A novel main criterion for generator inadvertent energizing protection using the fan-shaped low impedance principle and its setting calculation method are proposed.When inadvertent energizing happens under different operating conditions，different characteristics of generator active power and measuring impedance at the protection installation point are analyzed.It can be concluded that the measuring impedance may fluctuate in any one or two of the four quadrants of the R-X plane at the initial moment of the inadvertent energizing.The generator inadvertent energizing protection using the fan-shaped low impedance principle has a large enough action area in each of the four quadrants of the R-X plane.It can not only reliably prevent the misoperation of the protection equipment when the generator is connected to the grid normally，but also can ensure highly sensitive tripping when inadvertent energizing happens any time of the stop or start process.The protection equipment using the novel criterion has been put into operation.