Voltage gradient energy ratio-based full-line quick-acting single-ended protection method for flexible direct-current line

The invention belongs to the field of power system relay protection, and discloses a voltage gradient energy ratio-based flexible direct-current line full-line quick-acting single-terminal protection method, which comprises the following steps of: constructing multiple protection criteria by using a single-terminal voltage measurement value of a flexible direct-current transmission line, improving voltage gradient calculation and energy value calculation; and fault poles can be effectively selected and faults can be isolated when different faults occur in the line. Constructing improved voltage gradient calculation by analyzing voltage fault characteristics inside and outside a flexible direct current line area, eliminating interference influence, and constructing a protection starting criterion; the quadratic sum of the voltage gradient of each sampling point is calculated to construct a voltage gradient energy value, and a whole-line fault protection criterion of energy value longitudinal comparison is cons