51377137).